46 research outputs found
Roles of non-equilibrium conduction electrons on magnetization dynamics of ferromagnets
The mutual dependence of spin-dependent conduction and magnetization dynamics
of ferromagnets provides the key mechanisms in various spin-dependent
phenomena. We compute the response of the conduction electron spins to a
spatial and time varying magnetization within the
time-dependent semiclassical transport theory. We show that the induced
non-equilibrium conduction spin density in turn generates four spin torques
acting on the magnetization--with each torque playing different roles in
magnetization dynamics. By comparing with recent theoretical models, we find
that one of these torques that has not been previously identified is crucial to
consistently interpret experimental data on domain wall motion.Comment: References correcte
Tensor form of magnetization damping
A tensor form of phenomenological damping is derived for small magnetization
motions. This form reflects basic physical relaxation processes for a general
uniformly magnetized particle or film. Scalar Landau-Lifshitz damping is found
to occur only for two special cases of system symmetry.Comment: Paper HA-03 presented at MMM'01, to be published in J. Appl. Phy
Domain Structure of Co/Pd multilayers
The observations of submicron domain structure of Co/Pd multilayers at various parts of the M-H loop and after different magnetization cycles designed to approach the global-equilibrium domain width are reported. The wall energy densities were estimated from comparison of the measured equilibrium domain width with the anhysteretic model predictions and also compared with the wall energy densities determined from the slope of major M-H loo
Domain wall theory and exchange stiffness in Co/Pd multilayers
The stripe model of domain structure in multilayers is studied by micromagnetic simulation. The results indicate a strong reduction of the effective domain wall energy (by dipolar effects). Domain width measurements on sputtered Co/Pd multilayers are compared with the theory. The estimated exchange stiffness is comparable with that of bulk Co. The effects of interface roughness and of interlayer exchange are discussed
Analysis of torque measurements on films with oblique anisotropy
A measurement method is discussed to determine the magnetic anisotropy energy in a sample without assuming an a priori model for the origins of the anisotropy. The measurement procedure involves torque measurements in five different planes. Since it is especially useful for films with an oblique anisotropy axis, the method is illustrated on an obliquely evaporated Co80Ni20 fil
Macrospin Models of Spin Transfer Dynamics
The current-induced magnetization dynamics of a spin valve are studied using
a macrospin (single domain) approximation and numerical solutions of a
generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative
comparison with experiment [Kiselev {\it et al.} Nature {\bf 425}, 380 (2003)],
we calculate the resistance and microwave power as a function of current and
external field including the effects of anisotropies, damping, spin-transfer
torque, thermal fluctuations, spin-pumping, and incomplete absorption of
transverse spin current. While many features of experiment appear in the
simulations, there are two significant discrepancies: the current dependence of
the precession frequency and the presence/absence of a microwave quiet magnetic
phase with a distinct magnetoresistance signature. Comparison is made with
micromagnetic simulations designed to model the same experiment.Comment: 14 pages, 14 figures. Email [email protected] for a
pdf with higher quality figure
Magnetic relaxation in metallic films: Single and multilayer structures
The intrinsic magnetic relaxations in metallic films will be discussed. It will be shown that the intrinsic damping mechanism in metals is
caused by incoherent scattering of itinerant electron-hole pair
excitations by phonons and magnons. Berger [L. Berger, Phys. Rev. B
54, 9353 (1996)] showed that the interaction between spin waves and
itinerant electrons in multilayers can lead to interface Gilbert
damping. Ferromagnetic resonance (FMR) studies were carried out using
magnetic single and double layer films. The FMR linewidth of the Fe
films in the double layer structures was found to always be larger than
the FMR linewidth measured for the single Fe films having the same
thickness. The increase in the FMR linewidth scaled inversely with the
film thickness, and was found to be linearly dependent on the microwave
frequency. These results are in agreement with Berger's predictions.
(C) 2002 American Institute of Physics
Magnetization reversal driven by spin-injection : a mesoscopic spin-transfer effect
A mesoscopic description of spin-transfer effect is proposed, based on the
spin-injection mechanism occurring at the junction with a ferromagnet. The
effect of spin-injection is to modify locally, in the ferromagnetic
configuration space, the density of magnetic moments. The corresponding
gradient leads to a current-dependent diffusion process of the magnetization.
In order to describe this effect, the dynamics of the magnetization of a
ferromagnetic single domain is reconsidered in the framework of the
thermokinetic theory of mesoscopic systems. Assuming an Onsager
cross-coefficient that couples the currents, it is shown that spin-dependent
electric transport leads to a correction of the Landau-Lifshitz-Gilbert
equation of the ferromagnetic order parameter with supplementary diffusion
terms. The consequence of spin-injection in terms of activation process of the
ferromagnet is deduced, and the expressions of the effective energy barrier and
of the critical current are derived. Magnetic fluctuations are calculated: the
correction to the fluctuations is similar to that predicted for the activation.
These predictions are consistent with the measurements of spin-transfer
obtained in the activation regime and for ferromagnetic resonance under
spin-injection.Comment: 20 pages, 2 figure
Interaction Of Electrons With Spin Waves In The Bulk And In Multilayers
The exchange interaction between electrons and magnetic spins is considerably
enhanced near interfaces, in magnetic multilayers. As a result, a dc current
can be used to generate spin oscillations. We review theory and experimental
evidence. The s-d exchange interaction causes a rapid precession of itinerant
conduction-electron spins s around the localized spins S of magnetic electrons.
Because of the precession, the time-averaged interaction torque between s and S
vanishes. An interface between a magnetic layer and a spacer causes a local
coherence between the precession phases of differnt electrons, within 10 nm
from the interface, and restores the torque. Also, a second magnetic layer with
pinned S is used to prepare s in a specific direction. the current-induced
drive torque of s on S in the active layer may be calculated from the spin
current (Slonczewski) or from the spin imbalance Delta-mu (Berger). Spin
current and Delta-mu are proportional to each other, and can arise from
Fermi-surface translation, as well as from expansion/contraction.Comment: Invited paper at Seattle MMM01 Conference, Nov. 2001 (to appear in J.
Appl. Phys.