103 research outputs found
Chapter VII The Rare Earth Garnets
International audienc
Magnetic Structure Investigations at the Nuclear Center
The magnetic structure of the compounds UOS, ß-CoSO4, YCO5, and HoCO5 is briefly described.
UOS is antiferromagnetic. The Néel temperature is Tn=55°K. The magnetic cell is doubled in the c direction with a ++ - - sequence of U moments along c. The apparent spin is S∼1. The negative interaction corresponds to U-O-U links.
In ß-CoSO4 (high-temperature modification, space group Pbnm), Co atoms are in 000, 00½, ½½½, ½½0. Here three different antiferromagnetic spin modes, mutually perpendicular, Ax(+ - - +), Gy(+-+-), and Cz(++ - - ), in the Wollan-Koehler notation, are coupled. Direction cosines are 0.71, 0.50, and 0.50, respectively. The Co moment is about 3,84 µB at 4.2°K. A field-induced spin flip to the configuration Fx, Cy, Gz is predicted.
YCO5 is ferromagnetic at room temperature with a moment value of Co practically equal to that of metallic Co and moment direction along c, which is conserved down to 4.2°K.
In HoCO5 the moment of Ho is opposite to those of the Co atoms. When cooling from room to liquid helium temperature, the direction of easy magnetization changes from near c to a direction in the basal plane and the Ho moment increases from 4 to about 9 µB. The compensation temperature is 70°K
Brillouin scattering studies in FeO across the Verwey transition
Brillouin scattering studies have been carried out on high quality single
crystals of FeO with [100] and [110] faces in the temperature range of
300 to 30 K. The room temperature spectrum shows a surface Rayleigh wave (SRW)
mode at 8 GHz and a longitudinal acoustic (LA) mode at 60 GHz. The SRW mode
frequency shows a minimum at the Verwey transition temperature of 123 K.
The softening of the SRW mode frequency from about 250 K to can be
quantitatively understood as a result of a decrease in the shear elastic
constant C, arising from the coupling of shear strain to charge
fluctuations. On the other hand, the LA mode frequency does not show any
significant change around , but shows a large change in its intensity. The
latter shows a maximum at around 120 K in the cooling run and at 165 K in the
heating run, exhibiting a large hysteresis of 45 K. This significant change in
intensity may be related to the presence of stress-induced ordering of
Fe and Fe at the octahedral sites, as well as to stress-induced
domain wall motion.Comment: 14 pages, 3 figures, accepted in Physical Review B 200
Symmetric hysteresis curves of rare earth-cobalt magnets measured by high magnetic fields
SmCo5magnets show generally asymmetrical hysteresis curves in the region of ordinary magnetic field strength. However, if these magnets are magnetized by a field of over 14 T, hysteresis loop becomes symmetric, except for the experimental errors caused by the thermal fluctuation after effect. The influence of the after effect on the magnetization curve and the coercive forceIHCare discussed.</p
Theory of Current-Induced Magnetization Precession
We solve appropriate drift-diffusion and Landau-Lifshitz-Gilbert equations to
demonstrate that unpolarized current flow from a non-magnet into a ferromagnet
can produce a precession-type instability of the magnetization. The fundamental
origin of the instability is the difference in conductivity between majority
spins and minority spins in the ferromagnet. This leads to spin accumulation
and spin currents that carry angular momentum across the interface. The
component of this angular momentum perpendicular to the magnetization drives
precessional motion that is opposed by Gilbert damping. Neglecting magnetic
anisotropy and magnetostatics, our approximate analytic and exact numerical
solutions using realistic values for the material parameters show (for both
semi-infinite and thin film geometries) that a linear instability occurs when
both the current density and the excitation wave vector parallel to the
interface are neither too small nor too large. For many aspects of the problem,
the variation of the magnetization in the direction of the current flows makes
an important contribution.Comment: Submitted to Physical Review
Stability of equidimensional pseudo-single-domain magnetite over billion-year timescales
Interpretations of paleomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of paleomagnetic signals in rocks are usually larger, nonuniformly magnetized particles, for which there is no empirically complete, thermally activated model. This study has developed a thermally activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological timescales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that, contrary to previously published theories, such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, nonuniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain high-fidelity magnetic recordings over billions of years
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