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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