Combinatorial Fe–Co thin film magnetic structures obtained by thermionic vacuum arc method

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Combinatorial Fe-Co thin film magnetic structures obtained by ThermionicVacuum Arc Method

I. Jepu, C. Porosnicu, C.P. Lungu, I. Mustata, C. Luculescu, V. Kuncser,G. Iacobescu, A. Marin, V. Ciupina

PII: S0257-8972(13)01205-XDOI: doi: 10.1016/j.surfcoat.2013.12.050Reference: SCT 19095

To appear in: Surface & Coatings Technology

Received date: 5 August 2013Accepted date: 19 December 2013

Please cite this article as: I. Jepu, C. Porosnicu, C.P. Lungu, I. Mustata, C. Luculescu,V. Kuncser, G. Iacobescu, A. Marin, V. Ciupina, Combinatorial Fe-Co thin film magneticstructures obtained by Thermionic Vacuum Arc Method, Surface & Coatings Technology(2013), doi: 10.1016/j.surfcoat.2013.12.050

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Combinatorial Fe-Co thin film magnetic structures obtained by Thermionic Vacuum

Arc Method

I. Jepu1*

, C. Porosnicu1, C. P. Lungu

1, I. Mustata

1, C. Luculescu

1, V. Kuncser

2,

G. Iacobescu3, A. Marin

4, V. Ciupina

5

1National Institute for Lasers, Plasma and Radiation Physics, Magurele-Bucharest, RO

2National Institute of Materials Physics, Bucharest-Magurele, RO

3University of Craiova, Faculty of Physics, Craiova, RO

4 Institute of Physical Chemistry,”Ilie Murgulescu”, Bucharest, RO

5 “Ovidius” University, Faculty of Physics, Constanta, RO

*Corresponding author. Address: National Institute for Lasers, Plasma and Radiation

Physics, Atomistilor No. 409 Street, Magurele, Ilfov, 077125, Romania

Tel: +1 858 405 8398

Fax: +4021 457 4468

Email address: [email protected] (Ionut Jepu)

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Combinatorial Fe-Co thin film magnetic structures obtained by Thermionic Vacuum

Arc Method

I. Jepu1*

, C. Porosnicu1, C. P. Lungu

1, I. Mustata

1, C. Luculescu

1, V. Kuncser

2,

G. Iacobescu3, A. Marin

4, V. Ciupina

5

1National Institute for Lasers, Plasma and Radiation Physics, Magurele-Bucharest, RO

2National Institute of Materials Physics, Bucharest-Magurele, RO

3University of Craiova, Faculty of Physics, Craiova, RO

4 Institute of Physical Chemistry,”Ilie Murgulescu”, Bucharest, RO

5 “Ovidius” University, Faculty of Physics, Constanta, RO

Abstract

Magnetoresistive Fe-Co based thin film structures were produced using thermionic vacuum

arc method. The purpose of this work was to obtain significant magnetic response on different

granular combinatorial structures. The Giant Magnetoresistive (GMR) and combination

between GMR and Tunneling Magnetoresistive (TMR) properties of the thin film structures

were the aim of the work. The proposed method in order to obtain the desired granular

structures is based on electron beam emitted by an externally heated cathode, accelerated by a

high anodic potential.

The work consisted in preparing two sets of samples, first being a combination between Fe-

Co as magnetic materials embedded in a Cu matrix, with a total thickness of 200nm. The

second structure was a combination between Fe-Co (50%-50%) alloy, embedded in a matrix

of Cu combined with MgO with a thickness of 200nm. Both sets of samples were obtained by

three simultaneously types of discharges. Because of the substrate positioning in respect with

the three anode-cathode systems, different material concentrations were obtained, confirmed

by Energy Dispersive Spectroscopy (EDS) measurements results. Structural and

morphological properties were investigated using Scanning Electron Microscopy; Atomic

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Force Microscopy and EDS. Electrical properties of the obtained samples were studied using

the 4 point measurement method. The magnetic properties were first studied using a non-

destructive optical method called MOKE (Magneto-Optical Kerr Effect). Electrical resistance

behavior of the granular type structures was studied for different values of the magnetic field,

up to 0.3 T, at different values of the sample temperature. The magnetoresistive effect

obtained for the two sets of samples varied from 1.5% to 81 % in respect with the substrate

positioning and samples temperature for a constant magnetic field.

Keywords: Thermionic Vacuum Arc, Giant Magnetoresistive (GMR), Tunneling

Magnetoresistive (TMR), Magneto-Optical Kerr Effect (MOKE)

I. Introduction

Magnetorezistive structures are of a high interest in the magnetic memory technology,

bringing an improvement to the memory devices and in the same time a significant increase of

the reading speed of this type of storage devices. The known magnetoresistive structures are

(1) Giant Magneto Resistence (GMR) multilayer type, which consists in two ferromagnetic

layers separated by a non magnetic highly conductive metal; (2) GMR granular type, where

the magnetic metal is embedded in the non magnetic material matrices; (3) Tunnel Magneto

Resistance (TMR) multilayer where the magnetic thin film structures are separated by thin

non magnetic electrically insulating layers; (4) TMR granular type where the magnetic

elements are embedded in a non magnetic non conductive element matrices; (5) combinatorial

GMR+TMR multilayer structures, and (6) combinatorial GMR+TMR granular type of

structures [1-5]. All of these thin films can be easily obtained by Thermionic Vacuum Arc

(TVA) deposition method due to its possibility of simultaneously multiple discharges. This

deposition technique is using the energetic ions released from the anode by igniting the pure

material vapors in high vacuum atmosphere [6-10]. The plasma formed in this way is in

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contact with the high positive anode and receives, in turn, a high positive potential too.

Therefore the plasma ions flying from this high positive region towards the grounded

substrate probes are able to bombard the just forming layer with high energy. By easily

controlling the external plasma parameters like the heating of the cathode, potential applied

on anode, inter-electrode distance, the flux of energetic ions from the plasma can be as well

controlled. This (intense) ionic bombardment has a beneficial contribution to the formation of

the deposited layers, by increasing their compactness, and by ensuring a uniform distribution

of the magnetic domains of almost the same size within the non magnetic network [11-14].

It is known that magnetic structures are highly sensitive to the purity and thickness of each

contained individual layer (TMR type of structures) and to the ferromagnetic granular

concentration embedded in a non-magnetic matrix (GMR type of structures) [15-17]. The lack

of any buffer gas inclusions inside the structure represents one of the main advantages of this

deposition technique. Another positive aspect about TVA method, which led to using it in

obtaining the desired structures, consists in the possibility of simultaneously igniting [6]

inside the deposition chamber of different punctual evaporation sources that allow obtaining

of different relative elemental concentrations structures in respect with the distance between

the samples and the anode-cathode systems during the deposition process.

The thin layers obtained in different discharge conditions are analyzed in terms of the

magnetic effect and in term of the electric resistance at a zero value of the magnetic field.

This characteristic is very interesting to be studied on GMR-TMR type structures. As it is

known from literature, the GMR-type layers have a low value of the electrical resistance at a

null magnetic field applied, being made only of metals. When an electric current is passed

perpendicular on the multilayered structure (CPP case), the magnetoresistive effect is at the

highest values (as compared with CIP case). In order to obtain a significant magnetic effect in

this case there are needed high values of the electric current, which makes the GMR structure

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improper for the development of storage magnetic devices, but ideal for obtaining reading

heads of memory devices. Instead, the storage magnetic memories can be produced by using

TMR type of structures. They have higher values of the electric resistance at a null magnetic

field and have a higher magnetic effect in the presence of the magnetic field, which means in

the end that the magnetic elements of the storage devices are sensible to low values of the

electric currents used. The reason for building a combination of the GMR+TMR structure

with a Fe-Co base was to vary another important parameter of these types of layers, the null

magnetic field value of the electrical resistance, around which the value of the electric

resistance in the presence of the magnetic field oscillates. With other words, it has been tried

to obtain the same high values of the magnetoresistive effect for different null magnetic field

values of the electric resistance that is in close connection with the non magnetic insulating

relative concentration of the material present in the thin film structure. That is why, two types

of Fe-Co structures were prepared, one a GMR granular structure containing Fe, Cu and Co,

and the second structure, a GMR-TMR granular combination of Fe-Co 50%-50% alloy, Cu

and MgO. Both structures were obtained by three simultaneously discharges, as it will be seen

in the experimental set-up chapter.

II. Experiment

Thermionic vacuum arc method was used to obtain both granular GMR and the combinatorial

GMR+TMR structures. This technique is described elsewhere [18-21]. It consists in an

electronic bombardment of the emitted electrons from a heated tungsten filament that plays

the role of the grounded cathode. The electrons are focused using a Whenelt cylinder on the

anode, by a high voltage potential applied on it being forced to heat and evaporate the anode

material. The interest material is placed on the anode, on a special highly thermal resistant

crucible made of graphite and inner coated with tungsten.

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The accelerated electrons focused on the anode start the evaporation process that afterwards,

leads to the ignition of a bright plasma discharge in pure anodes vapors, all process taking

place in high vacuum conditions [6]. For the present work, three anode-cathode independent

systems were used (Fig.1). Each of the systems functioned as described above, simultaneously

in the high vacuum deposition chamber. For the GMR structure, one anode contained the iron

crucible, the second anode contained the cooper crucible and the third anode contained the

cobalt crucible. For the combinatorial GMR+TMR structure, a Fe-Co alloy having a 50-50%

ratio was previously prepared using the same deposition technique. Due to iron and cobalt

respectively near melting temperature points, this alloy was easily produced. The second

anode contained cooper as in the first structure, while the third one contained magnesium

oxide. MgO is the non magnetic electric insulating material of the GMR+TMR thin film

production. During the deposition process, it was used an evaporation rate as low as possible

in order to precisely monitor the deposition of very thin layers. For the present work, the

coating rate and the thickness of the structures were in situ monitored using a FTM7 micro-

quartz balance. The total thickness of both types of thin films was of 200 nm. The substrates

used were silicon and glass, placed at a distance of 25 cm above the anode-cathode systems in

a specially designed rectangular holder with 4 rows and 12 columns, each sample being

numbered “a-b” where a stands for the row number and b for column number. Samples in the

same column have the same morphological structural electric and magnetic properties. Due to

substrates position in respect with each element to be deposited, different elemental

concentrations were obtained in the same deposition batch. The entire coating process was

performed in a high vacuum chamber, with a base pressure in the deposition time of 5x10-6

Torr. This high vacuum pressure assured the obtaining of high purity structures, made only of

the interest materials, without any other unwanted inclusions.

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III. Results and discussion

III.1 Structural and morphological analysis of the obtained thin films

SEM and EDS measurements were performed on both FeCo-based structures in order to

analyze the surface morphology and the elemental concentration distribution of the materials

used for each type of thin film using the Scanning Electron Microscope, FEI Co., model

Inspect S, with an accelerating voltage range of 0-30 kV, working distance in the range of 0-

30 mm, in low vacuum conditions, EDS capable with an EDAX SiLi detector.

The EDS measurements revealed different elemental concentration on each sample in respect

with the sample positioning. For GMR Fe+Cu+Co structure, it was obtained an iron relative

atomic concentration from 25% to 11.5 %, cooper relative atomic concentration between 50.9

and 69% and a cobalt relative atomic concentration between 6.9% and 31.2%. The highest

relative concentration of each material was obtained for the sample positioned at the

minimum distance of the anode-cathode system with the specific material. This different

elemental ratio influenced, as it can be seen later on this work, the electric and magnetic

response of each sample. This is considered to be one important advantage of this deposition

method, consisting in obtaining within the same deposition process many types of thin films

with different properties.

From SEM images it can be seen the compact shape without major imperfection for both

structures. Again, due to different positioning in respect with the anode-cathode systems it can

be seen for the Fe+Cu+Co structure how the high iron concentration on the sample 4-2,

influenced the surface morphology of the thin film. As the iron concentration decreases, for

the 4-11 samples it can be observed a change of the surface morphology as well (Fig. 2-3).

For the FeCo+Cu+MgO structure, the SEM images show high compact smooth surfaces for

each studied thin film, in respect with the position and elemental concentration.

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The atomic force microscopy (AFM) measurements were recorded using the Park XE-100

model in a non-contact operating mode (NC-AFM). It was used a silicon cantilever with a

nominal length of 125 mm, with an oscillation range of 275-328 kHz. The sample scanned

surface was 5x5 μm, with the horizontal “line-by-line” flattening as planarization method.

The surface topographies carried out on both GMR and combinatorial GMR+TMR structures

coated on silicon substrates have shown different roughness values in respect with the sample

positioning and samples relative elemental concentration consequently. For the Fe+Cu+Co

structure, the highest value of the roughness was of 17.17 nm, on the sample placed over the

iron anode-cathode system. The roughness decreases significantly with the decrease of the

iron concentration in the structure, to a minimum of 1.35 nm for the sample with the lowest

iron concentration (Fig.4). For the FeCo+Cu+MgO structure, the value of the measured

roughness was in the range of 3.06 nm to 1.36 nm in respect with the sample positioning

(Fig.5-6). The roughness values for both types of structures are in perfect concordance with

the obtained SEM images.

The X-Ray Photoelectron Spectroscopy (XPS) surface analysis performed on the

combinatorial GMR+TMR structure composed of FeCu+Cu+MgO was carried out with a

Quantera SXM equipment, with a base pressure in the analysis chamber of 10-9

Torr. The X-

ray source was Al Kα radiation (1486.6eV, monochromatized) and the overall energy

resolution is estimated at 0.65 eV by the full width at half maximum (FWHM) of the Au4f7/2

line. In order to consider the charging effect on the measured Binding Energies (BEs) the

spectra were calibrated using the C1s line (BE = 284.8 eV, C-C (CH)n bondings) of the

adsorbed hydrocarbon on the sample surface. A dual beam neutralizing procedure (e- and Ar

+

ion beams) has been used to compensate the charging effect in insulating samples [22-26].

The XPS analysis was used to determine the chemical states of the elements present on the

surface and, after quantitative analysis, to find the elemental and chemical state relative

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concentrations as well. After scanning survey XPS spectra, the high resolution photoelectron

spectra of the most prominent XPS transitions (C 1s, O 1s, Cu2p3/2, Co2p, Fe2p, Mg1s and

MgKLL) were recorded for the FeCo+Cu+MgO structure.

It is appropriate to note here that all the calculations were performed assuming that the

samples were homogeneous within the XPS detected volume. We have to emphasize that the

errors in our quantitative analysis (relative concentrations) were estimated in the range of ± 10

%, while the accuracy for Binding Energies (BEs) assignments was ± 0.2 eV.[22-26]

From the XPS (Fig.7) spectra it can be seen that for the high relative concentration of the

FeCo alloy, present in the sample 4-3, after Argon Ion etching, a high amount of carbon is

still present on top of the surface. Sampling depth is estimated according to the corresponding

sputter rates, as: Sputtering rate=1.4nm/min; but analysis depth = 6.5 nm/after 1 min

sputtering 1kV (3x3); 2 min sputtering ~ 2.8nm, analysis depth=7.9nm; 3 min sputtering ~ 4.2

nm, analysis depth=9.3nm. The rather high C relative concentration suggests that the surface

could be contaminated during deposition process beyond the unavoidable carbon

contamination in the atmosphere. One possible explanation could be the pumping system

used, formed by preliminary and diffusion oil pumps which assured an inside working

pressure of 5x10-6

Torr. We can notice the presence of the hydrocarbon layer on the top of the

surface progressively decreasing after Ar ion etching. The contaminant present in the first

mono-layers of the surface is removed by the argon ion beam.

The relative carbon concentration decreases from sample 4-3 to sample 4-11, and it can be

explained also by the fact that sample 4-3 was placed over the FeCo anode-cathode system.

Because both iron and cobalt have a relative high melting temperature (around 15000C), the

energy needed to melt and evaporate this alloy was quite high. Because the Fe-Co alloy was

placed in a graphite crucible, it can be assumed that part of the carbon concentration is due to

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the crucible contamination during the deposition process. With a higher Ar ion etching time

(over 3 min) it can be assumed that the relative carbon concentration decreases considerably.

The XPS results show that the O1s band like spectra reveals two features: oxygen bounded

into the lattice oxidizing Fe, Mg and a tiny amount of Cu and OH groups confined on the

outermost layer of the surface leading to formation of Co(OH)2. The spectra reveal a

combination of metallic Fe and Fe2O3. The Co2p3/2 binding energy and the presence of OH

groups adsorbed on the top surface can be attributed to Co(OH)2. MgO can be assigned from

characteristic binding energies of Mg1s photoelectron line and Mg KLL Auger line which are

chemically shifted.

The atomic relative composition of each element contained by the GMR+TMR combinatorial

structure, after 3 min etching (~4.2nm/9.3nm) when considering the sample positioning is

presented in fig. 8.

III.2 Electric and magnetic measurements

Before any electric measurements, which consisted in the four points contact method using

silver conductive pasta, a series of non destructive Magneto-Optic Kerr Effect (MOKE)

measurement were performed on both types of structure using a longitudinal magneto-Kerr

effect magnetometer (type AMACC), with laminated sheets, zero remanence and an incident

p-polarized He-Ne laser light with λ=640 nm. The incident light made a 450 angle with the

sample plane, being linearly polarized perpendicular to the incidence plane, by a polarizer.

The MOKE effect describes the electromagnetic wave interaction with magnetic materials,

which corresponds to the rotation or changing the linear polarized light intensity by reflection

on magnetic surfaces placed in a magnetic field. This effect is similar with Faraday Effect,

which implies the rotation of linear polarized light when it passes through a transparent

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material placed in a constant magnetic field [27-30]. The Kerr effect is proportional with the

magnetization of the magnetic materials.

Figure 9 shows the MOKE signals for the ternary granular Fe+Cu+Co structure for each

studied sample in respect with its position to the anode-cathode systems. For the high relative

iron concentration the absolute value of the MOKE signal was 10.31 mdeg, with a coercive

force of 7.6 mT. As the iron relative concentration decreases, the MOKE signal decreases as

well to 1.85 mdeg, for the 3-7 sample and to 0.704 mdeg for the 3-12 sample. The coercive

force decreases as well for the 3-7 sample to 6.7 mT, but it increases to 16.4 mT for the 3-12

sample. A possible explanation can be that the domains orientation of the sample 3-2 with a

higher relative iron concentration is made much easier than in the case of the magnetic

domains of cobalt, for the case of sample 3-12 which has a higher cobalt relative

concentration.

Figure 10 shows the MOKE signals of the combinatorial GMR+TMR structures for each

studied sample in respect with the position to the anode-cathode systems. Sample 2-2 shows

the MOKE signal for the sample with the highest FeCo alloy relative concentration. The

rectangular shape can be translated by the presence of specific values of the magnetic field

that can influence the magnetic behavior of the structure. The MOKE signal for this sample

was 31.68 mdeg, with a coercive force of 2.9 mT. For the 2-6 sample, that has a lower alloy

concentration and a higher magnesium oxide relative concentration, the MOKE signal was of

41.51 mdeg. The coercive force increases as well to 9.3 mT, due to the presence of the non

conductive insulating element –MgO. The shape is also rectangular. As in the 2-2 sample, it

can be said that magnetic domains of the FeCo alloy are easier oriented with a lower

insulating relative concentration. With the increase of the magnesium oxide relative

concentration, the magnetic domains are no longer easily oriented, which can explain the high

values of the coercive force for the 2-6 sample. For sample 2-11, the one with the lowest

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relative alloy concentration and with the highest magnesium oxide presence, the hysteresis

shape is no longer rectangular and it can be seen a double opening of it. This double opening

of the hysteresis shape can be attributed to the low ferromagnetic grains concentration

embedded in the high relative concentration insulating matrix, and also to the oxidation of the

alloy due to the presence of the magnesium oxide. For this sample, the MOKE signal was of

6.25 mdeg, with a coercive force of 17.6 mT. This high value of the coercive force is

attributed to the low value of the alloy concentration in the non magnetic matrix.

Electric measurements

In order to determine the electrical resistance of the studied structures, it was used the four

probe measurement technique [31-35]. This method consists in having four metallic probes,

linearly and equidistant arranged on the surface of the studied structure. Two of the probes

were used to pass through the sample a constant electric current, and the other two probes

were used to read the dropping voltage on the sample. Fig 13 shows a schematic

representation of the electric circuit made for both sets of structures. The contact between the

interest structure and the probe was made using a highly conductive silver based paste. The

used probes were cooper thin wires, with 0.2-0.3 mm in diameter. For both sets of structures,

the electrical measurements circuit (Fig. 11) had a known resistance of 10 Ω, the calculus

formula in order to determine the value of the electric resistance was: x

x e

e

UR R

U

where Rx is the electric resistance of the interest structure; Re - known electric resistance; Ux –

the value of the dropping voltage measured on the studied structure and Ue – the value of the

measured dropping voltage on the known resistance. In order to have a precise electric

measurement and to have a transversal orientation of the electric current through the sample,

the electric contact were made between a cooper electrode coated using TVA technology

beneath the entire structure, with a total thickness of 500 nm and the surface of the measured

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thin film. In this way, it was certain that the recorded value of the dropping voltage was

entirely measured through the whole sample. In order to determine the electric resistance

through both structures, without any presence of a magnetic field, a constant current of 1 μA

was used, with an increasing ramp of 0.5μA/s. until a maximum value of 10 μA was reached.

It can be seen from fig. 12 a) and b) that for both sets of samples the current-voltage

characteristics are similar like shape, the variation of the dropping voltage with the increasing

of the electric current being linear, a normal behavior for a classic electric resistance. For the

Fe+Cu+Co structure, the value of the measured electric resistance on the sample placed in the

middle of iron and cobalt anode-cathode systems was of 1.004 Ω. This low value of the

electric resistance is due to a maximum relative concentration of cooper present in this

sample, which was positioned exactly above the cooper anode-cathode system. By increasing

the value of the relative concentration of cobalt, and a lower relative concentration of cooper,

the measured electric resistance becomes 6.91 Ω. As presented above, the sample positioning

in respect with the anode-cathode systems is crucial in obtaining optimum elemental relative

concentrations, with different values of the electric resistance. For the FeCo+Cu+MgO

structure, it can be seen an obvious difference between each studied thin film. For the sample

with the highest iron and cobalt concentration, and with a low magnesium oxide relative

concentration, the recorded electric resistance was of 0.576Ω. With the increase of the

nonconductive material present in this structure, the electric resistance had a value of 1.192Ω

for the sample placed over the cooper anode-cathode systems. The relative high value of the

cooper present in this sample explains the relative low value of the measured electric

resistance. The highest value of the electric resistance on the combinatorial GMR+TMR

structure was obtained having a value of 5.065Ω, and how it was expected, on this sample the

relative concentration of the nonconductive material had the highest value.

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Magnetic measurements were made on both types of structures in order to observe the

magnetoresistive effect on each studied sample. As the previous electric measurements were

performed, it was used the same four probe measuring technique due to its advantage in

having low errors. Two of the electric probes were used to pass a constant electric current

through the sample, and the other two probes were used to read the dropping voltage on the

interest sample. The formula used to determine the magnetoresistive effect is written below:

(0) ( )(%) .100

(0)

R R H

R

where R(0) is the value of the electric resistance measured

without any presence of a magnetic field and R(H) is the value of the electric resistance

measured at a given value of the magnetic field. In order to determine the variation of the

electric resistance of each sample in the presence of the magnetic field, a constant current of 5

μA was used. Each sample was placed in a constant magnetic field with a value of 0.3 T. The

magnetic field was perpendicular to the direction of the constant electric current through the

sample. In this way, first it was recorded the value of the electric resistance without an applied

magnetic field and then, a 0.3T magnetic field was applied, and again it was recorded the

value of the electric resistance. After every set of readings, the sample temperature was

increased and the measurements took place following the same steps. For this study, the

sample temperature was varied from 280C to a maximum of 63

0C. This way it was also

studied the variation of the magnetoresistive effect in respect with the sample temperature. As

it can be seen from fig. 13, both GMR and GMR+TMR structures have significant variation

of the magnetorezistive effect in respect with the sample temperature on one hand and on the

other hand, a variation of the MR effect in respect with each relative elemental composition.

For the GMR, Fe+Cu+Co structure, the MR effect on the sample with the lowest iron relative

concentration (sample 3-11) varies between 1.4% to a maximum of 32.28%. It is interesting to

observe that at a given sample temperature of 280C, the percentage MR effect is 1.47%. By

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increasing the sample temperature to 430C, the MR effect increases to 32.28%. A further

increase of the sample temperature with another 100C decreases the MR effect to a value of

7%. An even further increase of the sample temperature decreases the MR effect to 1.9%.

With other words, the measured MR effect increases up to a maximum value at a specific

temperature. Beyond this value, the MR effect decreases. For the central sample (3-6) the

general value of the recorded MR effect is much higher that in the case of the previous sample

and varies from 26% to a maximum of 80%. This high value of the MR effect is reached at

the same sample temperature around 400C. With further increase of this temperature, the MR

effect decreases as well.

For the combinatorial GMR+TMR FeCo+Cu+MgO structure, the same evolution of the MR

effect in respect with each sample relative elemental concentration and variation with

temperature was observed. For the sample with the highest ferromagnetic relative

concentration (FeCo) sample, the recorded MR effect varied from 1.12% to 16.45% This

maximum value of the magnetic effect was recorded for a temperature of 520C. A further

increase beyond this temperature leaded to a decrease of the MR effect. For the sample with

the lowest ferromagnetic relative concentration (3-11) but with the highest nonconductive

concentration (MgO), the MR effect varies from 7.3% to 69.2%. The maximum value was

reached at a sample temperature of 430C, as in the case of the central sample of this structure

and also as in the case of the other studied structure. It is interesting how for both sets of

samples, it was found an optimum sample temperature, around 400C, for which the MR effect

has the greatest values.

This temperature behavior of the magneto-resistive effect could be explained when

considering two counteracting physical phenomena present in these measurements. The

magnetic domain clustering determined by the temperature which can increase the GMR-

TMR effect of the granular type structures on one side [21], and the destructive action of the

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temperature concerning the magnetic domains alignment along the external magnetic field, on

the other side.

Therefore, for lower temperatures the magnetic domain clustering is lower together with

GMR-TMR effect and when the temperature increases, the percentage clustered magnetic

domains increases together with GMR-TMR effect of the granular structure. For higher

temperatures the delineating effect of the temperature becomes preponderant and decreases

the magneto resistance effects.

Conclusions

Thermionic vacuum arc method was successfully used in obtaining two types of FeCo base

structures. One, a GMR combination where the ferromagnetic grains of iron and cobalt were

embedded in a pure cooper matrix and the other one, a combinatorial GMR+TMR structure

which contained as a ferromagnetic material the FeCo alloy embedded in a nonmagnetic

semi-conductive cooper-magnesium oxide matrix. Due to different positioning of each sample

in respect with the anode-cathode systems position, different elemental concentrations were

obtained in a single batch of samples as the EDS and XPS measurements showed. Different

elemental concentration influenced also the surface morphology of both structures as it was

seen in the AFM and SEM measurements. It was obtained a variation of roughness from

17.169nm to 1.35 nm for the GMR structure and from 3.06 nm to 1.36 nm for the

combinatorial GMR+TMR structure. The electric measurements showed the normal electric

behavior of each type of structure. The current – voltage characteristics were similar to a

classic resistance behavior. MOKE measurements were first to highlight the magnetic

properties of each structure. It was seen the different hysteresis shape, with different values of

the MOKE signal and coercive force in respect with different positioning of the samples.

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The magneto-resistive effect obtained for each type of structure had a variation with the

relative elemental concentration from 1.31% to 26.4% for the Fe+Cu+Co thin films, and from

4.5% to 19% for the FeCo+Cu+Mgo structure, at a given sample temperature of 270C. It was

found for both types of structures an influence of the magneto-resistive effect in respect with

the sample temperature, at a constant magnetic field of 0.3T.

The highest MR effect of 80% for the Fe+Cu+Co structure was obtained at 400C. For the

FeCo+Cu+MgO structure the highest MR value of 69% was obtained at a temperature of

430C. The results obtained in this work prove the efficiency of TVA method in obtaining

combinatorial magnetic structures. Due to its unique advantage of having no buffer gas inside

the deposition chamber and having the possibility to ignite in pure vapors several materials,

different elemental concentrations were obtained with different values of the recorded MR

effect. Moreover, an optimum temperature was observed for which the magnetic response of

each sample had a maximum value.

Acknowledgments

This work was carried out with the financial support of National Institute for Laser, Plasma

and Radiation Physics, Romania, under the contract PN 09 39 04 01.

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Figure Captions

Figure 1: Schematic view of both GMR and combinatorial GMR+TMR structures.

Figure 2: SEM images for Fe+Cu+Co structure (4-2 left, 4-11 right)

Figure 3: SEM images for FeCo+Cu+MgO structure (3-2 left, 3-12 right)

Figure 4: AFM images of Fe+Cu+Co structure (2-3 left, 2-12 right)

Figure 5: AFM images for FeCo+Cu+MgO structure (4-2 left, 4-12 right)

Figure 6: Roughness variation of the Fe+Cu+Co and FeCo+Cu+MgO structures

Figure 7: XPS spectra for the combinatorial GMR+TMR structure

Figure 8: Relative atomic concentration from the XPS spectra of the FeCo+Cu+MgO

structure

Figure 9: MOKE for Fe+Cu+Co

Figure 10: MOKE for FeCo+Cu+MgO

Figure 11: Schematic representation of the electric measurements set-up

Figure 12: a) Fe+Cu+Co and b) FeCo+Cu+MgO I-V characteristics

Figure 13: a) MR effect function of temperature for the Fe+Cu+Co structure and b) MR effect

function of temperature for the FeCo+Cu+MgO structure

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Figure 1

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Figure 2 4-2 left; 4-11 right

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Figure 3 3-2 left; 3-12 right

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Figure 12 a)

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Figure 12 b)

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Figure 13 a)

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Figure 13 b)

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Highlights to manuscript

Combinatorial Fe-Co thin film magnetic structures obtained by Thermionic Vacuum

Arc Method

by I. Jepu, C. Porosnicu, C. P. Lungu, I. Mustata, C. Luculescu, V. Kuncser,

G. Iacobescu, A. Marin, V. Ciupina

Magnetic Fe-Co based thin films obtained by Thermionic Vacuum Arc method

were studied;

A pure GMR and a combinatorial GMR+TMR type of structure were produced;

The magnetic response was highly influenced by the relative elemental

concentration;

Sample temperature influenced the magneto resistive effect on both structures;

A 69% MR effect was reported for the combinatorial GMR+TMR thin film;