Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions (original) (raw)
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Physical Review Letters, 2005
We present a scaling theory of magneto-transport in Anderson-localized disordered ferromagnets. Within our framework a pronounced magnetic-field-sensitive resistance peak emerges naturally for temperatures near the magnetic phase transition. We find that the resistance anomaly is a direct consequence of the change in localization length caused by the magnetic transition. For increasing values of the external magnetic field, the resistance peak is gradually depleted and pushed towards higher temperatures. Our results are in good agreement with magneto-resistance measurements on a variety of disordered magnets.
Strong Magnetoresistance Induced by Long-Range Disorder
Physical Review Letters, 1999
We calculate the semiclassical magnetoresistivity ρxx(B) of non-interacting fermions in two dimensions moving in a weak and smoothly varying random potential or random magnetic field. We demonstrate that in a broad range of magnetic fields the non-Markovian character of the transport leads to a strong positive magnetoresistance. The effect is especially pronounced in the case of a random magnetic field where ρxx(B) becomes parametrically much larger than its B = 0 value. 73.50.Jt, 05.60.+w The magnetoresistance (MR) is one of the most frequently studied characteristics of the two-dimensional electron gas (2DEG). When the effect of disorder is described by a collision integral within the semiclassical Boltzmann equation approach, the resistivity tensorρ(B) for an isotropic system has the Drude form
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The European Physical Journal B - Condensed Matter, 2002
Peculiarities of transport properties of three-and two-dimensional half-metallic ferromagnets are investigated, which are connected with the absence of spin-flip scattering processes. The temperature and magnetic field dependences of resistivity in various regimes are calculated. The resistivity is proportional to T 9/2 for T < T * and to T 7/2 for T > T * , T * being the crossover temperature for longitudinal scattering processes. The latter scale plays also an important role in magnetoresistance. The contribution of non-quasiparticle (incoherent) states to the transport properties is discussed. It is shown that they can dominate in the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity.
Effects of Disorder on Ferromagnetism in Diluted Magnetic Semiconductors
Physical Review Letters, 2001
We present results of a numerical mean-field treatment of interacting spins and carriers in doped diluted magnetic semiconductors, which takes into account the positional disorder present in these alloy systems. Within our mean-field approximation, disorder enhances the ferromagnetic transition temperature for metallic densities not too far from the metal-insulator transition. Concurrently, the ferromagnetic phase is found to have very unusual temperature dependence of the magnetization as well as specific heat as a result of disorder. Unusual spin and charge transport is implied.
Localisation and scaling of resistance of disordered systems including magnetic field effects
Application of High Magnetic Fields in Semiconductor Physics
The concept of localisation is briefly reviewed with special emphasis on the scaling properties of the conductance. The influence of a magnetic field is discussed. Numerical results for the localisation length in two-and three-dimensional systems are reported. It is found that it obeys a one-parameter scaling law, which depends on the dimensionality, and which allows conclusions on the temperature dependence of the resistance. The influence of a magnetic field on localisation is studied with the same numerical procedure as in the case of zero magnetic field. The localisation length is found to obey a two parameter scaling law. A negative magnetoresistance is obtained. Evidence is presented that even for strong magnetic field all quantum states remain localised. An interpretation of recent high field magnetotransport data obtained from inversion layers is attempted.
Asymmetric metal-insulator transition in disordered ferromagnetic films
Physical Review Letters, 2011
We present experimental data and a theoretical interpretation on the conductance near the metalinsulator transition in thin ferromagnetic Gd films of thickness b ≈ 2 − 10nm. A large phase relaxation rate caused by scattering of quasiparticles off spin wave excitations renders the dephasing length L φ b in the range of sheet resistances considered, so that the effective dimension is d = 3. The observed approximate fractional temperature power law of the conductivity is ascribed to the scaling regime near the transition. The conductivity data as a function of temperature and disorder strength collapse on to two scaling curves for the metallic and insulating regimes. The best fit is obtained for a dynamical exponent z ≈ 2.5 and a correlation length critical exponent ν ′ ≈ 1.4 on the metallic side and a localization length exponent ν ≈ 0.8 on the insulating side.
Free magnetic moments in disordered metals
Journal of Experimental and Theoretical Physics Letters, 2006
The screening of magnetic moments in metals, the Kondo effect, is found to be quenched with a finite probability in the presence of nonmagnetic disorder. Numerical results for a disordered electron system show that the distribution of Kondo temperatures deviates strongly from the result expected from random matrix theory. A pronounced second peak emerges for small Kondo temperatures, showing that the probability that magnetic moments remain unscreened at low temperatures increases with disorder. Analytical calculations, taking into account correlations between eigenfunction intensities yield a finite width for the distribution in the thermodynamic limit. Experimental consequences for disordered mesoscopic metals are discussed. PACS: 72.10.Fk, 72.15.Qm, 75.20.Hr, 72.15.Rn In a metal with antiferromagnetic exchange interaction between a local magnetic moment and the conduction electrons, correlations cause a change in the Fermi liquid ground state. The screening of the localized spin by the formation of a Kondo singlet below the Kondo temperature, T K , is affected by disorder in various ways. Fluctuations in the exchange coupling due to random positioning of magnetic moments results in a dispersion of T K [1]. Since T K is defined by an integral equation similar to the BCS equation for the critical temperature of a superconductor, one could expect, by analogy, the Anderson theorem [2] to be valid. In that case, the leading correction to T K would be of order T
Physical Review B, 2007
We consider the Anomalous Hall Effect (AHE) in thin disordered ferromagnetic films. Using a microscopic model of electrons in a random potential of identical impurities including spin-orbit coupling, we develop a general formulation for strong, finite range impurity scattering. Explicit calculations are done within a short range but strong impurity scattering to obtain AH conductivities for both the skew scattering and side jump mechanisms. We also evaluate quantum corrections due to interactions and weak localization effects. We show that for arbitrary strength of the impurity scattering, the electron-electron interaction correction to the AH conductivity vanishes exactly due to general symmetry reasons. On the other hand, we find that our explicit evaluation of the weak localization corrections within the strong, short range impurity scattering model can explain the experimentally observed logarithmic temperature dependences in disordered ferromagnetic Fe films.
World Journal of Condensed Matter Physics, 2014
Electronic transport properties of magnetically disordered R(-3)c phase 1.5 0.5 3 Fe Ti O −δ thin films epitaxially grown on Al 2 O 3 (0001) substrates have been studied. The measured magnetization in configurations with the magnetic field perpendicular and parallel to the film plane shows weak values of 0.1µ B /formula compared to the theoretical value of 2µ B /formula and a strong anisotropy with no saturation in perpendicular configuration. These properties are associated with the atomic scale disorder of Ti/Fe ions along c-axis. At zero-magnetic field and within the temperature range of 80 K to 400 K, the conduction mechanism appears to be Efros-Shklovskii variable range hopping with a carrier localization length of 0 0.86 nm = ξ. Magneto-resistance (MR) is positive in perpendicular configuration, while it is negative in parallel configuration, with significant values of 27%-37% MR = at room temperature at 9 Tesla. Electron localization lengths were deduced from experiment for different external magnetic fields. The origin of magneto-resistance observed in experiment, is discussed.