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A MÖSSBAUER STUDY OF THE HEUSLER ALLOY SERIES Ni2 Mnx Ti1-x Sn
J. Williams, D. Danson
To cite this version:
J. Williams, D. Danson. A MÖSSBAUER STUDY OF THE HEUSLER ALLOY SERIES Ni2 Mnx Ti1-x Sn. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-169-C2-171.
�10.1051/jphyscol:1979260�. �jpa-00218658�
A MOSSBAUER STUDY OF THE HEUSLER ALLOY SERIES N i
2M n
xT i ^ S
nJ. M. Williams and D.P. Danson
Department of Phyeios, The University, Sheffield, England
Abstract.- Measurements of the magnetic hyperfine field in the series of Heusler alloys Ni2Mnn Tij-x Sn (0 -$ x .<: 1) are presented. The spectra are analysed in terms of a continuous distribution of ma- gnetic hyperfine fields. Values of 0 T and 9.5 T at 10 K are reported for Ni2 Ti Sn and Ni2 Mn Sn res- pectively.
1. Introduction.- Heusler alloys are magnetically and chemically ordered with the L2i structure, com- prising of four inter-penetrating face-centred-cubic sub-lattices. The typical composition is X2YZ, where X is normally a transition metal, Y is usually Mn and Z is a metal such as Al, In, Sn, Sb, etc.
Most Heuslers order ferromagnetically and magnetization studies /l/ indicate that the magnetic moment per molecule is ~4lir. and is confined to the Mn sites (except in those alloys containing Co). The Mn atoms have a large separation (>4A) so the cou- pling is thought to arise from an indirect interac- tion, via the conduction electrons, either on the basis of an RKKY interaction /2/ or following the double resonance method of Caroli and Blandin /3/.
Recently attempts have been made to extend both models to cover interactions not restricted to asym- ptotically large magnetic ion-ion separations /4-7/.
In order to test such theories, a large num- ber of Heusler alloys have been investigated /8/
using Mossbauer spectroscopy and other techniques.
Attention has now turned to the possibility of investigating series of alloys, where one element is gradually substituted for another, in the ordered structure. This has been achieved for the X site /9, 10/, the Y site /ll, 12/ and Z site /13/. Here we report measurements on the series N i2 Mn Tij Sn (0 < n -S l)j which are of particular interest, since the magnetic moment bearing Mn ions are gradually substituted into the non-magnetic N i2 Ti Sn alloy.
2. Experimental results and analysis.- The samples investigated, were prepared by melting together the appropriate quantities of spectrographically pure elements in an argon arc furnace and then annealing
for 24 hours at 800°C, with final quenching. X-ray analysis shows that the lattice constant is approxi- mately 6A across the series, with a gradual decrea- se as more Mn is substituted.
The Mossbauer spectra were obtained at 10 K with a 5 mCi 1 1 9Sn source (Ca Sn O3) using a con- ventional constant acceleration spectrometer. The spectra shown in figure 1 were fitted in terms of a continuous distribution of magnetic hyperfine fields using the method of Hesse and Rubartsch /14/.
Fig. 1 : The l l 9Sn Mossbauer absorption spectra measured for the Heusler series N i2 Mnx Tii-x Sn at
10 K.
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-169
Résumé.- Des mesures du champ magnétique hyperfin dans des alliages de Heusler du type Ni2Mnx Tij-x Sn(0 4 x 4 \) ont été réalisées. Les spectres Mossbauer sont analysés en supposant qu'il existe une distribution continue de champ magnétique hyperfin. A 10 K des valeurs de 0 et 9,5 T ont été mesu- rées pour Ni2Ti Sn et Ni2 Mn Sn respectivement.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979260
C2- 170
JOURNAL DE PHYSIQUE The r e s u l t i n g P(H) curves shown i n f i g u r e 2 c l e a r l yshow a main d i s t r i b u t i o n peak, which i s e a s i l y d i s - t i n g u i s h e d from t h e s m a l l e r , p h y s i c a l l y meaningless s u b s i d i a r y o s c i l l a t i o n s , which a r e a f e a t u r e of t h e f i t t i n g method.
0 5 10 15 TESLA
F I E L D OlSTRlBUTlDN
Fig. 2 : The magnetic f i e l d d i s t r i b u t i o n s o b t a i n e d by computer f i t t i n g t h e s p e c t r a of f i g u r e 1 .
The expected i n c r e a s e i n h y p e r f i n e f i e l d , w i t h i ~ c r e a s e i n c o n c e n t r a t i o n of t h e magnetic Mn i o n s i s d i s p l a y e d . A t t h e extremes of t h e s e r i e s , t h e w e l l d e f i n e d s p e c t r a g i v e v a l u e s of z e r o f i e l d f o r N i z T i Sn and 9.5 T f o r N i z Mn Sn. The o t h e r s p e c t r a show v a r y i n g degrees of r e s o l u t i o n , i n d i c a - t i n g a range of magnetic i n t e r a c t i o n s c o n t r i b u t i n g t o t h e a b s o r p t i o n . These f e a t u r e s a r e a l s o v i s i b l e i n t h e d i s t r i b u t i o n p l o t s , w i t h a maximum a t z e r o f i e l d f o r x=O, a w e l l d e f i n e d peak f o r x=1.0 and a g r a d u a l broadening of t h e f i e l d range, as x i s in- c r e a s e d .
3. Discussion.- It i s c l e a r t h a t t h e " ' ~ n n u c l e i sample , a v a r y i n g range of magnetic f i e l d s a s we move a c r o s s t h e s e r i e s . I f t h e p o s i t i o n of t h e d i s t r i b u - t i o n curve maximum and t h e range of f i e l d s a r e p l o t - t e d , a s i n f i g u r e 3. s e v e r a l f e a t u r e s become appa- r e n t . F i r s t l y t h e most probable f i e l d f o r samples i n
t h e range 0 4 x ,< 0.6 remains a t a low v a l u e a n d t h e n i n c r e a s e s d r a m a t i c a l l y towards ~ ~ 1 . 0 . Even though t h e r e i s no obvious maximum f o r x=0.8 ( t h e average f i e l d v a l u e i s used h e r e ) , t h e o t h e r maxima a r e c l e a r l y d i s t i n g u i s h e d , s o t h e g e n e r a l t r e n d s shown must be p r e s e n t , even though t h e abrupt change around x=0.6 t o 1.0 may n o t have e x a c t l y t h e form shown.
F i g . 3 : The dependence of f i e l d d i s t r i b u t i o n on composition. The shaded a r e a r e p r e s e n t s t h e range o f f i e l d s , X r e p r e s e n t t h e v a l u e s a t t h e maximum of each d i s t r i b u t i o n , A r e p r e s e n t s t h e weighted avera- ge f i e l d .
The n e a r l y c o n s t a n t maximum f i e l d v a l u e f o r ~ 0 . 6 , i s t e n t a t i v e l y a s c r i b e d t o some s o r t o f p a r t i a l se- g r e g a t i o n w i t h i n t h e a l l o y although i t i s n o t y e t c l e a r t h a t t h i s i s t h e c a s e . Secondly, t h e range of f i e l d s p r e s e n t i n c r e a s e s n e a r l y l i n e a r l y up t o x=0.6 and t h e n d e c r e a s e s t o t h e end of t h e s e r i e s a t x=l.O.
This change a t x=0.6 a l s o shows t h a t some s o r t of t r a n s i t i o n may be t a k i n g p l a c e around t h i s v a l u e . Also p l o t t e d on f i g u r e 3 i s t h e p o s i t i o n o f t h e c e n t r e of g r a v i t y of each of t h e d i s t r i b u t i o n peaks. These average v a l u e s show an approximately l i n e a r i n c r e a s e w i t h i n c r e a s i n g x and t h e y c o r r e l a t e w e l l w i t h t h e b u l k magnetization measurements repor-
t e d i n r e f e r e n c e / 1 1 / . Such agreement i s t o be ex- p e c t e d s i n c e b u l k m a g n e t i z a t i o n measurements g i v e an "average" r e s u l t a c r o s s t h e sample.
Acknowledgements.- We a r e g r a t e f u l t o D r . P e t e r Webster of S a l f o r d U n i v e r s i t y f o r supplying t h e a l l o y s . One of us (D.P.D) g r a t e f u l l y acknowledges t h e award of an S.R.C. r e s e a r c h s t u d e n t s h i p .
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