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Cooling field dependence of exchange bias in phase-separated
La0.88Sr0.12CoO3
Yan-kun Tang, Young Sun,a兲and Zhao-hua Cheng
State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics,
Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China
共Received 9 December 2005; accepted 13 May 2006; published online 31 July 2006兲
We report the observation of exchange bias phenomena in the hole-doped perovskite cobaltite
La0.88Sr0.12CoO3in which a spontaneous phase separation occurs. When the sample is cooled in a
static magnetic field through a freezing temperature, the magnetization hysteresis loops shift to the
negative field. Moreover, the exchange bias strongly depends on the cooling field. These results
highlight the important role of a glassy interface between the intrinsic inhomogeneous phases in a
phase-separated system. © 2006 American Institute of Physics.关DOI: 10.1063/1.2219698兴
I. INTRODUCTION
The exchange bias phenomenon refers to a shift of the
magnetization hysteresis loop away from zero field due to an
unidirectional anisotropy. This anisotropy is usually caused
by the exchange coupling at the interface between ferromag-
netic 共FM兲and antiferromagnetic 共AFM兲spin structures after
the system is cooled in a static magnetic field through the
Néel temperature of the AFM.1So far the research on ex-
change bias has been mainly focused on FM/AFM thin films
where a well defined and controllable interface exists.2,3 In
addition to FM/AFM interfaces, exchange bias has also been
observed in other types of interfaces involving a ferrimagnet
共FI兲共e.g., FI/AFM and FI/FM兲共Refs. 4 and 5兲or a spin glass
共SG兲phase 共e.g., FM/SG, AFM/SG, and FI/SG兲.6–9 These
interfaces are usually achieved through artificially designed
phase inhomogeneity, for example, by making artificial mag-
netic bilayers or mixing magnetic particles with a different
matrix 共granular systems兲. In this paper, we report the obser-
vation of exchange bias phenomenon in a spontaneously
phase-separated cobaltite in which intrinsic phase inhomoge-
neity plays a crucial role.
The hole-doped cobaltites La1−xSrxCoO3共LSCO兲have
been proved to exhibit a particularly clear form of phase
separation for a broad range of doping level xby many ex-
perimental evidences obtained using various techniques in-
cluding electron microscopy,10,11 nuclear magnetic resonance
共NMR兲,12,13 and small-angle neutron scattering 共SANS兲.14 It
has been well recognized that the phase separation in LSCO
is in the form of coexistence of FM metallic regions and
non-FM insulating regions. Furthermore, with this form of
phase separation, it has been proposed that the spontaneously
phase-separated LSCO is analog to the artificial granular
films that are composed of FM particles embedded in a
non-FM matrix.14 In this work, we have studied the magne-
tization hysteresis loops of a La0.88Sr0.12CoO3sample with
zero-field-cooling and field-cooling processes. Our results
demonstrate exchange bias phenomena that strongly depend
on the cooling field.
II. EXPERIMENTS
The La0.88Sr0.12CoO3sample was prepared with solid
state reaction method. A stoichiometric mixture of SrCO3,
Co3O4, and La2O3powders was well ground and calcined
twice at 800 and 950 ° C for 24 h. Then, the resulting powder
was pressed into pellets and sintered at 1100 and 1150 °C
for 24 h, respectively. X-ray diffraction patterns show that
the sample is single phase with rhombohedral structure. The
magnetization measurements were performed using a com-
mercial Quantum Design superconducting quantum interfer-
ence device 共SQUID兲magnetometer.
III. RESULTS AND DISCUSSION
Figure 1 shows the temperature dependence of magneti-
zation in a low magnetic field 共H=10 Oe兲with the zero-
field-cooled 共ZFC兲and field-cooled 共FC兲processes for
La0.88Sr0.12CoO3. At 240 K, the magnetization starts to in-
crease rapidly with decreasing temperature. Below 240 K,
the ZFC and FC magnetizations separate with each other.
These results imply that the onset of FM ordering within the
a兲Author to whom correspondence should be addressed; electronic mail:
youngsun@aphy.iphy.ac.cn
FIG. 1. Temperature dependence of magnetization with ZFC and FC
processes for La0.88Sr0.12CoO3. The inset illustrates the freezing temperature
of 50 K with ZFC process for La0.88Sr0.12CoO3.
JOURNAL OF APPLIED PHYSICS 100, 023914 共2006兲
0021-8979/2006/100共2兲/023914/3/$23.00 © 2006 American Institute of Physics100, 023914-1
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clusters occurs at Tc=240 K. As illustrated in the inset of
Fig. 1, the ZFC magnetization exhibits a peak around 50 K
共Tf兲and a wide shoulder around 170 K. The wide shoulder
could be due to the intercluster interactions, and the sharp
peak around Tfindicates a collective freezing of magnetic
moments.15
Figure 2 shows the hysteresis loops of La0.88Sr0.12CoO3
at 5 K with both the ZFC and FC processes. For the ZFC
process, the sample was cooled in zero magnetic field from
360 to 5 K. For the FC process, the sample was cooled in
10 kOe magnetic field from 360 to 5 K. Then the hysteresis
loops were measured between ±50 kOe. While the ZFC
magnetization has a normal hysteresis loop centered at zero
field, it is clear that the FC hysteresis loop shifts to the nega-
tive field. The magnitude of the shift is known as exchange
bias HE=−共HC1+HC2兲/2, where HC1and HC2are the left
and right coercive fields, respectively. With the coercive
fields obtained from the FC loop, we get the value of ex-
change bias, HE⬇500 Oe. The FC hysteresis loop also has
an increased coercivity, HC=兩HC1−HC2兩/2 =6640 Oe, com-
pared to the coercivity of 5430 Oe for the ZFC loop. All
these behaviors are the characteristics of exchange bias
phenomenon.
The appearance of exchange bias in La0.88Sr0.12CoO3in-
dicates that an unidirectional anisotropy is built up after the
sample is cooled in a magnetic field. In some compounds,
such as Gd2CuO4,16 a shift of hysteresis loops is also ob-
served, which is interpreted as the presence of unidirectional
anisotropies due to Dzyaloshinskii-Moriya interactions.
However, it seems that this mechanism is not appropriate for
our sample considering the large magnetization of our
sample and the shape of the initial magnetization curve. We
argue that this unidirectional anisotropy in La0.88Sr0.12CoO3
is due to interfacial exchange interaction associated with
phase separation. As we already mentioned above, the phase-
separated LSCO is analogous to the artificial granular films
that are composed of FM particles embedded in a non-FM
matrix. It has been well known that a spin-disordered
interface/surface layer is usually formed when a FM particle
is embedded in a non-FM matrix8or the magnetic particle is
small enough 共the finite size effect兲.7Such spin-disordered
interfaces/surfaces lead to exchange bias in many artificial
granular systems.9,17 Similarly, we believe that spin-
disordered regions could exist at the interfaces between the
FM clusters and the non-FM matrix in phase-separated
LSCO. In fact, 59Co NMR and magnetic relaxation experi-
ments have revealed that SG regions coexist with FM and
non-FM regions in LSCO.12,13,18 Therefore, the exchange
bias in LSCO can be qualitatively understood with the fol-
lowing picture. When the sample is cooled in the presence of
a magnetic field, an energy favorable spin configuration at
the interface will be selected through the exchange interac-
tion between the spin glass regions and the FM clusters. The
exchange interaction competes with thermal fluctuation. As a
result, the interfacial spin configuration is frozen below the
freezing temperature and becomes more stable at lower tem-
peratures. When the measuring field is reversed, the spins of
FM particles start to rotate. However, the spin configuration
at the interface may remain unchanged. Therefore, the spins
at interface exert a microscopic torque on the FM spins to
keep them in their original position. Thus, the field needed
to reverse the FM spins will be larger because an extra
field is required to overcome the microscopic torque. As a
result, the magnetization hysteresis loop shifts toward the
negative field. Based on this picture, exchange bias in
La0.88Sr0.12CoO3can be qualitatively interpreted in terms of
the interfacial coupling associated with intrinsic phase inho-
mogeneity. We also measured the FC hysteresis loops at
60 K 共above Tf兲. No exchange bias is observed, which is
consistent with the above picture. We note that this effect
was also recently observed in other phase-separated perov-
skite oxides,19,20 which indicates that the exchange bias as-
sociated with phase separation is an intrinsic property.
We then studied the influence of cooling field on the
exchange bias in La0.88Sr0.12CoO3. The sample was cooled
down from 360 to 5 K under different applied fields 0
艋Hcool艋50 kOe. After the temperature was stable, the field
was set to H=50 kOe and the hysteresis loop was measured.
Figure 3 shows the cooling field dependence of the loop
parameters, exchange bias HE, remanent magnetization Mr,
coercivity HC, and magnetization at H=50 kOe M50 kOe.At
small cooling field, Mr,HC, and HEincrease with increasing
cooling field whereas M50 kOe remains nearly constant. The
constant M50 kOe indicates that the proportion of FM clusters
changes little at small cooling fields. With the increase of
cooling field, the alignment degree of the moments of FM
clusters along a preferential direction is enhanced, which re-
duces the effect of averaging of the anisotropy due to ran-
domness. Therefore, Mr,HC, and HEincrease with the in-
crease of cooling field. At high cooling field, Mrtends to be
saturated but HCand HEdecrease with increasing cooling
field. The decrease in HEis accompanied with the increase in
M50 kOe. This indicates that the growth of FM clusters in high
cooling field could contribute partly to the decay of HE.At
high cooling field, not only the alignment degree of the mo-
ments of FM clusters is enhanced but also the size of FM
clusters increases. As the FM clusters grow up, exchange
FIG. 2. Hysteresis loops of La0.88Sr0.12CoO3at 5 K measured after zero-
field cooling and field cooling in 10 kOe field. Inset: enlarged view of the
central region of the loops.
023914-2 Tang, Sun, and Cheng J. Appl. Phys. 100, 023914 共2006兲
Downloaded 01 Aug 2006 to 159.226.36.159. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
bias is reduced. This effect is qualitatively analogous to that
in FM/AFM thin films where exchange bias is inversely pro-
portional to the thickness of the FM layer and becomes neg-
ligible when the FM layer is too thick.1Meanwhile, the spins
at the interfaces between the FM clusters and the surround-
ing non-FM matrix also tend to orient along the field direc-
tion when the Zeeman coupling overcomes the interfacial
exchange coupling. Both the growth of FM clusters and the
diminishment of spin disorder at the interfaces weaken the
exchange bias.
IV. CONCLUSIONS
In summary, we have discovered exchange bias phenom-
ena associated with phase separation in LSCO. The exchange
bias is strongly dependent on cooling field. These results
suggest that, in a spontaneously phase-separated system, the
exchange coupling at the interfaces between the FM regions
and the surrounding non-FM phase may create an exchange
anisotropy when the sample is cooled in a static magnetic
field.
ACKNOWLEDGMENTS
The authors are grateful to Professor Jian-wang Cai for
helpful discussion and Professor Tong-yun Zhao for assis-
tance in experiments. This work was supported by the State
Key Project of Fundamental Research and the National
Natural Science Foundation of China.
1J. Nogués and I. K. Schuller, J. Magn. Magn. Mater. 192,203共1999兲.
2J. Nogués, D. Lederman, T. J. Moran, and I. K. Schuller, Phys. Rev. Lett.
76,4624共1996兲.
3M. R. Fitzsimmons, P. Yashar, C. Leighton, I. K. Schuller, J. Nogués, C. F.
Majkrzak, and J. A. Dura, Phys. Rev. Lett. 84,3986共2000兲.
4P. J. van der Zaag, R. M. Wolf, A. R. Ball, C. Bordel, L. F. Feiner, and R.
Jungblut, J. Magn. Magn. Mater. 148, 346 共1995兲.
5W. C. Cain and M. H. Kryder, J. Appl. Phys. 67, 5722 共1990兲.
6B. Aktas, Y. Öner, and H. Z. Durusoy, J. Magn. Magn. Mater. 119 , 339
共1993兲.
7R. H. Kodama, A. E. Berkowitz, E. J. McNiff, and S. Foner, Phys. Rev.
Lett. 77, 394 共1996兲.
8R. H. Kodama, S. A. Makhlouf, and A. E. Berkowitz, Phys. Rev. Lett. 79,
1393 共1997兲.
9L. Del Bianco, D. Fiorani, A. M. Testa, E. Bonetti, and L. Signorini, Phys.
Rev. B 70, 052401 共2004兲.
10R. Caciuffo et al., Phys. Rev. B 59, 1068 共1999兲.
11J. Mira, J. Rivas, G. Baio, G. Barucca, R. Caciuffo, D. Rinaldi, D. Fiorani,
and M. A. Señarís-Rodríguez, J. Appl. Phys. 89, 5606 共2001兲.
12P. L. Kuhns, M. J. R. Hoch, W. G. Moulton, A. P. Reyes, J. Wu, and C.
Leighton, Phys. Rev. Lett. 91, 127202 共2003兲.
13M. J. R. Hoch, P. L. Kuhns, W. G. Moulton, A. P. Reyes, J. Wu, and C.
Leighton, Phys. Rev. B 69, 014425 共2004兲.
14J. Wu, J. W. Lynn, C. J. Glinka, J. Burley, H. Zheng, J. F. Mitchell, and C.
Leighton, Phys. Rev. Lett. 94, 037201 共2005兲.
15J. Mira, J. Rivas, R. D. Sánchez, M. A. Señarís-Rodríguez, D. Fiorani, D.
Rinaldi, and R. Caciuffo, J. Appl. Phys. 81, 5753 共1997兲.
16J. Mira et al., Phys. Rev. B 52, 16020 共1995兲.
17L. Del Bianco, D. Fiorani, A. M. Testa, E. Bonetti, L. Savini, and S.
Signoretti, Phys. Rev. B 66, 174418 共2002兲.
18Y.-K. Tang, Y. Sun, and Z.-H. Cheng, Phys. Rev. B 73, 012409 共2006兲.
19Y. Sun, Y.-K. Tang, and Z.-H. Cheng, Phys. Rev. B 73, 174419 共2006兲.
20D. Niebieskikwiat and M. B. Salamon, Phys. Rev. B 72, 174422 共2005兲.
FIG. 3. The cooling field dependence of 共a兲exchange bias HE,共b兲remanent
magnetization Mr,共c兲coercivity HC,and共d兲magnetization at 50 kOe
M50 kOe at 5 K.
023914-3 Tang, Sun, and Cheng J. Appl. Phys. 100, 023914 共2006兲
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