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MÖSSBAUER STUDY OF THE TWO-DIMENSIONAL ANTIFERROMAGNET IN THE CRITICAL REGION :
Rb2CoF4 : 57Fe AND Rb2Co0.95Mg0.05F4 : 57Fe
A. Ito, M. Horiike
To cite this version:
A. Ito, M. Horiike. MÖSSBAUER STUDY OF THE TWO-DIMENSIONAL ANTIFERROMAG-
NET IN THE CRITICAL REGION : Rb2CoF4 : 57Fe AND Rb2Co0.95Mg0.05F4 : 57Fe. Journal de
Physique Colloques, 1979, 40 (C2), pp.C2-323-C2-324. �10.1051/jphyscol:19792113�. �jpa-00218483�
JOURNAL DE PHYSIQUE
Colloque C2, supplkment au n
O3, Tome 40, mars 1979, page ~ 2 - 3 2 3
M ~ S S B A U E R STUDY OF THE TWO-DIMENSIONAL ANTIFERROMAGNET IN THE CRITICAL REGION : R ~ ~ C O F ~ : AND Rb2C00 g5Mg0 - 0 5 ~ ~ : " ~ e
A. Ito and M. Horiike
Department of Physics, Faculty of Science, Ochmromizu University, Otsuka-machi, Bwrkyo-ku, Tokyo, Japan RQsm6.- On a realis6 une Qtude par effet Mhsbauer du comportement d'un antiferromagnQtique type Ising 2D (Rb2CoF4 dope au S 7 ~ e ) en fonction de la templrature au voisinage de la region critique. On observe un elargissement des raies plusieurs degres au-dessus de TN, ce qui indique l'existence d'un ordre B courte distance dans 1'Btat paramagnBtique. Cet Blargissement, maximum juste au-dessus de T
(T %T
%10'~s) est encore detectable environ
10K en dessous de TN. N'
W L
Abstract.- The temperature variation of Gssbauer spectra of 5 7 ~ e doped in a typical two-dimensional Ising antiferromagnet Rb2CoF~ has been observed to investigate the critical behavior around TN. With decreasing temperature a line broadening due to magnetic interactions is observed from temperatures several degrees above TN, which indicates the existence of a short-range order in a paramagnetic re- gion. The line broadening is drastic just above TN
(T%T %10-~s) and is still considerably noticed at temperatures about ten degrees below TN. W
LRb2CoF4 is known to be a typical two-dimen- sional Ising antiferromagnet. There is a great deal of interest in M6ssbauer study on the critical behavior around T of this compound. The hyperfine
N
interaction of doped "Fe nuclei (%l at%) in Rb2CoF4 was measured from room temperature to 4.2 K using the Mb'ssbauer spectroscopy. Especially in the vicinity of TN of 101.96 K (see below), a detailed temperature varLation of the spectrum was observed at small intervals of 0.1W.2 K.
Temperature was kept stable throuzhout the experi- ment to less than 0.05 K. The temperature inhomo- geneity in a sample was estimated to be less than 0.05 K.
Figure I shows some typical spectra observed for a single crystal sample with incident y-rays parallel to the axially symmetric crystallographic c-axis. Except for the region near TN, well resol- ved spectra were observed. As seen in the figure, the spectrum consists of a quadrupole doublet with an intensity ratio about 1:3 in a paramagnetic region and it consists of four lines in an anti- ferromagnetic region, indicating the absence of the lines corresponding to the Am=O transitions of 5 7 ~ e . These spectra show that the quadrupole interaction e2qq is positive and that the direction of the magnetic hyperfine field at 7 ~ e nuclei is parallel to the c-axis which is the antiferro- magnetic axis of co2+ ions: Q =O and q=O. The fact
H
that OH=O and
0'11means that a sub-level-mixing of the excited state of 5 7 ~ e in Rb2CoFr is not caused by the magnetic hyperfine interaction. This fact makes it easy to assign spectra and to treat
critical phenomena theoretically.
-<. ,/"?.
!---'-- (a) R.T.- 2 - 1 0 1 2 3 4 5 6 7 8 V E L O C I T Y
( m m / s 15 7
Fig.
1 :RbnCoFi,
: lat% Fe: The temperature variation of Mb'ssbauer spectra observed for a single crystal sample with the surfaces perpendi- cular to the tetragonal c-axis. Transmitted y-rays were detected along the c-axis.
Now we pay attention to the broadened spectra observed around 102 K. It should be emphasized that the observed line broadening was ascertained not
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792113
c2-324
JOURNAL DE PHYSIQUEt o be due t o a d i s t i b u t i o n o f TN i n a sample. S i n c e t h e h i g h e r v e l o c i t y l i n e o f t h e q u a d r u p o l e d o u b l e t c o r r e s p o n d s t o t h e t r a n s i t i o n s ( i 3 / 2 +
+
1/21 o f ' ' ~ e and s p l i t s i n t o two o u t e r m o s t l i n e s a t low tempera- t u r e s ( s e e F i g . l (K) ), a n i n f l u e n c e o f c r i t i c a l f l u c t u a t i o n s , i f i t e x i s t s , i s c o n s i d e r e d t o a p p e a r much more s t r o n g l y on t h i s l i n e t h a n o n t h e lower v e l o c i t y l i n e . I n t h e s p e c t r u m a t 103.5 K ( F i g . l ( b ) ), i n f a c t , i t i s c l e a r l y s e e n t h a t t h e h i g h e r v e l o c i t y l i n e i s c o n s i d e r a b l y b r o a d e n e d : t h i s shows t h e growing o f t h e m a g n e t i c s p l i t t i n g . T h i s f e a t u r e i s n o t i c e d even a t 106.5 K , s e v e r a l d e g r e e s above TN. A s t e m p e r a t u r e i s d e c r e a s e d down t o T N' t h e l i n e w i d t h o f t h e h i g h e r v e l o c i t y l i n e i n c r e a - s e s v e r y r a p i d l y and t h e l i n e i s r e a d y t o s p l i t i n t o two b r o a d l i n e s a t 102.0 K ( F i g . l ( g ) ) a l i t t l e above TN. With d e c r e a s i n g t e m p e r a t u r e t h r o u g h T t h e w i d t h o f t h e l i n e s d e c r e a s e s r a p i -N
d l y and a t t a i n s t o a v a l u e o f a b o u t 0.3 m/s a t a t e m p e r a t u r e a b o u t t e n d e g r e e s below T F i g u r e
N ' 2 shows t h e t e m p e r a t u r e dependence o f t h e l i n e w i d t h o f t h e h i g h e r v e l o c i t y l i n e .
4 1 1 1 1 1 1 1 1
0'80 85 90 95 100105 110 T E M P E R A T U R E
( K )
F i g . 2 : Temperature v a r i a t i o n o f t h e l i n e w i d t h o f s p e c t r a f o r RbscoF~, : l a t % "Fe
0 TL102.25 K, t h e f u l l w i d t h a t h a l f maximum o f a n u n r e s o l v e d l i n e .
Ts102.0 K , t h e f u l l w i d t h a t h a l f maximum o f a m a g n e t i c a l l y s p l i t l i n e on t h e e x t r e m e r i g h t . Although t h e meaning o f t h e l i n e w i d t h p l o t t e d is d i f f e r e n t i n t h e two t e m p e r a t u r e r e g i o n s ,
T>102.25
-
K and T5102.0 K ( s e e f i g u r e c a p t i o n ) , i t i s s e e n t h a t t h e l i n e w i d t h i s broadened remar- k a b l y a s t e m p e r a t u r e a p p r o a c h e s t o TN from b o t hs i d e r e d t o b e a t t r i b u t e d t o a two-dimensional s h o r t - r a n g e o r d e r i n a p a r a m a g n e t i c s t a t e . As t e m p e r a t u r e d e c r e a s e s and a p p r o a c h e s TN, a d e g r e e o f t h e s h o r t - r a n g e o r d e r i n c r e a s e s and a c h a r a c t e r i s t i c t i m e T
W r e l a t e d t o a s p i n f l u c t u a t i o n may become l o n g . A s T~ a p p r o a c h e s t o t h e Larmor p r e c e s s i o n t i m e T
L O f
n u c l e a r moments o f 5 7 ~ e , t h e n u c l e u s f e e l s a n e f f e c - t i v e m a g n e t i c f i e l d and t h e h i g h e r v e l o c i t y l i n e b e g i n s t o b e b r o a d e n e d f i r s t i n t h e p r e s e n t c a s e . I n t h e r e g i o n o: T ~ = T ~ . t h e l i n e b r o a d e n i n g w i l l b e s t r i k i n g ( s e e F i g . 1 ( g ) ) . A s t o c h a s t i c model / l / was a p p l i e d t o u n d e r s t a n d a c r i t i c a l b e h a v i o r around TN, and T~ was e s t i m a t e d t o b e % 1 . 2 x 1 0 - ~ s f o r t h e s p e c t r u m o b t a i n e d a t 102.00 K('!
I n a compound d i l u t e d by non-magnetic 14g i o n s , RbzCoo. 95Mg0.05F4:0. 2 a t % 7 ~ e t h e t e m p e r a t u r e r a n g e where t h e l i n e b r o a d e n i n g was o b s e r v e d \!as t w i c e a s wirie a s t h o t i a t h e p u r e s y r tern RbrCoFq:
I at;b 5 7 ~ e .
'Ihe h y p e r f i n e f i e l d o b s e r v e d f o r Rb2CoFb: ' ' ~ e was found t o f o l l o w t h e power law H ~ ~ = ( I - T / T ~ ) ~ i n t h e r a n g e 0.002<(1-T/TN)<O.l f o r 6=0.14 and
5 TN=101.96 K. I f one assumes t h a t Hhf i s p r o p o r t i o - n a l t o a s u b l a t t i c e m a g n e t i z a t i o n , t h e v a l u e o f 6=0.145 s h o u l d b e compared w i t h 6=0.119 o b t a i n e d by t h e n e u t r o n d i f f r a c t i o n s t u d y / 2 / and w i t h 6=0.125 t h e o r e t i c a l l y p r e d i c t e d f o r t h e 2D I s i n g s y s t e m w i t h s p i n 112. This d e v i a t i o n may b e concer- ned w i t h t h e phenomena c a u s e d by t h e s h o r t - r a n g e o r d e r n e a r TN mentioned above.
R e f e r e n c e s
/ l / Blume, M. and T j o n , J. A . , Phys. Rev.
165
(1968) 446
/ 2 / Samuelsen, E . J . , Phys. Rev. L e t t .
2
(1973) 936t h e h i g h and low t e m p e r a t u r e s i d e s . P a r t i c u l a r l y , --
' I ) R e f e r t o t h e a r t i c l e "MGssbauer s t u d y o n t h e a b r o a d e n i n g s e e n i n a s m a l l i n t e r v a l o f a b o u t c r i t i c a l dynamics o f t h e two-dimensional a n t i - 2 K a r o u n d T i s d r a s t i c . f e r r o m a g n e t " by I t o A. and H o r i i k e ?I. ( i n t h i s
N p r o c e e d i n g s )
The l i n e b r o a d e n i n g o b s e r v e d above TN i s con-
