HIGH PRESSURE MAGNETISM

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HIGH PRESSURE MAGNETISM

R. Ingalls

To cite this version:

R. Ingalls. HIGH PRESSURE MAGNETISM. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-

174-C2-179. �10.1051/jphyscol:1979262�. �jpa-00218660�

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HIGH PRESSURE MAGNETISM

R. I n g a l l s

Physics Department, University of Washington, Seattle, Wa. 98195, U.S.A.

Abstract.- A study of the Mossbauer effect of 57Fe in magnetic materials subjected to high pressures is described. Specific systems reviewed are FeF2 , ct, s and -y phases of iron, invar, stain- less steel and a metallic glass.

1. Introduction.- Especially since P.W. Bridgman's pioneering work with high pressures has the capabi-

lity to vary interatomic spacing significantly en- hanced the study of matter /I/. Perhaps the most striking aspect of such work is one's ability to induce crystallographic, magnetic, or electronic phase changes with high pressure. Thus, for example, cubic materials can be made hexagonal, ferromagnetic made paramagnetic, and insulating made conducting.

In many instances the new structures or phases that appear may only be stable at high pressure, while in other cases they may persist after the pressure is released. Even where new phases, per se, are not generated by high pressures, material properties may still be significantly altered, since a varia- tion of the interatomic spacing affects energy levels in matter. In particular, optical, X-ray and even y_ r ay energies have been observed to have a pressure dependence. Moreover, such a dependence is fundamentally related to our microscopic under- standing of materials.

The first Mossbauer experiment at high pressure, by R.V. Pound et al. 12/, was a precision study of the isomer shift of metallic iron to a pressure of 3 kbar. This was soon followed by an extensive research program by H.G. Drickamer /l/

and coworkers, mainly with the isotope 5 7Fe, but to pressures which were as much as two orders of magnitude higher. This began with an extension of

the above work on b.c.c. iron (a-Fe) /3/ and then on to dilute iron alloys I hi and iron compounds/1/, with special emphasis on the isomer shift. The ba-

sic pressure-induced decreases in isomer shift (in- crease in ijj2(0)) observed in virtually all 57Fe systems, seems quite predictable in the metallic systems. There the main effect is a compression

of the 4s-like conduction electron charge density, almost as if it were due to free electrons /5/.

Theoretical treatments have verified this feature /6/.

The isomer shift in iron compounds and its pressure dependence has a more complex interpreta- tion and has prompted a large effort in molecular orbital calculations /7,8/. The basic feature is the importance of the covalent 4s admixture and overlap contributions from the ligands as well as, in some cases, back donation of 3d electrons from the ligands. Pressure work here has been instrumental in obtaining a satisfactory calibration constant, a, defined by /5/

More recently, high pressure studies have been made on many other Mossbauer isotopes /9/.

Compared with pressure studies of the other Mossbauer parameters, it is probably fair to say

that the isomer shift work has given us the most hard information for comparison with theory, to da-

te. Yet, the other parameters are certainly sensi- tive to pressure as well, i.e., recoilless fraction, second-order Doppler shift, quadrupole splitting and, of course, the magnetic hyperfine splitting.

The latter is the topic of this paper. Because of length limitations, here we mainly review some of the 57Fe work performed in our laboratory at the University of Washington.

In contrast to the isomer shift studies re- ported above, the pressure dependence of the hyperfine field has not shed as much light on the rele- vant nuclear calibration constants, e.g., the magnetic moments of the Mossbauer levels. Instead it has drawn us very deeply into questions involving the origin of magnetism in solids, and the effect

JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 3, Tome 40, mars 1979, page C2-174

Résumé.- On décrit ici l'effet Mossbauer de 57Fe dans des matériaux soumis à de hautes pressions.

Les systèmes spécifiques passés en revue sont FeFj, les phases a, £ et y du fer, des invars, des aciers inoxydables et un verre métallique.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979262

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of p r e s s u r e on t h i s magnetism. H i s t o r i c a l l y , a s a b a s i c thermodynamic v a r i a b l e , p r e s s u r e h a s played a secondary r o l e t o t h e magnetic f i e l d s t r e n g t h , H.

Of c o u r s e p r e s s u r e may be used t o produce new magne- t i c a l l y i n t e r e s t i n g phases, b u t i t s main use h e r e i s i n i t s e f f e c t upon magnetic c r i t i c a l temperatures and magnetization, M ( u s u a l l y a t H = 0 ) . A complete d e s c r i p t i o n of any magnetic system should, a s a mat- t e r of p r i n c i p l e , i n c l u d e t h e p r e s s u r e .

The h y p e r f i n e magnetic f i e l d , Hhf, g e n e r a l l y p r o v i d e s a measure of t h e magnetization, o r sub- l a t t i c e magnetization i n t h e c a s e of an a n t i f e r r o - magnet. This i s because, i n l a r g e p a r t , t h e main c o n t r i b u t i o n t o

ylf

i s t h e c o n t a c t s p i n d e n s i t y which i s p o l a r i z e d by t h e same magnetic e l e c t r o n s

t h a t give r i s e t o t h e magnetization. Most of t h e o t h e r c o n t r i b u t i o n s a l s o go a s t h e n e t s p i n of t h e magnetic e l e c t r o n s . This i s n o t t r u e o f t h e o r b i t a l term, however. Because of unquenched o r b i t a l angu- l a r momentum i n f o r example COO / l o / , t h e h y p e r f i n e f i e l d d e c r e a s e s a t low temperature i n c o n t r a s t t o t h e s u b l a t t i c e magnetization. Thus one may say t h e

" f a c t o r of p r o p o r t i o n a l i t y " between Hhf and M i s somewhat dependent upon temperature and p r e s s u r e I l l / . Time dependent e f f e c t s such a s e l e c t r o n i c re- l a x a t i o n o r superparamagnetism may a l s o give r i s e t o s i t u a t i o n s i n which t h e r e i s a h y p e r f i n e f i e l d and no magnetization o r v i c e v e r s a .

N e v e r t h e l e s s , t h e presence of a magpetic hy- p e r f i n e s p l i t t i n g g e n e r a l l y s i g n i f i e s a s t a t e of l o n g range magnetic o r d e r . For t h i s r e a s o n t h e M ~ S - sbauer thermal scan technique 1121 may b e used w i t h advantage i n determining t h e o n s e t of such a s t a t e . It r e l i e s on t h e g e n e r a l f a c t t h a t t h e y-ray t r a n s - mission through a MEssbauer a b s o r b e r changes d i s - continuously i f e i t h e r t h e source o r a b s o r b e r , mo- v i n g a t some c o n s t a n t v e l o c i t y (such a s zero) w i t h r e s p e c t t o t h e o t h e r , undergoes a magnetic t r a n s i - t i o n . We have found t h i s t o be a r a t h e r e f f e c t i v e means of observing t h e v a r i a t i o n of magnetic t r a n - s i t i o n s w i t h p r e s s u r e .

9.

Experimental.- The h i g h p r e s s u r e methods used h e r e a r e c l o s e l y r e l a t e d t o those of Drickamer and coworkers / 3 , 1 3 / . The sample f i l m o r f o i l i s embedded u p r i g h t i n a g a s k e t of B + LiH which i s p l a c e d b e t - ween t h e t r u n c a t e d t i p s of t a p e r e d t u n g s t e n c a r b i d e p i s t o n s (Bridgman-anvils). The r a d i a t i o n then p a s s e s r a d i a l l y through t h e g a s k e t , p e r p e n d i c u l a r t o t h e p i s t o n axes. A s o r i g i n a l l y used, t h e tapered por- t i o n s of t h e p i s t o n s were supported a s w e l l . I n such a c a s e , t h e p i s t o n s a r e g e n e r a l l y c a l l e d " D r i -

ckamer a n v i l s " i n fond t r i b u t e t o t h e i r d e s i g n e r . We have subsequently i n c o r p o r a t e d t h e above high p r e s s u r e technology i n t o a low temperature ap- p a r a t u s i n which t h e f o r c e on t h e p i s t o n s i s mutu- a l l y orthogonal t o t h e c o l d f i n g e r of a h e l i u m cryo- s t a t a s w e l l a s t h e d i r e c t i o n of t h e r a d i a t i o n 1141.

U n t i l q u i t e r e c e n t l y , s o l i d a n g l e l i m i t a t i o n s i n t h i s d e s i g n made i t only f e a s i b l e t o apply p r e s s u r e t o t h e MGssbauer source. However, i n t h e c a s e of thermal s c a n s i t was p o s s i b l e t o a l s o squeeze ab- s o r b e r s f o r which a ~ i j s s b a u e r resonance l i n e i n t h e paramagnetic s t a t e happened t o o v e r l a p t h e source l i n e a t z e r o r e l a t i v e v e l o c i t y 1151. I n such a n a r - rangement the s o u r c e i s p l a c e d i r e c t l y behind t h e magnetic a b s o r b e r and b o t h a r e placed u n d e r p r e s s u r e . The e m i t t e d r a d i a t i o n i s then simply recorded a s a f u n c t i o n of temperature.

I n a subsequent m o d i f i c a t i o n , Bridgman an- v i l s , t h a t i s , a n v i l s w i t h o u t a supported t a p e r , have a l s o been used t o study e i t h e r MGssbauer sources o r a b s o r b e r s . When an a b s o r b e r i s t o be s t u d i e d , a moving source may be brought i n t o t h e low tempe- r a t u r e r e g i o n c l o s e t o t h e a b s o r b e r (Fig. l ) , a s i n t h e e a r l i e r room temperature d e s i g n /13/. This de- velopment i s accomplished by means of a bellows i n t h e c r y o s t a t w a l l (C.M. Liu and R. I n g a l l s , Rev.

S c i . Instrum.

49

(1978) 1680). Such arrangement re- q u i r e s a c e r t a i n amount of c a r e i n c o l l i m a t i n g t h e r a d i a t i o n and p o s i t i o n i n g t h e moving s o u r c e r e l a t i v e 'to t h e a b s o r b e r .

F i g . 1 : Schematic diagram of t h e t h r e e - a x i s a r r a n - gement f o r s i m u l t a n e o u s l y applying pressu- r e and c o o l i n g a ~ E s s b a u e r absorber.

With g r e a t c a r e i n l o a d i n g such c e l l s , t h e sample p r e s s u r e , f o r a given a p p l i e d f o r c e , i s q u i t e r e p r o d u c i b l e . Pres_sure c a l i b r a t i o n may be checked by measuring e l e c t r i c a l r e s i s t a n c e s d i s c o n t i n u i t i e s

13

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JOURNAL DE PHYSIQUE

o f , f o r example, bisimuth, which i s loaded i n a geo- m e t r i c a l arrangement s i m i l a r t o t h a t of t h e Mgss- bauer sample.

Many o t h e r high p r e s s u r e techniques have been developed, some of which a r e based on an a x i a l arrangement i n which t h e y-rays p a s s through a n v i l s made of a m a t e r i a l of low atomic number. S u i t a b l e m a t e r i a l s a r e boron c a r b i d e , c u b i c boron n i t r i d e , o r diamond. Recently, workers a t t h e U.S. N a t i o n a l Geophysical Laboratory, u s i n g diamond a n v i l s , c l a i m t o have reached a s t a t i c p r e s s u r e of 1.72 megabar 1161. A t t h e same i n s t i t u t i o n have been Mzssbauer experiments on Fe20s and FesOt, t o about 600 k b a r 1171. One g r e a t advantage of t h e diamond t e c h n i q u e i s t h a t t h e p r e s s u r e may be continuously monitored by observing t h e s h i f t i n wavelength of a f l u o r e s - c e n t m a t e r i a l , such a s ruby, contained i n t h e gaske- t e d r e g i o n between t h e a n v i l s 1181.

3.Superexchange i n t e r a c t i o n i n Fey2.- From t h e e a r l y work of Abragam and Boutron /19/, and Wertheim 1201, FeF2 has been of fundamental i n t e r e s t t o MEssbauer s p e c t r a c o p i s t s . As one of t h e more i o n i c of t h e t r a n - s i t i o n metal systems i t s temperature dependent quadrupole s p l i t t i n g h a s a r a t h e r simple phenomenologi- c a l i n t e r p r e t a t i o n based upon simple c r y s t a l f i e l d t h e o r y 1211. The same may be s a i d of t h e a n i s o t r o p y c o n s t a n t s , D and E , and g f a c t o r s a p p e a r i n g i n i t s s p i n Hamiltonian.

An a t t e m p t a t a more fundamental i n t e r p r e t a - t i o n of both t h e e f f e c t of temperature and p r e s s u r e 1221 on t h i s quadrupole s p l i t t i n g was subsequently made. This was a c o n f i g u r a t i o n - i n t e r a c t i o n molecular o r b i t a l c a l c u l a t i o n 1231. I t h a s r e c e n t l y been r e f i - ned and a l s o used t o t h e o r e t i c a l l y determine t h e important p r e s s u r e dependence of t h e isomer s h i f t a s w e l l 171.

A t atmospheric p r e s s u r e J?e'F2 i s a n t i f e r r o - m a g n e t i c a l l y ordered below 78 K. Noreover, i t i s presumably a good example of a " l o c a l i z e d " magnetic system, w i t h t h e magnetic p r o p e r t i e s completely do- minated by. t h e superexchange i n t e r a c t i o n between a Fe 2+ i o n a t t h e c e n t e r of a u n i t c e l l and a second a t t h e c o r n e r . Although S i l v a ' s c a l c u l a t i o n 1231 of t h i s i n t e r a c t i o n was n o t complete he e s t i m a t e d t h a t i t would i n c r e a s e according t o B l o c h l s l110/3 r u l e "

1241. That i s

D r . Garcia, of our l a b o r a t o r y , h a s v e r y recen- t l y r e f i n e d t h i s a n a l y s i s 1251 by a d a p t i n g a forma- l i s m developed by R i m e r 1261. I n t h i s d e t a i l e d c a l -

c u l a t i o n , t h e above superexchange coupling parame- t e r , J , c o n s i s t s of s e v e r a l terms. These i n c l u d e t h e dominant k i n e t i c exchange term of Anderson 1271, a s w e l l a s s e v e r a l s m a l l e r terms. The c a l c u l a t i o n was l a r g e l y based upon covalency parameters of S i l - va 1231, p l u s a d d i t i o n a l o v e r l a p and t r a n s f e r i n t e - g r a l s . A l l were c a l c u l a t e d a s a f u n c t i o n of pres- s u r e based upon p r e s s u r e dependent X-ray d a t a of C h r i s t o e and Drickamer /22/.

I n performing t h i s c a l c u l a t i o n i t was d i s - covered t h a t t h e exchange i n t e g r a l , a s c a l c u l a t e d , was ve8ry s e n s i t i v e t o t h e r e l e v a n t r a t h e r poorly known covalency parameters, and was e a s i l y overes- timated by a s much a s a f a c t o r of f i v e ! However, t h e parameter, y was, by way of c o n t r a s t , q u i t e

J

i n s e n s i t i v e t o t h e v a l u e s of t h e covalency parame- t e r s . G a r c i a ' s b e s t v a l u e i s yJ = 3 . 5 .

An experiment was conducted t o measure t h e p r e s s u r e dependence of t h i s NLel temperature 1151.

A t t h a t time i t was n o t p o s s i b l e t o s t u d y t h e con- p l e t e Mzssbauer s p e c t r a of a b s o r b e r s i n our appa- r a t u s . However i n t h i s f o r t u i t o u s c a s e i t was pos- s i b l e t o perform t h e thermal scan measurements a s d e s c r i b e d i n s e c t i o n 2. This was because, f o r t u n a - t e l y , a t low temperature (P < 50 k b a r ) a s t a i n l e s s s t e e l s o u r c e n i c e l y overlapped one of t h e l i n e s of t h e quadrupole d o u b l e t i n FeF2 a t ^%80 K. The e x p e r i m e n t a l r e s u l t was t h a t TN i n c r e a s e d by 0.27 2 0 . 0 3 Klkbar, o r

Since TN i s expected t o go a s J , t h i s r e s u l t i s q u i t e c o n s i s t e n t w i t h t h e o t h e r r e s u l t s .

As a s i d e l i g h t on t h e above, t h e r e have been e s t i m a t e s of t h e e f f e c t of p r e s s u r e on t h e a n i s o t r o p y c o n s t a n t s , s p i n - o r b i t coupling, g fac- t o r s , h y p e r f i n e magnetic f i e l d and quadrupole s p l i t - t i n g below TN 1281. With t h e r e c e n t m o d i f i c a t i o n of our a p p a r a t u s t o handle a b s o r b e r s , we s h a l l soon perform t h e r e q u i r e d Mzssbauer s t u d i e s . 4. a and E i r o n . - I n an e x t e n s i o n of t h e e a r l i e r work on i r o n /2,3/ i t has been l e a r n e d t h a t t h e p r e s s u r e induced d e c r e a s e i n h y p e r f i n e f i e l d i s e s s e n t i a l l y t h e same a t 82 K and 20 K a s a t room temperature 1291. Such a d e c r e a s e has been a t t r i - b u t e d i n p a r t t o an enhancement of t h e c o n t a c t d e n s i t y of t h e conduction e l e c t r o n s which a r e po- l a r i z e d o p p o s i t e l y t o t h e c o r e 161. The s a t u r a t i o n m a g n e t i z a t i o n i t s e l f has been found t o d e c r e a s e even more r a p i d l y t h a n t h e h y p e r f i n e f i e l d 1301.

This i s a n o t h e r example where t h e two f i e l d s a r e

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n o t s t r i c t l y p r o p o r t i o n a l . It may be added t h a t t h e Curie temperature i s independent of p r e s s u r e . These r e s u l t s l e n d some support t o t h e p i c t u r e of a i r o n behaving much l i k e a moderately weak i t i n e r a n t magnet. (A v e r y "weak" system would b e one t h a t b a r e l y s a t i s f i e s a c r i t e r i o n f o r magnetic o r d e r i n g t o occur.

I n v a r o r ZrZn, may be such examples /31/. )

We and o t h e r s have a l s o cooled t h e h.c.p. form of i r o n (E-Fe)., which i s s t a b l e above 110 kbar a t room temperature /3,29,32/. No magnetic o r d e r i n g ( i . e . no h y p e r f i n e magnetic s p l i t t i n g ) was o b s e r v e d , down t o 2.2 K. This has been i n t e r p r e t e d a s agree- i n g w i t h an i t i n e r a n t p i c t u r e 1331. That i s , t h e band s t r u c t u r e f o r t h i s system a p p a r e n t l y y i e l d s a d e n s i t y of s t a t e s t h a t i s too low t o s a t i s f y t h e Stoner-Wohlfarth c r i t e r i o n f o r magnetic s t a b i l i t y . 5. y-iron.- T h e - f . c . c . form of i r o n (y-Fe) which i s only s t a b l e i n pure form above 1180 K a t P = 0, may be s t a b i l i z e d a s f i n e a n t i f e r r o m a g n e t i c p r e c i p i - t a t e s i n copper. The magnetic o r d e r i n g i s comensu- r a t e w i t h t h e l a t t i c e . The NBel temperature i s 67 K f o r t h e l a r g e s t s t a b l e p a r t i c l e s and d e c r e a s e s somewhat w i t h p a r t i c l e s i z e 1341. We have r e c e n t l y s t u - d i e d such a system a s a ~ g s s b a u e r a b s o r b e r (C.M. Liu and R. I n g a l l s , J. Appl. Thys., t o b e p u b l i s h e d ) .

Since samples w i t h p r e c i p i t a t e diameter l a r g e r

0

than roughly 60 A t r a n s f o r m t o the a phase w i t h p r e s - s u r e , one has had t o s t u d y samples w i t h s m a l l e r d i a - meter and, a s a consequence, lower N g e l t e m p e r a t u r e ,

4 6 K. Our thermal scan r e s u l t s (Fig. 2) y i e l d

Although t h e t h e o r e t i c a l i n t e r p r e t a t i o n of such a r e s u l t i s n o t a v a i l a b l e , s e v e r a l remarks may be made. The Ndel temperature of t h e p r e s e n t system i s n o t g r e a t l y d i f f e r e n t from e x t r a p o l a t i o n s based upon y-Fe-Kn and y-Fe-Ni-Cr a l l o y s /35/. Moreover, s i n c e band c a l c u l a t i o n s 1361 s u c c e s s f u l l y p r e d i c t a n t i - ferromagnetism t o occur f o r y-Fe ( a s opposed t o f e r - romagnetism i n a-Fe) we presumable have a r e s u l t t h a t may b e a p p l i e d t o pure y-Fe. A q u e s t i o n may a- r i s e connected w i t h t h e r e d u c t i o n of t h e Ndel ten- p e r a t u r e w i t h p a r t i c l e s i z e a t P = 0. S e v e r a l causes have been suggested f o r such a r e d u c t i o n /34/ such a s copper contamination, changes i n t h e l a t t i c e parameter o r s t r a i n e f f e c t s . I n a d d i t i o n t h e m i s t h e p o s s i b i l i t y of superparamagnetism (D.L. Williamson, p r i v a t e communication). I n g e n e r a l , however, i t i s d o u b t f u l t h a t any of t h e s e causes could a l t e r our g e n e r a l conclusion : p r e s s u r e a p p a r e n t l y weakens t h e

20 40

6 0 80

TEMPERATURE ( K )

Fig. 2 : Thermal s c a n s obtained a t v a r i o u s p r e s s u r e s f o r a b s o r b e r s of y-Fe p r e c i p i t a t e s i n a copper ma- t r i x . The f i x e d source i s Co-57 i n rhodium.

a n t i f e r r o m a g n e t i c S t a t e of y-Fe. However, t h e de- gree t o which i t i s weakened may conceivably indeed be dependent upon p a r t i c l e s i z e , e t c .

It may a l s o be mentioned t h a t t h e Ndel tem- p e r a t u r e of y-Fe f i t s i n w i t h t h e dependence of T

N upon l a t t i c e parameter i n a s e r i e s of y-iron a l l o y s 1351. I n t h e incommensurate systems, y-FeMn and C r , p r e s s u r e a l s o reduces TN. However, so f a r t h e r e i s no theory t o s a y , f o r example, t h a t y-Fe behaves like a "weak" i t i n e r a n t system j u s t because of t h e n e g a t i v e p r e s s u r e d e r i v a t i v e of T N'

6. f . c . c . i r o n a l l o y s . - Our l a b o r a t o r y h a s a l s o examined s e v e r a l i r o n a l l o y s which s t a b i l i z e i n t h e f . c.c. phase, namely i n v a r s 1371 and s t a i n l e s s 1381.

They have mainly been examined a s M k s b a u e r sources.

The i n v a r systems, Fe,-xNi,, examined a t high p r e s s u r e were f o r c o n c e n t r a t i o n s x = 30, 34and 36. They a r e ferromagnetic a t atmospheric p r e s s u r e , and, a s remarked e a r l i e r , t h e i r Curie temperature d e c r e a s e s with i n c r e a s i n g p r e s s u r e . A t very h i g h p r e s s u r e s (100 k b a r t o 200 k b a r ) t h e two a l l o y s , x = 0.30 and 0.34, appear t o o r d e r antiferromagne- t i c a l l y w i t h N6el temperatures i n c r e a s i n g w i t h p r e s s u r e . (The a l l o y w i t h x = 0.36 d i d n o t show any o r d e r i n g below 238 k b a r , b u t i t was only s t u - d i e d a t room temperature). From t h e s e experiments we have a g a i n shown t h a t the ferromagnetic s t a t e

i s l e s s s t a b l e w i t h g r e a t e r i r o n c o n t e n t /39/ o r g r e a t e r p r e s s u r e , whereas t h e . o p p o s i t e i s t r u e f o r t h e a n t i f e r r o m a g n e t i c s t a t e .

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c2-

178 JOURNAL DE _PHYSIQUE

It should be remarked t h a t t h e f e r r o m a p e t i s m i n i n v a r h a s prompted welcome t h e o r e t i c a l i n t e r e s t /31,39/ i n terms of t h e i t i n e r a n t p i c t u r e . However, a phenomenological l o c a l i z e d d e s c r i p t i o n may a l s o b e made /37/. Perhaps magnetic i n t e r a c t i o n s between atom p a i r s , a t l e a s t i n t h e s e n s e of Moriya / 4 0 / , h a s v a l i d i t y .

Two m a t e r i a l s which a r e s t a b l e a n t i f e r r o m a g n e t s a t atmospheric p r e s s u r e a r e t h e s t a i n l e s s s t e e l s type 302 (% F e 7 6 N i 1 2 C r 1 ~ ) and t y p e 310 ^('1.FessNinlCrzo) We have a l s o s t u d i e d them u s i n g o u r thermal s c a n me- thod. The r e s p e c t i v e P = 0 Ndel t e m p e r a t u r e s a r e 5 8 K and 31 K. The type of a n t i f e r r o m a g n e t i c o r d e r i n g i s i d e n t i c a l w i t h t h a t of y-Fe. At v e r y h i g h p r e s s u r e s we f i n d t h e N6el temperature of S.S. 302 t o i n c r e a s e g r e a t l y a s w i t h t h e i n v a r s / 3 8 / . However, a t low p r e s s u r e s i t s b e h a v i o r h a s n o t been c l e a r l y measured, i t may even somewhat d e c r e a s e . The Ndel t e m p e r a t u r e o f S.S. 310 c l e a r l y d e c r e a s e s w i t h p r e s s u r e i n t h e low p r e s s u r e r e g i o n (P

<

5 0 k b a r ) a s w i t h y-Fe (C.M.

L i u and R. I n g a l l s , J. Appl. P h y s . , t o b e published).

As w i t h t h e l a t t e r we have n o t gone t o v e r y h i g h p r e s s u r e s . Here a g a i n a s w i t h t h e i n v a r s , i n c r e a s i n g Fe c o n t e n t s t a b i l i z e s t h e a n t i f e r r o m a g n e t i c s t a t e , and t h e more s t a b l e t h e system, a n t i f e r r o m a g n e t i c a - l l y , t h e more s t a b l e i t i s w i t h p r e s s u r e . However, more i n f o r m a t i o n i s r e q u i r e d t o g i v e a complete p i e t u r e of t h e s e systems.

7. I r o n m e t a l l i c g l a s s e s . - Very r e c e n t l y we have be- gun a s t u d y o f f e r r o m a g n e t i c m e t a l l i c g l a s s e s con- t a i n i n g i r o n . They a r e n o t o n l y i n t e r e s t i n g techno- l o g i c a l l y because of t h e i r low c o e r c i v i t y , e t c . , b u t a l s o t h e o r e t i c a l l y , where t h e y appear t o be good examples of i d e a l weak i t i n e r a n t f e r r o m a g n e t s /41/.

Using a b s o r b e r s of amorphous Fe,2Ni,6Crl,P,2B6 (METGLAS 2826 A , A l l i e d Chemical) we have measured t h e MEssbauer e f f e c t and i t s dependence upon p r e s s u - r e (C.Y. L i u , R. I n g a l l s J.E. Whitmore, K.V. Rao, and S.M. Bhagat, J. Appl. Phys. t o b e p u b l i s h e d ) . Above t h e C u r i e t e m p e r a t u r e , 245K, t h e P = 0 _spec-

trum i s a b r o a d , asymmetric d o u b l e t , presumably c a w s e d by a range o f isomer s h i f t s and quadrupole s p l i - t t i n g ~ . At 78 K, w e l l beow Tc, t h e spectrum c o n s i s t s o f t h i s same d o u b l e t superposed on a d i f f u s e hyper- f i n e p a t t e r n . From t h e o v e r a l l w i d t h o f t h i s magne- t i c component we e s t i m a t e t h e l a r g e s t s i g n i f i c a n t h y p e r f i n e f i e l d t o b e approximately 210 kOe. The i n t e n s e p a r a a g n e t i c component below T i s unique f o r a m e t g l a s . However, such a f e a t u r e i s n o t s u r - p r i s i n g i n view of t h e o b s e r v a t i o n of a l a r g e , low t e m p e r a t u r e h i g h f i e l d , s u s c e p t i b i l i t y f o r t h e same m a t e r i a l /42/.

The p r e s s u r e dependence of t h i s s y s t e m t o 110 k b a r i s v e r y s i m i l a r t o t h e f e r r o m a g n e t i c i n v a r a l l o y s d i s c u s s e d e a r l i e r . The C u r i e t e m p e r a t u r e r e - duces a t an approximate r a t e of

-

0.5 K/kbar, w h i l e t h e h y p e r f i n e f i e l d i n i t i a l l y d e c r e a s e s a t a r a t e of ^%-1 kOe/kbar. At h i g h e r p r e s s u r e s t h e r a t e of d e c r e a s e i s s m a l l e r . Work on t h i s and o t h e r s i m i l a r i n t r i g u i n g systems i s c o n t i n u i n g .

8.Summary.- The Mijssbauer e f f e c t i s a n e x t r e m e l y powerful t o o l f o r o b s e r v i n g t h e pronounced e f f e c t s of p r e s s u r e on a l a r g e v a r i e t y o f magnetic systems.

However, a t p r e s e n t , experiments i n t h i s f i e l d re- main demanding. But t h e same may b e s a i d f o r t h e

t h e o r y o f magnetism i t s e l f e s p e c i a l l y a t f i n i t e tem- p e r a t u r e s . R e g r e t f u l l y , t o make t h i n g s worse, most such c a l c u l a t i o n s f a i l t o i n c l u d e p r e s s u r e ( o r vo- lume) i n a fundamental way, a l t h o u g h t h e measured p r e s s u r e b e h a v i o r i s a p o t e n t i a l v e r i f i c a t i o n of t h e i r v a l i d i t y . O p t i m i s t i c a l l y , i t i s hoped t h a t a t some p o i n t i n t h e n e a r f u t u r e t h e t h e o r e t i c a l advances, can b e g i n t o keep pace w i t h t h e e x p e r i - mental p r o g r e s s !

Acknowledgements,- The a u t h o r i s d e e p l y i n d e b t e d t o t h i s c o l l e a g u e s , H.G. Drickamer, D.L. Willamson, D.R. R h i g e r , D.M. S i l v a , G.A. G a r c i a , C.M. L i u and K.V. Rao. T h i s work was s u p p o r t e d by t h e U.S. De- p a r t m e n t of Energy C o n t r a c t EY-76-S-06-2225 and U.S. N a t i o n a l Science Foundation Grant DMR-76-21053.

References

/ 1 / Drickamer, H.G. and Franck, C.W., " E l e c t r o n i c T r a n s i t i o n s and t h e High P r e s s u r e Chemistry and P h y s i c s of S o l i d s , (Chapman and H a l l , London) 1973 and r e f e r e n c e s t h e r e i n . / 2 / Pound, R.V., benedek, G.B, and Drever, R.,

Phys. Rev. L e t t .

1

(1961) 405.

/ 3 / P i p k o r n , D.N., Edge, C.K., Debrunner, P . , De P a s q u a l i , G., Drickamer, H.G. and F r a u e n f e l d e r , H., Phys. Rev.

135

(1964) A1604.

/ 4 / I n g a l l s , R., Drickamer, H.G. and DePasquali,G., PHys. Rev.

155

(1967) 165.

/ 5 / I n g a l l s , R., Phys. Rev.

155

(1967) and S.S.

Comm.

2

(1974) 1 1 .

/ 6 / Duff, K.J. and Das, T.P., Phys. Rev. (1971) 2294.

/ 7 / Reschke, R., Trautwein, A., and H a r r i s , F.E., Phys. Rev. B15 (1977) 2708.

-

/ 8 / Van d e r Woude, F., Sawatzky, G.A. ans I n g a l l s , R., i n "&ssbauer Isomer S h i f t s " , Shenoy G.K.

and Qagner F., eds. (North H o l l a n d , Amsterdam) 1978 and r e f e r e n c e s t h e r e i n .

/ 9 / Williamson, D.L., i b i d .

/ l o / Ok, H.N. and Mullen, J . G . , Phys. Rev.

168

(1968) 563.

(7)

/ I I / Benedek, G.B., "Magnetic Resonance a t High P r e s s u r e " ( I n t e r s c i e n c e , New York) 1963, 47.

/12/ P r e s t o n , R.S., Hanna, S.S. and Heberle, J., Phys. Rev.

128

(1962) 2207.

/13/ Debrunner, P., Vaughan, R.W., Champion, A.R., Cohen, J . , Moyzis, J. and Drickamer, H.G., Rev.

S c i . Instrum.

2

(1966) 1310.

/ I 4 1 Williamson, D.L., Bukshpan, S., I n g a l l s , R. and S h e c h t e r , H., Rev. S c i . Instrum.

43

(1972) 194.

/ I 5 1 G a r c i a , G.A. and I n g a l l s , R., J. Phys. Chem.

S o l i d s

2

( 1976) 2 1 1 .

/ I 61 Mao, H.K. and B e l l , P.M., Science

200

(1978) 1145.

/ I 7 1 Mao, H.K., Virgo, D, and B e l l , P.M., Carnegie I n s t . Washington Yearb.

76

(1977) 522.

/18/ B a r n e t t , J . D . , Block, S. and P i e r m a r i n i , G . J . , Rev. S c i . Instrum. (1973) 1 . ^I /19/ Abragam, A. and Boutron, F., Compt. Rend.

252

(1961) 2404.

/20/ Wertheim, G.K., Phys.

121

(1961) 63.

/21/ Johnson, D.P. and I n g a l l s , R. Phys.,Rev.

(1970) 1013.

/ 2 2 / C h r i s t o e , C.W. and Drickamer, H.G., Phys. Rev.

B l (1970) 1813.

-

/23/ S i l v a , D.M. and I n g a l l s , R., Phys. Rev.

(1972) 3725.

/24/ Bloch, D . , J. Phys. Chem. S o l i d s

2

(1966) 881.

1251 G a r c i a , G.A., Ph. D . , D i s s e r t a t i o n , Univ. Wash., S e a t t l e (1977).

/26/ Rimmer, D.E., J. Phys.

2

(1969) 329.

/27/ Anderson, P.W., S o l i d S t a t e Phys.

16

(1963) 99.

/28/ S i l v a , D.M., Phys. Rev. (1973) 3306.

/29/ Williamson, D.L., Bukshpan, S. and I n g a l l s , R., Phys. Rev. (1972) 4194.

/30/ Kouvel, J.S. and Wilson, R.H., J. Appl. Phys.

32 (1961) 435.

-

/31/ Wohlfarth, E.P., I.E.E.E. Trans. Magn.

fi

(1975) 1638.

1321 Koniq, K., Wortmann and K a l v i u s , G.M., Proc.

I n t . Conf. on M k s b a u e r Spect., Cracow, Poland, 1975, Vol. 1 , p. 189.

/33/ F l e t c h e r , G.C. and Addis, R.P., J. Phys.

3

(1974) 1951.

1341 Gonser, U., Meechan, C . J . , Muir, A.H. and Wie- d e r s i c h , H., J. Appl. Phys.

34

(1963) 2373.

1351 Ishikawa, Y., S c i . Repts. Tohoku Univ.

58

(1975) 151.

/36/ Asano, S. and Yamashita,

J.,

J. Phys. S o c . J a p a n 31 (1971) 1000.

-

/37/ R h i g e r , D.R. and I n g a l l s , R., Phys. Rev. L e t t . 28 (1972) 749.

-

/38/ R h i g e r , D.R., I n g a l l s , R. and L i u , C.M., S o l i d S t a t e Commun.

2

(1976) 681.

/39/ Kanamori, J . , Teraoka, Y. and J o , T., A.I.P.

Conf. Proc.

26

(1975) 16.

/40/ Moriya, T., Prog. Theor. Phys.

34

⁽¹⁹⁶⁵⁾ ^329.

/41/ W o h l f a r t h , E.P., P r o c . Intermag. Conf. F l o r e n c e (1978) t o b e p u b l i s h e d .

/42/ F i g u e r o a , E . , Lundgren, L . , Beckman, 0. and Bhagat S.M., S o l i d S t a t e Commun.

20

(1976) 961.