On the determination of the magnetic hyperfine field distribution in an amorphous alloy

Journal of Magneusm and Magneuc Matermls 31-34 (1983) 1521-1522 ON THE DETERMINATION IN AN AMORPHOUS S.J. C A M P B E L L ,

1521

OF THE MAGNETIC

HYPERFINE

FIELD DISTRIBUTION

ALLOY

G.L. WHITTLE

and A.M. STEWART

Department of Sohd State Ph),stcs, Research School of Physwal Scwnce~, A ustrahan Nattonal Umverst(~, Canberra, A C T 2600, A ustraha

The magnetic hyperfine field distribution P ( H ) in Fe32N136Crl4 Pi2 B6 has been determined by several methods The P ( H ) curves obtained over the range 1.5 K to the ordering temperature T~ = 233 K, confirm the existence of two distinct regions of strong and weak magnetic interactions

The evaluation of the distribution of magnetic and electric hyperflne parameters which are generally present in the M o s s b a u e r spectrum of a magneUcally ordered a m o r p h o u s alloy is a difficult task For those materials which exhibit a d o m i n a n t d x s m b u u o n of magn e u c hyperfine fields P ( H ) , a n u m b e r of methods have been proposed (eq. refs. [1,2]) which enable an approximate solutmn for P ( H ) to be obtained. D e s p n e the widespread acceptance of such analytical methods, there remain uncertainties a b o u t the a p p h c a b d n y of these methods for the study of certain classes of materials and the p a r a m e t e r constraints reqmred for data analysis These u n c e r t a m u e s are exemphfied by the current controversy concerning the magnetic hyperflne field d l s m b u t l o n in the a m o r p h o u s f e r r o m a g n e t Fe32NI36CrI4PI2B6, Chlen [3] has obtained a blmodel P ( H ) distribution at 4 2 K m which the low field peak is considered to arise from won atoms that have primarily c h r o m i u m atoms as neighbours, whereas Schaafsma [4] has derived a single m a x i m u m f u n c u o n which he assocmted with a r a n d o m d~strlbuuon of c h r o m m m a t o m s Our a p p r o a c h m determining the m a g n e u c hyperfine field d l s t n b u u o n of Fe32 B136Cr14 PI2 B6 has been to test for consistency by deriving P ( H ) curves using several different m e t h o d s [5] In thxs paper we o u t h n e our a p p r o a c h to the analysis of the 1.5 and 4 2 K data [5] a n d present the m m a l results o b t a i n e d for the P ( H ) & s t r , b u t l o n of Fe32NI36Crj4PI2B6 up to the ordering temperature. The analysis is based on the Fourier series m e t h o d of W i n d o w [1] with the o p t i m u m values for the three main parameers N, b a n d F being determined by minlm~sat m n of the quality-of-fit p a r a m e t e r X 2 N is the n u m b e r of terms of the Fourier series, b the factor which describes the intensity ratios 1~.6 12,5 : 13 4 =- 3. b 1 of the six-line pattern comprising a u m q u e magnetic hyperfme field (b = 4 sin20/(1 + cos20) where 0 is the angle between the magnetic m o m e n t and the 3,-ray d~rectIon) a n d F the h n e w l d t h This method, labelled A, was tested by analysing magnetically saturated 4.2 K spectra for which the m t e n s n y raUos of the M o s s b a u e r hnes are k n o w n [5] The results confirmed that the shape of the 0304-8853/83/0000-0000/$03

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Fig 1 (a) Mossbauer spectra of Fe32Nt~6Cr14PI2B 6 at 45 1 and 128 8 K The full hne represents the fit to the data as described l n t h e t e x t ( N = 1 5 , F = 0 4 2 m m s - I b(45 1 K ) = I 75, b(1288 K) = 1 8), (b) the P(H) curves corresponding to the fns to the spectra m fig la The arrows indicate H u and H~efor the P ( H ) curve at 45 1 K P ( H ) curve remained u n c h a n g e d u n d e r the influence of an a p p h e d magnetic field. The second method, B, is based on Vlncze's spectrum s u b s t r a c u o n method [2] in wh,ch the p a r a m e t e r b xs eliminated by analysing the difference between spectra taken in zero and a p p h e d fields These difference data were fitted both by the Fourier method a n d also by using pseudo-Lorentzmn

1522

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h n e s h a p e s to m o d e l a G a u s s l a n d i s t r i b u t i o n o f L o r e n t z l a n d o u b l e t s [5] T h e P ( H ) curves o b t a i n e d bv m e t h o d B in the study of zero a n d applied field spectra at 4 2 K were in g o o d a g r e e m e n t with the curve oht a m e d by m e t h o d A and c o n f i r m e d that this latter m e t h o d , which is e x p e r i m e n t a l l y m o r e s t r a i g h t f o r w a r d than B, can be a d o p t e d for the investigation o f zero field, variable t e m p e r a t u r e spectra Typical e x a m p l e s of M o s s b a u e r spectra o b t a i n e d in our variable t e m p e r a t u r e (1 5 - 3 0 0 K) study of Metglas '~ 2826A (Allied C h e m i c a l Corp.) are s h o w n in fig la T h e s p e c t r a were o b t a m e d using a s t a n d a r d M o s s b a u e r spect r o m e t e r ~ t h a variable t e m p e r a t u r e helium cryostat A R h s7 C o source was used and the velocity scale was c a l i b r a t e d against a 25 btm foil o f natural a - F e , g w m g a line width of 0 24 m m s 1. The full lines t h r o u g h the d a t a show the fits o b t a i n e d using m e t h o d A with F = 0 4 2 m m s ~, the value c o m m o n to the r o o m t e m p e r a ture s p e c t r u m and the o p t i m i s a U o n p r o c e d u r e for the 4 2 K s p e c t r u m T h e c o r r e s p o n d i n g P ( H ) curves are s h o w n m fig lb T h e s e results, t o g e t h e r with the analysis of 1 5 a n d 4 2 K s p e c t r a [5], c o n f i r m the b l m o d a l n a t u r e of the h~perflne field d i s t r i b u t i o n in the magnetically o r d e r e d region of Fea2Nl~6Crl4Pl2B 6, in agreem e n t with the findings of C h l e n [3] Fig 2a is a g r a p h o f the o p t l n m m values of b as d e t e r m i n e d in the fitting p r o c e d u r e , versus t e m p e r a t u r e T h e s e values show an essentially c o n s t a n t b e h a v i o u r over the t e m p e r a t u r e range = 20 150 K with an average value of = 1 8, this is similar to the value b = 2 exp e c t e d for a r a n d o m d i s t r i b u t i o n of spins 111 the a m o r p h o u s alloy F r o m fig 2a we note that h exhibits a t e n d e n c y to increase a b o v e = 160 K This b e h a v l o u r is

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c o n s i d e r e d to be the result of the rapid r e d u c t i o n m the m a g n e t m a n i s o t r o p y w h i c h occurs as the Curie p o i n t is a p p r o a c h e d a n d is c o n s i s t e n t with the b e h a v l o u r of the ac m a g n e t i c susceptibility as m e a s u r e d o n a sample taken from the s a m e role of Metglas ~ 2826A ribbon [6] W h e n this r e d u c t i o n in m a g n e t i c a m s o t r o p ) takes place the direction of the m o m e n t s will be d e t e r m i n e d b'r the d e m a g n e t i s m g fields and the spins will theretore tend to lie m the p l a n e of the specimen, resulting m an mcre,tsc in the value of h Below = 20 K we note that h decreases to the value 1 5 o b t a i n e d on analysis of the 1 5 a n d 4 2 K spectra This indicates a p r e l e r e n c e toi the m a g n e h s a t i o n d l r e c n o n to be o r i e n t e d p e r p e n d i c u l a r to the r i b b o n surface Fig 2b is a graph of Hi, a n d t t : versus t e m p e r a t u r e w h e r e 1t~ and H : are the fields of m a x i m u m p r o b a h h ties for the u p p e r a n d lower peaks in the P( H ) distributions (see e.g fig. lb) T h e s e data show a significant increase ( > 15%) in these p a r a m e t e r s below = 2 0 K A b o v e this t e m p e r a t u r e the variations of 1t U and tt~ e x t r a p o l a t e s m o o t h l y to an o r d e r i n g t e m p e r a t u r e I~ = (232 8 _+ 0 5) K o b t a i n e d by ac m a g n e t i c s u s c e p t l b l h t y m e a s u r e m e n t s on the same r i b b o n [6] This increase m m a g n e t i c h y p e r f m e field b e l o w = 20 K could be associated with the o r d e r m g of individual m a g n e t i c a t o m s or clusters that are only weakl> c o u p l e d to the bulk magn e h s a t l o n T h e horizontal d a s h e d line a r o u n d ~ m fig 2b indicates the m a g n i t u d e o f the a s y m m e t r i c quadrt,pole s p h t t m g which we observe m the high t e m p e r a t u r c ( T > T ) n o n - m a g n e t i c spectra of Fe~xNI~6CrHPleB ~, This splitting is c o n s i d e r a b l y less than the s p h t t m g o b t a i n e d from the lower field peak H~, (fig 2b), wc c o n s i d e r that Hg reflects the m a g n e t i c mter,tctions m the s a m p l e rather than an e n h a n c e m e n t o f the q u a d r u p o l e electrostatic term as suggested by Plecuch et al [7] The main c o n c l u s i o n is thai the m a g n e t i c h y p e r f m e held d i s t r i b u t i o n of Fe~2NI~6CrlaPI2B ~ exhibits a Nm o d a l structure which persists up to the o r d e r i n g temp e r a t u r e in a g r e e m e n t with the results o f Chien [31 A fuller d e s c r i p t i o n of the variable t e m p e r a t u r e 17(11) curves and the m a g n e t i c c h a r a c t e r of Fe~:Nl ~,,Cri,; PI~ B~ wdl be r e p o r t e d elsewhere

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Fig 2 (a) A graph of the ophmum values of b as determined in the fitting process versus temperature, (b) a graph of H u and Hg versus temperature H~ and H e are explained in the lext (see fig lb) The full hnes indicate the trends of the data, the horizontal dashed hne is explained m the text

[1] B Wmdov,,J Ph'~,, E4(1971)401 [2] 1 Vmcze, Sohd State Commun 25 (1978) 689, Ph,,s Re~ B to be pubhshed [3] C L ( h m n Ph'~s Rev B 19(1979) 81, B 23 (19811 4788 [4] A S Schaafsma, Phys Re', B 23 (1981) 4784 [5] G L Whittle, S J Campbell and A M Stewart, Ph,~s Star Sol (a) 71 (1982) 245 [6] P J Back and S J Campbell, J Magn Magn Mat 31 :14 (1983l 1543 [7] M Pmcuch, G Marchal, Ph Mangm and Chr Janot, Ph\~ Stat Sol (a) 62(1980) K99