Spin-polarized tunneling in the half-metallic ferromagnetsLa0.7−xHox

Pratap Raychaudhuri

¹

,

Karthik Sheshadri

²

,

Praveen Taneja

³

et al.

Abstract

Abstract:The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La 0.7 Sr 0.3 MnO 3 , and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.

Temperature dependence of transport spin polarization inNdNi5from point-contact Andreev reflection

“…Since much of the practical interest is in the value of spin polarization close to room temperature, the bulk spin polarization at elevated temperature is often estimated based on the assumption P͑T͒ ϰ M s ͑T͒, namely, that the spin polarization of the electrons close to the Fermi level is proportional to the spontaneous magnetization ͓M s ͑T͔͒ of the ferromagnet. [14][15][16][17] Though the spin polarization as a function of temperature has been measured using spin-polarized photoemission for a few ferromagnets, 18,19 this simple relation has so far remained experimentally unverified even for relatively simple ferromagnets. 20 The experimental verification of this relation is important for two reasons.…”

Transition Metal-Nonmetal in Conductivity of Ceramic Hole-Doped Cobaltites

“…The given model assumes that when an electron is tunnelling through a boundary between two granules with the antiparallel magnetization, it will meet a potential barrier of order of the exchange energy J in size which fact reduces the probability of tunneling, i.e., the conductivity, by the factor of e -J/kT in comparison with the case of ferromagnetic orientation of vectors of the magnetization of granules. As it was later shown in [9], the model predicts sharp increase in the considered contribution to conductivity even in low magnetic fields  1 kOe, in case the initial (in the zero field) distribution of directions of the magnetizations of granules was random (paramagnetic). The effect should have obviously been absent if the distribution in the zero field was close to antiferromagnetic (АFМ).…”

“…This fact allows us to assume AFM orientation of the magnetizations of granules in the zero field in these samples. Indications of АFМ exchange between the granules are also available in [9,15,16].…”

“…In [9], the expression for intergranule conductivity for two granules conditional upon the tunneling of spinpolarized electrons was derived as…”

“…The idea is based on the fact that in order to generate an electron in a neutral granule and carry it over to the next granule which was originally neutral, it is necessary to break a certain energy barrier connected with Coulomb repulsion and characterized by certain electrostatic charging energy E C and/or to overcome an exchange interaction by E m in value if the granules possess noncompensated spontaneous magnetic moment. These reasons provided the basis for various models of tunneling, such as the model of intergranule spin-dependent Coulomb gap [8] or the model of spinpolarized tunneling between magnetic granules [9].…”