M A G N E T I C S T R U C T U R E O F A L N I C O A L L O Y

D. A . L a p t e i a n d V . E . S h c h e r b a k o v UDC 669.405.538

Domain s t r u c t u r e as obse rved in the alnico al loy in a h igh-coe rc ive s ta te was desc r ibed . The o b s e r - vations were c a r r i e d out for different magne t ic s t a t e s of the a l loy with s imul taneous use of the magne to - optical K e r r effect and powder suspens ion methods .

Since the fo rma t ion m e c h a n i s m of s t r u c t u r e , r e v e a l e d by the powder method, is desc r ibed in detai l in [1, 2], and in [3] only the bas ic p r inc ip les of the use of the magne to -op t i ca l method in the domain s t r u c - tu re s tudies of alnico al l0y a re layed out, it is of in te res t to d i scuss only those magne t ic c h a r a c t e r i s t i c s of this a l loy that have been poss ib le to r e v e a l by the s imul taneous use of the two methods and a lso to d i s - cus s the s t r u c t u r e s that de se rve the a t tent ion f r o m the point of view of the i r phys ica l s t a te and which con- ta in novelty e l e m e n t s .

The single c ry s t a l spec imens , cut in the f o r m of pla tes (14 • 9 x 1 ram) with longer s ide coinciding with the d i rec t ion of the field H T applied during t h e r m a l t r e a t m e n t , we re subjec ted to tes t ing (H c = 600 Oe).

The magne t iza t ion r e v e r s a l p r o c e s s when the r e v e r s e magnet iz ing field H coincided with the d i r e c - tion of the p r ev ious ly applied field H T is shown in Fig. l a , b, c. It can be seen that magne t i za t ion r e v e r s a l of the spec imen as a whole occurs by fo rma t ion of a cons iderab le number of nuclei of r e v e r s e m a g n e t i z a - tion that grow along the field cover ing the whole of the s p e c i m e n .

It is known that in alnico al loy during magne t iza t ion r e v e r s a l p r o c e s s m a c r o s c o p i c a l domains appea r each of which is f o r m e d by a l a rge number of in terac t ing f e r r o m a g n e t i c p a r t i c l e s . In a conventional u n d e r - standing the domain wails were not obse rved . By presen t ing the pa r t i c l e s cons idered in the f o r m of dipoles , a r r a n g e d pa ra l l e l to one of the axes , C r a i k [4] has shown on the ba s i s of the magne tos ta t i c in te rac t ion that the tendency of the walls to dr if t in the t r a n s v e r s a l d i rec t ion will be insignificant as c o m p a r e d with the domain growth in the d i rec t ion of the r e v e r s e magnet iz ing field. The r ea l i ty of such a p r o c e s s is i l lus t ra ted in Fig. l a , b, c.

Fig. 1. Domain s t r u c t u r e during magne t i za t ion r e v e r s a l pa r a l l e l and under an angle to H T.

Branch of K r a s n o y a r s k Poly techntca l Inst i tute . T r a n s l a t e d f r o m [zves t iya Vysshtkh Uchebnyk_h Z a v e - denii, Fiz ika , No.4 , pp. 136-137, Apri l , 1973. Original a r t i c l e submi t ted F e b r u a r y 7, 1972; r ev i s ion sub- mi t red June 6, 1972.

�9 1975 Plenum Publishing Corporation, 227 West 17th 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.

567

Fig. 2 Fig. 3

Fig.2. Domain structure as revealed at the same site by different methods: a) magnetic suspen- sion method; b) magneto-optical method.

Fig. 3. Domain structure in a demagnetized state and during magnetizing along HT: a, b) magneto- optical method; c, d) powder method.

Considering the crystal structure (shape, size, misorientation of easy axes, density of particles), magnetic state, and relative posRion of the easy axis with the field direction, the character of the structure changes can be different. This is illustrated in the figures presented. On Fig. id, e, f are shown structural changes during magnetization reversal at an angle to H T. From the figure R is seen that in this case along wRh the domain growth in the direction close to HT, change in the structure on account of the domain dis- placement in the transversal direction is observed.

Comparing the structures corresponding to one and the same state, but revealed by different methods, enables a qualitative esthnate of the I s vector orientations in domains to be made. The presence of bright domains , cor responding to one and the s a m e s i te , F ig .2 , conf i rms that the vec to rs I s in these domains lie in the spec imen plane. The dark domains as r evea l ed by the magne to -op t i ca l method and powder suspen - sion, indicate that the vec to rs I s f o r m in these domains longitudinal and perpendicu la r components . D i rec - t ion of the longitudinal component in these domains is c lose to the di rect ion of the r e v e r s e magnet iz ing field.

F igure 3a, b shows the magnet ic s t r u c t u r e cor responding to the demagnet ized s ta te of the spec imen . Domains as r evea l ed by the magne to -op t i ca l means have the shape of per iod ica l ly a l te rna ted da rk and br ight bands pa ra l l e l to the axis of easy magnet iza t ion . This indicates the an t ipara l le l or ienta t ion of the vec to r s I s in the neighboring domains . The perpendicu la r component in these domains is c lose to ze ro . This can be ver i f ied by an ana lys i s of Fig. 3c in which the p a r t of the s a m e s t ruc tu re is shown with the aid of the magnet ic suspension. Powder p rec ip i t a t e s only at the domain boundar ies forming t rans i t iona l l a y e r s which appear much like the block boundar ies . In this case the magnet iza t ion r e v e r s a l p roce s s dif- f e r s s ignif icant ly f rom that desc r ibed above. According to the field inc rease along the ea sy axis the do- main configurat ion and the i r posi t ion in r e s p e c t to the spec imen sur face r e m a i n unchanged (Fig . 3b, d). At the s a m e t ime con t r a s t of the bands cor responding to the an t ipara l le l domains as r evea led by the m a g - neto-opt ica l method, fades away and d i sappears at fields cons iderably exceeding H c. When observa t ion is made with the magnet ic suspension the powder p rec ip i t a t e s , concent ra ted at the boundar ies of these do- mains , sp r ead along the spec imen su r face with increas ing field. C lea r ly , this can be a t t r ibuted to the fact that the magnet iz ing p roce s s occurs by ro ta t ion of the vec tor I s in the spec imen plane in domains where it is or iented opposi te ly to the field direct ion.

Thus the r e su l t s p resen ted showed:

a) the type of magnet ic s t ruc tu re in alnico al loy depends on t r e a t m e n t by the field;

b) depending on the magnet ic s ta te the magnet iza t ion and magnet ic r e v e r s a l p roceed differently;

568

! .

2. 3. 4.

c) the presence of magnetic structure in the demagnetized state allows to assume that in this mate- rial along with the magnetostatic occurs volume interaction between particles.

L I T E R A T U R E C I T E D

Ya. S. Shur and M. G. Luzhinskaya, Lzv. Akad. Nauk SSSR, Seriya Fiz., 3, 1022 (1966). M. G. Luzhinskaya, T. Z. Puzauova, and Ya. S. Shut, Fiz. Metal. i Metal., 23, No. 3, 495 (1967). M. G. Luzhinskaya, T. Z. Puzanova, and Ya. S. Shut, Fiz. Metal. i Metal., 25, No. 1, 191 (1968). D. J . Craik, Z. Angew. Phys., ]321, No. 1, 27 (1966).

569