S o f t X - r a y 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 o f c ( 2 × 2 ) O r d e r e d S u r f a c e A l l o y S o f t X - r a y 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 o f c ( 2 × 2 ) C u M n O r d e r e d S u r f a c e A l l o y Fi g. 1. Su rf ac e st ru ct ur e vi ew ed fr om th e to p (a ) an d fr om th e si de (b ) of c( 2 × 2) CuMn / Cu (001) 2-D ordered surface alloy as determined by LEED I-V analysis [3]. F i g . 1 Table I Crystalline Mn is known as an antiferromagnetic o r a p a r a m a g n e t i c m a t e r i a l w i t h q u i t e a s m a l l m a g n e t i c m o m e n t . H o w e v e r , o n c e M n a t o m s crystallize with non-magnetic elements, such as Sb a n d e v e n a n o x y g e n a t o m , f e r r o m a g n e t i s m appears as is found in MnSb or in La 1-x Sr x MnO 3 [1]. Very small amount s of Mn in a semiconductor can a l s o d e r i v e a d e g r e e o f f e r r o m a g n e t i s m , a s discovered in (Ga, Mn )As [2]. One may extend t his idea to a low - dimensional case. An example is a n Mn - based alloy fabricated on noble metal surfaces. W u t t i g e t a l . d i s c o v e r e d t h a t M n b a s e d t w o - dimensional (2-D) ordered alloy can be formed on a c l e a n C u ( 0 0 1 ) s u r f a c e a t a c o v e r a g e o f 0 . 5 M L , w h e r e M n a n d s u b s t r a t e a t o m s a r e a l t e r n a t i v e l y pl ac ed fo rm in g a c( 2 × 2) “c he cke rb oa rd ” str uc tu re , a s s h o w n i n [ 3 ] . A l o w e n e r g y e l e c t r o n diffraction ( LEED ) I-V measurement shows that the c ( 2 × 2 ) C u M n s u r f a c e a l l o y h a s a p r o n o u n c e d corrugation in which Mn atoms in the first layer are displaced outwards by δ z = 0.30 ± 0.02 Å, which is 17% with respect to the atomic distance in the bulk [ 3 ] . S u c h a r e m a r k a b l e a t o m i c d i s p l a c e m e n t i s s u r p r i s i n g b e c a u s e t h e o t h e r c ( 2 × 2 ) o r d e r e d surface alloy systems with non-magnetic elements, n a m e l y c ( 2 × 2 ) C u A u / C u ( 0 0 1 ) a n d c ( 2 × 2 ) Cu Pd / Cu (0 01 ) sh ow on ly sm al l re la xa ti on s of 6% and 1%, respectively, instead of the larger atomic rad ii of Au (1.442 Å) and Pd (1.375 Å) compared to that of Mn (1.365 Å) as listed in . A theoretical band structure calculation predicts t h a t t h e m o s t s t a b l e m a g n e t i c s t a t e f o r c ( 2 × 2 ) C u M n o r d e r e d s u r f a c e a l l o y i s a f e r r o m a g n e t i c structure in the ground state [3]. The theory also ex pl ai ns th at th e ob se rv ed la rg e re la xa ti on of th e M n a t o m s a r e d e r i v e d f r o m t h e m a g n e t i s m [ 3 ] . H o w e v e r , n o e x p e r i m e n t a l e v i d e n c e o f t h e ferr omag neti c stat e of this surf ace allo y has been o b t a i n e d s o f a r . T h e l a c k o f t h e e x p e r i m e n t a l e v i d e n c e p o s s i b l y c o m e s f r o m t h e l o w e r f e r r o m a g n e t i c t r a n s i t i o n t e m p e r a t u r e ( C u r i e temperature) as usually found in ultra-thin films with 48 ( a ) T o p V i e w 1 s t l a y e r M n C u C u 2 n d l a y e r ( b ) S i d e V i e w δ z Figure 2 F i g . 2 . M n 2 p X A S ( u p p e r ) a n d X M C D ( l o w e r ) s p e c t r a o f 0 . 4 6 M L M n u l t r a t h i n f i l m o n Ni (110). The inset shows the Ni 2p XAS (upper) and XMCD (lower) spectra of the same system. The ferromagnetic coupling between the Mn layer and the Ni substrate is clear. (see text) Table I. Atomic relaxation values determined by the LEED I-V analysis for several c(2 × 2) 2-D surface alloys [3]. Fig. 3 a c o u p l e o f 3 d t r a n s i t i o n m e t a l m o n o l a y e r s . F o r this reason, we have tried to observe the soft X-ray magnetic circular dichroism ( XMCD ) spectra in the Mn 2 p core absorption region at low temperature at beamline BL25SU . Soft X-ray magnetic circular dichroism ( XMCD ) in core level absorption spectrum provides us with u s e f u l i n f o r m a t i o n r e l a t e d t o e l e c t r o n i c s t a t e s o f magnetic materials. It is noted that the XMCD not only gives us element - specific magnetic moments , b u t a l s o t e l l s u s h o w m u c h a n o r b i t a l ( a s p i n ) m a g n e t i c mo me nt con tr ib ut es to t he ir tot al mag ne ti c mo me nt s. shows an example of a submonolayer Mn grown on Ni (110) substrate [4]. One finds a clear XMCD of Mn and Ni 2 p edges , and the polarities of the XMC D sig nal s are the sam e for bot h Mn and N i , m e a n i n g t h e d i r e c t i o n s o f t h e m a g n e t i c m o m e n t s a r e p a r a l l e l b e t w e e n t h e M n a n d t h e su bs tr at e Ni [4 ]. Th us on e ca n su re ly ob ta in th e e l e m e n t - s p e c i f i c i n f o r m a t i o n o f t h e m a g n e t i c m o m e n t s b y c o n d u c t i n g c o r e - e x c i t e d X M C D experiments. T h e M n 2 p X A S a n d X M C D s p e c t r a o f t h e c(2 × 2) CuMn / Cu (001) are shown in [5]. We ha ve ob se rv ed se ve ra l fi ne st ru ct ur es on al l sp in - orbit split components of the Mn 2 p XAS spectrum. 49 N i 2 p e d g e 8 5 0 8 6 0 8 7 0 8 8 0 8 9 0 P h o t o n E n e r g y ( e V ) X A S ( a r b . u n i t s ) X M C D ( a r b . u n i t s ) × 2 0 . 4 6 M L M n / N i ( 1 1 0 ) 0 . 4 6 M L M n / N i ( 1 1 0 ) M n 2 p e d g e 6 4 0 6 5 0 6 6 0 6 7 0 6 8 0 P h o t o n E n e r g y ( e V ) X A S ( a r b . u n i t s ) X M C D ( a r b . u n i t s ) × 1 . 5 2 p 1 / 2 2 p 3 / 2 S t r u c t u r e c ( 2 × 2 ) C u A u / C u ( 0 0 1 ) c ( 2 × 2 ) C u P d / C u ( 0 0 1 ) c ( 2 × 2 ) C u M n / C u ( 0 0 1 ) A t o m i c r a d i u s ( Å ) c ( 2 × 2 ) N i M n / N i ( 0 0 1 ) R e l a x a t i o n δ z ( Å ) 0 . 1 0 . 0 2 ± 0 . 0 3 0 . 3 0 ± 0 . 0 3 0 . 2 5 ± 0 . 0 3 6 % 1 % 1 6 . 6 % 1 4 . 2 % 1 . 4 4 2 1 . 3 7 5 1 . 3 6 5 1 . 3 6 5 Fig. 4. Calculated Mn 2p XAS (upper) and X M C D ( l o w e r ) s p e c t r a w i t h M n 2 p 5 3 d 6 final state multiplets. Fig. 3 Fig. 4 Fig. 3. Mn 2p XAS (upper) and XMCD (lower) spectra o f c ( 2 × 2 ) C u M n / C u ( 0 0 1 ) 2 - D o r d e r e d s u r f a c e a l l o y measured at T = 25 K. The XAS spectra are normalized by the incident photon flux. The XAS ( XMCD ) intensity scale is indicated at the left (right) axis. We f ou nd two shoulder states at energies ~1 and ~2 eV higher than that of the 2 p 3 /2 main peak. Furthermore, a doublet peak st ru ct ur e wa s fou nd for the 2 p 1 /2 com po ne nt . However, we have clearly observed the XMCD signal at T = 25 K as shown in the lower part of , w h i c h i n d i c a t e s t h e e x i s t e n c e o f t h e l o n g - range ferromagnetic order under the external magnetic field (~1.4 T). The present XMCD s p e c t r u m s h o w s t h e n e g a t i v e s t r u c t u r e followed by th e we ak er po si ti ve st ru ct ur e wi th i n c r e a s i n g h ν i n t h e 2 p 3 / 2 c o r e e x c i t a t i o n r e g i o n , w h e r e a s t h e d o u b l e p e a k s t r u c t u r e with the positive sign exits in the 2 p 1 /2 region. W e now compare the experimental Mn 2 p XA S an d XM CD sp ec tr a wi th th e ca lc ul at ed M n 2 p 5 3 d 6 f i n a l s t a t e m u l t i p l e t s w i t h a s s u m i n g t h e 3 d 5 a s t h e g r o u n d s t a t e configuration [5]. As shown in , we find e x c e l l e n t c o r r e s p o n d e n c e w i t h t h e e x p e r i m e n t a l X M C D s p e c t r u m a s w e l l a s t h e X A S o n e , w h i c h s h o w s t h a t t h e o b s e r v e d s e v e r a l f i n e s t r u c t u r e s a r e d e r i v e d f r o m t h e m u l t i p l e t effects. This result clearly indicates an almost hal f-f ill ed ele ctr on nat ure lea din g to the hig h spin magnetic moment of the Mn atom. Akio Kimura Hiroshima University E-mail: akiok @ hiroshima-u.ac.jp References [1] M. Imada, A. Fujimori and Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [ 2 ] H . O h n o , J . M a g n . M a g n M a t e r . 2 0 0 (1999) 110. [3] M. Wuttig et al. , Surf. Sci. 292 (1993) 189; M. Wuttig et al. , Phys. Rev. Lett. 70 (1993) 3619. [4] T. Xie et al. , Jpn. J. Appl. Phys. 42 (2003) - in press. [5] A. Kimura, T. Kanbe, T. Xie, S. Qiao, M. T a n i g u c h i , T . M u r o , S . I m a d a a n d S . S u g a , Jpn. J. Appl. Phys. 42 (2003) - in press. 50 0 . 1 0 0 . 0 9 0 . 0 8 0 . 0 7 0 . 0 6 0 . 0 5 0 . 0 4 6 3 5 6 4 0 6 4 5 6 5 0 6 5 5 6 6 0 6 6 5 P h o t o n E n e r g y ( e V ) – 2 – 1 0 1 2 3 × 1 0 - 3 X M C D T o t a l P h o t o e l e c t r o n Y i e l d ( a r b . u n i t s ) 2 p 1 / 2 c ( 2 × 2 ) C u M n / C u ( 0 0 1 ) 2 p 3 / 2 μ - – μ + R e l a t i v e P h o t o n E n e r g y ( e V ) I n t e n s i t y X M C D – 1 0 – 5 0 5 1 0 M u l t i p l e t c a l c . – 2 0 0 2 0 4 0 6 0 – 3 0 – 2 0 – 1 0 0 1 0 2 0 3 0 M n 3 d 5 → 2 p 5 3 d 6
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