Magnetocapacitance

About: Magnetocapacitance is a research topic. Over the lifetime, 497 publications have been published within this topic receiving 23846 citations.

Magnetocapacitance Measurement of Selectively Doped n-AlGaAs/GaAs Heterojunction with InGaAs Quantum Dots

Abstract: The magnetocapacitance between a two-dimensional electron gas (2DEG) and a gate electrode was investigated in a selectively doped n-AlGaAs/GaAs heterojunction with embedded InGaAs quantum dots (QDs). The complex capacitance C was studied as a function of the modulation frequency f ranging from 60 to 12 kHz at the filling factor ν= 2 of a Landau level. The experiment was well explained by a resistive plate model, where the bulk conductance σxx of the 2DEG has a frequency-dependent component proportional to fS (S = 0.51). This result suggests that the QDs induce potential valleys in a 2DEG channel, in which electron transfers occur between the valleys with their relaxation times extended over a wide range.

Pressure Tuning of Magnetism and Drastic Increment of Thermal Conductivity under Applied Magnetic Field in HgCr2S4

Abstract: HgCr2S4 is a typical compound manifesting competing ferromagnetic (FM) and antiferromagnetic (AFM) exchanges as well as strong spin-lattice coupling. Here we study these effects by intentionally choosing a combination of magnetization under external hydrostatic pressure and thermal conductivity at various magnetic fields. Upon applying pressure up to 10 kbar at 1 kOe, while the magnitude of magnetization reduces progressively, the AFM ordering temperature TN enhances concomitantly at a rate of about 1.5 K/kbar. Strikingly, at 10 kOe the field polarized FM state is found to be driven readily back to an AFM one even at only 5 kbar. In addition, the thermal conductivity exhibits drastic increments at various fields in the temperature range with strong spin fluctuations, reaching about 30% at 50 kOe. Consequently, the results give new experimental evidence of spin-lattice coupling. Apart from the colossal magnetocapacitance and colossal magnetoresistance reported previously, the findings here may enable new promising functionalities for potential applications.

Large magnetocapacitance beyond 420% in epitaxial magnetic tunnel junctions with an MgAl2O4 barrier

Abstract: Magnetocapacitance (MC) effect has been observed in systems where both symmetries of time-reversal and space-inversion are broken, for examples, in multiferroic materials and spintronic devices. The effect has received increasing attention due to its interesting physics and the prospect of applications. Recently, a large tunnel magnetocapacitance (TMC) of 332% at room temperature was reported using MgO-based (001)-textured magnetic tunnel junctions (MTJs). Here, we report further enhancement in TMC beyond 420% at room temperature using epitaxial MTJs with an MgAl2O4(001) barrier with a cation-disordered spinel structure. This large TMC is partially caused by the high effective tunneling spin polarization, resulted from the excellent lattice matching between the Fe electrodes and the MgAl2O4 barrier. The epitaxial nature of this MTJ system sports an enhanced spin-dependent coherent tunneling effect. Among other factors leading to the large TMC are the appearance of the spin capacitance, the large barrier height, and the suppression of spin flipping through the MgAl2O4 barrier. We explain the observed TMC by the Debye-Fröhlich modelled calculation incorporating Zhang-sigmoid formula, parabolic barrier approximation, and spin-dependent drift diffusion model. Furthermore, we predict a 1000% TMC in MTJs with a spin polarization of 0.8. These experimental and theoretical findings provide a deeper understanding on the intrinsic mechanism of the TMC effect. New applications based on large TMC may become possible in spintronics, such as multi-value memories, spin logic devices, magnetic sensors, and neuromorphic computing.

Coexistence of the electric polarization and conductive current in the bismuth–neodymium ferrite garnet films

Abstract: The Nd1Bi2Fe5O12/Nd2Bi1Fe4Ga1O12 polycrystalline films on the glass substrate and the Nd0.5Bi2.5Fe5O12 epitaxial films on the single-crystal gadolinium gallium garnet substrate have been investigated by impedance and dielectric spectroscopy. The inductive contribution to the impedance and two relaxation channels related to ferroelectric domains and migration polarization have been established. The magnetocapacitance and magnetoimpedance have been determined. The conductive and polarization currents and the phase difference between them for the films of two types have been determined. The critical temperatures of the polarization disappearance and hysteresis I–V have been found. A model of the polarization caused by the piezoelectric effect and flexoelectric interaction has been proposed. I–V hysteresis is explained by the presence of ferroelectric domains near the interface and is associated with the hysteresis of the electric polarization.

Magnetostructural and magnetodielectric coupling in spinel oxides

Abstract: Author(s): Kemei, Moureen C | Advisor(s): Seshadri, Ram | Abstract: Spinels oxides are of great interest functionally as multiferroic, battery, and magnetic materials as well as fundamentally because they exhibit novel spin, structural, and orbital ground states. Competing interactions are at the heart of novel functional behavior in spinels. Here, we explore the intricate landscape of spin, lattice, and orbital interactions in magnetic spinels by employing variable-temperature high-resolution synchrotron x-ray powder diffraction, total neutron scattering, magnetic susceptibility, dielectric, and heat capacity measurements. We show that the onset of long-range magnetic interactions often gives rise to lattice distortions. We present the complete crystallographic descriptions of the ground state structures of several spinels, thereby paving the way for accurate modeling and design of structure-property relationships in these materials. We also report the emergence of magnetodielectric coupling in the magnetostructural phases of some of the studied spinels. We begin by examining spin-lattice coupling in the Jahn-Teller active systems NiCr2O4 and CuCr2O4. Orbital ordering yields a cubic to tetragonal lattice distortion in these materials above their magnetic ordering temperatures, however, we find that magnetic ordering also drives structural distortions in these spinels through exchange striction. We provide the first orthorhombic structural descriptions of NiCr2O4 and CuCr2O4. Our observation of strong spin-lattice coupling in NiCr2O4 and CuCr2O4 inspired the study of magnetodielectric coupling in these spinels. Magnetocapacitance measurements of NiCr2O4 reveal multiferroic behavior and new magnetostructural distortions below the Neel temperature. This observation illustrates the sensitivity of dielectric measurements to magnetostructural transitions in spinel materials. Finally, in the examination of NiCr2O4 we show that magnetodielectric coupling is well described by Ginzburg-Landau theory. In addition to exchange striction, geometric frustration couples spin interactions to the lattice of the spinels MgCr2O4 and ZnCr2O4. Novel spin ground states that are important for memory and quantum computing applications are predicted to exist in these spinels. However, their structural and spin ground states are not well understood. We find that tetragonal and orthorhombic phases coexist in antiferromagnetic MgCr2O4 and ZnCr2O4. The structural deformations in these materials lift spin degeneracy by primarily distorting the pyrochlore Cr sublattice. In subsequent studies, we probe the effect of adding dilute spins on the non-magnetic cation sites of MgCr2O4 and ZnCr2O4. Substitution of Co2+ cations in Zn1-xCoxCr2O4 completely suppress the spin-Jahn-Teller distortion of ZnCr2O4 while, Cu2+ substitutions in Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 induce Jahn-Teller distortions at temperatures above their magnetic ordering temperatures. The Jahn-Teller distortions of Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 do not lift spin degeneracy, therefore magnetic ordering is still suppressed down to low temperatures. We show that only more than 20% magnetic A substituents can lift spin degeneracy in MgCr2O4 and ZnCr2O4.We have also examined the magnetostructural phase transition of the spinel Mn3O4. We show that Mn3O4 undergoes a magnetostructural phase transition from tetragonal I41/amd symmetry to a phase coexistence regime consisting of tetragonal I41/amd and orthorhombic Fddd symmetries. Phase coexistence in Mn3O4 is mediated by strain due to a significant lattice mismatch between the low temperature orthorhombic phase and the high temperature tetragonal phase. We propose that strain could be used to control the structure and properties of Mn3O4. Our investigations of spin-driven lattice distortions in spinel oxides illustrate that structural phase coexistence is prevalent for spinels with Neel temperatures below 50 K.