219 research outputs found
Magnetically-driven electronic phase separation in the semimetallic ferromagnet EuB
From measurements of fluctuation spectroscopy and weak nonlinear transport on
the semimetallic ferromagnet EuB we find direct evidence for
magnetically-driven electronic phase separation consistent with the picture of
percolation of magnetic polarons (MP), which form highly conducting
magnetically-ordered clusters in a paramagnetic and 'poorly conducting'
background. These different parts of the conducting network are probed
separately by the noise spectroscopy/nonlinear transport and the conventional
linear resistivity. We suggest a comprehensive and 'universal' scenario for the
MP percolation, which occurs at a critical magnetization either induced by
ferromagnetic order at zero field or externally applied magnetic fields in the
paramagentic region
Electrical Noise From Phase Separation In Pr2/3Ca1/3MnO3 Single Crystal
Low frequency electrical noise measurements have been used to probe the
electronic state of the perovskite-type manganese oxide Pr2/3Ca1/3MnO3 versus
temperature and in the vicinity of the field-induced transition from the
insulating, charge-ordered state (I-CO) to the metallic, ferromagnetic state
(M-F). At high temperature we have observed a high level of the excess noise
with mainly a gaussian distribution of the resistance fluctuations, and the
associated power spectral density has a standard 1/f dependence. However, in
the hysteretic region, where the electrical resistance depends dramatically on
the sample history, we have observed a huge non-gaussian noise characterized by
two level fluctuator-like switching (TLS) in the time domain. We discuss the
origin of the noise in terms of percolative behavior of the conductivity. We
speculate that the dominant fluctuators are manganese clusters switching
between the M-F and the I-CO phases.Comment: RevTeX, 6 pages with 3 figure
Magnetization Reversal in Elongated Fe Nanoparticles
Magnetization reversal of individual, isolated high-aspect-ratio Fe
nanoparticles with diameters comparable to the magnetic exchange length is
studied by high-sensitivity submicron Hall magnetometry. For a Fe nanoparticle
with diameter of 5 nm, the magnetization reversal is found to be an incoherent
process with localized nucleation assisted by thermal activation, even though
the particle has a single-domain static state. For a larger elongated Fe
nanoparticle with a diameter greater than 10 nm, the inhomogeneous magnetic
structure of the particle plays important role in the reversal process.Comment: 6 pages, 6 figures, to appear in Phys. Rev. B (2005
Measurement of the spin polarization of the magnetic semiconductor EuS with zero-field and Zeeman-split Andreev reflection spectroscopy
We report measurements of the spin polarization (\textbf{\textit{P}}) of the
concentrated magnetic semiconductor EuS using both zero-field and Zeeman-split
Andreev reflection spectroscopy (ARS) with EuS/Al planar junctions. The
zero-field ARS spectra are well described by the modified (spin-polarized) BTK
model with expected superconducting energy gap and actual measurement
temperature (no additional spectral broadening). The fittings consistently
yield \textbf{\textit{P}} close to 80% regardless of the barrier strength.
Moreover, we performed ARS in the presence of a Zeeman-splitting of the
quasiparticle density of states in Al. To describe the Zeeman-split ARS
spectra, we develop a theoretical model which incorporates the solution to the
Maki-Fulde equations into the modified BTK analysis. The method enables the
determination of the magnitude as well as the sign of \textbf{\textit{P}} with
ARS, and the results are consistent with those from the zero-field ARS. The
experiments extend the utility of field-split superconducting spectroscopy from
tunnel junctions to Andreev junctions of arbitrary barrier strengths.Comment: 6 pages, 4 figure
Noise Probe of the Dynamic Phase Separation in La2/3Ca1/3MnO3
Giant Random Telegraph Noise (RTN) in the resistance fluctuation of a
macroscopic film of perovskite-type manganese oxide La2/3Ca1/3MnO3 has been
observed at various temperatures ranging from 4K to 170K, well below the Curie
temperature (TC = 210K). The amplitudes of the two-level-fluctuations (TLF)
vary from 0.01% to 0.2%. We use a statistical analysis of the life-times of the
TLF to gain insight into the microscopic electronic and magnetic state of this
manganite. At low temperature (below 30K) The TLF is well described by a
thermally activated two-level model. An estimate of the energy difference
between the two states is inferred. At higher temperature (between 60K and
170K) we observed critical effects of the temperature on the life-times of the
TLF. We discuss this peculiar temperature dependence in terms of a sharp change
in the free energy functional of the fluctuators. We attribute the origin of
the RTN to be a dynamic mixed-phase percolative conduction process, where
manganese clusters switch back and forth between two phases that differ in
their conductivity and magnetization.Comment: 15 pages, PDF only, Phys. Rev. Lett. (in press
Signatures of electronic phase separation in the Hall effect of anisotropically strained La0.67Ca0.33MnO3 films
Systematic transport measurements have been performed on a series of
La0.67Ca0.33MnO3 (LCMO) thin films with varying degrees of anisotropic strain.
The strain is induced via epitaxial growth on NdGaO3(001) substrates and varied
by controlling the thermal annealing time. An antiferromagnetic insulating
(AFI) state, possibly associated with charge ordering, emerges upon thermal
annealing. The Hall effect in these materials exhibits features that are
indicative of a percolative phase transition and correlate closely with the
emergence of the AFI state. In the paramagnetic phase, the Hall resistivity
takes on two slopes in all samples: a decreasing negative slope with increasing
temperature at low fields, which is attributed to the carrier hopping motion,
and an almost temperature independent positive slope at high fields due to
diffusive transport of holes. Significantly, the crossover fields of the Hall
resistivity slope at different temperatures correspond to the same
magnetization, which is interpreted as the critical point of a magnetic
field-driven percolative phase transition. At lower temperatures near the
zero-field metal-insulator transition, pronounced enhancement of the Hall
coefficient with the development of the AFI state is observed. The enhancement
peaks near the magnetic field-driven percolation; its magnitude correlates with
the strength of the AFI state and is suppressed with the melting of the AFI
state by an in-plane magnetic field. The observations resemble many features of
the enhancement of the Hall coefficient in granular metal films near the
composition-driven percolation
Spin-related magnetoresistance of n-type ZnO:Al and Zn_{1-x}Mn_{x}O:Al thin films
Effects of spin-orbit coupling and s-d exchange interaction are probed by
magnetoresistance measurements carried out down to 50 mK on ZnO and
Zn_{1-x}Mn_{x}O with x = 3 and 7%. The films were obtained by laser ablation
and doped with Al to electron concentration ~10^{20} cm^{-3}. A quantitative
description of the data for ZnO:Al in terms of weak-localization theory makes
it possible to determine the coupling constant \lambda_{so} = (4.4 +-
0.4)*10^{-11} eVcm of the kp hamiltonian for the wurzite structure, H_{so} =
\lambda_{so}*c(s x k). A complex and large magnetoresistance of
Zn_{1-x}Mn_{x}O:Al is interpreted in terms of the influence of the s-d
spin-splitting and magnetic polaron formation on the disorder-modified
electron-electron interactions. It is suggested that the proposed model
explains the origin of magnetoresistance observed recently in many magnetic
oxide systems.Comment: 4 pages, 4 figure
High Temperature Thermopower in La_{2/3}Ca_{1/3}MnO_3 Films: Evidence for Polaronic Transport
Thermoelectric power, electrical resistivity and magnetization experiments,
performed in the paramagnetic phase of La_{2/3}Ca_{1/3}MnO_3, provide evidence
for polaron-dominated conduction in CMR materials. At high temperatures, a
large, nearly field-independent difference between the activation energies for
resistivity (rho) and thermopower (S), a characteristic of Holstein Polarons,
is observed, and ln(rho) ceases to scale with the magnetization. On approaching
T_c, both energies become field-dependent, indicating that the polarons are
magnetically polarized. Below T_c, the thermopower follows a law S(H) prop.
1/rho (H) as in non saturated ferromagnetic metals.Comment: 10 pages, 5 .gif figures. Phys. Rev B (in press
Transition Rates between Mixed Symmetry States: First Measurement in 94Mo
The nucleus 94Mo was investigated using a powerful combination of
gamma-singles photon scattering experiments and gamma-gamma-coincidence studies
following the beta-decay of 94mTc. The data survey short-lived J^pi=1+,2+
states and include branching ratios, E2/M1 mixing ratios, lifetimes, and
transition strengths. The mixed-symmetry (MS) 1+ scissors mode and the 2+ MS
state are identified from M1 strengths. A gamma transition between MS states
was observed and its rate was measured. Nine M1 and E2 strengths involving MS
states agree with the O(6) limit of the interacting boson model-2 using the
proton boson E2 charge as the only free parameter.Comment: 9 pages, 3 PostScript figures included, ReVTeX, accepted for
publication in Physical Review Letters, tentatively scheduled for August 9,
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