Magnetoresistance Response Research Articles

A multifunctional molecular spintronic platform with magnetoresistive and memristive responses via a self-assembled monolayer

We report the spin-dependent transport and the I-V hysteretic characteristics in molecular-level organic spin valves containing a self-assembled-monolayer (SAM) barrier of 1,4 benzenedimethanethiol (BDMT). X-ray photoelectron spectroscopy confirms the establishment of an ordered self-assembled monolayer of BDMT with the phosphonic groups coordinated onto the ferromagnet surface. The magnetoresistive (MR) and the I-V curves characterize the transport properties of the SAM-based organic spin valves, which exhibit both types of non-volatile memory switching, i.e., the magnetoresistive and the memristive switching. The results reveal the possibility of integrating organic SAM into the future multifunctional molecular-level spintronic device applications.

Magnetoresistance in the in-plane magnetic field induced semimetallic phase of inverted HgTe quantum wells

In this study we have measured the magnetoresistance response of inverted HgTe quantum wells in the presence of a large parallel magnetic field up to 33 T is applied. We show that in quantum wells with inverted band structure a monotonically decreasing magnetoresistance is observed when a magnetic field up to order 10 T is applied parallel to the quantum well plane. This feature is accompanied by a vanishing of non-locality and is consistent with a predicted modification of the energy spectrum that becomes gapless at a critical in-plane field $B_{c}$. Magnetic fields in excess of $B_c$ allow us to investigate the evolution of the magnetoresistance in this field-induced semi-metallic region beyond the known regime. After an initial saturation phase in the presumably gapless phase, we observe a strong upturn of the longitudinal resistance. A small residual Hall signal picked up in non-local measurements suggests that this feature is likely a bulk phenomenon and caused by the semi-metallicity of the sample. Theoretical calculations indeed support that the origin of these features is classical and a power law upturn of the resistance can be expected due to the specifics of two-carrier transport in thin (semi-)metallic samples subjected to large magnetic fields.

Anisotropic Magnetoresistance in NiFe-Based Polymer Spin Valves.

Precise evaluation of magnetoresistance (MR) with the identified transport mechanism is one of the key points in organic spin-valve (OSV) devices. To investigate the origin of the spin-valve-like signal in polymer spin valve with a vertical structure of NiFe/P3HT/AlO x/Co, the magnetic response measurements in the rotated magnetization direction were established in different well-designed device configurations. We identified the significant influence of anisotropic MR (AMR) and further induced tunneling AMR in NiFe electrodes contributing to the total MR signal. These findings suggest that the mechanisms responsible for the transport mode in polymer spin-valve devices also strongly depend on the interface between the ferromagnetic electrodes and organic semiconductor layer. Even for the spin-valve-resembled MR response with distinct parallel and antiparallel states, carefully controlled experiments such as temperature-dependent and angle-dependent measurements should always be performed to track down the injected spin-polarized carriers. Our thorough experiments and analyses may shed light on the effective MR signal evaluation in OSVs and spin-related parameters with transport mechanism identification.

Fluoride ion assisted growth of hierarchical flowerlike nanostructures of Co/Ni ferrites and their magnetoresistive response.

One-dimensional nanorod arrays exhibiting hierarchical flowerlike morphologies, of Co and Ni based ferrites were synthesized by hydrothermal treatment and using ammonium fluoride (NH4F) as a mineralizing agent. The effects of NH4F concentration and synthesis temperature were probed to control the morphology of these nanorods that were formed as a result of crystal nucleation. It was observed that a higher concentration of NH4F leads to several other nucleation sites above these nanorods while controlled concentration of precursors and NH4F results in the synthesis of floral patterns. The specific geometries of these nanorods leads to a shape anisotropy effect resulting in increased magnetic coercive fields. To study the effect of magnetic field on the resistance and current density, impedance spectroscopy and I–V–R characteristics, respectively, were performed. Nanorods show enhanced values for resistance with the increase in magnetic field confirming the effect of magnetoresistive coupling while a decrease in current densities with increasing magnetic field highlights the potential of these structures for magnetoresistive applications.

PicoTesla magnetic tunneling junction sensors integrated with double staged magnetic flux concentrators

Ultra-sensitive solid-state magnetic sensors are in strong demand in many applications where currently available sensors are inadequate. We have used high performance magnetic tunneling junction (MTJ) sensors and pushed the magnetic sensing limit to a high level. We have incorporated double-staged magnetic flux concentrators, one on the MTJ chip level and the other on a more macroscopic level, to amplify the external field of interest. With this approach and undergoing a process of optimization on the flux concentrators, we have increased the sensitivity of the MTJ sensor by a large factor of 517 to 775.4%/Oe in terms of magnetoresistance response. The coercivity of the sensor is only 0.12 Oe. We have achieved a detectable field limit of 30 pT/Hz at 10 kHz. We have presented the noise spectrum and the sensitivity spectrum up to a maximum frequency of 100 kHz.

Physical nature of electrically detected magnetic resonance through spin dependent trap assisted tunneling in insulators

We show that electrically detected magnetic resonance (EDMR), through spin dependent trap assisted tunneling (SDTT) in amorphous SiC, exhibits approximately equal amplitudes at very high (8.5 T) and very low (0.013 T) magnetic fields at room temperature. This result strongly supports an SDTT/EDMR model in which spins at two nearby sites involved in a tunneling event are coupled for a finite time in circumstances somewhat analogous to spin pair coupling in the spin dependent recombination/EDMR model of Kaplan, Solomon, and Mott (KSM) [Kaplan, Solomon, and Mott, J. Phys. Lett. 39, 51 (1978)]. Since a comparable near zero magnetic field change in resistance is also observed in these samples, our results support the idea that this magnetoresistance response is also the result of a KSM-like mechanism involving SDTT. Additionally, we observe a large enhancement in SDTT/EDMR at high field (8.5 T) for temperatures below 50 K, which suggests the potential utility of SDTT in spin based quantum computation and other spintronic applications.

Effects of the band-filling and Fe/Mo disorder on physical properties of Ca2FeMoO6**Project supported by the National Natural Science Foundation of China (Grant No. U1504107) and the Doctoral Scientific Research Foundation (Grant No. qd15214).

Both the band filling effect and Fe/Mo disorder have a close correlation with the physical properties of the double perovskite Ca2FeMoO6. Two series of Ca2FeMoO6 and Nd0.3Ca1.7FeMoO6 ceramics sintered at (1050 °C, 1200 °C, and 1300 °C) were specially designed to comparatively investigate the band-filling effect and Fe/Mo disorder on the physical properties of Ca2FeMoO6. The x-ray diffraction indicates that Fe/Mo disorder is sensitive to the sintering temperature. The magnetization behavior is mainly controlled by the Fe/Mo disorder not by the band filling effect, manifested by a close correlation of saturated magnetization (Ms) with the Fe/Mo disorder. Interestingly, magnetoresistance (MR) property of the same composition is dominantly contributed by the grain boundary strength, which can be expressed by the macroscopic resistivity values. However, the band filling effect caused by the Nd-substitution can decrease the spin polarization, and thus suppress the MR performance fundamentally. Contrary to the MR response, the Curie temperature (TC) shows an obvious optimization due to the band filling effect, which increases the carrier density near the Fermi level responsible for the ferromagnetic coupling interaction strengthen. Maybe, our work can provoke further research interests into the correlation of the band-filling effects and Fe/Mo disorder with the physical properties of other Fe/Mo-based double perovskites.

Sign reversal of spin-polarized tunnelling magnetoresistance in 99.95% La0.7Sr0.3MnO3-0.05% Paraffin wax nanocomposite: An effect of spin-flip scattering at intergranular Paraffin wax interface

Spin-dependent electronic transport has been investigated on granular 99.95% La0.7Sr0.3MnO3-0.05% Paraffin wax nanocomposite. Although, the composite exhibits usual signature of spin-polarized tunnelling magnetoresistance (MR) of polycrystalline La0.7Sr0.3MnO3 (LSMO) within the temperature range of 50–300 K; it differs significantly at low temperature region (T < 50 K) where decrease in low field MR (LFMR) with decrease in temperature has been observed. Interestingly, we have even found a change in the sign of LFMR from negative to positive at T = 10 K with the effect gets enhanced with decreasing temperature. Finally, at T = 5 K, MR is found to be positive at the entire applied field regime. Calculated surface or boundary spin susceptibility has been found to follow similar nature of temperature dependence of MR indicating interface magnetization governing MR. Our conjecture is the blocking of superparamagnetic nanoscopic LSMO impurities, possibly present at intergranular Paraffin wax layers, at lower temperature (T < 50 K) resulting in spin-flip scattering of tunnelling electrons. These spin-flipped tunnelling electrons, in turn experience strong spin-dependent scattering at the minority spin band of LSMO. This leads to enhancement of resistance with applied field and hence yields positive MR. Significantly, with the incorporation of cheap, easily available, organic Paraffin wax tunnel barrier, an overall MR response of ∼200% has been achieved at T = 5 K, at 5 T applied magnetic field and with ac excitation of 111 Hz frequency.

Role of conducting polyaniline interphase on the low field magnetoresistance for LSMO-PANI nanocomposites

The aim of the present report is to show the synergistic effect of conducting polyaniline and manganite nanoparticles on the low field magnetoresistance response. We specially focused on the role of the conducting polyaniline interphase in a broad range of temperatures and low applied magnetic fields. Our results showed an increment on the electrical conductivity and enhancement on the magnetoresistance for lower amounts of conducting polyaniline additions with respect to the manganite unloaded samples. We also found that close to room temperature, the changes in magnetoresistance are mostly due to an increment of the spin polarization and spin disorder due to the presence of polyaniline interphase. However, at intermediate temperatures the polyaniline interphase intrinsic magnetoresistance showed an additional contribution to the total negative magnetoresistance response. These results, suggest that conducting polymer interphase shows both passive and active roles in the enhancement of the low field magnetoresistance of inorganic-organic nanocomposites.

Nanobismuth: Fabrication, Optical, and Plasmonic Properties—Emerging Applications

Along the twentieth century, the electronic properties of bismuth have been widely studied, especially in relation with its magnetoresistive and thermoelectric responses. In this context, a particular emphasis has been made on electronic confinement effects in bismuth nanostructures (or nanobismuth). In the recent years, the optical properties of bismuth nanostructures are focusing a growing interest. An increasing number of reports point at the potential of such nanostructures to support plentiful optical resonances over an ultrabroad spectral range: “interband plasmonic” resonances in the ultraviolet, visible, and near-infrared; dielectric Mie resonances in mid- and far-infrared; and conventional free-carrier plasmonic resonances in the far-infrared and terahertz. With the aim to provide a comprehensive basis for exploiting the full optical potential of bismuth nanostructures, we review the current progress in their controlled fabrication, the trends reported (from theoretical calculations and experimental observations) for their optical and plasmonic response, and their emerging applications, including photocatalysis and switchable metamaterials.

Temperature and current dependence of the magnetoresistive behavior of poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS)

We investigate the magnetic field effects in thin-film diodes made of the conducting polymer poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) as a function of temperature and electrical current. Magnetoresistance of these devices can be measured to high precision on two distinct magnetic field scales: <3 mT, where a pronounced nonmonotonic magnetoresistance response can be resolved, owing to weak hyperfine coupling, and at intermediate magnetic fields, ranging between 3 and 10 mT, where strong monotonic magnetoresistance is seen. The detailed examination of the magnetoresistance effects in both regimes allows one to scrutinize the accuracy of the underlying models for the behavior of these kinds of materials.

Effect of NiFeCr seed and capping layers on exchange bias and planar Hall voltage response of NiFe/Au/IrMn trilayer structures

Thin films and cross junctions, based on NiFe/Au/IrMn structures, were grown on Ta and NiFeCr seed layers by magnetron sputtering. The effects of substitution of Ta with NiFeCr in seed and capping layers on an exchange bias field are studied. A threefold improvement of the exchange bias value in the structures, grown with NiFeCr seed and capping layers, is demonstrated. The reasons for this effect are discussed. Formation of clusters in the NiFeCr capping layer is proved by atomic force microscopy technique. Ta replacement on NiFeCr in the capping layer results in the enhancement of magnetoresistive response and a reduction of noise.

Confinement of picosecond timescale current pulses by tapered coplanar waveguides

Tapered coplanar waveguides with integrated photoconductors were designed, fabricated, and measured, with pulsed transmission results comparing well with High Frequency Structure Simulator simulations which predict increased confinement and electric field concentration in the tapered region. Devices made with titanium/gold metallisation were used to demonstrate transmission and confinement, while the magnetoresistive properties of devices with cobalt/copper multilayers were used to demonstrate the field concentration. In the latter case, a mathematical framework was developed to understand the relationship between tapering effects and the picosecond magnetoresistance response.

Evolution of the spin hall magnetoresistance in Cr2O3/Pt bilayers close to the Néel temperature

We study the evolution of magnetoresistance with temperature in thin film bilayers consisting of platinum and antiferromagnet Cr2O3 with its easy axis out of the plane. We vary the temperature from 20 °C to 60 °C, in the vicinity of the Néel temperature of Cr2O3 of approximately 37 °C. The magnetoresistive response is recorded during rotations of the external magnetic field in three mutually orthogonal planes. A large magnetoresistance having a symmetry consistent with a positive spin Hall magnetoresistance is observed in the paramagnetic phase of Cr2O3, which however vanishes when cooling to below the Néel temperature. Compared to analogous experiments in a Gd3Ga5O12/Pt bilayer, we conclude that a paramagnetic moment in the insulator induced by an applied magnetic field is not sufficient to explain the observed magnetoresistance. We speculate that the type of magnetic moment at the interface qualitatively impacts the spin angular momentum transfer, with the 3d moments of Cr sinking angular momentum much more efficiently as compared to the more localized 4f moments of Gd.

Free and forced Barkhausen noises in magnetic thin film based cross-junctions

Barkhausen noise, driven by thermal fluctuations in stationary magnetic field, and Barkhausen jumps, driven by sweeping magnetic field, are demonstrated to be effects of different orders of magnitude. The critical magnetic field for domain walls depinning, followed by avalanched and irreversible magnetization jumps, is determined. Magnetoresistive response of NiFe/M/NiFe (M = Au, Ta, Ag) trilayers to stationary and sweeping magnetic field is studied by means of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) measurements. Thermal fluctuations result in local and reversible changes of magnetization of the layers in thin film magnetic junctions, while the sweeping magnetic field results in reversible and irreversible avalanched domain motion, dependently on the ratio between the values of sweeping magnetic field and domain wall depinning field. The correlation between AMR and PHE responses to Barkhausen jumps is studied. The value of this correlation is found to be dependent on the α angle between the directions of magnetic field and current path.

Phase shift of oscillatory magnetoresistance in a double-cross thin film structure of La0.3Pr0.4Ca0.3MnO3 via strain-engineered elongation of electronic domains

The subtle balance between the competing electronic phases in manganites due to complex interplay between spin, charge, and orbital degrees of freedom could allow one to modify the properties of electronically phase separated systems. In this paper, we show that the phase shift in the oscillatory magnetoresistance $\ensuremath{\rho}(\ensuremath{\theta})$ can be modified by engineering strain driven elongation of electronic domains in ${\mathrm{La}}_{0.3}{\mathrm{Pr}}_{0.4}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ (LPCMO) thin films. Strain-driven elongation of magnetic domains can produce different percolation paths and hence different anisotropic magnetoresistance responses. This tunability provides a unique control that is unattainable in conventional $3d$ ferromagnetic metals and alloys.

Interplay between magnetic domain patterning and anisotropic magnetoresistance probed by magneto-optics

We study the correlation between the magnetic reversal and the anisotropic magnetoresistance (AMR) response in magnetic hybrid structures that were created by local modification of magnetic properties induced by ion implantation. The stripe pattern has been investigated simultaneously by dual-wavelength Kerr microscopy and magnetoresistance measurements. We observe that the switching of the stripe pattern introduces an additional AMR maximum. The domain wall in between the stripes provides a positive resistance contribution, whereas domains at the stripe edges lead to an asymmetric AMR response. A method for calculating the AMR response from the quantitative Kerr micrographs is demonstrated that allows the reconstruction of the AMR value within a region of interest only.

Influence of the magnetic proximity effect on spin-orbit torque efficiencies in ferromagnet/platinum bilayers

Current-induced spin-orbit torques in Co$_2$FeAl/Pt ultrathin bilayers are studied using a magnetoresistive harmonic response technique, which distinguishes the dampinglike and fieldlike contributions. The presence of a temperature-dependent magnetic proximity effect is observed through the anomalous Hall and anisotropic magnetoresistances, which are enhanced at low temperatures for thin platinum thicknesses. The fieldlike torque efficiency decreases steadily as the temperature is lowered for all Pt thicknesses studied, which we propose is related to the influence of the magnetic proximity effect on the fieldlike torque mechanism.

Mechanism of enhancement in magnetoresistance properties of manganite perovskite ceramics by current annealing

Studies on spintronics have provided solid evidence that the grain boundaries (GBs) in polycrystalline manganite can produce a strong extrinsic magnetoresistance (MR). This type of MR, called Low-field MR (LFMR), is larger than the intrinsic MR and can be triggered over a wide range of temperature. However, the existence of more GBs would bring about the weakening of magnetism and decrease the magnitude of MR simultaneously. Here we show that during annealing the application of electric-current to a representative ferromagnetic manganite perovskite, polycrystalline La2/3Sr1/3MnO3 (LSMO), can produce more GBs and improve low-field magnetization, which leads to enhanced MR effect and field-response sensitivity as compared to the traditional-annealed sample. By using static micromagnetic models combined with the theories of spin-polarized intergrain tunneling and charge carrier hopping across domain wall, the observed enhancement of magnetoresistive response in current-annealed LSMO can be well explained.

Magnetic solitons and magnetic phase diagram of the hexagonal chiral crystal CrNb3S6 in oblique magnetic fields

We investigate the magnetic torque and magnetoresistance (MR) responses in oblique magnetic fields in micrometer-sized specimens of the hexagonal chiral magnetic crystal ${\mathrm{CrNb}}_{3}{\mathrm{S}}_{6}$. The results exhibit hysteresis over a wide range of applied field angles, while reversible behavior appears only when the magnetic field is closely aligned to the helical axis of the crystal. Stepwise changes of the magnetic torque and MR detected in the hysteresis region indicate the existence of chiral solitons in the oblique magnetic fields. A magnetic phase diagram is derived from the experimental results, and the stability of the chiral magnetic phases, such as the chiral soliton lattice and chiral conical phase, and the nature of the phase transition between them are discussed.