4.25 GPa 23.9 GPa Absorption ( μ μ t + + μ μ t – )/2 (a) 23.9 GPa 19.5 GPa 0.35 GPa 9.85 GPa A.P. (without DAC ) (b) 0.006 0.004 0.002 0.000 – 0.002 7.10 7.11 7.12 7.13 7.14 7.15 7.16 Photon Energy ( keV ) XMCD ( μ μ t + – μ μ t – ) In Fe 4 N perovskite, hybridization between the e l e c t r o n w a v e - f u n c t i o n s a n d t h e l i g a n d f i e l d prese nts an impor tant view poin t for under stand ing m a g n e t i c p r o p e r t i e s . T h e i n s e r t i o n o f a n i t r o g e n atom into the center of fcc cell of Fe expands the c e l l v o l u m e , a n d t h e e x p a n d e d F e 4 N c h a n g e s t o s h o w a f e r r o m a g n e t i c s t a t e w i t h a C u r i e temperature of 761 K from antiferromagnetic γ -Fe. T h e s m a l l o v e r l a p o f F e 3 d e l e c t r o n s o r i g i n a t i n g f r o m t h e l a t t i c e e x p a n s i o n i s w h a t i s p r o b a b l y responsible for the transition. If so, then how does XMCD Measurements of Fe 4 N under High-pressure b y O d i n e t a l . [ 3 ] s o f a r . T o o v e r c o m e t h i s l i m i t a t i o n , a p a i r o f t h i n n e r d i a m o n d a n v i l a n d i n t e n s e X - r a y s w i t h a h i g h d e g r e e o f c i r c u l a r polarization were utilized; consequently XMCD was su cc es sf ul ly me as ur ed un de r hi gh -p re ss ur e up to 26 GPa at Fe K -edge (7.111 keV ) in Fe 4 N [4]. T h e h e l i c i t y - r e v e r s a l m e t h o d u s i n g a q u a r t e r - w a v e p l a t e d i a m o n d ( 1 1 1 ) s l a b w a s a p p l i e d t o r e c o r d t h e h i g h - p r e s s u r e X M C D s p e c t r u m a t bea mli ne BL3 9XU . The cir cul arl y pol ari zed X-r ay passed through a pair of thinner diamond anvils of 2.0 mm in total thickness. A sufficient intensity of t h e t r a n s m i t t e d b e a m e n a b l e d u s t o m e a s u r e t h e XM CD sp ec tr um ac cu ra te ly by da ta ac cu mu la ti on f o r o v e r a p e r i o d o f f o u r t o f i v e h o u r s . F i g u r e 1 shows the pressure variation of the XMCD spectra th e ma gn et ic pr op er ty of Fe 4 N be ha ve under compression? In order to understand that, use can b e m a d e o f h i g h -p re ss u re w h i ch i s a n e f f e c t i v e e x t e r n a l f i e l d t o m o d i f y t h e lattice constant. The pressure variation o f t h e h y b r i d i z a t i o n a n d t h e c h a r g e t r a n s f e r b e t w e e n t h e i r o n a n d t h e n i t r o g e n p r o v i d e s i n f o r m a t i o n w h i c h i s u s e f u l t o u n d e r s t a n d t h e i n f l u e n c e s o f t h e s e s u b s t a n c e s o n t h e m a g n e t i c property of Fe 4 N. S p e c t r o s c o p i c a n a l y s i s s u c h a s Mös sba uer spe ctro sco py [1] and X-ra y e m i s s i o n s p e c t r o s c o p y [ 2 ] h a v e b e e n applied to study magnetism under high- p r e s s u r e . I n r e c e n t y e a r s , X - r a y m a g n e t i c circular d i c h r o i s m ( X M C D ) h a s a l s o b e e n a p p l i e d to probe magnetically polarized electron states. However, XMCD under h i g h - p r e s s u r e u s i n g t h e d i a m o n d - a n v i l c e l l h a s b e e n l i m i t e d t o a r a n g e o f p h o t o n e n e r g y a b o v e 1 0 k e V , w h e r e t h e X - r a y a b s o r p t i o n b y t h e d i a m o n d c r y s t a l s i s a l m o s t n e g l i g i b l e . T h e r e h a v e b e e n o n l y a f e w r e p o r t s o n P t L 3 - edge shows (11.564 keV ) in Fe 72 Pt 28 Fig. 1. (a) X-ray absorption spectra of Fe 4 N. (b) Pressure variation of XMCD spectra in Fe 4 N. Each s pectrum is shifted 0.002 steps in the y-direction. The error bars correspond to the standard deviation of runs of XMCD measurement. 43 Pressure ( GPa ) 0 20 10 0 2 4 6 8 Integrated XMCD ( a.u. ) [ × 10 -6 ] References [1] C. L. Yang et al. , J. Mag. Mag. Mat. 151 (1995) L19. [2 ] J. -P . Ru ef f et al . , Ph ys . Re v. Le tt . 82 (1 99 9) 3284. [3] S . O d i n e t a l . , J . A p p l . P h y s . 8 3 ( 1 9 9 8 ) 7 2 9 1 . [4] N . I s h i m a t s u , Y . O h i s h i , M . S u z u k i , N . K a w a m u r a , M . I t o , H . M a r u y a m a a n d O . S h i m o m u r a , i n p r e p a r a t i o n ; N u c l . I n s t r u m . M e t h . A 4 6 7 - A 4 6 8 (2001) 1061. [5] J. Igarashi and K. Hirai, Phys. Rev. B 24 (1994) 17820. un de r a ma gn et ic fi el d of 0. 6 T at ro om te mp er at ur e. The XMCD spectrum is characterized by a positive peak at 7.111 keV, which first increases in intensity as the pressure increases and then reduces under f u r t h e r h i g h - p r e s s u r e s . O n t h e o t h e r h a n d , t w o n e g a t i v e p e a k s s h o w a m o n o t o n o u s d e c r e a s e i n intensity. The unit-cell of Fe 4 N consists of two Fe si te s; on e is Fe (I ) at th e co rn er po si ti on an d th e o t h e r i s F e ( I I ) a t t h e f a c e - c e n t e r e d p o s i t i o n . Mös sbauer spectroscopy shows that these two Fe sites give rise to different pressure dependences of hyper-fine field and isomer shift [1]. Therefore, the p r e s s u r e v a r i a t i o n i n t h e X M C D p r o f i l e m a y b e a s c r i b e d t o t h e d i f f e r e n t r e s p o n s e s o f t h e s e F e sites. T h e p r e s s u r e d e p e n d e n c e o f X M C D i s d e m o n s t r a t e d b y t h e p l o t o f i n t e g r a t e d X M C D shown in Fig. 2 . The integrated XMCD corresponds to the area of the two negative peaks represented b y t h e r e g i o n c o l o r e d i n g r a y i n F i g . 1 . T h e rep rod uci bil ity of the dat a was con fir med wit h two s e r i a l m e a s u r e m e n t s . A t f i r s t , a s t h e p r e s s u r e i n c r e a s e s , t h e i n t e n s i t y g r a d u a l l y r e d u c e s , t h e n dec rea ses abo ve 15 GPa to eve ntu all y van ish at around 24 GPa. This behavior is evidence for the p r e s s u r e - i n d u c e d t r a n s i t i o n f r o m a f e r r o m a g n e t i c s t a t e t o a p a r a m a g n e t i c o n e , a s e c o n d - o r d e r transition without pressure hysteresis. According to an X- ra y di ff ra ct io n ex pe ri me nt wh ic h wa s ca rr ie d out separately, this transition is not accompanied by structural transformation. XMCD at the K -edge in pure 3 d transition metals is strongly affected by small 3 d orbital moments on t h e n e i g h b o r i n g s i t e [ 5 ] . S i n c e s p i n - o r b i t a l interaction induces a 3 d orbital moment, the XMCD spectrum reflects bulk magnetization in addition to i t s e l e c t r o n i c s t a t e . I t s h o u l d b e n o t e d t h a t h i g h - p r e s s u r e X M C D m e a s u r e m e n t c a n d e v e l o p t h e basic studies of magnetism in 3 d transition metals and/or 4 f rare-earth metals. Fig. 2. Pressure dependences of integrated XMCD. T h e s o l i d c i r c l e s , o p e n c i r c l e s a n d s o l i d s q u a r e s correspond to the first run in pressure increase, the second run in pressure increase and the second run in pressure decrease, respectively. The solid line is guide for the eyes. Naoki Ishimatsu Hiroshima University E-mail: naoki @ sci.hiroshima-u.ac.jp 1 st run 2 nd run(pressure increase) 2 nd run(pressure decrease) guide to eyes 44
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