APPLIED PHYSICS LETTERS 94, 042505 ͑2009͒
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Ingyu Kim, Yoon Seok Oh, Yong Liu, Sae Hwan Chun, Jun-Sik Lee, Kyung-Tae Ko,
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Jae-Hoon Park, Jae-Ho Chung, and Kee Hoon Kim
1
Department of Physics and Astronomy & FPRD, Seoul National University, Seoul 151-747,
Republic of Korea
2
Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 790-784,
Republic of Korea
3
Department of Physics, Korea University, Seoul 136-713, Republic of Korea
͑
Received 5 December 2008; accepted 7 January 2009; published online 27 January 2009͒
Single crystals of multiferroic cobalt chromite Co͑Cr Co ͒O have been grown via several
2
−x
x
4
methods to have different Co3 doping levels ͑x=0.0, 0.14, and 0.18͒. Under magnetic fields, all the
crystals display electric polarization reversal below their spiral spin ordering temperatures. We find
that both saturated electric polarization and magnetization under magnetic fields increase
significantly with the increase in x. This result can be qualitatively explained by a broken balance
between at least two electric polarization contributions existing in CoCr O and is expected to be
+
2
4
Recently, multiferroics, which have two or more ferroic
orders in a material system, have attracted considerable
both magnetization ͑M͒ and P increase significantly, while
the magnetoelectric coercive field decreases. These observa-
tions suggest that Cr-site mixing causes an imbalance be-
tween at least two competing P components in the stoichio-
metric compound.
attention owing to their potential applications in various de-
vices such as multibit memories or sensitive magnetic field
sensors. Furthermore, the scientific interest in understanding
the origin of multiferroicity and related strong magnetoelec-
tric coupling has triggered a surge of research activities in
this field. A newly emerging multiferroic system that exhib-
Single crystalline Co͑Cr2−xCo ͒O samples were grown
x
4
by the flux method and chemical vapor transport method. In
the former, CoO and Cr O powders in a 1:1 molar ratio
2
3
its such strong magnetoelectric coupling is the spiral magnet,
were mixed with PbO, PbO , and PbF flux, heated in a Pt
2
2
crucible at 1260 °C for 12 h, and then slowly cooled to
ordering.
Spinel cobalt chromite CoCr O is a new spiral magnet
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00 °C. All of the resultant crystals had octahedral shape
2
4
with a typical lateral size of ϳ3–4 mm and were confirmed
that has been recently found to exhibit strong magnetoelec-
tric coupling. It shows concomitant long-range spiral
to be cubic spinels at room temperature by x-ray
diffraction. Different amounts of Co ͑xϾ0͒ could be doped
into the Cr site by controlling the cooling speed between
2 °C/h ͑sample F#1͒ and 1 °C/h ͑sample F#2͒. The stoi-
chiometric crystal ͑C#1; xϷ0͒ could be obtained by the
͑
conical-type͒ spin ordering and ferroelectric transitions at
TS=24–27 K ͑Refs. 7–10͒ and a subsequent lock-in transi-
L
commensurate magnetic transition occurs.
Thus, a finite
chemical vapor transport method with Cl gas as the trans-
2
electric polarization ͑P͒ develops at T , accompanied by a
S
port agent.
sharp decrease in the dielectric constant . An additional
The valence states of the Co ions in the three different
samples were examined by Co L2,3-edge x-ray absorption
spectroscopy ͑XAS͒, as shown in Fig. 1. The XAS measure-
ments were performed at the 2A beamline at Pohang Light
Source ͑PLS͒. The XAS spectra of C#1 show a multiplet
change in is also observed at T . More interestingly, P
L
S
ϳ0.5 T is applied. This feature can be advantageous for
designing novel devices such as H-controlled ferroelectric
memories. However, the magnitude of P still needs to be
enhanced for practical applications. Due to the intrinsically
small P values as often observed in spiral magnets, the satu-
2+
structure observed typically in the Co ion in a tetrahedral
site symmetry; this pattern agrees well with that of a refer-
ence sample. However, in the spectra of F#1 and F#2, an
2
rated value of P at H=0.5 T reaches only ϳ2 C/m in
intense additional peak corresponding to the spectral weight
CoCr O . Thus, it is necessary to increase P value of the
3+
2
4
from Co ions is observed. In particular, the spectral weight
spiral magnet by modifying their material properties.
In this letter, we report the observation of enhanced elec-
3+
of Co is greater in F#2 than in F#1. Considering the fact
2+
3+
that in a normal spinel structure, Co and Co ions prefer to
reside in the tetrahedral A-site and the octahedral B site,
respectively, it is likely that the B-sites in the flux-grown
tric polarization in Co͑Cr2−xCo ͒O single crystals with Cr-
x
4
site mixing. By following different crystal growth methods,
we control the Co ion content ͑x͒ in the Cr-sites to obtain
x=0.0, 0.14, and 0.18. We find that with an increase in x,
3
+
3+
samples are occupied by both Cr and Co ions, which
result in the B-site ionic mixing. The chemical compositions
of the obtained crystals were determined by electron probe
microanalysis, and the flux-grown crystals were confirmed to
contain large excess Co ions. The average values of x in our
a͒Author to whom correspondence should be addressed. Electronic mail:
khkim@phya.snu.ac.kr.
0
003-6951/2009/94͑4͒/042505/3/$25.00
94, 042505-1
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