Chapter 4 Magnetic amorphous alloys

Abstract

The structural disorder of magnetic M-T, M-R and R-T alloys (T: magnetic transition metals, R: rare earths, M: not T and R) gives rise to significant changes of the mechanical, electrical, magnetic and magneto-optical properties as compared to the crystalline counterparts. Although it is not yet possible to explain many properties in terms of the relevant band structure of amorphous alloys, many new concepts and theories were generated and an overwhelming amount of experimental work was performed, leading to new physical insights concerning the atomic structure and short-range order, the relation between chemical bonding and magnetism, magnetic structures, anisotropic magnetic properties and transport properties on the one hand and the development of materials with a unique combination of properties on the other hand which makes these alloys attractive for a variety of applications. Generally, amorphous Fe-based alloys behave differently from Co- and Ni-based alloys due to their stronger (sp)-d hybridization and weaker covalent p-d bonding, resulting in broader valence bands. Also, they exhibit a sensitive dependence of the exchange coupling on the atomic distance, leading even to antiferromagnetic bonds for Fe Fe distances below 0.25 nm. The distribution of atomic distances inferred from the radial distribution function suggests the presence of a concentration-dependent portion of negative exchange interactions. Thus, various amorphous M 1 − x Fe x alloys show noncollinear magnetic structures, and for M = Zr, Hf, Y, La, Ce or Lu even speromagnetic or spin-glass-like behavior occurs with a tricritical point for x ≥ 0.9 and Curie temperatures ranging between 100 and 200 K. In amorphous M-Co alloys, small Co-Co distances are favorable for the magnetic moment formation and tend to increase the exchange interaction and thus T C . Therefore, amorphous Co-based alloys are predominantly strong ferromagnets, although significant differences in the magnetic properties are observed for Co-based alloys containing metalloids or other elements. The magnetic moment variation was interpreted in terms of the magnetic valence model or the environment model. In both cases, the experimental data were well described for certain classes of alloys. Most transition-metal-metalloid alloys exhibit good corrosion resistance, high electrical resistivity, good mechanical properties and are soft-magnetic materials. Ferich alloys exhibit the highest saturation flux density, and the Co-based alloys show low magnetostriction, high permeabilities and very low magnetic losses. These properties make various magnetic glasses attractive candidates for commercial applications such as power supplies, transformers, sensors, transducers, magnetic heads, magnetic shielding or magnetometers. Amorphous rare-earth-transition-metal alloys reveal pronounced differences in their magnetic properties as compared to M-T alloys due to the different electronic structure of the rare earths and the presence of two magnetic sublattices formed from elements of different groups. The negative exchange coupling between the 5d rare-earth electrons and the 3d transition-metal electrons leads to a parallel alignment of the R and T moments for the light rare earths and an antiparallel alignment for the heavy rare earths. A further difference in the amorphous R-T alloys with respect to M T alloys is the strong influence of the structural disorder on the local direction of the R moments which are coupled via the strong spin-orbit coupling to the randomly varying axes of the electrostatic field. This leads to sperimagnetic structures except for the Gd (S-state) based alloys exhibiting ferrimagnetic order. Amorphous R Fe and R Co alloys reveal the same differences as observed for M-Fe and M-Co alloys. Fe-rich R-Fe alloys exhibit a very low T C , below 200 K, due to competing positive and negative exchange interactions. Therefore, all R 1 − x Fe x alloys show a maximum in the concentration dependence of T C around x ≅ 0.7, followed by a strong turndown of T C at large x , in contrast to R-Co alloys revealing a steep increase of T C for alloy compositions with x above the critical composition. Amorphous R-T alloys containing non-S-state rare earths are characterized by strong uniaxial anisotropies and high coercivities but low magnetizations in the case of sperimagnetic order. The presence of a compensation temperature for the antiferro-magnetically coupled alloys gives rise to strong influences on the temperature and the concentration dependence of the magnetic, magneto-optical and transport properties. Amorphous R T alloys also show a large extraordinary Hall effect arising from side jumb and skew scattering processes. The favorable magnetic and magneto-optical properties have led to the development of GdTb-Fe, Tb-FeCo or Dy-FeCo alloys for magneto-optical data storage.