STUDY OF ELECTRONIC STATES IN 3d TRANSITION-METAL OXIDES BY X-RAY RESONANCE MAGNETIC SCATTERING

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Fig. 1. Energy dependence of the resonance magnetic scattering intensity at the K-absorption edge of nickel. Polarization of the incident beam and scattered beam are in the π−π configuration. Intensities are corrected for the absorption. The K-absorption spectrum is designated by the solid line. Transition-metal oxides such as NiO, CoO, and M n O h a v e b e e n r e c e n t l y r e c o g n i z e d a s c h a r g e t r a n s f e r t y p e i n s u l a t o r s o f w h i c h t h e b a n d g a p e x i s t s b e t w e e n t h e p - b a n d o f O 2 - a n d t h e u p p e r Hubbard d -band of transition metal M 2 + . The lower Hubbard band and the p -band overlap each other, while the degree of overlap and the energy level of e a c h b a n d d i f f e r a m o n g N i O , C o O , a n d M n O . I n f o r m a t i o n r e g a r d i n g t h e m e c h a n i s m o f t h e s p i n s ta te c h a n g e s o f th e s e m a te r i a l s i s c r u c i a l i n th e d i s c u s s i o n o f h i g h l y c o r r e l a t e d a n t i f e r r o m a g n e t i c i n s u l a t o r s o n t h e b a s i s o f t h e c h a r g e t r a n s f e r m o d e l . I n t h i s c o n t e x t , t h e p - t y p e b a n d o f t h e s e materials displays interesting magnetic features. D u e t o t h e l a c k o f a n e f f e c t i v e e x p e r i m e n t a l m e t h o d , s u c h i n f o r m a t i o n o f t h e s p i n s t a t e s h a s never been elucidated. X-ray resonance magnetic sc at te ri ng is a ne w sp ec tr os co pi c me th od , pr ov id in g STUDY OF ELECTRONIC STATES IN 3 d TRANSITION-METAL OXIDES BY X-RAY RESONANCE MAGNETIC SCATTERING s p i n r e s o l v e d i n f o r m a t i o n r e g a r d i n g u n o c c u p i e d s t a t e s . T h i s m e t h o d c a n b e a p p l i e d t o a n t i f e r r o m a g n e t i c m a t e r i a l s , e v e n t h o u g h m a g n e t i c c i r c u l a r X - r a y d i c h r o i s m o r s p i n r e s o l v e d p h o t o e m i s s i o n spectroscopy are not applicable for these materials. Moreover, this method enables us to separate the e l e c t r o n i c s t a t e s a c c o r d i n g t o s y m m e t r y u t i l i z i n g p o l a r i z a t i o n d e p e n d e n c e o f t h e s c a t t e r i n g amplitudes. T h e f i r s t o b s e r v a t i o n o f X - r a y r e s o n a n c e m a g n e t i c scattering from NiO was reported by Hill et al. [1] , i n t r o d u c i n g t h e o b s e r v a t i o n o f a s t r o n g enhancement of magnetic scattering at the energy c o r r e s p o n d i n g t o t h e p r e - p e a k p o s i t i o n i n t h e K - absorption spectrum. This peak was claimed to be due to the quadrupole resonant magnetic scattering c o r r e s p o n d i n g t o t h e d - b a n d , w h i c h w a s a l s o confirmed by Neubeck et al. [2] . Furthermore, they 0 0.01 0.02 0.03 0.04 0 0.5 1 1.5 2 8.31 8.32 8.33 8.34 8.35 8.36 π−π Absorption Corrected Intensity Energy μτ Fig. 2. Energy dependence of the resonance magnetic scattering intensity at the K-absorption edge of nickel. Polarization of the incident beam and scattered beam are in the σ−σ configuration. Intensities are corrected for the absorption. The K-absorption spectrum is designated by the solid line. f o u n d d i p o l e r e s o n a n t m a g n e t i c s c a t t e r i n g a t energy corresponding to the main peak position in t h e K - a b s o r p t i o n s p e c t r u m , a l o n g w i t h a n o b s e r v a t i o n o f C o O [ 3 ] , w h e r e b o t h d i p o l e a n d quadrupole resonant magnetic scattering were also r e p o r t e d . T h e r e s p e c t i v e o r i g i n s o f t h e d i p o l e scattering amplitude of NiO and CoO, however, still h a v e n o t b e e n s a t i s f a c t o r i l y d e t e r m i n e d . T h e 4 p - bands of the transition metal in both NiO and CoO are believed to be empty, suggesting no existence o f e x c h a n g e s p l i t t i n g . W e p l a n n e d t o i n v e s t i g a t e the spin polarized spectrum of the vacant states of anti ferr omag neti c NiO and MnO usin g pola riza tion a n a l y z e d X - r a y r e s o n a n c e m a g n e t i c s c a t t e r i n g . T h i s i s t h e f i r s t s t e p i n o u r i n v e s t i g a t i o n o f t h e tr an si ti on me ta l ox id es , to wa rd th e ob se rv at io n of the spin polarized spectrum of vacant states in the antiferromagnetic NiO [4] . T h e e x p e r i m e n t w a s p e r f o r m e d a t B L 3 9 X U w h e r e t h e u n d u l a t o r X - r a y b e a m i s m o n o c h r o m a t i z e d b y a S i ( 1 1 1 ) m o n o c h r o m a t o r . A t o t a l r e f l e c t i o n mirror is used to suppress higher harmonics. The X-ray beam is highly polarized in π polarization. To make σ polarized incident beam, a diamond phase r e t a r d e r i s u s e d . A π o r σ c o m p o n e n t o f t h e s c a t t e r e d b e a m w a s a n a l y z e d b y a n S i ( 3 3 1 ) a n a l y z e r c r y s t a l . A s a m p l e i s a s l a b - l i k e ( 1 1 1 ) si ng le T- do ma in cr ys ta l. Bo th co mp on en ts of th e m a g n e t i z a t i o n p a r a l l e l a n d p e r p e n d i c u l a r t o t h e s c a t t e r i n g p l a n e a r e p o s s i b l e t o a p p e a r , a s n o magnetic field has been applied. Rocking curves of a half integer order 5/2 5/2 5/2 reflection are measured in the π−π and the σ−σ po la ri za ti on co nf ig ur at io ns at en er gi es ne ar th e K - a b s o r p t i o n e d g e . I n t e g r a t e d i n t e n s i t y p l o t s , corrected for absorption, are shown in the figures. The spectrum shown by open triangles in Fig. 1 is m e a s u r e d i n t h e π − π c o n f i g u r a t i o n , w h i l e t h e s p e c t r u m s h o w n b y o p e n t r i a n g l e s i n F i g . 2 i s measured in the σ−σ configuration. The solid lines 0 0.03 0.06 0.09 0.12 0 0.5 1 1.5 2 8.31 8.32 8.33 8.34 8.35 8.36 σ−σ Absorption Corrected Intensity μ μ t Energy Fig. 3. A difference spectrum between the π−π configuration a n d t h e σ − σ c o n f i g u r a t i o n , a f t e r t h e i n t e n s i t y h a d b e e n scaled at the maximum. Large p-type polarization appears a t t h e r e g i o n , w h i c h c o i n c i d e s w i t h t h e m a i n a b s o r p t i o n p e a k . T h e s p e c t r u m i s b i p o l a r , w h i l e t h e a b s o r p t i o n i s unipoler type. A small hump can be seen at the slightly high energy side of the pre-edge absorption peak. References [1] J.P. Hill et al. , Phys. Rev. B 55 (1997) R8662. [2] W. Neubeck et al. , to be published. [3] W. Neubeck et al. , Phys. Rev. B 60 (1999) R9912. [4] K. Namikawa, H. Maruyama, M. Ito, E. Arakawa, N . K a w a m u r a , M . S u z u k i , A . K o i z u m i a n d M . M i z u m a k i , The 3 rd Int. Forum Harima Conf. (2000) 16. i n b o t h f i g u r e s d e s i g n a t e a b s o r p t i o n s p e c t r a m e a s u r e d o n N i O p o w d e r . W e observed a strong intensity enhancement i n b o t h r e s o n a n t m a g n e t i c s c a t t e r i n g spectrum at 8.332 keV, corresponding to t h e p r e - p e a k p o s i t i o n o f t h e a b s o r p t i o n s p e c t r u m . A n o t h e r s t r o n g i n t e n s i t y enhancement in the π−π configuration at 8 . 3 4 7 k e V , c o r r e s p o n d i n g t o t h e m a i n - peak position of the absorption spectrum, wa s ob se rv ed , as we ll as an an ti -p ha se d i p a t t h e s a m e e n e r g y i n t h e σ − σ c o n f i g u r a t i o n . I n t e r e s t i n g l y , t h i s a n t i - phase behavior between the π−π and the σ−σ configurations appears again on the h i g h e r e n e r g y s i d e o f t h e 4 p a b s o r p t i o n peak. We recognize this phenomenon in Fi g. 3 , wh er e th e sp ec tr um in di ca te d by o p e n t r i a n g l e s i s a d i f f e r e n c e s p e c t r u m b e t w e e n π − π c o n f i g u r a t i o n a n d σ − σ c o n f i g u r a t i o n a f t e r b e i n g s c a l e d a t t h e m a x i m u m . A n a n t i - p h a s e o s c i l l a t o r y b e h a v i o r i n t h e d i f f e r e n c e s p e c t r u m s u g g e s t s t h e e x i s t e n c e o f s o m e o t h e r m e c h a n i s m b e y o n d t h e s i m p l e 4 p p o l a r i z a t i o n m o d e l . A t t h e p r e - p e a k p o s i t i o n , t h e p e a k w i d t h s o f t h e π − π c o n f i g u r a t i o n a n d t h e σ − σ c o n f i g u r a t i o n s p e c t r a i n t e r e s t i n g l y a p p e a r t o d i f f e r , w h e r e h i g h e r e n e r g y s i d e o f t h e π − π configuration spectrum is wider than that o f t h e σ − σ c o n f i g u r a t i o n s p e c t r u m . A small hump in the difference spectrum at th e pr e- ed ge po si ti on in Fi g. 3 in di ca te s this fact. Our immediate future direction i s t o p r o c e e d t o t h e m e a s u r e m e n t o f M n O . A comparison of the spectra between NiO and MnO is highly expected. Kazumichi Namikawa a , Hiroshi Maruyama b and Masahisa Ito c (a) Tokyo Gakugei University (b) Okayama University (c) Himeji Institute of Technology E-mail: namikawa @ u-gakugei.ac.jp -0.6 0 0.6 1.2 0 0.5 1 1.5 2 8.31 8.32 8.33 8.34 8.35 8.36 Subtracted Intensity μ μ τ τ Energy