Grain Boundary Magnetoresistance Research Articles

Atomic order, magnetic and transport phenomena in half-Heusler CoMnSb0.9Z0.1 alloys (Z = Si, Al, Sn, and Bi)

In this study, we investigate the effects of doping with ∼10 at% of Si, Al, Sn, and Bi on the atomic order, microstructure, and magnetic properties of CoMnSb samples prepared using the arc-melting method. X-ray diffraction, scanning electron microscopy, and magnetometry techniques were employed to derive the structural and magnetic parameters. The samples demonstrate a superstructure with an Fm-3m space group. They exhibit a saturation moment of 3.71–4.71 μB per formula unit at 5 K and a Curie temperature ranging from 474 to 507 K. Rietveld refinement performed on the XRD data revealed a possible occupation of magnetic atoms in the vacant 32f (x’, x’, x’) sites and atomic swapping, both of which may affect the magnitudes of the magnetic moment and Curie temperature. The electrical measurement results show that the grain boundaries and voids govern the conductivity of the samples. The spin polarization of the Si-doped sample was determined based on the grain-boundary magnetoresistance effect, which showed that this sample has a high degree of atomic order, consistent with the low magnetic moment, Curie temperature, and lattice constant.

Enhancement of room-temperature magnetoresistance in La0.5Sm0.2Sr0.3MnO3/(Ag2O) x/2

La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2 (x = 0.00, 0.04, 0.08, 0.25, 0.30) samples were prepared by the solid-state reaction method, and their transport behaviors, transport mechanism, and magnetoresistance effect were studied through the measurement and fitting of ρ-T curves. The results show that the element Ag takes part in reaction when the doping amount is small. Ag is mainly distributed at the grain boundary of the host material and is in metallic state when the doping amount is relatively large; then the system becomes a two-phase composite. A small amount of Ag doping can apparently increase grain-boundary magnetoresistance induced by the spin-dependent scattering. The resistivity of the sample doped with 30 mol% Ag is one order of magnitude smaller than that of low-doped samples, and its magnetoresistance in the magnetic field of 0.5 T and at 300 K is strengthened apparently reaching 9.4%, which is connected not only with the improvement of the grain-boundary structure of the host material but also with the decrease of material resistivity.

Large grain-boundary magnetoresistance in MnFe 2O 4 nanoparticle compact

The low-field magnetoresistance (MR) has been investigated on the ferrite MnFe 2O 4 nanoparticle compact. At 300 K, a relatively large MR of 6% has been observed in an applied field of 5 kOe. With temperature decreasing, the MR ratio increases to a maximum of 12% at 100 K. These large MR are likely attributed to the spin-polarized tunneling of conducting electrons through the grain boundaries. The rapid drop of MR below 100 K reveals that there is a spin glass transition in this kind of compound.

Dependence of the high-field grain-boundary magnetoresistance of ferromagnetic manganites on Curie temperature

We analyze the high-field magnetoresistance of polycrystalline ferromagnetic manganites of varied composition in magnetic fields up to μ0H=47T. Small to medium deviations from a linear field dependence of the conductance {as discovered for a La0.7Sr0.3MnO3 film recently, [N. Kozlova et al., J. Magn. Magn. Mater. 261, 48 (2003)]} are typical. The conductance is always well described by a quadratic polynomial G(H)=a+bH+cH2 with temperature-dependent coefficients a, b, and c. Both b and c increase with decreasing Curie temperature (TC) of the samples. The conductance slope 1∕G0 dG∕dH is related to an average susceptibility of the magnetically disturbed layer at the grain boundaries. The nonsaturated magnetic order of grain-boundary (GB) spins at 5K and 47T means the presence of strong antiferromagnetic interactions. The observation of a systematic correlation between GB magnetoresistance and bulk TC indicates that magnetic order at GBs is governed by parameters such as doping and average ionic size (electronic band width). Furthermore, the temperature dependence of linear and quadratic magnetoconductance contributions is investigated; here, the weak temperature dependence for compounds with high TC is notable.

Grain-boundary magnetoresistance enhancement induced by network self-optimization

A random conductance network model is proposed to study the global magnetoresistance effects of granular systems of half-metallic oxides. Different transport mechanisms resulting from grain-boundary effects are considered and their contributions to the total magnetoresistance of the network are discussed. It is found that magnetoresistance can be enhanced obviously when the spatial current localization is large enough to produce a percolative conductance path along conductances of high magnetoresistance in the network.

Grain-boundary magnetoresistance up to 42 T in cold-pressed Fe3O4 nanopowders

The magnetoresistance (MR) in cold-pressed magnetite nanopowders has been studied using pulsed magnetic field up to 42 T and steady field up to 12 T. Ball milling in air produces pure and stoichiometric Fe3O4 grains of nanometric size coated by a thin layer of Fe2O3, which electrically isolates the magnetite and acts as a tunnel barrier. Therefore, the intergrain magnetoresistance of magnetite grain boundaries can be analyzed regardless of the bulk transport properties. At high fields and high temperature, the MR depends linearly on the field, whereas at lower fields a direct tunneling contribution governed by the surface magnetization appears. Below the Verwey transition (T<120K) the linear high-field MR disappears. We interpret these results in terms of the grain-boundary properties.

Magnetoresistance in bicrystal grain-boundary junctions analyzed within the Stoner–Wohlfarth model

Experimental data of bicrystal grain-boundary magnetoresistance hysteresis shows that the resistance at remanence after saturation depends on whether the magnetic field was applied parallel or perpendicular to the grain boundary. In the latter case the behavior can be described by the Stoner–Wohlfarth magnetization reversal model for coherent rotation of the magnetization, which leads to an alignment along the easy magnetization directions of the bicrystal in zero field. In the parallel configuration the magnetization directions of the two grains remain parallel in zero field. We have used this information to deduce an effective spin polarization of P≈40% of the grain boundary in a bicrystal of La0.7Sr0.3MnO3.

Step-edge magnetoresistance of magnetite films

Summary form only given. The phenomenon of grain-boundary magnetoresistance in half-metallic oxides has attracted an intense research activity in recent years. The Curie temperature of ferromagnetic oxides, however, is too low in order to realize magnetoresistive devices operating at room temperature. There is only one notable exception: the half-metallic ferrimagnet magnetite (Fe/sub 3/O/sub 4/) with a Curie temperature of 858 K. It is shown here that grain boundaries introduced at step edges in magnetite films yield a considerable magnetoresistance.

Mechanism of grain-boundary magnetoresistance in Fe O films

The magnetotransport properties of magnetite films with different microstructures were investigated in order to identify prerequisites for the attainment of a large tunnelling magnetoresistance in polycrystalline samples. Epitaxial films on MgAl2O4, polycrystalline films on Al2O3 and rough MgAl2O4 substrates and a polycrystalline La0.7Ca0.3MnO3 film on MgO were compared. Although grain boundaries induce a large high-field magnetoresistance in magnetite films, the low-field magnetoresistance characteristic for spin-polarized tunnelling was virtually absent in these samples. Two factors might be responsible for this behaviour: (1) grain boundaries in magnetite are conducting and do not form tunnelling barriers and (2) the spin-polarization near grain boundaries is suppressed due to non-stoichiometry.

Grain-boundary magnetoresistance in manganites: Spin-polarized inelastic tunneling through a spin-glass-like barrier

The grain-boundary magnetoresistance of various ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ samples was investigated by resistance, magnetoresistance, and dynamic conductance measurements. Data on five samples, namely an epitaxial film, a polycrystalline film, a step-edge array, a mechanically induced grain boundary, and a chromium-manganite tunneling contact, are presented. A model of spin-polarized inelastic tunneling through an insulating barrier with spin-glass-like characteristics is proposed that explains the experimental observations.

Anisotropic magnetoresistance of thin La0.7Ca0.3MnO3 films

The low-field magnetoresistance of films deposited on different substrates has been measured. Whereas post-annealed films on and exhibit a clear anisotropic magnetoresistance (AMR), the low-field magnetoresistance of as-deposited films on and Si is dominated by grain-boundary magnetoresistance. At low temperatures the anisotropic magnetoresistance is temperature independent with a value of about . A simple atomic d-state model can explain the sign of the anisotropic magnetoresistance.

Low-field magnetoresistance in the pyrochlore Tl2Mn2O7

The discovery of colossal magnetoresistance in perovskite manganites derived from LaMnO3 has renewed interest in these and related materials for possible technological applications1,2,3,4,5,6,7. But the high magnetic fields and narrow temperature windows where the large magnetoresistive responses are observed present daunting limitations. A possible avenue for overcoming these limitations is to improve the low-field response by modifying the manganites to enhance the contribution to magnetoresistance from spin-polarized tunnelling of the conduction electrons; such tunnelling, which can be very sensitive to an applied magnetic field, takes place across grain boundaries (natural or artificial) or between the manganite planes of the layered derivatives of the perovskite compounds8,9,10,11,12,13. Here we show that low-field (H) grain-boundary magnetoresistance can also be realized in a non-perovskite magnetoresistive oxide, the pyrochlore Tl2Mn2O7. Moreover, this low-field response, which persists for all temperatures below the transition to the ferromagnetic state, does not show the strong temperature-dependent decay characteristic of the perovskite-based systems. We suggest that this improved response is in part due to weaker electron-spin coupling and the lack of strong electron–lattice interactions in the pyrochlore.