Full text of DOI:10.1016/0375-9601(88)90820-1

Volume 134, number 3
PHYSICS LETTERSA
26 December 1988
EFFECT OF PRESSURE ON THE CURIE TEMPERATURE OF C0Cr2O4
T. KANOMATA
Department ofApplied Physics, Faculty of Engineering, Tohoku Gakuin University, Tagajo, Miyagi 985, Japan
T. TSUDA
Department of NaturalScience, Facultyof Education, Saitama University, Urawa 338, Japan
H. YASUI and T. KANEKO
Institutefor Materials Research, Tohoku University, Katahira, Sendai 980, Japan
Received 25 August 1988; accepted for publication 26 October 1988
Communicated by D. Bloch
The pressure effect on the Curie temperature T ~of CoCr2O4 was measured under pressures up to 11.5 kbar. The pressure
derivative of T ~is found to be + 0.25 K/kbar. On the basis of this result, the relationship between the magnetic transition tem -
perature and volume is discussed.
Bloch [1] reported that there is an emperical rule
for the relationship between the magnetic transition
temperature TCN (Ta: Curie temperature, TN: Néel
temperature) and volume V in insulating magnetic
compounds such as CoO, FeO, several garnets and
ferrites:
The compound CoCr2O4 crystalizes in the spine!
lattice with a normal cation distribution, where Co2+
ions locate at tetrahedral (A) sites and Cr3 + ions at
octahedral (B) sites. It becomes ferrimagneticbelow
the Curie temperature (Ta) of 97 K [31.There is a
magneticphase transition (Ta) around27 K [3]. The
P
(
spontaneous magnetization at 4.2 K is about 0.1 UB
Au lo g ic N /u/lAo1g V=— T,
or
~l,
.
per formular unit [31,which is so low that it cannot
be explained by the simple Néel model. Menyuk et
dT /d
—
—
~
icT
al. [4] and Plumier [51proposed a ferrimagnetic
spiral structure withthree magnetic sublattices A, B-
C,N
P
~
C .N ,
‘
‘
in which K expresses the isothermal compressibility.
Recently, Kanomata et a!. [2] also reported that
the value of d log TCN/dlog V for chalcogenide spi-
I and B-Il for CoCr2O4 from their neutron diffrac-
tion (N.D.) experimental data. Then, Tsuda et a!.
[61 observed the NMR signals from Co59 and Cr53
nuclei in CoCr2O4. From the magnetic field depen-
dence of these resonance frequencies up to 50 kOe,
they estimated the half cone angles of the ferrimag-
netic spiral for low temperature magneticphaseto be
nels FeCr2S4 and CoCr2S4 are given by about
—
~.
Furthermore they reported that eq. (1) is obtained
by assuming that the dominant part of the super-
exchange interaction is the kinetic one and the band
width W (the transfer integral I) is proportional to
180
±
7
0
,
1230 ± 4 0 and 1230 ± 4 0 for Co, Cr-I and
V
— 5 /3 ~ However, it is not clear at present whether
Cr-I! sublattices, respectively. These angles are
somewhat different from those determined by N.D.
Furthermore, they reported that the local moment of
Co2+ in the high temperature magnetic phase seems
to be parallel to the net magnetization. The main
purpose of the present note is to measure the pres-
Bloch’s rule is adaptable for all insulating magnetic
compounds. So, it is necessary to examine system-
atically the pressure effect on the transition temper-
ature of many magnetic compounds besides those
mentioned above,
196
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Volume 134, number 3
PHYSICS LETTERS A
26 December 1988
surechange of the Curie temperature of CoCr2O4and
to investigate the interatomic distance dependence
of the superexchange interaction of CoCr2O4.
For the preparation of the sample, desired pro-
portions of powdered Co304 and Cr203 were mixed
in alcohol, and, after preheating at 900°C for six
hours and 10009C for twelve hours, this product was
fired at 1400°Cin air for a day and then quenched
104
102
.
CoCr2O4
-
to room temperature.
The X-ray diffraction pattern which was obtained
at room temperature using copper radiation m di-
cated a single-phase, normal spine! structure. The
lattice parameter and u parameter are found to be
8.322 A and 0.386, respectively. This value of the u
parameteris in good agreement withthat previously
observed by Menyuk et al. [4].
98
The value of dT ~/dpwas measured with the mag-
netic induction method at hydrostatic pressures de-
scribed in a previous work [7J~Initial permeability
versus temperature curves for CoCr2O4 were ob-
I
0
10
p
k bar)
Fig. I. Shift of the Curie temperature versus pressure curve.
tamed at different pressures: initial permeability de-
creases rapidly just below T ~ with increasing
temperature and then takes a nearly constant value
with further rise in temperature. T ~was defined as
the point of intersection oflinear extrapolations from
both high and low temperature ranges. The Curie
temperature for CoCr2O4 was found to shift from
100.2 K at normal pressure to 103.1 K under a pres-
NiO [12,14,15], La2CuO4~ [16,17], Mn—Zn fer-
rite [9], FeCr2S4 [2], CoCr2S4 [21 and the rare earth
iron garnets [10,18]. The relation (1’) suggested by
Bloch is expressed by the straight line in fig. 2. As
shown in fig. 2, the relation between dTc,N/dp and
‘CTCN for the present compound is well consistent
with the relation proposed by Bloch [1]. For the
sure of 11.5 kbar. The Curie temperature at normal
pressure is somewhat larger than the value (97 K)
reported by Menyuk et al. [3]. The pressure shift of
compressibility of Mn—Zn ferrite, GdIG and ErIG,
we assumed that the compressibility of these corn-
pounds are equal to that of YIG [18].
T ~is shown against applied pressure in fig. 1. For the
present compound, the Curie temperature wasfound
to increase linearly withapplied pressure in the pres-
As mentioned above, the spin arrangement of
CoCr2O4 is a complicated ferrimagnetic spiral type.
sure range investigated. The value of dT ~ /dpdeter-
mined from the results in fig. 1 is found to be + 0.25
K/kbar. This value is smaller than those of Mn—Zn
and Ni—Zn ferrites. That is, dT ~ /dpof Ni—Zn fer-
rites range from +0.73±0.06 to + 1.16±0.07K/
kbar [8]. Furthermore, dT ~ /dpofMn05Zn05-Fe204
was found to be +0.9± 0.04K/kbar [9]. The value
i.s
~
Mn~,Znn,Fe20~
ErIC
‘.
1.0
0.5
LoIcuO~.y_~<0
FeO 0Co~~.
of d log T ~ /dlog V for CoCr2O4 was obtained to be
0COCI.2S.
—3.1 using the present result on dT~/dp.In this cal-
culation,
we
used
the
compressibility
K = (0.80 ± 0.21) x 1 0 ~ kbar’ for CoCr2S4 [2] as
CoCr1~
01
the compressibility of CoCr2O4. In fig. 2, we plot
0 ~
0.3
dTCN/dp against ICTCN for CoCr2O4 together with
those for CoO [10,12], FeO [11,12], MnO [12,13],
k ~ (K / k bar)
Fig. 2. P1otsofdT~~/dpagainsitcTCN.
197

Volume 134, number 3
PHYSICS LETTERS A
26 December 1988
Lyons and Kaplan [191 studied theoretically the
ground state spin configuration of cubic normal spi-
nels. They proposed a ferrimagnetic spiral structure
with three magnetic sublattices A, B-I and B-IT, when
the isotropic exchange interaction between the near-
est neighbour B—B ions becomes comparable with
that between nearest neighbour A—B ions. Tsuda et
ions withoppositely directed moment. In these corn-
pounds, the only magnetic interactions of antifer-
romagnetic spinels are those of A site Co ions. The
distance between A site Co ions is very large and the
A—A interaction involves two or three intervening
ions in superexchange interactions. In this case, it is
interesting to investigate whether the relationship
al. [6] discussed the relation between the ferrimag-
netic spiral structure and the exchange parameters
for CoCr2O4 on the basis of the experimental results
between the magnetic transition temperature and
volume follows Bloch’s empirical rule mentioned
above.
of their NMR measurement. According to their re-
sults, the nearest neighbour A—B and B—B interac-
tions are almost equal and are consistent with the
theoretical result by Lyons and Kaplan. These two
interactions are an order of magnitude larger than
many other exchange interactions between distant
neighbour ions in this oxide. Therefore, the pressure
dependence of the Curie temperature is probably
caused by the variation in the nearest neighbour A—
References
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surrounded by four nearest neighbour same site Co
198