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Influence of Fano effect on magnetic circular dichroism due to core electron transition in Ni and Gd metals
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Solid State Communications,Vol. 83, NO. 1, pp. 41--45, 1992. Printed in Great Britain.
0038-1098/9255.00 + .00 Pergamon Press Ltd
INFLUENCE OF FANO EFFECT ON MAGNETIC CIRCULAR DICHROISM DUE TO CORE ELECTRON TRANSITION IN Ni AND Gd METALS S.Muto Department of Synchrotrnn Radiation Science The Graduate University for Advanced Studies Oho 1-1, Tsukuba-shi, Ibaraki 305, JAPAN Y.Kagoshima, T.Miyahara, S.Yamamoto, and H.Kitamura Photon Factory, National Laboratory for High Energy Physics Oho 1-1, Tsukuba-shi, Ibaraki 305, JAPAN (Received on 19 February 1992 by H.Kamimura) M a g n e t i c c i r c u l a r d i c h r o i s m s (MCD) of f e r r o m a g n e t i c Ni and Gd thin films h a v e b e e n m e a s u r e d in the v i c i n i t y of the 3p-3d e x c i t a t i o n for Ni and the 4d-4f e x c i t a t i o n for Gd, respectively. The p h o t o a b s o r p t i o n e x p e r i m e n t s w e r e p e r f o r m e d by u s i n g an u l t r a h i g h v a c u u m c h a m b e r i n s t a l l e d at BL-28 of the P h o t o n F a c t o r y in Japan. Through these e x p e r i m e n t s a p p a r e n t MCD signals due to core e l e c t r o n e x c i t a t i o n s w e r e observed. In a d d i t i o n to the MCD due to core e l e c t r o n excitations, an a n o m a l o u s feature was found, w h i c h extends far b e y o n d the r e g i o n of core e l e c t r o n e x c i t a t i o n both for Ni and for Gd.
materials, e s p e c i a l l y f e r r o m a g n e t i c materials. In more detail the symmetry b r e a k i n g of orbital and spin m o m e n t u m s of the initial state of atoms are s e n s i t i v e l y r e f l e c t e d to the MCD, w h i c h has b e e n p r e d i c t e d by recent t h e o r i e s 4. In the r e s e a r c h on ferromagnetism, Ni has been one of the most a t t r a c t i v e materials to i n v e s t i g a t e the 3d e l e c t r o n system. The 3d e l e c t r o n s in t r a n s i t i o n metals have b e e n e x t e n s i v e l y u n d e r s t o o d to be itinerant, and its m a g n e t i c p r o p e r t i e s such as fractional number of m a g n e t i c moment at an atom has been s u c c e s s f u l l y e x p l a i n e d by the Stoner model s (itinerant model). From the v i e w point of a band theory Erskine and Stern p r e d i c t e d MCD of Ni due to 3p-3d transition. R e c e n t l y h o w e v e r an atomic model has been p r e s e n t e d in o r d e r to interpret the experimental results obtained by h i g h e n e r g y s p e c t r o s c o p i e s w i t h respect to 3d e l e c t r o n s in t r a n s i t i o n metals and their compounds, v C . T . C h e n et al. c a r r i e d out an e x p e r i m e n t on MCD of Ni at the 2p-3d e x c i t a t i o n region by means of total y i e l d m e a s u r e m e n t and p r e s e n t e d an i m p o r t a n t p h y s i c a l s u g g e s t i o n that the 3d band is m o d u l a t e d by spin-orbit i n t e r a c t i o n of 3d electrons. 8 The 3p(2p) core hole level is split into 3p(2p)z/2 and 3p(2p)3/2 core hole levels by s p i n - o r b l t i n t e r a c t i o n of 3p(2p)
R e c e n t l y the m a g n e t i c c i r c u l a r dic h r o i s m due to core e l e c t r o n e x c i t a t i o n has a t t r a c t e d m u c h a t t e n t i o n in unders t a n d i n g more c l e a r l y in b o t h t h e o r e t i c a l and e x p e r i m e n t a l aspects the e l e c t r o n i c s t r u c t u r e s of f e r r o m a g n e t i c m a t e r i a l s such as t r a n s i t i o n metals, rare e a r t h m e t a l s 1.2 and their compounds. The s y n c h r o t r o n sources w h i c h can emit high e n e r g y circ u l a r l y p o l a r i z e d light w i t h c o n t i n u o u s s p e c t r u m has made it p o s s i b l e to p e r f o r m the m e a s u r e m e n t s of m a g n e t i c c i r c u l a r dic h r o i s m (MCD) due to core e l e c t r o n excitations. Core e l e c t r o n e x c i t a t i o n s sensit i v e l y reflect the l o c a l i z e d e l e c t r o n i c s t r u c t u r e t h r o u g h the dipole s e l e c t i o n rule w h i c h r e s t r i c t s optical excitations. Especially the d i f f e r e n c e due to the d i p o l e s e l e c t i o n rule b e t w e e n right- and l e f t - c i r c u l a r l y p o l a r i z e d lights classifies e l e c t r o n e x c i t a t i o n s with respect to the m a g n e t i c q u a n t u m number under the c o n d i t i o n Am=+l,-I respectively. The MCD due to core e l e c t r o n e x c i t a t i o n s is det e c t e d w i t h core e l e c t r o n s p e c t r o s c o p y in c o n j u n c t i o n w i t h right- and left- circularly p o l a r i z e d light and w i t h an e x t e r n a l m a g n e t i c field p a r a l l e l to the p r o p a g a t i o n d i r e c t i o n of the light. We can d i r e c t l y get m u c h i n f o r m a t i o n to d e t e r m i n e the localized magnetic structure of cons t i t u e n t atoms in m a g n e t i c a l l y ordered 41
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core hole, w h i c h d i s t i n g u i s h e s the contrib u t i o n s to M C D due to 3 p ( 2 p ) z n - 3 d transition and 3p(2p)~/2-3d transition. If it is a s s u m e d that the s y m m e t r y of 3d o r b i t a l m o m e n t u m is not b r o k e n and only the symm e t r y of spin m o m e n t u m is broken, the cont r i b u t i o n s to M C D from 3 p ( 2 p ) z n - 3 d tran s i t i o n and 3 p ( 2 p ) ~ 2 - 3 d t r a n s i t i o n are the same i n t e g r a t e d v a l u e s but w i t h d i f f e r e n t • 9 slgns , w h i c h c o r r e s p o n d s to the case of the p r e d i c t e d MCD by E r s k i n e and Stern.6 H o w e v e r the c o n t r i b u t i o n to the o b s e r v e d M C D from 2P3/2-3d t r a n s i t i o n is l a r g e r than the one from 2p1~-3d transition. This experimental result suggests that the o r b i t a l m o m e n t u m of 3d e l e c t r o n s dose not v a n i s h and c o n t r i b u t e s to the m a g n e t i c m o m e n t p a r a l l e l to t h e i r spin momentum. The MCD s p e c t r a c a l c u l a t e d by Jo et al. for Ni due to 2p-3d t r a n s i t i o n s b a s e d on an a t o m i c p i c t u r e w i t h 3d c o n f i g u r a t i o n i n t e r a c t i o n s i n c l u d e d e x p l a i n s the e x p e r i m e n t a l r e s u l t s m o r e s u c c e s s f u l l y than the Erskine's model. I° This fact directly s u g g e s t s that the e l e c t r o n i c state of 3d e l e c t r o n in t r a n s i t i o n m e t a l s can be well d e s c r i b e d by the a t o m i c m o d e l in c o n t r a s t w i t h the i t i n e r a n t m o d e l s w h i c h o n l y take the spin m o m e n t u m s into a c c o u n t to e x p l a i n the f e r r o m a g n e t i s m of 3d t r a n s i t i o n m e t a l s and alloys. In the c a l c u l a t i o n p e r f o r m e d by Jo et al. the c o n t r i b u t i o n to the m a g n e t i c m o m e n t from the o r b i t a l m o m e n t u m of 3d e l e c t r o n is q u a l i t a t i v e l y estimated. W h i l e the f e r r o m a g n e t i s m of Ni metal is n o w u n d e r s t o o d to be due to b o t h the spin and the o r b i t a l m o m e n t u m s of 3d electrons, ferromagnetic Gd is a m a t e r i a l w h e r e the o r b i t a l m o m e n t u m v a n i s h e s due to the o c c u p a t i o n in h i g h l y localized 4f o r b i t a l a c c o r d i n g to the H u n d ' s rule and the m a g n e t i c m o m e n t is a t t r i b u t e d o n l y to the spin m o m e n t u m of the 4f electrons. In a n a l y s i s of the M C D of rare e a r t h m e t a l s the a t o m i c m o d e l is f r e q u e n t l y used bec a u s e of the l o c a l i z a t i o n of 4f states. 4 The c o m p a r i s o n of the MCDs b e t w e e n Ni and Gd will c l a r i f y the d i f f e r e n c e b e t w e e n 3d e l e c t r o n s y s t e m s and h i g h l y l o c a l i z e d 4f e l e c t r o n systems.
In this p a p e r we p r e s e n t the M C D s p e c t r a of Ni and Gd m e t a l s with a suggestion that the Fano n e f f e c t should be cons i d e r e d to u n d e r s t a n d the spectra. The photoabsorption experiments have been carried out in the v i c i n i t y of 3 p - 3 d e x c i t a t i o n for Ni and 4d-4f e x c i t a t i o n for Gd, respectively. The e x p e r i m e n t s have been p e r f o r m e d using an u l t r a h i g h vacuum chamber ins t a l l e d at BL-28 of the P h o t o n F a c t o r y in Japan. The BL-28 is c o n n e c t e d to an elliptic m u l t i p o l e w i g g l e r (EMPW#28) from w h i c h c i r c u l a r l y p o l a r i z e d light is obtained. 12 In F i g u r e 1 the e x p e r i m e n t a l setup is illustrated, n The s y n c h r o t r o n r a d i a t i o n is monochromatized by a c o n s t a n t - d e v i a t i o n grating monochromator to cover photon energies from 5 to 250 eV. The v a c u u m c h a m b e r for MCD m e a s u r e m e n t has an elect r o n - g u n for v a c u u m evaporation, a quartz t h i c k n e s s m o n i t o r for v a c u u m evaporation, a p e r m a n e n t m a g n e t flipper to s w i t c h the m a g n e t i c field on a sample, a p h o t o m u l t i p lier (R595 HAMAMATSU) covered with a m a g n e t i c shield to avoid the i n f l u e n c e of m a g n e t i c field from the m a g n e t flipper, and an a i r - l o c k s y s t e m to i n t r o d u c e s a m p l e s u b s t r a t e s from a t m o s p h e r e into the chamber w i t h o u t b r e a k i n g the u l t r a h i g h vacuum. The b a s i c v a c u u m in the c h a m b e r was b e t t e r t h a n 2x10 "I° torr. A sample was d e p o s i t e d to t h i c k n e s s a b o u t i00 A on a p o l y c a r b o n a t e film subs t r a t e c o o l e d to liquid n i t r o g e n temperature. A f t e r e v a p o r a t i o n the m e a s u r e m e n t was s t a r t e d immediately. The d i r e c t i o n of i n c i d e n t light was normal to the sample s u r f a c e and the t r a n s m i t t e d light t h r o u g h the s a m p l e was d e t e c t e d by the p h o t o m u l t iplier. The s t a t i c m a g n e t i c field a p p l i e d to the s a m p l e by the p e r m a n e n t m a g n e t f l i p p e r is p a r a l l e l to the i n c i d e n t light. The direction of the m a g n e t i c field can be r e v e r s e d at the sample p o s i t i o n in a few s e c o n d s by a linear m o t i o n of the permanent m a g n e t w h i l e the s t r e n g t h s of the each magnetic field are the same (1.2 Tesla). The field m o d u l a t i o n by u s i n g a
VERTICAL CDM ToMONITOR GRATING ¢;Aup/~" SLIT MONOCHROMATOR |I ...... LIGHT 0 TOROI ~ DAL P ~ L i I H E R ~[=, SOURCE o
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Figure 1 S c h e m a t i c d e s c r i p t i o n of the e x p e r i m e n t a l setup for M C D measurement.
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p e r m a n e n t m a g n e t flipper is the first attempt for m e a s u r e m e n t of MCD and makes it p o s s i b l e to m e a s u r e MCD at each p h o t o n e n e r g y in a few seconds. Then any systematic error caused by i n s t a b i l i t y of light source and e s p e c i a l l y the i n f l u e n c e of d e g r a d a t i o n of sample by residual gases is largely reduced, w h i c h is the s u p e r l a t i v e a d v a n t a g e of the p e r m a n e n t m a g n e t flipper. The spurious MCD signal was found to be smaller than 0.01% w h i c h was e s t i m a t e d t h r o u g h m e a s u r e m e n t b e f o r e e v a p o r a t i o n of a sample. The o b s e r v e d spectra for altern a t i v e d i r e c t i o n s of m a g n e t i c field w i t h fixed h a n d e d n e s s of c i r c u l a r p o l a r i z a t i o n c o r r e s p o n d to those for right- and leftc i r c u l a r l y p o l a r i z e d light. More d e t a i l e d d e s c r i p t i o n s of the a p p a r a t u s and the m e t h o d s for m e a s u r e m e n t have been a l r e a d y p r e s e n t e d e l s e w h e r e z4. Figure 2(a) shows the a b s o r p t i o n spec t r a of 3p-3d e x c i t a t i o n r e g i o n of Ni for right- (solid curve) and left- (broken curve) c i r c u l a r l y p o l a r i z e d light w i t h the same scale. The c o n s t a n t back ground is s u b t r a c t e d in o r d e r to e m p h a s i z e the edge structure. These spectra w e r e taken w i t h the energy r e s o l u t i o n better than 70 meV. By the s p i n - o r b i t i n t e r a c t i o n of a 3p hole the top of the 3p-Ed giant r e s o n a n c e is a p p a r e n t l y split into two peaks one of w h i c h lying at lower e n e r g y is due to the 3p3/z-3d t r a n s i t i o n and the other lying at h i g h e r ~ n e r g y is due t o the 3pzn-3d transition. The d i f f e r e n c e in i n t e n s i t y b e t w e e n rightand leftcircularly polarized light is also observed. The MCD w h i c h is d e f i n e d by (pL--pR)/(pL+pR), w h e r e PR and PL are the a b s o r p t i o n c o e f f i c i e n t s for rightand leftcircularly polarized light, r e s p e c t i v e l y is d i s p l a y e d in Figure 2(b). In the lower e n e r g y range MCD w i t h the p o s i t i v e sign was observed. The largest s t r u c t u r e was o b s e r v e d w i t h the n e g a t i v e sign at the p o s i t i o n w h e r e the 3p3,2-3d t r a n s i t i o n occurs, w h i l e at the 3pzSz-3d e x c i t a t i o n e n e r g y the s t r u c t u r e was m u c h smaller and w i t h the p o s i t i v e sign. In e v e n higher e n e r g y region is e x t e n d i n g small MCD w i t h the p o s i t i v e sign. The MCD signal becomes closer to zero as the incident p h o t o n energy becomes far beyond the giant r e s o n a n c e energy. It was confirmed that structures w e r e not due to a s p u r i o u s signal g e n e r a t e d by i n f l u e n c e of m a g n e t i c field on the d e t e c t o r s or magnetic field error on the sample etc., b e c a u s e the sign of all these s t r u c t u r e s was rev e r s e d and the a b s o l u t e v a l u e s was almost c o n s e r v e d w h e n the h a n d e d n e s s of circularly p o l a r i z e d light was changed. The s t r u c t u r e s o b s e r v e d at the 3p 3/2 3d t r a n s i t i o n and at the 3pz/z-3d transition are d i f f e r e n t from those p r e d i c t e d by E r s k i n e and Stern, 6 but are q u a l i t a t i v e l y c o n s i s t e n t w i t h the results c a l c u l a t e d by Y o s h i d a and Jo. 9 This fact s u g g e s t s that in a d d i t i o n to the 3d m a j o r i t y spin the orbital m o m e n t u m of 3d e l e c t r o n contributes to the total m a g n e t i c moment. This s u g g e s t i o n was also p r o p o s e d t h r o u g h the
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RCP
---LCP
Ni
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I
I
6O
I
Photon Energy ( eV )
T5
Figure 2(a) The a b s o r p t i o n spectra of Ni in the vic i n i t y of 3p-3d e x c i t a t i o n for the two a l t e r n a t i v e m a g n e t i c fields. The solid curve c o r r e s p o n d s to the a b s o r p t i o n spectrum for right c i r c u l a r l y p o l a r i z e d light. The b r o k e n curve c o r r e s p o n d s to the abs o r p t i o n s p e c t r u m for left c i r c u l a r l y polarized light.
LCP- RCP Ni LCP+ RCP A
60
Photon Energy (eV)
75
Figure 2(b) MCD of Ni, d e f i n e d by (~L--~R)/(~L+~R), where PR and PL are the a b s o r p t i o n c o e f f i c i e n t s for right- and left- c i r c u l a r l y p o l a r i z e d light, respectively.
e x p e r i m e n t a l result p r e s e n t e d by C.T.Chen. 8 In the c a l c u l a t i o n the d i f f e r e n c e in c o n t r i b u t i o n to MCD from b e t w e e n 3p3/2and 3p~z-3d t r a n s i t i o n s reflects th~ extent o f LS c o u p l i n g of 3d electrons.We have also tried to m e a s u r e the MCD of Gd metal by using the p e r m a n e n t m a g n e t flipper in spite of too small m a g n e t i c field to s a t u r a t e the m a g n e t i c m o m e n t of Gd thin film. The m a g n e t i c field o b t a i n e d at the sample p o s i t i o n is 1.2 Tesla, w h i l e 3.5 Tesla is n e e d e d to s a t u r a t e the magn e t i c m o m e n t of Gd thin film to the normal d i r e c t i o n of the film surface at 120°K. F i g u r e 3(a) shows the a b s o r p t i o n spectra of the 4d-4f t r a n s i t i o n r e g i o n of Gd m e a s u r e d at liquid n i t r o g e n t e m p e r a t u r e for right- (solid curve) and left- (broken
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curve) circularly polarized light. The scale for the a b s o r p t i o n c o e f f i c i e n t s are the same for b o t h spectra. T h e s e s p e c t r a w e r e t a k e n w i t h r e s o l u t i o n b e t t e r than 70 meV. As seen in the F i g u r e the Fano profile and a n o t i c e a b l e d i f f e r e n c e b e t w e e n these two s p e c t r a w e r e c l e a r l y o b s e r v e d in spite of the small m a g n e t i c field. T h o u g h an a p p a r e n t shift of peak p o s i t i o n of the 3d-6f giant resonance between the two s p e c t r a was observed, it is i n t e r p r e t e d in the c o n v e n t i o n a l a t o m i c c a l c u l a t i o n as the c h a n g e s in i n t e n s i t y of each m u l t i p l e t component. The M C D is s h o w n in F i g u r e 3(b), w h e r e the d e f i n i t i o n of MCD is the same as in the case of Ni. The sign of all t h e s e s t r u c t u r e s have been c o n f i r m e d to r e v e r s e w i t h a l m o s t the same a b s o l u t e v a l u e s w h e n the h a n d e d n e s s of c i r c u l a r l y polarized light was changed. But the M C D in the r e g i o n b e f o r e p r e t h r e s h o l d is not reliable, b e c a u s e the d e p o s i t e d film was not t h i c k e n o u g h to take p r e c i s e a b s o r p t i o n coefficient. The o b s e r v e d MCD is m u c h s m a l l e r than c a l c u l a t e d MCD z6 b e c a u s e the m a g n e t i c m o m e n t of s a m p l e m i g h t not be s a t u r a t e d enough. In spite of i n c o m p l e t e m a g n e t i c s a t u r a t i o n we found new p h y s i c a l p r o p e r t y on the MCD in Gd as well as in completely magnetically saturated Ni. T h e r e is an e x t e n d e d MCD in Gd in the e n e r g y r e g i o n h i g h e r than the 3 d - 6 f g i a n t resonance, w h i l e t h e r e is also e x t e n d e d M C D in Ni in the e n e r g y r e g i o n lower than the 3p-6d giant resonance. The above e x t e n d e d MCD in Ni dose not a p p e a r in b o t h the r e s u l t s of the c a l c u l a t i o n by E r s k i n e and S t e r n 6, and that by Y o s h i d a and Jo 9. In t h e s e c a l c u l a t i o n s only the 3p-3d e x c i t a t i o n was t a k e n into account. H o w e v e r in the e n e r g y r e g i o n of 3 p - 3 d t r a n s i t i o n for Ni and 4d-4f t r a n s i t i o n for Gd t h e r e are 3 d - 6 f and 4 f - c o n t l n u o u s level transitions, respectively. Here we should take into a c c o u n t the Fano e f f e c t for the i n t e r f e r e n c e of a d i s c r e t e e x c i t a t e d s t a t e w i t h a c o n t i n u u m t h r o u g h super C o s t e r K r o n i g transition. The Fano e f f e c t c a u s e s the b r o a d and a s y m m e t r i c p r o f i l e s of abs o r p t i o n s p e c t r u m w h i c h give rise to the e x t e n d e d MCD. For Ni the a b s o r p t i o n due to 3p3/2-3d t r a n s i t i o n by left c i r c u l a r l y p o l a r i z e d light is s m a l l e r than by right c i r c u l a r l y p o l a r i z e d light. Then by the Fano e f f e c t in the energy sufficiently below the t h r e s h o l d the a b s o r p t i o n c o e f f i c i e n t for left c i r c u l a r l y p o l a r i z e d light b e c o m e s l a r g e r t h a n for right c i r c u l a r l y p o l a r i z e d light. This gives the c h a n g e in the sign of M C D and c o n s e q u e n t l y the e x t e n d e d M C D w i t h the o p p o s i t e sign to the MCD due to 3P3/z-3d t r a n s i t i o n c o u l d be observed. E s p e c i a l l y for Ni in a d d i t i o n to the Fano e f f e c t t h e r e is a n o t h e r p o s s i b i l i t y w h i c h c a u s e s the e x t e n d e d MCD. From the s t r u c t u r e of the M C D o b s e r v e d at the 3p-6d giant resonance, it is s u g g e s t e d that the o r b i t a l m o m e n t u m of 3d e l e c t r o n c o n t r i b utes to the m a g n e t i c m o m e n t to the same
- - RCP - - - LCP
Gd
~o
I
I
120
I
I
I
I
I
I
I
Photon Energy ( eV )
170
Figure 3 ( a ) The a b s o r p t i o n s p e c t r a of Gd in the vic i n i t y of 4d-4f t r a n s i t i o n for the two alternative magnetic fields. The solid c u r v e c o r r e s p o n d s to the a b s o r p t i o n spectrum for right c i r c u l a r l y p o l a r i z e d light. The b r o k e n c u r v e c o r r e s p o n d s to the abs o r p t i o n s p e c t r u m for left c i r c u l a r l y pol a r i z e d light.
LCP - RCP Gd LCP ~ RCP
A
c=O
,
,
-15
120
Photon Energy (eV)
Figure 3(b) MCD o f Gd, d e f i n e d by the same formula t h e MCD o f N i .
170
for
d i r e c t i o n of the spin momentum. This symm e t r y b r e a k i n g also gives rise to the ext e n d e d MCD w i t h the p o s i t i v e sign t h r o u g h 3d-6f transition. H o w e v e r in Gd the e x t e n d e d MCD due to 4f-continuous level t r a n s i t i o n does not o c c u r b e c a u s e t h e r e is not s y m m e t r y breaking of the orbital m o m e n t u m of 4f electrons. V e r y r e c e n t l y O g a s a w a r a and Kotani have c a l c u l a t e d the MCD for 4d-4f transition in Gd i n c l u d i n g the Fano effect, z6 T h o u g h the o b s e r v e d MCD is m u c h s m a l l e r than c a l c u l a t e d MCD, the r e l a t i v e shape of the o b s e r v e d MCD is v e r y s i m i l a r to the c a l c u l a t e d one b o t h in the p r e t h r e s h o l d r e g i o n and the h i g h e r e n e r g y region. In summary, we have p e r f o r m e d experim e n t s On MCD of f e r r o m a g n e t i c Ni and Gd in the v i c i n i t y of the 3p-3d t r a n s i t i o n and the 4 d - 4 f transition, respectively. The
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MAGNETIC CIRCULAR DICHROISM
results on the MCD due to 3p core electron excitation in Ni are consistent with the experimental results on the MCD due to 2p excitation previously presented by C.T.Chen. From these results it is suggested that LS coupling of 3d electrons in Ni
45
exists. In addition to the MCD due to 3p core electron excitation an extended MCD and the anomalous sign change of MCD is observed in both Ni and Gd. The extended MCD is considered to be due to the Fano effect.
References
i. G.SchQtz,W.Wagner,W.Wilhelm, P.Kienle, R.Zeller,R.Frahm, and G.Materlik, Phys. Rev. Lett.58,737(1987) 2. T.Jo, and S.Imada,J.Phys.Soc. Jap.58, 1922(1989) 3. B.T.Thole,R.D.Cowan,G.A.Sawatzky, J.Fink, and J.C.Fuggle,Phys.Rev.B31, 6856(1985) 4. S.Imada, and T.Jo,J.Phys. Soc. Jpn 59, 3358(1990) 5. C.Stoner,Proc.Roy.Soc. Lond.A165,372 (1938) 6. J.L.Ersklne, and E.A.Stern,Phys.Rev. B12,5016(1975) 7. S. J.Oh, J.W.Allen, I.Lindau, and J.C.Mikkelsen, Jr.,Phys.Rev.B26,4845(1982) 8. C.T.Chen,F.Sette,Y.Ma, and S.Modesti, Phys.Rev.B42,7262(1990)
9. A.Yoshida, and T.Jo,J.Phys.Soc. Jap.60, 2098(1991) i0. T.Jo, and G.A.Sawatzky, Phys.Rev.B43, 8771(1991) ii. U.Fano, Phys.Rev.124,1866(196i) 12. S.Yamamoto,T.Shioya,S.Sasaki, and H.Kitamura,Rev.Sci. Instrum.60,1834 (1989) 13. Y.Kagoshima,S.Muto,T.Miyahara,T.Kolde, S.Yamamoto, and H.Kitamura,Rev.Sci. Instrum. to be published 14. S.Muto,Y.Kagoshlma, and T.Miyahara, Rev.Sci. Instrum. to be published. 15. F.C.Brown,C.GMhwiller, and A.B.Kunz, Solid State Commun.9,487(1971) 16. H.Ogasawara and A.Kotani, private communication
Solid State Communications,Vol. 83, NO. 1, pp. 41--45, 1992. Printed in Great Britain.
0038-1098/9255.00 + .00 Pergamon Press Ltd
INFLUENCE OF FANO EFFECT ON MAGNETIC CIRCULAR DICHROISM DUE TO CORE ELECTRON TRANSITION IN Ni AND Gd METALS S.Muto Department of Synchrotrnn Radiation Science The Graduate University for Advanced Studies Oho 1-1, Tsukuba-shi, Ibaraki 305, JAPAN Y.Kagoshima, T.Miyahara, S.Yamamoto, and H.Kitamura Photon Factory, National Laboratory for High Energy Physics Oho 1-1, Tsukuba-shi, Ibaraki 305, JAPAN (Received on 19 February 1992 by H.Kamimura) M a g n e t i c c i r c u l a r d i c h r o i s m s (MCD) of f e r r o m a g n e t i c Ni and Gd thin films h a v e b e e n m e a s u r e d in the v i c i n i t y of the 3p-3d e x c i t a t i o n for Ni and the 4d-4f e x c i t a t i o n for Gd, respectively. The p h o t o a b s o r p t i o n e x p e r i m e n t s w e r e p e r f o r m e d by u s i n g an u l t r a h i g h v a c u u m c h a m b e r i n s t a l l e d at BL-28 of the P h o t o n F a c t o r y in Japan. Through these e x p e r i m e n t s a p p a r e n t MCD signals due to core e l e c t r o n e x c i t a t i o n s w e r e observed. In a d d i t i o n to the MCD due to core e l e c t r o n excitations, an a n o m a l o u s feature was found, w h i c h extends far b e y o n d the r e g i o n of core e l e c t r o n e x c i t a t i o n both for Ni and for Gd.
materials, e s p e c i a l l y f e r r o m a g n e t i c materials. In more detail the symmetry b r e a k i n g of orbital and spin m o m e n t u m s of the initial state of atoms are s e n s i t i v e l y r e f l e c t e d to the MCD, w h i c h has b e e n p r e d i c t e d by recent t h e o r i e s 4. In the r e s e a r c h on ferromagnetism, Ni has been one of the most a t t r a c t i v e materials to i n v e s t i g a t e the 3d e l e c t r o n system. The 3d e l e c t r o n s in t r a n s i t i o n metals have b e e n e x t e n s i v e l y u n d e r s t o o d to be itinerant, and its m a g n e t i c p r o p e r t i e s such as fractional number of m a g n e t i c moment at an atom has been s u c c e s s f u l l y e x p l a i n e d by the Stoner model s (itinerant model). From the v i e w point of a band theory Erskine and Stern p r e d i c t e d MCD of Ni due to 3p-3d transition. R e c e n t l y h o w e v e r an atomic model has been p r e s e n t e d in o r d e r to interpret the experimental results obtained by h i g h e n e r g y s p e c t r o s c o p i e s w i t h respect to 3d e l e c t r o n s in t r a n s i t i o n metals and their compounds, v C . T . C h e n et al. c a r r i e d out an e x p e r i m e n t on MCD of Ni at the 2p-3d e x c i t a t i o n region by means of total y i e l d m e a s u r e m e n t and p r e s e n t e d an i m p o r t a n t p h y s i c a l s u g g e s t i o n that the 3d band is m o d u l a t e d by spin-orbit i n t e r a c t i o n of 3d electrons. 8 The 3p(2p) core hole level is split into 3p(2p)z/2 and 3p(2p)3/2 core hole levels by s p i n - o r b l t i n t e r a c t i o n of 3p(2p)
R e c e n t l y the m a g n e t i c c i r c u l a r dic h r o i s m due to core e l e c t r o n e x c i t a t i o n has a t t r a c t e d m u c h a t t e n t i o n in unders t a n d i n g more c l e a r l y in b o t h t h e o r e t i c a l and e x p e r i m e n t a l aspects the e l e c t r o n i c s t r u c t u r e s of f e r r o m a g n e t i c m a t e r i a l s such as t r a n s i t i o n metals, rare e a r t h m e t a l s 1.2 and their compounds. The s y n c h r o t r o n sources w h i c h can emit high e n e r g y circ u l a r l y p o l a r i z e d light w i t h c o n t i n u o u s s p e c t r u m has made it p o s s i b l e to p e r f o r m the m e a s u r e m e n t s of m a g n e t i c c i r c u l a r dic h r o i s m (MCD) due to core e l e c t r o n excitations. Core e l e c t r o n e x c i t a t i o n s sensit i v e l y reflect the l o c a l i z e d e l e c t r o n i c s t r u c t u r e t h r o u g h the dipole s e l e c t i o n rule w h i c h r e s t r i c t s optical excitations. Especially the d i f f e r e n c e due to the d i p o l e s e l e c t i o n rule b e t w e e n right- and l e f t - c i r c u l a r l y p o l a r i z e d lights classifies e l e c t r o n e x c i t a t i o n s with respect to the m a g n e t i c q u a n t u m number under the c o n d i t i o n Am=+l,-I respectively. The MCD due to core e l e c t r o n e x c i t a t i o n s is det e c t e d w i t h core e l e c t r o n s p e c t r o s c o p y in c o n j u n c t i o n w i t h right- and left- circularly p o l a r i z e d light and w i t h an e x t e r n a l m a g n e t i c field p a r a l l e l to the p r o p a g a t i o n d i r e c t i o n of the light. We can d i r e c t l y get m u c h i n f o r m a t i o n to d e t e r m i n e the localized magnetic structure of cons t i t u e n t atoms in m a g n e t i c a l l y ordered 41
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MAGNETIC CIRCULAR DICHROISM
core hole, w h i c h d i s t i n g u i s h e s the contrib u t i o n s to M C D due to 3 p ( 2 p ) z n - 3 d transition and 3p(2p)~/2-3d transition. If it is a s s u m e d that the s y m m e t r y of 3d o r b i t a l m o m e n t u m is not b r o k e n and only the symm e t r y of spin m o m e n t u m is broken, the cont r i b u t i o n s to M C D from 3 p ( 2 p ) z n - 3 d tran s i t i o n and 3 p ( 2 p ) ~ 2 - 3 d t r a n s i t i o n are the same i n t e g r a t e d v a l u e s but w i t h d i f f e r e n t • 9 slgns , w h i c h c o r r e s p o n d s to the case of the p r e d i c t e d MCD by E r s k i n e and Stern.6 H o w e v e r the c o n t r i b u t i o n to the o b s e r v e d M C D from 2P3/2-3d t r a n s i t i o n is l a r g e r than the one from 2p1~-3d transition. This experimental result suggests that the o r b i t a l m o m e n t u m of 3d e l e c t r o n s dose not v a n i s h and c o n t r i b u t e s to the m a g n e t i c m o m e n t p a r a l l e l to t h e i r spin momentum. The MCD s p e c t r a c a l c u l a t e d by Jo et al. for Ni due to 2p-3d t r a n s i t i o n s b a s e d on an a t o m i c p i c t u r e w i t h 3d c o n f i g u r a t i o n i n t e r a c t i o n s i n c l u d e d e x p l a i n s the e x p e r i m e n t a l r e s u l t s m o r e s u c c e s s f u l l y than the Erskine's model. I° This fact directly s u g g e s t s that the e l e c t r o n i c state of 3d e l e c t r o n in t r a n s i t i o n m e t a l s can be well d e s c r i b e d by the a t o m i c m o d e l in c o n t r a s t w i t h the i t i n e r a n t m o d e l s w h i c h o n l y take the spin m o m e n t u m s into a c c o u n t to e x p l a i n the f e r r o m a g n e t i s m of 3d t r a n s i t i o n m e t a l s and alloys. In the c a l c u l a t i o n p e r f o r m e d by Jo et al. the c o n t r i b u t i o n to the m a g n e t i c m o m e n t from the o r b i t a l m o m e n t u m of 3d e l e c t r o n is q u a l i t a t i v e l y estimated. W h i l e the f e r r o m a g n e t i s m of Ni metal is n o w u n d e r s t o o d to be due to b o t h the spin and the o r b i t a l m o m e n t u m s of 3d electrons, ferromagnetic Gd is a m a t e r i a l w h e r e the o r b i t a l m o m e n t u m v a n i s h e s due to the o c c u p a t i o n in h i g h l y localized 4f o r b i t a l a c c o r d i n g to the H u n d ' s rule and the m a g n e t i c m o m e n t is a t t r i b u t e d o n l y to the spin m o m e n t u m of the 4f electrons. In a n a l y s i s of the M C D of rare e a r t h m e t a l s the a t o m i c m o d e l is f r e q u e n t l y used bec a u s e of the l o c a l i z a t i o n of 4f states. 4 The c o m p a r i s o n of the MCDs b e t w e e n Ni and Gd will c l a r i f y the d i f f e r e n c e b e t w e e n 3d e l e c t r o n s y s t e m s and h i g h l y l o c a l i z e d 4f e l e c t r o n systems.
In this p a p e r we p r e s e n t the M C D s p e c t r a of Ni and Gd m e t a l s with a suggestion that the Fano n e f f e c t should be cons i d e r e d to u n d e r s t a n d the spectra. The photoabsorption experiments have been carried out in the v i c i n i t y of 3 p - 3 d e x c i t a t i o n for Ni and 4d-4f e x c i t a t i o n for Gd, respectively. The e x p e r i m e n t s have been p e r f o r m e d using an u l t r a h i g h vacuum chamber ins t a l l e d at BL-28 of the P h o t o n F a c t o r y in Japan. The BL-28 is c o n n e c t e d to an elliptic m u l t i p o l e w i g g l e r (EMPW#28) from w h i c h c i r c u l a r l y p o l a r i z e d light is obtained. 12 In F i g u r e 1 the e x p e r i m e n t a l setup is illustrated, n The s y n c h r o t r o n r a d i a t i o n is monochromatized by a c o n s t a n t - d e v i a t i o n grating monochromator to cover photon energies from 5 to 250 eV. The v a c u u m c h a m b e r for MCD m e a s u r e m e n t has an elect r o n - g u n for v a c u u m evaporation, a quartz t h i c k n e s s m o n i t o r for v a c u u m evaporation, a p e r m a n e n t m a g n e t flipper to s w i t c h the m a g n e t i c field on a sample, a p h o t o m u l t i p lier (R595 HAMAMATSU) covered with a m a g n e t i c shield to avoid the i n f l u e n c e of m a g n e t i c field from the m a g n e t flipper, and an a i r - l o c k s y s t e m to i n t r o d u c e s a m p l e s u b s t r a t e s from a t m o s p h e r e into the chamber w i t h o u t b r e a k i n g the u l t r a h i g h vacuum. The b a s i c v a c u u m in the c h a m b e r was b e t t e r t h a n 2x10 "I° torr. A sample was d e p o s i t e d to t h i c k n e s s a b o u t i00 A on a p o l y c a r b o n a t e film subs t r a t e c o o l e d to liquid n i t r o g e n temperature. A f t e r e v a p o r a t i o n the m e a s u r e m e n t was s t a r t e d immediately. The d i r e c t i o n of i n c i d e n t light was normal to the sample s u r f a c e and the t r a n s m i t t e d light t h r o u g h the s a m p l e was d e t e c t e d by the p h o t o m u l t iplier. The s t a t i c m a g n e t i c field a p p l i e d to the s a m p l e by the p e r m a n e n t m a g n e t f l i p p e r is p a r a l l e l to the i n c i d e n t light. The direction of the m a g n e t i c field can be r e v e r s e d at the sample p o s i t i o n in a few s e c o n d s by a linear m o t i o n of the permanent m a g n e t w h i l e the s t r e n g t h s of the each magnetic field are the same (1.2 Tesla). The field m o d u l a t i o n by u s i n g a
VERTICAL CDM ToMONITOR GRATING ¢;Aup/~" SLIT MONOCHROMATOR |I ...... LIGHT 0 TOROI ~ DAL P ~ L i I H E R ~[=, SOURCE o
HORIZONTAL SLIT
HORIZONTAL DIAPHRAGM
Vol. 83, No. 1
POSTFOCUSING I PERMAN~-~T MIRRORL MAGNET='"
GPIB~
GPIB
MICROCOMPUTER
Figure 1 S c h e m a t i c d e s c r i p t i o n of the e x p e r i m e n t a l setup for M C D measurement.
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MAGNETIC CIRCULAR DICHROISM
p e r m a n e n t m a g n e t flipper is the first attempt for m e a s u r e m e n t of MCD and makes it p o s s i b l e to m e a s u r e MCD at each p h o t o n e n e r g y in a few seconds. Then any systematic error caused by i n s t a b i l i t y of light source and e s p e c i a l l y the i n f l u e n c e of d e g r a d a t i o n of sample by residual gases is largely reduced, w h i c h is the s u p e r l a t i v e a d v a n t a g e of the p e r m a n e n t m a g n e t flipper. The spurious MCD signal was found to be smaller than 0.01% w h i c h was e s t i m a t e d t h r o u g h m e a s u r e m e n t b e f o r e e v a p o r a t i o n of a sample. The o b s e r v e d spectra for altern a t i v e d i r e c t i o n s of m a g n e t i c field w i t h fixed h a n d e d n e s s of c i r c u l a r p o l a r i z a t i o n c o r r e s p o n d to those for right- and leftc i r c u l a r l y p o l a r i z e d light. More d e t a i l e d d e s c r i p t i o n s of the a p p a r a t u s and the m e t h o d s for m e a s u r e m e n t have been a l r e a d y p r e s e n t e d e l s e w h e r e z4. Figure 2(a) shows the a b s o r p t i o n spec t r a of 3p-3d e x c i t a t i o n r e g i o n of Ni for right- (solid curve) and left- (broken curve) c i r c u l a r l y p o l a r i z e d light w i t h the same scale. The c o n s t a n t back ground is s u b t r a c t e d in o r d e r to e m p h a s i z e the edge structure. These spectra w e r e taken w i t h the energy r e s o l u t i o n better than 70 meV. By the s p i n - o r b i t i n t e r a c t i o n of a 3p hole the top of the 3p-Ed giant r e s o n a n c e is a p p a r e n t l y split into two peaks one of w h i c h lying at lower e n e r g y is due to the 3p3/z-3d t r a n s i t i o n and the other lying at h i g h e r ~ n e r g y is due t o the 3pzn-3d transition. The d i f f e r e n c e in i n t e n s i t y b e t w e e n rightand leftcircularly polarized light is also observed. The MCD w h i c h is d e f i n e d by (pL--pR)/(pL+pR), w h e r e PR and PL are the a b s o r p t i o n c o e f f i c i e n t s for rightand leftcircularly polarized light, r e s p e c t i v e l y is d i s p l a y e d in Figure 2(b). In the lower e n e r g y range MCD w i t h the p o s i t i v e sign was observed. The largest s t r u c t u r e was o b s e r v e d w i t h the n e g a t i v e sign at the p o s i t i o n w h e r e the 3p3,2-3d t r a n s i t i o n occurs, w h i l e at the 3pzSz-3d e x c i t a t i o n e n e r g y the s t r u c t u r e was m u c h smaller and w i t h the p o s i t i v e sign. In e v e n higher e n e r g y region is e x t e n d i n g small MCD w i t h the p o s i t i v e sign. The MCD signal becomes closer to zero as the incident p h o t o n energy becomes far beyond the giant r e s o n a n c e energy. It was confirmed that structures w e r e not due to a s p u r i o u s signal g e n e r a t e d by i n f l u e n c e of m a g n e t i c field on the d e t e c t o r s or magnetic field error on the sample etc., b e c a u s e the sign of all these s t r u c t u r e s was rev e r s e d and the a b s o l u t e v a l u e s was almost c o n s e r v e d w h e n the h a n d e d n e s s of circularly p o l a r i z e d light was changed. The s t r u c t u r e s o b s e r v e d at the 3p 3/2 3d t r a n s i t i o n and at the 3pz/z-3d transition are d i f f e r e n t from those p r e d i c t e d by E r s k i n e and Stern, 6 but are q u a l i t a t i v e l y c o n s i s t e n t w i t h the results c a l c u l a t e d by Y o s h i d a and Jo. 9 This fact s u g g e s t s that in a d d i t i o n to the 3d m a j o r i t y spin the orbital m o m e n t u m of 3d e l e c t r o n contributes to the total m a g n e t i c moment. This s u g g e s t i o n was also p r o p o s e d t h r o u g h the
--
RCP
---LCP
Ni
-
-
I
I
6O
I
Photon Energy ( eV )
T5
Figure 2(a) The a b s o r p t i o n spectra of Ni in the vic i n i t y of 3p-3d e x c i t a t i o n for the two a l t e r n a t i v e m a g n e t i c fields. The solid curve c o r r e s p o n d s to the a b s o r p t i o n spectrum for right c i r c u l a r l y p o l a r i z e d light. The b r o k e n curve c o r r e s p o n d s to the abs o r p t i o n s p e c t r u m for left c i r c u l a r l y polarized light.
LCP- RCP Ni LCP+ RCP A
60
Photon Energy (eV)
75
Figure 2(b) MCD of Ni, d e f i n e d by (~L--~R)/(~L+~R), where PR and PL are the a b s o r p t i o n c o e f f i c i e n t s for right- and left- c i r c u l a r l y p o l a r i z e d light, respectively.
e x p e r i m e n t a l result p r e s e n t e d by C.T.Chen. 8 In the c a l c u l a t i o n the d i f f e r e n c e in c o n t r i b u t i o n to MCD from b e t w e e n 3p3/2and 3p~z-3d t r a n s i t i o n s reflects th~ extent o f LS c o u p l i n g of 3d electrons.We have also tried to m e a s u r e the MCD of Gd metal by using the p e r m a n e n t m a g n e t flipper in spite of too small m a g n e t i c field to s a t u r a t e the m a g n e t i c m o m e n t of Gd thin film. The m a g n e t i c field o b t a i n e d at the sample p o s i t i o n is 1.2 Tesla, w h i l e 3.5 Tesla is n e e d e d to s a t u r a t e the magn e t i c m o m e n t of Gd thin film to the normal d i r e c t i o n of the film surface at 120°K. F i g u r e 3(a) shows the a b s o r p t i o n spectra of the 4d-4f t r a n s i t i o n r e g i o n of Gd m e a s u r e d at liquid n i t r o g e n t e m p e r a t u r e for right- (solid curve) and left- (broken
44
Vol. 83, No. 1
M A G N E T I C C I R C U L A R DICHROISM
curve) circularly polarized light. The scale for the a b s o r p t i o n c o e f f i c i e n t s are the same for b o t h spectra. T h e s e s p e c t r a w e r e t a k e n w i t h r e s o l u t i o n b e t t e r than 70 meV. As seen in the F i g u r e the Fano profile and a n o t i c e a b l e d i f f e r e n c e b e t w e e n these two s p e c t r a w e r e c l e a r l y o b s e r v e d in spite of the small m a g n e t i c field. T h o u g h an a p p a r e n t shift of peak p o s i t i o n of the 3d-6f giant resonance between the two s p e c t r a was observed, it is i n t e r p r e t e d in the c o n v e n t i o n a l a t o m i c c a l c u l a t i o n as the c h a n g e s in i n t e n s i t y of each m u l t i p l e t component. The M C D is s h o w n in F i g u r e 3(b), w h e r e the d e f i n i t i o n of MCD is the same as in the case of Ni. The sign of all t h e s e s t r u c t u r e s have been c o n f i r m e d to r e v e r s e w i t h a l m o s t the same a b s o l u t e v a l u e s w h e n the h a n d e d n e s s of c i r c u l a r l y polarized light was changed. But the M C D in the r e g i o n b e f o r e p r e t h r e s h o l d is not reliable, b e c a u s e the d e p o s i t e d film was not t h i c k e n o u g h to take p r e c i s e a b s o r p t i o n coefficient. The o b s e r v e d MCD is m u c h s m a l l e r than c a l c u l a t e d MCD z6 b e c a u s e the m a g n e t i c m o m e n t of s a m p l e m i g h t not be s a t u r a t e d enough. In spite of i n c o m p l e t e m a g n e t i c s a t u r a t i o n we found new p h y s i c a l p r o p e r t y on the MCD in Gd as well as in completely magnetically saturated Ni. T h e r e is an e x t e n d e d MCD in Gd in the e n e r g y r e g i o n h i g h e r than the 3 d - 6 f g i a n t resonance, w h i l e t h e r e is also e x t e n d e d M C D in Ni in the e n e r g y r e g i o n lower than the 3p-6d giant resonance. The above e x t e n d e d MCD in Ni dose not a p p e a r in b o t h the r e s u l t s of the c a l c u l a t i o n by E r s k i n e and S t e r n 6, and that by Y o s h i d a and Jo 9. In t h e s e c a l c u l a t i o n s only the 3p-3d e x c i t a t i o n was t a k e n into account. H o w e v e r in the e n e r g y r e g i o n of 3 p - 3 d t r a n s i t i o n for Ni and 4d-4f t r a n s i t i o n for Gd t h e r e are 3 d - 6 f and 4 f - c o n t l n u o u s level transitions, respectively. Here we should take into a c c o u n t the Fano e f f e c t for the i n t e r f e r e n c e of a d i s c r e t e e x c i t a t e d s t a t e w i t h a c o n t i n u u m t h r o u g h super C o s t e r K r o n i g transition. The Fano e f f e c t c a u s e s the b r o a d and a s y m m e t r i c p r o f i l e s of abs o r p t i o n s p e c t r u m w h i c h give rise to the e x t e n d e d MCD. For Ni the a b s o r p t i o n due to 3p3/2-3d t r a n s i t i o n by left c i r c u l a r l y p o l a r i z e d light is s m a l l e r than by right c i r c u l a r l y p o l a r i z e d light. Then by the Fano e f f e c t in the energy sufficiently below the t h r e s h o l d the a b s o r p t i o n c o e f f i c i e n t for left c i r c u l a r l y p o l a r i z e d light b e c o m e s l a r g e r t h a n for right c i r c u l a r l y p o l a r i z e d light. This gives the c h a n g e in the sign of M C D and c o n s e q u e n t l y the e x t e n d e d M C D w i t h the o p p o s i t e sign to the MCD due to 3P3/z-3d t r a n s i t i o n c o u l d be observed. E s p e c i a l l y for Ni in a d d i t i o n to the Fano e f f e c t t h e r e is a n o t h e r p o s s i b i l i t y w h i c h c a u s e s the e x t e n d e d MCD. From the s t r u c t u r e of the M C D o b s e r v e d at the 3p-6d giant resonance, it is s u g g e s t e d that the o r b i t a l m o m e n t u m of 3d e l e c t r o n c o n t r i b utes to the m a g n e t i c m o m e n t to the same
- - RCP - - - LCP
Gd
~o
I
I
120
I
I
I
I
I
I
I
Photon Energy ( eV )
170
Figure 3 ( a ) The a b s o r p t i o n s p e c t r a of Gd in the vic i n i t y of 4d-4f t r a n s i t i o n for the two alternative magnetic fields. The solid c u r v e c o r r e s p o n d s to the a b s o r p t i o n spectrum for right c i r c u l a r l y p o l a r i z e d light. The b r o k e n c u r v e c o r r e s p o n d s to the abs o r p t i o n s p e c t r u m for left c i r c u l a r l y pol a r i z e d light.
LCP - RCP Gd LCP ~ RCP
A
c=O
,
,
-15
120
Photon Energy (eV)
Figure 3(b) MCD o f Gd, d e f i n e d by the same formula t h e MCD o f N i .
170
for
d i r e c t i o n of the spin momentum. This symm e t r y b r e a k i n g also gives rise to the ext e n d e d MCD w i t h the p o s i t i v e sign t h r o u g h 3d-6f transition. H o w e v e r in Gd the e x t e n d e d MCD due to 4f-continuous level t r a n s i t i o n does not o c c u r b e c a u s e t h e r e is not s y m m e t r y breaking of the orbital m o m e n t u m of 4f electrons. V e r y r e c e n t l y O g a s a w a r a and Kotani have c a l c u l a t e d the MCD for 4d-4f transition in Gd i n c l u d i n g the Fano effect, z6 T h o u g h the o b s e r v e d MCD is m u c h s m a l l e r than c a l c u l a t e d MCD, the r e l a t i v e shape of the o b s e r v e d MCD is v e r y s i m i l a r to the c a l c u l a t e d one b o t h in the p r e t h r e s h o l d r e g i o n and the h i g h e r e n e r g y region. In summary, we have p e r f o r m e d experim e n t s On MCD of f e r r o m a g n e t i c Ni and Gd in the v i c i n i t y of the 3p-3d t r a n s i t i o n and the 4 d - 4 f transition, respectively. The
Vol. 83, No. I
MAGNETIC CIRCULAR DICHROISM
results on the MCD due to 3p core electron excitation in Ni are consistent with the experimental results on the MCD due to 2p excitation previously presented by C.T.Chen. From these results it is suggested that LS coupling of 3d electrons in Ni
45
exists. In addition to the MCD due to 3p core electron excitation an extended MCD and the anomalous sign change of MCD is observed in both Ni and Gd. The extended MCD is considered to be due to the Fano effect.
References
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9. A.Yoshida, and T.Jo,J.Phys.Soc. Jap.60, 2098(1991) i0. T.Jo, and G.A.Sawatzky, Phys.Rev.B43, 8771(1991) ii. U.Fano, Phys.Rev.124,1866(196i) 12. S.Yamamoto,T.Shioya,S.Sasaki, and H.Kitamura,Rev.Sci. Instrum.60,1834 (1989) 13. Y.Kagoshima,S.Muto,T.Miyahara,T.Kolde, S.Yamamoto, and H.Kitamura,Rev.Sci. Instrum. to be published 14. S.Muto,Y.Kagoshlma, and T.Miyahara, Rev.Sci. Instrum. to be published. 15. F.C.Brown,C.GMhwiller, and A.B.Kunz, Solid State Commun.9,487(1971) 16. H.Ogasawara and A.Kotani, private communication