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Abstract:
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Research on spintronics has galvanized the design of new devices that
exploit the electronic spin in order to augment the performance of current
microelectronic technologies . The sucessful implementation of these devices
is largely contingent on a quantitative understanding of nonequilibrium magnetism
in conducting ferromagnets . This thesis is largely devoted to expanding
the microscopic theory of magnetization relaxation and current -induced spin
torques in transition metals ferromagnets as well as in (III ,Mn )V dilute magnetic
semiconductors .
We start with two theoretical studies of the Gilbert damping in electric
equilibrium , which treat disorder exactly and include atomic -scale spatial
inhomogeneities of the exchange field . These studies enable us to critically review
the accuracy of the conventional expressions used to evaluate the Gilbert
damping in transition metals . We follow by generalizing the calculation of the Gilbert damping to
current -carrying steady states . We find that the magnetization relaxation
changes in presence of an electric current . We connect this change with the
non -adiabatic spin transfer torque parameter , which is an elusive yet potentially
important quantity of nonequilibrium magnetism . This connection culminates
in a concise analytical expression that will lead to the first ab initio
estimates of the non -adiabatic spin transfer torque in real materials .
Subsequently we predict that in gyrotropic ferromagnets the magnetic
anisotropy can be altered by a dc current . In these systems spin -orbit coupling ,
broken inversion symmetry and chirality conspire to yield current -induced spin
torques even for uniform magnetic textures . We thus demonstrate that a
transport current can switch the magnetization of strained (Ga ,Mn )As .
This thesis concludes with the transfer of some fundamental ideas from
nonequilibrium magnetism into the realm of superconductors , which may be
viewed as easy -plane ferromagnets in the particle -hole space . We emphasize
on the analogies between nonequilibrium magnetism and superconductivity ,
which have thus far been studied as completely separate disciplines . Our
approach foreshadows potentially new effects in superconductors . |