140 research outputs found
Field dependence of the switching field for nonellipsoidal single domain particles
Experimental data on a model system of a two-dimensional array of single domain garnet particles, switching by incoherent rotation, are presented to show that the switching field of individual particles, H-sw, and the coercivity of the major hysteresis loop for similar to1000 particles, H-c, depend on the previously applied saturating field. For the system measured the asymptotic, "true" value of H-c in large fields is 321 Oe, in contrast with H-c=225 Oe, measured in an applied field of H-sat=188 Oe, i.e., the smallest field adequate to close the major loop. Statistical data were collected on switching of a single particle, with an asymptotic value of H-sw=150 Oe. After the application of H-sat=160 Oe H-sw decreased to 111 Oe. Due to the nonellipsoidal shape of the particles, a significant canting of the magnetization near corners and edges persists up to very high fields. The torque, due to these canted magnetic moments, facilitates premature switching in lower fields. It is proposed that defects are responsible for the irreversibility of the process
Major loop reconstruction from switching of individual particles
Major hysteresis loops of groups of isolated 60 mm square garnet particles of a regular
two-dimensional array, have been measured magnetooptically. Individual loops for each particle
were measured, and the statistics of the distribution of coercivities and interaction fields was
determined. It is shown that from the measured coercivity distribution and calculated magnetostatic
interaction fields the major hysteresis loop can be reconstructed. The switching sequence, and the
major loop of an assembly of 535 particles were calculated numerically for two cases: first, when
calculating the magnetostatic interaction, the 25 particles were assumed to be isolated; second, the
major loop of the same 25 particles, embedded into a 939 square, was reconstructed taking into
account the interactions among all 81 particles. The numerically simulated major hysteresis loops
agree very well with the measured loops, demonstrating the reliability of numerical modeling
Defect related switching field reduction in small magnetic particle arrays
An array of 42 mum square, 3 mum thick garnet particles has been studied. The strong crystalline uniaxial anisotropy of these particles results in the stable remanent state being single domain with magnetization parallel to the film normal. Magneto-optic measurements of individual particles provide distribution statistics for the easy-axis switching field H-sw, and the in-plane hard-axis effective anisotropy field, H-eff, which induces the formation of a metastable stripe domain structure. Both H-sw and H-eff are much smaller than the crystalline anisotropy field. Micromagnetic simulations show that the small H-sw cannot be attributed to shape anisotropy, but is consistent with smooth, localized reductions in the crystalline anisotropy caused by defects in either the particles or the substrate
New magnetostatic modes in small nonellipsoidal magnetic particles
Magnetostatic normal modes are investigated here in elongated rods. The
dipolar field resulting from the dipole-dipole interactions is calculated
numerically in points of the axis connecting opposite rod face centers
(\emph{central axis}) by collecting individual contributions to this field
coming from each of the atomic planes perpendicular to the central axis. The
applied magnetic field is assumed to be oriented along the central axis, and
the magnetization to be uniform throughout the sample. The \emph{frequency}
spectrum of magnetostatic waves propagating in the direction of the applied
field is found numerically by solving the Landau-Lifshith equation of motion
with the spatially \emph{nonhomogeneous} dipolar field taken into account; the
mode amplitude \emph{profiles} are depicted as well. While energetically
highest modes have \emph{bulk-extended} character, the modes forming the lower
part of the spectrum are localized in the subsurface region (\emph{bulk-dead
modes}). Between these two mode types, magnetostatic modes of a new type
(\emph{comb modes}) are found to occur, characterized by two clearly
discernible regions: a zone of fast amplitude oscillations inside the rod, and
narrow slow-oscillation regions at the borders. Absorbing virtually no energy
from an applied alternating field, comb modes will have no significant
contribution to the magnetic noise.Comment: 16 pages, 6 figures, conferences paper: Physics of Magnetism'05,
Poznan, Polan
Magnetic dot arrays modeling via the system of the radial basis function networks
Two dimensional square lattice general model of the magnetic dot array is
introduced. In this model the intradot self-energy is predicted via the neural
network and interdot magnetostatic coupling is approximated by the collection
of several dipolar terms. The model has been applied to disk-shaped cluster
involving 193 ultrathin dots and 772 interaction centers. In this case among
the intradot magnetic structures retrieved by neural networks the important
role play single-vortex magnetization modes. Several aspects of the model have
been understood numerically by means of the simulated annealing method.Comment: 16 pages, 8 figure
Photoproduction of Long-Lived Holes and Electronic Processes in Intrinsic Electric Fields Seen through Photoinduced Absorption and Dichroism in Ca_3Ga_{2-x}Mn_xGe_3O_{12} Garnets
Long-lived photoinduced absorption and dichroism in the
Ca_3Ga_{2-x}Mn_xGe_3O_{12} garnets with x < 0.06 were examined versus
temperature and pumping intensity. Unusual features of the kinetics of
photoinduced phenomena are indicative of the underlying electronic processes.
The comparison with the case of Ca_3Mn_2Ge_3O_{12}, explored earlier by the
authors, permits one to finally establish the main common mechanisms of
photoinduced absorption and dichroism caused by random electric fields of
photoproduced charges (hole polarons). The rate of their diffusion and
relaxation through recombination is strongly influenced by the same fields,
whose large statistical straggling is responsible for a broad continuous set of
relaxation components (observed in the relaxation time range from 1 to about
1000 min). For Ca_3Ga_{2-x}Mn_xGe_3O_{12}, the time and temperature dependences
of photoinduced absorption and dichroism bear a strong imprint of structure
imperfection increasing with x.Comment: 20 pages, 10 figure
Fabrication of Flexible Oriented Magnetic Thin Films with Large in-plane Uniaxial Anisotropy by Roll-to-roll Nanoimprint Lithography
Here, we report wafer scale fabrication of densely packed Fe nanostripe-based
magnetic thin films on a flexible substrate and their magnetic anisotropy
properties. We find that Fe nanostripes exhibit large in-plane uniaxial
anisotropy and nearly square hysteresis loops with energy products (BH)max
exceeding 3 MGOe at room temperature. High density Fe nanostripes were
fabricated on 70 nm flexible polyethylene terephthalate (PET) gratings, which
were made by roll-to-roll (R2R) UV nanoimprintlithography technique. Observed
large in-plane uniaxial anisotropies along the long dimension of nanostripes
are attributed to the shape. Temperature dependent hysteresis measurements
confirm that the magnetization reversal is driven by non-coherent rotation
reversal processes.Comment: 17 pages, 6 figure
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