SPECIFIC HEAT OF MTa2O6 (M = Co, Ni, Fe, Mg) EVIDENCE FOR LOW DIMENSIONAL MAGNETISM

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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:

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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 ( I ) , W. Dai (I) l , M. A. White (3), _{S. M. Eicher (2)} and K. J. Lushington (2)

( I ) 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 crystalllizing in the well known tri-rutile structure type [4,5] are possible candi- dates for low-dimensional magnetism. We carried out detailed heat capacity studies to measure the exchange constants and to enlighten the issue of the dimension- ality of the magnetic lattice.

Experimental

Powder samples of MTazOs(M = Co, Ni, Mg) were prepared according to reference 16, 71. The prepara- tion 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 adi- abatic calorimeter [8]. The magnetic part Cm in C, of the title compounds (except Mg) was obtained by sub- tracting 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 Cm persist up to at least 6.Tc. The total entropy S = Cm/TdT

1

I"

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 =

4

Ising model (Onsager's so- lution) 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-

0 5 10 15 M 25 30 35 Temperature I K J

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.

a2

f

/ a ~ ~

by a numerical evaluation of the integrals. The sign of

the exchange constants is not provided by this anal- ysis 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 a t Tc = 10.55(5)K indicating 3D magnetic ordering (cp. [I, 31) and contributions above Tc expand- ing over a wide range up to several Tc. They again re- veal the presence of short range magnetic correlations

W. D. gratefully acknowledges financial support by the Max Planck Society - Academia Sinica exchange program.

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C8 - 1496 JOURNAL DE PHYSIQUE

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 anal- ogy t? CoTa206 very well be fitted by the anisotropic

1

S =

-

Ising model. The parameter set

1

Jll = 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 t o detect experimen- tally because of the uncertainties that rose when the dominating Clat is subtracted.

0 I0 20 30 40 W

Temperature I K )

Fig. 2.

-

Magnetic part Cm of 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 ~ i3~~gound state. How- 2+ ever, 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

estimating Cl,t may result in considerable errors in

c m

-

FeTa206.

-

Cm exhibits a broad peak with the max- imum at Tc = 8.1 ( 1 ) K indicative for 3D ordering [2].

A shoulder evolves above T, that extends up t o 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 the pure Ising case 1 ) ( 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.

"

D

l

0 10 20 30 ' W 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

I

JI = 8 K (:dashed curve) and

a S = 1 afm chain with Ji/JII = 0.25 and JIl = -16 K

(full curve). For details see text.

[ 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).

[ l o ] 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