Hysteretic Magnetoresistance Research Articles

Hysteretic magnetoresistance and unconventional anomalous Hall effect in the frustrated magnetTmB4

We study TmB_4, a frustrated magnet on the Archimedean Shastry-Sutherland lattice, through magnetization and transport experiments. The lack of anisotropy in resistivity shows that TmB_4 is an electronically three-dimensional system. The magnetoresistance (MR) is hysteretic at low-temperature even though a corresponding hysteresis in magnetization is absent. The Hall resistivity shows unconventional anomalous Hall effect (AHE) and is linear above saturation despite a large MR. We propose that complex structures at magnetic domain walls may be responsible for the hysteretic MR and may also lead to the AHE.

Emergent vortices at a ferromagnetic superconducting oxide interface

Understanding the cohabitation arrangements of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface remains an open challenge. Probing this coexistence with sub-Kelvin magnetotransport experiments, we demonstrate that a hysteretic in-plane magnetoresistance develops below the superconducting transition for 0.15 T, independently of the carrier density or oxygen annealing. This hysteresis is argued to arise from vortex depinning within a thin ( nm) superconducting layer, mediated by discrete ferromagnetic dipoles located solely above the layer. The pinning strength may be modified by varying the superconducting channel thickness via electric field-effect doping. No evidence is found for bulk magnetism or finite-momentum pairing, and we conclude that ferromagnetism is strictly confined to the interface, where it competes with superconductivity. Our work indicates that oxide interfaces are ideal candidate materials for the growth and analysis of nanoscale superconductor/ferromagnet hybrids.

Ferromagnetism and spin-dependent transport inn-type Mn-doped bismuth telluride thin films

We describe a detailed study of the structural, magnetic, and magneto-transport properties of single-crystal, n-type, Mn-doped Bi2Te3 thin films grown by molecular beam epitaxy. With increasing Mn concentration, the crystal structure changes from the tetradymite structure of the Bi2Te3 parent crystal at low Mn concentrations towards a BiTe phase in the (Bi2Te3)m(Bi2)n homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temperature in the range 13.8 K - 17 K in films with 2 % - 10 % Mn concentration. Magnetization hysteresis loops reveal that the magnetic easy axis is along the c-axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68 nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain wall scattering. Ordinary Hall effect measurements show that the carrier density is n-type, increases with Mn doping, and is high enough (> 2.8 x 10^{13} cm^{-2}) to place the chemical potential in the conduction band. Thus, the observed ferromagnetism is likely associated with both bulk and surface states. Surprisingly, the Curie temperature does not show any clear dependence on the carrier density but does increase with Mn concentration. Our results suggest that the ferromagnetism probed in these Mn-doped Bi2Te3 films is not mediated by carriers in the conduction band or in an impurity band.

Nanopillar spin filter tunnel junctions with manganite barriers.

The potential of a manganite ferromagnetic insulator in the field of spin-filtering has been demonstrated. For this, an ultrathin film of Sm0.75Sr0.25MnO3 is integrated as a barrier in an epitaxial oxide nanopillar tunnel junction and a high spin polarization of up to 75% at 5 K has been achieved. A large zero-bias anomaly observed in the dynamic conductance at low temperatures is explained in terms of the Kondo scattering model. In addition, a decrease in spin polarization at low bias and hysteretic magneto-resistance at low temperatures are reported. The results open up new possibilities for spin-electronics and suggest exploration of other manganites-based materials for the room temperature spin-filter applications.

Topological insulator based spin valve devices: Evidence for spin polarized transport of spin-momentum-locked topological surface states

Spin-momentum helical locking is one of the most important properties of the nontrivial topological surface states (TSS) in 3D topological insulators (TIs). It underlies the iconic topological protection (suppressing elastic backscattering) of TSS and is foundational to many exotic physics (e.g., majorana fermions) and device applications (e.g., spintronics) predicted for TIs. Based on this spin-momentum locking, a current flowing on the surface of a TI would be spin-polarized in a characteristic in-plane direction perpendicular to the current, and the spin-polarization would reverse when the current direction reverses. Observing such a spin-helical current in transport measurements is a major goal in TI research and applications. We report spin-dependent transport measurements in spin valve devices fabricated from exfoliated thin flakes of Bi2Se3 (a prototype 3D TI) with ferromagnetic (FM) Ni contacts. Applying an in-plane magnetic (B) field to polarize the Ni contacts along their easy axis, we observe an asymmetry in the hysteretic magnetoresistance (MR) between opposite B field directions. The “polarity” of the asymmetry in MR can be reversed by reversing the direction of the DC current. The observed asymmetric MR can be understood as a spin-valve effect between the current-induced spin polarization on the TI surface (due to spin-momentum-locking of TSS) and the spin-polarized ferromagnetic contacts. Our results provide a direct transport evidence for the spin helical current in TSS.

Giant ferromagnetism and exchange bias in tensile strained and Cr modified CaRuO3 thin films

We present an unified approach of epitaxial strain and chemical substitution to induce ferromagnetic order in otherwise non-magnetic CaRuO3. The tensile and compressive strained CaRu1−xCrxO3 (0 < x ≤ 0.2) (CRCO) films deposited on SrTiO3 (100) and LaAlO3 (100) substrates, respectively, exhibit a magnetic moment larger than their bulk compositions. The compressive strained x = 0.15 film displays unusually large magnetic moment ∼1.4 μB/f.u. which is about one order of magnitude larger than that of its bulk counterpart. We show that this giant magnetic moment manifests in the realization of (i) a giant exchange bias of ∼700 Oe in CaRu0.85Cr0.15O3/Pr0.5Ca0.5MnO3 bilayer and (ii) a large hysteretic magnetoresistance; both potential attributes for the spintronic applications.

Electrical Spin Injection and Detection in Mn5Ge3/Ge/Mn5Ge3 Nanowire Transistors

In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.

Reversible room-temperature ferromagnetism in Nb-doped SrTiO3single crystals

The search for oxide-based room-temperature ferromagnetism has been one of the holy grails in condensed matter physics. Room-temperature ferromagnetism observed in Nb-doped SrTiO3 single crystals is reported in this Rapid Communication. The ferromagnetism can be eliminated by air annealing (making the samples predominantly diamagnetic) and can be recovered by subsequent vacuum annealing. The temperature dependence of magnetic moment resembles the temperature dependence of carrier density, indicating that the magnetism is closely related to the free carriers. Our results suggest that the ferromagnetism is induced by oxygen vacancies. In addition, hysteretic magnetoresistance was observed for magnetic field parallel to current, indicating that the magnetic moments are in the plane of the samples. The x-ray photoemission spectroscopy, the static time-of-flight and the dynamic secondary ion mass spectroscopy and proton induced x-ray emission measurements were performed to examine magnetic impurities, showing that the observed ferromagnetism is unlikely due to any magnetic contaminant.

Gate-Tunable Large Negative Tunnel Magnetoresistance in Ni–C60–Ni Single Molecule Transistors

We have fabricated single C(60) molecule transistors with ferromagnetic Ni leads (FM-SMTs) by using an electrical break junction method and investigated their magnetotransport. The FM-SMTs exhibited clear gate-dependent hysteretic tunnel magnetoresistance (TMR) and the TMR values reached as high as -80%. The polarity of the TMR was found to be always negative over the entire bias range studied here. Density functional theory calculations show that hybridization between the Ni substrate states and the C(60) molecular orbitals generates an antiferromagnetic configuration in the local density of states near the Fermi level, which gives a reasonable explanation for the observed negative TMR.

Electrical Probing of Magnetic Phase Transition and Domain Wall Motion in Single-Crystalline Mn5Ge3 Nanowire

In this Letter, the magnetic phase transition and domain wall motion in a single-crystalline Mn(5)Ge(3) nanowire were investigated by temperature-dependent magneto-transport measurements. The ferromagnetic Mn(5)Ge(3) nanowire was fabricated by fully germaniding a single-crystalline Ge nanowire through the solid-state reaction with Mn contacts upon thermal annealing at 450 °C. Temperature-dependent four-probe resistance measurements on the Mn(5)Ge(3) nanowire showed a clear slope change near 300 K accompanied by a magnetic phase transition from ferromagnetism to paramagnetism. The transition temperature was able to be controlled by both axial and radial magnetic fields as the external magnetic field helped maintain the magnetization aligned in the Mn(5)Ge(3) nanowire. Near the magnetic phase transition, the critical behavior in the 1D system was characterized by a power-law relation with a critical exponent of α = 0.07 ± 0.01. Besides, another interesting feature was revealed as a cusp at about 67 K in the first-order derivative of the nanowire resistance, which was attributed to a possible magnetic transition between two noncollinear and collinear ferromagnetic states in the Mn(5)Ge(3) lattice. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric, and stepwise axial magnetoresistance of the Mn(5)Ge(3) nanowire. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn(5)Ge(3) nanowire and the annihilation of domain walls driven by the magnetic field. The Kurkijärvi model was used to describe the domain wall depinning as thermally assisted escape from a single energy barrier, and the fitting on the temperature-dependent depinning magnetic fields yielded an energy barrier of 0.166 eV.

Reconstruction and Electronic Properties of Interface between Carbon Nanotubes and Ferromagnetic Co Electrodes

The reconstruction and electronic structures of the interfaces between single wall carbon nanotubes and ferromagnetic Co electrodes were studied in the framework of density functional theory. The obtained results revealed that there is a strong interaction between carbon nanotubes and Co electrodes. At the interface region, Top layers of Co surface have been significantly reconstructed. The nature of chemical bonds at the Co-C interface is covalent bonding. The increase of the electron density occurs mainly at the interface where a substantial concentration of electron accumulates in Co-C bonds. A small amount of charge transfer from Co electrode to carbon nanotube junction was found. In addition, the spin polarization of Co atoms at the interface region has been suppressed due to the interaction with a carbon nanotube. It implies that the spin transport through this layer is low, which results in the small hysteretic magneto-resistance of carbon nanotube devices.

Direct observation of magnetization reversal and low field magnetoresistance of epitaxial La0.7Sr0.3MnO3/SrTiO3 (001) thin films at room temperature

We have observed the in-plane magnetic domain arrangement during magnetization reversal in a 40 nm thick La0.7Sr0.3MnO3/SrTiO3 (001) thin film patterned into 500 μm long microbridges of width 50 or 100 μm. Magneto-optical Kerr effect microscopy was used at room temperature and magnetic hysteresis loops were deduced from local averaging of intensity over the microbridge areas. Magnetization reversal proceeds by nucleation and propagation of 180° domain walls. When the magnetic field was applied parallel to the bridge, we observed the nucleation of only one or two domain walls and the reversal occurred by the propagation of them. When the magnetic field was applied perpendicular to the bridge, the reversal occurred mostly by the nucleation of several domain walls. The low field magnetoresistance (MR) and the low frequency noise at zero magnetic field were measured at room temperature. In addition to the linear and reversible colossal MR effect, hysteretic MR versus magnetic field curves could be observed, showing two maxima (minima) when the magnetic field is parallel (perpendicular) to the bridge length. The observed hysteretic MR behaviour is attributed to anisotropic MR inside the 180° Néel domain walls.

Hysteretic magnetoresistance and thermal bistability in a magnetic two-dimensional hole system

Colossal negative magnetoresistance and the associated field-induced insulator-to-metal transition, the most characteristic features of magnetic semiconductors, are observed in n-type rare earth oxides and chalcogenides, p-type manganites, n-type and p-type diluted magnetic semiconductors (DMS) as well as in quantum wells of n-type DMS. Here, we report on magnetostransport studies of Mn modulation-doped InAs quantum wells, which reveal a magnetic field driven and bias voltage dependent insulator-to-metal transition with abrupt and hysteretic changes of resistance over several orders of magnitude. These phenomena coexist with the quantised Hall effect in high magnetic fields. We show that the exchange coupling between a hole and the parent Mn acceptor produces a magnetic anisotropy barrier that shifts the spin relaxation time of the bound hole to a 100 s range in compressively strained quantum wells. This bistability of the individual Mn acceptors explains the hysteretic behaviour while opening prospects for information storing and processing. At high bias voltage another bistability, caused by the overheating of electrons10, gives rise to abrupt resistance jumps.

Hysteretic giant magnetoresistance curves induced by interlayer magnetostatic coupling in [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves

The giant magnetoresistance (GMR) characteristics of hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves with applied field perpendicular and parallel to the film plane are studied. Linear and nonhysteretic GMR behaviors are observed for the two spin valves in perpendicular-to-plane applied fields due to the coherent rotation of the Co layer driven by the in-plane shape anisotropy. The GMR value of the dual spin valve is 1.8 times as large as that of the single spin valve. The correlations between the magnetic configuration of the devices and the resistance evolvement are analyzed. In-plane GMR curves with maximum applied field of 30 kOe deviate from the linear behavior and can be well described by the Stoner–Wohlfarth model including high order anisotropy terms. Moreover, obvious hysteresis behavior is observed in the in-plane GMR curve of the dual spin valve. This phenomenon results from the hindered rotation of the Co/Pd moments due to the magnetostatic coupling between the top and bottom Co/Pd multilayers in the dual spin valve.

Quantum phase transitions in NbFe2 and Ca3Ru2O7

AbstractWe examine the low temperature states of two transition metal compounds: (i) NbFe2 is poised on the threshold of ferromagnetism and can be pushed into a spin‐aligned state at low temperature by modifying the composition slightly. Stoichiometric NbFe2 has been reported as a rare example of low‐temperature spin density wave order in a d‐metal system. We have used pressure, field and composition tuning to examine the phase diagram of NbFe2. Near the quantum critical point, we find distinct non‐Fermi liquid forms of the resistivity and heat capacity, whereas we observe strong, hysteretic magnetoresistance effects deep in the ordered phase. (ii) Ca3Ru2O7 undergoes first a magnetic transition (TN = 56 K) and then a structural transition (TS = 48 K) on cooling. Most of the Fermi surface is gapped out at low temperature, leading to a very low carrier density and small Fermi surface pockets. Pressure suppresses both TN and TS and, for p > 3.5 GPa, induces a third low temperature state, which is robust up to at least 7.5 GPa.

Magnetic vortex structures probed by magnetotransport in sub-micron permalloy disks

We have successfully used magneto-transport measurement to study magnetization reversal of domain structure in single sub-micron magnetic disks of various sizes, 300nm ≤ diameter ≤ 5μm and 36nm ≤ thickness ≤ 48nm. The magnetoresistance (MR) resulted from the anisotropic magnetoresistance effect can reveals the magnetization reversal of a single disk as confirmed by the magnetic force microscopic results. For a fixed thickness, the remanent domain state can transits from the multi-domain to vortex states by decreasing the disk diameter. Here, the nucleation and annihilation fields of vortex core are determined from disk's hysteretic MR and analytically described by the micromagnetic LLG calculation incorporated with the "rigid" vortex model developed by Guslienko et al. under the consideration of magnetostatic, exchange, and Zeeman energies.

Spin Torques in Point Contacts to Exchange-Biased Ferromagnetic Films

Hysteretic magneto-resistance of point contacts formed between non-magnetic tips and single ferromagnetic films exchange-pinned by antiferromagnetic films is investigated. The analysis of the measured current driven and field driven hysteresis agrees with the recently proposed model of the surface spin-valve, where the spin orientation at the interface can be different from that in the bulk of the film. The switching in magneto-resistance at low fields is observed to depend significantly on the direction of the exchange pinning, which allows identifying this transition as a reversal of interior spins of the pinned ferromagnetic films. The switching at higher fields is thus due to a spin reversal in the point contact core, at the top surface of the ferromagnet, and does not exhibit any clear field offset when the exchange-pinning direction or the magnetic field direction is varied. This magnitude of the switching field of the surface spins varies substantially from contact to contact and sometimes from sweep to sweep, which suggests that the surface coercivity can change under very high current densities and/or due to the particular microstructure of the point contact. In contrast, no changes in the effect of the exchange biasing on the interior spins are observed at high currents, possibly due to the rapid drop in the current density away from nanometer sized point contact cores.

Spin-dependent intergranular transport in highly spin-polarized Co1−xFexS2 thin films

Magnetic and magnetotransport measurements on polycrystalline thin film Co1−xFexS2, a system that exhibits tunable conduction electron spin polarization in bulk, are reported. The films exhibit a low field hysteretic magnetoresistance (MR) due to spin-dependent intergranular tunneling, thus providing a simple probe of the spin polarization. The MR increases rapidly with x, saturating as the magnetization reaches 1.0μB/Co, demonstrating that high, doping tunable, spin polarization can be readily achieved in thin films synthesized by simple methods. Quantitative estimates yield spin polarizations as high as 90%.

Magnetoresistance in Hybrid Organic Spin Valves at the Onset of Multiple-Step Tunneling

By combining experiments with simple model calculations, we obtain new insight in spin transport through hybrid, CoFeB/Al2O3(1.5 nm)/tris(8-hydroxyquinoline)aluminium (Alq3)/Co spin valves. We have measured the characteristic changes in the I-V behavior as well as the intrinsic loss of magnetoresistance at the onset of multiple-step tunneling. In the regime of multiple-step tunneling, under the condition of low hopping rates, spin precession in the presence of hyperfine coupling is conjectured to be the relevant source of spin relaxation. A quantitative analysis leads to the prediction of a symmetric magnetoresistance around zero magnetic field in addition to the hysteretic magnetoresistance curves, which are indeed observed in our experiments.

Hysteretic magnetoresistance in polymeric diodes

AbstractWe report on hysteretic organic magnetoresistance (OMAR) in polymeric diodes. We found that magnitude and lineshape of OMAR depend strongly on the scan speed of the magnetic field and on the time delay between two successive measurements. The time‐dependent OMAR phenomenon is universal for diodes made with various polymers. However, the width and magnitude of OMAR varied with the polymeric material. The suggestive reason for this hysteretic behavior is trapped carriers, which in presence of a magnetic field change the ferromagnetic ground‐state of the polymer leading to a long spin relaxation time. These experimental observations are significant for clarification of the OMAR phenomenon. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)