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Spin-torque diode have shown great potentials and performance in many applicative fields, from microwave detectors to energy harvesters. In this work, we use micromagnetic simulations to study, at room temperature, a state-of-the-art non-resonant low-frequency-tail spin-torque diode in terms of dc output voltage as a function of the amplitude of an in-plane external field applied along different directions. We find that there exists a threshold value of the injected ac current that promotes a linear behavior of the output voltage of field down to the pT range, and we suggest exploiting such a behavior for the design of a magnetic field sensor. [ABSTRACT FROM AUTHOR]

Magnetic sensors are key elements in many industrial, security, military, and biomedical applications. Heusler alloys are promising materials for magnetic sensor applications due to their high spin polarization and tunable magnetic properties. The dynamic field range of magnetic sensors is strongly related to the perpendicular magnetic anisotropy (PMA). By tuning the PMA, it is possible to modify the sensing direction, sensitivity and even the accuracy of the magnetic sensors. Here, we report the tuning of PMA in a Co2MnGa Heusler alloy film via argon (Ar) ion irradiation. MgO/Co2MnGa/Pd films with an initial PMA were irradiated with 30 keV 40Ar+ ions with fluences (ions·cm−2) between 1 × 1013 and 1 × 1015 Ar·cm−2, which corresponds to displacement per atom values between 0.17 and 17, estimated from Monte-Carlo-based simulations. The magneto optical and magnetization results showed that the effective anisotropy energy (Keff) decreased from ~153 kJ·m−3 for the un-irradiated film to ~14 kJ·m−3 for the 1 × 1014 Ar·cm−2 irradiated film. The reduced Keff and PMA are attributed to ion-irradiation-induced interface intermixing that decreased the interfacial anisotropy. These results demonstrate that ion irradiation is a promising technique for shaping the PMA of Co2MnGa Heusler alloy for magnetic sensor applications. [ABSTRACT FROM AUTHOR]

The electric-field modulation of magnetization switching is an energy-efficient method for the design of potential spintronic devices. In this study, a magnetic tunnel junction (MTJ)/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) magnetoelectric heterostructure with the memory configuration of the MTJ was constructed for electric-field control of magnetization switching through angular-dependent magnetotransport measurements. It was observed that the electric-field control of tunneling magnetoresistance (TMR) depends on the direction of the applied magnetic field, and the corresponding tunability (T) of the TMR ratio will also change as the angle between the applied magnetic field and the x-axis changes. When a maximal electric field of 8.6 kV/cm is applied to the PMN-PT layer with a magnetic field (~50 Oe) along the major axis direction of the MTJ (i.e., PMN-PT [100]), the tunability Tmajor (the subscript refers to the major axis) is approximately +0.17‰. However, when a magnetic field (~50 Oe) is applied along the minor axis direction of the MTJ (i.e., PMN-PT [01-1]), the tunability Tminor is approximately −1.8‰. Furthermore, the tunability Tminor (the subscript refers to the minor axis) first increases with a positive sign and then decreases with a negative sign as the applied magnetic fields increase. It further increases negligibly as the magnitude of the applied magnetic field increases. The corresponding transitional region is in the range of approximately 15–40 Oe. It is conjectured that the competition among electric-field-induced magnetoelastic anisotropy, magnetic shape anisotropy, and Zeeman energy induced by the external magnetic field contributes to the magnetization switching of the free layer in the MTJ. This results in the aforementioned electric-field control of the TMR behavior. The findings from this study can help in understanding the mechanism of electric-field-induced magnetic switching in storage-mode MTJs through strain-mediated magnetoelectric coupling. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs), a prominent type of spintronic device based on the spin valve effect, have facilitated the development of numerous spintronic applications. The technical appeal for the next‐generation MTJ devices has been proposed in two directions: improving device performance by utilizing advanced two‐dimensional (2D) ferromagnetic materials or extending device functionalities by exploring the gate‐tunable magnetic properties of ferromagnets. Based on the recent development of 2D magnets with the ease of external stimuli, such as electric field, due to their reduced dimensions, reliable prospects for gate‐tunable MTJ devices can be achieved, shedding light on the great potential of next‐generation MTJs with multiple functionalities for various application environments. While the electrical gate‐tunable MTJ device is highly desirable for practical spintronic devices, it has not yet been demonstrated. Here, we demonstrate the experimental realization of a spin valve device by combining a vertical Fe3GeTe2/h‐BN/Fe3GeTe2 MTJ with an electrolyte gate. The magnetoresistance ratio (MR ratio) of 36% for the intrinsic MTJ confirms the good performance of the device. By electrolyte gating, the tunneling MR ratio of Fe3GeTe2/h‐BN/Fe3GeTe2 MTJ can be elevated 2.5 times, from 26% to 65%. Importantly, the magnetic fields at which the magnetoresistance switches for the MTJ can be modulated by electrical gating, providing a promising method to control the magnetization configuration of the MTJ. Our work demonstrates a gate‐tunable MTJ device toward the possibility for gate‐controlled spintronic devices, paving the way for performing 2D magnetism manipulations and exploring innovative spintronic applications. [ABSTRACT FROM AUTHOR]

We investigated the effects of sputtering conditions for the deposition of an Iridium (Ir) layer in a [Co/Pt]m/Co/Ir/[Co/Pt]n/Co/W/CoFeB synthetic antiferromagnetic reference layer (Ir-SyF) on the magnetic properties and tunnel magnetoresistance ratio (TMR ratio) of magnetic tunnel junctions (MTJs) stacks annealed at 400 °C for 1 h. The exchange coupling field ($H_{\mathrm {ex}}$) of Ir-SyF was improved by reducing the energy of Ir recoil ions and two times larger than that with Ru-SyF. Energy dispersive X-ray (EDX) spectrometry line analysis revealed greater interlayer diffusion in Ir when Ir was sputtered by using a conditions with large recoiled energy. This could cause the deterioration of the $H_{\mathrm {ex}}$ of the Ir-SyF. Despite the larger $H_{\mathrm {ex}}$ , the TMR ratio of the MTJ with Ir-SyF is smaller than that with Ru-SyF. The $m$ – $H$ curve of MTJ with Ru-SyF showed a large plateau region around zero magnetic field, whereas that with Ir-SyF did not. These results indicated the degradation of perpendicular magnetic anisotropy (PMA) in the top part of the Co/Pt multilayer with CoFeB reference layer and a large biquadratic coupling effect in the thin Ir layer. This causes the deterioration of the TMR ratio of the MTJ with Ir-SyF. TEM image of the Co/Pt layer in the MTJ with Ir shows some lattice defects. The EDX line analysis revealed that a large amount of Pt in the top Co/Pt layer diffused toward CoFeB reference layer in the Ir-SyF, resulting in the degradation of PMA. The structural analysis by X-ray diffraction showed the lattice spacing of CoPt (111) in Ir-SyF to be larger than that in Ru-SyF, indicating the occurrence of strain relaxation at the Co/Pt interface. These crystallographic changes in Ir-SyF might be related to a larger Pt diffusion. Suppression of Pt diffusion as well as low damage Ir deposition in the reference layer is crucial to utilize Ir-SyF. [ABSTRACT FROM AUTHOR]

Synthetic antiferromagnets (SAFs), formed through an interlayer antiferromagnetic exchange coupling of ferromagnetic layers, exhibit intriguing potential in next‐generation spintronic devices due to the zero net magnetization and the high thermal stability. Compared to ferromagnets that are conventionally widely employed, SAFs are expected to significantly improve the data density and stability due to the suppression of the net stray field. Thus far, it has been well established that the spin‐orbit torque (SOT) switching of SAF requires an in‐plane effective magnetic field to break inversion symmetry and thereby assist the Landau–Lifshitz dynamics. This field can either be externally applied or achieved through the exchange coupling given by an adjacent ferromagnetic layer. Such requirement hinders the application of SAFs since a net magnetization cannot be ruled out. Here, the field‐free SOT switching of an all‐SAF system with a significantly reduced net magnetization is demonstrated. The switching is demonstrated to be robust up to ≈460 K. This work paves the way for the practical applications of the SAFs in the SOT‐based memory devices. [ABSTRACT FROM AUTHOR]

The integration of data sensing and processing within a single device is of great significance in the future intelligent application scenarios due to its advantages in reducing power consumption and time delay. Nevertheless, most of the current investigations focus on sensing a single type of external stimuli, which is a hindrance for an accurate and complete sensing and processing of information. Here, based on the combination of optically controlled phase-change material (vanadium dioxide, VO2) and magnetoresistive sensing component (magnetic tunnel junction, MTJ), a device with in-sensor computing ability of optical and magnetic signals is demonstrated. To ensure the functionality integration, the thickness of VO2 is optimized in terms of resistance matching with MTJ. The device exhibits multiresistance states under the stimuli of light illumination and magnetic field. Further, the reconfigurable logic functions of the two types of captured information are achieved, realizing a multifunctional sensory processing integrated system. This work could pave the way for high-performance intelligent sensing and the Internet of Things. [ABSTRACT FROM AUTHOR]

We developed perpendicular magnetic tunnel junctions (MTJs) with four synthetic anti-ferromagnetically coupled Co/Pt layers (quad-SyF) and investigated their magnetic and transport properties. The quad-SyF comprised four Co/Pt layers and three 0.9 nm-thick Ru coupling layers, which consisted of Co/[Co/Pt] ${}_{a}$ /Ru/Co/[Co/Pt] ${}_{b}$ /Ru/Co/[Co/Pt] ${}_{c}$ /Ru/Co/[Co/Pt] ${}_{d}$ from top to bottom. The exchange coupling field ($H_{\mathrm {ex}}$) reached a maximum of 1 T when the values of $a$ , $b$ , $c$ , and $d$ were 1, 2, 2, and 1, respectively. The tunnel magnetoresistance ratio of the MTJ with the quad-SyF and the second-peak conventional double-SyF increased as the annealing temperature was increased up to 400 °C, whereas that of the MTJ with the first-peak conventional double-SyF degraded at temperatures of more than 350 °C in blanket films. A 55 nm diameter MTJ with quad-SyF was found to be stable even against an external magnetic field up to 300 mT. On the contrary, in the conventional double-SyF, the reference-layer (RL) magnetization direction flips at around 250 mT. The shift magnetic field of the MTJ with quad-SyF becomes approximately zero when the values of $a$ , $b$ , $c$ , and $d$ are 1, 4, 1, and 2, respectively. No back-hopping of the MTJ with quad-SyF was observed even for the write pulsewidth ($t_{W}$) down to 10 ns. On the contrary, an MTJ with conventional double-SyF exhibited back-hopping. In the patterned MTJ with conventional double-SyF, as the MTJ size decreases, the coercive field of Co/Pt significantly increases and $H_{\mathrm {ex}}$ decreases, causing the $m$ – $H$ curve of the RL to cross the zero magnetic field. This enables both parallel and antiparallel configurations for the top and bottom Co/Pt layers in double-SyF at the zero magnetic field, which could induce back-hopping. However, the $m$ – $H$ curves of the RL in the patterned MTJ with quad-SyF are far from the zero magnetic field owing to the high $H_{\mathrm {ex}}$ and low $H_{c}$ , which could lead to the suppression of back-hopping. [ABSTRACT FROM AUTHOR]

Highlights • Magnetic tunnel junction comprised of free Heusler alloy and pinned CoFe • Finite open-circuit voltage develops on the order of several 10 μ V in the anti-parallel state • Reversal of magnetic dipolar field generates an electromotive force of order 10 μ V Abstract We investigate the tunneling magnetoresistance (TMR) of the magnetic tunnel junction (MTJ) comprised of the Heusler alloy ( Co 2 MnSi) and the pinned CoFe to probe the spin polarization of Co 2 MnSi. From the TMR value thus obtained, we estimate 40 % polarization of Co 2 MnSi , much less than 100 % expected for the suspected half metal. Remarkably, upon applying the weak switching magnetic field to the MTJ in the parallel state, our system exhibits a finite open-circuit voltage on the order of several 10 μ V in the anti-parallel state. We have demonstrated that the reversal of magnetic dipolar field induced by the Heusler alloy as a free layer can generate an electromotive force of the same order and can clearly explain the observed polarity and sample size dependence of the voltage offset at zero bias current. [ABSTRACT FROM AUTHOR]

The processing-in-memory (PIM) paradigm has been considered as a promising alternative to break the bottlenecks of conventional von-Neumann architecture by realizing the unity of data storage and processing in the same die. On the road toward implementing such an architecture, finding a novel memory that can support both dense data storage and efficient logic processing is the critical step. In this letter, we report a voltage-controlled magnetic tunnel junction (MTJ), which is a potential candidate for PIM implementation. Stateful Boolean logic functions can be realized with a single device through the memory-like write/read operations. The device was fabricated and characterized at room temperature. Afterwards, typical Boolean logic operations, e.g., ?OR?, ?AND?, and ?NXOR?, were experimentally demonstrated with the fabricated MTJ device. The proposed approach opens up a new way for PIM implementation in spintronic memories. [ABSTRACT FROM PUBLISHER]

Usually, charge and spin transport properties in tunnel junctions are studied in the DC bias regime and/or in the adiabatic regime of time-varying magnetic fields. In this letter, the temporal dynamics of charge and spin currents in a tunnel junction induced by pulsed magnetic fields is considered. At low bias voltages, energy and momentum of the conduction electrons are nearly conserved in the tunneling process, leading to the description of the junction as a spin-1/2 fermionic system coupled to time-varying magnetic fields. Under the influence of pulsed magnetic fields, charge and spin current can flow across the tunnel junction, displaying oscillatory behavior, even in the absence of DC bias voltage. A type of spin capacitance function, in close analogy to electric capacitance, is predicted. [ABSTRACT FROM AUTHOR]

We report low-frequency electrical resistance noise of Co40Fe40B20/MgO/Co20Fe60B20-based magnetic tunnel junction field sensors with reference and sensing layer magnetization directions along the out-of-plane and in-plane directions, respectively. The devices are fabricated using the sputter deposition and conventional lithography techniques with a short axis of \sim 6~\mu \textm long and long axes of \sim 6 –12 \mu \text{m} long. Noise power spectra under different bias currents indicate increase in noise power with increasing bias current. A close 1/ f dependence of the noise according to Hooge’s relation, SV(f)= AV^{2}/f^{\alpha } ( A is Hooge’s like constant) is observed with an average value of \alpha = 1.03 \pm 0.08 . In smaller devices, the magnetic field dependence of noise amplitude follows similar trend of magnetoresistance (MR) behavior without any distinct peak near antiparallel–parallel states transition of the devices, which is generally observed for magnetic defects-induced resistance fluctuation. The experimental observations infer the resistance noises in those devices that are related to the defects associated with the spin independent charge trapping at structural defects near or in the barrier layer. The device with the smallest lateral size exhibits the Lorentzian contribution, whereas the device with the largest size shows magnetic contribution in total noise. Numerically deduced magnetic fluctuations \Delta B_{\mathrm{rms}} of all the devices shows an average magnetic fluctuation of \sim 10$ nT except for one device. The devices show the highest MR% of $\sim 27$ % under perpendicular magnetic field with a dynamic range of $\sim \pm 25$ Oe and a sensitivity of $\sim 0.3$ %/Oe. The observed low-frequency noise levels are suitable for the perpendicular field detection application and can be reduced more by improving the crystallinity of the device films stacks and their interfaces. [ABSTRACT FROM PUBLISHER]

Tunneling magnetoresistance (TMR) phenomena in hybrid Fe/GaAlAs/GaMnAs magnetic tunnel junctions (MTJs) were investigated by rotating a magnetic field of constant strength in the film plane. When a strong field (e.g., 4000 G) is used, the magnetization in GaMnAs and Fe coherently rotates in both layers, resulting in a smooth angular dependence of TMR. In contrast, abrupt transition steps and plateaus are observed in TMR, when a weak field (below 100 G) is rotated. The behavior observed in strong fields is ascribed to tunneling anisotropic magnetoresistance, an effect that occurs when magnetizations in both magnetic layers in the MTJ are aligned parallel to each other. The tunneling behavior observed in weak fields, on the other hand, is caused by differences in relative magnetization alignments in the two layers that arise from differences in their magnetocrystalline anisotropies. The latter behavior provided the anisotropic TMR that involved with parallel and antiparallel alignments at specific crystallographic directions. [ABSTRACT FROM PUBLISHER]

We investigate magnetic properties of CoFeB/Ta/[Co/Pd (Pt)] multilayers and properties of magnetic tunnel junctions (MTJs) with the structures as a recording layer. CoFeB/Ta/[Co/Pd] multilayer shows a lower damping constant $\alpha $ than that of CoFeB/Ta/[Co/Pt] multilayer. We evaluate current-induced magnetization switching (CIMS) properties of the MTJs with CoFeB/Ta/[Co/Pd (Pt)] multilayers with the junction diameter of 15 (13) nm that show similar thermal stability factor. CIMS is observed for the MTJ with a CoFeB/Ta/[Co/Pd] multilayer at zero magnetic field, whereas it is observed for the MTJ with the CoFeB/Ta/[Co/Pt] multilayer only in the presence of an external field. [ABSTRACT FROM PUBLISHER]

Abstract 1 1 Ferromagnetic resonance (FMR), magnetic tunnel junctions (MTJ). We studied the rectification function of microwaves in CoFeB/MgO-based magnetic tunnel junctions using voltage-induced ferromagnetic resonance (FMR). Our findings reveal that the shape of the structure of the spectrum depends on the rotation angle of the external magnetic field, providing clear evidence that FMR dynamics are excited by voltage-induced magnetic anisotropy changes. Further, enhancement of the rectified voltage was demonstrated under a DC bias voltage. In our experiments, the highest microwave detection sensitivity obtained was 350 mV/mW, at an RF frequency of 1.0 GHz and field angle of θ H =80°, ϕ H =0°. The experimental results correlated with those obtained via simulation, and the calculated results revealed the magnetization dynamics at the resonance state. [ABSTRACT FROM AUTHOR]

We systematically measured the spin torque diode spectra of a magnetic tunnel junction nanoelement as a function of external magnetic field and field angle using a homemade quasi-omnidirectional broadband electrical spectrometer. The electrical spectroscopies reveal a significant higher-order spin wave modes excited in the free layer, which is substantiated by an analytical model. [ABSTRACT FROM PUBLISHER]

Abstract: Different phenomena that give rise to a spin-polarized current in some systems with magnetic tunnel junctions are considered. In a manganite-based magnetic tunnel structure in CIP geometry, the effect of current-channel switching was observed, which causes bias-driven magnetoresistance, rf rectification, and the photoelectric effect. The second system under study, ferromagnetic/insulator/semiconductor, exhibits the features of the transport properties in CIP geometry that are also related to the current-channel switching effect. The described properties can be controlled by a bias, a magnetic field, and optical radiation. At last, the third system under consideration is a cooperative assembly of magnetic tunnel junctions. This system exhibits tunnel magnetoresistance and the magnetic-field-driven microwave detection effect. [Copyright &y& Elsevier]

Abstract: The effect of magnetically driven high-frequency rectification in a polycrystalline La0.7Ca0.3MnO3 manganite has been measured at different frequencies of microwave radiation. The magnetic field dependence of a rectified voltage has a broad peak resembling an absorption line, whose shape and position are determined by the radiation frequency. The rectification effect in a polycrystalline manganite sample is related to a ramified network of magnetic tunnel junctions, which is formed by ferromagnetic conducting grains with insulator boundaries. The results of measurements are consistent with a model for the magneto-dependent rectification effect based on the interplay between a spin-polarized current through the tunnel junctions and magnetic resonance induced in the grains forming the junctions. [ABSTRACT FROM AUTHOR]

Devices consisting of single or double magnetic tunnel junctions (MTJs) with cross-magnetization configurations were proposed to enable simultaneous electrical detection of both the structure and motion of a domain wall (DW), and the operation of these devices was confirmed through micromagnetic simulation. Through the tunnel magnetoresistance (TMR) effect of the MTJs, two types of domain-wall structure formed in a ferromagnetic wire were clearly identified: a transverse wall (TW) in which the magnetization at the center of the wall is directed transverse to the wire axis, and a vortex wall (VW) in which the magnetization circulates in the plane about a small perpendicular vortex core. In addition to the structural difference between TW and VW, the velocity of the DW motion was detected through the time response of the TMR. [ABSTRACT FROM AUTHOR]

Abstract: The minimum applied field HSW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power PHP and duration δ. For the same power of the heating pulse PHP (or, equivalently, for the same temperature of the storage layer), HSW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of HSW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application. [Copyright &y& Elsevier]

Abstract: An angular magnetic field sensor for automotive applications based on magnetic tunnel junctions has been designed. Using magnetic tunnel junctions, the power consumption has been reduced by a factor of 1000 with respect to the one based on Hall sensors and the working distance has been increased. The standard Wheatstone bridge configuration is replaced by an electrical circuit based on a current loop layout. Using this configuration, the balancing of resistance is not necessary as for a Wheatstone bridge that can be a bottleneck when using magnetic tunnel junctions. The field to be measured is produced by a multipolar magnetic seal integrated in a wheel bearing. To obtain an angular sensor, four blocks of tunnel junctions in series have been used and with an appropriate sensor design, the two signals with a 90° phase shift required for an high accuracy angular measurement have been obtained. [Copyright &y& Elsevier]

Abstract: We comment on both recent progress and lingering puzzles related to research on magnetic tunnel junctions (MTJs). MTJs are already being used in applications such as magnetic-field sensors in the read heads of disk drives, and they may also be the first device geometry in which spin-torque effects are applied to manipulate magnetic dynamics, in order to make non-volatile magnetic random access memory. However, there remain many unanswered questions about such basic properties as the magnetoresistance of MTJs, how their properties change as a function of tunnel-barrier thickness and applied bias, and what are the magnitude and direction of the spin-transfer-torque vector induced by a tunnel current. [Copyright &y& Elsevier]

Scanning tunneling microscope (STM) images of highly charged ion (HCI) synthesized nano-features formed on the Au(111) surface are presented as well as transport studies of HCI modified magnetic tunnel junctions (MTJs). HCIs can carry a significant amount of neutralization energy (51 keV per ion for Xe44+, used in most of the results presented) in addition to their kinetic energy. This potential energy amplifies the violence of the HCI's interaction with the surface by generating massive secondary electron and sputtered atom yields and can displace thousands of substrate atoms. This paper presents STM images of surface features observed on the Au(111) surface with emphasis on the characteristics of the physical structure and formation mechanisms. In separate experiments, the HCIs are used to ablate small portions of the oxide barrier in MTJs to allow nano-scale metal shunts to be created through the barrier. Electronic measurements of HCI irradiated MTJs demonstrate that the MTJ total conductance increases linearly with the HCI dose for thin barriers and quadratically for thicker barriers. Initial results from a second generation of MTJs demonstrate that magnetoresistance is diminished but not destroyed when HCIs are used to modify MTJ barrier layer. [ABSTRACT FROM AUTHOR]

Abstract: Careful measurements of highly charged ions (HCIs) colliding with gases and surfaces have provided glimpses of intense electronic interactions, but a comprehensive model for the interaction mechanisms, time scales, and resultant nano-features that bridges materials systems is yet to be realized. At the National Institute of Standards and Technology (NIST) electron beam ion trap (EBIT) facility, new apparatus is now connected to the HCI beamline to allow preparation of clean, atomically flat surfaces of single crystals, e.g. gold, tungsten and silicon, and deposition and patterning of thin films, e.g. high resistivity oxides, ferromagnetic metals, normal metals and superconductors. Experiments reported here focus on the electronic and morphological structure of HCI induced nano-features. Current activities are focused on using in situ scanning tunneling microscope (STM) on Au(111) and (separately) ex situ transport measurements to study electronic properties within HCI modified magnetic multilayer systems. Specifically, we are fabricating magnetic multilayers similar to magnetic tunnel junctions (MTJs) (important in advanced magnetic field sensors and superconducting Josephson junction devices) and using HCIs to adjust critical electronic properties. The electrical response of the tunnel junction to HCIs provides a novel approach to performing HCI-induced nanostructure ensemble measurements. [Copyright &y& Elsevier]

Abstract: Both single-barrier magnetic tunnel junctions (SBMTJs) and double-barrier magnetic tunnel junctions (DBMTJs) with an amorphous hardcore structure of Co60Fe20B20/Al–O/Co60Fe20B20 were microfabricated. A high TMR ratio of 102.2% at 4.2K was observed in the SBMTJs after annealing at 265°C for 1h. High TMR ratio of 56.2%, low junction resistance-area product RS of 4.6kΩμm2, small coercivity H C=25Oe, and relatively large bias-voltage-at-half-maximum TMR with the value V 1/2 greater than 500mV at room temperature (RT) had been achieved in such Co–Fe–B SBMTJs. Whereas, high TMR ratio of 60% at RT and 89% at 30K, low junction resistance-area product RS of 7.8kΩμm2 at RT and 8.3kΩμm2 at 30K, low coercivity H C=8.5Oe at RT and H C=14Oe at 30K, and relatively large bias-voltage-at-half-maximum TMR with the value V 1/2 greater than 1150mV at RT had been achieved in the Co–Fe–B DBMTJs. Temperature dependence of the TMR ratio, resistance, and coercivity from 4.2K to RT, and applied voltage dependence of the TMR ratio and resistance at RT for such amorphous MTJs were also investigated. [Copyright &y& Elsevier]

Abstract: Conventional magnetic sensors, easy to use, are supposed to work mainly well over the nanotesla range, as due to the large magnetic environmental noise occurring in urban and industrial environments. On the other hand, a strange world exists, well below the nanotesla range, where the very efficient magnetic properties of superconducting materials have been used. It is the world investigated using cryogenic sensors, especially those of the SQUID''s family. During a long time starting from the 1960s, SQUID people have refined their technologies, together with the use of advanced signal processing both analogue and digital, in order to input couple various external magnetic sources at room temperature, such the bio magnetic ones. State of the art of SQUID sensors is given. In the early 1990s, the dramatic improvement of the operating temperature led to the hope of lighter and lower costs systems with reduced cryogenic mount, designed to operate in open environment. An important target of multi SQUID systems using high critical temperature superconductors was and still is the magnetocardiography (MCG) mapping that could be daily used for improved diagnosis, as compared to conventional electrocardiography. It is known that such an important application is realistic only with noise spectral densities referred at the input lower than 100fT/√Hz in a frequency bandwidth lying in between 1Hz and 1kHz and with a spatial resolution lower than 1cm. The talk will review the recent advances in room temperature solid state sensors that could reach the above specified noise level. The review includes: magnetoresistive devices (AMR, GMR, spin valve, and spin dependent tunnelling device), Giant magneto-inductive devices. Non-solid, atomic vapor laser magnetometers, which have recently shown their ability to deliver very clear MCG signals, and start to be used to map the MCG signal above the chest just like SQUIDs systems, are reviewed. A simple, convenient energy resolution—volume is proposed, which allows a convenient way to compare high sensitivity magnetic sensors. [Copyright &y& Elsevier]

Abstract: In this work, we demonstrate that tunnel magnetoresistive (TMR) elements can be used as sensitive built-in current sensors in the μA–mA range for IC testing. The sensor can be integrated in CMOS and uses a technology similar to the magnetic random access memory (MRAM) technology; therefore the implementation of the sensor is expected to be cost-effective in ICs containing embedded MRAM. A sensitivity of 2.5 (mV/V)/mA and a current resolution of about 5μA have been achieved. The design, fabrication process and measurement results of different sensor geometries are presented and discussed. [Copyright &y& Elsevier]

Abstract: We present proof-of-concept experiments and modeling towards a high-sensitivity magnetic microarray which “tags” a DNA fragment (or other biological samples) with a high-moment magnetic nanoparticle (NanoTag), which is in turn detected by a high-sensitivity spin valve (SV) or magnetic tunnel junction (MTJ) detector array. The detector can count the number of magnetic tags with a resolution of 1–20 magnetic NanoTags, potentially counting individual biomolecules. [Copyright &y& Elsevier]

Local topographical and electrical properties were simultaneously measured for a magnetic tunnel junction formed by Ta (50 Å)/Ni-Fe (20 Å)/Cu (50 Å)/Mn75 Ir25 (100 Å)/Co70 Fe30 (40 Å)/Al-O (8-15 Å)/Co70 Fe30 (40 Å)/Ni-Fe (100 Å)Ta (50 Å). Local current-voltage (I-V) characteristic curves were obtained for different contrast levels in the electrical current distribution images on the test sample. With the purpose of obtaining quantitative values for the barrier characteristics, data was analyzed by the Simmons' equation from -1.0 to 1.0 V. The magnetoresistance ratio values were estimated to be 35.02%, with a bias voltage of 0.36 V, when applying a magnetic field of ±200 Oe. In addition, a study on the ramping effect on the dielectric tunneling capacitance and analytical resistance-capacitance (RC) model were carried out. [ABSTRACT FROM AUTHOR]

Abstract: We report the growth of ultra-thin indium oxide layers using the dc-magnetron sputtering method. We demonstrate that good quality tunnel barriers made of indium oxide can be routinely fabricated and employed in spintronic-related devices. Simple magnetic tunnel junctions (MTJs) were fabricated in a cross geometry using ex situ thermally evaporated cobalt and permalloy. Our best junctions obey the Rowell criteria for tunneling and exhibit a tunnel magnetoresistance of 15% at 100 K. [Copyright &y& Elsevier]

This paper experimentally verifies that for low bias voltages &lT; 2 kT/q, the noise of a saturated magnetic tunnel junction (MTJ) device is caused by thermally assisted barrier crossings (thermal noise), whereas for voltages &gT; 2 kT/q, it stems from field-assisted barrier crossings (shot noise). This can also be used as a criterion for an MTJ with a healthy, defect-free barrier. A new dual-channel ultralow-noise amplifier is described allowing noise measurements for very low resistance (large area) devices. A "barrier defect model" is developed and verified for noise, tunneling magnetoresistance ratio (TMR), and defective area, using healthy MTJ devices, which ate given controlled defects by means of pulsed electrical overstress. It is shown that MTJ devices are very electrostatic-damage prone; narrow voltage pulses (0.5 ns) can damage the sensors at relatively low voltages (1.5 V). Relationships are shown between pulse voltage, pulse energy, duration, number of pulses, and the drop in sensor resistance. Finally, the sensor signal-to-noise, bandwidth, and electronic readout are discussed. [ABSTRACT FROM AUTHOR]

We propose a two-input configuration of a magnetic tunnel junction (MTJ) for use as a programmable logic element. The MTJ intrinsically performs four logic functions—NAND, NOR, IMP, and NIMP—based on two physically independent inputs whose values are determined by magnetic field and the voltage bias. By applying the surface magnetoelectric effect, four distinct input states are determined to perform these logic operations through switching of the MTJ. With predetermined logic inputs, a single MTJ can serve as a programmable logic gate that represents all fundamental logic functions. [ABSTRACT FROM AUTHOR]

Spin-dependent tunneling structures are widely used in many spintronic devices and sensors. This paper describes the magnetic tunnel junction (MTJ) characteristics caused by the inhomogeneous magnetic field of ferromagnetic layers. The extremely oblate magnetic ellipsoids have been used to mimic these layers. The strong effect of an inhomogeneous magnetic field on the magnetoresistive layers' interaction was demonstrated. The magnetostatic coupling coefficient is also calculated. [ABSTRACT FROM AUTHOR]

We report on the magneto-transport measurements of MgO magnetic tunnel junctions (MTJs) composed of NiFeSiB/CoFeB as the free layer for two different structures (top-type and bottom-type pinning). The magneto-transport properties of these MTJs were investigated by varying the thickness of the amorphous NiFeSiB layer for a fixed C0FeB thickness. The tunnel magnetoresistance (TMR), measured in both type of structures, exhibit the same or a higher amplitude (up to 230% measured at room temperature in the case of top-type device), comparing to the case of a single CoFeB free layer. These results suggest that hybrids free layers can be used as good candidates for MTJs with reduced saturation magnetization while keeping a high TMR ratio. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) are currently replacing spin valves as read sensors in high density media. To be implemented, MTJs must have low resistance-area product (R × A) and reasonable tunnel magnetoresistance (TMR) which are usually obtained using ultra-thin barriers. The use of thicker, underoxidized AlOx barriers can also lead to similarly suitable MTJ-characteristics. Here we study the temperature dependence of the transport properties of 9 A underoxidized magnetic tunnel junctions, revealing the dominance of different transport mechanisms in different samples of the same wafer: tunnel, metallic or a mixture of the two, depending on MTJ- magnetic state. Such differences likely arise from slight structural differences inherent to the oxidation process. We further show that the MR-temperature dependence is stronger for MTJs displaying a metallic-like behavior (dR/dT > 0) and compare such behavior with the enhanced MR(T) observed in current perpendicular to the plane (CPP) spin valves. [ABSTRACT FROM AUTHOR]

Abstract: Switching behavior and spin configuration of single-free layer and SAF-free layer with rough edge and under stray field are studied with micromagnetic simulation using Landau–Lifshize–Gilbert equation. From the simulation results, the R–H loops of single free layer in MTJs have some kinks. However, junctions with SAF-free layers exhibit kink-free R–H loops. Because of the enclosed magnetic flux of the SAF-free layer, unbalanced SAF-free layer shows kink-free switching, even with pronounced fabrication defects at the memory element boundaries. [Copyright &y& Elsevier]

Abstract: We propose and demonstrate an edge magnetic tunnel junction (MTJ) structure that uses the edge part of the magnetic thin film and the barrier layer parallel to the edge surface. The shape of an edge MTJ becomes a rectangle whose area is defined as the product of the thickness of one magnetic thin film and the line width of another magnetic film. The thickness of a thin film is reduced in a controlled manner so that the edge junction structure can have a smaller area as well as a smaller variation in area. In the case of the edge junction, a rectangular aspect ratio is enlarged without increasing the area of the cell. An in situ ion milling etching apparatus is connected to the sputtering system in order to avoid the oxidation of the edge surface. In our fabrication process of the edge MTJs, magnetic electrodes are not exposed to water, oxygen plasma, or organic solvent. The edge MTJs consist of CoFeB/MgO/CoFeB. We obtain both a high magnetoresistance ratio (MR) of 71% and a small area of 0.3×0.04μm in the edge MTJ. [Copyright &y& Elsevier]

Abstract: The magnetoresistance effect for a magnetic tunnel junction (MTJ) with Co2FeAl0.5Si0.5 electrodes on Cr buffered MgO (001) substrate has been investigated. The MTJ with B2-type Co2FeAl0.5Si0.5 electrode exhibited tunnel magnetoresistance (TMR) ratio of 76% at room temperature (RT) and 106% at 5K, while that with L21 structure showed 51% and 78% at RT and 5K, respectively. For both cases, the bias voltage dependence of TMR ratio is almost the same, but only for the MTJs with a Co2FeAl0.5Si0.5 electrode annealed at 600°C shows a dip around −200mV. [Copyright &y& Elsevier]

Abstract: We investigated the influence of insulating barrier thickness and the Ti composition dependence of the band structure of Al-oxide on the resistance and tunneling magnetoresistance (TMR) behavior of the magnetic tunnel junction (MTJ). Low resistance × area (RA) value (1.1MΩμm2) was achieved by decreasing the Al-oxide thickness down to 1.0nm. However, this led to the partial oxidation of the bottom ferromagnetic (FM) electrode of the junction and non-continuous thin barriers by the occurrence of pinholes, with low TMR ratio of 8.3%. For an alternative for low RA value, we developed a new Ti-alloyed Al-oxide (TiAlO x ) that had lower band gap than Al-oxide as an insulating barrier of MTJ. As the Ti concentration increased up to 5.33at.% Ti in Al, the RA value of the MTJs was reduced from 9.5 to 0.69MΩμm2, owing to the band-gap reduction of TiAlO x caused by the formation of extra bands, mainly composed of Ti-3d orbitals, within the band gap. It was analyzed that TiAlO x has localized d states in the band gap below the conduction band. In addition, the TMR ratio increased with the Ti concentration and reached a maximum of 49% at 5.33at.% Ti owing to the microstructural evolution of Ti–Al alloy film in the pre-oxidation state. [Copyright &y& Elsevier]

• Circle-shaped magnetic tunnel junction (MTJ) have higher sensitivity at zero field. • Sputtering condition including process pressure and Argon flow rate of Mu-metal (NiFeCuMo) thin film could be optimized and achieve high magnetic susceptibility. • Argon flow rate mainly affect the coercivity of the NiFeCuMo thin film within range 5–20 sccm. • Process pressure of sputtering affect both coercivity and magnetic remanence. • Hysteresis behavior shows a trend of changing between hard axis switching and easy axis switching in range from 6 mTorr to 2 mTorr. Weak magnetic field detection is always a challenging topic and sensing systems face a great challenge to be very sensitive in response to magnetic field, in the meantime, to maintain very low overall system noise. Some external auxiliary system could provide significant enhancement to overall signal-to-noise ratio (SNR) such as a magnetic flux concentrator made with soft magnetic material. In this paper, we have systematically studied and built an integrated magnetic tunnel junction (MTJ) sensing system to meet these challenges. MTJ sensor design is optimized to achieve high zero-field sensitivity of 5% MR/Oe, which is defined by the slope of the device's transfer loops at zero field, equivalent to 20 mV/Oe with 10 mA driving current. Both deposition conditions and fabrication techniques are optimized for magnetic flux concentrator (MFC) made of soft magnetic material to achieve a magnetic susceptibility of χ = 30,800 and a gain of 4 for a 100 nm thick prototype. [ABSTRACT FROM AUTHOR]

Thanks to their high magnetoresistance and integration capability, magnetic tunnel junction-based magnetoresistive sensors are widely utilized to detect weak, low-frequency magnetic fields in a variety of applications. The low detectivity of MTJs is necessary to obtain a high signal-to-noise ratio when detecting small variations in magnetic fields. We fabricated serial MTJ-based sensors with various junction area and free-layer electrode aspect ratios. Our investigation showed that their sensitivity and noise power are affected by the MTJ geometry due to the variation in the magnetic shape anisotropy. Their MR curves demonstrated a decrease in sensitivity with an increase in the aspect ratio of the free-layer electrode, and their noise properties showed that MTJs with larger junction areas exhibit lower noise spectral density in the low-frequency region. All of the sensors were able detect a small AC magnetic field (Hrms = 0.3 Oe at 23 Hz). Among the MTJ sensors we examined, the sensor with a square-free layer and large junction area exhibited a high signal-to-noise ratio (4792 ± 646). These results suggest that MTJ geometrical characteristics play a critical role in enhancing the detectivity of MTJ-based sensors. [ABSTRACT FROM AUTHOR]

A new multibit magnetic random access memory (MRAM) cell of toggle switching type is presented. The multibit stack consists of multiple conventional toggle MRAM cells connected in series. These cells are identical except that their cell geometry, anisotropy, and pinned directions are oriented differently from each other. The selective write of the individual cell is achieved by using three phases of word and bit line current generating a contiguous magnetic field rotating past the easy axis of the targeted cell. Two-, four-, and six-bit toggle cell configurations are found to be feasible. This approach has the potential to greatly enhance the density of the MRAM cell for a given lithography ground rule. [ABSTRACT FROM AUTHOR]

This paper reports the magnetic, electrical, and microstructural properties of epitaxial thin films with an ordered double-perovskite structure. Sr2 FeMoO6 (SFMO) and Sr2 CrReO6 (SCRO) films have been grown by sputtering onto the lattice-matched substrates of Ba0.4 Sr0.6 TiO3 -buffered and bare SrTiO3, respectively. These films exhibit high saturation magnetization M8 values (3.8 μB/f.u. for SFMO and 0.9 μB/f.u. for SCRO), which are close to the expected values for their half-metallicity, and high curie temperature Tc values (385 K for SFMO and 620 K for SCRO). Surface analyzes by AFM and XPS indicate that these films have atomically flat and well-defined surfaces, which are free from any surface precipitates. This enables us to employ a standard photolithographic process for fabricating magnetic tunnel junctions based on these films. SFMO junctions with a native barrier formed by surface oxidation of a SFMO base-electrode and a Co counterelectrode have shown a tunnel magnetoresistance ratio of 10% at 4.2 K. [ABSTRACT FROM AUTHOR]

We studied the thermal stability of new amorphous ZrAl-based magnetic tunnel junctions (MTJs) with a ZrAl-oxide barrier replacing the Ta layers traditionally used for the under and capping layers. The MTJs were compared with similar conventional MTJs (Ta-based MTJ with Al-oxide). After annealing at various temperatures up to 450 °C, the ZrAl-based MTJs still had a significant tunnel magnetoresistance signal of nearly 21%. The thermal stability of amorphous ZrAl-based and conventional Ta-based MTJs differs dramatically, mainly because of the different microstructural evolution. The noncrystalline ZrAl-alloy film had superior surface uniformity and an induced microstructure that resisted interdiffusion, with dense, equiaxed grains making up the upper stacks' films. By contrast, the conventional Ta-based MTJ had a broad columnar structure with less dense boundaries, which act as a source of interdiffusion, resulting in barrier deformation at elevated temperatures. [ABSTRACT FROM AUTHOR]

The thermal and electrical stability of a magnetic tunnel junction (MTJ) with a Zr-alloyed Al-oxide (ZrAl-oxide) tunnel barrier were investigated. A significant tunneling magnetic resistance (TMR) value of 10% was maintained in the MTJ with a ZrAl-oxide barrier (9.89 at.% Zr) annealed at 450 °C. The breakdown voltage of 1.75 V was highest for the MTJ with a 9.89 at.% Zr-alloyed Al-oxide barrier. The quality of the barrier material microstructure in the pre-oxidation state had a dominant effect on the thermal and electrical stability of the MTJ. In addition, plasma oxidation of an ultrathin 0.6-nm-thick ZrAl barrier led to an MTJ with 8% TMR and a junction resistance × area (RA) of 0.72 MΩμm². [ABSTRACT FROM AUTHOR]

To improve the sensitivity of the magnetic tunnel junction(MTJ)sensor, a novel architecture for a double-gap magnetic flux concentrator (MFC) was studied theoretically and experimentally in this paper. The three-dimensional finite element model of magnetic flux was established to optimize the magnetic field amplification factor, with different gaps. The simulation results indicate that the sensitivity of an MTJ sensor with a double-gap MFC can be significantly better than that of a sensor with a traditional single-gap MFC, due to the fact that the magnetic magnification sharply increases with the decrease in effective gap width. Besides this, the half-bridge MTJ sensors with the double-gap MFC were fabricated using photolithography, ion milling, evaporation, and electroplating processes. Experimental results show that the sensitivity of the MTJ sensor increased by ten times compared to the sensor without the double-gap MFC, which underlines the theoretical predictions. Furthermore, there is no significant increase in the sensor noise. The work in this paper contributes to the development of high-performance MTJ sensors. [ABSTRACT FROM AUTHOR]

Abstract: Resistance change in tunneling magnetoresistive (TMR) devices due to mechanical stress was observed experimentally. Without any stripe-height change, the resistance rose up to 45% during lapping, and returned to its original value gradually after lapping. The resistance change was accompanied by changes in ΔR/R and breakdown voltage. Besides lapping, bias voltage and heat treatment (HT) both affect mechanical stress, hence the TMR resistance. [Copyright &y& Elsevier]

Abstract: The thermal stability and the spin transportation phenomenon at room temperature and 140 °C of a series of magnetic tunneling junctions with the structure of bottom electrode/PtMn/Pinned layer/ AlO x /CoFe/NiFe/top electrode have been investigated. The MR ratio decreases from 33.5% at room temperature to 29% at 140 °C. The MR ratio at room temperature increases roughly 0.8% after thermal treatment at temperatures above 60 °C. This is related to the thermal relaxation of the strains existing in the samples. [Copyright &y& Elsevier]

Abstract: Magnetic tunnel junctions (MTJs) consisting of Si/SiO2/Ta/Ru/IrMn/CoFe/Ru/CoFe/Al-O/CoFe/NiFe/Ru with different surface roughness of bottom electrode were prepared, and the dependence of tunneling magnetoresistance (TMR) ratio and resistance area product (RA) on surface roughness of tunnel barrier and the tunneling characteristics of these junction devices were investigated. The MTJ with rough tunnel barrier of 12Å root mean square roughness (R rms) showed TMR ratio of 4% and RA of 2.2kΩμm2. In contrast, the MTJ with uniform tunnel barrier of R rms=3Å showed TMR ratio of 40% and RA of 14kΩμm2. As MTJs had more uniform tunnel barrier, TMR ratio and resistance were higher, while the interlayer coupling field decreased. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJs. [Copyright &y& Elsevier]