Itinerant ferromagnetism

The discussion of the preceding two sections relied on the presumption that localized (atomic-like) moments were present. However, valence s and p electrons are always best described by Bloch fimctions, while 4/electrons are localized and 5/are intermediate. Valence d electrons, depending on the intemuclear distance, are also intermediate -neither free nor atomic-Uke. In such cases, the dilemma is that the Heisenberg exchange interaction of Eq. 8.43, which is the physical basis for the Weiss field, is not strictly applicable in the case of delocalized electrons in metallic systems, in spite of the success of the Weiss model. 

Recall that, for atoms, only electrons outside of closed shells need to be considered. Analogously, in solids with delocalized states, only those electrons in partially filled 

TABLE 8.6. Stoner Parameters for Several Elements in Rydbergs (1 Ry = 13.6 eV). Obtained by the Korringa-Kohn-Rostoker (Green Function) Nonspin Polarized Density-Functional Calculations in the Local-Spin Density Approximation 

Source Zeller, R. (2006) and the spin-polarized exchange-correlation integral, I c, calculated by the local spin density approximation. The Stoner parameter is, to a first approximation, element-specific and independent of the atom s local environment. 

The d bands in palladium have a DOS A/(Ef) value of 2.28 states/eV/atom. Use data from Table 8.6 to predict whether Pd should be magnetic or nonmagnetic. 

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By using this low temperature superimposed susceptibility, the transition temperature and the Curie constant immediately above Tc, Dunlap et al. found that NpOs2 fulfilled the Murata-Doniach requirements for a weak itinerant ferromagnet ... 

I shows a magnetic moment of 2.71 between 100 and 300 K. Since there is no specific short contact between the molecules, there is no ferromagnetic interaction in the solid state. Among the nonmetallic molecular magnets known, Cgo TDAE where TDAE = (Mc2N)2 C=C(NMe)2, which orders into an itinerant ferromagnetic state with a of... 

Since a charge transfer from TDAE to C60 leads to a partially filled molecular tlu orbital (Fig. 2), and because of the close contacts of the C60 units, it might be expected that TDAE-C60 is a metal with a highly anisotropic electronic structure, displaying itinerant ferromagnetism [2]. However, powder infrared absorption... 

There are a few exceptions, such as very weak itinerant ferromagnets (for example ZrZn2) and low-spin high-spin transition in fee iron. 

Our elucidation of the structure helps in the understanding of the ferromagnetism in this material. It was argued previously that TDAE-Cso is an itinerant ferromagnet that is, the unpaired spins are carried by delocalized electrons. We have measured the electron paramagnetic resonance signal in this material, and find a relatively low value for the g-factor, g = 2.0008. This implies that the open-shell species reside principally on the C. and to a lesser extent on the TDAE. 

Very weak itinerant ferromagnets, defined as materials for which the splitting in energy between the + and - bands is very small, are predicted to have a number of distinctive properties. In particular, the value of the ratio qc/q% should be much greater than unity and roughly inversely proportional to the Curie temperature (Rhodes and Wohlfarth 1963). Here qc is obtained from the observed paramagnetic susceptibility using the relationship... 

This compound has the bet Mn12Th structure. AC susceptibility measurements (Jin et al. 1982) as a function of pressure up to 19 kbar suggest the phase diagram is as depicted in fig. 4, where Tcl is taken to be the transition between itinerant ferromagnetism and paramagnetism and Tc2 is thought to be associated with an impurity (< 10%). The variation of Tcl as a function of pressure follows the Wohlfarth form... 

The current view of these and of the off-stoichiometric (Fe0 5 Co0 5), >,Ti y alloys seems to be that they can be correctly represented by an itinerant part which is well-described in terms of the SEW model of weak-itinerant ferromagnetism but that there is additional moment formation associated with Fe or Co atoms on Ti sites (antistructure atoms) in the region of 0 < x < 0.35 leading to localized effects (Buis et al. 1981b). Parviainen (1982) has shown that the observation of a negative... 

UNi2 behaves as a weak itinerant ferromagnet. The value Tc — 20-21 K was determined on polycrystals (Sechovsky et al. 1980a, Havela et al. 1980) while Tc = 27 K (Frings et al. 1986) was found on a single crystal. The most appropriate value, however, would be Tc = 24 K which corresponds to a peak in the C(T) data (Frings et al. 1986). A very low spontaneous moment jus = 0.08jaB/f.u. locked in the... 

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