46 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
Lattice strain accompanying the colossal magnetoresistance effect in EuB
The coupling of magnetic and electronic degrees of freedom to the crystal
lattice in the ferromagnetic semimetal EuB, which exhibits a complex
ferromagnetic order and a colossal magnetoresistance (CMR) effect, %, very
likely involving magnetic polarons, is studied by high-resolution thermal
expansion and magnetostriction experiments. EuB may be viewed as a model
system, where pure magnetism-tuned transport and the response of the crystal
lattice can be studied in a comparatively simple environment,i.e., not
influenced by strong crystal-electric field effects and Jahn-Teller
distortions. We find a very large lattice response, quantified by (i) the
magnetic Gr\"uneisen parameter, (ii) the spontaneous strain when entering the
ferromagnetic region and (iii) the magnetostriction in the paramagnetic
temperature regime. Our analysis reveals that a significant part of the lattice
effects originates in the magnetically-driven delocalization of charge
carriers, consistent with the scenario of percolating magnetic polarons. A
strong effect of the formation and dynamics of local magnetic clusters on the
lattice parameters is suggested to be a general feature of CMR materials.Comment: 5 pages and 3 figure
Spin transport and accumulation in the persistent photoconductor AlGaAs
Electrical spin transport and accumulation have been measured in highly Si
doped Al0.3Ga0.7As utilizing a lateral spin transport device. Persistent
photoconductivity allows for the tuning of the effective carrier density of the
channel material in situ via photodoping. Hanle effect measurements are
completed at various carrier densities and the measurements yield spin
lifetimes on the order of nanoseconds, an order of magnitude smaller than in
bulk GaAs. These measurements illustrate that this methodology can be used to
obtain a detailed description of how spin lifetimes depend on carrier density
in semiconductors across the metal-insulator transition
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
Micromechanical Thermal Assays of Ca2+-Regulated Thin-Filament Function and Modulation by Hypertrophic Cardiomyopathy Mutants of Human Cardiac Troponin
Microfabricated thermoelectric controllers can be employed to investigate mechanisms underlying myosin-driven sliding of Ca2+-regulated actin and disease-associated mutations in myofilament proteins. Specifically, we examined actin filament sliding—with or without human cardiac troponin (Tn) and α-tropomyosin (Tm)—propelled by rabbit skeletal heavy meromyosin, when temperature was varied continuously over a wide range (∼20–63°C). At the upper end of this temperature range, reversible dysregulation of thin filaments occurred at pCa 9 and 5; actomyosin function was unaffected. Tn-Tm enhanced sliding speed at pCa 5 and increased a transition temperature (Tt) between a high activation energy (Ea) but low temperature regime and a low Ea but high temperature regime. This was modulated by factors that alter cross-bridge number and kinetics. Three familial hypertrophic cardiomyopathy (FHC) mutations, cTnI R145G, cTnI K206Q, and cTnT R278C, cause dysregulation at temperatures ∼5–8°C lower; the latter two increased speed at pCa 5 at all temperatures