SPECIFIC HEAT OF MTa2O6 (M = Co, Ni, Fe, Mg)
EVIDENCE FOR LOW DIMENSIONAL MAGNETISM
R. Kremer, J. Greedan, E. Gmelin, W. Dai, M. White, S. Eicher, K.
Lushington
To cite this version:
R. Kremer, J. Greedan, E. Gmelin, W. Dai, M. White, et al.. SPECIFIC HEAT OF MTa2O6
(M = Co, Ni, Fe, Mg) EVIDENCE FOR LOW DIMENSIONAL MAGNETISM. Journal de
Physique Colloques, 1988, 49 (C8), pp.C8-1495-C8-1496. <10.1051/jphyscol:19888688>. <jpa00228921>
HAL Id: jpa-00228921
https://hal.archives-ouvertes.fr/jpa-00228921
Submitted on 1 Jan 1988
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JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
SPECIFIC HEAT OF MTa206 (M = Co, Ni, Fe, Mg) EVIDENCE FOR LOW
DIMENSIONAL MAGNETISM
R. K. Kremer (I), J. E. Greedan (2), E. Gmelin
and K. J. Lushington (2)
(I)
(I),
W. Dai
(I) l ,
M. A. White
(3),
S. M. Eicher (2)
Max-Planck-Institut fiir Festk6rperforschlmg D-7000 Stuttgart 80, F.R.G.
(2) Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada, L8S 4MI
(3) Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 355
Abstract. - We report the heat capacities of MTa206 (M = Co,Ni,Fe,Mg). Antiferromagnetic ordering is observed at
6.67 K (Co), 10.55 K (Ni) and 8.1 K (Fe). Short range correlations above Tc contribute substantially to the heat capacity
of the magnetic compounds. The mtgnetic part in the heat capacity of the Co and Ni compound can be described by a
very anisotropic square planar S = 1 Ising model. Magnetic ordering in FeTa206 takes place in a S = 1 triplet ground
2
state. The short range order contributions are compared to the heat capahities of S = 1 afm chains.
Introduction
We have shown in previous publications [I-31 that
oxides of composition MTa206 crystalllizingin the well
known tri-rutile structure type [4,5] are possible candidates for low-dimensional magnetism. We carried out
detailed heat capacity studies to measure the exchange
constants and to enlighten the issue of the dimensionality of the magnetic lattice.
0
5
10
15
M
25
30
35
Temperature I K J
Experimental
Powder samples of MTazOs(M = Co, Ni, Mg) were
prepared according to reference 16, 71. The preparation of FeTa206 has been described in reference 121.
Specific heats of pressed and sintered pellets of the
samples or powder samples were determined in an adiabatic calorimeter [8]. The magnetic part Cm in C, of
the title compounds (except Mg) was obtained by subtracting a lattice part Clat calculated from the Debye
temperature OD ( T )of MgTa,06 after a proper scaling.
Results
CoTa206. - A sharp spike is observed that marks the
transition t o 3D long range ordering at Tc = 6.67(3)K
in best agreement with values observed previously
[I, 31. Short range order contributions t o Cmpersist up
to at least 6.Tc. The total entropy S =
I"
Cm/TdT
1
is about 0.57.R indicating a S = ;; doublet ground
1,
state. The major part (91 % ) of S is removed by the
short range correlations pbove Tc. A comparison with
the square planar S = Ising model (Onsager's solution) was made t o anzflyze our data. As shown in
figure 1 the experimental data are well approximated
if we chose a very anisotropic exchange parameter set
lJll = 11.8 K and 1521 = 0.2 K. The differences be-
4
Fig. 1. - Magnetic part C , of the specific heat of CoTazOs
1
(0) compared to the anisotropic square planar S = - Ising
2
model with exchange constants 51 = 11.8 K and 52/51 =
0.018 (full curve).
tween theory and experiment observed above Tc might
be due t o an improper subtraction of the lattice heat
capacity. The latter might as well be the reason for the
deviation of the entropy from In 2. The heat capacity C of the sq Ising net was calculated from the free
energy per spin f [9] according to C = -T. a2f / a
by a numerical evaluation of the integrals. The sign of
the exchange constants is not provided by this analysis but from the negative 0 observed in Curie-Weiss
plots 16, 71 we conclude that Ji < 0 i.e. afm coupling.
The J; found must be interpreted in the sense that
although the magnetic cations are arranged in layers,
superexchange via the 02- anions dominantly takes
place along a chain direction favouring a I D magnetic
lattice. Coupling between the chains within the same
layer is about two orders of magnitude smaller.
NiTazO6. - Cm is characterized by a X-shaped
anomaly at Tc = 10.55(5)K indicating 3D magnetic ordering (cp. [I, 31) and contributions above Tc expanding over a wide range up to several Tc. They again reveal the presence of short range magnetic correlations
W. D. gratefully acknowledges financial support by the Max Planck Society - Academia Sinica exchange program.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888688
~ ~
JOURNAL DE PHYSIQUE
C8 - 1496
characteristic for low dimensional magnetic behaviour.
S is 0.65.R close to the expected value for a doublet
ground state. 26 9% of S are acquired below Tc. As
shown in figure 2, the experimental data can, in analogy t? CoTa206 very well be fitted by the anisotropic
1
S = Ising model. The parameter set 1Jll = 15.0 K
2
and lJ2l = 0.4 K indicates a very similar situation as
for CoTa206 with dominantly Ising chains present and
only a weak coupling between them.
The observation of a doublet ground state for Ni2+
in NiTa206 points to a substantial single ion zero-field
1
splitting D. S: - 2.S (S I ) ] ) of the S = 1 triplet
-
+
(
[
ground state [lo] which usually is observed for a 3ds
ion in an octahedral symmetry. In case of D < 0 the
doublet remains lowest. In figure 2 we have drawn
the results obtained for the specific heat of an afrn
S = 1 chain with isotropic exchange coupliiig and additional single ion anisotropy (111. For Dl (JI = -20
and J = -15.0 K the dashed curve in figure 2 results
1
which for low T is identical to the C of an S = 2
chain. For higher T a n additional Schottky term is expected which might be difficult to detect experimentally because of the uncertainties that rose when the
dominating Clat is subtracted.
estimating Cl,t may result in considerable errors in
c m
-
FeTa206. - Cm exhibits a broad peak with the maximum at Tc = 8.1 ( 1 ) K indicative for 3D ordering [2].
A shoulder evolves above T, that extends up to about
5.Tc and again reveals the presence of short range order contributions. S = 1.17.R which is close to In 3
for a S = 1 spin system. About 1 / 3 of S are gained
below Tc.In order to get an estimate of the exchange
constants we have compared (see Fig. 3) Cm with the
theoretical predictions [12]for various S = 1 afrn chain
models with exchange couplings intermediate between
the pure Heisenberg ( J I = ~ ~ and
1 ) the pure Ising case
( J L = 0 ) . A good fit of the high T part of Cm is
achieved for Jll sz -16 K and JL = (11.25.Jll(121.
"
0
10
D
20
30
'W
l
W
Temperature i K l
Fig. 3. - Magnetic part C , of the specific heat of FeTazOs
( 0 ) compared to a S = 1 afm Heisenberg chain J = -12 K
(dash-dot curve), Ising chain IJI = 8 K (:dashedcurve) and
a S = 1 afm chain with Ji/JII = 0.25 and JIl = -16 K
(full curve). For details see text.
0
I0
20
30
40
W
Temperature I K )
Fig. 2. - Magnetic part Cmof the specific heat of NiTa206
1
( 0 ) compared to the anisotropic square planar S = ;
;Ising
model with exchange constants Jl = 15 K and J:/JI =
0.28 (full curve). The dashed curve shows the specific heat
of a S = 1 antiferromagnetic Heisenberg chain with single
site anisotropy D / fJ( = -20 and J = -15 K .
ID1 sz 300 K implied by the fit in figure 2 is of an
unprecedented magnitude for Ni2+ compounds and in
fact much higher than values observed hitherto for the
zero-field splitting of the ~ i 3 ~~ gound
2+
state. However, it seems clear from the results presented that a
1
S = - model is consistent with the total magnetic
2
entropy and the fit of the Cm versus T behaviour. To
explain this obvious discrepancy the subtraction of the
lattice contribution needs to be carefully reexamined
especially in the higher temperature region where Clat
dominates the total heat capacity and small errors in
[ I ] Eicher, S . M., Thesis, McMaster University
(1984).
[2]
- .Either, S. M., Greedan, J. E. and Lushington, J.
Solid State Chem. 62 (1986) 220.
[3] Kremer, R. K., Greedan, J. E., J. Solid State
Chem., J. Solid State Chem. 73 (1988) 579.
[4] Heidenstamm, 0. V., Ark. Kemi 28 (1968) 375.
[5] Miiller-Buschbaum, Hk. and Wichmann, R., 2.
Anorg. allg. Chem. 536 (1986) 15.
[6] Takano, M. and Takada, T., Mater. Res. Bull. 5
(1970) 449.
[7] Bernier, J.-C., C.R. C 273 (1971) 1166.
[8] Gmelin, E . and Ripka, K., Cryogenics 21 (1981)
177.
[9] e.g. Mattis, D. C., The Theory of Magnetism I1
(Springer Ser.) Solid-State Sci. (1985).
[lo] e.g. Abragam, A. and Bleaney, B'., Electron Par*
magnetic Resonance of Transition Ions (Oxford
University Press) 1970.
[Ill Blote, H. W. J., Physdca 79B (1975) 479.
[12] C for a = J 1 / J I I= 0.25 is not given in [ll]. We
have used the mean value of a := 0 and (Y = 0.5
instead.