49 research outputs found
Current dependence of grain boundary magnetoresistance in La_0.67Ca_0.33MnO_3 films
We prepared epitaxial ferromagnetic manganite films on bicrystal substrates
by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was
measured as a function of magnetic field, temperature and current. At low
temperatures hysteretic changes in resistivity up to 70% due to switching of
magnetic domains at the coercitive field are observed. The strongly non-ohmic
behavior of the current-voltage leads to a complete suppression of the MR
effect at high bias currents with the identical current dependence at low and
high magnetic fields. We discuss the data in view of tunneling and mesoscale
magnetic transport models and propose an explicit dependence of the spin
polarization on the applied current in the grain boundary region.Comment: 5 pages, to appear in J. Appl. Phy
Epitaxy and magnetotransport of Sr_2FeMoO_6 thin films
By pulsed-laser deposition epitaxial thin films of Sr_2FeMoO_6 have been pre-
pared on (100) SrTiO_3 substrates. Already for a deposition temperature of 320
C epitaxial growth is achieved. Depending on deposition parameters the films
show metallic or semiconducting behavior. At high (low) deposition temperature
the Fe,Mo sublattice has a rock-salt (random) structure. The metallic samples
have a large negative magnetoresistance which peaks at the Curie temperature.
The magnetic moment was determined to 4 mu_B per formula unit (f.u.), in
agreement with the expected value for an ideal ferrimagnetic arrangement. We
found an ordinary Hall coefficient of -6.01x10^{-10} m^3/As at 300 K,
corresponding to an electronlike charge-carrier density of 1.3 per Fe,Mo-pair.
In the semiconducting films the magnetic moment is reduced to 1 mu_B/f.u. due
to disorder in the Fe,Mo sublattice. In low fields an anomalous holelike
contribution dominates the Hall voltage, which vanishes at low temperatures for
the metallic films only.Comment: Institute of Physics, University of Mainz, Germany, 4 pages,
including 5 pictures and 1 Table, submitted to Phys. Rev.
Charge carrier density collapse in La_0.67Ca_0.33MnO_3 and La_0.67Sr_0.33MnO_3 epitaxial thin films
We measured the temperature dependence of the linear high field Hall
resistivity of La_0.67Ca_0.33MnO_3 (T_C=232K) and La_0.67Sr_0.33MnO_3
(T_C=345K) thin films in the temperature range from 4K up to 360K in magnetic
fields up to 20T. At low temperatures we find a charge carrier density of 1.3
and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In
this temperature range electron-magnon scattering contributes to the
longitudinal resistivity. At the ferromagnetic transition temperature T_C a
dramatic drop in the number of current carriers down to 0.6 holes per unit
cell, accompanied by an increase in unit cell volume, is observed. Corrections
of the Hall data due to a non saturated magnetic state will lead a more
pronounced charge carrier density collapse.Comment: 5 pages, 5 EPS figures, submitted to Eur. Phys. J.
Epitaxial growth and magnetic properties of Sr2CrReO6 thin films
The double perovskite Sr2CrReO6 is an interesting material for spintronics,
showing ferrimagnetism up to 635 K with a predicted high spin polarization of
about 86%. We fabricated Sr2CrReO6 epitaxial films by pulsed laser deposition
on (001)-oriented SrTiO3 substrates. Phase-pure films with optimum
crystallographic and magnetic properties were obtained by growing at a
substrate temperature of 700 degree C in pure O2 of 6.6x10-4 mbar. The films
are c-axis oriented, coherently strained, and show less than 20% anti-site
defects. The magnetization curves reveal high saturation magnetization of 0.8
muB per formula unit and high coercivity of 1.1 T, as well as a strong magnetic
anisotropy.Comment: accepted for publicatio
Role of Orbital Degeneracy in Double Exchange Systems
We investigate the role of orbital degeneracy in the double exchange (DE)
model. In the limit, an effective generalized ``Hubbard''
model incorporating orbital pseudospin degrees of freedom is derived. The model
possesses an exact solution in one- and in infinite dimensions. In 1D, the
metallic phase off ``half-filling'' is a Luttinger liquid with
pseudospin-charge separation. Using the solution for our effective
model, we show how many experimental observations for the well-doped () three-dimensional manganites can be qualitatively
explained by invoking the role of orbital degeneracy in the DE model.Comment: 8 pages, 2 figures, submitted to Phys. Rev.
A model for spin-polarized transport in perovskite manganite bi-crystal grain boundaries
We have studied the temperature dependence of low-field magnetoresistance and
current-voltage characteristics of a low-angle bi-crystal grain boundary
junction in perovskite manganite La_{2/3}Sr_{1/3}MnO_3 thin film. By gradually
trimming the junction we have been able to reveal the non-linear behavior of
the latter. With the use of the relation M_{GB} \propto M_{bulk}\sqrt{MR^*} we
have extracted the grain boundary magnetization. Further, we demonstrate that
the built-in potential barrier of the grain boundary can be modelled by
V_{bi}\propto M_{bulk}^2 - M_{GB}^2. Thus our model connects the
magnetoresistance with the potential barrier at the grain boundary region. The
results indicate that the band-bending at the grain boundary interface has a
magnetic origin.Comment: 9 pages, 5 figure
Grain boundary effects on magnetotransport in bi-epitaxial films of LaSrMnO
The low field magnetotransport of LaSrMnO (LSMO) films
grown on SrTiO substrates has been investigated. A high qualtity LSMO film
exhibits anisotropic magnetoresistance (AMR) and a peak in the
magnetoresistance close to the Curie temperature of LSMO. Bi-epitaxial films
prepared using a seed layer of MgO and a buffer layer of CeO display a
resistance dominated by grain boundaries. One film was prepared with seed and
buffer layers intact, while a second sample was prepared as a 2D square array
of grain boundaries. These films exhibit i) a low temperature tail in the low
field magnetoresistance; ii) a magnetoconductance with a constant high field
slope; and iii) a comparably large AMR effect. A model based on a two-step
tunneling process, including spin-flip tunneling, is discussed and shown to be
consistent with the experimental findings of the bi-epitaxial films.Comment: REVTeX style; 14 pages, 9 figures. Figure 1 included in jpeg format
(zdf1.jpg); the eps was huge. Accepted to Phys. Rev.
Structural and doping effects in the half-metallic double perovskite CrWO
he structural, transport, magnetic and optical properties of the double
perovskite CrWO with have been studied. By
varying the alkaline earth ion on the site, the influence of steric effects
on the Curie temperature and the saturation magnetization has been
determined. A maximum K was found for SrCrWO having an almost
undistorted perovskite structure with a tolerance factor . For
CaCrWO and BaCrWO structural changes result in a strong
reduction of . Our study strongly suggests that for the double perovskites
in general an optimum is achieved only for , that is, for an
undistorted perovskite structure. Electron doping in SrCrWO by a
partial substitution of Sr by La was found to reduce both
and the saturation magnetization . The reduction of could be
attributed both to band structure effects and the Cr/W antisites induced by
doping. Band structure calculations for SrCrWO predict an energy gap in
the spin-up band, but a finite density of states for the spin-down band. The
predictions of the band structure calculation are consistent with our optical
measurements. Our experimental results support the presence of a kinetic energy
driven mechanism in CrWO, where ferromagnetism is stabilized by a
hybridization of states of the nonmagnetic W-site positioned in between the
high spin Cr-sites.Comment: 14 pages, 10 figure
Extrinsic Magnetotransport Phenomena in Ferromagnetic Oxides
This review is focused on extrinsic magnetotransport effects in ferromagnetic
oxides. It consists of two parts; the second part is devoted to an overview of
experimental data and theoretical models for extrinsic magnetotransport
phenomena. Here a critical discussion of domain-wall scattering is given.
Results on surfacial and interfacial magnetism in oxides are presented.
Spin-polarized tunnelling in ferromagnetic junctions is reviewed and
grain-boundary magnetoresistance is interpreted within a model of
spin-polarized tunnelling through natural oxide barriers. The situation in
ferromagnetic oxides is compared with data and models for conventional
ferromagnets. The first part of the review summarizes basic material
properties, especially data on the spin-polarization and evidence for
half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed.
An outlook on the further development of oxide spin-electronics concludes this
review.Comment: 133 pages, 47 figures, submitted to Rep. Prog. Phy