Finite-temperature spin polarization in half-metallic ferromagnets

Abstract

The temperature dependence of the spin polarization of half-metallic ferromagnets is investigated. A unitary spinor transformation shows that the corresponding spin mixing goes beyond finite-temperature smearing of the Fermi level, leading to a nonzero density of states in the gap of the insulating spin channel. As a consequence, the resistance ratio of the two spin channels changes from infinity to some finite value and, in a strict sense, half-metallic ferromagnetism is limited to zero temperature. Bloch-type spin waves and crystal imperfections contribute to the density of states in the gap but only partly explain the pronounced changes at about 0.2 T C observed in various half-metallic magnets. In the case of NiMnSb, the spin structure depends on a nearly dispersionless transverse optical mode that occurs at about 28 meV. In terms of 3 k B T, this corresponds to 103 K-very close to the temperature at which there is a dramatic loss in the Ni and Mn magnetization in NiMnSb. Similar modes exist in other potential half-metallic systems.

Are half-metallic ferromagnets half metals? (invited)

“…As noted now by several groups, [41][42][43][44] spin disorder has a profound influence on the persistence of half-metallic character at finite temperature. Spin-mixing effects in ferromagnetic metals have been investigated for many years, for example, in the context of finite-temperature excitations in elemental nickel.…”

“…However, the behavior of half-metallic ferromagnets cannot be reduced to a smearing of the Fermi level by a relative amount of the order of k B T/E F . [41][42][43][44] Figure 1 shows typical finite-temperature ↑ and ↓ densities of state. 41,42 The distortion of the ↑ DOS, D ↑ (E), has the character of a quantitative correction ͑dark region I͒, but in the ↓ gap the DOS changes qualitatively, from zero to nonzero ͑dark region II͒.…”

“…[41][42][43][44] Figure 1 shows typical finite-temperature ↑ and ↓ densities of state. 41,42 The distortion of the ↑ DOS, D ↑ (E), has the character of a quantitative correction ͑dark region I͒, but in the ↓ gap the DOS changes qualitatively, from zero to nonzero ͑dark region II͒. This spin mixing yields nonzero D ↓ (E) values in the middle of the spin-down gap, well above the highest energy level occupied at zero temperature.…”

“…In a very crude approximation, the magnitude of this spin-mixing contribution to the DOS is proportional to M o ϪM s (T), where M s (T) is the spontaneous magnetization and M o ϭM s (0). 41,42 Considering a ferromagnet-insulatorferromagnet tunnel-junction geometry, Itoh, Ohsawa, and Inoue 43 have calculated the spin polarization at interfaces with and without spin fluctuations considered. Figure 2 indicates that the effect of spin fluctuations is quite significant.…”

“…͑1͒ to model half-metallic ferromagnets 41,42 and to compare with ordinary itinerant ferromagnets. As in other systems, 32,33 the description of finitetemperature spin disorder involves the standard spin-1/2 rotation matrix Û ( , ), which describes the rotation of a ↑ spin by the angles and .…”