MAGNETIC ANISOTROPIC BEHAVIOUR OF RbNi1-xCoxF3

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MAGNETIC ANISOTROPIC BEHAVIOUR OF RbNi1-xCoxF3

G. Elbinger, E. Jäger, W. Keilig, R. Perthel

To cite this version:

G. Elbinger, E. Jäger, W. Keilig, R. Perthel. MAGNETIC ANISOTROPIC BEHAVIOUR OF RbNi1-xCoxF3. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-625-C1-626.

�10.1051/jphyscol:19711212�. �jpa-00214038�

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JOURNAL DE PHYSIQUE Colloque C I , supplbment au no 2-3, Tome 32, Fburier-Mars 1971, page C 1

-

625

MAGNETIC ANISOTROPIC BEHAVIOUR OF

RbNi,-,Co,F3

G. ELBINGER, E. JAGER, W. KEILIG and R. PERTHEL Zentralinstitut fiir Festkorperphysik und Werkstofforschung der DAW,

Institutsteil fiir magnetische Werkstoffe Jena, G D R

Rksumk. - On a determine les valeurs 9 87 OK de KI, K2 et de l'aimantation a saturation des composes RbNi I - ~ C O ~ F ~ (0 < x < 0,20). L'aimantation saturation prkente une anisotropic prononu%. Les variations de KI et K2 en fonction de H sont analogues B celles observks par Suits et al. A l'aide d'un modele de champ cristallin a un ion on a calcule pour RbNiF3 les constantes K I et K2 et l'aimantation a saturation. Pour celle-ci I'accord avec la valeur mesurke est bon, alors que pour KI et K2 on obtient seulement I'ordre de grandeur. Pour R ~ N ~ I - ~ C O Z F ~ , la distribution des catlons &ant incon- nue, on ne peut pas obtenir de resultats quantitatifs.

Abstract. - K I , K2 and the magnetic moment were measured on RbNi I - ~ C O ~ F ~ (0 ^$x < 0.20) at 87 OK by means of magnetization and torque curves. The magnetic moment shows a pronounced anisotropy. The K I and K2 values as a function of x show the same behaviour as observed by Suits et al. but the quantitative agreement is not good. By a one ion crystalline field model it is possible to calculate for RbNiF3 K I and K2 values which agree in sign and magnitude and p-values which agree quantitatively with the experimental values. For R ~ N ~ I - ~ C O ~ F , the unknown cation distribution does not allow quantitative calculations.

I. Introduction. - In the course of a study of the

anisotropic magnetic properties of d"-ions in magnetic

-

^v ^.< ^.. ^v ^{K l . C}

compounds with various crystal structures we per- 80.

- . .

^{II c}

formed magnetization and anisotropy measurements on hexagonal fluorides of the composition

RbCo,Ni,-,F, (0 ,< x

<

^0.2)

I ?

in order to compare magnetic moment and anisotropy $ ⁶⁰

values with those obtained by a one ion crystalline _; field model. Single crystal samples were prepared

2

either by the well-known combination of chemical reaction and flux melt [I]

3 RbHF,

+

(1 - x)NiC12.xCoC12 =

= RbNil-,Co,F3

+

^{2 RbCl}

+

^{3 H F}

_%

or by the Bridgmau-Stockbarger method. The C o : Ni

5

ratio was determined by usual chemical analysis &

procedures. Magnetization and anisotropy measurements were made ballistically in different crystallo- 2.1.

graphic directions and also by the torque method in the (l0i0) plane as a function of field strength a t T = 87 OK. The maximum field strength for the magne- tization measurements was 30 kOe and for the torque measurements 21 kOe.

0 .I0 2 0 3 0

IT. Experimental Results. - The magnetization Applied f i l d H [kOc]

-

curves show besides a large crystal anisotropy a pro-

nounced anisotropic behaviour of the magnetic Fro. 1. - Magnetization curves for RbNiF3 at ⁸⁷OK.

moment especially for x < 0.05 and x > 0.15 (e. g.

Fig. 1 for x = 0). Between x = 0.05 and x = 0.15 K1, K,, k, and k2 are the constants of magnetocrys- the anisotropic effects are smaller, due to the change talline anisotropy and anisotropy of magnetic moment of the easy magnetization axis from the a-plane to resp., 8, and 8 the angle between c-axis and H or the C-axis. In order to interpret Our measurements We M, resp. and the differential susceptibility. By start from the following expression for the free minimizing this expression with respect to 0 we get energy, assuming that the magnetic moment obeys the the stable position of M, for a given H and 8, and same symmetry relations as the magnetocrystalline therefrom we can calculate the magnetization and

energy [2, 31 : torque curves in the well-known manner.

1 Neglecting k2 both the experimental magnetiza-

' F

v

=

- v

^(FK

+

F H ) = K O

+

K1 sin2 8 K2 sin4 9 - tion curves and torque curves can be fitted very well by the calculated curves based upon the same values - HM,,,(l - k, sin2 8

-

k2 sin4 8) cos (8, - 8) of M,, K,, K2, k, and X . M, and

x

change only by a

-

1 X ~ 2 B c o s 2 (8, - 8) (1) small amount, from 74 to 76 G and from 0.17 to

2 0.20 x resp., in the studied composition range.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711212

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The dependence of K,, K2 and k, from the Co-content of the free ion of each of these two Ni2+ ions is split is shown in figure 2. Our K1- and K,-values d o not by the following Hamiltonian

agree well with those obtained by Suits et al. [3].

The discrevancv in the anisotrovv constants for X ⁼'U,,

+

2 PB Hex St

+

E.CS

+

^/.lBH(C

+

2 S) (3) x < 0.05 and x

>

0.15 may be due% the corrections W,, is the crystalline field operator, 2 p, Hex St the which are caused by taking into account an anisotro- exchange term in molecular field approach, where (

means the direction of

m,

ICS is the spin-orbit

1 s 4,0 coupling term and pB H(f

+

2 S) the Zeeman term.

?' The crystalline field potential was calculated by a

$ 4 simple point charge model, taking into account 7

(NiI) and 8 (NiII) surroundings of the central ion

*

F

3 considered (electro-neutrality). As the lowest state

s

we get the singlet A, for both NiI and NiII, the next

higher states are a singlet and then a doublet for

5

2 NiI and vice versa for NiII. From the cubic part of

S the crystalline field follows a 10 Dq-value which is

-2 1

c about six times smaller than that obtained experi-

2 mentally 151.

$ 0 0 Taking the values 10 Dq ⁼6 390 cm-' from 151,

U % 200 cm-' for the trigonal splitting calculated by

$1 the point charge model, 170 cm-' and 50 cm-'

-..

L for the exchange splitting for NiI and NiII resp. cited

0

.3 -2 in [6] and the free ion value 3. = - 335 cm-I we get

T r -40s the values given in table I1 for the anisotropy con-

stants and the magnetic moment in RbNiF, at T = 0.

TABLE I1

0 QOS 0,40 41.5 Kl K2 PI1 PI

Co - c o n t e n t [XI

-

[cm- '/ion]

-

--

[PBI

_-

Fro. 2. - Dependence of anisotropy constants K I , K2 and kl on the Co-content in R ~ N ~ I - ~ C O ~ F ~ at 87 ^OK.

pic moment. In the intermediate region we get, con- trary to Suits et al., a monotonous change of the easy direction from the a-plane to the c-axis. This transi- tion is field dependent. From eq. (1) follows for the cone angle 8,

2(K2

+

H M , k2) ' (2) Table I shows a comparison of calculated and measured 8,-values for x = 0.12 at 4 different field strengths.

H [kOe] 6.55 11.7 17.0 21.7

- -

-

^- -

8,, calc. 30.60 25.30 18.70 10.40 O,, obs. 26.80 22.80 18.60 12.80 111. Theoretical Considerations. - The experimen- tally obtained values will be compared with those calculated by an one ion crystalline field model.

In RbNiF, the Ni2+ ions occupy two different lattice sites, one with point symmetry D,, (Nil) and the other with point symmetry C3" (NiIQ, which are coupled antiferromagnetically. The ground state

NiI 1.160 - 0.004 2.396 2.378

NiII - 0.905 0.026 2.366 2.377

[erg/cm31 [e. m. u./gl RbNiF,

-

6.10 x lo5 4.46 x lo4 21.6 22.0

11 and 1 denotes parallel and normal to c-axis resp.

The values for RbNiF, were obtained by assuming that Ni = 113 NiI

+

213 NiII and that the magnetic moments and therefore also Hex are directed opposi- tely for the two kinds of Ni2+ ions. Sign and magnitude for K, and K2 and the absolute value as well as the anisotropy of magnetic moment are in very good agreement with the experimental values.

The same calculations were performed for Co2+.

The results obtained are in good qualitative agreement with experiment. For example the K,-values of CoI and CoII have the opposite sign and are one order larger compared with Ni. Unfortunately, the exact distribution of Co2+ ions on the two different lattice sites in RbNi, -,Co,F3 is not yet known, so that it is not worth calculating values for K,, K2 and the magnetic moments in different crystallographic directions.

Acknowledgements. - The authors are indebted to Mr. P. Rosemann for preparing the single crystals and to Mr A. Funke for the chemical analyses.

References

[I] SMOLENSKI (G. A.), YUDIN (V. M.), S~RNIKOV (P: P.), [4] SUITS (J. C.), MCGUIRE (T. R.) and SHAFER (M. W.), and SHERMAN (A. B.), Fiz. Tverd. Tela, 1966, 8, Appl. Phys. Lett., 1968, 12, 406.

2965. ^[S]TYLICKY (J.), YEN (W. M.), VAN DER ZIEL (J. P.) and [2] PERTHEL (R.), ELBINGER (G.) and KEILIG (W.), GUGGENHEIM (H. J.), Phys. Rev., 1969, 187,

Phys. stat. sol., 1966, 17, 151. 758.

[3] SCHELLENG (J. H.) and RADO (G. T.), Phys. Rev., [6] CHINN (S. R.), ZEIGER (H. J.), Phys. Rev. Lett., 1968,

1969, 179, 541. 21, 1589.