Dynamic study of the internal magnetic order of Mn3O4 nanoparticles

J Nanopart Res (2011) 13:5653–5659

DOI 10.1007/s11051-011-0314-2

Dynamic study of the internal magnetic order of Mn3O4 nanoparticles

E. Winkler1, J. P. Sinnecker

2, M. A. Novak

3 and R. D. Zysler

1

1Centro Atómico Bariloche, Av. Bustillo km 9.500 RN Argentina;

2Centro Brasileiro de Pesquisas Físicas - CBPF, Rua Xavier Sigaud 150, 22290-180, Rio de

Janeiro, RJ, Brazil; 3 Universidade Federal do Rio de Janeiro, Instituto de Física, Rio de Janeiro-RJ 21941-972,

Brazil

Abstract

The dynamic magnetic properties of Mn3O4 nanoparticles with mean diameter <φ> = 15 nm

have been investigated by frequency and dc-field (HDC) dependence of the in phase (χ’) and

out of phase (χ’’) ac-susceptibility. The studies were performed in non-interacting and

interacting systems of Mn3O4 nanoparticles diluted in a polymer with concentrations 1.5 %

and 17.6 %. The ac-susceptibility of the non-interacting system, measured with HDC =0,

presents only one maximum located at TC = 42 K associated to the paramagnetic (PM)-

ferrimagnetic (FiM) transition. In contrast, the susceptibility of the interacting system

shows two anomalies. One frequency independent peak associated to TC and a second low

temperature maximum was observed in χ’’, located at TP. The position of the TP maximum

shifts to higher temperature when the frequency increases. The relation between the

relaxation time and TP was well described by the Vogel-Fulcher law. When the

susceptibility was measured with HDC = 20 kOe, the PM-FiM transition was observed in

both systems. Remarkably, in the non-interacting system, the low temperature anomaly is

evidenced by the magnetic field. This anomaly is present as a well defined maximum,

which shifts to higher temperature when the frequency increases.

Introduction

The manganese oxides are very interesting materials that continue to surprise the scientific

community for their wide variety of properties. In particular the magnetic properties of

hausmannite Mn3O4 where studied since 1960. It was established that the material orders

ferrimagnetically at TC = 42 K and develop a complex magnetic ordered structure at low

temperature (Dwight et al. 1960, Jensen et al. 1974, Srinivasan et al. 1983). Below 39 K the

spins rearrange in a helicoidal structure and at 32 K a second reorientation transition

occurred. Recently the search for new materials with magnetoelectric properties has

renewed the interest in frustrated magnetic spinels such as Mn3O4 (Cheong et al 2007). It

was found that the dielectric anomalies were associated to the spin-ordering transition and

an important magnetoelectric coupling is present at the helicoidal phase (Tackett et al 2007,

Suzuki et al 2008).

When the size of the material is reduced to nanometric scale it is well known that the

magnetic properties are modified, being mainly determined by surface effects. Therefore

the influence of the interfaces and the inter/intra-particle interaction on the surface spins is

decisive factors (Fiorani 2005, Dormann 1992). For example, when the Mn3O4 phase

covers AFM nanoparticles of MnO in a core-shell structure, the magnetic spinel preserves

the magnetic transitions although the reorientation transition temperatures are shifted, i.e.

from 39 K and 32K to 36 K and 20 K, respectively. (Ortega et al 2010) However, Regmi et

al. have reported that, when the system is formed by Mn3O4 nanoparticles of 15-25 nm, the

reorientation magnetic transitions below TC are suppressed. (Regmi et al 2009). In this last

system approximately 30-50 % of the spins do not contribute to the magnetic order,

therefore the suppression of the low temperature transitions is attributed to the surface

disorder. In these examples two different situations were mentioned where the interface in

one case maintain the bulk low temperature spin structure and in the other case the spin

reorientation transition is suppressed by the size effects and surface disorder.

Another situation is found when the Mn3O4 nanoparticles are dispersed in a polymer.

(Winkler et al 2004) In this reference, the nanoparticles are diluted enough to be considered

as non-interacting, and below TC = 42 K a low temperature anomaly appears. It was

estimated that most of the spins in this system (approximately 80%) do not contribute to the

magnetic order core and the low temperature anomaly was associated to the surface spin

freezing. Recently, by ac-susceptibility measurements on 13-16 nm Mn3O4 nanoparticles,

Tackett et al. derive that the low temperature anomaly is associated to the onset of

superparamagnetic relaxation (Tackett et al 2010) and not to a spin-glass transition as was

previously assumed.

With this picture in mind we studied moderately interacting and non-interacting Mn3O4

nanoparticles in order to shed light on the nature of the low temperature anomaly. With this

aim we performed a dynamic study of the magnetic moment by ac-susceptibility on diluted

Mn3O4 nanoparticles at different concentrations.

Experimental

The Mn3O4 nanoparticles were prepared by chemical precipitation at room temperature by

mixing Mn(NO3)2 aqueous solution and aqueous sodium hydroxide NaOH solution at pH =

12.

The obtained brown powder (manganese hydroxide) was washed with distilled water to

remove the residual ions. The final solution was boiled in reflux during 5 days and a

particle suspension was obtained. Diluted dispersions of Mn3O4 particles in a

polyvinylpyrrolidone (PVP) aqueous solution were prepared with 17.6% and 1.5% of

particles by weight. The x-ray powder diffraction pattern shows that the sample consists of

a single Mn3O4 phase with tetragonal I41/amd symmetry. The nanoparticle average size,

estimated from the full width at half maximum of the [hkl] reflection by the Scherrer

equation was <φ> = 15 nm. This value is in agreement with the size determined by

transmission electron microscopy (TEM) and light scattering experiments. The measured

distribution is approximately log-normal with a mean diameter <φ> = 15 nm. Details of the

sample preparation and characterization are described in (Winkler et al 2004). Taking into

account the concentration of the particle dispersions and the particle size, the average

interparticle distances were estimated as 40 nm and 100 nm for the 17.6% and 1.5%

samples, respectively. In the more dispersed sample the particles are far enough to neglect

dipole-dipole interactions and therefore the particles can be considered as non-interacting

The magnetic properties were investigated in a commercial SQUID magnetometer in

applied fields up to 5 T. The ac-susceptibility measurements were performed at different

frequencies f, between 10 Hz and 10 kHz, applying an ac-field of 10 Oe and a dc-field of 0

Oe and 20 kOe. The ac-susceptibility measurements were performed in all cases after

cooling the sample in zero field.

Results First we are going to present the results for the 1.5 % Mn3O4 nanoparticles diluted in PVP

system. This system provides information about the intrinsic magnetic properties of single

nanoparticles without the influence of the interparticle interaction. Figure 1 shows the

temperature dependence of the dc magnetization measured under zero–field-cooling (ZFC)

and field-cooling (FC) conditions with 50 Oe and 50 kOe applied magnetic field. When the

measurement was performed under 50 Oe an anomaly is observed at T = 42 K associated to

the ferrimagnetic transition temperature. At lower temperature the onset of another anomaly

is evidenced. This anomaly is better defined when the magnetization is measured applying

larger magnetic field, as can be observed in the derivative of the magnetization showed in

the inset of figure 1.

0 20 40 60 80 1000

20

40

0 20 40 60 80-0.04

0.00

0.04

0 20 40 60 800

1

2

(b)

T (K)

0.0

0.5

1.0

1.5

2.0 (a)

M (

em

u/g

)

-dM

/dT

T (K)

-dM

/dT

T (K)

Figure 1: Temperature dependence of the ZFC and FC magnetization as measured with a)

HDC =50 Oe and b) HDC = 50kOe corresponding to the 1.5 % diluted in PVP Mn3O4

nanoparticles system. The inset shows the temperature derivative of the ZFC

magnetization.

Figure 2: Temperature dependence of the in phase χ´and out of phase χ´´ measured ac-

susceptibility at different frequencies from 10 Hz to 10 kHz under HDC = 0 Oe

corresponding to the 1.5 % diluted in PVP Mn3O4 nanoparticles system.

In order to obtain more information about the origin of the magnetization anomalies we

studied the dynamic susceptibility. Figure 2 shows the real (χ′) and imaginary (χ′′)

components of the ac susceptibility measured at different frequencies from 10 Hz to 10 kHz

by applying an ac field of 10 Oe measured with HDC = 0 Oe. As can be observed, both χ′

and χ′′ show a frequency independent peak located at 42 K. This behaviour confirms, once

again, that this maximum is originated by the magnetic transition associated to the Mn3O4

paramagnetic (PM)-ferrimagnetic (FiM) order as previously reported (Winkler et al 2004,

Zysler et al 2007, Regmi et al 2009, Tackett et al 2010) and not by a change of regime from

superparamagnetic to the blocked regime. At low temperature an onset of relaxation is

observed in χ′′ when the measurement is performed at f > 1 kHz, while for lower

frequencies the curve does not indicate any dissipative process. On the other hand, when

the ac-measurement is performed under an applied dc-field of 20 kOe, two maxima are

observed. One of them is frequency independent located at 44 K and the second one shown

0.0

1.0

2.0

3.0

4.0

0 10 20 30 40 50 60

0.0

1.0

2.0

χ´(

10

-5 e

mu

/g)

10 Hz

30 Hz

100 Hz

300 Hz

1 kHz

3 kHz

10 kHz

χ''

(10

-6 e

mu

/g)

T(K)

in figure 3 presents a frequency dependence at low temperature.

0

4

8

12

0 5 10 15 20 25 30

0

2

4

χ' (

10

-6 e

mu/g

)

10 Hz

30 Hz100 Hz

300 Hz

1 kHz

3 kHz 10 kHz

χ''

(10

-6 e

mu/g

)

T(K)

Figure 3: Low temperature dependence of the in phase χ´and out of phase χ´´ as measured

at different frequencies from 10 Hz to 10 kHz under HDC = 20 kOe corresponding to the

non-interacting nanoparticles system.

The results of the interacting Mn3O4 nanoparticles in the sample diluted at 17.6 % in PVP

are presented in figure 4. As can be observed, both χ′ and χ′′ show a frequency

independent peak located at 42 K associated to the Mn3O4 FiM transition. On the contrary,

at low temperature χ′ and χ′′ present a very different behaviour. While χ′ shows a

frequency independent behaviour down to the lowest measured temperature; χ′′ exhibits a

small maximum at TP which shifts to higher temperature when the frequency increases.

Similar behaviour was previously observed on the strongly interacting Mn3O4 powder

system where a low temperature peak was only observed on the dissipative component of

the susceptibility. (Regmi et al. 2009, Tackett et al. 2010). Figure 5 shows the dynamic

susceptibility measured under an applied magnetic dc-field H=20 kOe. In this case χ′

presents the frequency independent peak corresponding to the FiM transition at TC ~ 44 K

and the low temperature TP maximum which shift to higher temperature when the

frequency increases. On the other hand χ′′ only displays the frequency dependent low

temperature maximum.

Figure 6 compares the dependence of the relaxation time, τ =1/2πf, with the temperature of

the χ′′ low temperature maximum for HDC = 0 and 20 kOe. From this figure a clear shift of

τ toward lower temperature is observed when the measurement is performed with an

applied magnetic field.

0 10 20 30 40 50 60

0

2

4

6

0

4

8

12

χ´´

(10

-5 e

mu/g

)

T (K)

10 Hz

30 Hz

100Hz

300Hz 1kHz

3kHz

10kHz

χ´

(10

-4 e

mu

/g)

0 10 20

-0.4

0.0

0.4

0.8

T (K)

χ''

(10

-5 e

mu

/g)

Figure 4: χ´and χ´´ versus T for the 17.6% concentration, measured at different

frequencies from 10 Hz to 10 kHz under HDC = 0 Oe and 10 Oe ac field.

0.5

1.0

1.5

0 20 40 60

0.0

2.0

4.0

10 Hz

30 Hz

100 Hz

300 Hz

1 kHz

3 kHz

10 kHz

χ'(10

-4 e

mu/g

)χ''

(10

-5em

u/g

)

T(K)

Figure 5: χ´and χ´´ versus T, measured at different frequencies from 10 Hz to 10 kHz under

HDC = 20 kOe corresponding to the interacting nanoparticles system.

Discussion

The main experimental results found by dynamic susceptibility measurements can be

summarized as follows. In the non-interacting system only one magnetic anomaly is

observed at 42 K associated to the PM-FiM transition. When the measurement is performed

in presence of a strong magnetic field a second anomaly is unveiled by the magnetic field.

In fact a well defined maximum at the dc and ac-susceptibility is observed when the

measurement is performed by applying high dc-magnetic fields. Moreover, the maximum

shifts to higher temperatures when the frequency increases.

5 10 15

0.00

0.02

0.04

HDC

= 20 kOe HDC

=0 Oe

τ (s

)

T (K)

Figure 6: Relaxation time versus the low temperature χ´´ maximum T position, measured at

different dc-fields.

0.12 0.16 0.20 0.241.0xe

-12

1.0xe-11

1.0xe-10

1.0xe-9

1.0xe-8

1.0xe-7

1.0xe-6

1.0xe-5

τ (s

)

1/(T-To) (K

-1)

Figure 7: Relaxation time as a function of 1/(T-To) . The solid line correspond to the best

fit with the Vogel-Fulcher law, where EB/kB = 56 K, and To =6 K.

As for the more concentrated nanoparticles system, where the interparticle interactions are

present, a TP low temperature anomaly is observed in χ′′ even when the measurement is

performed with HDC = 0. In this case we can analyze the dynamic behaviour of the system

without the influence of the external magnetic field. The relaxation time τ = 1/2π f as a

function of the χ′′ maximum position, can be well described by the Vogel-Fulcher law

(Souletie et al. 1985):

−=

)(exp

oB

Bo

TTk

Eττ

The above phenomenological law is used for the relaxation time of the superparamagnetic

system in presence of interparticle interaction, where τo is a characteristic time constant, EB

is the anisotropy energy barrier, and To is a phenomenological parameter that describes the

interparticle interaction. In figure 7 the measured relaxation time as a function of 1/(T-To)

is represented. From the fit we have obtained EB/kB= 56 (3) K, To = 6.0 (5) K and τo ~10-9

s. These parameters are within the range of expected values for interacting

superparamagnetic system. The relaxation time can be also phenomenologically described

by the power law (Souletie et al. 1985, Binder et al. 1986): ν

ττ

z

g

oT

T−

−= 1

The power law assumes an equilibrium phase transition where Tg corresponds to the

freezing temperature, ν and z are the critical and dynamic exponents, respectively.

However, our attempt to adjust the measured relaxation time with the power law

dependences yield unphysical parameters, as τo = 10-6

s and zν = 4. These results suggest

that the low temperature maximum presented by the interacting nanoparticle system is

originated by a superparamagnetic blocking behaviour. In addition, the ac-peak shifts to

lower temperature when the measurement is performed with an applied field, shown in

figure 6, as expected for a lower effective energy barrier.

0 10 20 30 40 500

1

2

3

4

5

6

Mr (

em

u/g

)

T (K)

0 10 20 30 40 500.0

0.2

0.4

0.6

-dM

r/dT

T (K)

Figure 8: Remanent magnetization, Mr, corresponding to the 1.5 % Mn3O4 nanoparticles

diluted in PVP system measured at zero field after field cooling the sample with H=50

kOe. It is also included Mr obtained from the hysteresis loops (solid symbols). The inset

shows the Mr temperature derivative curve, which evidence the peak corresponding to TC

and a bump (signal with an arrow) associated to a blocking process.

At this point an open question remains: why the non-interacting system shows only the low

temperature anomaly (associated to a blocking temperature) when the measurement is

performed under an applied magnetic field? In order to obtain additional information to

clarify this issue we measured the remanent magnetization, Mr, at zero field. The results,

presented in figure 8, were obtained after cooling the sample from room temperature with

an applied magnetic field HDC=50 kOe. This figure also includes Mr obtained from the

hysteresis loops. As can be observed Mr increase at TC and shows another slight increase at

low temperature. These features are more evident from the temperature derivative curve

which shows a peak at T ~ 41 K and a bump at T ~ 8 K. The low temperature anomaly can

be associated to a blocking process at zero field which could not be resolved from the

dynamic measurements. In fact the results of the χ” at HDC=0 (figure 2) show a small

frequency dependent signal that could be attributed to a relaxation process, but this

response is small and blurred by the low temperature signal increase.

Conclusion We have studied the magnetic dynamic properties of interacting and non-interacting Mn3O4

FiM nanoparticles. Both systems present the PM-FiM transition at TC = 42 K. On the

contrary, the low temperature reorientation transition observed in bulk Mn3O4 is suppressed

in agreement with previous results (Regmi 2009, Tackett 2010). When the ac-susceptibility

measurement is performed with HDC =0 a second low temperature anomaly is observed

only in the interacting system. This anomaly is present as a frequency dependent maximum

in the out of phase dynamic susceptibility and is strongly influenced by the magnetic field.

The relaxation times of the TP maxima, observed in the interacting system, are well

described by a Vogel-Fulcher law. In the presence of a dc-magnetic field, the TP maxima

shift to lower temperature in agreement with the usual behaviour observed in

superparamagnetic systems.

The results suggest that the systems can be described by a nanoparticle core that orders

magnetically at TC. Previous results indicate that the ordered volume is larger in presence

of interparticle interactions (Winkler 2004). From the calculated core magnetic moment, it

is shown that in the non-interacting system approximately 85 % of the nanoparticle spins

do not contribute to the ordered core. In presence of interparticle interactions the ordered

core increases and the nanoparticle spins that remain fluctuating is reduced to 60 %. It was

observed that at T ~ 11 K the fraction of the spins that thermally fluctuate is greatly

reduced. This result is consistent with the anomaly observed in the ac-susceptibility and the

remanent magnetization associated to the change from superparamagnetic to a blocking

regime.

ACKNOWLEDGEMENTS

This work was accomplished with partial support of ANPCyT Argentina through Grant

Nos. PICTs 2007-832; CONICET Argentina through Grant No. PIP 200801-01333, and U.

N. Cuyo through Grant No. 882/07.

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