ANNEALING EFFECTS ON PERMEABILITY RELAXATION IN AMORPHOUS FERROMAGNETS

Abstract

The re laxa t ion of t h e magnetic permeability has been measured a s a funct ion of temperature i n amorphous ribbon samples annealed a t d i f f e r e n t temperatures and f o r d i f f e r e n t annealing times. The r e s u l t s a r e analyzed to ge t informations on t h e very na ture of t h e ac t iva ted processes giving r i s e t o permeability re laxa t ion e f f e c t s . INTRODUCTION The relaxation of the magnetic permeability in amorphous ferromagnets of the type (Fe-Co-Ni) 30 (P-B) displays features closely related to the 20 structural properties of these materials (1 ) (2 ) (3 ) Permeability relaxation experiments give consequently informations on processes taking place in amorphous systems. In order to c l a r i fy the role of these processes, the magnetic permeabi 1 i t y relaxation has been studied between 4..2 K and 550 K i n amorphous ribbon samples annealed a t d i f ferent temperatures and fo r different annealitig times. In a l l casesathe curves of the permeability relaxation vs. measurement temperature of a l l studied samples have been found to be significantly affected by high temperature anneals. As i s known, permeability relaxation ef fec ts are related to the presence of atoms, or atomic clust e r s , interacting w i t h the local magnetization di rection and able to directionally order through thermally activated processes, thus hindering Bloch walls motion. This e f f ec t i s referred to as diffusion af ter -ef fec t (4) . The magnetic permeability value i s generally not affected by the i r revers ib le fluctuation a f t e r e f f e ~ t ( ~ ) . The permeability relaxation in crys ta l l ine a-iron i s due to i n t e r s t i t i a l diffusion of impurities, such as C or N ( ~ ) . These processes a re .characterized by sharp activation energies. The permeability relaxation in amorphous materials i s instead supposed to be related to activated processes characterized by a very broad distr ibution of activation energies. As described elsewhere) these processes can suitably be pictured as small irreversible changes i n the amorphous structure or as structural i n s t ab i l i t i e s . These ins t ab i l i t i e s , inherent t o the amorphous stateshave been described as atoms o r group of atoms periodica l l y moving between two or more equivalent configurations. As a f i r s t approximation, these activated processes are assumed to be largely independent. This assumption i s probably bet ter verif ied below and about room temperature, whi 1 e a t higher temperatures correlations between activated processes (7) are l ike ly to occur . EXPERIMENTAL Samples obtained from Fe40Ni40P1 4B6 and Fe80B20 ribbons (314 cm long, 152 mm wide) were annealed in vacuum a t d i f ferent temperatures and f o r different annealing times (Table I ) . All anneals were performed without magnetic f i e ld . The annealing temperatures were chosen to be lower than the crystall ization ones f o r these materials. The values of the reversible permeability of a l l samples were only s l ight ly modified by annealing. Moreover, no relevant differences in permeabi 1 i ty were found between samples annealed fo r 2 and f o r 6 hours,the major part of the permeabi 1 i t y changes occurring