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The ability of an insulating solid to conduct heat is rarely effected by the application of a magnetic field. We have found, however, that the low temperature heat conduction of some solids increases by more than a factor of two with the application of a modest magnetic field. The effect occurs in low-dimensional magnetically ordered materials when a small gap, delta, in the acoustic magnon (spin wave) spectra is closed using a magnetic field H > delta/gmu_B. Since all magnetically ordered materials must have a gap in the magnon spectra for magnons with k = 0, this may be a very general effect. Extra heat is carried through the solid only when the magentic field exceeds the critical value (H > delta/gmu_B). At this critical field the tiny atomic magnets in the solid abruptly change the direction they point which results in more heat flowing through the material. The magnetic field thus acts as a heat switch. We have observed this effect in three quite different magnetically ordered materials: K_2V_3O_8, Nd_2CuO_4 and Pr_2CuO_4. Several possible explanations for these effects will be discussed.
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The spin valve effect is a quantum phenomenon so far only realized in multilayer thin films or heterostructures. Here we report a strong spin valve effect existing in bulk single crystals of Ca3(Ru1-xCrx)2O7 having an anisotropic, bilayered crystal structure. This discovery opens new avenues to understand the underlying physics of spin valves, and fully realize its potential in practical devices.
Magnetism arising from coupled spin and spatial degrees of freedom underlies the properties of a broad array of physical systems. We study here the interplay between correlations in spin and space for the quantum compass model in a finite external field, using quantum Monte Carlo methods. We find that finite temperatures cant the spin and space (bond) correlations, with increasing temperature even reorienting spin correlations between orthogonal spatial directions. We develop a coupled mean field theory to understand this effect in terms of the underlying quantum critical properties of crossed Ising chains in transverse fields and an effective field that weakens upon increasing temperature. Thermal canting offers an experimental signature of spin-bond anisotropy.
Recent experimental measurements of magnetoresistance in dual spin valves [A. Aziz et al., Phys. Rev. Lett. 103, 237203 (2009)] reveal some nonlinear features of transport, which have not been observed in other systems. We propose a phenomenological model describing current-dependent resistance (and giant magnetoresistance) in double spin valves. The model is based on a modified Valet-Fert approach, and takes into account the dependence of bulk/interface resistance and bulk/interface spin asymmetry parameters for the central magnetic layer on spin accumulation, and consequently on charge current. Such a nonlinear model accounts for recent experimental observations.
We present a study of the effects of inelastic scattering on the transport properties of various nanoscale devices, namely H$_2$ molecules sandwiched between Pt contacts, and a spin-valve made by an organic molecule attached to model half-metal ferromagnetic current/voltage probes. In both cases we use a tight-binding Su-Schrieffer-Heeger Hamiltonian and the inelastic effects are treated with a multi-channel method, including Pauli exclusion principle. In the case of the H$_2$ molecule, we find that inelastic backscattering is responsible for the drop of the differential conductance at biases larger than the excitation energy of the lower of the molecular phonon modes. In the case of the spin-valve, we investigate the different spin-currents and the magnetoresistance as a function of the position of the Fermi level with respect to the spin-polarized band edges. In general inelastic scattering reduces the spin-polarization of the current and consequently the magnetoresistance.
Longitudinal spin Seebeck effect has been investigated for an uniaxial antiferromagnetic insulator Cr2O3, characterized by a spin-flop transition under magnetic field along the c-axis. We have found that temperature gradient applied normal to Cr2O3/Pt interface induces inverse spin Hall voltage of spin current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr2O3. The observed voltage shows significant enhancement for the lower temperature region, which can be ascribed to the phonon-drag effect on the relevant spin excitations. The above results establish that antiferromagnetic spin waves with high frequency above 100 GHz can be an effective carrier of spin current.
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