MAGNETIC ORDERING OF TWO-DIMENSIONAL ANTIFERROMAGNETIC FORMATES

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MAGNETIC ORDERING OF TWO-DIMENSIONAL ANTIFERROMAGNETIC FORMATES

M. Matsuura, H. Blöte, W. Huiskamp, K. Takeda, T. Haseda

To cite this version:

M. Matsuura, H. Blöte, W. Huiskamp, K. Takeda, T. Haseda. MAGNETIC ORDERING OF TWO-

DIMENSIONAL ANTIFERROMAGNETIC FORMATES. Journal de Physique Colloques, 1971, 32

(C1), pp.C1-757-C1-758. �10.1051/jphyscol:19711263�. �jpa-00214094�

MAGNETISME A UNE ET A DEUX DIMENSIONS

MAGNETIC ORDERING

OF TWO-DIMENSIONAL ANTIFERROMAGNETIC FORMATE S

M. MATSUURA, H. W. J. BLOTE, W. J. HUISKAMP Kamerlingh Qnnes Laboratory, Leiden, Nederland

K. TAKEDA and T. HASEDA

Department of Physics, Kyoto University, Kyoto, Japan

R6sum6. - La formation de l'ktat magndtique ordonnk de formiate de manganese et de cobalt est etudiCe par des mesures de la chaleur spkcifique. Les rksultats au-dessous de ^{T N} demontrent que le systeme des spins dans le plan

(100) est ordonnk

grande distance et dans le plan B (200) reste dans l'htat paramagnktique de sorte qu'ils sont polarisks comme sous l'influence d'un champ extkrieur. L'effet d'anisotropie d'kchange sur la transition de phase est aussi discuth.

Abstract. - Magnetic ordering of manganese and cobalt formates has been studied by heat capacity measurements.

The results show that the spin system in the

(100) plane comes into long range order at T N and that in the B (200) plane remains paramagnetic below T N and orders gradually with decreasing temperature just as normal paramagnetic satura- tion in external field. The effect of anisotropy of the exchange interaction on the phase transition is also discussed.

I. Introduction.

A series of transition metal for- tely below TN. This spin system behaves paramagneti- mates M(HC00), .2 H,Q (M : Mn, Co, Ni and Cu) cally with no entropy gain to far below TN (N TN/3) [7].

are crystallographically isomorphous and also their For these reasons, we measured the heat capacity of magnetic properties show a great deal of similarity. Mn-formate below 1.8 OK and also that of Co-formate The schematic picture of the crystal structure is shown in the temperature from 0.06 OK to 20 OK. The exchange in figure 1. From the experimental results so far interaction in the Co-formate is expected to be quite anisotropic in view of the anisotropic g values [a].

t

FIG. 1. - A

schematic picture of crystal structure of

11. Experimental result. - 1 . Mn-formate. The heat capacity is shown in figure

A Schottky type anomaly was observed having its maximum at 0.23 OK, which

M(HC0 0)2.2 H20.

- HCOO-bond between A ions, - - - - HCOO-bond between A and

ions,

M2+ at A sites,

at B sites.

2.0- 1.0r

obtained [I], especially from neutron scattering [2], manganese (Mn-) formate is a good example of the two-dimensional Heisenberg (2 dH) model. An anti- ferromagnetic ordering takes place among the spins in the (100) plane (A plane) at TN

3.7

However, the appearance of spontaneous magnetization [2, 3, 41 and the similarity of some critical phenomena [4, 5, 61 to those of the two-dimensional Ising (2 dT) model seems to imply the large influence of the anisotropy on the phase transition. On the other hand, the spins in the alternate (200) plane (B plane') do not noticeably participate in the magnetic ordering at least immedia-

C/R Mn(HC00)22H20 a

FIG.

The heat capacity of (a) Mn- and (b) Co-formate.

The data plotted above 1.8

for Mn-formate are those of Pierce and Friedberg

The drawn lines show the hyperfine

contribution.

.: 1.

0.5

0.2 0.1

-

_-.

- . ,&... -

-. _{. .} . ^{- .}

- -

_... ...

. .

. .

\ ^{... ... .:.f}

^T

^Kl

J ' 1 1 ' I I 1 1 1 1 1 1 1 I I I L . 1 I I l l

0.1 1.0 10

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711263

C 1 - 7 5 8

M. MATSUURA, H. W.

BLOTE, W.

HUISKAMP, K. TAKEDA AND

HASEDA

is appreciably lower than TN. The total entropy was estimated as 1.68 R which is 94 % of R In 6 for S

5.

2. Co:formate. The heat capacity is shown in figure 2b.

The results show a sharp peak at TN

5.12OKand a Schottky type anomaly having its maximum at 0.5 OK. The total entropy was estimated as 0.67 R which agrees with R In 2 for S

4 within experimental error.

111. Discussion.

The heat capacities of Mn- and Co-formates were successfully divided into two parts.

One of these is associated with the peak at TN and the other one with a broad Schottky type anomaly.

These originate from the ordered A system and the paramagnetic B-system respectively. The Schottky type anomaly can be explained by considering the parama- gnetic B system under the influence of the local exchange field caused by the ordered A spins. From the best fit of the experimental points with the computed curve,

the interlayer exchange interaction JAB/k was estimated as about 0.025 OK for Mn-formate and 0.6 OK for Co-formate. The intralayer exchange interaction JAA/k was estimated, both from the heat capacity tail at high T and from the total energy of the A system, to be about 0.35 OK [7] for Mn-formate and 4.3 OK for Co-formate. Thus, the ratio JAB/JAA is very small in these salts. The anisotropy of the exchange inter- action is expected to be quite different in the two compounds. The ratio kTN/JAA is 10.4 for Mn-formate and 1.19 for Co-formate. These are compared with 9.9 for the 2 d H model of S

3 (Stanley-Kaplan [9]) and 1.13 for the 2 d I model of S

3 (Onsager [lo]).

The broad anomalous heat capacity (concave upward) of Mn-formate just above TN may be related to the properties of the 2 d H model for S

3 since we cannot find s ~ ~ c h anomaly in Co-formate and besides one would not expect such anomaly for Mn-formate from the crystalline field splitting in this temperature

range.

References

rll As for the references see for examwle

- ^71. 161 TAKEDA - - (K.). MATSUURA (M.) and HASEDA (T.). .

[2j SKALYO Jr (J.), SHIRANE

and F&ED;ERG (s. A.), J. ~ h ) s .

Japan, 1970,

29.

Phys. Rev., 1969,

1037. [7] PIERCE (R. D.) and FRIEDBERG (S. A.), Phys. Rev., [3] YAMAGATA .-,, (K.)

and ABE (H.), J. Phys. Soc. Japan, 1968,

680.

IYbb, Ll, 4ui5.

[4] AJIRO (Y.), TERADA

MATSUURA (M.) and HASE-

(H.)' private communication.

DA

(T.), J. Phys. Soc. Japan, 1970,

1587. [91 STANLEY (H. E.1 and KAPLAN (T. A.), Phys. Rev- [5] MATSUURA (M.), AJIRO (Y.) and HASEDA (T.), J. Letters, 1966,

913.

Phys.

Japan, 1969, 26, 665.

ONSAGER (L.), Phys. Rev., 1944, 65, 90.