84 research outputs found
Experimental observation of an enhanced anisotropic magnetoresistance in non-local configuration
We compare non-local magnetoresistance measurements in multi-terminal Ni
nanostructures with corresponding local experiments. In both configurations,
the measured voltages show the characteristic features of anisotropic
magnetoresistance (AMR). However, the magnitude of the non-local AMR signal is
up to one order of magnitude larger than its local counterpart. Moreover, the
non-local AMR increases with increasing degree of non-locality, i.e., with the
separation between the region of the main current flow and the voltage
measurement region. All experimental observations can be consistently modeled
in terms of current spreading in a non-isotropic conductor. Our results show
that current spreading can significantly enhance the magnetoresistance signal
in non-local experiments
Anisotropic Hall Effect in Single Crystal Heavy Fermion YbAgGe
Temperature- and field-dependent Hall effect measurements are reported for
YbAgGe, a heavy fermion compound exhibiting a field-induced quantum phase
transition, and for two other closely related members of the RAgGe series: a
non-magnetic analogue, LuAgGe and a representative, ''good local moment'',
magnetic material, TmAgGe. Whereas the temperature dependent Hall coefficient
of YbAgGe shows behavior similar to what has been observed in a number of heavy
fermion compounds, the low temperature, field-dependent measurements reveal
well defined, sudden changes with applied field; in specific for a
clear local maximum that sharpens as temperature is reduced below 2 K and that
approaches a value of 45 kOe - a value that has been proposed as the
quantum critical point. Similar behavior was observed for where a
clear minimum in the field-dependent Hall resistivity was observed at low
temperatures. Although at our base temperatures it is difficult to distinguish
between the field-dependent behavior predicted for (i) diffraction off a
critical spin density wave or (ii) breakdown in the composite nature of the
heavy electron, for both field directions there is a distinct temperature
dependence of a feature that can clearly be associated with a field-induced
quantum critical point at persisting up to at least 2 K.Comment: revised versio
Colossal Positive Magnetoresistance in a Doped Nearly Magnetic Semiconductor
We report on a positive colossal magnetoresistance (MR) induced by
metallization of FeSb, a nearly magnetic or "Kondo" semiconductor with 3d
ions. We discuss contribution of orbital MR and quantum interference to
enhanced magnetic field response of electrical resistivity.Comment: 5 pages, 5 figure
Ballistic anisotropic magnetoresistance
Electronic transport in ferromagnetic ballistic conductors is predicted to
exhibit ballistic anisotropic magnetoresistance (BAMR) - a change in the
ballistic conductance with the direction of magnetization. This phenomenon
originates from the effect of the spin-orbit interaction on the electronic band
structure which leads to a change in the number of bands crossing the Fermi
energy when the magnetization direction changes. We illustrate the significance
of this phenomenon by performing ab-initio calculations of the ballistic
conductance in ferromagnetic Ni and Fe nanowires which display a sizable BAMR
when the magnetization changes direction from parallel to perpendicular to the
wire axis
Synthesis and characterization of core-shell structure silica-coated Fe29.5Ni70.5 nanoparticles
In view of potential applications of magnetic particles in biomedicine and
electromagnetic devices, we made use of the classical Stober method
base-catalysed hydrolysis and condensation of tetraethoxysilane (TEOS) to
encapsulate FeNi nanoparticles within a silica shell. An original stirring
system under high power ultrasounds made possible to disperse the otherwise
agglomerated particles. Sonication guaranteed particles to remain dispersed
during the Stober synthesis and also improved the efficiency of the method. The
coated particles are characterized by electron microscopy (TEM) and
spectroscopy (EDX) showing a core-shell structure with a uniform layer of
silica. Silica-coating does not affect the core magnetic properties. Indeed,
all samples are ferromagnetic at 77 K and room temperature and the Curie point
remains unchanged. Only the coercive force shows an unexpected non-monotonous
dependence on silica layer thickness.Comment: Regular paper submited to international peer-reveiwed journa
Surface Magnetoelectric Effect in Ferromagnetic Metal Films
A surface magnetoelectric effect is revealed by density-functional
calculations that are applied to ferromagnetic Fe(001), Ni(001) and Co(0001)
films in the presence of external electric field. The effect originates from
spin-dependent screening of the electric field which leads to notable changes
in the surface magnetization and the surface magnetocrystalline anisotropy.
These results are of considerable interest in the area of
electrically-controlled magnetism and magnetoelectric phenomena
Diffusive and ballistic current spin-polarization in magnetron-sputtered L1o-ordered epitaxial FePt
We report on the structural, magnetic, and electron transport properties of a
L1o-ordered epitaxial iron-platinum alloy layer fabricated by
magnetron-sputtering on a MgO(001) substrate. The film studied displayed a long
range chemical order parameter of S~0.90, and hence has a very strong
perpendicular magnetic anisotropy. In the diffusive electron transport regime,
for temperatures ranging from 2 K to 258 K, we found hysteresis in the
magnetoresistance mainly due to electron scattering from magnetic domain walls.
At 2 K, we observed an overall domain wall magnetoresistance of about 0.5 %. By
evaluating the spin current asymmetry alpha = sigma_up / sigma_down, we were
able to estimate the diffusive spin current polarization. At all temperatures
ranging from 2 K to 258 K, we found a diffusive spin current polarization of >
80%. To study the ballistic transport regime, we have performed point-contact
Andreev-reflection measurements at 4.2 K. We obtained a value for the ballistic
current spin polarization of ~42% (which compares very well with that of a
polycrystalline thin film of elemental Fe). We attribute the discrepancy to a
difference in the characteristic scattering times for oppositely spin-polarized
electrons, such scattering times influencing the diffusive but not the
ballistic current spin polarization.Comment: 22 pages, 13 figure
Nanosized superparamagnetic precipitates in cobalt-doped ZnO
The existence of semiconductors exhibiting long-range ferromagnetic ordering
at room temperature still is controversial. One particularly important issue is
the presence of secondary magnetic phases such as clusters, segregations,
etc... These are often tedious to detect, leading to contradictory
interpretations. We show that in our cobalt doped ZnO films grown
homoepitaxially on single crystalline ZnO substrates the magnetism
unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior
was investigated by SQUID magnetometry, x-ray magnetic circular dichroism, and
AC susceptibility measurements. The results were correlated to a detailed
microstructural analysis based on high resolution x-ray diffraction,
transmission electron microscopy, and electron-spectroscopic imaging. No
evidence for carrier mediated ferromagnetic exchange between diluted cobalt
moments was found. In contrast, the combined data provide clear evidence that
the observed room temperature ferromagnetic-like behavior originates from
nanometer sized superparamagnetic metallic cobalt precipitates.Comment: 20 pages, 6 figures; details about background subtraction added to
section III. (XMCD
Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation
A model to treat the anomalous Hall effect is developed. Based on the Kubo
formalism and on the Dirac equation, this model allows the simultaneous
calculation of the skew-scattering and side-jump contributions to the anomalous
Hall conductivity. The continuity and the consistency with the
weak-relativistic limit described by the Pauli Hamiltonian is shown. For both
approaches, Dirac and Pauli, the Feynman diagrams, which lead to the
skew-scattering and the side-jump contributions, are underlined. In order to
illustrate this method, we apply it to a particular case: a ferromagnetic bulk
compound in the limit of weak-scattering and free-electrons approximation.
Explicit expressions for the anomalous Hall conductivity for both
skew-scattering and side-jump mechanisms are obtained. Within this model, the
recently predicted ''spin Hall effect'' appears naturally
Martensitic transition and magnetoresistance in a Cu-Al-Mn shape memory alloy. Influence of aging
We have studied the effect of ageing within the miscibility gap on the
electric, magnetic and thermodynamic properties of a non-stoichiometric Heusler
Cu-Al-Mn shape-memory alloy, which undergoes a martensitic transition from a
-based (-phase) towards a close-packed structure (-phase).
Negative magnetoresistance which shows an almost linear dependence on the
square of magnetization with different slopes in the - and -phases,
was observed. This magnetoresistive effect has been associated with the
existence of Mn-rich clusters with the CuAlMn-structure. The effect of an
applied magnetic field on the martensitic transition has also been studied. The
entropy change between the - and -phases shows negligible dependence
on the magnetic field but it decreases significantly with annealing time within
the miscibility gap. Such a decrease is due to the increasing amount of
CuMnAl-rich domains that do not transform martensitically.Comment: 9 pages, 9 figures, accepted for publication in PR
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