Physica B 448 (2014) 162–166
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Physica B
journal homepage: www.elsevier.com/locate/physb
Magnetization reversal phenomena and bipolar switching
in La1.9Bi0.1FeCrO6
J. Krishna Murthy n, A. Venimadhav
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur-721302, India
art ic l e i nf o a b s t r a c t
Available online 18 April 2014 A magnetization reversal phenomenon in double perovskite La1.9Bi0.1FeCrO6 has been investigated.
Keywords: Polycrystalline sample was prepared by conventional solid state reaction method and it was found to
Canted antiferromagnetism stabilize in orthorhombic crystal structure with Pnma space group. Temperature dependent magnetiza-
Weak ferromagnetism tion study at low fields has shown the spontaneous magnetization reversal phenomena below the
Magnetocystalline anisotropy compensation temperature (Tcomp 186 K) and it is attributed to the antiferromagnetic arrangement of
Zero magnetization uncompensated weak ferromagnetic clusters of Fe and Cr. The field dependent tunable bipolar magnetic
Magnetic switching switching and observation of normal and inverse magnetocaloric effects evidence the magnetization
Magnetocaloric effect reversal phenomena in La1.9Bi0.1FeCrO6.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction phenomenon is also observed in the itinerant ferromagnetic SrRuO3
[20]. Based on the sign reversal of magnetization property, the
It is well known that in FM materials, the magnetization reversal system can exhibit both conventional and inverse magnetocaloric
is obtained by reversing the field direction. In some of special effect which is useful to maintain constant temperature bath in solid
magnetic materials, the magnetization reversal effect (MRE) state magnetic refrigeration devices [14].
(i.e., flipping the negative magnetization to positive magnetization) In the single perovskite LaFe0.5Cr0.5O3 (LFCO) system, the
is obtained without actually reversing the external magnetic field [1]. understanding of the magnetic properties is unsettled. From the
The phenomenon was first predicted by Néel, in 1948 in ferrimag- Neutron diffraction study, Azad et al. have reported that LFCO is
netic systems and this phenomenon was assigned to the presence of collinear antiferromagnetic ordering with a weak uncompensated
different magnetic sublattices with different temperature dependent magnetic moment near to TN 265 K [21] and a weak intensity of
magnetic behaviors [2]. Recently, MRE has been reported in variety of the magnetic reflections persisted up to 550 K. On the other
systems such as orthovanadates [3,4], orthoferrites [5,6], orthochro- hand Belayachi et al. have assigned the Néel temperature 550 K
mites [7–10], perovskite manganites [11,12], garnets [13] and mole- from the high temperature susceptibility measurements [22].
cular magnets [14–16]. The origin of MRE is different in each of these Further, Vijayanandhini et al. have reported uncompensated weak
systems. In orthovanadate (RVO3) systems, the MRE is due to the first ferromagnetic (WFM) ordering in (La1 x/2Bix/2) Fe0.5Cr0.5O3 with
order structural transition associated with unquenched orbital angu- an unusual zero magnetization behavior within 100–160 K regime
lar moment [3,4] and in rare earth chromites the opposite alignment depending on the Bi content; while the ferrimagnetic (FiM) or
of Cr ions with neighboring paramagnetic rare earth ions at low WFM to paramagnetic (PM) transition was found above 400 K [23].
temperature induces MRE [17,18]. In YFe0.5Cr0.5O3 and BiFe0.5Mn0.5O3 MRE has also been found in (La1 x/2Bix/2) Fe0.5Cr0.5O3 system and
systems, MRE has been attributed to the competition of single-ion was attributed to the clusters of canted antiferromagnetic
magnetocystalline anisotropy and antisymmetric Dzyaloshinsky– La1 xBixCrO3 and La1 xBixFeO3 which are coupled antiferromag-
Moriya exchange interactions [5,19]. Recently, chromite systems have netically through dipolar interactions [23]. The (La1 x/2Bix/2)
received renewed research interest due to its magnetization reversal Fe0.5Cr0.5O3 system received renewed research interest due to rich
switching effect, a consequence of MRE that can be potential in physics involved in the MRE and such negative magnetization
magnetic memories and thermomagnetic switching applications. effect can be tuned with bismuth doping. With complete replace-
Such a temperature induced positive and negative magnetization ment of Bi, one can expect the ferroelectricity in this system.
However, the consequences of negative magnetization effect, like
magnetic reversal switching or the magnetocaloric effect were not
n
Corresponding author. Tel.: þ 03222269958. studied. In this study, we present the results obtained on the
E-mail address: 09CR9701@iitkgp.ac.in (J. Krishna Murthy). crystal structure and magnetic properties of the double perovskite
http://dx.doi.org/10.1016/j.physb.2014.04.020
0921-4526/& 2014 Elsevier B.V. All rights reserved.
J. Krishna Murthy, A. Venimadhav / Physica B 448 (2014) 162–166 163
La1.9Bi0.1FeCrO6 polycrystalline system. Importantly, we report the refined parameters are listed in Table 1. The refined parameters of
structural, magnetic and MRE studies and show the bipolar LBFCO are close to the parent compound LaFe0.5Cr0.5O3 and Bi
switching in magnetization and conventional, inverse magneto- doped (La1 x/2Bix/2) Fe0.5Cr0.5O3 system [21,23]. The FESEM mono-
caloric effects (MCE). graph of LBFCO (Fig. 1(c)) ceramic sample shows the homogeneous
distribution of cubic shaped particles with particle size of the
order of 2–10 μm. The SAED pattern of bulk sample (Fig. 1(c))
2. Experimental details indicates the dot pattern confirming the crystalline nature within
the grains and inset shows the bright field image of the TEM
Polycrystalline La1.9Bi0.1FeCrO6 (LBFCO) sample was prepared pattern which indicates the particle size is larger than 1 μm.
by conventional heat treatment of solid state reaction method. The
high purity precursor materials of La2O3, Bi2O3, Fe3O4 and Cr2O3 3.2. Magnetization study
were weighted according to their stoichiometric ratios and mixed
then grained with a motor and pestle for 2 h and then heated at The temperature variation of magnetization (M (T)) study of the
800 1C for 10 h. The obtained granulated powder was then pressed present sample was performed in zero-field cooled (ZFC), field-
into pellets and sintered to 1000 1C for 24 h with an intermediate cooled cooling (FCC) and field-cooled warming (FCW) modes
grinding. Final sintering was done at 1050 1C for 24 h. The phase under 0.01 T magnetic field and is shown in Fig. 2(a). With
analysis was carried out using a Phillips powder high resolution decreasing temperature, a number of magnetic features were
X-ray diffractometer (HRXRD) with Cu-Kα radiation and a JEOL- observed. As shown in Fig. 2(a), all the three modes showed finite
JEM2100 transmission electron microscope (TEM) used for elec- magnetization which is independent of temperature from 400 K
tron diffraction analysis. The surface morphology was studied down to 300 K. The PM to FiM or WFM transition expected at
using a JEOL-JSM5800 field emission scanning electron microscope TC 450 K for LBFCO is beyond the measured temperature window
(FESEM). The temperature and magnetic field dependence of of 3–400 K. With decreasing temperature below 300 K, in all three
magnetization study was done with an Evercool Quantum Design modes the magnetization increases and shows broad maxima and
SQUID-VSM magnetometer. bifurcation i.e., irreversibility between FCC and ZFC magnetization
below (Tirr) 265 K. Further, MFCW decreases with decreasing
temperature due to the reorientation of magnetic clusters and
3. Results and discussion vanishes at compensation temperature (Tcomp) 186 K. In MFCW,
below Tcomp negative magnetization i.e., resultant magnetization
3.1. X-ray diffraction analysis of the sample opposite to the applied field can be noticed and
further decreases linearly with temperature as shown in Fig. 2(a).
The room temperature HRXRD pattern of as prepared powder While the MZFC increases with decreasing temperature below Tirr
sample has been analyzed with Rietveld structural refinement and is positive for all temperatures down to 3 K. The negative
using Fullprof suite [24]. The HRXRD profile data, the Rietveld magnetization of MFCC becomes positive while cooling the system
fitting profile, the Bragg positions and the difference between under 0.1 T and moreover, a mirror like variation of MFCW is
experimental and simulated result are shown in Fig. 1(a). Here, no observed for 70.1 T (inset to Fig. 2(a)). Here the magnetization is
trace of impurities indicates the single phase of the sample. positive for T rTcomp, while it is negative above that temperature
Rietveld study suggests the orthorhombic crystal structure with under the applied field of 0.1 T. It is interesting to note that the
Pnma space group. The complete crystal structure details and absence of thermal hysteresis in between MFCC and MFCW curves
around the Tirr 265 K is like a second order type magnetic
transition [6].
Temperature induced magnetization reversal or negative mag-
netization phenomena can be investigated further by applying
different field cooled (μ0HFC) values (see Fig. 2(b)). In MFCC versus
T curve, with increasing μ0HFC from 0.005 T to 0.05 T, the magni-
tude of negative magnetization at low temperature increases and
then decreases for μ0HFC 40.05 T and it can be noticed that the
negative magnetization vanishes for μ0HFC Z0.3 T. As shown in
Fig. 2(b), the Tcomp shifts to low temperature side, while the Tirr is
independent of μ0HFC. The variation of negative magnetization
with μ0HFC and magnetization reversal process can be discussed
later. It is evident that the polarity change in magnetization from
negative to positive is gradual and this can be attributed to the
presence of magnetic inhomogenity. It is interesting to note that
the magnetization reversal with applied μ0HFC is gradual in
perovskite oxides with multiple magnetic ions at the B-site
[5,19], while with single ion at the B site like in ReCrO3 (Re – rare
earth ion) this variation is rather sudden [17,18]. This implies that
the disorder on B-site seems to play a role.
The large thermomagnetic irreversibility between MZFC and
MFCW curves at low fields (Fig. 2(a)) suggests strong magnetocrys-
talline anisotropy and to overcome this barrier a strong external
field is required. We have measured isothermal field dependent
magnetization (M (H)) measurements at different temperatures as
shown in Fig. 3. Here the M (H) loop at 2 K illustrates hysteresis
Fig. 1. (a): XRD pattern, (b): FESEM monograph and (c): SAED pattern of LBFCO behavior at low fields with large coercive field (μ0Hc) 0.8 T and
polycrystalline sample. non-zero remnant magnetization of 0.6 10 2μB/f.u. With
164 J. Krishna Murthy, A. Venimadhav / Physica B 448 (2014) 162–166
Table 1
List of structural parameters from the Rietveld refinement with a reliability factors of Rp ¼ 13.4 and χ2 ¼2.07.
Lattice parameters
a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg)
5.5249(2) 7.8124(2) 5.5364(2) 90 90 90
Atomic positions
Wyckoff positions x y z U [Å2]
La/Bi 4c 0.0217(19) 0.2500 0.9961(10) 0.0044(2)
Fe/Cr 4b 0.0000 0.0000 0.5000 0.0029(3)
O1 4c 0.4914(14) 0.2500 0.0710(3) 0.0020(5)
O2 8d 0.2910(4) 0.0140(3) 0.7130(4) 0.0260(5)
Bond distance and bond angles
Fe/Cr–O1 (Å) Fe/Cr–O2 (Å) Fe/Cr–O1–Fe/Cr (deg) Fe/Cr–O2–Fe/Cr (deg)
1.9997(35) 1.9931(22) 155.22(16) 157.66(12)
Fig. 3. Magnetization versus magnetic field at different temperatures. Inset
(a) shows initial M (H) curve at 2 K and using this raw data, in Eq. (1), the AFM
and WFM contributions were extracted. Inset (b) shows temperature variation
of ss.
observed WFM component is attributed to the departure of spin
structure i.e., canting of the magnetic moments from the collinear
AFM ordering. The observed inhomogeneous magnetic state with
different magnetic phases below 300 K can be represented as total
magnetization,
M ¼ χ AFM H þ ss ; ð1Þ
Fig. 2. (a): Magnetization versus temperature in different modes: MZFC, MFCC and where χAFMH is the AFM contribution and ss is the saturation
MFCW for 0.01 T and inset: MFCC versus temperature for 7 0.1 T; (b): MFCC versus magnetization of the WFM phase [5]. The WFM contribution at
temperature for different dc fields. different temperatures can be obtained by fitting the initial
magnetization curves to Eq. (1) and subtracting the AFM linear
increasing the measured magnetic field ( Z3 T), a linear variation part (extrapolated from high fields) from the experimental raw
of magnetization without saturation tendency is noticed. The data of initial curve. Inset (a) to Fig. 3 shows the field dependent
value of highest magnetization (Ms) at the highest applied mag- experimental data of initial curve and extracted AFM and WFM
netic field (7 T) is 0.23 μB/f.u. at 2 K which is smaller than the components. The obtained ss plotted as a function of temperature
theoretically calculated value of 1.0 μB/f.u. for collinear antiparallel shows the linear variation as shown in the inset (b) of Fig. 3.
ordering of Fe3 þ (S¼ 5/2) and Cr3 þ (S ¼3/2) using Goodenough– The magnetic irreversibility between MFCC and MZFC just below
Kanamori superexchange interactions [23]. From M (H) loops at (TN) and observation of zero magnetization in the present system
different temperatures, it was observed that μ0Hc remains constant can be explained based on the phenomenological model first
up to 100 K, while Ms is strongly depends on temperature. The low proposed by Tamine et al. [25]. As said above, the Fe and Cr
field hysteresis and high field linear dependence of magnetization clusters have WFM interaction above the Neel temperature, but
are the features of WFM component within the AFM matrix. The below TN, the clusters are reorienting and become antiparallel
J. Krishna Murthy, A. Venimadhav / Physica B 448 (2014) 162–166 165
with respect to each other. At compensation temperature cycled several times to examine the reproducibility of the switching
(Tcomp 186 K) where these two substructures of ordering and and the switching without noticeable decay in magnetization is
magnetization are equivalent then resultant magnetization is zero. attractive for switching applications [14]. This phenomenon was
Below Tcomp, magnetic moment of Fe3 þ clusters (aligned antipar- also verified with another field set of combination (0.14 T and
allel to the applied field) can increase faster than the Cr3 þ clusters 0.01 T) at the same temperature. This measurement was also
(aligned parallel to the field) and total magnetization of the checked at different temperatures, in the vicinity of Tirr (265 K)
sample is opposite to the field direction. Such a configuration is after cooling the sample under 0.02 T field. Here, below Tcomp at
retained due to the local anisotropy field (as discussed in 130 K and 170 K, the observed magnetization data is negative while
Figs. 2 and 3) and magnetization completely switched to positive it is positive for T4Tcomp at 210 K and 230 K. Once again at 300 K,
direction for μ0HFC Z 0.3 T, with both the clusters aligned parallel the magnetization value starts to decrease. At all temperatures the
to the field. magnetization data is stable and the time dependent magnetic
relaxation measurements are consistent with temperature depen-
dence of magnetization data i.e., M (T), as shown in Fig. 2(a).
3.3. Magnetization switching study
3.4. Magnetocaloric effect
To understand the MRE clearly, the best experimental char-
acterization is the isothermal magnetic switching measurement
In addition to the bipolar switching property, the system also
and the results are shown in Fig. 4(a). The protocol of such
exhibits magnetocaloric effect (MCE). In general, with decreasing
measurement is Ref. [19], initially the sample has to be cooled
temperature the negative slope variation of magnetization gives
under magnetic field ( 0.01 T) to the measuring temperature (say,
conventional MCE, while the positive slope variation of magneti-
150 K) oTcomp and then the magnetization was measured as a
zation gives the inverse MCE. Since, the present sample exhibits
function of time. Further the magnetization can be switched to
both positive and negative slope in magnetization in response to
positive value with same magnitude by increasing field to
the external magnetic field, one can expect both conventional and
0.095 T. As shown in Fig. 3(a), without changing the applied
inverse MCE in LBFCO. Since the MCE results from the variation of
field direction, the polarity of magnetization can be changed. As per
magnetoentropy with application of magnetic field, we have
the sequence of 0.01 T-0.095 T-0.01 T, the magnetization
calculated thermal variation of ΔSM from the μ0HFC dependent
changes its polarity from positive to negative and this process was
MFCC data (Fig. 2(b)) with the well known Maxwell equation [26],
" #
∂M ∂M ΔH i
jΔSM j ¼ ∑ þ ð2Þ
∂T Hi ∂T Hi þ 1 2
here, ð∂M=∂TÞHi and ð∂M=∂TÞHi þ 1 are the first derivative of MFCC at
H i and H i þ 1 fields respectively. The calculated temperature varia-
tion of MCE for different fields is shown in Fig. 5. Here, the polarity
variation of MCE with temperature at low fields i.e., conventional
and inverse
MCE are the direct consequences of the change in the
sign of ΔM=ΔT . As decreasing the temperature from 400 K, the
MCE increases with positive sign and becomes maxima at Tirr
265 K and decreases to zero at Tcomp. Now below Tcomp it changes
its sign and shows inverse MCE. The field independent maximum
value of positive MCE at 265 K indicates that the reorientation of
magnetic clusters or AFM ordering is a second order type in nature
as evidenced from the M (T) curve [27]. The observation of normal
as well as inverse MCE in LBFCO makes it a promising material for
coolant or constant temperature bath applications [14].
Fig. 4. (a): Magnetization versus time at 150 K to show the switching property
with magnetic field, and (b): magnetization versus time at different temperatures Fig. 5. Magnetic entropy change (ΔSM) as function of temperature (3r T r 400 K)
after cooling the sample under 0.1 T and 0.02 T fields respectively. for different dc fields.
166 J. Krishna Murthy, A. Venimadhav / Physica B 448 (2014) 162–166
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