Heavy ion induced modifications on morphological, magnetic and magneto-transport behaviour of exchange biased Fe/NiO and NiO/Fe bilayers with Si substrate for spintronic applications

Heavy ion induced modifications on morphological, magneticand magneto-transport behaviour of exchange-biased Fe/NiOand NiO/Fe bilayers with Si substrate for spintronic applications

Neelabh Srivastava1,2 • P. C. Srivastava1

Received: 7 July 2015 / Accepted: 31 July 2015 / Published online: 12 August 2015

� Springer Science+Business Media New York 2015

Abstract Exchange-coupled interfacial structures of Fe/

NiO and NiO/Fe with pSi substrate have been studied and

also the effect of swift heavy ion irradiation on the mor-

phological, structural, transport and magnetic behaviour is

reported. The interfacial structures have been characterised

from X-ray diffraction (XRD), magnetic force microscopy/

atomic force microscopy, X-ray photoelectron spec-

troscopy and magnetisation characteristics. XRD and X-ray

photoelectron spectroscopy studies have shown the for-

mation of various silicide and oxide phases due to the

interfacial intermixing across the interfaces which is found

to affect the transport and magnetic behaviour. A signifi-

cant enhancement in exchange bias field and coercivity has

been observed for Fe/NiO/pSi interfacial structure on the

irradiation (as compared to unirradiated ones). The

observed enhanced exchange bias and coercivity on the

irradiation has been understood due to creation of

uncompensated surface/pinned interfacial spins. Magnetic

field-induced enhanced current has been observed at low

temperatures (50–250 K) for the irradiated structure sug-

gesting the spin-mixing effect. Low temperature magneto-

transport study across the irradiated interface has shown

negative magnetoresistance (MR) as compared to unirra-

diated ones for which positive MR is observed. The

observed change in MR at low temperatures has been

understood in terms of diffuse scattering at grain

boundaries/spin-disorder scattering and/or magnetic polar-

ons. Role of interfacial modification/changes in chemical

environment across the interfaces is invoked for the

observed changes in magnetic and transport behaviour of

the structures. A possible explanation for the observed

changes is given.

Introduction

Exchange coupling phenomenon between magnetic thin

films/multilayers in which the magnetisation of one layer is

influenced by the proximity of another layer can be

exploited to make spin-dependent electronic devices such

as spin valves, magnetoresistive random access memory

and giant magnetoresistance read heads [1, 2]. The phe-

nomenon of exchange bias has been extensively studied

since its discovery for more than 50 years ago by Meik-

lejohn and Bean in ferromagnetic (FM) Co particles sur-

rounded by a layer of antiferromagnetic (AF) CoO [3]. It is

well known that when a system of FM/AF is cooled

through the Neel temperature (TN) of AF with Curie tem-

perature, TC[ TN, exchange bias (EB) is induced in the

system. In spite of several intensive experimental and

theoretical investigations, several aspects of the underlying

mechanism are still lacking due to the lack of information

about the structural and chemical environments across the

interface which may affect the interface magnetic structure.

Different models [4–8] have been proposed earlier to

explain the exchange bias phenomenon which claims that

the exchange bias effect is of interfacial origin, but it also

involves several layers of AF moments. Therefore, it is

very sensitive to the microstructure of the bilayer and also

to its interface. So, the arrangement of spins at the interface

often plays a very crucial role in determining their

& Neelabh Srivastava

[email protected]

1 Department of Physics, Banaras Hindu University, Varanasi,

UP 221005, India

2 Institut fur Halbleiteroptik und Funktionelle Grenzflachen

(IHFG), Allmandring 3, Universitat Stuttgart,

70569 Stuttgart, Germany

123

J Mater Sci (2015) 50:7610–7626

DOI 10.1007/s10853-015-9321-5

magnetic properties and has much profound consequences

for its possible practical applications. Hence, one might

expect that small structural modifications across the inter-

face by any means could cause considerable change in the

magnetic property such as exchange bias field (Hex) and

coercivity (Hc).

Ion irradiation is known to be an excellent tool to

modify the magnetic properties of magnetic thin films and

multilayers, such as exchange coupling between the layers

by intermixing at the interfaces thereby offering an

important tool for device optimisation [9]. The magnetic

properties of ultrathin magnetic films and multilayered

structures e.g. magnetic anisotropies and exchange cou-

pling are strongly dependent on the film microstructure,

quality of the interfaces [10, 11] as well as also on the

interfacial interactions between the layer and the substrate,

and hence the magnetic properties of such materials can be

tailored by microstructural modifications [9]. When the

swift heavy ions (SHIs) traverse through the target mate-

rial, the ions lose their energy to the material by two dif-

ferent processes called as nuclear energy loss (Sn) and

electronic energy loss (Se) and then this energy will be

transferred to the lattice atoms via ‘electron–phonon cou-

pling’ causing the thermal spike phenomenon [12] for

creating the defects and structural modifications. The loss

of energy at low ion energies (\10 keV/nucleon) is by

elastic collisions referred as nuclear energy loss, whereas

the energy loss at higher energies ([1 MeV/nucleon) is by

inelastic collisions resulting in excitation or ionisation of

atoms, referred as electronic energy loss. The electronic

energy loss for SHI is, generally, about two orders of

magnitude higher than the nuclear energy loss [13]. SHIs

during its passage through material can cause defect

annealing, cluster of point defects and columnar type of

defects, phase transformation and intermixing at the

interfaces [14] depending on the mass and energy of the ion

and the material. Therefore, SHIs can be used for engi-

neering the defects in the materials. Swift heavy ions, in a

controlled manner, can also produce changes at the bulk,

thin film surfaces, interfaces and is becoming increasingly

important in basic and applied research. The effects of ion

irradiation on the magnetic properties of exchange biased

bilayers have been widely reported [15–17].

Recently, an increase in the exchange bias effect

accompanied with a complete reorientation of exchange

bias direction with the fluence through He? irradiation in

an IrMn/Cu/Co structure has been reported [18]. Local

manipulation of the exchange bias has been achieved by He

ion irradiation under an external magnetic field in FeNi/

FeMn bilayers [19]. Magnetic properties such as magnetic

anisotropy and exchange bias can be modified by means of

ion irradiation in Co/Pt multilayers [20–22] and FePt alloys

[23]. The effect of 40 keV C? ion irradiation on NiFe–

NiMn, CoFe–PtMn and CoFe–IrMn exchange bias systems

has been investigated by Lai and co-workers [24, 25]. Ion

irradiation has also been used for the study of transport

properties in relation to ferromagnetic/antiferromagnetic

bilayers. Increase in magnetoresistance has been found in

Fe/Cr multilayers by Xe ion irradiation [26]. Lin et al. [27]

have shown the decrease in magnetoresistance in a spin

valve structure by Ni ion irradiation. Kac et al. [28] studied

the effect of swift iodine modification of the structural and

magneto-transport properties of Fe/Cr systems where they

observed a decrease of magnetoresistance with increasing

irradiation fluence. An enhancement in magnetoresistance

data after swift heavy ion irradiation has been reported for

La0.5Pr0.2Sr0.3MnO3 epitaxial thin films grown by pulsed

laser deposition [29]. Effects of swift heavy ion bom-

bardment on magnetic tunnel junction structure have

shown the irreversible decrease in magnetoresistance with

increasing ion fluence [30].

In view of the above, in this report, we have studied the

effect of swift heavy ion irradiation on the structural,

magnetic and magneto-transport behaviour of Fe/NiO and

NiO/Fe exchange-biased bilayer, interfaced with pSi sub-

strate. It has been found from the study that there is a

significant increase in exchange bias field and coercivity

for Fe/NiO/pSi interfacial structure as compared to unir-

radiated ones. Magneto-transport study has shown the

positive magnetoresistance (MR) data at low temperatures

for unirradiated structure which in turn changed to negative

MR after ion irradiation. The results were discussed in the

realm of interfacial modifications of chemical/structural

environment across the interface (in the antiferromagnetic

layer of NiO) for the observed change in magnetic property

and magnetoresistance modification.

Experimental details

The interfacial structures of Fe/NiO and NiO/Fe were

realised by the sequential deposition of Fe (of thickness

*50 nm) and NiO (of thickness *50 nm) on pSi substrate

(of orientation: (100) and resistivity: 8–10 X cm) by

electron beam (e-beam) evaporation technique at ambient

temperature under the base pressure of better than 10-6

torr. For metallisation, Fe was used as a pure metal

(purity[ 99.99 %, metals basis) and NiO (purity[99.999 %, powder) in a pressed pellet form. Prior to met-

allisation, Si substrates were properly cleaned ultrasoni-

cally in a solution of trichloroethylene (TCE) followed by

rinsing in acetone in order to remove the adsorbed impu-

rities on the surface of wafer. After this, the cleaned Si

substrates were etched in a solution of HF:HNO3 (1:30) for

*20 s followed by rinsing with distilled water for

removing the organic contamination and any trace of

J Mater Sci (2015) 50:7610–7626 7611

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naturally formed oxide layer over Si surface and then dried

in a vacuum chamber. These fabricated bilayer structures

of Fe/NiO and the reverse interface of NiO/Fe on pSi

substrates were irradiated at ambient conditions. The irra-

diation of the structures were performed with 100 MeV

Fe7? ions using the national facility of 15UD Pelletron

accelerator housed at Inter University Accelerator Centre

(IUAC, formerly Nuclear Science Centre), New Delhi with

a fluence of 5 9 1013 ions/cm2. For irradiation, the sam-

ples were mounted on a copper target ladder and glued with

a conducting silver paste for a better thermal conduction

between the samples and holder. The incidence angle of the

ion beam was kept slightly off-normal during the irradia-

tion for a wider exposure of the sample’s surface. The size

of ion beam was focussed to a spot of*1 mm in width and

raster scanned over the entire area of sample’s surface

using a magnetic scanner for uniform irradiation of the

sample. The typical beam current during irradiation was

maintained between 3 and 5 pna (particle nano-ampere) to

avoid the heating effect in the as-prepared films, i.e. pris-

tine (or unirradiated) films.

These irradiated interfacial structures were characterised

from XRD (Philips PW-1710 diffractometer), atomic force

microscopy (AFM, Digital Instruments Nanoscope IIIa),

magnetic force microscopy (MFM, Digital Instruments

Nanoscope IIIa) and vibrating sample magnetometer

(VSM) (EV7 ADE Technologies) facilities. Magnetisation

(M) vs magnetic field (H) measurement was performed for

both the field orientations, i.e. for in-plane field (magnetic

field applied along the plane of the interface, ||) and out-of-

plane field (magnetic field applied perpendicular to the

interface plane, \). The diamagnetic contribution arising

from the Si substrate was subtracted from the measured

data by performing M–H loop of the Si substrate of similar

dimensions as that of thin film samples. The magnetic

domain structures were analysed by MFM technique.

Temperature variation (from room temperature to 9 K)

electronic and magneto-transport measurements were also

carried out across the interfaces for its magneto-electronics

application point of view.

Results and discussion

XRD study

To investigate the structural modifications induced by swift

heavy ion irradiation, we have performed XRD study.

Figure 1a, b shows the XRD patterns of the irradiated

interfacial structures of Fe/NiO and NiO/Fe, realised on pSi

substrates, respectively. The observed diffraction peaks

have been identified from JCPDS card data and are tabu-

lated in Tables 1 and 2.

The XRD patterns have shown the formation of various

phases of silicides and oxides as a result of irradiation-

induced interfacial intermixing (tabulated in Tables 1, 2).

From Fig. 1, it is also clear that the diffraction peaks of Si

(400) peak, observed in the case of pristine sample (dis-

cussed elsewhere [31]) has completely diminished out and

a broad peak has emerged out with a prominent intensity on

the irradiation, for all the structures.

The emergence and formation of oxide phases have been

also discussed in our earlier study [31] on unirradiated

structures. The formation of silicide phases on the irradi-

ation can be further understood in the realm of irradiation-

induced interfacial intermixing. The irradiation-induced

compound formation could be understood due to the huge

electronic energy loss (Se) of the irradiating ions

(*100 MeV Fe7? ions) in the interfacial region. The

electronic energy loss (Se) and nuclear energy loss (Sn) are

*1.259 9 104 and 2.275 9 101 keV/lm, respectively, for

100 MeV Fe irradiating ions in Fe–Ni target material

(calculated from SRIM data [13]). The large electronic

energy loss causes the thermal spike phenomenon [12]

which leads to the intermixing and alloying. It seems that

the irradiation-induced interfacial intermixing has caused

the broadening in the XRD peaks and the formation of the

observed silicide phases.

MFM/AFM study

The morphology of the structures has been studied because

the magnetic property also depends on the morphological

details of the structures. AFM and MFM have been carried

out from Digital Instruments Nanoscope IIIa facility. The

MFM images were recorded for a lift height of *120 nm

in order to avoid any topographical contrast due to AFM

signal. The two-dimensional (2D) AFM/MFM images of

the pristine and as well as irradiated Fe/NiO/pSi structures

are shown in Fig. 2. The grain size, root mean square (rms)

roughness data (RMS), domain size and magnetic signal

strength could be estimated from the software attached

with the AFM/MFM facility.

Figure 2a, a0 shows the AFM image of the pristine and

irradiated Fe/NiO/pSi interfacial structures, respectively.

The AFM micrograph of the pristine Fe/NiO/pSi bilayer

structure (Fig. 2a) clearly reveals that there are polycrys-

talline grains where the grain boundary is clearly observed.

It is observed that the grains are being modified to ellip-

soidal-shaped grains (Fig. 2a0) without any grain boundary

on the irradiation. The root mean square (rms) roughness

has been observed to increase to *15.5 nm (from 10 nm,

for unirradiated ones; Table 3). The grain size has been

found to enhance on the irradiation and the average grain

size has been found to be in the range of *150–200 nm. It

can also be observed from the AFM micrograph (Fig. 2a0)

7612 J Mater Sci (2015) 50:7610–7626

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that the grains seem to be aligned along a specific direction

on the irradiation, as marked by a line on the micrograph.

Such evolution of shape on the irradiation takes place due

to the recrystallisation phenomenon [32] of swift heavy ion

irradiation.

Figure 2b, b0 shows the MFM images for the pristine

and irradiated Fe/NiO/pSi interfacial structures, respec-

tively. The MFM images (Fig. 2b, b0) show the similar

feature as observed in the AFM micrographs for both the

pristine as well as irradiated structure. The estimated

domain size has been found to be same as of the grain size

(Table 3). Thus, it looks like that as if the grains were of

single-domain in nature (for both pristine and irradiated

structures). However, such observation may also be

understood due to the phenomenon of ‘cluster-edge effect’

[33].

Similar study has also been performed for the reverse

interfacial structure, i.e. NiO/Fe bilayer interfaced with pSi

substrate. Figure 3a, b shows the AFM micrographs of the

unirradiated and irradiated NiO/Fe/pSi interfacial struc-

tures, respectively. The images were scanned over a scan

area of 5 9 5 lm2. Surface topography of unirradiated

structure (Fig. 3a) shows the feature of aligned grains

along a line with an average grain size distribution of

*80–600 nm. The rms roughness was found to be of

*22 nm. The line scan analysis of the corresponding AFM

Fig. 1 XRD pattern of the unirradiated [31] and irradiated (with a fluence of 5 9 1013 ions/cm2) a Fe/NiO/pSi, and b NiO/Fe/pSi interfacial

structures

Table 1 XRD data of the irradiated Fe/NiO/pSi (with a fluence of 5 9 1013 ions/cm2) interfacial structure

Sample description Angle (2h) d value (A) Peak width (2h) Identified possible phases

Fe/NiO/pSi 16.005 5.5331 0.600 Cello tape

17.655 5.0195 0.600 b00-Fe2O3(102)

24.695 3.6022 0.700 b00-Fe2O3(017)

44.435 2.0372 0.200 Fe2O3(410)/Ni(111)/NiSi(210)/Ni3Si(220)/FeNi3(111)

64.050 1.4526 0.600 FeSi2(103)/Fe5Si3(400)/Ni3Si(223)/b-Fe2O3(541)

67.980 1.3779 0.800 Fe5Si3(222)/Ni3Si(512)

Table 2 XRD data of the irradiated NiO/Fe/pSi (with a fluence of 5 9 1013 ions/cm2) interfacial structures

Sample description Angle (2h) d value (A) Peak width (2h) Identified possible phases

NiO/Fe/pSi 15.985 5.5712 0.300 Cello tape

24.500 3.6304 0.300 Cello tape

37.575 2.3918 0.300 NiFe2O4(222)/FeSi2(101)

44.490 2.0348 0.200 Fe2O3(410)/Ni(111)/NiSi(210)/Ni3Si(220)/FeNi3(111)

J Mater Sci (2015) 50:7610–7626 7613

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image (Fig. 3a) is shown in Fig. 3a0 which clearly shows

the grain size of *441 nm. AFM image of the irradiated

structure (Fig. 3b) shows the nice spherical-shaped grains

with a uniform distribution. The average grain size and

roughness data calculated from the AFM micrograph was

found to be in the range of *100–500 and *8 nm,

respectively (Table 3). The line scan sectional analysis of

the AFM image for the irradiated structure (Fig. 3b0) showsthe grain size of *392 nm.

Figure 4a, b shows the MFM images of the unirradiated

and irradiated NiO/Fe/pSi structures, respectively. MFM

images for both the unirradiated and irradiated structures

show the similar feature as observed in AFM images. The

average domain size calculated for the unirradiated

Fig. 2 AFM image of a unirradiated, and (a0) irradiated (with a fluence of 5 9 1013 ions/cm2) Fe/NiO/pSi interfacial structure. b Corresponding

MFM image of (a), and b0 corresponding MFM image of a0 for a lift height *120 nm

Table 3 Comparison of the data obtained from AFM and MFM for unirradiated and irradiated (with a fluence of 5 9 1013 ions/cm2) Fe/NiO

and NiO/Fe interfacial structures on pSi substrate

AFM/MFM Fe/NiO/pSi NiO/Fe/pSi

Unirradiated Irradiated Unirradiated Irradiated

Average grain size distribution (in nm) *50–100 *150–200 *80–600 *100–500

Roughness (in nm) *10 *15.5 *22 *8

Domain size (in nm) Same as of the grain size Same as of the grain size *80–500 *30–150

Magnetic signal strength (in degree) – – *0.575� *0.479�

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structure from the line scan analysis of the MFM image

(Fig. 4a0) has been found to be in the range of

*80–500 nm with a phase shift of 0.575� (Table 3).

Moreover, for the irradiated structure, the average domain

size calculated from its line scan analysis (Fig. 4b0) has

been found to be in the range of*30–150 nm with a phase

shift of *0.479� (Table 3). It is also noteworthy to men-

tion here that on the irradiation, there is a decrease in

average particle size, domain size and magnetic signal

strength as compared to the unirradiated ones.

The observed increase and decrease in roughness on the

irradiation could be understood due to the two phenomena

of swift heavy ion irradiation. When the film is irradiated,

two phenomena of grain growth [34] and intermixing of

atoms [19] can take place. Grain growth leads to the

increase in interface roughness while intermixing of atoms

causes it to decrease. Moreover, the increase in roughness

on the irradiation could also be attributed to several phe-

nomena of swift heavy ions, such as mass transport through

atomic displacements in the surface region which will lead

to increase in mobility of adatoms and sputtering from the

surface, evaporation from the hot surface and modifications

due to melting and recrystallisation. Thermal spike phe-

nomenon [12] of swift heavy ion irradiation also causes

modifications due to the melting and recrystallisation [32]

in the structures. The thermal spike phenomenon is due to

high electronic energy loss of swift heavy ions in near

surface/interfacial regions to produce a very high temper-

ature for a very short time of pico-second (*10-12 s).

Magnetisation study

Magnetic hysteresis loops (M–H characteristics) were

measured for the irradiated interfacial structures from VSM

facility at room temperature by sweeping the applied field

from ?17.5 to -17.5 kOe and back to ?17.5 kOe. Fig-

ure 5a, b shows the M–H characteristics for irradiated Fe/

NiO/pSi and NiO/Fe/pSi interfacial structures, respec-

tively. The magnetisation characteristics have been cor-

rected for the diamagnetic contribution of the silicon

Fig. 3 AFM image of a unirradiated, and b irradiated (with a fluence of 5 9 1013 ions/cm2) NiO/Fe/pSi interfacial structure. a0 line scan

analysis of the corresponding AFM image (a). b0 Line scan analysis of the corresponding AFM image (b)

J Mater Sci (2015) 50:7610–7626 7615

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substrate. The magnetisation characteristics have been

measured for both magnetic field orientations, i.e. for in-

plane field (magnetic field applied along the plane of the

interfacial structure) and out-of-plane field (magnetic field

applied perpendicular to the plane of the interfacial struc-

ture). The magnetic parameters have been estimated for the

irradiated structures for both the field orientations and are

tabulated in Table 4 and compared with the magnetic

property data of the pristine, i.e. unirradiated structures.

Comparing the data for Fe/NiO/pSi interfacial structures

(prior to and after irradiation; Table 4), it can be seen that

coercivity (Hc) is increased from *41.5 to 66 Oe for in-

plane orientation whereas for out-of-plane orientation

coercivity is increased to *90 from 39 Oe. Moreover, no

significant exchange bias field (Hex) is observed (both for

irradiated and unirradiated structures). The coercivity has

been found to be significantly enhanced on the irradiation

for out-of-plane orientation as compared to in-plane ori-

entation. The observed magnetic behaviour can be corre-

lated with the observed shape of the grains. It could be

understood due to ellipsoidal shape of the grains (on the

irradiation) with a larger dimension along the interfacial

plane (Fig. 2a0) and shorter dimension along the direction

perpendicular to the interface. From the magnetisation data

(Table 4), it seems that the easy axis of magnetisation is

along the interfacial plane whereas out-of-interfacial plane

behaves as a hard axis. It is also interesting and significant

to observe that other magnetic parameters such as satura-

tion magnetisation (Ms) and vertical shift have also

enhanced on the irradiation.

The M–H characteristics recorded for the irradiated

NiO/Fe/pSi interfacial structures (Fig. 5b) show coercivity

(Hc) of *68 Oe and exchange bias (Hex) of *10 Oe for

in-plane orientation whereas for out-of-plane orientation,

the observed coercivity and exchange bias field is *108

and 22 Oe, respectively.

The magnetic property of the structures on the irradia-

tion (and also prior to irradiation) could be understood due

to the different interfacial exchange coupling mechanisms

across Fe/NiO and NiO/Fe bilayers, interfacial chemistry

Fig. 4 MFM image a of unirradiated, and b irradiated (with a fluence of 5 9 1013 ions/cm2) NiO/Fe/pSi interfacial structure. a0 The line scan

sectional analysis of the magnetic image (a). b0 The line scan sectional analysis of the magnetic image (b)

7616 J Mater Sci (2015) 50:7610–7626

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and also the associated charge carriers of p-type silicon

substrates.

The value of remanence (Mr) and vertical shift has also

been observed to increase on the irradiation. The increase

of remanence on the irradiation shows the presence of

strong exchange coupling. Moreover, this increase in

remanence and vertical shifting could be understood due to

the increase in the number of pinned interfacial spins.

However, the presence of metallic Ni has also been

suggested to be decisive for exchange bias and increased

coercivity which was related to the interfacial uncompen-

sated surface spins [35]. The presence of metallic Ni phase

has been confirmed by us from the XRD data (Tables 1, 2)

and also from our XPS data (discussed in next section).

Since, a significant magnetic behaviour has been observed

for Fe/NiO/pSi interfacial structures (before and after

irradiation) so further measurements (viz. XPS and trans-

port study) have been done only for Fe/NiO/pSi structure.

Fig. 5 M–H characteristics of the irradiated (with a fluence of 5 9 1013 ions/cm2) a Fe/NiO/pSi, and b NiO/Fe/pSi interfacial structures for both

applied magnetic field orientations (i.e. parallel and perpendicular to the interfacial plane)

Table 4 Comparison of the magnetic property data between unirradiated [31] and irradiated (with a fluence of 5 9 1013 ions/cm2) Fe/NiO and

NiO/Fe interfacial structures on pSi substrate

Magnetic field orientation Magnetic parameters Fe/NiO/pSi NiO/Fe/pSi

Unirradiated [31] Irradiated Unirradiated [31] Irradiated

In-plane field (0�) Ms 3.4 9 10-4 emu 6.5 9 10-4 emu 0.6 9 10-4 emu 6.4 9 10-4 emu

Hc 41.5 Oe 66 Oe

Not perceptible

68 Oe

Hex 1.5 Oe 2 Oe 10 Oe

Mr 2.2 9 10-5 emu 3.5 9 10-5 emu 3.7 9 10-5 emu

Vertical shift 2.0 9 10-6 emu 1 9 10-6 emu 1 9 10-6 emu

Squareness 0.065 0.053 0.057

Out of plane field (90�) Ms 2.8 9 10-4 emu 3.7 9 10-4 emu 0.4 9 10-4 emu 3.4 9 10-4 emu

Hc 39 Oe 90 Oe

Not perceptible

108 Oe

Hex 11 Oe 14 Oe 22 Oe

Mr 3.8 9 10-6 emu 8.4 9 10-6 emu 1.7 9 10-5 emu

Vertical shift 0.3 9 10-6 emu 5.2 9 10-6 emu 2.0 9 10-5 emu

Squareness 0.014 0.023 0.052

J Mater Sci (2015) 50:7610–7626 7617

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XPS study

To study the chemical processes and bonding states

information about the elements across the interfaces for

both unirradiated and irradiated structures, X-ray photo-

electron spectroscopy study has been carried out. XPS

study was performed at room temperature using VSW-

ESCA photoelectron spectrometer with Alka (energy,

ht * 1486.6 eV) X-ray source. An energy analyzer was

operated at pass energy *40 eV resulting in an energy

resolution of *0.9 eV. The source emission current and an

anode voltage were maintained at *10 mA and *9 kV,

respectively, during the measurement. The samples were

properly grounded (with conducting silver paste) in order

to avoid any charging effect. The surface of the samples

was also sputter cleaned using low energy (*3.5 keV) Ar?

ion gun (in a separate vacuum chamber) which was

equipped with the XPS machine and XPS data were col-

lected for various sputter durations, i.e. for 25 min. (e25)

and again further 25 min. (e50). The shift in binding

energy (B.E.) positions of the core level signals was cor-

rected by taking C1s line *284.6 eV as an internal ref-

erence. Quantitative information about the films/interfaces

has been analysed by collecting the integrated intensities of

C1s, O1s, Fe2p, Ni2p and Si2p signals. The peaks of the

XPS binding energies were deconvoluted with Gaussian

peak shapes using freely available XPSPEAK4.1 software

[36]. Prior to peak fitting, Shirley background was sub-

tracted and then peaks were deconvoluted. The B.E. posi-

tions thus obtained were used for interpretation of the

spectra.

Figure 6a, b shows the survey scan XPS spectra of Fe/

NiO/pSi interfacial structures, prior to and after irradiation

for various sputter durations. It can be clearly seen that

prior to sputter etching (e0), the spectra contain the

prominent photoemission signals due to carbon (C1s) and

oxygen (O1s) for both the structures (prior to and after

irradiation) as compared to other signals of iron (Fe2p) and

nickel (Ni2p). Such prominent signals due to C and O can

be understood due to atmospheric exposure of the sample’s

surface during the sample transfer to vacuum chamber. It is

also noteworthy to mention here that carbon is an ubiqui-

tous contaminant and is present on nearly any surface. It is

almost always present before the transfer to high vacuum

chamber but could also originate due to the adsorption of

contaminant species from the ambient gases in vacuum.

Moreover, for unirradiated structure (Fig. 6a), one can see

that the contamination due to carbon and oxygen is almost

negligible and the signals due to Fe and Ni are emerging

out with a prominent intensity only after sputter cleaning of

50 min. (e50) whereas for the irradiated structure (Fig. 6b)

the signals due to Fe and Ni are observed only after 25 min

sputter etching (e25) which is further more pronounced

after 50 min cleaning. Thus, it looks that after sputter

etching, the atmospheric impurities (like O and C) are

getting removed from the sample’s surface. For irradiated

structure (Fig. 6b), it is also interesting to observe that after

sputter cleaning (for e25 and e50), the signal due to Si is

also coming out with a significant intensity which was

absent prior to irradiation (Fig. 6a). So, the presence of Si

signal within the probe depth and emergence of Fe and Ni

signal with a prominent intensity only after 25 min etching

(e25) for the irradiated structure also confirms the role of

irradiation-induced interfacial intermixing across the

interfaces.

To gain more insight about the observed elements (Fe,

Ni, O and Si) and to analyse the variation in content of Fe

and Ni either in the form of oxide and silicides, separate

detail scans corresponding to each elements have been

recorded and further deconvoluted. As from the survey

scan spectra, it is evident that after 50 min sputter cleaning,

the signal due to contaminants is less and signals for the

relevant species (such as Fe, Ni and Si) are more prominent

so hereafter we discuss the deconvoluted spectra only after

50 min. surface cleaning (e50) for both the structures (prior

to and after irradiation).

Figure 7a, b shows the deconvoluted Fe2p core level

signals due to Fe2p3/2 and Fe2p1/2 corresponding to unir-

radiated and irradiated interfacial structure, respectively, in

a narrow scan between 700 and 730 eV. The deconvoluted

Fe2p spectra prior to irradiation (Fig. 7a) show four dis-

tinct B.E. peaks at *707.8, 709.9, 720.8 and 722.7 eV.

The observed peak positions correspond to the presence of

both metallic and oxide phases of iron. Peaks at B.E.

position of *707.8 eV (Fe02p3/2) and 720.8 eV (Fe02p1/2)

relate to the metallic phase of iron. Whereas the B.E.

positions *709.9 eV (Fe2?2p3/2) and 722.7 eV (Fe2?2p1/2)

correspond to oxide phase of iron, i.e. due to Fe2O3.

Moreover, for irradiated structure (Fig. 7b), the Fe2p

spectrum has also been deconvoluted into four peaks at

*707.8, 711.5, 720.9 and 723.0 eV. The B.E. positions at

*707.8 eV (Fe02p3/2) and 720.9 eV (Fe02p1/2) correspond

to metallic phase of iron, and *711.5 eV (Fe2?2p3/2) and

723.0 eV (Fe2?2p1/2) correspond to oxide phase of iron

(Fe2O3 phase). The spin–orbit doublet separation between

2p3/2 and 2p1/2 for metallic phase is observed to be

*13.0 eV (for unirradiated structure) and *13.1 eV (for

irradiated structure) which is close to the earlier reported

separation value *12.8 eV [37]. It also looks that after

irradiation the B.E. positions are slightly shifted towards

higher binding energy side which is a signature that some

chemical modifications have taken place across the inter-

face. Moreover, after irradiation it is also clear that con-

tribution due to oxide component is dominant over metallic

phase as the peak width and intensity of oxide phase of iron

are getting enhanced as compared to prior to irradiation

7618 J Mater Sci (2015) 50:7610–7626

123

(where metallic phase is dominant). This observation can

be understood due to chemical reaction at Fe/NiO interface

which is given below:

Feþ NiO ! Niþ FeO

or; 2Feþ 3NiO ! 3Niþ Fe2O3:

So, it looks that oxygen from NiO is diffusing towards

Fe to form iron oxide phase after irradiation and leaving

behind the metallic phase of nickel (also confirmed from

the nickel spectra; discussed later). The reaction is also

thermodynamically favourable which is earlier shown by

Yu et al. [38]. Moreover, on careful observation from the

survey scan spectra of irradiated structure, it can be seen

that intensity of oxygen peak is enhanced (for e50 relative

to e25) which also confirms that this is not surface oxygen

but is coming from the NiO interface as a result of irra-

diation-induced mixing.

Figure 8a, b shows the deconvoluted Ni2p spectra for

unirradiated and irradiated interfacial structures, respec-

tively. The spectra of Ni2p prior to irradiation have been

deconvoluted in four different narrow scans viz.

851–858 eV (Fig. 8a-i), 857–865 eV (Fig. 8a-ii),

868–874 eV (Fig. 8a-iii) and 873–881 eV (Fig. 8a-iv). The

recorded spectrum has shown the Ni2p signals at B.E.

positions of *853.8 eV (due to Ni2?2p3/2) and 872.2 eV

(Ni2?2p1/2) along with their satellite peak positions at

Fig. 6 Core level XPS survey scan spectra of a unirradiated, and b irradiated (with a fluence of 5 9 1013 ions/cm2) Fe/NiO/pSi interfacial

structure for various sputter etching durations of the surface

Fig. 7 Deconvoluted XPS spectra of Fe2p for a unirradiated, and b irradiated structures

J Mater Sci (2015) 50:7610–7626 7619

123

*862.4 and 879.6 eV, respectively. The observed B.E.

difference between Ni2p spin–orbit doublet spectra, i.e.

between 2p3/2 and 2p1/2 signals is *18.4 eV which mat-

ched very well with the standard doublet separation

between Ni2p3/2 and Ni2p1/2 is *18.4 eV for NiO whereas

the doublet separation is *17.4 eV for metallic Ni phase.

The B.E. position at *853.8 eV (Fig. 8a-i) mainly corre-

sponds to Ni2?2p3/2 due to NiO or NiSi [39] along with a

satellite peak at 860.1 eV (Fig. 8a-ii). The other observed

multiplet-split B.E. positions at 855.0 and 857.8 eV

(Fig. 8a-i) seem to correspond to the silicide phase of

nickel, NiSi2 [40] along with a satellite peak at 862.4 eV

(Fig. 8a-ii). The multiplet-split due to NiO corresponding

to Ni2?2p1/2 is observed at *870.6 eV (Fig. 8a-iii) along

with the satellite peaks at *877.4 and 879.6 eV (Fig. 8a-

iv). Moreover, for the irradiated structure, the spectra have

been deconvoluted into two different narrow scans viz.

848–864 eV (Fig. 8b-i) and 867–876 eV (Fig. 8b-ii). The

spectra contain the main signals at *853.7 eV (due to

Ni2?2p3/2) and 870.9 eV (due to Ni2?2p1/2) along with a

shoulder and satellite peak at *854.5 and 860.6 eV

(Fig. 8b-i), respectively. The shoulder peak at *852.0 eV

(Fig. 8b-i) also corresponds to the metallic phase of Ni

(Ni02p3/2) which could be in support to the outward dif-

fusion of oxygen atom towards Fe to form iron oxide (as

discussed earlier).

Figure 9a, b shows the deconvoluted spectra of O1s for

unirradiated and irradiated structures, respectively. The

deconvoluted spectra of Fe2p for unirradiated structure

(Fig. 9a) show two distinct peaks at *530.2 and 531.2 eV

which mainly correspond to signal due to NiO and ele-

mental nature, respectively. Similar feature is observed for

the irradiated structure (Fig. 9b) with a slight shift in B.E.

value towards higher side at *530.4 and 531.8 eV,

respectively, as compared to unirradiated ones.

Figure 10 shows the deconvoluted spectra of Si2p for

the irradiated structure only as we could not observe the Si

signal prior to irradiation. The Si2p spectra have been

deconvoluted in two different narrow scans viz. 90–105 eV

(Fig. 10-i) and 108–113 eV (Fig. 10-ii). The deconvoluted

spectrum shows the B.E. positions at *91.7, 94.4, 100.8,

110.4 and 111.9 eV. The B.E. at *91.7 eV along with a

satellite signal *94.4 eV corresponds to Fe3s due to oxide

phase of iron (Fig. 10-i). The other B.E. peaks at

*110.4 eV seem to correspond to Ni3s signal and

*111.9 eV (Fig. 10-ii) might be due to other phase of

nickel oxide. The B.E. peak at *100.8 eV (Fig. 10-i) is

due to Si2p3/2 signal.

Thus, XPS study has confirmed the formation of various

phases of oxides and silicides as a result of irradiation-

induced interfacial intermixing (also discussed in XRD

data) which also affects the transport study across the

interfaces and magnetisation behaviour of the structures.

Electronic transport (I–V) study

I–V characteristics across the interfaces of Fe/NiO/pSi (for

both unirradiated and irradiated structures) were measured

by using top–bottom contacts in CPP (i.e. current perpen-

dicular to the plane of the interface) configuration, as shown

in Fig. 11. For applying the bias voltage and to measure the

corresponding current, computer-controlled Keithley 2400

Source Measure Unit was employed. The top–bottom con-

tacts were made by contacting the conducting wires using

conducting silver paste and the back ohmic contact on Si

substrate has been realised by sandblasting the back surface

Fig. 8 Deconvoluted XPS spectra of Ni2p for a unirradiated (i–iv), and b irradiated (i, ii) structures

7620 J Mater Sci (2015) 50:7610–7626

123

before making the contact. The applied bias voltage has been

kept with respect to metal for all the measurements, i.e.

positive (?ve) voltage on the voltage axis refers to metal

positive and semiconductor negative and vice versa.

I–V measurements were performed in a closed cycle liquid

helium cryostat (M/s. Janis Research Co., USA, HC-4E)

where the temperature could be varied up to 9 K. The mea-

surement temperature was well controlled and monitored

using a cryogenic temperature controller (Lake Shore 331)

which was attached to the cryostat. The room temperature

I–V data were collected before the start of cooling process

(just after loading the sample into cryostat and recorded the

I–V data at ambient conditions).

Figure 12a, b shows the I–V characteristics of unirra-

diated and irradiated Fe/NiO/pSi interfacial structure,

respectively measured from room temperature (306 K) to

9 K. It can be seen that for unirradiated structure, the value

of forward current has decreased with decrease in tem-

perature. Moreover, on the irradiation the value of forward

current is also decreased as compared to unirradiated ones

whereas only at 250 K and room temperature (RT) an

increase in reverse current is observed (inset of Fig. 12b).

The observed decrease in current value on the irradiation

could be understood due to the formed irradiation-induced

defects, structural damage or electrically active defects etc.

Such decrease in conductivity after swift heavy ion

Fig. 9 Deconvoluted XPS spectra of O1s for a unirradiated, and b irradiated structures

Fig. 10 Deconvoluted XPS spectra of Si2p for irradiated structures

J Mater Sci (2015) 50:7610–7626 7621

123

irradiation is also earlier observed by our group [41]. The

decrease in current value with temperature shows the

semiconducting nature of the structures. It is also very

interesting and significant to observe that both the struc-

tures (unirradiated and irradiated structure) show strong

temperature dependence in forward current regime (in the

temperature range of 9–306 K). Moreover, the reverse

current shows nearly temperature-independent behaviour

for unirradiated structure whereas for irradiated structure, a

significant change in reverse current is observed for 250 K

and RT (306 K). Since, it is well known that under forward

bias condition, the injection of charge carriers takes place

from semiconductor to metal side whereas for reverse bias

condition, it follows the opposite path, i.e. from metal to

semiconductor side. Thus, it looks that change in reverse

current at 250 K and RT after irradiation is due to transport

of carriers from metal to semiconductor side. This tem-

perature dependency of reverse current after irradiation

only at 250 K and RT (inset of Fig. 12b) also shows the

possibility of tunnel transport from metal to semiconductor.

On careful analysis of I–V data, it is also observed that data

recorded at RT for irradiated structure shows nearly the

same current value for both bias polarities (?ve and -ve

voltage) which signifies the ‘Ohmic’ nature of the interface

which is in support of tunnel transport phenomenon across

the interface as compared to I–V data recorded at low

temperatures (9–250 K). Moreover, the I–V curve of

unirradiated structures show the rectification effect at all

measured temperatures (9 K to RT). Thus, it seems that a

conductivity type change has occurred after irradiation at

RT as compared to unirradiated ones.

Figure 13 shows the I–V characteristics of unirradiated

and irradiated (with 5 9 1013 ions/cm2) Fe/NiO bilayer

on pSi substrate, respectively measured at RT. I–V char-

acteristic of unirradiated structure shows the respective

diode like behaviour of the p-type silicon substrate.

However, on the irradiation, the I–V characteristic shows

conductivity type change (i.e. looks from p-type to

n-type) behaviour of the respective silicon substrate. Such

conductivity type change has also been observed earlier in

the irradiated metal/silicon and detector diodes [41, 42].

The phenomenon has been understood due to the role

Fig. 11 Schematic set-up for

measuring transport (electronic

and magneto-transport)

measurement in CPP

configuration

Fig. 12 I–V characteristics of a unirradiated, and b irradiated (with a fluence of 5 9 1013 ions/cm2) Fe/NiO/pSi interfacial structures as a

function of temperature

7622 J Mater Sci (2015) 50:7610–7626

123

played by irradiation-induced deep acceptors and donors

in the materials. The diffusion of Ni atoms in silicon

behaves as deep acceptors and oxygen in silicon behaves

as deep donors. The irradiation-induced diffusion of such

impurities can cause the observed conductivity type

change of interfaces with respective silicon substrate on

the irradiation.

Effect of magnetic field on the electronic transport

(I–V) study across unirradiated and irradiated Fe/

NiO/pSi interfacial structures

To study the effect of magnetic field sensitivity on the

electronic transport (I–V), I–V characteristics have been

recorded in an external applied magnetic field (H) of

*8 kG and the temperature was also varied from room

temperature to 9 K. The external magnetic field was

applied from an electromagnet. The interfacial structures

were placed between the poles of an electromagnet and

magnetic field was applied along the plane of the bilayer

structure and thus magnetic field also perpendicular to

current flow direction (CPP mode).

Figure 14a, b shows the I–V characteristics recorded for

unirradiated and irradiated Fe/NiO/pSi interfacial struc-

tures from 9 K to RT, respectively measured without field

and in presence of external magnetic field (8 kG). It is

evident from the I–V plots that magnetic field sensitivity

could be observed for both the structures (unirradiated and

irradiated) only at low temperatures (below RT), but not at

RT. Moreover, it is also interesting to observe that for

irradiated structures (Fig. 14b) the value of current is

increased in presence of magnetic field whereas for prior to

irradiation (Fig. 14a), the value of current in presence of

magnetic field is decreased as compared to the current data

recorded for without field. So, it looks that formed inter-

mixed compounds at the interface after irradiation can

change the NiO interface to other phases of oxides/silicides

which is somehow responsible to control the transport

process and external magnetic field is allowing them to

reorient their spins to cause less scattering at the interfaces

(present due to impurities or defects within the magnetic

layers) and thus increase in current value. However, it can

also be clearly seen that at low temperatures (at 9 and

50 K), the value of current data for both the structures is

little bit fluctuating which could be understood due to spin-

flip electron–magnon scattering. Generally, in ferromag-

netic metals, at low temperatures the spin-flip scattering of

the conduction electrons by magnons is frozen out and the

spin relaxation time is much larger than the momentum

relaxation time [43]. Moreover, at higher temperatures

(50–250 K), two mechanism of spin-mixing effect (by

electron–magnon collisions), i.e. spin-flip scattering and

propagation through alternating magnetisations of com-

pounds at the interfaces seems responsible for the observed

increase in current.

Magnetoresistance calculation

From the above reported study of magnetic field sensitivity

across both unirradiated and irradiated Fe/NiO/pSi inter-

facial structures from RT to 9 K (Fig. 14), the magne-

toresistance (MR) data have been calculated using the

following relation:

%MR ¼ RH � R0

R0

� �� 100;

Fig. 13 Comparison of room

temperature (RT) I–V

characteristics of unirradiated

and irradiated (with a fluence of

5 9 1013 ions/cm2) Fe/NiO/pSi

interfacial structures

J Mater Sci (2015) 50:7610–7626 7623

123

where RH is the resistance in magnetic field and R0 is the

resistance without magnetic field.

Figure 15 shows the variation of %MR data with tem-

perature for unirradiated and irradiated structures from RT

to 9 K. The %MR data has been calculated at an applied

voltage of -2.0 V. From the magnetoresistance data, fol-

lowing observations can be pointed out.

(i) For unirradiated structure, the MR data decrease

exponentially with temperature. A positive MR of

*100–42 % at low temperatures (from 9 to 200 K)

has been observed.

(ii) After irradiation, the MR data also follow the

similar trend (exponential variation) with temper-

ature. However, the values of MR changes sign

from positive to negative (as compared to unirra-

diated ones) and a negative MR of *-245 to

-38 % at low temperatures (9–250 K) has been

observed.

Fig. 14 Temperature variation study of I–V characteristics for a unirradiated, and b irradiated Fe/NiO/pSi structure measured in presence of

external magnetic field

Fig. 15 Variation of

magnetoresistance (%MR) data

with temperature for

unirradiated and irradiated Fe/

NiO/pSi interfacial structure

7624 J Mater Sci (2015) 50:7610–7626

123

Such variation of %MR data with temperature is sig-

nificant and interesting to observe that the unirradiated

structure shows a positive MR which becomes negative on

the irradiation. The negative and positive MR has been

understood in the realm of spin-dependent structure-related

scattering. The reason for the observed MR is not very well

understood at this stage; however, the negative MR can be

attributed to the spin-disorder scattering which is being

suppressed by the applied external magnetic field [44]. So,

it looks that grain boundaries/structural defects (created

after ion irradiation) are favouring paths for the spins to

align themselves along the direction of external magnetic

field causing less scattering. Another possible mechanism

for negative MR on irradiation at low temperatures

(50–250 K) could also be the diffuse scattering at the

crystalline boundaries and/or by magnetic polarons.

Conclusions

In summary, we have studied the ion irradiation-induced

modifications on exchange-coupled bilayers of Fe/NiO and

NiO/Fe interfaced with pSi substrate. XRD data have

revealed the formation of various silicides/oxides phases of

Fe and Ni which have been understood in the realm of

irradiation-induced interfacial intermixing. XPS study has

confirmed the interfacial chemistry modification as a result

of ion irradiation to form oxide/silicide phases. Magnetic

property data have shown the significantly enhanced

exchange bias and coercivity after irradiation for Fe/NiO/

pSi structure. The electronic transport study across Fe/NiO/

pSi bilayer structure has shown conductivity type change

on the irradiation which has been understood due to the

role played by irradiation-induced deep acceptors (trap

states) in the materials. Significant increase in current value

has been observed for the irradiated Fe/NiO/pSi structure

in presence of magnetic field at low temperature which

could be due to spin-mixing effect (electron–magnon col-

lision). Low temperature magneto-transport study across

unirradiated and irradiated Fe/NiO/pSi bilayer structure has

shown positive MR for unirradiated ones which changes to

negative MR on the irradiation. This observed change in

MR at low temperatures could be understood in terms of

diffuse scattering at boundaries/spin-disorder scattering

and/or magnetic polarons.

Acknowledgements The authors would like to acknowledge the help

received from Pelletron group, Inter University Accelerator Centre

(IUAC), New Delhi, India during the irradiation experiments. The

authors are thankful to Dr. D. K. Avasthi and Dr. D. Kabiraj for their

involvement during irradiation experiments and Dr. Indra Sulania,

Scientist (IUAC, New Delhi, India) for performing MFM/AFM mea-

surements. The authors are also thankful to Dr. T. Shripathi (Scientist

‘H’, UGC-DAE-CSR, Indore, India) for providing the access to XPS

measurement and related fruitful discussions. One of the authors (N.

Srivastava) is grateful to Council of Scientific and Industrial Research

(CSIR), NewDelhi, India for providing the financial support in the form

of Senior Research Fellowship (CSIR-SRF).

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