Magnetic susceptibility of single-crystal hexagonal nickel sulphide

Solid State Communications,

Vol. 9~pp. 41—43, 1971.

Pergamon Press.

Printed in Great Britain

MAGNETIC SUSCEPTIBILITY OF SINGLE-CRYSTAL HEXAGONAL NICKEL SULPHIDE* J.L. Horwood and M.G. Townsend Mines Branch, Department of Energy, Mines and Resources, Ottawa (Received 9 October 1970; in revised form 22 October 1970

by

M.F. Collins)

The magnetic susceptibilities of five single-crystals of hexagonal NiS, with compositions ranging from NiS—NiS1~have been measured in the temperature range 78—300°K.The susceptibility curves have a markedly different shape from that normally predicted for an antiferromagnet. This results from the first-order character of the phase transition at TN.

SPARKS and Komoto determined the magnetic susceptibility, neutron diffraction pattern and resistivity of powdered of nickel arsenide structure. Theyhexagonal observed NiS a phase transition from a low-temperature antiferromagnetic semiconductor to a paramagnetic metallic phase at a temperature TN which depended on sulphur content. These measurements, together with the observed discontinuous change in lattice parameters and thermal hysteresis at TN, led to the conclusion that the phase transition was a firstorder transition. The powder neutron-diffraction measurements showed that the spins within a given hexagonal layer are coupled ferromagnetically and point in the c-axis direction. The moments on adjacent layers are coupled antiferroinagnetically. x powder increased by only 15 per cent on cooling through TN, although neutron diffraction showed that the magneti~ation was within 90 per cent of the saturation value immediately below TN.’

that this transition may be typical example of a metal-to-semiconductor transition entirely due to 3Mott on the other the magnetic interaction.~’ hand suggested a ‘Hubbard gap’ below TN, with the onset of metallic conduction destroying the long-range magnetic order.4 Our magnetic susceptibility measurements made using the Faraday method in the temperature range 82—300°Kon five single crystals of composition from NiS—NiS ~ shown in Fig. 1. No correction for diamagnetism has been made, a value of 0.7 x 10~gr1 is estimated for T < TN. ~

Evidently, for T < TN, x~ < 0, we deduce the spins point in the c-axis direction in agreement with the neutron-diffraction results, but in contrast with earlier torque measurements, on a single crystal with TN = 150°K,from which it was concluded that the spins were i.c.5 The small increase in the powder value of x on cooling through 7~is seen to arise from an abrupt increase in x~ and decrease in x,,~Careful examination of Fig. 1 shows that, on heating just below 2~,a marked•increase in the value of ~, occurs~at a lower temperature than the —

Because the crystallographic space group, P6mmc, is unchanged at TN it has been proposed ___________

*

Suiphide Research Contribution No. 30, Mineral Sciences Division. Crown Copyrights reserved. 41

42

SINGLE-CRYSTAL HEXAGONAL NICKEL SULPHIDE

Vol. 9, No. 1

40 I ~.

5’

~ FIG. 1.

~

vs. T for (A) NiS

>~‘~ and

E 20 0

D01~S KILVIN

TCMPEIATUIE

~,

(B) NiS

~,

(C) NiS ~

(D) NiS~ O~5~(E) NiS

1

O2~

—

-

I—

I-

v’ 15

-

uJ

z

10— 8000

FIG. 2. A typical run for H

I

12000

I

i

16000 20000 24000 TEMPERATURE (DEG. KELVIN)

I

28000

c, showing reproducibility obtainable in our measurements. The output on punched tape was plotted in a computer.

corresponding decrease in >~. This may result partially from anisotropy in the lattice strain.

The observed temperature independence of is predicted by a spin wave model. The nearly saturated magnetic moment immediately

Vol. 9, No. 1

SINGLE-CRYSTAL HEXAGONAL NICKEL SULPHIDE

below TN implies a low ~

at

as is observed.

A general conclusion to be drawn from these results is that the observation of low, temperatureindependent susceptibilities in powders need not necessarily imply the absence of long-range magnetic order.

43

In a forthcoming, detailed publication Hall, resistivity, thermoelectric power, and further magnetic measurements on single-crystal NiS will be reported and discussed.6 Acknowledgements The authors thank Dr. A.H. Webster for a useful discussion and Mr. L.G. Ripley for growing the crystals. —

REFERENCES 1.

SPARKS J.T. and KOMOTO T., Rev, mod. Phys., 40, 752 (1968).

2.

ADLER D. and BROOKS H., Phys. Rev. 155, 826 (1967).

3.

ADLER D., Rev, mod. Phys. 40, 714 (1968).

4.

MOTT N.F., Phil. Mag. 20, 1 (1969);

5. 6.

TSUBOKAWA I., J. Phys. Soc. Japan 13, 1432 (1958). HORWOOD J.L., RIPLEY L.G., TREMBLAY J.R. and TOWNSEND MG., (to be published).

Nous avons effectué des mesures de susceptibilités magnétiques de 78 è 300°K sur cinq cristaux uniques de la phase hexagonale du sulfure de nickel dont la composition variait entre NiS 1 ~ et NiS1 o2~ Les courbes de susceptibilités ont une forme très différente de celle normalement anticipêe pour un antiferromagnetique. Ccci est une consequence directe du fait que le changement de phase au point de Ned soit de premiere espéce.