70 CHAPTER 7. ITINERANT-ELECTRON MAGNETISMferromagnets is equal to zero because field-induced electron transfer into an already
completely filled subband is not possible.- Many metal systems consist of a combination of a 3d transition metal (T) with a non
magnetic metal (A). Frequently, ferromagnetism disappears when the concentration
of the T component becomes too low. This happens, for instance, in the series of
intermetallic compounds formed by combining the non-magnetic element yttrium with
cobalt: and The first four compounds are ferro
magnetic with Curie temperatures much higher than room temperature, whereas the
last compound does not show magnetic ordering at any temperature. It is wrong to say
that the Co moment in the latter compound has disappeared because electron transfer
from Y to the more electronegative Co has led to a filling up of the 3d band of the
latter, preventing 3d magnetism. More realistic is the explanation that mixing of the
Y valence-electron states with the Co 3d-electron states has led to a decrease of and
to a broadening of the 3d band and a concomitant lowering of The result is that
3d-band splitting will not occur, leaving the compound paramagnetic. Charge-transfer
effects, where the valence electrons of A decrease the depletion of the 3d band of T
do occur to some extent, but have a comparatively modest effect on the 3d-moment
reduction upon alloying. - The application of the itinerant-electron model to the description of magnetism in
3d-electron systems does not necessarily mean that the 3d-electron spin polarization
extends uniformly through the whole crystal. The small width of the 3d-electron band
implies that the 3d electrons are rather strongly localized at the 3d atoms, and this
holds a fortiori for their spin polarization. This justifies to some extent the use of
local moments in molecular-field approximations for describing the magnetic coupling
between 3d moments. It follows from the discussion given above that the moment of 3d
atoms consists to a first approximation only of a spin moment. It is common practice
to use the relation
7.4. INTERSUBLATTICE COUPLING IN ALLOYS OF
RARE EARTHS AND 3d METALSMetallic systems composed of magnetic rare-earth elements and magnetic 3d elements
have found their way into many modern applications such as high-performance permanent
magnets (Chapter 11), magneto-optic-recording materials (Chapter 13), and magneto-
acoustic devices (Chapter 16). The favorable properties of these materials are partly due to
the rare-earth sublattice (high magnetocrystalline anisotropy, high magnetostriction, high
magnetic moments) and partly due to the 3d sublattice (high magnetic-ordering temper
ature). In order to have this combination of favorable properties in one and the same
compound, it is of paramount importance that there be a strong magnetic coupling between
the two magnetic sublattices involved.
There are several hundred intermetallic compounds composed of rare-earth metals and
3d metals and their magnetic properties are fairly well known and have been reviewed
by Franse and Radwanski (1993). Without exception it is found that the rare-earth-spin
moment couples antiparallel to the 3d-spin moment. This feature can be understood by