MAGNETIC TORQUE STUDY OF MnCr2O4 AT LOW TEMPERATURES

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MAGNETIC TORQUE STUDY OF MnCr2O4 AT LOW TEMPERATURES

S. Miyahara, T. Miyadai, S. Horiuti

To cite this version:

S. Miyahara, T. Miyadai, S. Horiuti. MAGNETIC TORQUE STUDY OF MnCr2O4 AT LOW TEMPERATURES. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-57-C1-58.

�10.1051/jphyscol:1971113�. �jpa-00213947�

JOURNAL DE PHYSIQUE

Col2oque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 57

MAGNETIC TORQUE STUDY OF MnCr204 AT LOW TEMPERATURES

S. MIYAHARA, T. MIYADAI and S. HORIUTI

Dept. of Physics, Faculty of Science, Hokkaido Univ., Sapporo, Japan

Resume.

On a mesurk l'anisotropie magnetique par mesures de couple de 4,2 OK jusqu'a 45 OK

Tc). A 20 OK.

K I

- 4,l x 104 erglcc. Une valeur negative de K I ne peut s'expliquer par un modele d'anisotropie a un ion et par l'interaction des dip6les magnetiques dans le cas d'un arrangement colinkaire des spins. Au-dessous de 18 OK, une hyste- resis de rotation indkpendante de l'intensite du champ magnetique est observee. Une tentative d'explication du mecanisme de cette hystkrksis est proposke.

Abstract.

Magnetic torque measurement from 4.2 OK to 45 OK

Tc) are described. At 20 OK,

Negative

can not be explained from single ion anisotropy and dipolar interaction, provided that colinear spin arran- gement is assumed. Below 18 OK rotational hysteresis, independent of magnetic field intensity, was observed. A tentative of mechanism of this rotational hysteresis is given.

1. Introduction. - Chromites with a formula MCr204 (M

Mn, Fe, Co, Ni, Cu) are of normal spinel structure with cubic or tetragonal symmetry.

They are all ferrimagnets and show non-colinear spin structures at low temperatures. MnCr20, is a cubic spinel, its Curie temperature (Tc) is 45 OK and the absolute saturation magnetization is 1.2 p,/

molecule. Neutron diffraction data [I], [2] show that MnCr204 has a triple conical spin structure with three magnetic sublattices (Mn, CrI, CrII) below 18 OK (hereafter we denote by Ts) and that the pro- pagation vector (k) is perpendicular to the cone axis.

Above Ts, satellite-lines disappear and so there are no more evidence of long range order of conical spin structure.

2. Experimental results and discussions. - Magne- tocrystalline anisotropy in MnCr,04 was measured on single crystal sphere samples (-- 2 mm in diameter) using a torque magnetometer with three wings (made of phosphor bronze) on which strain gauges were glued. Above Ts (high temperature phase : HTP) we obtained ordinary torque curves without hyste- resis, while below Ts (low temperature phase : LTP) large rotational hysteresisses were observed.

1. RESULTS

HTP (ABOVE 18 OK). - An exam- ple of torque curves is shown in figure 1. Obtained

FIG. I. - The torque curve in

plane of

single

at

OK in

kOe.

cubic anisotropy constants, K, and K2 at 20 OK are K,

- 1.5 x l o d 2 ~rn-~/molecule

- 4.1 x

x 104 ergslcc and

K2

- 1 x lo-' cm-l/molecule

- 3 x

x lo4 ergslcc.

We have calculated individual contributions to K , from single ion anisotropies of Mn2+ and Cr3+, accor- ding to the theory of Yosida and Tachiki [3], and Wolf [4], assuming a colinear spin structure above Ts. Using as values of crystal field splittings, a(Mn2+ at A-sites)

=

0.1 cm-I and D ( C ~ ~ + at B-sites)

0.93 cm-l, we have K,

+ 3 x lo-' ~m-~/molecule at 20 OK.

The contribution of Cr3+ is dominant, while that of Mn2' is negligible and negative. We, further, esti- mated the contribution at OOK from dipolar inter- action and obtained a positive K , with the same order of magnitude as the observed. Thus we cannot explain the observed Kl from the above model. However, a calculation shows a possibility of explaining this discrepancy in sign between observed K, and that calculated from crystal field splitting. If we assume that, even in HTP, transverse components of magnetic moment of individual ions remain non-zero but are disordered, then the sign of observed Kl can be, for suitable cone angles, opposite to that of calculated Kl from the assumption of colinear spin arrangement.

2. RESULTS

LTP. - (a) Rotational hysteresis in high fields. We observed large rotational hysteresis losses with relaxation phenomenon in torque curves as shown in figure 2. As a measure to characterize these losses, we take a value, AL, as indicated in figure 2. Features of this rotational hysteresis are : (1) it exists even in as high fields as 20 kOe, much higher than saturation-field (-- 2 kOe) : (2) AL disap- pears at T s (Fig. 3) : (3) AL consists of relaxing and non relaxing parts, both being of comparable order of magnitude.

The facts (1) and (2) may be explained if we assume that k-domains exist even in high magnetic fields and that the rotation of the direction of net magne- tization would be accompanied by movements of boundaries of k-domains through magnetoelastic

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971113

( 2 1 - 5 8

S. MIYAHARA, T. MIYADAI AND S. HOiUUTI

0" 9 0" 180"

FIG. 2. - The torque curve in (110) plane of MnCrzO4 single crystal in 20 kOe at 4.2 OK, in which a measure of rotational hysteresis, AL, is shown. Dashed curve shows the torque curve

measured very slowly.

Erratum.

-

x

x

a o single crystal

i : + A

0. Oo

10 20

TEMPERATURE

FIG.

-The temperature dependence of AL in 20 kOe.

coupling. If this assumption is true, the decrease in domain boundaries would cause a decrease in AL.

Measurements on powder-samples of MnCr20, show that this is the case (in powder sample the number of boundaries is expected to decrease). We have also made measurements on powder sample of CoCr204, being similar to MnCr,04(Ts

26

Tc

96

and obtained results which can be interpreted to be essentially the same as for MnCr,O,.

(b) Annealing efSects. - We obtained a torque curve (dashed curve in Fig. 4) at 4.2 OK without AL by annealing above Ts before each measurent of torque at each angle (8,). Thus obtained torque curve is very similar to those above Ts. It should be noted that this curve indicates a negative K , ; [ I l l ] is an easy direction of magnetization. This is neither consis- tent with the result of neutron diffraction nor with magnetization data. The reason is not clear. In figure 4,

FIG. 4. - Torque curve in (110) plane of MnCr204 at 4.2

after annealing at 20 OK.

are seen several oblique lines. These were obtained by small angle-rotation of magnetic field around the direction of O i (after annealing). These oblique lines can be explained by the following phenomeno- logical model ; after annealing the net magnetization appears along the direction of an applied field, it remains fixed in the initial direction during rotation of field by smaller angles

than a critical value, say

(this fixing is due to some kind of friction) and finally it begins rotate following the field when the rotating angle exceeds

This friction may take place in movement of boundaries of k-domain.

The authors wish to thank Drs T. Tsushima and Y. Kino for supplying single crystals of MnCr,O,.

References

[ I ] KAPLAN (T. -4.)

al.,

Phys., 1961, 32, 13 S . [31 YOSHIDA (K.)

al., Prog. Theor. Phys., 1957, 17, 331.

[2] HASTINGS (J. M.)

al., Phys. Rev., 1962, 126, 556. [4] WOLF

P.), Phys. Rev., 1957,

1152.