THESIS
2017
viii, 126 pages : illustrations ; 30 cm
Abstract
In this thesis, we studied two phenomena involved with the magnon-electron scattering due
to the s-d interaction: the thermal spin injection and the magnetoresistance in the ferromagnetic
materials.
The spin current injection and spin accumulation near a ferromagnetic insulator (FI)/nonmagnetic
metal (NM) bilayer film under a thermal gradient is investigated theoretically. By using the
Fermi golden rule and the Boltzmann equations, we find that FI and NM can exchange spins
via interfacial electron-magnon scattering which is described by the s-d interaction because
of the imbalance between magnon emission and absorption caused by either the deviation of
the magnon number from the equilibrium Bose-Einstein distribution, or difference in magnon
temperature and electron temperature...[
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In this thesis, we studied two phenomena involved with the magnon-electron scattering due
to the s-d interaction: the thermal spin injection and the magnetoresistance in the ferromagnetic
materials.
The spin current injection and spin accumulation near a ferromagnetic insulator (FI)/nonmagnetic
metal (NM) bilayer film under a thermal gradient is investigated theoretically. By using the
Fermi golden rule and the Boltzmann equations, we find that FI and NM can exchange spins
via interfacial electron-magnon scattering which is described by the s-d interaction because
of the imbalance between magnon emission and absorption caused by either the deviation of
the magnon number from the equilibrium Bose-Einstein distribution, or difference in magnon
temperature and electron temperature.
When two heat baths are attached to the bilayer structure longitudinally (the heat flows perpendicular
to the interface), a temperature distribution is determined by the thermal parameters
and the sizes of the materials. A linear-response transport theory shows that a temperature gradient
in FI and/or a temperature difference across the FI/NM interface generates a spin current
which carries angular momenta parallel to the magnetization of FI from the hotter side to the
colder one. Interestingly, the spin current induced by a temperature gradient in NM is negligibly
small due to the nonmagnetic nature of the non-equilibrium electron distributions. The results
agree well with all existing experiments.
Such spin current across the interface is converted into the spin current carried by conduction
electrons in the nonmagnetic metal layer, which can be affected by the spin-orbital coupling and converted to an electric voltage along a direction perpendicular to both the thermal gradient or
the magnetization orientation. And the value of such voltage is determined by the spin Hall
angle and other parameters. Such heat-to-electricity conversion devices can be used in the area
of waste heat recovery. Besides of other possible applications, the results should be useful in
extracting material parameters such as spin Hall angle from experimental measurements.
Another phenomena involved the magnon-electron scattering is the intrinsic magnonic magnetoresistance
(MMR) of magnetic metals. To get a pure results, we limited our study in the
nanowires. We theoretically investigated the MR due to magnon scattering in metals. MMR
at room temperature is found to be linear in magnetic field along the magnetization direction
for typical ferromagnetic materials. The slope can be either positive or negative, depending
on whether the field is parallel or anti-parallel to the magnetization direction. Surprisingly, the
MMR increases (decreases) with temperature below (above) a critical temperature, determined
by the ratio of resistivity from impurity scattering and the thermal resistivity coefficient due
to the electron-magnon scattering. The comparison between the theoretical results and recent
experiment (PRL 107, 136605 (2011)) is also made.
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