magnetic anisotropy and coercivity of alnico-alloy films

3
MAGNETIC ANISOTROPY AND COEROIVITY OF AlNiCo-ALLOY FILMS R. N. Ganzha, A. v. Dolyuk, D. A. Laptei, and N. M. Salanskii UDC 539.216.2:538.6 Alloys of the Fe--Ni--A1 system with additions of Co, Ti, Cu constitute a group of hard magnetic ma- terials, alnico and tieonal. These materials are of primary importance in the modern manufacture of permanent magnets. Their high eoercivity is attained by special thermomagnetic treatment [1, 2]. One believes now that the breakdown of the high-temperature phase in a magnetic field results in the formation of particles with shape anisotropy and dimensions which meet the single-domain criterion. The nature of the eoereivity, the mechanism of magnetization reversal, the crystalline state, and the phase composition of these alloys are currently the subjects of many experimental and theoretical studies [3-5]. In this re- portwe will continue dealing with the magnetic properties of these alloys in film form [6]. Film specimens with a thickness up to 103 A were produced by vacuum evaporation (under 5" 10 -s mm Hg) of an alloy with a composition close to grade YuNDK-24. The condensate was deposited on the Ha, Oe ' 395 3 ,h-I I I t t I I t ] I I I He, Oe Ku-10"Zerg/em3 6751 ' v , l 325 3#0. 45 - - KU" 30~ "- leO, gO- 2aJ "~"'~---- " "11 "5'x =q l 2~a. m - , t55 ~ I /60' eO 1 / Ilc.t 135 I" leo 15 nc~ t. ~ ~o - ~ ' Jo j j I "1 1 sJ o Fig. 1 'Fig. 2 Fig. 1. Coercive force H e (Oe), anisotropy coefficient K u. 10 -3 (erg/cm3), and ratio Hc• I as functions of the thermomagnetic treatment temperature (~ for AlNiCo films (He• and HelI are the coercive forces of a film, measured respectively perpendicular and parallel to the axis of easy magnetization). Fig. 2. Coercive force of a film He (Oe) as a function of the mea- surement angle r Bottom curve refers to initial state, other curves refer to thermomagnetie treatment at indicated tempera- tures. Branch of Krasnoyarsk Polytechnic Institute. L. V. Kirenskii Institute of Physics, Siberian Branch of the Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 137-139, March, 1975. Original article submitted July 4, 1974. 76 Plenum Publishing Corporation, 22 7 West 1 7th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00. 412

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Page 1: Magnetic anisotropy and coercivity of AlNiCo-alloy films

M A G N E T I C A N I S O T R O P Y A N D C O E R O I V I T Y

O F A l N i C o - A L L O Y F I L M S

R. N. G a n z h a , A . v . D o l y u k , D . A . L a p t e i , a n d N. M. S a l a n s k i i

UDC 539.216.2:538.6

Alloys of the Fe--Ni--A1 s y s t e m with additions of Co, Ti , Cu const i tute a group of hard magnet ic m a - t e r i a l s , alnico and tieonal. These m a t e r i a l s a re of p r i m a r y impor tance in the modern manufacture of pe rmanen t magnets . The i r high eoerc iv i ty is at tained by special the rmomagne t ic t r ea tmen t [1, 2]. One be l i eves now that the breakdown of the h igh - t empe ra tu r e phase in a magnet ic field resu l t s in the fo rmat ion of pa r t i c l e s with shape anisotropy and d imensions which mee t the s ing le-domain c r i t e r ion . The nature of the eoere iv i ty , the mechan i sm of magnet iza t ion r e v e r s a l , the c rys ta l l ine s ta te , and the phase composi t ion of these al loys a re cu r ren t ly the subjects of many exper imenta l and theore t ica l s tudies [3-5]. In this r e - po r twe will continue dealing with the magnet ic p r o p e r t i e s of these al loys in f i lm f o r m [6].

F i lm spec imens with a th ickness up to 103 A were produced by vacuum evapora t ion (under 5" 10 -s m m Hg) of an alloy with a composi t ion c lose to grade YuNDK-24. The condensate was deposited on the

Ha, Oe ' 395 3 ,h-I I I t t I I t ]

I I I He, Oe Ku-10"Zerg/em 3 6751 ' v , l �9 325

3#0. 45 - - KU " 30~ "-

leO, gO- 2aJ " ~ " ' ~ - - - -

" "11 "5'x =q l 2~a. m - , t55 ~ I

/ 6 0 ' eO 1 / I lc . t 135 I " leo 15 nc~ t . ~ ~o - ~ '

Jo

j j I "1 1 sJ o Fig. 1 'Fig. 2

Fig. 1. Coerc ive force H e (Oe), anisotropy coeff icient K u. 10 -3 (erg/cm3), and rat io Hc• I as functions of the thermomagnet ic t r e a t m e n t t e m p e r a t u r e (~ for AlNiCo f i lms (He• and Hel I are the coerc ive f o r c e s of a f i lm, measu red respec t ive ly pe rpend icu la r and para l le l to the axis of easy magnetizat ion).

Fig. 2. Coerc ive force of a f i lm He (Oe) as a function of the m e a - s u r em en t angle r Bottom curve r e f e r s to initial s ta te , o ther cu rves r e f e r to the rmomagne t ie t r ea tmen t at indicated t e m p e r a - tu res .

Branch of K r a s n o y a r s k Polytechnic Inst i tute. L. V. Kirenski i Institute of Phys ics , Siberian Branch of the Academy of Sciences of the USSR. Trans la t ed f rom Izves t iya Vysshikh Uchebnykh Zavedenii , Fizika, No. 3, pp. 137-139, March, 1975. Original ar t ic le submit ted July 4, 1974.

�9 76 Plenum Publishing Corporation, 22 7 West 1 7th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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Page 2: Magnetic anisotropy and coercivity of AlNiCo-alloy films

s u r f a c e of p o l i s h e d q u a r t z , NaC1 c r y s t a l s , o r s i t a l . The f i l m s w e r e then t h e r m o m a g n e t i c a l l y t r e a t e d in a s p e c i a l h e a t - - v a c u u m s y s t e m at v a r i o u s t e m p e r a t u r e s f r o m 200 to 750~C and in a m a g n e t i c f i e ld 2500 Oe s t r o n g . The v a c u u m l e v e l d u r i n g th i s t h e r m o m a g n e t i c t r e a t m e n t w a s 5 �9 10 -~ m m Hg.

The d e g r e e of induced un iax ia l a n i s o t r o p y w a s c a l c u l a t e d f r o m to rque c h a r a c t e r i s t i c s which had b e e n m e a s u r e d in a u n i f o r m m a g n e t i c f i e ld of an i n t e n s i t y equal to 8000 Oe. The s e n s i t i v i t y of the a n i s o m e t e r f i l a m e n t w a s 6.2 �9 10 -4 dyn - c m / d i v i s i o n . The c o e r c i v e f o r c e and the m a g n e t i c s t r u c t u r e of the f i l m s w e r e d e t e r m i n e d on an a p p a r a t u s o p e r a t i n g with the F a r a d a y e f fec t .

RESULTS AND DISCUSSION

The magnetic properties of the films, immediately after deposition, were close to those of quenched bulk material [3]

Fig. 3. Magnetic structure of films, (H c ~ 5-90e, K u ~ 2 �9 103 erg/cm3). An electron-microscope examination by these authors [6] has established that the films

in demagnetized state, after various have also the polycrystalline structure of a one-phase solid

modes of thermomagnetic treatment. solution with a bcc crystal lattice (~-phase). Their mean- grain size does not exceed 350-400 A. Some film specimens yield reflexes indicating aweak grain orientation.

As the temperature of thermomagnetic treatment is raised, H e and K u increase while the mechanism of magnetization-reversal processes changes. The trends of H c and K u with treatment temperature are shown in Fig. I. On the same diagram is also shown the trend of the ratio He• I (ratio of coercive forces measured respectively perpendicular and parallel to the axis of easy magnetization). The curve indicates that low-temperature thermomagnetic treatment, to about 500~ does not produce substantial changes in the said parameters relative to the initial state of a film. Within this temperature there takes place a grain growth, anneal, and relief of internal stresses, as well as also a partial redistribution of ele- ments in the one-phase solid solution. The least effect has been revealed by eIectronographic analysis. Such structural changes do not affect appreciably the magnitude of the coercive force and the degree of induced uniaxial anisotropy. Magnetization and magnetization reversal in such films are effected by dis- placement of domain walls. The validity of these conclusions has been confirmed by direct observation of changes in the domain structure; it also follows from the manner in which the coercive force varies with the measurement angle within the plane of the film (Fig. 2) as well as from the magnitude of the ratio Hc• Hcl I (larger than unity, where displacement processes predominate).

Thermomagnetic treatment at temperatures within the 550-600~ range is characterized by the beginning of a ubiquitous precipitation of the ionic phase (7-phase with an fcc crystal lattice), a thorough redistribution of alloy elements between phases, and a larger difference between their magnetic characteris- tics. This process of phase transformations corresponds to the highest rate of increase of H c and Ku, which continues with rising treatment temperatures up to 700-750~ As the treatment temperature is raised, there form magnetically shielded fragments of the strongly magnetic ~'-phase amid the weakly magnetic 7-phase occupying an increasingly larger fraction of the total volume. Naturally, this must also produce changes in the processes of magnetization reversal in films. The angular characteristic of H c (Fig. 2) and the v:ariation of ratio Hc• I with treatment temperature (the ratio is smaller than unity for the given range of treatment temperatures) indicate that magnetization reversal in films of the said structural content occurs through a mechanism which also involves processes (largely nonuniform) as- sociated with the rotation of magnetization vectors in isolated regions of the strongly magnetic ~'-phase. An optomagnetic examination of changes in the domain structure during magnetization reversal has also confirmed the dominant role of rotation processes here (Fig. 3).

These particular trends of H c and Ku, as functions of the tbermomagnetic treatment temperature, can be explained on the assumption that the processes of magnetization reversal in films are determined es- sentially by the resultant effect of two anisotropies: crystallographic (Ka) and induced uniaxial (Ku). In the high-coercivity state, according to measurements made on bulk specimens [3], the coefficient of crystallo- graphic anisotropy in single-domain particles of the highly magnetic phase is about 105 ergs/cm 3, i.e., by

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Page 3: Magnetic anisotropy and coercivity of AlNiCo-alloy films

one o r d e r of magnitude l a r g e r than the coeff icient of induced anisotropy in f i lms . It is this c rys ta l lograph ic anisot ropy which contr ibutes p r i m a r i l y to the coerc ive force H c. Induced uniaxial anisot ropy is , apparent ly , due to the l e s s - p r o m i n e n t grain or ienta t ion and in turn affects the dependence of H c and K u on the t r ea tmen t tempe ra tu re .

In conclusion, it ought to be noted that Fe--Ni--A1--Co f i lms have much more in fe r ior h igh-coerc iv i ty c h a r a c t e r i s t i c s than bulk ma te r i a l . In o r d e r to explain this , it is n e c e s s a r y to make seve ra l assumpt ions . The grain or ienta t ion is weak in f i lms and a lmos t nonexistent in the s t r u c t u r e - f o r m i n g e lements of the s t rongly magnet ic phase . The content of f i lms dif fers somewhat f rom that of the initial ma te r i a l (especial ly in t e r m s of aluminum), so that, consequently, the mechan i sm of phase t r ans fo rma t ions is also different in f i lms and in bulk ma te r i a l . El iminat ing the causes of this infer ior i ty of Fe--Ni--A1--Co f i lms will , un- doubtedly, enhance the ef fec t iveness of the i r t he rmomagne t i c t r e a t m e n t and thus g rea t ly improve the i r h igh-coere iv i ty c h a r a c t e r i s t i c s .

1.

2, 3. 4. 5.

6.

7.

LITERATURE CITED

Ya. M. Dovgalevski i , Alloying and Heat T r e a t m e n t of Hard Magnetic Mate r ia l s [in Russian], Meta l - lu rg iya , Moscow (1971), p. 175. T. I. Bulygina and V. V. Sergeev, Fiz. Met. i Metal loved. , 27, No. 4, 703-709 (1969). A. S. Ermolenko , Izv. Akad. Nauk SSSR, Ser. Fiz . , 30, No. 6, 1046-1049 (1966). V. R. Kran tman , E lek t ronnaya Tekh., Ser .6 , Mater ia ly , No. 8, 29-42 (1972). Ya. S. Shur, "Ways of producing h igh-coerc iv i ty m a t e r i a l s for pe rmanen t magne t s , " Vestnik Akad. Nauk SSSR, No. 7 (1971). D. A. Laptei, N. M. Salanskii, et al., in: Thin Magnetic Films for Radio Engineering and Computer Engineering [in Russian], Vol. 2, Krasnoyarsk (1972). E. I. Kondorskii and P. P. Denisov, "Magnetization reversal in indirectly deposited cobalt films," Trans. Internationl Symposium, Irkutsk (1968), p. 99.

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