59 research outputs found
Hot electron driven enhancement of spin-lattice coupling in 4f ferromagnets observed by femtosecond x-ray magnetic circular dichroism
Femtosecond x-ray magnetic circular dichroism was used to study the
time-dependent magnetic moment of 4 fs electrons in the ferromagnets Gd and Tb,
which are known for their different spin-lattice coupling. We observe a
two-step demagnetization with an ultrafast demagnetization time of 750 fs
identical for both systems and slower times which differ sizeably with 40 ps
for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the
electronically excited state is enhanced up to orders of magnitude compared to
equilibrium.Comment: added reference 24, clarified the meaning of photo-induced,
emphasized that XMCD probes the magnetic moment localized at 4f electron
Phase separation in the non-equilibrium Verwey transition in magnetite
We present equilibrium and out-of-equilibrium studies of the Verwey
transition in magnetite. In the equilibrium optical conductivity, we find a
step-like change at the phase transition for photon energies below about 2 eV.
The possibility of triggering a non-equilibrium transient metallic state in
insulating magnetite by photo excitation was recently demonstrated by an x-ray
study. Here we report a full characterization of the optical properties in the
visible frequency range across the non-equilibrium phase transition. Our
analysis of the spectral features is based on a detailed description of the
equilibrium properties. The out-of-equilibrium optical data bear the initial
electronic response associated to localized photo-excitation, the occurrence of
phase separation, and the transition to a transient metallic phase for
excitation density larger than a critical value. This allows us to identify the
electronic nature of the transient state, to unveil the phase transition
dynamics, and to study the consequences of phase separation on the
reflectivity, suggesting a spectroscopic feature that may be generally linked
to out-of-equilibrium phase separation
Nonlinear Magnetization Dynamics Driven by Strong Terahertz Fields
We present a comprehensive experimental and numerical study of magnetization
dynamics triggered in a thin metallic film by single-cycle terahertz pulses of
MV/m electric field amplitude and ps duration. The
experimental dynamics is probed using the femtosecond magneto-optical Kerr
effect (MOKE), and it is reproduced numerically using macrospin simulations.
The magnetization dynamics can be decomposed in three distinct processes: a
coherent precession of the magnetization around the terahertz magnetic field,
an ultrafast demagnetization that suddenly changes the anisotropy of the film,
and a uniform precession around the equilibrium effective field that is relaxed
on the nanosecond time scale, consistent with a Gilbert damping process.
Macrospin simulations quantitatively reproduce the observed dynamics, and allow
us to predict that novel nonlinear magnetization dynamics regimes can be
attained with existing table-top terahertz sources.Comment: 6 pages, 4 figure
Electronic structure and spectroscopy of the quaternary Heusler alloy CoCrFeAl
Quaternary Heusler alloys CoCrFeAl with varying Cr to Fe
ratio were investigated experimentally and theoretically. The electronic
structure and spectroscopic properties were calculated using the full
relativistic Korringa-Kohn-Rostocker method with coherent potential
approximation to account for the random distribution of Cr and Fe atoms as well
as random disorder. Magnetic effects are included by the use of spin dependent
potentials in the local spin density approximation.
Magnetic circular dichroism in X-ray absorption was measured at the
edges of Co, Fe, and Cr of the pure compounds and the alloy in order to
determine element specific magnetic moments. Calculations and measurements show
an increase of the magnetic moments with increasing iron content. Resonant
(560eV - 800eV) soft X-ray as well as high resolution - high energy (keV) hard X-ray photo emission was used to probe the density of the
occupied states in CoCrFeAl.Comment: J.Phys.D_Appl.Phys. accepte
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