Magnetoresistance Fluctuations Research Articles

The magnetization and magnetoresistance of Ni46Mn23Ga22Co5Cu4 shape memory microwires after mechanical training

Magnetization and magnetoresistance are significant for the application of Ni-Mn-Ga shape memory alloys. The Ni46Mn23Ga22Co5Cu4 microwires with excellent mechanical properties were selected to investigate the influence of mechanical training on the microstructure and magnetic properties. The largest change of magnetization (ΔM) and magnetoresistance fluctuations were found after training 10k cycles. Furthermore, the microstructure and magnetic configuration were studied by TEM, GPA, and MFM to investigate the mechanical training effect and the internal mechanism. Nano-precipitated phases were found embedded in the matrix and the dislocation stacking around the precipitate induced a local strain field which could be released during mechanical training. The magnetic domain was transformed from indistinct into strip-type after training, which affects the magnetic properties of microwires. This work provides an idea for improving the magnetic and magnetoresistance performance and building a link between microstructures and electric signals of shape memory alloys.

Magnetoresistance Dynamics in Superparamagnetic Co−Fe−B Nanodots

Individual disk-shaped $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ nanodots are driven into a superparamagnetic state by a spin-transfer torque, and their time-dependent magnetoresistance fluctuations are measured as a function of current. A thin layer of oxidation at the edges has a dramatic effect on the magnetization dynamics. A combination of experimental results and atomistic spin simulations shows that pinning to oxide grains can reduce the likelihood that fluctuations lead to reversal, and can even change the easy-axis direction. Exchange-bias loop shifts and training effects are observed even at room temperature after brief exposure to small fields. The results have implications for studies of core-shell nanoparticles and small magnetic tunnel junctions and spin-torque oscillators.

Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO3/SrTiO3/SmTiO3 Quantum Well Structures

Heterointerfaces of SrTiO3 with other transition metal oxides make up an intriguing family of systems with a bounty of coexisting and competing physical orders. Some examples, such as LaAlO3/SrTiO3, support a high carrier density electron gas at the interface whose electronic properties are determined by a combination of lattice distortions, spin-orbit coupling, defects, and various regimes of magnetic and charge ordering. Here, we study electronic transport in mesoscale devices made with heterostructures of SrTiO3 sandwiched between layers of SmTiO3, in which the transport properties can be tuned from a regime of Fermi-liquid like resistivity (ρ ∝ T2) to a non-Fermi liquid (ρ ∝ T5/3) by controlling the SrTiO3 thickness. In mesoscale devices at low temperatures, we find unexpected voltage fluctuations that grow in magnitude as T is decreased below 20 K, are suppressed with increasing contact electrode size, and are independent of the drive current and contact spacing distance. Magnetoresistance fluctuations are also observed, which are reminiscent of universal conductance fluctuations but not entirely consistent with their conventional properties. Candidate explanations are considered, and a mechanism is suggested based on mesoscopic temporal fluctuations of the Seebeck coefficient. An improved understanding of charge transport in these model systems, especially their quantum coherent properties, may lead to insights into the nature of transport in strongly correlated materials that deviate from Fermi liquid theory.

Thermal stability of the geometrically constrained magnetic wall and its effect on a domain-wall spin valve

We studied thermal fluctuations in magnetoresistance (MR) signals originating from geometrically confined magnetic walls (GCMWs) of nanometer size. To this end, we developed a novel numerical simulation method which quantitatively evaluates the magnitude of thermal fluctuations in MR signals of magnetic nanostructures. Using the method, we first investigated the case when the twist angle Θ between the magnetization in a fixed layer and that in a free layer is 180°. We found that the thermal fluctuations of the magnetic structure of the 180° GCMW do not induce any crucial fluctuations in the MR signal because there is no significant difference among the MR values of the magnetic structures caused by the thermal fluctuations. We next investigated the dependence on the twist angle Θ of thermal fluctuations in MR signals. Since the GCMW is stabilized by decreasing Θ from 180°, the standard deviation (SD) of the MR signal is reduced with decreasing Θ. On the contrary, the SD/M ratio (M is the mean of the MR signal) monotonically increases with decreasing Θ because the attenuation of the mean value of MR is faster than that of the standard deviation. We also found that the SD/M ratio was not large for any of the temperatures (from 300 to 600 K) and twist angles (from 90° to 180°) we examined. The maximum value of the ratio, which was obtained when T = 600 K and Θ = 90°, was about 13%. This result indicates that thermal fluctuations do not cause significant noise in MR devices that utilize GCMWs of nanometer size.

High-field magneto-transport at charge neutrality point in monolayer graphene

We have investigated the magneto-transport around filling factor ν = 0 in monolayer graphene sheet. Zero Hall plateau of Hall conductivity and minimum of longitudinal conductivity have been clearly observed despite the highly fluctuated behaviour of magneto-resistance above 10 T. These phenomena can be understood by the transport of counter-propagating electron and hole edge states at the charge neutrality point. We tried to explain the origin of remarkable fluctuations in magneto-resistance by the energy relaxations among the counter-propagating edge channels.

The effects of current-path patterns on magnetotransport in spatially-confined structures by Monte Carlo simulation

Simulations are performed on clusters of finite size to study the effects of size and current-path structure on magnetotransport in spatially-confined samples. Magnetotransport networks are established and calculated based on fractal structures including Koch curves and percolation backbones extracted from regular lattices. The structure pattern of clusters is shown to play an important role in the magnetotransport behaviours by affecting the magnetoresistance fluctuations due to spin disorder in the systems of small size, which suggests the possibility of controlling the magnetotransport by the design of current-path configurations.

Magnetoresistance fluctuations in a weak disorder indium nitride nanowire

We report measurements of magnetoresistance (MR) fluctuations in a weak disorder indium nitride nanowire. The MR fluctuations are reproducible, aperiodic and symmetric in magnetic field but are asymmetric upon reversal of bias direction. The fluctuations are analysed for both perpendicular and parallel external magnetic field configurations in the light of tunnel magnetoresistance at low field and impurity scattering at higher field. The asymmetry in bias reversal is caused by breakdown of time reversal symmetry.

Mesoscopic anisotropic magnetoconductance fluctuations in ferromagnets

The conductance of a ferromagnetic particle depends on the relative orientation of the magnetization with respect to the direction of current flow. This phenomenon is known as "anisotropic magnetoresistance". Quantum interference leads to an additional, random dependence of the conductance on the magnetization direction. These "anisotropic magnetoresistance fluctuations" are caused by spin-orbit scattering, which couples the electron motion to the exchange field in the ferromagnet. We report a calculation of the dependence of the conductance autocorrelation function on the rotation angle of the magnetization direction.

Electron transport in InAs/AlGaSb ballistic rectifiers

Nonlinear transport properties of a ballistic rectifier fabricated from InAs/AlGaSb heterostructures are reported. The operation of the ballistic rectifier is based on the guidance of carriers by a square anti-dot structure. The structure was defined by electron beam lithography and wet chemical etching. The DC characteristics and magneto-transport properties of the ballistic rectifier have been measured at 77 K and 4.2 K. Rectification effects relying on the ballistic transport were observed. From the four-terminal resistance measured at low magnetic fields, we also observed magneto-resistance fluctuations corresponding to the electron trajectories and symmetry-breaking electron scattering, which are influenced by the magnetic field strength.

Quantum-interference characteristics of a 25 nm trench-type InGaAs/InAlAs quantum-wire field-effect transistor

We study the quantum-interference characteristics of a 25 nm, trench-type, InGaAs quantum-wire field-effect transistor realized by selective epitaxy, and find very different behavior from that typically exhibited by disordered wires. The amplitude of the magnetoresistance fluctuations is exponentially suppressed at high fields, where evidence of an Aharonov–Bohm effect is observed. The exponential suppression appears to be consistent with theoretical predictions for the influence of magnetic field on the scattering rate in clean wires, while the Aharonov–Bohm effect points to an interference process in which the one-dimensional subbands of the wire themselves constitute well-resolved paths for electron interference.

Magneto-transport properties of InAs/AlGaSb open quantum dot structures

We report on the magneto-transport properties of InAs/AlGaSb open quantum dot structures. The open dot structures have been fabricated by using electron beam lithography. The open dot structures are characterized by magnetoresistance measurements at 4.2 K. In addition to the Shubnikov-de Hass oscillations, aperiodic magnetoresistance fluctuations have been observed at low magnetic fields below 1 T. The amplitude of the fluctuations was quite large even at 4.2 K in comparison with the case of the GaAs/AlGaAs system. Electron-wave interference effect and phase-breaking times in InAs mesoscopic structures are discussed from the analyses using correlation functions of magneto-conductance fluctuations.

Resistance oscillations and magnetic fingerprints in superconductingAu0.7In0.3cylinders

Magnetoresistance of hollow ${\mathrm{Au}}_{0.7}{\mathrm{In}}_{0.3}$ cylinders of submicron diameter was measured in the superconducting transition regime. Depending on the amount of disorder present in the samples, resistance oscillations were either absent or found only below a certain temperature in the transition regime. For those samples where the resistance oscillations were completely suppressed, reproducible sample-specific magnetoresistance fluctuations (magnetic fingerprints) were observed, also in the low-temperature part of the transition regime. The amplitude of the conductance fluctuation exceeded that of the universal conductance fluctuation in normal metals by several orders of magnitude. The fluctuation disappeared as superconductivity in the samples was suppressed by increasing either temperature or magnetic field. Thermal cycling to temperatures much higher than the superconducting critical temperature resulted in irreversible changes in the fingerprint pattern. In addition, a negative magnetoresistance was found just above the transition temperature in some samples. The physical origin of these observations is discussed in the context of mesoscopic effects in disordered superconductors.

Magnetoresistance fluctuations in shortn-type Si/SiGe heterostructure wires

Magnetoresistance fluctuations in short quasiballistic Si/SiGe wire segments have been investigated as a function of magnetic field and temperature. The segments are measured in a four-probe geometry and the voltage probe distances L are taken smaller or larger than the phase coherence length l f ~’1.5 m ma tT 50.1 K! and the electron mean free path l e ~’0.8 mm!. At magnetic fields smaller thanB51 T, the amplitude dR and the correlation fieldBc of both the symmetric and antisymmetric part of the resistance fluctuations have been studied as a function of probe distance and temperature. It is found that, despite the quasiballistic character of electron transport in our samples, the behavior of the amplitude and correlation field with probe separation is in good qualitative agreement with expressions derived for the diffusive regime. The observed magnitude of Bc , however, is much larger than expected for the diffusive transport regime. A better agreement for Bc is obtained using an expression adapted for the quasiballistic regime. The temperature dependence of the correlation field cannot be explained by expressions appropriate for the diffusive or the quasiballistic transport regime.

Selective MBE growth of GaAs wire and dot structures using atomic hydrogens and their electronic properties

The selective growth of GaAs wire and dot structures by molecular beam epitaxy in the presence of atomic hydrogen was studied on a GaAs substrate with stripe and window shaped SiN x mask patterns. It was found that 20 nm ridges were successfully formed on long stripe patterns but the ridge width increased substantially when grown on short stripes of less than several μm. This is ascribed to the longitudinal migration of Ga flowing from the two ends of a stripe to its central zone. The incorporation of Si dopants into a 0.3 μm-wide wire was achieved. Negative magnetoresistance and conductance fluctuations indicative of the one-dimensional nature of electrons were observed. On a 2 x 2 μm square window pattern, a pyramidal structure with two-fold symmetry was formed, on which a GaAs dot with the lateral dimension of 100 nm was grown. Photoluminescence (PL) spectra from this dot exhibited a shoulder structure 15 meV, suggesting the filling of several discrete levels and the slow relaxation of carriers.

The Magnetic Field Dependent Characteristics of Conductance Fluctuations in Ballistic Quantum Dots

We study magneto-resistance fluctuations in GaAs/AlGaAs, ballistic quantum dots. At low temperatures, and at sufficiently low magnetic fields, the fluctuations obscure any average features in the magneto-resistance. As the magnetic field is increased, such that the cyclotron orbit size becomes much smaller than the dot dimensions, however, a strong decay in their high frequency content is observed. We associate this behaviour with the formation of well defined edge states in the dot, and in order to account for our observations apply a simple model, which considers the flux enclosed by skipping orbits localised at the dot walls.

Magnetotransport in a chaotic scattering cavity with tunable electron density

We have measured the resistance versus magnetic field and Fermi energy for two stadium-shaped cavities in which chaotic scattering is expected. We fit the power spectra of the magnetoresistance fluctuations to semiclassical chaotic scattering theory and find a characteristic field which scales with cavity dimensions as predicted by classical simulations. Measurements of conductance versus Fermi energy indicate a significant contribution from short, non-chaotic paths. We have also studied the energy-averaged conductance as a function of magnetic field. We observe a ballistic weak localization effect similar to that predicted by recent calculations.

Experimental studies of the localization transition in the quantum Hall regime.

We study experimentally the localization-to-delocalization transition in the transport regime between adjacent integer-quantum-Hall plateaus. We use small Hall bar geometries at millikelvin temperatures so that the phase-breaking length exceeds the sample size. Under these conditions the width \ensuremath{\Delta}B of the transition region scales with the size W of the sample according to W\ensuremath{\propto}(\ensuremath{\Delta}B${)}^{\mathrm{\ensuremath{-}}\ensuremath{\nu}}$. We obtain a universal scaling exponent \ensuremath{\nu}=2.3\ifmmode\pm\else\textpm\fi{}0.1. This result agrees with the predictions of several theoretical approaches to the metal-insulator transition in the integer-quantum-Hall regime. The numerical result agrees with the findings of the trajectory network model (\ensuremath{\nu}=2.5\ifmmode\pm\else\textpm\fi{}0.5), the percolation picture including quantum tunneling (\ensuremath{\nu}=7/3), and recent numerical studies (e.g., \ensuremath{\nu}=2.34\ifmmode\pm\else\textpm\fi{}0.04). The temperature exponent of the inelastic-scattering rate can be measured in the same experiment. We obtain results in the range from p=2.7\ifmmode\pm\else\textpm\fi{}0.3 to p=3.4\ifmmode\pm\else\textpm\fi{}0.4, which are considerably larger than commonly assumed values. Small reproducible magnetoresistance fluctuations are observed, which do not substantially influence the scaling behavior. By studying the effect of current heating, it is shown that noise heating does not play a role in the measurements. We discuss the present results in comparison with previous experimental and theoretical investigations.

Thermally induced characteristic-magnetoresistance-pattern changes on a mesoscopic gold wire.

Mesoscopic systems exhibit magnetoresistance fluctuations that are caused by interference of the conduction electrons moving along different paths as they scatter off impurities and have their phase altered by a magnetic field. We have produced gold wires of submicrometer widths and have studied their magnetoresistance below 1 K. Clear, characteristic magnetoresistance patterns (CMRP's) were obtained that were stable over a period of several weeks. We have observed no changes of resistance as a function of time. We have observed changes on the CMRP's after warming up to 1 K. We interpret those changes as indications of thermally assisted hops of impurities. An estimate of the size of the barriers between impurity sites is made based on the frequency of the CRMP changes and on the theory of universal conductance fluctuations.

Quantum magnetoresistance fluctuations in an amorphous metal.

We report an observation of aperiodic magnetoresistance fluctuations in an amorphous system. We have observed fluctuations in amorphous ${\mathrm{Pd}}_{80}$${\mathrm{Si}}_{20}$ wires with macroscopic lengths, up to L=1 mm (L/${\mathit{L}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$=12 000, where ${\mathit{L}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$ is the phase-coherence length). The root-mean-square amplitude of the resistance fluctuations is proportional to ${\mathit{L}}^{1/2}$, in agreement with theory. We observe a reduction in the fluctuation amplitude at low magnetic fields, which we attribute to magnetic scattering, possibly in the Kondo regime. Two simple models are presented to describe this behavior, one based on a single population of magnetic scatterers with temperature-dependent magnetic moment, and the other based on two scatterer populations, each with fixed moment. Both models agree with the data. We use the high-field amplitude of the magnetoresistance-fluctuation structure to predict the characteristic field scale; the prediction is larger than the observed field scale by a factor of 2.

Low temperature magnetoresistance in a narrow wire of GaAs/AlGaAs

Aperiodic magnetoresistance (MR) fluctuations in narrow GaAs/AlGaAs wires are studied up to 8 T by means of analyzing their Fourier transform. It is found that decay of the spectrum shifts toward low frequencies with increasing the magnetic field region where Fourier transformation is done. This suggests that the effective area for the quantum interference may be changed by the magnetic field. Shubnikov-de Haas (SdH) oscillations are also described.