124 research outputs found
Two-dimensional behavior of the sublattice magnetization in three-dimensional Ising antiferromagnets
A three-dimensional layered Ising-Antiferromagnet with a ferromagnetic
intra-layer coupling to z neighbors, zJ > 0, and an antiferromagnetic
interlayer coupling to z' neighbors, z'J' < 0, is investigated by Monte Carlo
simulations on a hexagonal lattice. The physical nature of the anomalous
temperature bahavior of the sublattice magnetizations, which is found for
certain values of r=zJ/z'J' and z' in magnetic fields is explained in terms of
successive phase transitions. They take place on the ferromagnetic
2-dimensional spin-down sublattice at T = T_c^{2d}, smeared by a finite
stabilizing molecular field, and on both antiferromagnetically coupled
sublattices at T_c^{3d} > T_c^{2d}.Comment: 8 pages (TeX), 6 figures (eps), submitted to World Scientific:
Proceedings SDHS'99 Duisbur
Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/AlO
The magnetization and magnetic ac susceptibility, ,
of superferromagnetic systems are studied by numerical simulations. The
Cole-Cole plot, vs. , is used as a tool for classifying
magnetic systems by their dynamical behavior. The simulations of the
magnetization hysteresis and the ac susceptibility are performed with two
approaches for a driven domain wall in random media. The studies are motivated
by recent experimental results on the interacting nanoparticle system
CoFe/AlO showing superferromagnetic behavior. Its
Cole-Cole plot indicates domain wall motion dynamics similarly to a disordered
ferromagnet, including pinning and sliding motion. With our models we can
successfully reproduce the features found in the experimental Cole-Cole plots.Comment: 8 pages, 6 figure
Superparamagnetic nanoparticle ensembles
Magnetic single-domain nanoparticles constitute an important model system in
magnetism. In particular ensembles of superparamagnetic nanoparticles can
exhibit a rich variety of different behaviors depending on the inter-particle
interactions. Starting from isolated single-domain ferro- or ferrimagnetic
nanoparticles the magnetization behavior of both non-interacting and
interacting particle-ensembles is reviewed. A particular focus is drawn onto
the relaxation time of the system. In case of interacting nanoparticles the
usual Neel-Brown relaxation law becomes modified. With increasing interactions
modified superparamagnetism, spin glass behavior and superferromagnetism is
encountered.Comment: Corrected formula: Eq. (1
Interaction effects and transport properties of Pt capped Co nanoparticles
We studied the magnetic and transport properties of Co nanoparticles (NPs)
being capped with varying amounts of Pt. Beside field and temperature dependent
magnetization measurements we performed delta-M measurements to study the
magnetic interactions between the Co NPs. We observe a transition from
demagnetizing towards magnetizing interactions between the particles for an
increasing amount of Pt capping. Resistivity measurements show a crossover from
giant magnetoresistance towards anisotropic magnetoresistance
Domain wall propagation in Permalloy nanowires with a thickness gradient
The domain wall nucleation and motion processes in Permalloy nanowires with a
thickness gradient along the nanowire axis have been studied. Nanowires with
widths, w = 250 nm to 3 um and a base thickness of t = 10 nm were fabricated by
electron-beam lithography. The magnetization hysteresis loops measured on
individual nanowires are compared to corresponding nanowires without a
thickness gradient. The Hc vs. t/w curves of wires with and without a thickness
gradient are discussed and compared to micromagnetic simulations. We find a
metastability regime at values of w, where a transformation from transverse to
vortex domain wall type is expected
Fingerprinting the magnetic behavior of antiferromagnetic nanostructures using remanent magnetization curves
Antiferromagnetic (AF) nanostructures from Co3O4, CoO and Cr2O3 were prepared
by the nanocasting method and were characterized magnetometrically. The field
and temperature dependent magnetization data suggests that the nanostructures
consist of a core-shell structure. The core behaves as a regular
antiferromagnet and the shell as a two-dimensional diluted antiferromagnet in a
field (2d DAFF) as previously shown on Co3O4 nanowires [Benitez et al., Phys.
Rev. Lett. 101, 097206 (2008)]. Here we present a more general picture on three
different material systems, i.e. Co3O4, CoO and Cr2O3. In particular we
consider the thermoremanent (TRM) and the isothermoremanent (IRM) magnetization
curves as "fingerprints" in order to identify the irreversible magnetization
contribution originating from the shells. The TRM/IRM fingerprints are compared
to those of superparamagnetic systems, superspin glasses and 3d DAFFs. We
demonstrate that TRM/IRM vs. H plots are generally useful fingerprints to
identify irreversible magnetization contributions encountered in particular in
nanomagnets.Comment: submitted to PR
Magnetic phase diagram of the diluted metamagnet Fe\u3csub\u3e0.95\u3c/sub\u3eMg\u3csub\u3e0.05\u3c/sub\u3eBr\u3csub\u3e2\u3c/sub\u3e
The axial magnetic phase diagram of the antiferromagnet Fe0.95Mg0.05Br2 is studied by specific heat, superconducting quantum interference device, and Faraday rotation techniques. The diamagnetic impurities give rise to random-field criticality along the second-order phase line Hc(T) between TN=13.1 K and a multicritical point at Tm≈5 K, and to a spin-flop line between Tm and the critical end-point temperature Te≈3.5 K. The phase line H1(T)c(T) ending at Tm is probably due to symmetric nondiagonal exchange
Magnetic properties and spin structure of MnO single crystal and powder
Zero field cooled (ZFC)/Field Cooled (FC) magnetization curves of a bulk MnO
single crystal show a peculiar peak at low temperatures (~40K) similar to the
low temperature peak observed in MnO nanoparticles. In order to investigate the
origin of this peak, the spin structure of a MnO single crystal has been
studied and compared with a single phase powder sample using magnetometry and
polarized neutron scattering. Both magnetometry and polarized neutron
diffraction results confirm the antiferromagnetic (AF) phase transition at the
N\'eel temperature T_N of 118K, in both powder and single crystal form.
However, the low temperature peak in the ZFC/FC magnetization curves is not
observed in single phase MnO powder. To better understand the observed
behavior, ac susceptibility measurements have been employed. We conclude that
the clear peak in the magnetic signal from the single crystal originates from a
small amount of ferrimagnetic (FiM) Mn2O3 or Mn3O4 impurities, which is grown
at the interfaces between MnO crystal twins
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