CONVERSION ELECTRON MÖSSBAUER SPECTROSCOPY DOWN TO LIQUID HELIUM TEMPERATURE

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CONVERSION ELECTRON MÖSSBAUER SPECTROSCOPY DOWN TO LIQUID HELIUM

TEMPERATURE

O. Massenet

To cite this version:

O. Massenet. CONVERSION ELECTRON MÖSSBAUER SPECTROSCOPY DOWN TO LIQ- UID HELIUM TEMPERATURE. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-26-C2-28.

�10.1051/jphyscol:1979207�. �jpa-00218465�

JOURNAL DE PHYSIQUE

Collogue C2, supplement au n° 3, Tome 40, mars 1979, page C2-26

CONVERSION ELECTRON MOSSBAUER SPECTROSCOPY DOWN TO L I Q U I D HELIUM TEMPERATURE

0 . Massenet

Groupe des Transitions de Phases, C.N.R.S., B.P. 166, 38042 Grenoble Cedex, Franoe

Résumé.- On a réalisé un spectromètre Mossbauer à électrons de conversion -qui permet d'effectuer des mesures jusqu'à la température de l'hélium liquide. Le détecteur d'électrons de conversion fonctionne sous vide; il comporte un analyseur d'électrons à champ longitudinal et un multiplica- teur tubulaire d'électrons (channeltron). Des mesures Mossbauer à 4,2 K et à la température ambian- te, effectuées sur des couches minces d'alliages amorphes Fe Gej sont présentées.

Abstract.- A conversion Electron Mossbauer Spectrometer is described which allows measurements to be performed down to liquid helium temperature. The conversion electron detector operates under vacuum. It is made of a longitudinal field electron analyser and a tubular electron multiplier

(channeltron). Mossbauer measurements at room temperature and 4.2 K, made on thin films, 1000 A, of amorphous FexGe!_x alloys are presented.

Introduction.- Conversion Electron Mossbauer Spec- troscopy (G.E.M.S.) has been used for a large number of studies. The conventional technique to detect the conversion electrons uses a gas counter filled with a mixture of helium and methane /1-2/. This method, convenient at room temperature and above, is diffi- cult to operate much below 77 K /3/. In order to extend down to 4.2 K the range of temperature in which Mossbauer measurements could be done by C.E.M.S., we realized a conversion electron detector operating under vacuum, similar in its principle to that described by Schrunk et al. / 4 / , and in which the sample is under vacuum, in thermal contact with the helium bath of a cryostat.

Description of the spectrometer.- The apparatus is schematically represented on figure 1.

Fig. 1 : Schematic diagram of the conversion elec- tron Mossbauer spectrometer.

The Mossbauer source and its driving motor are loca- ted outside the vacuum chamber. The beam of the

Y rays, horizontally oriented, penetrates inside the vacuum chamber through an air-tight beryllium window. It hits at incidence of about 45° the sur- face of the sample which is stuck on a copper sam- ple holder in thermal contact with the helium tank of the cryostat.

A longitudinal magnetic field electron analyser selects the conversion electrons whose trajectories initially makes an angle of 30 +_ 5° with the axis of the analyser. The value of the uniform magnetic field inside the analyser is set to the correct value to focalize the conversion electrons on the entrance of a tubular electron multiplier (channel- tron) . The other end of the channeltron is connec- ted to a high voltage source of 2500 volts. The pulses given by each electron reaching the channel- tron are amplified by a charge sensitive amplifier, counted and stored with a classical Mossbauer electronic equipment. Classical vacuum was maintai- ned inside the vacuum chamber with an oil diffusion pump. A liquid nitrogen baffle was added to limit oil condensation on the sample during low tempera- ture measurements. A decrease in the counting rate of about 30 %, due to this effect was observed after a 48 hours run with the sample at 4.2 K.

We used a 100 mCi 57Co radioactive source with a rhodium matrix, located about 4 cm away from the sample surface. Under these conditions, the coun- ting rate showed a fairly broad maximum of about 20.000 counts/ minute for a field in the magnetic analyser of about 60 gauss. The broad maximum results from the fact that the energy resolution of the magnetic analyser has been made intentionally

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979207

v e r y poor i n o r d e r to i n c r e a s e t h e counting r a t e and t o s h o r t e n correspondingly t h e c o l l e c t i n g time ne- c e s s a r y t o g e t Mzssbauer s p e c t r a w i t h c o r r e c t s t a - t i s t i c .

The spectrum o b t a i n e d on t h e s u r f a c e of 0.5 mm t h i c k i r o n s h e e t , a f t e r 20 hours of counting i s shown on f i g u r e 2. The p e r c e n t a g e of e f f e c t on t h e e x t e r n a l peak of t h e s e x t u p l e t i s 10 %. A more complete des- c r i p t i o n of t h i s a p p a r a t u s w i l l b e published e l s e - where 151.

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F i g . 2 : ~ E s s b a u e r spectrum of a m e t a l l i c i r o n s h e e t 0.5 mm t h i c k ; c o u n t i n g time : 20 hours.

Mhsbauer measurement on FexGe t h i n f i l m s . -

1 -x

~ Z s s b a u e r measurements have been done w i t h t h i s a p p a r a t u s a t room temperature and 4.2 K on t h i n f i l m s , 1000 t h i c k , of amorphous FexGel-x a l l o y s f o r 0.4 < x < 0.6 w i t h n a t u r a l , non e n r i c h e d i r o n , i n o r d e r t o s t u d y t h e o n s e t of ferromagnetism i n t h e s e m a t e r i a l s . These amorphous a l l o y s become f e r - romagnetic f o r x > 0 . 4 161.

C h a r a c t e r i s t i c s p e c t r a o b t a i n e d a f t e r 24 o r 48 h o u r s of counting a r e shown on f i g u r e s 3 and 4 f o r two compositions, x = 0.47 and x = 0.55.

F i g . 3 : Mijssbauer s p e c t r a o f a F e o - s - ~ G e ~ 5 3 amor- phous f i l m , 1000 i( t h i c k , measured a t roo& tempera- t u r e and 4.2 K.

F i g . 4 : Mijssbauef; s p e c t r a of a Feo,55Geo,ss amor- phous f i l m , 1000 A t h i c k , measured a t room tempe- r a t u r e and 4 . 2 K.

They show t h a t a t 4.2 K, t h e r e i s a wide d i s t r i b u - t i o n i n t h e v a l u e s of t h e magnetic h y p e r f i n e f i e l d s seen by d i f f e r e n t Fe atoms. The corresponding com- p u t e r c a l c u l a t e d histograms of t h e d i s t r i b u t i o n of magnetic h y p e r f i n e f i e l d s a r e p r e s e n t e d on f i g u r e 5 . This d i s t r i b u t i o n i s a t t r i b u t e d t o t h e s t a t i s - t i c a l f l u c t u a t i o n s i n t h e n e a r e s t neighbour envi- ronment of Fe atoms.

F i g . 5 : Histograms of t h e d i s t r i b u t i o n s of magn t i c h y p e r f i n e f i e l d v a l u e s f o r F e o . ~ 7 G e o , s 3 and F e 0 . s ~ Ge0.s~ deduced from t h e i r a s s b a u e r s p e c t r a measured a t 4.2 K.

Conclusion.- These r e s u l t s demonstrate t h e i n t e r e s t of e x t e n d i n g t h e C.E.M.S. i n t h e l i q u i d helium temperature range. Besides measurements on t h i n f i l m s , t h i s a p p a r a t u s a l l o w s measurements t o b e performed d i r e c t l y on t h e s u r f a c e of bulk mate- r i a l s , f o r c o r r o s i o n s t u d i e s o r f o r G s s b a u e r ana- l y s i s of implanted s u r f a c e s / 7 , 8 / .

JOURNAL DE PHYSIQUE

R e f e r e n c e s

/ I / Fenger, J . , N u c l . Instrum. Methods

69

/2/ Swanson, K . R . , Spijkerman, J . J . , J . Appl. Phys.

41

(1970) 3155.

131 S a w i c k i , J . A . , Sawicka, B . P . , Stamer, J . , Nucl. I n s - trum. Methods

138

/ 4 / Schunck, J . P . , F r i e d t , J.M., Llabador, Y . , Rev. Phy- s i q u e Appl.

2

151 Massenet, O., N u c l . Instrum. Methods

153

25

917.

/ 7 / Sawicka, B.D., S a w i c k i , J . , Stanek, J . , J . Phys.

C2

(1974) 879.

/ 8 / Longworth, G . , J a i n , R . , 3 . Phys. !F (1978) 3 5 1 .