Volume 30A, number 8 P H Y S I C S L E T T E R S 15 December 1969

At 3620 A, we h a v e a p e a k in t he Hg e x c i t a t i o n l a m p e m i s s i o n s p e c t r u m . One of two P r 3+ p r o - c e s s e s i s p r o b a b l y r e s p o n s i b l e f o r the l a r g e P l i n e s : 3P 1 + 3F 3 o r l I 6 + 3F3 w i t h subsec~uent r e l a x a t i o n of t he l I 6 l e v e l to t he 3 P 1 a n d ° P o l e v e l s by m u l t i p h o n o n e m i s s i o n . In a d d i t i o n , 3 6 2 0 / ~ c o r r e s p o n d s to a d i r e c t e x c i t a t i o n of the 3H 5 l e v e l of Ho 3+, w h i c h c o u l d s u b s e q u e n t l y r e l a x to the 5 F 5 l e v e l . F l u o r e s c e n c e of t h i s w a v e - l e n g t h h a s b e e n o b s e r v e d in Ho : LaC13, bu t h a s no t b e e n p o s i t i v e l y i d e n t i f i e d a s c o m i n g f r o m t h i s t r a n s i t i o n [5]. The H o : 5F 5 - 5I 8 s p e c t r u m ob - s e r v e d i s s i m i l a r in s h a p e to t h a t in 2% Ho : LaC13 u n d e r 5G 6 e x c i t a t i o n . T h i s g r o u p of l i n e s a l s o c o n t a i n s the Ho 3+ t r a n s i t i o n l i n e s 5F 3 - 5I 7.

The two i n f r a r e d P r 3+ t r a n s i t i o n s 1G 4 - 3H 4 and 3F 3 - 3H 4 h a v e b e e n s e a r c h e d f o r bu t no t o b s e r v e d , e v e n when p o p u l a t i n g t he l e v e l s v i a s i n g l e P r 3+ ion a b s o r p t i o n . B o t h t he s e c o n d and t he t h i r d of t he d o u b l e - i o n p r o c e s s e s i n v o l v e a r e l a x a t i o n to t he Ho 3+ l e v e l 5F3 and the f l u o r e s - c e n t d e c a y of t h i s l e v e l to the ~I 8 g r o u n d l e v e l h a s b e e n o b s e r v e d f o r i n c i d e n t l i gh t n e a r 32 000 c m -1.

S ince i o n s w h i c h a r e e x c i t e d to the 3 P 2 l e v e l c a n d e c a y to the 3P1 , 3Po , and D l e v e l s by m u l - t i p h o n o n e m i s s i o n , the low a m o u n t of 3P o f l u o r - e s c e n c e r e l a t i v e to the 1D 2 f l u o r e x c e n c e at 3100 A e x c i t a t i o n v e r s u s t h a t a t 3620 A i s good e v i d e n c e t h a t we a r e i n d e e d s e e i n g the d i r e c t t w o - i o n p r o c e s s .

The a u t h o r s g r a t e f u l l y a c k n o w l e d g e the h e l p - ful c o m m e n t s of D r s . J. G. C a s t l e , J r . and J. E. Rush .

References 1. F. Varsanyi and G. H. Dieke. Phys. Rev. Le t te r s 7

(1961) 442. 2. J .S . Margol is , O. Stafsudd and E. Y. Wong. J. Chem.

Phys. 38 {1963) 2045. 3. J. P. Van der Ziel and L. G. Van Ui te r t , Phys. Rev.

180 (1969) 343. 4. E .Dorman . J. Chem. Phys. 44 (1966) 2910. 5. G.H. Dieke and B. Pandey, J. Chem. Phys. 41 (1964)

1952. 6. G.H. Dieke and E. Dorman, Phys. Rev. Le t te r s 11

(1936) 17.

* * * * *

FAR-INFRARED ABSORPTION IN FeCI 2

K. NAGATA and Y. TOMONO Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Japan

Received 28 October 1969

The f a r - i n f r a r e d t r a n s m i s s i o n was studied in single c rvs ta l FeCl 2 in the an t i fe r romagnet ic s tate and a new f ield-dependent absorpt ion line was observed at 32~8 cm -1 at 1.5°K.

T h e f a r - i n f r a r e d t r a n s m i s s i o n h a s b e e n s t u d i e d in s i n g l e c r y s t a l FeC12 in the a n t i f e r r o m a g n e t i c s t a t e . Two m a g n e t i c a b s o r p t i o n l i n e s w e r e ob- s e r v e d at 16.5 and 32.8 c m -1 at l i q u i d h e l i u m t e m - p e r a t u r e . T h e l ine at 16.5 c m -1 h a s b e e n i d e n t i - f i ed a s the a n t i f e r r o m a g n e t i c r e s o n a n c e (AFMR) m e d e d i s c o v e r e d by J a c o b s [1], bu t a n o t h e r l ine at 32 .8 c m -1 i s a new l i ne , w h i c h wi l l be r e p o r t e d in t h i s p a p e r .

The f a r - i n f r a r e d m e a s u r e m e n t s w e r e m a d e u s i n g H i t a c h i F IS -21 s p e c t r o m e t e r a s a m o n o - c h r o m a t o r t o g e t h e r wi th a g a l l i u m dopen g e r m a - n i u m b o l o m e t e r . T h e r a d i a t i o n f r o m t he ex i t s l i t of t he m o n o c h r o m a t o r w a s f ed in to a c o p p e r l i g h t

p ipe a n d c o n d u c t e d to the h e l i u m D e w a r p l a c e d in a 5 5 - m m m a g n e t gap p r o d u c i n g f i e l d up to 16 kOe. F o r m e a s u r e m e n t s in a h o r i z o n t a l m a g n e t i c f i e ld , 2 - m m b a s a l p l a n e c r y s t a l p l a t e s w e r e f ixed in the l i gh t p ipe w i th the h e x a g o n a l c - a x i s a t 45 ° w i th r e s p e c t to the b e a m d i r e c t i o n . In t h i s m a n n e r , t he a n g u l a r d e p e n d e n c e of l i n e s c a n be m e a s u r e d o v e r a r a n g e f r o m ~ = 45 ° to 0 = 90 ° , w h e r e ~) i s the a n g l e b e t w e e n the c - a x i s and the f i e l d d i r e c t i o n .

At z e r o f i e l d two l i n e s w e r e o b s e r v e d a t 16.5 + 0.3 ( A F M R m o d e ) and 32.8 ± 0.3 c m -1 ( the new l ine) a t 1 .5°K. W i t h i n c r e a s i n g t e m p e r a t u r e , b o t h l i n e s sh i f t s l i g h t l y to l o w e r f r e q u e n c i e s and b r o a d - en u n t i l a t ~ 2 0 ° K they a r e u n o b s e r v a b l e . It s h o u l d

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Volume 3OA, number 8 P H Y S I C S L E T T E R S 15 December 1969

38 I I I

36

"7

32 LL

3O

28

o

I I I 5 10 15

Magnetic Field [ k:Oe] Fig. 1. Magnetic field dependence of the absorption

lines in FeCI 2 at 1.5OK and 0 = 45 o.

be not iced that the new mode f requency is two t i m e s as high as the AFMR i :equency .

Fig. 1 shows the effect of a magnet ic f ie ld up to 16 kOe and p a r a l l e l to the d i rec t ion of 0 = 45 ° on the new line. The new line cons i s t s of two se t s of l ines : one is spli t l inear ly and the o ther i s spli t quadra t i ca l ly by an applied field. The l inear spli t t ing is propor t iona l to the magnitude of the c -componen t of the field, H cos0, and the s lopes c o r r e s p o n d to 2g, where g g ives the s lopes of the AFMR l ines and is expected as 3.8 + 0.2 for 0 = 0 and S = 1, which a g r e e s welI with the ea r ly works of Jacobs [1,2]. The qua- dra t ic spli t t ing is an o rde r of magnitude s m a l l e r than the l i nea r ' one at H= 16 kOe and 0 = 45 °, and i ts angular dependence is v e r y smal l .

The obse rved f ie ld dependence of these l ines may suggest that the l i nea r - and the quadra t i c - spli t l ines a re a s soc ia t ed with the exc i ta t ions of two magnons on one sublat t ice and on two d i f fe r - ent subla t t ices , r e spec t ive ly . This assumpt ion s e e m s to be cons i s ten t with the fact that the f r e -

quency of these l ines is exact ly two t i m e s as high as the AFMR f requency at z e r o field.

As is well known, FeC12 has the hexagonal l ayer type s t ruc tu re , i somorphous with CdC12, and o r d e r s an t i f e r romagne t i ca l ly at T N = 23.5OK. The magnet ic s t ruc tu re cons i s t s of the Fe 2+ m o m e n t s pointing along the + c - ax i s , in the a l t e r - nating ~0001) f e r r o m a g n e t i c sheet, r e spec t ive ly . The Fe ~+ ion in an approximate octahedron of C1- ions i s in the F 5 orb i ta l ground state which is spli t to th ree s inglet s t a tes and s ix doublet s t a tes by the r e s idua l t r igonal d is tor t ion and the L- S coupling. Accord ing to the MSssbaner study by Ono et al. [3], the lowest s tate i s doublet and is separa ted f rom a singlet f i r s t exc i ted state by about 14 cm -1. Th is spli t t ing is comparab le with the i n t r a l aye r f e r r o m a g n e t i c exchange in teract ion. The next l eve l l i e s 141 cm -1 higher. Then we can r e p r e s e n t the magnet ic p r o p e r t i e s of FeC12 using f ic t i t ious spins of S = 1, together with a single ion anisot ropy t e rm .

Since the i n t e r l aye r exchange in te rac t ion i s e s t ima ted as about 1 cm -1 f rom the metamagne t ic t r ans i t ion f ie ld [4], which i s an o rde r of magni - tude s m a l l e r than the e n e r g i e s of the obse rved l ines , the magnon d i spe r s ion for propagat ion in the c - d i r e c t i o n i s nea r ly independent on the wave vec tor . Then the exci ta t ion energy of two mag- nons at k = ( 0 , 0 , ~ / c ) is near ly two t i m e s l a r g e r than that of one magnon at k = 0.

The m e c h a n i s m for such two magnon exci ta t ion p roposed by Tanabe et al. [5] i s forbidden by sym- m e t r y in a pe r f ec t FeC12 c rys ta l . However , it i s sugges ted that the lower s y m m e t r y induced by im- pu r i t i e s , e.g. Fe 3+, may allow such two magnon exci ta t ion f r o m the observa t ion that the intensi ty of the new line i s dependent on the h i s tory of the c rys t a l .

We wish to thank Dr. M. Motokawa for suppling s ingle c r y s t a l s of FeC12.

References 1. I. s. Jacobs, S. Roberts and P. E. Lawrence, J. Appl.

Phys. 36 (1965) 1197. 2. Unpublished works refered by R. Albert, J. Phys. Soc.

Japan 26 (1969) 261. 3. K. Ono. A. Ito and T. Fujita. J. Phys. Soc. Japan 19

(1964) 2119. 4. A. Ito and K. Ono, J. Phys. Soc. Japan 20 (1965) 784;

I. S. Jacobs and P. E. Lawrence, Phys. Rev. 164 (1967) 1866.

5. Y. Tanabe, T. Moriya and S. Sugano, Phys. Rev. Let- ters (1965) 1023.

453