Giant Positive Magnetoresistance Research Articles

Evidence for an emergent anomalous metallic state in compressed titanium

The anomalous metallic state (AMS) emerging from a quantum superconductor-to-metal transition is a subject of great current interest since this exotic quantum state exhibits unconventional transport properties that challenge the core physics principles of Fermi liquid theory. As the AMS concept is historically derived from disordered two-dimensional (2D) systems, related studies have predominately concentrated on 2D materials. The AMS behaviors in three-dimensional (3D) systems have been rarely reported to date, which raises intriguing questions on the fundamental nature of pertinent physics. Here, we report experimental evidence for a 3D AMS in highly compressed titanium metal that exhibits superconductivity with a critical temperature (Tc) reaching near-record 25.1K among elemental superconductors, offering a favorable material template for exploring 3D AMS. At sufficiently strong magnetic fields, unusual transport behaviors set in over a wide pressure range, showcasing AMS hallmarks of a low-temperature saturation resistance below the Drude value and giant positive magnetoresistance. These findings reveal a 3D AMS in simple elemental systems and, more importantly, provide a fresh platform for probing the decades-long enigmatic underlying physics.

Anomalous metallic state regulated by magnetic field at LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterointerface

<sec>Since the discovery of two-dimensional electron gas with high mobility at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterointerface, many physical properties such as two-dimensional superconductivity, magnetism and spin-orbit coupling have been widely studied. The origin of the transition from quantum superconductor to metal at zero temperature in two-dimensional superconductor is still an open problem, which has been discussed intensely. According to the conventional theory, when the temperature is close to zero, the superconductor-insulator transition can be observed by applying a magnetic field or magnetic field effect of disorder, and the ground state should be superconducting or insulating.</sec><sec>However, when Jaeger et al. (Jaeger H M, Haviland D B, Orr B G, Goldman A M <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevB.40.182">1989 <i>Phys. Rev. B</i> <b>40</b> 182</ext-link>) studied the relationship between superconductivity evolution and thickness and temperature in a superconducting granular film, they found that there exists an intermediate metal state that can destroy the direct transition between superconducting and insulating. The intermediate metal state is characterized by the existence of saturation resistance at superconducting transition temperature, and the corresponding ground state is called anomalous metallic state. In addition to the saturation of resistance at low temperature, the characteristics of an anomalous metallic state also include the linear current-voltage (<i>I-V</i>) characteristics in the low current range, the giant positive magnetoresistance (MR), the vanishing of Hall resistance (<i>R</i><sub><i>xy</i></sub>), and the tuning capability adjusted by changing a variety of parameters including degree of disorder, gate voltage and magnetic field.</sec><sec>In this work, we systematically investigate the electrical transport properties of LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface in a perpendicular magnetic field at low temperature. The <i>R-T</i> curves and the <i>I-V</i> characteristics in zero magnetic field show that LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface is in a superconducting state. However, after a small magnetic field is applied, the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface has the characteristics of resistance saturation at low temperature, linear <i>I-V</i> characteristics, giant positive MR, abnormal Hall response, indicating the clear characteristics of an anomalous metallic state. The sample undergoes a transition from quantum superconductor to metal at temperatures approaching to zero.</sec><sec>In addition, we observe that the anomalous metallic state in an unusually large region under the action of magnetic field, and our main observations are summarized in the <i>H-T</i> phase diagram. By analyzing the relationship between the resistance of the anomalous metallic regime and the magnetic field, and the vanish of Hall resistance, we infer that the anomalous metallic state observed in LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface can be explained by Bose metal model. According to our findings, the magnetic field regulated LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (001) heterointerface appears as a special platform to study the details of anomalous metallic state in a controllable way.</sec>

In situ tunable giant electrical anisotropy in a grating gated AlGaN/GaN two-dimensional electron gas

Materials with in-plane electrical anisotropy have great potential for designing artificial synaptic devices. However, natural materials with strong intrinsic in-plane electrical anisotropy are rare. We introduce a simple strategy to produce extremely large electrical anisotropy via grating gating of a semiconductor two-dimensional electron gas (2DEG) of AlGaN/GaN. We show that periodically modulated electric potential in the 2DEG induces in-plane electrical anisotropy, which is significantly enhanced in a magnetic field, leading to an ultra large electrical anisotropy. This is induced by a giant positive magnetoresistance and a giant negative magnetoresistance under two orthogonally oriented in-plane current flows, respectively. This giant electrical anisotropy is in situ tunable by tailoring both the grating gate voltage and the magnetic field. Our semiconductor device with controllable giant electrical anisotropy will stimulate new device applications, such as multi-terminal memtransistors and bionic synapses.

Spin-orbit-derived giant magnetoresistance in a layered magnetic semiconductor AgCrSe2

Two-dimensional magnetic materials have recently attracted great interest due to their unique functions as the electric field control of a magnetic phase and the anomalous spin Hall effect. For such remarkable functions, a spin-orbit coupling (SOC) serves as an essential ingredient. Here we report a giant positive magnetoresistance in a layered magnetic semiconductor AgCrSe2, which is a manifestation of the subtle combination of the SOC and Zeeman-type spin splitting. When the carrier concentration approaches the critical value of 2.5\times10^18 cm^-3, a sizable positive magnetoresistance of ~400 % emerges upon the application of magnetic fields normal to the conducting layers. Based on the magneto-Seebeck effect and the first-principles calculations, the unconventional magnetoresistance is ascribable to the enhancement of effective carrier mass in the SOC induced J = 3/2 state, which is tuned to the Fermi level through the Zeeman splitting enhanced by the p-d coupling. This study demonstrates a new aspect of the SOC-derived magnetotransport in two-dimensional magnetic semiconductors, paving the way to novel spintronic functions.

Bipolar Conduction and Giant Positive Magnetoresistance in Doped Metallic Titanium Oxide Heterostructures

AbstractEmpowering conventional materials with unexpected magnetoelectric properties is appealing to the multi‐functionalization of existing devices and the exploration of future electronics. Recently, owing to its unique effect in modulating a matter's properties, ultra‐small dopants, for example, H, D, and Li, attract enormous attention in creating emergent functionalities, such as superconductivity, and metal–insulator transition. Here, an observation of bipolar conduction accompanied by a giant positive magnetoresistance in D‐doped metallic Ti oxide (TiOxDy) films is reported. To overcome the challenges in intercalating the D into a crystalline oxide, a series of TiOxDy is formed by sequentially doping Ti with D and surface/interface oxidation. Intriguingly, while the electron mobility of the TiOxDy increases by an order of magnitude larger after doping, the emergent holes also exhibit high mobility. Moreover, the bipolar conduction induces a giant magnetoresistance up to 900% at 6 T, which is ≈6 times higher than its conventional phase. This study paves a way to empower conventional materials in existing electronics and induce novel electronic phases.

Transition from giant linear positive to negative magnetoresistance as a result of the metal insulator transition in cobalt nanosheet decorated graphene with high coercivity

Integration of superior electronic property of graphene and magnetic property of transition metal (TM) to achieve promising materials with excellent electronic and magnetic properties is an interesting area of research. Intrinsically, charge transport in pristine graphene and magnetic property in nanophase TM are well studied subjects. However, these properties deviate drastically from their intrinsic counterpart due to strong interaction at the interface when TM nanostructures are grown on graphene surface. So far no serious effort has been given to look at this issue. Therefore, in the present work, graphene sheet is decorated by Co nanosheets of average lateral dimension of ~15 nm to explore the basic understanding of charge transport, magnetism and magnetoresistance in these integrated structures. Very interesting metal–insulator transition is observed. The charge transport is explained by electron–electron interaction in the insulating regime as arising due to quantum interference and electron scattering in the metallic regime. The materials also show a high coercivity and a transition from giant linear positive magnetoresistance to nonlinear negative magnetoresistance. High coercivity in the present sample arises due to charge transfer, surface pinning and strain created at the graphene/Co interface while positive and negative magnetoresistances are explained on the basis of electron–electron interaction and electron-spin scattering respectively.

Magnetization and Giant Magnetoresistance of Polycrystalline TbSb at Low Temperatures

Dependences of the magnetization M and the resistance R on the temperature T in the range from 2 to 300 K and on the magnetic field H up to 14 T were investigated in polycrystalline samples of terbium antimonide TbSb. A peculiarity (bending) was observed on the M(H) dependences at each temperature below the transition point of TbSb into the antiferromagnetic phase (TN = 14.8 K), which is due to a metamagnetic phase transition. Positive giant magnetoresistance was found in TbSb near the antiferromagnetic phase region.

Colloquium : Anomalous metals: Failed superconductors

The observation of metallic ground states in a variety of two-dimensional electronic systems poses a fundamental challenge for the theory of electron fluids. Here, we analyze evidence for the existence of a regime, which we call the "anomalous metal regime," in diverse 2D superconducting systems driven through a quantum superconductor to metal transition (QSMT) by tuning physical parameters such as the magnetic field, the gate voltage in the case of systems with a MOSFET geometry, or the degree of disorder. The principal phenomenological observation is that in the anomalous metal, as a function of decreasing temperature, the resistivity first drops as if the system were approaching a superconducting ground state, but then saturates at low temperatures to a value that can be orders of magnitude smaller than the Drude value. The anomalous metal also shows a giant positive magneto-resistance. Thus, it behaves as if it were a "failed superconductor." This behavior is observed in a broad range of parameters. We moreover exhibit, by theoretical solution of a model of superconducting grains embedded in a metallic matrix, that as a matter of principle such anomalous metallic behavior can occur in the neighborhood of a QSMT. However, we also argue that the robustness and ubiquitous nature of the observed phenomena are difficult to reconcile with any existing theoretical treatment, and speculate about the character of a more fundamental theoretical framework.

Намагниченность и гигантское магнетосопротивление поликристаллов TbSb при низких температурах

On the terbium antimonide TbSb polycrystals, the dependences of the magnetization M and the resistance R on the temperature T in the range of 2 ÷ 300 K and on the magnetic field H up to H = 14 T have been studied. On the dependences M (H) at each temperature, a feature (kink) was observed associated with the metamagnetic phase transition. Giant positive magnetoresistance of TbSb in the region of the antiferromagnetic phase has been found.

Large magnetoresistance by Pauli blockade in hydrogenated graphene

We report the observation of a giant positive magnetoresistance in millimetre scale hydrogenated graphene with magnetic field oriented in the plane of the graphene sheet. A positive magnetoresistance in excess of 200\% at a temperature of 300 mK was observed in this configuration, reverting to negative magnetoresistance with the magnetic field oriented normal to the graphene plane. We attribute the observed positive, in-plane, magnetoresistance to Pauli-blockade of hopping conduction induced by spin polarization. Our work shows that spin polarization in concert with electron-electron interaction can play a dominant role in magnetotransport within an atomic monolayer.

Spin valve-like magnetic tunnel diode exhibiting giant positive junction magnetoresistance at low temperature in Co2MnSi/SiO2/p-Si heterostructure

The rectifying magnetic tunnel diode has been fabricated by growing Co2MnSi (CMS) Heusler alloy film carefully on a properly cleaned p-Si (100) substrate with the help of electron beam physical vapor deposition technique and its structural, electrical and magnetic properties have been experimentally investigated in details. The electronic- and magneto-transport properties at various isothermal conditions have been studied in the temperature regime of 78–300 K. The current–voltage (I–V) characteristics of the junction show an excellent rectifying magnetic tunnel diode-like behavior throughout that temperature regime. The current (I) across the junction has been found to decrease with the application of a magnetic field parallel to the plane of the CMS film clearly indicating positive junction magnetoresistance (JMR) of the heterostructure. When forward dc bias is applied to the heterostructure, the I–V characteristics are highly influenced on turning on the field B = 0.5 T at 78 K, and the forward current reduces abruptly (99.2% current reduction at 3 V) which is nearly equal to the order of the magnitude of the current observed in the reverse bias. Hence, our Co2MnSi/SiO2/p-Si heterostructure can perform in off (Ioff)/on (Ion) states with the application of non-zero/zero magnetic field like a spin valve at low temperature (78 K).

Giant positive magnetoresistance in half-metallic double-perovskite Sr2CrWO6 thin films.

Magnetoresistance (MR) is the magnetic field-induced change of electrical resistance. The MR effect not only has wide applications in hard drivers and sensors but also is a long-standing scientific issue for complex interactions. Ferromagnetic/ferrimagnetic oxides generally show negative MR due to the magnetic field-induced spin order. We report the unusually giant positive MR up to 17,200% (at 2 K and 7 T) in 12-nm Sr2CrWO6 thin films, which show metallic behavior with high carrier density of up to 2.26 × 1028 m-3 and high mobility of 5.66 × 104 cm2 V-1 s-1. The possible mechanism is that the external magnetic field suppresses the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations, which enhance electronic scattering and lead to the giant positive MR. The high mobility may also have contributions to the positive MR. These results not only experimentally confirm that the giant positive MR can be realized in oxides but also open up new opportunities for developing and understanding the giant positive MR in oxides.

Giant magnetoresistance of PbSnTe:In films in the space-charge-limited current regime: Angular features and effect of the surface

In the space-charge-limited current regime at T = 4.2 K, the magnetoresistance of PbSnTe:In/(111)BaF2 films has been studied at various mutual orientation of the magnetic field B (up to 4 T), electric field E (up to ~103 V/cm), and normal to the surface n. At B ‖ n, the reduction of the current reaches a factor of ~105, whereas at B ‖ E, the current increases by a factor of ~103. The angular dependences of the magnetoresistance have been studied at the “rotation” of B in three different planes. The angular dependences of the magnetoresistance for the plane corresponding to the orientation B ⊥ E exhibit local maxima near the orientations B ⊥ n, at which charge carriers are deflected by the magnetic field to one of the boundaries of the film. At the deviation to the free surface, the half-width of maxima is several degrees. At the deviation to the interface with the substrate, the half-width of maxima is about an order of magnitude larger and their amplitude is one or two orders of magnitude smaller. Possible mechanisms of giant positive and negative magnetoresistance, as well as the effect of the boundaries of the film on the angular dependences of the magnetoresistance, have been discussed.

Microwave response to the magnetization switching of CoFeB/Ta/CoFeB spin valves and CoFeB films

Negative magnetoresistance modifying the quality factor of a microwave cavity under the magnetization switching of ferromagnetic layers has been discovered in a MgO/CoFeB/MgO/Ta film with a single ferromagnetic layer and a MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve consisting of two ferromagnetic CoFeB layers. The dependence of the first derivative dP/dH of the microwave absorption signal on the dc magnetic field of the spectrometer exactly reproduce the magnetic hysteresis loops of the sample. The slope of these dependences and the amplitude of dP/dH jumps under remagnetization of the layers are determined by the interplay of a negative magnetoresistance of individual layers and a positive giant magnetoresistance of the entire multilayer structure. The discovered phenomenon allows using microwave absorption for making a high-sensitivity contact-free indicator of the basic magnetization states of a spin valve.

Oxidized Mn:Ge magnetic semiconductor: Observation of anomalous Hall effect and large magnetoresistance

We report on the structural and magneto-transport properties of the as-grown and oxidized Mn:Ge magnetic semiconductors. Based on X-ray diffraction and X-ray photoelectron spectroscopy results, the samples annealed at 650 and 700°C became fully oxidized and the chemical binding energies of Mn was found to be Mn3O4. Thus, the system became Mn3O4 clusters embedded in Ge1−yOy. The as-grown sample showed positive linear Hall effect and negligible negative magnetoresistance (MR), which trend remained for the sample annealed up to 550°C. Interestingly, for the samples annealed at above 650°C, we observed the anomalous Hall effect around 45K and the giant positive MR, which are respectively 59.2% and 78.5% at 7kOe annealed at 650°C and 700°C.

Magnetic and transport properties of half-Heuslers, RPdSb (R = Gd and Tb)

We present the results of our studies on rare–earth half-Heuslers, RPdSb (R = Gd and Tb), crystallizing in a LiGaGe–type hexagonal structure, by means of magnetization, heat capacity, electrical resistivity, Seebeck coefficient, thermal conductivity and magnetoresistance measurements. The compounds exhibit antiferromagnetic order with transition temperatures of 17 and 2.5 K for Gd and Tb samples, respectively. The results of electrical transport measurements establish a metallic behavior for both the samples while the deviation from the linear temperature dependence was explained based on the additional contribution from the s-d inter-band scattering. Moreover, thermal transport measurements also indicate the contribution due to the s-d interaction where both the types of charge carriers involved in scattering. The positive sign of Seebeck coefficient confirms the dominant charge carriers as holes with the carrier concentration of ∼1021 cm−3 at room temperature while the magnitude decreases with temperature non-linearly. The most interesting feature is the observation of giant positive and non-saturating magneto-resistance at low temperatures which is essentially linear at higher fields and resembles the behavior of charge compensated materials. These results were explained on the basis of the temperature dependent changes in the population of electrons and holes.

Superconducting Ferromagnetic Nanodiamond.

Superconductivity and ferromagnetism are two mutually antagonistic states in condensed matter. Research on the interplay between these two competing orderings sheds light not only on the cause of various quantum phenomena in strongly correlated systems but also on the general mechanism of superconductivity. Here we report on the observation of the electronic entanglement between superconducting and ferromagnetic states in hydrogenated boron-doped nanodiamond films, which have a superconducting transition temperature Tc ∼ 3 K and a Curie temperature TCurie > 400 K. In spite of the high TCurie, our nanodiamond films demonstrate a decrease in the temperature dependence of magnetization below 100 K, in correspondence to an increase in the temperature dependence of resistivity. These anomalous magnetic and electrical transport properties reveal the presence of an intriguing precursor phase, in which spin fluctuations intervene as a result of the interplay between the two antagonistic states. Furthermore, the observations of high-temperature ferromagnetism, giant positive magnetoresistance, and anomalous Hall effect bring attention to the potential applications of our superconducting ferromagnetic nanodiamond films in magnetoelectronics, spintronics, and magnetic field sensing.

Lateral Magnetically Modulated Multilayers by Combining Ion Implantation and Lithography.

The combination of lithography and ion implantation is demonstrated to be a suitable method to prepare lateral multilayers. A laterally, compositionally, and magnetically modulated microscale pattern consisting of alternating Co (1.6 µm wide) and Co-CoO (2.4 µm wide) lines has been obtained by oxygen ion implantation into a lithographically masked Au-sandwiched Co thin film. Magnetoresistance along the lines (i.e., current and applied magnetic field are parallel to the lines) reveals an effective positive giant magnetoresistance (GMR) behavior at room temperature. Conversely, anisotropic magnetoresistance and GMR contributions are distinguished at low temperature (i.e., 10 K) since the O-implanted areas become exchange coupled. This planar GMR is principally ascribed to the spatial modulation of coercivity in a spring-magnet-type configuration, which results in 180° Néel extrinsic domain walls at the Co/Co-CoO interfaces. The versatility, in terms of pattern size, morphology, and composition adjustment, of this method offers a unique route to fabricate planar systems for, among others, spintronic research and applications.

Giant positive magnetoresistance and field-induced metal insulator transition in Cr2NiGa

We report the magneto-transport properties of the newly synthesized Heusler compound Cr2NiGa which crystallizes in a disordered cubic B2 structure belonging to the Pmm space group. The sample is found to be paramagnetic down to 2 K with metallic characteristics. On application of a magnetic field, a significantly large increase in resistivity is observed which corresponds to magnetoresistance as high as 112% at 150 kOe of field at the lowest temperature. Most remarkably, the sample shows a negative temperature coefficient of resistivity below about 50 K under the application of field ⩾80 kOe, signifying a field-induced metal to ‘insulating’ transition. The observed magnetoresistance follows Kohler’s rule below 20 K indicating the validity of the semiclassical model of electronic transport in metals with a single relaxation time. A multi-band model for electronic transport, originally proposed for semimetals, is found to be appropriate to describe the magneto-transport behavior of the sample.

Evidence of kinetic arrest in Se doped CoS2

A systematic study of thermomagnetic irreversibility CoS$_{1.76}$Se$_{0.24}$ has been carried out. Our study shows that the resistivity at low temperature can be tuned by cooling in different magnetic fields and the critical field required for paramagnetic (PM) to ferromagnetic (FM) transition varies non-monotonically with temperature. The field induced PM to FM transition results in giant positive magnetoresistance (MR) of about 160\% at 5 K. Measurements under CHUF (cooling and heating in unequal magnetic field) protocol show reentrant transition on warming under higher magnetic field (than that applied during cooling). It indicates that the glass like behaviour in this system can be explained in the framework of the kinetic arrest of first order transition. Among the growing list of diverse system showing glass like arrested magnetic states, the present system is the first example where, kinetic arrest is observed for a disordered (here PM) to ordered (here FM) first order transition.