Mössbauer effect studies of ferromagnetic relaxation in YbIG

Volume 24A, n u m b e r 2

MOSSBAUER

PHYSICS

EFFECT

STUDIES

OF

LETTERS

16 J a n u a r y 1967

FERROMAGNETIC

S.OFER

RELAXATION

IN

YbIG

* a n d I . N O W I K **

Bell Telephone Laboratories Incorporated, Murray Hill, New Jersey Received 13 D e c e m b e r 1966 The efIect of f e r r o m a g n e t i c relaxation on the MSssbauer spectra of the 84 keY t r a n s i t i o n of 170yb in YbIG was observed. The spin relaxation t i m e s at 20OK were found to be (9 ± 3) × 10 -11 sec in YbIG and (2 • 1) × × I0 -II in p a r a m a g n e t i c YbGaG.

In a r e c e n t p a p e r [1] Nowik a n d W i c k m a n h a v e s h o w n how t h e M S s s b a u e r e f f e c t c a n b e u s e d to s t u d y r e l a x a t i o n e f f e c t s in f e r r o m a g n e t i c c o m p o u n d s . T h e y d e m o n s t r a t e d it u s i n g t h e e x p e r i m e n t a l r e s u l t s of 1 6 6 E r in E r F e O 3 [2]. T h e c a s e of Yb in Y b I G is v e r y s u i t a b l e f o r s u c h a s t u d y s i n c e a l l the p a r a m e t e r s , like the ionic "g" tensor and exchange splittings, are known from ind e p e n d e n t e x p e r i m e n t a l r e s u l t s [ 3 , 4 ] . In a d d i t i o n t h e C u r i e p o i n t i s h i g h (560°K), the g r o u n d K r a mers doublet is well below any Stark level (550 c m -1) a n d i t s e x c h a n g e s p l i t t i n g i s a l m o s t t e m p e r a t u r e i n d e p e n d e n t i n t h e r e g i o n of i n t e r e s t (T.~ 50°K). T h u s t h e o n l y u n k n o w n p a r a m e t e r (the f e r r o m a g n e t i c r e l a x a t i o n t i m e ) i n f l u e n c i n g t h e s h a p e of t h e M S s s b a u e r s p e c t r u m c a n b e s t u d i e d in a w i d e r a n g e of t e m p e r a t u r e s . We r e p o r t h e r e s t u d i e s of t h e M S s s b a u e r s p e c ~ t r a of t h e 84 k e V (2 + ~ 0 +) t r a n s i t i o n of 1 7 0 y b in YbIG a n d Y b G a G at 4.2 a n d 20.3OKo T h e e x p e r i mental spectra agree well with theoretical spect r a in w h i c h o n l y o n e a d j u s t a b l e p a r a m e t e r w a s u s e d ; n a m e l y , t h e s p i n r e l a x a t i o n t i m e (fig. 1). T h e m a g n e t i c p r o p e r t i e s of YbIG h a v e b e e n studied both experimentally [3,4] and theoretic a l l y [5]. T h e d i r e c t i o n of e a s y m a g n e t i z a t i o n i n YbIG i s a l o n g t h e (111) d i r e c t i o n . In s u c h a c a s e two m a g n e t i c a l l y i n e q u i v a l e n t y b 3 + s i t e s exist. The magnetic hyperfine fields acting on t h e 170Yb n u c l e i i n t h e l i m i t of v e r y f a s t r e l a x a t i o n a r e g i v e n (at low t e m p e r a t u r e s ) by:

Hleff(T) = l(~hilSA ~ i ) / t a n h

(~i/2kT) ,

(1)

where A is the hyperfine tensor which is propor* P r e s e n t a d d r e s s : The Hebrew U n i v e r s i t y . J e r u s a lem, I s r a e l . ** P r e s e n t a d d r e s s : Yale U n i v e r s i t y , New Haven, Connecticut. 88

1000k. ''.

.

•

.

• .. "."

T=/*.2=K

.

. ...

~.~

". , ,

..

s96

~ooo"f~~..

•

T=203°K

998

996 I

-2.0

-1.5

]

-t0

]

I

]

-0.5 0 0.5 VELOCITY (cm/sec)

I

10

I

15

Fig. 1. R e c o i l l e s s absorption s p e c t r a of the 84 keV t r a n s i t i o n of 170yb in YbIG. The source was in the f o r m of 170TmA12. Solid lines a r e t h e o r e t i c a l s p e c t r a a s s u m ing the p a r a m e t e r s g2~nHeff/h = 467 M c / s , eqQ2/4h = = 35 M c / s and the relaxation t i m e s indicated in the figure. t i o n a l to t h e i o n i c " g " t e n s o r , ~ i i s t h e w a v e f u n c t i o n of e i t h e r of t h e l e v e l s of t h e g r o u n d K r a m e r s d o u b l e t a n d Ai i s t h e e x c h a n g e s p l i t t i n g (A 1 = = 3 3 . 1 ° K a n d A 2 = 3 8 . 2 ° K [ 4 ] ) . In t h e c a s e w h e r e t h e s p i n r e l a x a t i o n t i m e s a r e of t h e o r d e r of t h e i n v e r s e L a r m o r f r e q u e n c y (T ~--1/WL), t h e l a s t f o r m u l a h a s no m e a n i n g a n d d y n a m i c e f f e c t s a r e o b s e r v e d in t h e M ~ s s b a u e r s p e c t r a . T h e s p e c t r u m of t T O y b in YbIG a t 4 . 2 ° K is not s e n s i t i v e to r e l a x a t i o n e f f e c t s s i n c e only t h e l o w est level is populated at this temperature. Acc o r d i n g to eq. (1), it i s c o m p o s e d of two s p e c t r a d i f f e r i n g in H e f f b y 3%. T h e e x p e r i m e n t a l a v e r a g e v a l u e of H e f f a t 4 . 2 ° K i s 43% of t h e f r e e - i o n v a l u e a n d i s i n v e r y good a g r e e m e n t w i t h t h a t c a l c u l a t e d u s i n g e q . (1) a n d t h e k n o w n v a l u e of t h e m a g n e t i c

Volume24A, number 2

PHYSICS LETTERS

m o m e n t s of the 2 + n u c l e a r l e v e l [6]. The s p e c t r u m at 20.3°K in the l i m i t of v e r y f a s t spin r e l a x ation is c o m p o s e d of two i n e q u i v a l e n t s p e c t r a diff e r i n g in Her f by ~10?o (this is a r e s u l t of the diff e r e n c e b e t w e e n 41 and A2). This would s l i g h t l y s m e a r the M S s s b a u e r s p e c t r u m (fig. 1) but not enough to explain the actually o b s e r v e d s p e c t r u m . We used the technique d e s c r i b e d in r e f . 1 to c a l culate the e f f e c t of the r e l a x a t i o n b e t w e e n the ground s u b l e v e l s (split by the exchange i n t e r a c tion) on the Mt~ssbauer s p e c t r a of 170yb in YbIG. F o r s i m p l i c i t y the s a m e r e l a x a t i o n t i m e was a s s u m e d f o r the two i n e q u i v a l e n t s i t e s and good a g r e e m e n t b et ween t h e o r e t i c a l and e x p e r i m e n t a l c u r v e s was obtained with T = (9+3) × 10 -11 s e c , With a YbGaG a b s o r b e r at 4.2 and 20.3°K, uns p l i t a b s o r p t i o n l i n es with widths of 8.3 m m / s e c at half m a x i m u m height w e r e obtained f r o m which the spin r e l a x a t i o n t i m e was e s t i m a t e d to be (2+ 1) x 10 -11 s e c . T e a l e and T w e e d a l e [7] m e a s u r e d the f e r r o m a g n e t i c r e l a x a t i o n t i m e s of Yb 3+ doped into YiG using the f e r r o m a g n e t i c r e s o n a n c e technique and found T(20°K) ~ 2 × 10 -10 s e c . The m a i n c o n c l u s i o n s f r o m the p r e s e n t work a r e : (1) the r e c o i l l e s s a b s o r p t i o n s p e c t r a in YbIG at low t e m p e r a t u r e s can be explained in t e r m s of r e l a x a t i o n p r o c e s s e s between the ground s u b l e v -

RELAXATION PHENOMENA IN I N T E R M E T A L L I C

16 January 1967

el s (split by the exchange i n t e r a c t i o n ) . (2) The f e r r o m a g n e t i c r e l a x a t i o n t i m e s in YbIG and those of Yb doped into YIG a r e about the s a m e , i n d i c a t ing that the r a t e of c r o s s r e l a x a t i o n a r i s i n g f r o m r a r e e a r t h - r a r e e a r t h i n t e r a c t i o n s is s m a l l e r o r equal to the r a t e s of the o t h e r co n t r i b u t i o n s to f e r r o m a g n e t i c r e l a x a t i o n [8]. (3) The f a s t r e l a x a tion in YbGaG in c o m p a r i s o n to that in YbIG p r o v e s that the ap p l i cat i o n of a m a g n e t i c or exchange f i el d on the y b 3 + ions has an effect of damping t h e i r spin r e l a x a t i o n r a t e s .

References 1. I. Nowik and H.H.Wickman, PhysoRev. Letters 117 (1966) 949. 2. W.Wiedemann and W. Zin, Z.Angew. Phys. 20 (1966) 327. 3. K.A.Wickersheim and R.L.White, Phys.Rev. Letters 8 (1962) 483. 4. A . J . Sievers and M. Tinkham, Phys. Rev. 129 (1963) 1995. 5. P. Levy, Phys.Rev. 147 (1966)311. 6. A. Huller, W. Wiedemann, P. Kienle and S. Hufner, Phys. Letters 15 (1965) 269. 7. R.W. Teale and K. Tweedale, Phys. Letters 1 (1962) 298. 8. J.H. Van Vleck, Proc.Intern. Conf. on Magnetism, Nottingham, 1964, p. 401; and J.Appl. Phys. 35 (1964) 882.

IN MOSSBAUER SPECTRA OF COMPOUNDS OF YTTERBIUM

170yb

I. NOWIK * S. O F E R ** and J . H. WERNICK Bell Telephone Laboratories Incorporated, Murray Hill, New Jersey Received 20 December 1966

Spin relaxation phenomena were observed in the M~ssbauer spectra of intermetallic compounds of Yb. Relaxation times at 4.2°K are (2 :e 1) x 10-11 sec in YbPd 3 and (7 =L2) x 10-12 sec in YbNi 5. Complicated spectra were obtained with a YbNi 2 absorber.

R e l a x a t i o n p h e n o m e n a in M S s s b a u e r s p e c t r a have been p r e v i o u s l y o b s e r v e d in p a r a m a g n e t i c s a l t s and f e r r i m a g n e t i c i n s u l a t o r s . We r e p o r t h e r e the f i r s t o b s e r v a t i o n of r e l a x a t i o n phenom e n a in the r e c o i l l e s s a b s o r p t i o n s p e c t r a of int e r m e t a l l i c compounds. The a b s o r p t i o n s p e c t r a * Present address: Yale University, New Haven, Connecticut. ** Present address: The Hebrew University, Jerusalem, Israel.

of the 84 keV g a m m a r a y s of 170yb (2 + ~ 0 + t r a n sition) in YbA12, YbNi2, YbNi 5 and YbPd 3 w e r e m e a s u r e d at 4.2 and 20.3°K. The s o u r c e used in all the e x p e r i m e n t s was 170Tm in the f o r m of TmA12. The Yb in YbA12 is d i v a l e n t and thus d i a m a g netic [1 ]. The width at half m a x i m u m h e i g h t of the YbA12 a b s o r p t i o n line at 4.2°K was found to be 2.1 m m / s e c which is v e r y c l o s e to the n a t u r a l width (2 Fn). In YbNi2, YbNi 5 and YbPd 3 the Yb 89