Spin-dependent Magnetoresistance Research Articles

High-efficient and gate-tunable spin transport in GaN thin film at room temperature

The emerging semiconductor spintronics has offered a practical routine for developing high-speed and energy-efficient electronic and optoelectronic devices. GaN holds broad prospects for room-temperature spintronic applications due to its weak spin scattering and moderate spin–orbit coupling. However, the development of GaN-based spintronic devices is still hindered by the relatively low spin injection efficiency and gate controllability. In this study, gate-modulated spin transport was achieved in a highly spin-polarized GaN-based non-local spin valve. A maximum spin diffusion length of 510 nm and a high spin polarization of 14.1% was obtained with the CoFeB/MgO tunnel spin injector. By applying gate voltages from −3 to +3 V, the spin-dependent magnetoresistance can be tuned in the range of 1.6–3.9 Ω. The modulation is attributed to the controllable spin relaxation of electrons by the gate electric field. This work has demonstrated high spin polarization and exceptional electric controllability in GaN, pushing forward the research in spin field-effect transistors.

Research Progress of Current Sensor Based on Spin-Dependent Magnetoresistance Effect

This article reviews the physical mechanism of spin-dependent magnetoresistance and its early application in sensors. The magnetic field performance generated by the current to be measured is explained. According to the realization of the magnetoresistance measurement of this characteristic, seven main indicators of the current sensor are summarized. Starting with the structure of magnetoresistance devices and magnetoresistance units of current sensors based on spin-dependent magnetoresistance effect, several design methods of sensors and their advantages and disadvantages are analyzed. Starting from the role of AMR, GMR and TMR in magnetoresistance cells, the structure of series and parallel arrays, permanent magnet bias, coil bias, coil reset, flux aggregator and superconducting ring are analyzed, and several design methods of sensors are summarized as well as their advantages and disadvantages. Finally, the possible development direction of the current sensor is forecasted based on the recently discovered spin correlation effect.

Lateral silicon spin-valve devices with large spin-dependent magnetoresistance and output voltage

We investigated the spin transport in nanoscale silicon (Si)-based spin-valve devices with a 20 nm Si channel and a Fe/(Mg)/MgO/Ge stack as the spin injector/detector. By optimizing the MgO barrier thickness, we achieved a large spin-dependent output voltage of 25 mV at 15 K. Furthermore, by inserting an ultrathin (1 nm) Mg layer between the the Fe layer and the tunnel barrier MgO layer to prevent the formation of a magnetically-dead layer, we have increased the spin-valve ratio up to –3.6% at 15 K. These are the highest values reported in lateral Si-based spin-valve devices.

Observation of large spin accumulation voltages in nondegenerate Si spin devices due to spin drift effect: Experiments and theory

A large spin-accumulation voltage of more than 1.5 mV at 1 mA, i.e., a magnetoresistance of 1.5 {\Omega}, was measured by means of the local three-terminal magnetoresistance in nondegenerate Si-based lateral spin valves (LSVs) at room temperature. This is the largest spin-accumulation voltage measured in semiconductor-based LSVs. The modified spin drift-diffusion model, which successfully accounts for the spin drift effect, explains the large spin-accumulation voltage and significant bias-current-polarity dependence. The model also shows that the spin drift effect enhances the spin-dependent magnetoresistance in the electric two terminal scheme. This finding provides a useful guiding principle for spin metal-oxide semiconductor field-effect transistor (MOSFET) operations.

Fabrication and characterization of nanostructured Fe3S4, an isostructural compound of half-metallic Fe3O4

High-purity, well-crystallized spinel Fe3S4 nanoplatelets were synthesized by the hydrothermal method, and the saturation magnetic moment of Fe3S4 was measured at 1.83 μB/f.u. The temperature-dependent resistivity of Fe3S4 was metallic-like for T < 180 K: room-temperature resistivity was measured at 7.711 × 103 μΩ cm. The anomalous Hall conductivity of Fe3S4 decreased with increasing longitudinal conductivity, in sharp contrast with the accepted theory of the anomalous Hall effect in a dirty-metal regime. Furthermore, negligible spin-dependent magnetoresistance was observed. Band structure calculations confirmed our experimental observations that Fe3S4 is a metal and not a half metal as expected.

Large negative magnetoresistance and ferromagnetism induced by interfacial spins in granular Cr/C films

Cr/C composite films with 7.9 and 8.5at% Cr contents were fabricated by facing-target magnetron sputtering. Cr nanoparticles are embedded in amorphous carbon matrix, and Cr atoms bond with C atoms at the interface. The Cr/C films were found to exhibit ferromagnetism in the temperature range of 2–300K, which is probably induced by uncompensated spins at the surface of Cr nanoparticles. Spin-dependent magnetoresistance (up to −19.5% at 90kOe and 2K) was observed in the Cr/C films. The negative MR originates from the alignment of the uncompensated spins at the interface between nanoparticles and matrix. The spin polarization which comes from the interfacial p–d hybridization was calculated to be 21% and 17% in the Cr/C films with 7.9% and 8.5% Cr, respectively.

Magnetoresistance of nanogranular Ni/NiO controlled by exchange anisotropy

A link between exchange anisotropy and magnetoresistance has been found to occur in a Ni/NiO sample consisting of Ni nanocrystallites (mean size ∼13nm, Ni content ∼33vol%) dispersed in a NiO matrix. This material shows metallic-type electric conduction and isotropic spin-dependent magnetoresistance as well as exchange bias effect. The latter is the outcome of an exchange anisotropy arising from the contact interaction between the Ni phase and the NiO matrix. Combined analysis of magnetization M(H) and magnetoresistance MR(H) loops measured in the 5–250K temperature range after zero-field-cooling (ZFC) and after field-cooling (FC) from 300K reveals that the magnetoresistance is influenced by exchange anisotropy, which is triggered by the FC process and can be modified in strength by varying the temperature. Compared to the ZFC case, the exchange anisotropy produces a horizontal shift of the FC MR(H) loop along with a reduction of the MR response associated to the reorientation of the Ni moments. A strict connection between magnetoresistance and remanent magnetization of FC loops on one side and the exchange field on the other, ruled by exchange anisotropy, is indicated.

Study of the magnetic microstructure of Ni/NiO nanogranular samples above the electric percolation threshold by magnetoresistance measurements

Magnetoresistance measurements have been exploited to gain information on the magnetic microstructure of two Ni/NiO nanogranular materials consisting of Ni nanocrystallites (mean size of the order of 10 nm) embedded in a NiO matrix and differing in the amount of metallic Ni, ∼33 and ∼61 vol%. The overall conductance of both samples is metallic in character, indicating that the Ni content is above the percolation threshold for electric conductivity; the electric resistivity is two orders of magnitude smaller in the sample with higher Ni fraction (10−5 Ωm against 10−3 Ωm). An isotropic, spin-dependent magnetoresistance has been measured in the sample with lower Ni content, whereas both isotropic and anisotropic magnetoresistance phenomena coexist in the other material. This study, associated with magnetization loop measurements and the comparison with the exchange bias effect, allows one to conclude that in the sample with lower Ni content neither the physical percolation of the Ni nanocrystallites nor the magnetic percolation (i.e., formation of a homogeneous ferromagnetic network) are achieved; in the other sample physical percolation is reached while magnetic percolation is still absent. In both behaviors, a key role is played by the NiO matrix, which brings about a magnetic nanocrystallite/matrix interface exchange energy term and rules both the direct exchange interaction among Ni nanocrystallites and the magnetotransport properties of these nanogranular materials.

Symmetry-induced perfect transmission and inverse magnetoresistance in cascade junctions of ferromagnet and semiconductor

Within the Landauer framework of ballistic transport, we theoretically investigate spin-dependent resonant transmission and magnetoresistance in symmetric cascade junctions of ferromagnetic metal (FM) and semiconductor (SC). It is shown that spin-up and spin-down electrons possess different bandgap structures against the Rashba spin-orbit wave vector. Due to the mirror symmetry, multiple spin-dependent perfect transmissions of electrons can be obtained within the bandgap, thereafter, spin polarization has multiple reversals. Around each resonant wave vector, high spin polarization is achieved and the electrical conductance comes from one kind of spin electrons. The resonant transmissions originate from the spin-dependent quasi-bound states at energies above the potential barriers, which are demonstrated by the electronic charge distributions in the system. Furthermore, if we change the magnetization of FM in the centre of the junctions, inverse magnetoresistance can be observed. The investigations may have potential applications in spin filters and spin switches.

Ferromagnetism of low-dimensional Mn-doped III-V semiconductor structures in the vicinity of the insulator-metal transition

The structural and transport properties of GaAs/Mn/GaAs/InxGa1−xAs/GaAs quantum wells (x≈0.2) with Mn δ-layer (4–10 at. %), separated from the well by a GaAs spacer, have been studied. The hole mobility in the investigated structures has exceeded the values known for magnetic III-V heterostructures by two orders of magnitude. For structures with the conductivity of the metal type, we have succeeded to observe at low temperatures Shubnikov–de Haas oscillations just confirming the two dimensionality (2D) of the hole energy spectrum. Exactly those 2D holes promote the ferromagnetic ordering of the Mn layer. That has been proven by (i) observing maxima (at 25–40 K) in temperature dependencies of the resistance, which positions agree with calculated values of Curie temperatures (for structures with the indirect interaction of Mn atoms via 2D holes), and (ii) revealing the negative spin-dependent magnetoresistance (NMR) as well as the anomalous Hall effect (AHE), which values are also in good agreement with calculations relating to ferromagnetic 2D III-V systems. As for the structures with the insulator type of the conductivity, their NMR and AHE features evidence the phase separation—the sample fragmentation with the formation of mesoscopic ferromagnetic areas separated by paramagnetic strata of the high tunnel conductivity.

Ferromagnetism of low-dimensional structures based on III-Mn-V semiconductors in the vicinity of the insulator-to-metal transition

The structural and transport properties of the GaAs/Mn/GaAs/InxGa1 − xAs/GaAs quantum wells (QWs) with the Mn layer distant from the QW and the Mn content ranging from 4 to 10% that correspond to the reentrant metal-to-insulator transition observed in the bulk GaMnAs are studied. The X-ray diffractometry and reflectometry are used to reconstruct the distribution profile of the Mn layers and to demonstrate the absence of the Mn atoms in the QW volume. The mobility of holes in the objects under study is greater than the known mobilities for the GaMnAs magnetic heterostructures by more than two orders of magnitude, which makes it possible to observe the Shubnikov-de Haas oscillations, proving the 2D character of the hole energy spectrum. A significant role of the 2D holes in the ferromagnetic ordering of the Mn layer is demonstrated under such conditions. The supporting evidence results from the observation of a maximum on the curves of the resistance versus temperature (at 25–40 K), whose position is in agreement with the calculated Curie temperature for the structures with indirect exchange interaction via the 2D-hole channel and the observation of the negative spin-dependent magnetoresistance (NM) and the anomalous Hall effect (AHE) that correlates with the theoretically calculated results for the ferromagnetic 2D III-Mn-V systems. The features of the NM and AHE revealed in the structures with the dielectric conduction indicate the effects of phase separation lying in the fragmentation of the sample into mesoscopic ferromagnetic domains separated by tunnel transparent paramagnetic layers. Such NM and AHE features are also observed in the weakly dielectric InMnAs films, which points to an important role of such effects in the III-Mn-V systems in the vicinity of the insulator-to-metal transition.

Ferromagnetic transition in GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.

Quantum transport of spin-polarized carriers in quasi paramagnetic quantum wires: Green’s function formalism

The spin-dependent electrical conductance and magnetoresistance (MR) of a one-dimensional system are calculated with disorder paramagnetic impurities. The system contains a quasi-paramagnetic quantum wire (QPQW) device attached between two non-magnetic semi-infinite leads. The calculations are based on the tight-binding model and generalized Green’s function method within the phase coherent transport regime. In order to investigate spin-filtering effect of QPQW, we study the spin-polarized electrical transmission as a function of impurity concentration and molecular field acting on paramagnetic impurities. It is assumed that in the absence of the external field, magnetic moments of the impurities are compensated (oriented at random), whereas the magnetic field creates finite net magnetization. It is found that the spin-dependent transport and MR depend strongly on the concentration of magnetic disorder, the size of the system and the Fermi energy. It is shown that MR has an oscillating behavior with respect to the magnetic molecular field at a fixed disorder concentration. The application of the predicted results can be a base for the designing of spintronics and moletronics devices.

Upper critical fields and thermally-activated transport ofNdFeAsO0.7F0.3single crystal

We present detailed measurements of the longitudinal resistivity rho(xx)(T,H) and the upper critical field H(c2) of NdFeAsO(0.7)F(0.3) single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of H(c2) is comparable to H(c2)similar to 100 T of high-T(c) cuprates. H(c2)(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of H(c2)(perpendicular to)(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H(c2)(theta) as a function of the angle theta between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter gamma(T)=(m(c)/m(ab))(1/2)=H(c2)(parallel to)/H(c2)(perpendicular to) obtained from these measurements decreases as temperature decreases from gamma similar or equal to 9.2 at 44 K to gamma similar or equal to 5 at 34 K, where parallel to and perpendicular to correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity rho(xx)(T,H) measured below T(c) but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior rho=rho(0) exp[-E(a)(T,H)/T] over 4-5 decades of rho(xx). The activation energy E(a)(T,H) has very different field dependencies for H parallel to ab and H perpendicular to ab varying from 4x10(3) K at H=0.2 T to similar to 200 K at H=35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of H(c2), using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on H(c2)(T) in oxypnictides, which may significantly reduce H(c2)(0) as compared to H(c2)(0)similar to 200-300 T based on extrapolations of H(c2)(T) near T(c) down to low temperatures.

Structure and magnetotransport properties of Fe3O4–SiO2 composite films reactively sputtered at room temperature

( Fe 3 O 4 ) 1−x –( SiO 2 ) x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by log ρ∝T−1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH−ρ0)/ρ0} of about −4.7% for Fe3O4 films and −1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x⩾0.4.

Magnetoresistance of nanocrystallized amorphous Fe–Zr–(Ru) alloys

We have studied the magnetic and magnetotransport properties of melt-spun Fe 91Zr 8Cu 1 and Fe 81Zr 8Cu 1Ru 10. The Fe 91Zr 8Cu 1 alloy after nanocrystallization at 753 K for 14.4 ks shows conventional anisotropic magnetoresistance (MR). On the other hand, spin-dependent MR (Δ ρ≈−0.04 μΩ cm) is confirmed for the Fe 81Zr 8Cu 1Ru 10 alloy after the same nanocrystallization treatment. Our 57Fe Mössbauer results indicate that the residual amorphous phase in the Ru-free alloy is ferromagnetic, whereas that in the Ru-doped alloy is paramagnetic. The spin-dependent MR in Fe 81Zr 8Cu 1Ru 10 is considered to be due to the inhibition of the intergranular magnetic coupling by this paramagnetic amorphous phase. Our study shows that the magnetotransport in BCC-Fe/amorphous two-phase nanocrystalline systems can be spin-dependent.

Temperature dependence of tunnel conductance in ferromagnetic double barrier junctions

The spin-dependent conductance and magnetoresistance (MR) in ferromagnetic double junctions are studied theoretically, taking into account discrete energy levels and a finite spin relaxation time. The basic results are applied to hybrid tunnel junctions, in which nano-scale grains are embedded in the tunnel barrier. With increasing size quantization and spin relaxation time the MR is found to be enhanced at low temperatures.

Spin fluctuations and magnetoresistance in dirty metals

Effects or strong spin fluctuations induced by the Anderson localization are investigated on the quantum corrections to conductivity and magnetoresistance based on the Hubbard model together with long range Coulomb interactions. The present mechanism predicts a large spin dependent magnetoresistance, which will explain at least the trend observed in the various experiments which has been so far left unanswered.

Spin Fluctuations and Magnetoresistance in Dirty Metals

Effects or strong spin fluctuations induced by the Anderson localization are investigated on the quantum corrections to conductivity and magnetoresistance based on the Hubbard model together with long range Coulomb interactions. The present mechanism predicts a large spin dependent magnetoresistance, which will explain at least the trend observed in the various experiments which has been so far left unanswered.

Positive magnetoresistance due to impurity interactions in dilute AuCr alloys

The magnetoresistance of dilute AuCr alloys (2.3–322 at.ppm) has been measured in a wide region of temperature (5-3K) and of magnetic field (up to 60 kG). The “modified Kohler's rule” was used to extract the spin dependent magnetoresistance (ϱ M) from measured total magnetoresistance. The results has shown that, except for the most dilute specimen, ϱ M includes explicitly a positive component in addition to the ordinary negative magnetoresistance proportional to log ( H/T). We have attributed this positive ϱ M to the effect of the interactions between the Cr impurities by taking consideration of the generalized phase shift expression of magnetoresistance proposed by Soultie.