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MAGNETIC PROPERTIES OF CsNiCl3
P. Johnson, J. Rayne, S. Friedberg
To cite this version:
JOURNAL D E PHYSIQUE
Coffoque C6, suppf6ment au no 8, Tome 39, aolit 1978, page
C6-733
MAGNETIC PROPERTIES OF C s ~ i C l ~ (*)
P.B. Johnson, J.A. Rayne and S.A. Friedberg
Carnegie-MelZon University, Pittsburgh, Pennsylvania 15213, U.S.A.
RQsumi5.- Le moment magngtique et la susceptibilit6 de CsNiC1, ont 6t6 Qtudi6s jusqu'l 60 kG, entre
I,I et 7 K. Les mesures en champ parallsle 3 l'axe c indiquent nettement l'existence d'une transi- tion de renversement de spin pr@s de 20 kG. Les d6tails du diagramme magngtique de phase ont 6t6 d6termin6s, en ~articulier ceux qui se rapportent 2 la reorientation de spin qui a lieu pres de
4,4 K en champs faibles.
Abstract.- The magnetic moment and susceptibility of CsNiC13 have been studied up to 60 kG from 1.1
to 7 K. Clear evidence for a spinflop transition near 20 kG has been obtained with the applied field parallel to the c-axis. Details of the magnetic phase diagram have been clarified particularly with respect to the spin reorientation which occurs near 4.4 K at low fields.
INTRODUCTION.- There is considerable interest in the magnetic properties of the linear-chain antiferro- magnet CsNiCl
.
The magnetic susceptibility /l/ and acoustic attenuation /2/ both exhibit broad maxima near 30 K, corresponding to the onset of one-dimen- sional antiferromagnetic ordering of the nickel atoms in chains parallel to the c-axis. Below 5 K these chains exhibit three-dimensional ordering, which has been observed in specific heat 1 3 1 , nu- clear magnetic resonance /4/ and neutron diffraction/5/ studies in addition to the abovementioned ex- priments. This ordering involves a spin rearrange- ment in the range 4.4 K to 4.85 K, as well as an apparent spin-flop at a field of approximately 20
kG 161, /7/. The present paper gives the results of
an extensive series of measurements of both the ma- gnetic moment and magnetic susceptibility ofCsNiC1' from 1.1 K to 7 K in fields up to 60 kG. A more com- plete phase diagram is deduced from the data.
EXPERIMENT
.-
Measurements were carried out on sin- gle-crystal specimens of CsNiCl used in previous3
acoustic experiments /2/, 161. Magnetic moment ex- periments were performed by a ballistic technique, using a nickel wire as a standard. Data were obtain ned from 1.1
K
to 4.4 K with the specimen immersed directly in the liquid helium bath. Susceptibility measurements were made using a mutual inductance bridge operating at 33 Hz, the final detector beinga Princeton Applied Research Model 120 lock-in am-
plifier. For measurements above 4.2 K, the specimen (*) Work supported by U.S. National Science
Foundation.
was enclosed in an evacuated chamber and connected thermally to an electrically controlled stage, the temperature of which was swept slowly and which was measured with a calibrated carbon thermometer. For both series of measurements, magnetic fields up to
60 kG were produced by a Nb-Zr superconducting ma- gne t.
RESULTS AND DISCUSSION.- Figure I shows the isotherm mal magnetization curves for CsNiCl from 1.1 K to
4.4 K with the applied field parallel to the c-axis. The results are similar to those for a spin-flop transition in a weakly anisotropic uniaxial antifer- romagnet, the first-order character being indicated by the discontinuity in the magnetization as a func-
tion of internal field. As expected, the susceptibi- lity below the transition is smaller than that above the transition and the magnitude of the discontinui- ty at the spinflop field decreases with increasing temperature. Near 4.2 K the discontinuity in magne- tization becomes rather small, making it difficult to follow the transition to higher temperatures. However, in the region of overlap with the acoustic data /6/ there is a good agreement between the two experiments. The present work gives a spin-flop field of 19.9 f 0.3 kG at 1.6 K, which is just wi- thin the confidence limits of the value 19.3 f 0.3
kG given by antiferromagnetic resonance experiments / 7 / .
It is of interest that isothermal susceptibi-
-+
lity measurements below 4.2 K with
11
parallel to the c-axis give well defined susceptibility maxima at the spin-flop field, the values at the latterb e i n g i n good agreement w i t h t h o s e c o r r e s p o n d i n g t o t h e i n f l e c t i o n p o i n t s i n t h e m a g n e t i z a t i o n d a t a . + F o r H p e r p e n d i c u l a r t o t h e c - a x i s , t h e s u s c e p t i b i - l i t y d a t a do n o t g i v e any i n d i c a t i o n of a s p i n - f l o p t r a n s i t i o n n e a r 20 kG. F i g . 1 : M a g n e t i z a t i o n c u r v e s f o r
c s ~ i c l
w i t h f i e l d p a r a l l e l t o c - a x i s . 3 F o r f i x e d f i e l d s below t h e s p i n - f l o p v a l u e , t h e v a r i a t i o n o f s u s c e p t i b i l i t y w i t h t e m p e r a t u r e shows e v i d e n c e b o t h f o r t h e p a r a m a g n e t i c t o a n t i f e r - r o m a g n e t i c t r a n s i t i o n a t t h e N6el p o i n t TN, a s w e l l a s t h e s p i n r e a r r a n g e m e n t a t lower t e m p e r a t u r e s , T ~ . T h e e x t r a p o l a t e d z e r o - f i e l d v a l u e s a t TN = 4.88 f 0.05 K and TR = 4.40 f 0 . 0 5 K a r e i n r e a s o n a b l e agreement w i t h t h e NMR r e s u l t s . From f i g u r e 2 i t i s c l e a r t h a t T i n i t i a l l y d e c r e a s e s and TR i n c r e a s e sN
w i t h i n c r e a s i n g a p p l i e d f i e l d , t h e two a s s o c i a t e d s u s c e p t i b i l i t y p e a k s becoming v i r t u a l l y i n d i s t i n - g u i s h a b l e n e a r t h e s p i n - f l o p t r a n s i t i o n . The beha- v i o r of TR a g r e e s w e l l w i t h t h a t p r e v i o u s l y o b s e r v e d i n t h e a c o u s t i c a t t e n u a t i o n e x p e r i m e n t s / 6 / , i n which t h e low-temperature a t t e n u a t i o n peak was i n -c o r r e c t l y a s s o c i a t e d w i t h T N.
Above t h e s p i n - f l o p f i e l d , t h e t e m p e r a t u r e de- pendence of t h e s u s c e p t i b i l i t y e x h i b i t s l a r g e anoma- l i e s n e a r t h e t r a n s i t i o n t o t h e p a r a m a g n e t i c s t a t e . Both t h e p r e s e n t e x p e r i m e n t s and t h e a c o u s t i c measu- rements g i v e e s s e n t i a l l y s i m i l a r phase b o u n d a r i e s , t h e t r a n s i t i o n t e m p e r a t u r e i n c r e a s i n g r a p i d l y w i t h a p p l i e d f i e l d .
0 Present Oato A Slurqe ond Cohen
Clorh ono M o u l t o n F i g . 2 : Phase d i a g r a m f o r CsNiCl w i t h f i e l d p a r a l - 3 l e l t o c - a x i s showing a comparison w i t h o t h e r d a t a . T h i s b e h a v i o r a p p e a r s t o be c h a r a c t e r i s t i c o f li- n e a r - c h a i n a n t i f e r r o m a g n e t s / 8 / , t h e n o n c o l l i n e a r i t y o f t h e s p i n s t r u c t u r e g i v i n g a d d i t i o n a l i n t e r e s t i n g f e a t u r e s . The r e g i o n of t h e phase d i a g r a m n e a r t h e c o a l e s c e n c e of t h e phase b o u n d a r i e s i s c u r r e n t l y b e i n g examined i n g r e a t e r d e t a i l f o r e v i d e n c e of m u l t i c r i t i c a l b e h a v i o r . Ref e r e n c e s
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