Mechanism Of Magnetoresistance Research Articles

Interfacial modulation of magnetic relaxation and electrical characteristic in RuO2/CrO2 antiferromagnet-half metal bilayer

Half-metallic chromium dioxide (CrO2) is a promising candidate in magnetic random-access memory (MRAM) devices due to its nearly full spin polarization and ultralow Gilbert damping at room temperature. In this study, we succeeded in fabricating (110) and (100)-oriented RuO2/CrO2 bilayers with different thicknesses of CrO2 film using two-step method. The CrO2 film epitaxially grown on a RuO2 layer exhibits a perfect in-plane magnetic uniaxial anisotropy. The magnetic relaxation of CrO2 films can be strongly modulated via the proximity to RuO2 layer, which may relate to the enhancement of the two-magnon scattering due to effects of antiferromagnetic order in RuO2 film. In addition, electronic transport measurements indicate that the electronic characteristic is strongly related to the film growth orientation, and a linear magnetoresistance is observed stemming from intergrain tunneling magnetoresistance and double-exchange mechanism. Angle-dependent ferromagnetic resonance measurements indicates that the uniaxial anisotropy in CrO2 film is strongly related to the strain effect, which is consistent with reciprocal space mapping results. This work lays a foundation for current-drive MRAM based on RuO2/CrO2 bilayer and provides a feasible damping modulation method for CrO2 film.

Observation of higher-order contribution to anisotropic magnetoresistance of thin Pt/[Co/Pt] multilayered films

We studied the magnetoresistance mechanisms in a Pt/[Co/Pt]x5 film consisting of a ferromagnetic [Co/Pt]x5 layer with strong perpendicular magnetic anisotropy and a nonmagnetic Pt layer with strong spin–orbit coupling. We revealed two competing contributions of the sin2θ and cos4θ types to its angular and magnetic field dependences of electrical resistance at T = 10–250 K corresponding to the out-of-plane rotation of the magnetization M(θ) perpendicularly to the electric current. They were attributed to different magnetoresistance mechanisms. The higher-order cos4θ contribution, which emerges and increases with decreasing temperature, is attributed to the anisotropic magnetoresistance of the ferromagnetic layer, while the sin2θ contribution, which prevails at room temperature and then decreases, is mainly associated with the spin Hall magnetoresistance originating from the Pt layer. The analysis of the corresponding angular dependences of the Hall voltage revealed non-trivial periodic oscillations in the second harmonic. Their appearance is found to be consistent with the manifestation of higher-order angle-dependent contributions to the field-like spin–orbit torque. The revealed strong influence of the electric current on the magnetization of the film studied, which ensures the higher-order effects manifestation, is of high relevance for magnetic memory design technologies.

Colossal negative magnetoresistance in field-induced Weyl semimetal of magnetic half-Heusler compound

The discovery of topological insulators and semimetals triggered enormous interest in exploring emergent electromagnetic responses in solids. Particular attention has been focused on ternary half-Heusler compounds, whose electronic structure bears analogy to the topological zinc-blende compounds while also including magnetic rare-earth ions coupled to conduction electrons. However, most of the research in this system has been in band-inverted zero-gap semiconductors such as GdPtBi, which still does not fully exhaust the large potential of this material class. Here, we report a less-studied member of half-Heusler compounds, HoAuSn, which we show is a trivial semimetal or narrow-gap semiconductor at zero magnetic field but undergoes a field-induced transition to a Weyl semimetal, with a negative magnetoresistance exceeding four orders of magnitude at low temperatures. The combined study of Shubnikov-de Haas oscillations and first-principles calculation suggests that the exchange field from Ho 4f moments reconstructs the band structure to induce Weyl points which play a key role in the strong suppression of large-angle carrier scattering. Our findings demonstrate the unique mechanism of colossal negative magnetoresistance and provide pathways towards realizing topological electronic states in a large class of magnetic half-Heusler compounds.

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.

Origins of observational errors in field sweep DC measurements for unidirectional magnetoresistance

Understanding the mechanisms of unidirectional magnetoresistance (UMR) has become an important topic for its potential application of a two-terminal spin–orbit torque device. Field sweep DC measurements have been proposed and adopted to measure the value of UMR instead of second harmonic measurements. In this paper, potential measurement errors in conventional DC measurements are investigated. Oersted field and field-like torque usually do not influence the measurement, but a large field-like torque was found to lead to an anisotropic magnetoresistance difference when the sample is not perfectly aligned with the external field. The existence of ordinary magnetoresistance was also found to contribute to a large background. In this paper, an alternative measurement method for UMR was proposed and demonstrated to address those issues related to previous DC measurements. Our work may broaden the understanding of the error sources of UMR measurements and provide a reliable DC measurement method for the characterization of UMR.

On universal butterfly and antisymmetric magnetoresistances

Butterfly magnetoresistance (BMR) and antisymmetric magnetoresistance (ASMR) are about a butterfly-cross curve and a curve with one peak and one valley when a magnetic field is swept up and down along a fixed direction. Other than the parallelogram-shaped magnetoresistance-curve (MR-curve) often observed in magnetic memory devices, BMR and ASMR are two ubiquitous types of MR-curves observed in diversified magnetic systems, including van der Waals materials, strongly correlated systems, and traditional magnets. Here, we reveal the general principles and the picture behind the BMR and the ASMR that do not depend on the detailed mechanisms of magnetoresistance: 1) The systems exhibit hysteresis loops, common for most magnetic materials with coercivities. 2) The magnetoresistance of the magnetic structures in a large positive magnetic field and in a large negative magnetic field is approximately the same. With the generalized Ohm’s law in magnetic materials, these principles explain why most BMR appears in the longitudinal resistance measurements and is very rare in the Hall resistance measurements. Simple toy models, in which the Landau-Lifshitz-Gilbert equation governs magnetization, are used to demonstrate the principles and explain the appearance and disappearance of BMR in various experiments. Our finding provides a simple picture to understand magnetoresistance-related experiments.

Hydrodynamic magnetoresistance in graphene Corbino devices

We study hydrodynamic electron magnetotransport in graphene devices. We show that in these systems a distinct mechanism of magnetoresistance appears which is absent in systems with Galilean-invariant electron liquid. The resulting magnetoresistance depends on the intrinsic conductivity and viscosity of the electron liquid, and becomes especially pronounced near charge neutrality. We obtain analytic expressions for magnetotransport coefficients of Corbino devices and obtain estimates for the electrical and thermal magnetoresistances for monolayer and bilayer systems at charge neutrality. Magnetoresistance becomes strong (of order $100\phantom{\rule{0.16em}{0ex}}%$) at relatively weak fields, at which the kinetic coefficients of the electron liquid are practically unaffected by the magnetic field.

Longitudinal magnetoresistance of uniaxially deformed n-type silicon

The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. The reliability of the mechanism of negative magnetoresistance was verified using uniaxial elastic deformation of the studied crystals. Uniaxial deformation excludes interline transitions of electrons, as a result of which negative magnetoresistance disappears with an increase in uniaxial pressure. When cubic symmetry is violated, anisotropic phenomena occur in such crystals. The multipath of the isoenergetic surface of the bottom of the silicon conduction band causes anisotropies of the effective mass and relaxation time, which are associated with the features of the transfer phenomenon. In particular, magnetoresistance (piezoresistance), which is the most sensitive to the anisotropy of the iso energy surface. The influence of the latter on magnetoresistance is most clearly revealed in the region of strong magnetic fields, where the magnetoresistance is saturated since the longitudinal magnetoresistance is entirely due to the anisotropy of electron mobility.

Longitudinal magnetoresistance of uniaxially deformed n-type silicon

The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. The reliability of the mechanism of negative magnetoresistance was verified using uniaxial elastic deformation of the studied crystals. Uniaxial deformation excludes interline transitions of electrons, as a result of which negative magnetoresistance disappears with an increase in uniaxial pressure. When cubic symmetry is violated, anisotropic phenomena occur in such crystals. The multipath of the isoenergetic surface of the bottom of the silicon conduction band causes anisotropies of the effective mass and relaxation time, which are associated with the features of the transfer phenomenon. In particular, magnetoresistance (piezoresistance), which is the most sensitive to the anisotropy of the iso energy surface. The influence of the latter on magnetoresistance is most clearly revealed in the region of strong magnetic fields, where the magnetoresistance is saturated since the longitudinal magnetoresistance is entirely due to the anisotropy of electron mobility.

Anisotropic giant magnetoresistance and Fermi surface topology in the layered compound YbBi2

Magnetoresistance in novel materials has been attracting ever-increasing attention since its mechanism is still the subject of intense debate and the physics behind these emergent phenomena remains a wild space to be explored. Here, we grow ${\mathrm{YbBi}}_{2}$ single crystals and study their anisotropic giant magnetoresistance and Fermi surface topology via de Haas--van Alphen oscillation and Hall resistivity measurements, electronic band structure calculations, and so on. A detailed analysis of the angle-dependent quantum oscillations reveals the presence of nontrivial topological electronic states and several cylindrical Fermi surface sheets extended along the $b$ axis. Hall resistivity data suggest that multiple charge carriers participate in the transport, and electron and hole densities are nearly balanced. These findings are further confirmed by theoretical calculations. After checking several possible mechanisms, the giant magnetoresistance ($\ensuremath{\sim}1.2\ifmmode\times\else\texttimes\fi{}{10}^{3}%$ at 14 T and 2 K) in ${\mathrm{YbBi}}_{2}$ is ascribed to the carrier compensation instead of topological protection and open orbits. Additionally, we also find that Fermi surface anisotropy serves as a key element for the angular magnetoresistance in this compound. Our studies show that ${\mathrm{YbBi}}_{2}$ can be not only a topologically nontrivial material, but also a prototype system to check familiar magnetoresistance mechanisms.

Crossover magnetoresistance in non-transferred synthesized graphdiyne film

Two-dimensional layered carbon materials such as graphene constructed by sp2 hybridization have raised considerable interest due to their large, non-saturating magnetoresistance. As a new sp-sp2 hybridization structure layered carbon material, the transport performance of graphdiyne has been founded comparable with graphene, while the magnetoresistance remains to be clarified. In this work, we developed a modified non-transferred graphdiyne film synthesized method and studied its magnetoresistance effect. We observed an unexpected temperature-dependent crossover between positive and negative magnetoresistance in graphdiyne film. The film shows negative magnetoresistance at 5–19 K, the maximum intensity of the negative magnetoresistance was about 150% at 5 K under 3 T magnetic field. It presents positive magnetoresistance at 19–21 K under 3 T magnetic field, the magnetoresistance value reached 150% at 20 K. The mechanism of crossover positive and negative magnetoresistance originates from a superposition of the magnetic polaron model and weak localization.

Electric current noise in mesoscopic organic semiconductors induced by nuclear spin fluctuations

We demonstrate that nuclear spin fluctuations lead to the electric current noise in the mesoscopic samples of organic semiconductors showing the pronounced magnetoresistance in weak fields. For the bipolaron and electron-hole mechanisms of organic magnetoresistance, the current noise spectrum consists of the high frequency peak related to the nuclear spin precession in the Knight field of the charge carriers and the low frequency peak related to the nuclear spin relaxation. The shape of the spectrum depends on the external magnetic and radiofrequency fields, which allows one to prove the role of nuclei in magnetoresistance experimentally.

Theory of giant magnetoresistance caused by magnetic-ion pairs embedded in paramagnetic metals

To clarify a mechanism of a giant magnetoresistance (MR), we study the resistivity of a paramagnetic metal in which a magnetic-ion pair is embedded as impurities. In this pair, magnetic ions at the positions ${\mathbf{r}}_{1}$ and ${\mathbf{r}}_{2}$, respectively, are assumed to have localized spins ${\mathbf{S}}_{1}$ and ${\mathbf{S}}_{2}$, which are called $d$ spins. Conducting electrons, on the other hand, are assumed to have a light mass, being called $s$ electrons. The $s$ electrons are scattered by both the $d$ spins through the $s\ensuremath{-}d$ exchange interaction. Since interferences occur in the wave functions of electrons scattered by both the $d$ spins, transition probabilities between different electronic states are shown to depend strongly on $\mathbf{R}={\mathbf{r}}_{2}\ensuremath{-}{\mathbf{r}}_{1}$ and the spin configuration in the pair $({\mathbf{S}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{S}}_{2})$. The steady state of the $s$ electron system under an external electric field is obtained on the basis of the Boltzmann equation and the relaxation-time approximation. By using the obtained relaxation times, the resistivity of the metal is calculated to find its $\mathbf{R}$ and $({\mathbf{S}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{S}}_{2})$ dependencies. MRs are shown to appear when an antiparallel spin configuration realized by the Ruderman-Kittel-Kasuya-Yoshida interaction is changed to be a parallel one by a magnetic field. The maximum MR ratio ${[({\ensuremath{\rho}}_{\mathrm{ap}}\ensuremath{-}{\ensuremath{\rho}}_{\mathrm{p}})/{\ensuremath{\rho}}_{\mathrm{p}}]}_{\mathrm{MR}}$ is found to give a giant MR (GMR) of the order of ${10}^{2}\phantom{\rule{4pt}{0ex}}%, {\ensuremath{\rho}}_{\mathrm{ap}}$ and ${\ensuremath{\rho}}_{\mathrm{p}}$ being the resistivities in the metals with the antiparallel and parallel spin pairs, respectively. These results are extensively applied to the GMR in the metallic magnetic multilayers by taking into account possible magnetic-ion pairs in their nonmagnetic layers. This application shows that Mott's two-fluid model can exhibit a GMR of magnetic multilayers also in the case of the current in planes (CIP).

Study on magnetoresistance of Ni48Mn22Ga22Co4Cu4 microwires

Ni–Mn-Ga(Co, Cu) alloys have implicated magnetoresistance mechanisms as ferromagnetic shape memory alloys. The relationship between magnetoresistance and the martensitic transformation was researched. The magnetoresistance mechanism of microwires was related to the reorientation of martensite induced by magnetic fields. It is found that a negative MR of 4.81% was obtained in 20 kOe field in Ni48Mn22Ga22Co4Cu4 microwires with diameters of 0.28 mm at 327 K. The effect of diameters on the martensitic transformation observed in ρ-T curves could be explained as the effect of rapid quenching during the preparation process, that smaller diameters wires have faster cooling rate and lower atomic order.

Electronic and Magnetic Properties of Cation Ordered Sr2Mn2.23Cr0.77As2O2.

Several different mechanisms of magnetoresistance (MR) have been observed in 1111 LnMnAsO1-xFx oxypnictides (Ln = lanthanide) as a result of magnetic coupling between the Mn and Ln. Such phases also exhibit interesting magnetic phase transitions upon cooling. Sr2Mn2CrAs2O2 has been synthesized to investigate if it is possible to observe MR and/or magnetic phase transitions as a result of magnetic coupling between the Mn and Cr. Sr2Mn2CrAs2O2 crystallizes in the tetragonal space group I4/mmm containing alternating MO22- and M'2As22- layers, and neutron diffraction results demonstrate that the actual stoichiometry is Sr2Mn2.23Cr0.77As2O2. Cation order is present between Mn and Cr, with Cr predominantly occupying the square planar MO22- site. Below 410 K, the magnetic moments of the Mn/Cr ions in the M'2As22- sublattice exhibit G-type antiferromagnetic order. The Mn/Cr moments within the MO22- layer order below 167 K with a K2NiF4-type antiferromagnetic structure that simultaneously induces a spin flip of the magnetic moments in the M'2As22- layers from a G-type to a C-type antiferromagnetic arrangement. The results demonstrate that the superexchange interactions are finely balanced in Sr2Mn2.23Cr0.77As2O2. Sr2Mn2.23Cr0.77As2O2 is semiconducting, and there is no evidence of MR.

Microscopic theory of organic magnetoresistance based on kinetic equations for quantum spin correlations

The correlation kinetic equation approach is developed that allows describing spin correlations in a material with hopping transport. The quantum nature of spin is taken into account. The approach is applied to the problem of the bipolaron mechanism of organic magnetoresistance (OMAR) in the limit of large Hubbard energy and small applied electric field. The spin relaxation that is important to magnetoresistance is considered to be due to hyperfine interaction with atomic nuclei. It is shown that the lineshape of magnetoresistance depends on short-range transport properties. Different model systems with identical hyperfine interaction but different statistics of electron hops lead to different lineshapes of magnetoresistance including the two empirical laws $H^2/(H^2 + H_0^2)$ and $H^2/(|H| + H_0)^2$ that are commonly used to fit experimental results.

Spin-Momentum-Locking Inhomogeneities as a Source of Bilinear Magnetoresistance in Topological Insulators.

A new mechanism of bilinear magnetoresistance (BMR) is proposed and studied theoretically within the minimal model describing surface electronic states in topological insulators. The BMR appears as a consequence of the second-order response to electric field, and depends linearly on both magnetic field and current (electric field). The mechanism is based on the interplay of current-induced spin polarization and scattering processes due to inhomogeneities of spin-momentum locking, that unavoidably appear as a result of structural defects in topological insulators. The proposed mechanism leads to the BMR even if the electronic band structure is isotropic (e.g., absence of hexagonal warping), and is shown to be dominant at lower Fermi energies.

Electric Field-Tunable Giant Magnetoresistance (GMR) Sensor with Enhanced Linear Range.

The operation mechanism of giant magnetoresistance (GMR) sensors relies on the linear response of the magnetization direction to an external magnetic field. Since the magnetic anisotropy of ferromagnetic layers can be manipulated by a strain-mediated magnetoelectric coupling effect, we propose a tunable GMR magnetic field sensor design that allows for voltage tuning of the linear range and sensitivity. A spin valve structure Ru/CoFe/Cu/CoFe/IrMn/Ru is grown on a PMN-PT (011) substrate, and the magnetization directions of ferromagnetic layers can be controlled by an electric field. An adjustable linear magnetoresistance is therefore induced. Based on the magnetoelectric coupling effect and spin valve, we prepared tunable GMR magnetic field sensors with bridge structures. The linear sensing range of a DC magnetic field is enhanced 6 times by applying an electric field of 14 kV/cm. The electrically tunable GMR sensor fulfills the requirements to work at different magnetic field ranges in the same configuration, therefore exhibiting great potential for applications in the Internet of things.

Complex magnetic ordering in nanoporous [Co/Pd]5-IrMn multilayers with perpendicular magnetic anisotropy and its impact on magnetization reversal and magnetoresistance.

We have systematically investigated the magnetization reversal characteristics and magnetoresistance of continuous and nanoporous [Co/Pd]5-IrMn multilayered thin films with perpendicular magnetic anisotropy at different temperatures (4-300 K). For their nanostructuring, porosity was induced by means of deposition onto templates of anodized titania with small (∼30 nm in diameter) homogeneously distributed pores. The magnetization reversal and magnetoresistance of the porous films were found to be closely related to the splitting of the ferromagnetic material into regions with different magnetic properties, in correlation with the complex morphology of the porous system. Independent magnetization reversal is detected for these regions, and is accompanied by its strong impact on the magnetic order in the capping IrMn layer. Electron-magnon scattering is found to be a dominant mechanism of magnetoresistance, determining its almost linear field dependence in a high magnetic field and contributing to its magnetoresistance behavior, similar to magnetization reversal, in a low magnetic field. Partial rotation of IrMn magnetic moments, consistent with the magnetization reversal of the ferromagnet, is proposed as an explanation for the two-state resistance behavior observed in switching between high-resistive and low-resistive values at the magnetization reversal of the porous system studied.

Giant gate-controlled proximity magnetoresistance in semiconductor-based ferromagnetic–non-magnetic bilayers

The evolution of information technology has been driven by the discovery of new forms of large magnetoresistance, such as giant magnetoresistance1,2 and tunnelling magnetoresistance3,4, in magnetic multilayers. Recently, new types of this effect have been observed in much simpler bilayers consisting of ferromagnetic and non-magnetic thin films5–10. However, the magnitude of the change in resistance with magnetic field in these materials is very small, varying between 0.01 and 1%. Here, we demonstrate that non-magnetic–ferromagnetic bilayers consisting of a conducting non-magnetic InAs quantum well and an insulating ferromagnetic (Ga,Fe)Sb layer exhibit giant proximity magnetoresistance of approximately 80% at high magnetic field, and that its magnitude can be controlled by a gate. The mechanism for this large magnetoresistance is a strong magnetic proximity effect. The spin splitting in the InAs quantum well induced by the magnetic proximity effect can be varied between 0.17 meV and 3.8 meV by varying the gate voltage. In principle, this provides a mechanism to locally access Majorana fermions in InAs-based Josephson junctions11–14 and introduces a new concept of magnetic-gating spin transistors in which the non-magnetic channel current is modulated by both electrical and magnetic means. The authors demonstrate magnetoresistance of 80% from a two-dimensional electron gas proximity coupled to a ferromagnetic layer. This extends spintronics functionality to semiconductor devices.