Low-field magnetic properties of anhydrous NiCl2

Low-field magnetic properties of anhydrous NiCl2

Journal of Magnetism and Magnetic Materials 21 (1980) 187 - 190 © North-Holland Publishing Company LOW-FIELD MAGNETIC PROPERTIES OF ANHYDROUS NiCI2 D...

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Journal of Magnetism and Magnetic Materials 21 (1980) 187 - 190 © North-Holland Publishing Company

LOW-FIELD MAGNETIC PROPERTIES OF ANHYDROUS NiCI2 D. BILLEREY, C. TERRIER, A.J. POINTON * and J.P. REDOULES ** Laboratoire de Thermomagnetisme, Universit~de Nancy L 54037 Nancy Cedex, France

Received 3 March 1980; in amended form 2 July 1980

Results are reported for the low-field magnetic susceptibilities and the low temperature isothermal magnetization of anhydrous NiC12. The theoretical analysis, based on a molecular field model, leads to precise values for certain of the magnetic parameters.

consistent with there being an oblique spin-flop phase. Isotherms of the magnetization at 4.2 K were taken for the two principal directions and are shown in fig. 3. For the direction perpendicular to the C-axis, the existence of antiferromagnetic domains in weak fields (H < 2.2 kOe) is shown by the value of the mean initial susceptibility which is smaller than the differential susceptibility which is observed for H > 2.2 kOe. No hysteresis was observed. The single domain specimen does not continue to exist when the field is lowered: this result was confirmed by the neutron diffraction measurements as seen in fig. 4. However, the behaviour of the magnetization parallel to the C-axis is perfectly linear. The precision of the measurements aU0ws the susceptibilities to be quoted as:

An earlier study by neutron diffraction [1] has shown that the structure of anhydrous nickel chloride is analogous to that of CoCI~ [2]. The single crystal, on which the magnetic measurements were made, was produced from the powder used in the neutron diffraction measurements by sealing the powder in a quartz tube under an atmosphere of hydrogen chloride and subliming at a temperature from 5 to 10°C below the melting point of 1035°C. The magnetic measurements were made on a specimen 6 X 3 X 0.8 mm a using a Foner magnetometer. The temperature dependence of the low-field (H = 0.5 kOe) magnetization was measured parallel to the hexagonal C-axis of the crystal and perpendicular to it. The results are shown in fig. 1. The plane perpendicular to the C-axis appears as a plane of easy magnetization while the peak in the initial susceptibility corresponds to a transition temperature, TN, of (52.3 -+ 0.1) K, in good agreement with previous results [3]. The temperature dependence of the magnetization at a higher field (H = 20 kOe) for the easy plane is shown in fig. 2. The variation is now similar to that for the susceptibility along the axis in fig. 1 and is

×±c = (0.098 -+ 0.001) emu/mol,

Xiic

=

(0.094 -+ 0.001) emu/mol.

The magnetic state of NiC12 may be written [4] as a spin Hamiltonian

H = - g D"BJY" S, Zl,jS,. Sj z i>j -

i >]'

* Present address: Physics Department, Portsmouth Polytechnic, UK. ** Present address: I.N.S.A., Universit$ Paul Sabatier de Toulouse, France.

--

i

D' ~ t

187

(S{ = - SZ2).

D. Billerey et aL/ Low-field magnetic properties of anhydrous NiC72

188 ~rm emu/mol 4"

*÷~

4"+*4,++ ÷ 4' 4.4. it, 4'4,#



4"4,4. i~i,1~,~.

45

40 *

+llc ./C

•1

0 IQ QOQ 0Q 6 0

35 ~

NI1..ool.

30

I

Q

1110

Q QQ I I

~- .p

I

I

I

0

i

I

I

I

20

l

I

/

40

55

TK

Fig. 1. Temperature dependence of the magnetization along the two principal directions at low field (H = 0.5 kOe).

O'me=u/mol e'~lNb'~4''l''l'

t4,

~ '~

4'

4'

4" 4,

4.

150C

4,

% #

1000

4. %%

I

0

A

l

l

40

l

i

|

l

80

TK

Fig. 2. Temperature dependence of the magnetization at H = 20 kOe for the direction perpendicular to the C-axis.

189

D. BiUerey et al. /Low-fieM magnetic properties of anhydrous NiCI 2

(~rr emu/mole 2000

1000

5

10

15

20

v

H k0e

Fig. 3. Magnetization isotherms at 4.2 K.

400 300 zoo(

.H,~. OH'*"

100 [ " t

!

i

HkOe 5 10 15 Fig. 4. Intensity of the magnetic 015 M neutron diffraction line at 4.2 K for the magnetic field increasing and decreasing. (Arbitrary units for count N.)

The a-axis of the hexagonal system is chosen as the quantization axis. Ji] and J~i are the exchange constants for interactions within and between the planes, respectively. D and D' define the anisotropy and, if D = D' > 0, the plane perpendicular to the z-axis is a plane of easy megnetization. We work on this assumption because the anisotropy in the plane is negligible: this is shown by the low value found for the spin-flop field (i.e. 2HsF ~ 2.2 kOe) and the fact that the threshold energy of the lower branch of the spin waves is practically 0 K [5]. In the molecular field approximation we can write, with J~]., = J ' for nearest neighbours X±C = Ng21a~/-4zJ'

and Xllc = Ng~la~/( - 4 z f + 2D),

D. Billerey et al. /Low.field magnetic properties o f anhydrous NiCl 2

190

where z = 6 and g = 2.23 + 0.01 [6]. The values of the parameters are found as J ' = ( - 0 . 7 9 --- 0.02) K and D =

(0.40 -+ 0101) K ,

which are in good agreement with those reported by Ling~rd et al. [4], namely, J ' = - 0 . 7 7 K, D = 0.40 K.

References

[ 1 ] D. Billerey, C. Terrier, R. Mainard and P. Meriel, Compt. Rend. Acad. Sci. (Paris) 284 (1977) 495. [2] M.K. Wilkinson, J.W. Cable, E.O. Wollan and W.C. Koehler, Phys. Rev. 113 (1959) 497. [3] R.H. Busey and W.F. Giaque, J. Am. Chem. Soc. 74 (1952) 443. [4] P.A. Ling/ird, R.J. Birgenau, 1. Als-Neilsen and H.J. Guggenheim, J. Phys. C. 8 (1975) 1059. [5] R.J. Birgenau and L.W. Rupp, Jr., Proc. Intern. Conf. Magnetism, Moscow, 1973, Vol. 4 (Nauka, Moscow, 1974) p. 287. [6] M. Kozielski, I. Pollini and G. Spinolo, J. Phys. C5 (1972) 1253.