Magnetoresistance Sensitivity Research Articles

Universal material trends in extraordinary magnetoresistive devices

Extraordinary magnetoresistance (EMR) is a geometric magnetoresistance emerging in hybrid systems typically comprising a high-mobility material and a metal. Due to a field-dependent redistribution of electrical currents in these devices, the electrical resistance at room temperature can increase by 107% when applying a magnetic field of 5 T. Although EMR holds considerable potential for realizing sensitive, all-electronic magnetometers, this potential is largely unmet. A key challenge is that the performance of EMR devices depends very sensitively on variations in a vast parameter space where changes in the device geometry and material properties produce widely different EMR performances. The challenge of navigating in the large parameter space is further amplified by the poor understanding of the interplay between the device geometry and material properties. By systematically varying the material parameters in four key EMR geometries using diffusive transport simulations, we here elucidate this interplay with the aim of finding universal guidelines for designing EMR devices. Common to all geometries, we find that the sensitivity scales inversely with the carrier density, while the MR reaches saturation at low carrier densities. Increasing the mobility beyond 20 000 cm2 Vs−1 is required to observe strong EMR effects at 1 T with the optimal magnetoresistance observed for mobilities between 100 000–500 000 cm2 Vs−1. An interface resistance below cm2 between the constituent materials in the hybrid devices was also found to be a prerequisite for very high magnetoresistances in all geometries. By further simulating several high-mobility materials at room and cryogenic temperatures, we conclude that encapsulated graphene and InSb are amongst the most promising candidates for EMR devices showing high magnetoresistance exceeding 107% below 1 T at room temperature. This study paves the way for understanding how to realize EMR devices with record-high magnetoresistance and high sensitivity for detecting magnetic fields.

Coupling between magnetic order and charge transport in a two-dimensional magnetic semiconductor.

Semiconductors, featuring tunable electrical transport, and magnets, featuring tunable spin configurations, form the basis of many information technologies. A long-standing challenge has been to realize materials that integrate and connect these two distinct properties. Two-dimensional (2D) materials offer a platform to realize this concept, but known 2D magnetic semiconductors are electrically insulating in their magnetic phase. Here we demonstrate tunable electron transport within the magnetic phase of the 2D semiconductor CrSBr and reveal strong coupling between its magnetic order and charge transport. This provides an opportunity to characterize the layer-dependent magnetic order of CrSBr down to the monolayer via magnetotransport. Exploiting the sensitivity of magnetoresistance to magnetic order, we uncover a second regime characterized by coupling between charge carriers and magnetic defects. The magnetoresistance within this regime can be dynamically and reversibly tuned by varying the carrier concentration using an electrostatic gate, providing a mechanism for controlling charge transport in 2D magnets.

Effect of nonuniform microstructure on magnetoresistance and field sensitivity in Co–MgO nanocomposite films

Co–MgO metal-insulator nanocomposite films have attracted increasing interest for their high resistivity and negative magnetoresistance at room temperature. In this study, nonuniform-microstructure Co–MgO nanocomposite films were produced, in which superparamagnetic particles and ferromagnetic clusters coexist. At the Co content of 36 at.%, the film exhibits a high room-temperature negative magnetoresistance value of over 2.2%, and a high resistivity of 7.0 × 104 μΩ m. When Co content increases, the size and quantity of Co clusters gradually increase, which causes the peaks shifting of magnetoresistance field sensitivity to higher field. Moreover, we have established a mechanism mode to explain this phenomenon, by inducing the localized magnetization flux effect and inhomogeneous current paths effect. The high-resistivity nonuniform-microstructure Co–MgO nanocomposites are realized, which can be used in different application scenarios by modulating magnetoresistance field sensitivity.

Modulation of magnetoresistance and field sensitivity of Co–ZnO nanocomposite film by microstructure controlling

Recently, magnetic metal-semiconductor nanocomposite films with a uniform superparamagnetic structure have attracted increasing interest in negative magnetoresistance (MR) properties at room temperature (RT). In this work, we have designed and prepared an original nonuniform structure with coexistence of size-distributed ferromagnetic Co clusters and superparamagnetic Co particles in Co–ZnO nanocomposite films, with Co content between 41 and 65 at.%. The film with 49 at.% Co shows a large RT negative MR with a maximum value of over 9.5%. As Co content increasing, the ferromagnetic Co clusters grow both in quantity and size. Meanwhile, the curves of normalized MR become broader, and the peaks of the normalized MR field sensitivity (FS) shift to higher fields. It is noteworthy that the increase in ferromagnetic Co clusters can effectively modulate the MR FS, which is reasonably explained by establishing a mechanism model with the localized magnetization flux effect and inhomogeneous current paths based on size-distributed ferromagnetic Co clusters. This nonuniform structure can be extended to other nanocomposite materials for the modulation of MR FS by microstructure control in various application scenarios.

Low Field Optimization of a Non-Contacting High-Sensitivity GMR-Based DC/AC Current Sensor.

Many applications require galvanic isolation between the circuit where the current is flowing and the measurement device. While for AC, the current transformer is the method of choice, in DC and, especially for low currents, other sensing methods must be used. This paper aims to provide a practical method of improving the sensitivity and linearity of a giant magnetoresistance (GMR)-based current sensor by adapting a set of design rules and methods easy to be implemented. Our approach utilizes a multi-trace current trace and a double differential GMR based detection system. This essentially constitutes a planar coil which would effectively increase the usable magnetic field detected by the GMR sensor. An analytical model is developed for calculating the magnetic field generated by the current in the GMR sensing area which showed a significant increase in sensitivity up to 13 times compared with a single biased sensor. The experimental setup can measure both DC and AC currents between 2–300 mA, with a sensitivity between 15.62 to 23.19 mV/mA, for biasing fields between 4 to 8 Oe with a detection limit of 100 μA in DC and 100 to 300 μA in AC from 10 Hz to 50 kHz. Because of the double differential setup, the detection system has a high immunity to external magnetic fields and a temperature drift of the offset of about −2.59 × 10−4 A/°C. Finally, this setup was adapted for detection of magnetic nanoparticles (MNPs) which can be used to label biomolecules in lab-on-a-chip applications and preliminary results are reported.

High-sensitivity tunneling magneto-resistive micro-gyroscope with immunity to external magnetic interference

Micro-electro-mechanical system (MEMS) gyroscopes have numerous potential applications including guidance, robotics, tactical-grade navigation, and automotive applications fields. The methods with ability of the weak Coriolis force detection are critical for MEMS gyroscopes. In this paper, we presented a design of MEMS gyroscope based on the tunneling magneto-resistance effect with higher detection sensitivity. Of all these designed parameters, the structural, magnetic field, and magneto-resistance sensitivity values reach to 21.6 nm/°/s, 0.0023 Oe/nm, and 29.5 mV/Oe, thus, with total sensitivity of 1.47 mV/°/s. Multi-bridge circuit method is employed to suppress external magnetic interference and avoid the integration error of the TMR devices effectively. The proposed tunneling magneto-resistive micro-gyroscope shows a possibility to make an inertial grade MEMS gyroscope in the future.

Room-Temperature Colossal Magnetoresistance in Terraced Single-Layer Graphene.

Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, a room-temperature colossal MR of up to 5000% at 9 T is reported in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 -terminated SrTiO3 , a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of ≈1012 cm-2 . Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

Optimization of Pt composition with magnetic thin films for magnetic field sensor application

Heavy metals like Pt play an important role in the field of spintronics due to their high spin-orbit coupling. Yet optimization condition for using it in proximity to magnetic materials is still challenging. In this letter, using post-annealing of permalloy (Py)/Pt multilayers, different composition structures of Pt with Py have been fabricated. Structural analysis showed that at temperatures higher than 200 °C the Pt diffuses into the crystal structure of Py while at lower temperatures Pt is in proximity to the Py layer. At annealing temperatures below 200 °C the magnetoresistance (MR) ratio and sensitivity increases and at higher annealing temperatures the MR ratio and sensitivity decreases. Magnetic hysteresis loop measurement showed that at lower annealing temperature, magnetic moment and therefore MR ratio increases. Results can be considered for optimization of heavy metal mixing with magnetic materials for magnetic field sensor enhancement.

Negative differential resistance and unsaturated magnetoresistance effects based on avalanche breakdown

The negative differential resistance (NDR) effect and magnetoresistance (MR) effect attract a lot of attention since they have been widely applied in fields such as circuit amplifiers and information storage, respectively. In general, the NDR and MR effects derive from different physical mechanisms, which makes it difficult to obtain both an NDR effect and a large unsaturated MR effect in a device based on the same physical mechanism. In this paper, the NDR and unsaturated MR effects were observed simultaneously in In/SiO2/p-Si/SiO2/In hetero junction devices, and their physical mechanisms were investigated. It is found that under zero magnetic field conditions, the NDR effect can be observed up to 25 K which is much higher than the temperature of NDR obtaining in intrinsic silicon. The NDR effect is significantly enhanced by applying a small magnetic field of 0.1 T. Meanwhile, the unsaturated positive MR effect is obtained when an external magnetic field is applied. Our analysis shows that the NDR effect and the unsaturated MR effect can be explained on the basis of the same physical mechanism of the avalanche breakdown in the In/SiO2/p-Si hetero junction. The NDR effect mainly results from complex effects containing the carrier injection after avalanche breakdown in the hetero junction, the reduction of the Schottky barrier height and the Joule heating effect. The large unsaturated MR effect derives from the suppression of the plasma due to avalanche breakdown by the applied magnetic field. The largest value of MR based on avalanche breakdown can reach about 530%@1 T at 10 K, and MR sensitivity can be as high as about 29.4 T−1 under 0.05 T.

Electrical Joining of Paper-Based Multilayer Magneto-Resistive Sensor Stacks

Abstract With the increase in the new development of paper electronics, there is a great demand for paper-compatible and reliable electrical joining techniques. In this work, we particularly addressed the interconnecting of multilayers of paper-based magneto-electronics. We deployed three room-temperature electrical joining techniques: (i) through-paper via, (ii) mechanical caulking, and (iii) collapsible daisy chain to fabricate an anisotropic magnetoresistive five-layer thin film sensor stack for planar rotary motion sensing. We studied the interplay between the electrical joining technique and the sensor characteristics such as magnetoresistive sensitivity and asymmetries in the sensor signal within the magnetic field strength domain. Despite process complexity and the precision limitations of manual machining and positioning, deployed in this work, the sensor stacks prepared by the through-paper via technique exhibited the closest uniformity in the magnetization planes across the stack, and hence the highest cumulative magnetoresistive sensitivity and lowest unfavorable asymmetries. Last, using peeling tests, we verified the mechanical reliability of the interconnects prepared by the through-paper via and collapsible daisy chain techniques.

Directionally Sensitive Magnetoresistance

Directionally Sensitive Magnetoresistance

Manipulation of free-layer bias field in giant-magnetoresistance spin valve by controlling pinned-layer thickness

The manipulation of the bias field of the free-layer in giant magnetoresistance spin-valves is of great importance in sensor applications because this feature dominantly affects the low-field sensitivity of magnetoresistance. In this study, it is demonstrated that the bias field of the free-layer can be manipulated by controlling the thickness of the pinned-layer deposited afterward. The key to success is the utilization of the magnetostatic interactions between the free-poles formed on the Néel walls in both free- and pinned-layers. Magnetostatic interactions play a role in stabilizing the antiparallel magnetization state and hence in suppressing the magnetization switching of the free-layer from an antiparallel to a parallel state. A nearly zero bias field is achieved for a Ta-buffered sample with a pinned-layer thickness of 1.75 nm, where a very high low-field sensitivity of 7.7 mV/mA·Oe is obtained.

Fabrication of Stannate Perovskite Structure as Optoelectronics Material: An Overview

This paper presents a review of recent fabrication progress of perovskite-type material suited for the future optoelectronics applications. Wide varieties of optoelectronic devices include solar cell, liquid displays, transparent FETs, etc are becoming the mainstream for the future electronics global industry. In June 2015, the major breakthrough of perovskite structure in solar energy harvesting with PCE of 20.1% has achieved. Since then, numerous research has been conducted progressively to further enhance the performance of the perovskite structure as new alternative materials for optoelectronics applications. The perovskite-type oxide is having typical ABO3 crystallized structure. It is one of an important class of materials that have many exceptional physical properties such as superconductivity, colossal magnetoresistance, ferromagnetic, piezoelectric, high-transition-temperature superconductivity, ferroelectricity, piezoelectricity, and photoelectrochemical sensitivity. In this paper, we reviewed development progress one of the major classes of perovskite-type materials namely Stannate-based. Calculated data from simulation results such as DFT and first principle were excluded and only fabricated devices are covered in this paper.

La–Sr–Mn–Co–O Films for High Pulsed Magnetic Field Measurements at Cryogenic Temperatures

The magnetoresistance of nanostructured La1– x Sr x (Mn1– y Co y ) z $O_{3 \pm \delta}$ (La–Sr–Mn–Co–O) films with substitution of Co for Mn with amount of Co/(La + Sr) = 0.12 and different Mn excess Mn/(La + Sr) = 1.05, 1.07, and 1.11 was investigated at temperatures of 4–230 K in pulsed magnetic fields up to 60 T. It was found that the manganite–cobaltite films exhibit larger magnetoresistance in comparison with manganite films without Co doping. The largest magnetoresistance values and sensitivity to the magnetic field are obtained for La–Sr–Mn–Co–O films having Mn content close to the stoichiometric ratio for manganites: Mn/(La + Sr) = 1.05. It was found that magnetoresistance at high fields (20–60 T) has a minimum at (50–80 K) and increases with the decrease of temperature. The possibility to use these films for magnetic field measurements at cryogenic temperatures is demonstrated.

Organic Single-Crystal Spintronics: Magnetoresistance Devices with High Magnetic-Field Sensitivity.

Organic spintronics is a new emerging field that deals with the spin-related phenomena of organic materials under the influence of a magnetic field. However, there remain some challenges in organic spintronics including (i) low conductivity and massive disorders of organic thin films blocking the way to controllable spin transport, (ii) relatively low magnetic-field sensitivity of organic magnetoresistance (OMAR) devices with tangled working mechanisms and short of methods for sensitivity improvement. Here, we report the realization of OMAR devices based on organic single crystals. The lesser amount of impurities and defects in crystals guarantees a reduction in spin and charge scatterings, so that the OMAR devices exhibit both a small Lorentz function fitting parameter B0 of 2.3 mT and a non-Lorentz function fitting parameter B1 of 0.86 mT in the strictly limited bipolaron model. Moreover, we demonstrate the effect of aggregation and intrinsic trap states, pointing out a way for the improvement of the sensitivity.

High-Sensitive Sensing Elements Based on Spin Valves with Antiferromagnetic Interlayer Coupling

Spin valves Ta/(Ni80Fe20)60Cr40/Co70Fe20Ni10/Cu/Co70Fe20Ni10/Ru/Co70Fe20Ni10/Fe50Mn50/Ta have been prepared by magnetron sputtering. It was found that the shift of low-field hysteresis loop with respect to H = 0 oscillates as the copper layer thickness changes. Structural studies showed the high perfection of the layer microstructure. Films of spin valves with the copper layer thickness corresponding to the second antiferromagnetic RKKI interaction maximum have been synthesized. They exhibit zero shift of low-field hysteresis loop and are characterized by high magnetoresistance effect. Sensing elements in the form of meanders prepared using the films demonstrate the almost anhysteretical field dependence of magnetoresistance and a magnetoresistive sensitivity of 0.5%/Oe.

Separating interface magnetoresistance from bulk magnetoresistance in silicon-based Schottky heterojunctions device

Nonmagnetic semiconductor based magnetoresistance (MR) devices combining high performance and low cost have attracted a lot of attention. However, it has been a great challenge to separate the interface MR from the bulk MR in the devices composed of Schottky heterojunctions. In this paper, the MR effect of a silicon-based Schottky heterojunction device had been studied, and its mechanisms were investigated by separating the interface MR effects from the bulk MR effects through combining two-probe and four-probe methods. We find that the bulk MR value is significantly smaller than the total MR value in the avalanche breakdown region in the temperature range of 150 K to 300 K, indicating that the total MR effect mainly originates from the interface MR effect. Theoretical analysis shows that the bulk MR effect is a normal one due to the existence of the Lorentz force on the carriers, and the interface MR effect relates to the suppression of the local plasmas by applying magnetic fields, where the local plasmas form due to the avalanche breakdown in the Ag/SiO2/p-Si Schottky heterojunctions. The total MR effect at room temperature can be further enhanced by reducing the distance between electrodes, and the total MR reaches about 1847% under a magnetic field of 1 T and the MR sensitivity is as large as 118.5 T−1 under 0.1 T.

Tailoring of magnetic properties of giant magnetoresistance spin valves via insertion of ultrathin non-magnetic spacers between pinned and pinning layers

The low-field sensitivity of a giant magnetoresistance (GMR) spin valve can be enhanced by tailoring the bias field of the free layer because this sensitivity and bias field are known to show a strong correlation. In this study, the free-layer bias field is reduced considerably to almost zero via the insertion of an ultrathin nonmagnetic spacer between the pinned layer and the pinning layer. The spacer promotes an increase in the density of Néel walls in the pinned layer. This increase, in turn, induces domain-wall-induced magnetostatic interactions of the free poles formed on the Néel walls inside the free and pinned layers. The magnetostatic interactions result in the formation of flux closures that act as pinning sites during the magnetization reversal process and stabilize the antiparallel magnetization state between the free layer and the pinned layer by suppressing the switching of the free layer from the antiparallel state to the parallel state. Furthermore, the spacer offers an additional advantage of increasing the GMR ratio by inducing a specular scattering effect at its top and bottom interfaces. A highly improved low-field sensitivity of 12.01 mV/mA·Oe is achieved in the sample with a Cu/Pt dual spacer.

A decay time approach for linear measurement of electrical conductivity

Nitrogen contamination of weld material affects mechanical properties and results in small change of the electric conductivity. This small difference of resistivity needs to be measured since the material property is affected by the nitrogen contamination level. A major challenge in determining the contamination level is the accurate measurement of resistivity in the presence of lift-off variation, top oxide layer and surface irregularities. In this paper, a novel approach based on Decay Time (DT) using Pulsed Eddy Current (PEC) and a high sensitive magnetoresistive (MR) sensor is proposed to measure small changes in electrical conductivity of materials. The DT parameter defined herewith is linearly correlated with material conductivity allowing it to be used for estimating the conductivity from transient PEC measurements. Moreover, this method is less sensitive to lift-off variations. The operation principle of this method is demonstrated using finite element model (FEM) simulation and validated using a prototype probe comprising an excitation coil and MR sensor. Weld metal samples with varying levels of nitrogen contamination are tested using the prototype probe demonstrating the feasibility of the method.

Design Optimization of an Anisotropic Magnetoresistance Sensor for Detection of Magnetic Nanoparticles

Recent studies have shown that the magnetic field sensitivity of an anisotropic magnetoresistance (AMR) sensor using a single-layer Ni80Fe20 thin film can be considerably improved by increasing the shape anisotropy of the film. In this work, an effective approach for improving the sensitivity and reducing the magnetic coercive field as well as the thermal noise contribution in an AMR Wheatstone bridge sensor is proposed by combining multiple resistors in the series–parallel combination circuits. Four different AMR sensor designs, consisting of a single resistor, three and five resistors in series and six resistors in series–parallel connection, were fabricated by using Ta (10 nm)/Ni80Fe20 (5 nm)/Ta (10 nm) films grown on thermally oxidized Si substrates under the presence and the absence of a biasing magnetic field (900 Oe). The results showed that the sensors based on series–parallel combination gain a magnetic sensitivity (SH) 1.72 times higher than that of the sensor based on the series connection. This optimized sensor has improved the capacity of detecting various concentrations of magnetic nanoparticles with a detection limit of magnetic moments estimated to be about 0.56 μemu.