Conduction spin moment

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

As a result, nearly perfect interfaces between the ferromagnetic material and the semiconductor are not a prerequisite for efficient spin injection. It is for example possible to insert a non-magnetic seed layer between the ferromagnetic base layer and the semiconductor collector. Since hot electrons retain their spin moment while traversing the thin non-magnetic layer this will not drastically reduce the spin polarization of the injected current. Finally, since electron injection is ballistic in SVT and MTT devices the spin injection efficiency is not fundamentally limited by a substantial conductivity mismatch between metals and semiconductors [161, 162], The latter is the case in diffusive ferromagnetic metal/semiconductor contacts [163],... 

The bulk magnetization moments are lower due to the presence of negative conduction spin density. 

This prediction is a reasonable one for most cerium pnictides, namely CeP, CeAs, CeSb, and CeBi which, in fact, exhibit localized spin moments with an antiferromagnetic ordering of the 4/ electron remaining on each Ce [268]. CeN, however, is a metallic conductor with the corresponding magnetic properties and it only shows Pauli paramagnetism of the metallic electron gas, such that no local spin moment, characteristic for an unpaired electron, can be detected. This behavior leads to the possibility of an electronic formulation according to with one electron left in the conduction band, but... 

Europium metal, like ytterbium at the end of the lanthanide series, loses only 2 electrons to the conduction band, and so retains a half filled 4f shell. Thus the observed C must be caused by core polarization effects i.e. since L 0, the magnetic field produced at the nucleus of the Eu ion is due mainly to polarization of electrons in closed shells by the spin moments of the 4f electrons. In the other rare earths this interaction is completely masked by the much larger field due to the orbital angular momentum of the 4f electrons. 

GdFei2 does not exist as a stable compoimd but electronic structure calculations using the LMTO-ASA method by Trygg et al. (1992) have been performed. As was observed in the experiment on the RMn systems there is a pronoimced influence of localized 4f magnetism on the conduction-band magnetism (transition-metal sublattice) which gives noticeable changes in the local moment of the iron (transition element). The presence of the 4f spin moment is found to induce a redistribution of the spin moment between the rare-earth and iron sites, while the total conduction-electron moment remains constant. It seems that these conclusions have also some importance for the ternary materials. 

Fig. 6. (a) The calculated partial Co 3d, Gd 5d and total conduction electron moments of GdCo2 as a function of 4f-spin moment. The arrows denote the direction in which the 4f spin is changed, (b) The calculated total conduction electron moments of GdCoj-YbCoj. After Nordstrdm et al. (1992). 

For a standard lanthanide system, the localized 4f" spin configuration will interact with the surrounding 5d-electron spin cloud by means of local exchange-correlation and this will enhance the total 5d spin-down moment. Therefore the 5d spin-up band will be pushed away further from the majority bonding (3d) band, and the associated hybridization will be further decreased. This means that the local 3d spin-up moment will increase in size. However, since the initial state was assumed to be saturated, the total conduction electron moment is fixed, and only its distribution between the R and M atoms changes. Thus, although the individual 5d and 3d moments both increase in size, there is for this situation an exact cancellation between the two 4f spin induced changes of the local moments. 

Thus the presence of the 4f spin moment induces a redistribution of the spin moment between the lanthanide and M sites, while the total conduction electron moment remains constant. This cancellation between the two 4f-induced extra spin moments also occurs to a large extent for non-saturated magnetism, and explains the successful interpretation of experimental magnetic moment data in terms of a constant conduction electron spin moment and an atomic 4f magnetic moment for a series of compounds. 

In fig. 45 we show the calculated total conduction electron spin moment along the series and its decomposition into the 3d and 5d contributions (Brooks et al. 1991a). As expected from model considerations, these two d moments have opposite directions. The individual 5d and 3d moments depend much more strongly upon the... 

Fig. 45. The calculated conduction-electron spin contributions to the moments of the RFej series (full lines). A negative spin for the total and M-3d contribution was chosen because they are antiparallel to the R-4f moment and the total moment is therefore positive. The dashed lines marked Gd-4f-red are for GdFej with the Gd-4f spin moment reduced to that of the corresponding lanthanide - hence the effect of varying volume is removed.

The calculated total moments are compared with experiment in fig. 46. Here we have added the rare earth moment to the calculated conduction-electron moment. Due to the 3d-5d hybridization a significant spin density is produced at the R sites, even when the f moments are zero. This hybridization is believed to be responsible for the important coupling between the 4f and 3d spin directions (Brooks et al. 1991a). The essential point to realize is that in the local spin density approximation the R-4f and R-5d spins are coupled by local exchange interactions to give a parallel spin alignment. The interaction between the R-4f and Fe-3d spins is mediated by the R-5d spin and it aligns the 4f and 3d spins antiparaUel. 

Fig. 47. The total conduction-electron moments of the RCoj compounds. When the calculations are started with zero moment on the Co, only a small Co-3d moment is induced for R atoms heavier than Tb (full line). Yb When the calculations are started with a large spin on the Co, the high-moment state remains stable (dashed line).

Figure 3 shows the calculated conductivity for one of the channels when the cobalt moments on either side of the copper layer are aligned anti-parallel. The spin channel for which the conductivity is shown in Figure 3 is locally the majority channel in the cobalt layer to the left of the copper (spin parallel to the Co moment) and locally minority to the right of the copper (electron spin anti-parallel to the local Co moments). The non-local conductivity for the other spin channel for the case in which the cobalt moments are antiparallel is the mirror image of the conductivity shown in Figure 3. 

Figure 3 Non-local layer dependent conductivity for one spin channel for antiparallel alignment of the cobalt moments. This spin channel is locally the majority in the cobalt on the left side of the sample.

Recently it was pointed out by Zener7 that the atomic moments, in parallel orientation, might react with the electrons in the conduction band in such a way as to uncouple some of the pairs, producing a set of conduction electrons occupying individual orbitals, and with spins parallel to the spins of the atomic electrons. Zener assumed that the conduction band for the transition metals is formed by the 4.s orbitals of the atoms, and that there is somewhat less than one conduction electron per atom in iron, cobalt, and nickel. Like Slater, he attributed the atomic magnetic moments to the partially filled 3d subshell. 

The calculated energy of interaction of an atomic moment and the Weiss field (0.26 uncoupled conduction electrons per atom) for magnetic saturation is 0.135 ev, or 3070 cal. mole-1. According to the Weiss theory the Curie temperature is equal to this energy of interaction divided by 3k, where k is Boltzmann s constant. The effect of spatial quantization of the atomic moment, with spin quantum number S, is to introduce the factor (S + 1)/S that is, the Curie temperature is equal to nt S + l)/3Sk. For iron, with 5 = 1, the predicted value for the Curie constant is 1350°K, in rough agreement with the experimental value, 1043°K. 

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