HAL Id: jpa-00217826
https://hal.archives-ouvertes.fr/jpa-00217826
Submitted on 1 Jan 1978
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
FERROMAGNETISM IN NiSnF6.6H2O AND
NiSiF6.6H2O
M. Karnezos, S. Friedberg
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-814
FERROMAGNETISM IN NiS
nF .
6H o AND NiSiF .
6H 0
(*\
6 2 6 2 v '
M. Karnezos and S.A. Friedberg
Ccamegie-Mellon University, Pittsburgh, PA 15213, U.S.A.
Résumé.- Par des mesures de X// > Xi> et C à l % 0_,05 K dans un réfrigérateur à dilution 3He - ''He,
nous étudions l'apparition du ferromagnétisme dans des monocristaux de NiSnF6.6H20 (T = 0,164 ±
0,004 K) et de NiSiF(..6H20 (T = 0,135 ± 0,002 K) . C
Abstract.- Measurements of XII » Xi an<^ Cp down t o % 0-05 K in a 3He-'*He dilution refrigerator are
used to study ferromagnetic ordering in single crystal NiSnF6.6H20 (T = 0.164 ± 0.004 K) and
NiSiFj.eH^O (T = 0.135 + 0.002 K ) . C
INTRODUCTION.- A number of salts with the general formula NiMX6.6H 0 crystallize in the trigonal
NiSnCl6.6H20 structure. One [Ni(H20)g] complex, trigonally distorted along the trigonal axis, occu-pies the center of the rhombohedral unit cell with [MX I octahedra at the eight vertices. The axial
distortion and spin-orbit interaction split the 3A
++ . 2
ground state of the Ni complex into a singlet (m =o) and a doublet (m =±1). The magnetic and
ther-s ther-s
mal properties associated with these levels are des-cribed approximately by the single-ion spin Hamil-tonian, o\> = DS 2 + gu_ H.S, where z // trigonal axis, S=l, and g = 2.3.
The first NiMX .6H 0 compound to be studied
6 2
was NiSiF .6H 0. Below 20 K, it was found IM D/k
6 2
- -0.16 K. Thus the ground state is a magnetic dou-blet. Ferromagnetic interactions producing ordering below T = 0.14 K were also detected /2,3/. More
c
recently we have shown that in NiSnCl .6H 0 itself,
6 2
below 4 K, D - +0.6 K so that the ground state is a singlet. In this case interactions of antiferroma-gnetic sign occur. However, they are too weak rela-tive to |D| to cause magnetic ordering above 0 K in zero field /4/. We have also found /4/ that several compounds with M = Pd or Pt and X «= CI, Br or I have D>0 and antiferromagnetic interactions while
those with M = Ti or Zr and X = F have D<0 and be-come ferromagnetic /5,6/.
The sign of D, and with it the sign of the net spin interaction in these salts, appears to de-pend on the nature of the halogen atom X, D<0 being associated with the presence of F, the smallest and most electronegative of the series. There is also
(*) Work supported by NSF Grant DMR 75-13908
evidence that, for given X~, changing M changes
the magnitude of D but not its sign. To test these ideas, we have measured, between 4 and 0.05 K, the susceptibilities of NiSnF .6H 0, which differs from
6_ 2
NiSnCl . 6H 0 only in its X ion. We have also
mea-6 2
sured in this interval the susceptibilities and heat capacity of NiSiF .6H 0 for which most of the
e 2
published information near T /3,8/ has been deter-mined by indirect methods.
THE EXPERIMENTS.- Y a n d X w e r e determined by a
II 1
mutual inductance method at 70 Hz down to 0.05 K in a 3He - ''He dilution cryostat. The heat capacity of an 8.91 g single crystal of NiSiF .6H 0 was measured
6 2
by adiabatic calorimetry in the dilution cryostat for several values of H „ produced by a supercon-ducting solenoid.
RESULTS AND DISCUSSION.- Figure I shows the tempe-rature variation of Xn a nd Xl f o r NiSiF .6H 0. xn
> xj. everywhere indicating that D<0. Below T = 0.139 ± 0.004 K, Xll is constant at 0.56 cgs/cm3.
S)
The demagnetizing factor for the specimen was °*^
-1.86 so that lygO^ 0.54 = xII (T<T ) a s expected for
H.|| the easy axis of a ferromagnet without remanence. The X|| (T>T )data were corrected to infinite needle geometry and fitted with two models of a uniaxial ferromagnet ; a simple mean field model, and an Oguchi model /5/ in which pairs of exchange-coupled spins are treated exactly and interpair interactions are represented by a mean field. The fitted para-meters are essentially the same for both models, namely, D/k = -0.166 K, g = 2.24 and an effective exchange constant qJ/k = +0.084 K. The solid curve in figure 1 is calculated for the Oguchi model which fails below T where it consists of only a
s i n g l e domain. There i s good agreement between o u r v a l u e s of D , g and q J and t h o s e determined by o t h e r methods / I , 7 , 8 / .
The measured Cp(H=O) v a l u e s f o r NiSiF .6H20 a r e i n r e a s o n a b l e agreement w i t h e a r l i e r d a t a / 3 , 8 / . A l a t t i c e c o r r e c t i o n , C ( 1 a t t . ) 3.24 x 1 0 - ~ ~ ~ c a l / mole K, h a s been a p p l i e d t o t h e d a t a shown i n f i g u - r e 2. The c o o p e r a t i v e peak i n Cp(H=O) o c c u r s a t Tc = 0.135
+
0.002 K. The magnetic e n t r o p y change SmS 0 = 2.177 c a l / m o l e K"
Rln 3 a s expected f o rs p i n S = 1. With HI, = 1200 G, t h e X-peak is roun- ded-off and s h i f t e d t o h i g h e r t e m p e r a t u r e , a s pre- d i c t e d by t h e mean f i e l d models ( f i g u r e 2 i n s e t ) .
F i g . 1 : Magnetic s u s c e p t i b i l i t i e s of NiSiF6.6H,0. Data c o r r e c t e d f o r d e m a g n e t i z a t i o n a r e shown a s
open c i r c l e s . Note e n l a r g e d s c a l e f o r
a.
The Oguchi model w i t h D/k =-
0.166 K, g = 2.24 and q J / k = t0.120 K g i v e s t h e dashed c u r v e of f i g u - r e 2 which f i t s t h e d a t a w e l l .X
and X d a t a f o r NiSnF6.6H20, q u a l i t a t i v e l y1
s i m i l a r t o t h o s e of f i g u r e 1 , r e v e a l i t a l s o t o b e a u n i a x i a l ferromagnet having Tc = 0.164 ? 0.004 K. The Oguchi model, w i t h D/k =-2.55 K, g = 2.26 and q J / k = +0.084 K, p r o v i d e s an e x c e l l e n t f i t ,ID!
and g a g r e e i n g w i t h EPR r e s u l t s 191. Replacement o f C I - by F- i n NiSnX .6H 0 changes t h e s i g n s o f D6 2
and J a s p r e v i o u s l y c o n j e c t u r e d . With X = F and M = S i , T i , Sn, and Z r , we now f i n d t h a t ID1 v a r i e s
l i n e a r l y w i t h t h e M" i o n r a d i u s and t h e u n i t c e l l edge l e n g t h . I n t h e p r e s e n c e of I s i n g - l i k e s i n g l e - i o n a n i s o t r o p y (DiO) t h e d i p o l a r i n t e r a c t i o n i n t h e s e c r y s t a l s f a v o r s f e r r o m a g n e t i c o r d e r i n g accoun- t i n g i n p a r t f o r t h e c o n n e c t i o n between t h e s i g n s of D and t h e e f f e c t i v e c o u p l i n g c o n s t a n t J. F i g . 2 : Magnetic c o n t r i b u t i o n s t o t h e h e a t capa- c i t y of NiSiF,.6H20 f o r H I I = 0 and 1200 G. 5 About 25 % of t h e c o u p l i n g energy i s d i p o l a r w h i l e
I D / ~ J [
> I and t h e I s i n g c h a r a c t e r i s pronounced. T h i s may e x p l a i n t h e s u c c e s s of mean f i e l d models i n d e s c r i b i n g t h e p r o p e r t i e s of t h e NiMFB6H,0 s a l t s-
2-
NiSiF6.6H,0
3
i'!
----
R e f e r e n c e s-
Y 4 -,Y=O : 8,
I Oguchi Model:
i a,-
!i
0 E 3 - I - ::
\ I-
Of
\ / H ~ l = l2OOG 0.
k 2 - , I . Ii
'-.
01
'. \..
*-._._,
.-.-
.-.--.--
1-; I 2 T (K) I - - . - - 2/ I / Penrose, R.P. and S t e v e n s , K.W.H., Proc. Phys. Soc.
A63
( 1 950) 29./ 2 / I s h i g u r o , E., Kambe, K. and Usui, T., P h y s i c a 17 (1951) 310.
-
/ 3 / Ohtsubo, A., J . Phys. Soc. J a p a n
20
(1965) 76. / 4 / F r i e d b e r g , S.A., Karnezos, M. and Meier, D.L.,Proc. LT14, Otaniemi, F i n l a n d (1975) 224.
/5/ Karnezos, M., Meier, D.L. and F r i e d b e r g , S.A., Phys. Rev; (1978) t o a p p e a r .
/ 6 / Karnezos, M. and F r i e d b e r g , S.A., J. Appl. Phys (1978) t o a p p e a r .
/ 7 / S v a r e , I. and S e i d e l , G., Phys. Rev.
134
(1964) A172/8/ Giauque, W.F., Hornung, E.W., F i s h e r , R.A. and Brodale, G.E., J . Chem. Phys.
