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We applied Co20Fe60B20 layers with thicknesses from 1.2 to 1.5 nm to the sensing layers of MgO based magnetic tunnel junctions (MTJs) with spin-valve-type structures annealed at 350^\circC for magnetic field sensors. The CoFeB layer on MgO had in-plane magnetic anisotropy and also slight perpendicular magnetic anisotropy that originated at the MgO/CoFeB interface. MR characteristics of high reversibility and linearity were confirmed at a CoFeB thickness of 1.3 nm. We consider that the hysteresis-free properties of the sensing layer were obtained by the balance between in-plane magnetization and other components in magnetization including out-of-plane magnetization. [ABSTRACT FROM AUTHOR]

The design and statistical analysis of a magnetic tunnel junction with pinned-layer uniaxial anisotropy, slightly tilted with respect to the perpendicular magnetic anisotropy (PMA) of the free layer, are presented in the presence of thermally induced magnetic noise. The marginal tilting of the pinned-layer easy axis reduces the critical switching current density by almost 80%, as compared to a regular PMA device for a delay of 2 ns with a switching failure probability lower than \10^-9. Substantially lower switching current density in spin-transfer torque MRAM with tilted pinned-layer anisotropy enables the use of a higher resistance-area product with a thicker tunnel barrier that compensates for the tunneling-magnetoresistance rolloff due to the relative misalignment of free- and pinned-layer easy axes. [ABSTRACT FROM PUBLISHER]

We investigated gain and fan-out in a current-field driven spin transistor that is driven by a current-induced magnetic field. The basic structure of the transistor consisted of a coplanar waveguide for magnetic field application and an MgO-based magnetic tunnel junction with high magnetoresistance and low resistance area product. Under an assisting ac magnetic field and a dc bias voltage of 0.4 V, we demonstrated substantial power gain of 130, current gain of 4.9, and a fan-out value of 5.7. The scaling of the fan-out value in this device is discussed in detail. The current-field driven spin transistor is proven to be a promising candidate as a basic component of a nonvolatile logic device. [ABSTRACT FROM PUBLISHER]

The need for low-power alternatives to digital electronic circuits has led to increasing interest in logic devices where information is stored in nanomagnets. This includes both nanomagnetic logic, where information is communicated through magnetic fields of nanomagnets, and all-spin logic (ASL), where information is communicated through spin currents. A key feature needed for logic implementation is nonreciprocity, whereby the output is switched according to the input but not the other way around, thus providing directed information transfer. The objective of this paper is to draw attention to possible ASL-based schemes that utilize the physics of spin-torque to build in nonreciprocity, as in transistors, that could allow logic implementation without the need for special clocking schemes. We use an experimentally benchmarked coupled spin-transport/magnetization-dynamics model to show that a suitably engineered single ASL unit indeed switches in a nonreciprocal manner. We then present heuristic arguments explaining the origin of this directed information transfer. Finally, we present simulations showing that individual ASL devices can be cascaded to construct a ring oscillator circuit, which provides a clear signature of inbuilt directionality. [ABSTRACT FROM AUTHOR]

We prepared giant magnetoresistance samples comprising of pseudo spin valve structures and measured the current-perpendicular-to-plane magnetoresistance (MR) with applied magnetic field along the easy and hard axes of the samples. When the magnetic field was applied along the easy axis, high and low resistance states representing magnetization parallel and anti-parallel states were observed. However, when the magnetic field was applied along the hard axis, two stochastic MR curves were obtained. The two distinct MR curves correspond to the different magnetizations reversal processes via two kinds of the magnetization orientation states, fully parallel and anti-parallel, at remanence. In our case, the two distinct magnetization reversal processes occurred randomly. [ABSTRACT FROM AUTHOR]

MOSFET scaling is facing overwhelming challenges with increased parameter variations, exponentially higher leakage current, and higher power density. Thus, researchers have started looking at alternative switching devices and spintronics-based computing paradigms. Nanomagnet-based computing is one such paradigm with intrinsic switching energy close to thermal limits and scalability down to 5 nm. In this paper, we explore the possibility of nanomagnet-based design using nonmajority gates. The design approach can offer significant area, delay, and energy advantages compared to majority-gate-based designs. Moreover, new clock technologies and architectures are developed to improve computation robustness and power dissipation of nanomagnet systems. We also developed a comprehensive device/circuit/system compatible simulation framework to evaluate the functionality and architecture of a nanomagnet system and conducted a feasibility/comparison study to determine the effectiveness of the technology compared to standard digital electronics. Performance results from a nanomagnet-based 16-point discrete cosine transform (DCT) with enhanced clock architecture, narrow gap cladding of nanomagnets, or embedding nanomagnets in solenoid with steel core, together with near neighbor system architecture, show up to 10× improvement over subthreshold 15 nm CMOS (Vdd = 90 mV) design, using “energy-delay^0.5-area product (ED^0.5 A)” as comparison metric. Finally, we explored the scalability of nanomagnets and the effectiveness of field-based switching. [ABSTRACT FROM PUBLISHER]

In this study, the exchange coupling properties of submicron patterned MnIr/CoFe bilayers with strong exchange coupling strength were investigated. The large area of 2.5 mm\,\times\,2.5 mm of submicron sized bilayer rings were fabricated by electron beam lithography and ion-milling processes. The clear variations of normalized magnetic hysteresis loops of patterned films are the degradation of pinned CoFe at the MnIr/CoFe interfaces and the decrease of exchange bias field, comparing with sheet film. After post-field-annealing treatment, only the exchange field was improved. A magnetic tunnel junction ring with outer diameter/linewidth of 2/0.5 \mum was constructed as well to further certify the speculations. The magnetoresistance loops of as-fabricated and post-field annealed MTJ ring revealed that the slightly improved magnetoresistance ratio and exchange field. These results were attributed to the rearrangement of distorted spins at the sample edges. However, in comparison with sheet film, the great degradation of magnetoresistance ratio of patterned device, without respect to as-fabricated or post-field annealed, was not retrieved, revealing that the futility of post-field-annealing treatment to the pinning portion of CoFe layer. [ABSTRACT FROM AUTHOR]

We have investigated experimentally direct current (dc) effects on high-frequency properties of two different permalloy (Py) submicrometer patterns of 0.24 μm and 0.55 μm width, 10 μm length, and 100 nm thickness. The natural ferromagnetic resonance (FMR) frequencies for the two samples are about 8.5 and 11.5 GHz. A 50 mA dc produces a FMR frequency reduction of about 1 GHz in both samples. We extracted susceptibility spectra for the samples from the measurement data. We studied inductance variations of Py embedded transmission lines for different dc levels. With 50 mA dc, the operational frequencies of the inductances decreased by 9% and 12.5%. We also tested effects of magnetic fields generated by external magnets on the submicrometer patterns for comparison. To obtain the same magnetization rotation angle, the external magnetic field needs to be about five times larger than the Ampere field created by the direct current. This behavior is unique and may be associated with the increased thermal energy from the Joule heating effects. [ABSTRACT FROM AUTHOR]

For toggle magnetic random access memory, the saturation field represents the exterior limit of the work region. Exterior magnetic fields higher than the saturation field lead to an irremediable loss of the stored information because the antiferromagnetic state of the synthetic antiferromagnetic (SAF) structure becomes a ferromagnetic state and data stored in the magnetic memory cell are altered. At the saturation point, the two ferromagnetic-layer magnetizations became parallel oriented, in a direction close to that of the applied field. In this paper, we present a method to find an analytical formula for the magnetic saturation field that depends on the angle between the applied magnetic field related to the easy anisotropy axis, on the amplitude of the antiferromagnetic coupling field between the ferromagnetic layers, on the geometrical characteristics of the SAF structure, and on the second term of the series expansion of the anisotropy energy of the system. This method allows us to obtain an analytical formula for saturation field that depends only on the controllable parameters of the SAF structure. This analytically obtained critical saturation curve perfectly matches with the critical saturation curve reported in previous papers. [ABSTRACT FROM AUTHOR]

We have investigated the detailed magnetization reversal in 10-nm-thick Ni-Fe elliptical dots with several dot sizes using magnetic field sweeping (MFS)-magnetic force microscopy (FM). At the dot edges, the shape of the phase (the stray field) curve versus magnetic field changes from a stepped hysteresis loop to a almost nonstepped hysteresis loop with increasing dot size. On the other hand, at the center within the dot, sharp or weak decreases in phase are observed as the magnetic field is varied. From these results, it is found that, in the magnetization reversal of a 10-nm-thick Ni-Fe elliptical dot with several dot sizes, the dominant factor changes from the change in the magnetic domain configuration to the domain wall motion as the length of major axis increases. [ABSTRACT FROM AUTHOR]

We investigated the out-of-plane anisotropy of Co[sub50] Fe[sub50]/Tb multilayer film with changing the thickness of Tb layer and applying an inplane filed of 200 Oe during film deposition. A strong out-of-plane anisotropy was found only for the multilayer with small thickness of Tb layer (t[subTb] = 1 nm) and was suppressed significantly by the application of an inplane magnetic field of 200 Oe during deposition, resulting in an inplane anisotropy. It was also found that the application of the magnetic field caused the enhanced intermixing between CoFe and Tb layers to form Co-Tb and Fe-Tb alloys at the interface of the layers. While the origin of the out-of-plane anisotropy and the change of the out-of-plane to inplane anisotropy with the application of inplane field of 200 Oe are not understood completely, it seems that there is a strong correlation between the out-of-plane anisotropy and the formation of the alloys and the stress at the interface. [ABSTRACT FROM AUTHOR]

A precise evaluation technique was created for developing magnetoresistive random access memory (MRAM), especially memory that operates in a toggle-writing mode. This technique enables us to observe the detailed resistance transition of magnetic tunneling junction (MTJ) cells during complicated write operations. It was used to analyze incomplete operation and failed cells, and revealed that the MTJ characteristics in the third quadrant are significantly related to disturb robustness in megabit MRAM. To improve sensitivity to failed cells, we prepared 16-kbit MRAM test structures with a high-speed failed-cell check mode. We found our technique to be a powerful method of failure analysis and expect it to accelerate MRAM development. [ABSTRACT FROM AUTHOR]

In this paper, we report on the design, fabrication, and implementation of ultrasensitive micromechanical oscillators. Our ultrathin single-crystal silicon cantilevers with integrated magnetic structures are the first of their kind: They are fabricated using a novel high-yield process in which magnetic film patterning and deposition are combined with cantilever fabrication. These novel devices have been developed for use as cantilever magnetometers and as force sensors in nuclear magnetic resonance force microscopy (MRFM). These two applications have achieved nanometer-scale resolution using the cantilevers described in this work. Current magnetic moment sensitivity achieved for the devices, when used as magnetometers, is 10-15 J /T at room temperature, which is more than a 1000-fold improvement in sensitivity, compared to conventional magnetometers. Current room temperature force sensitivity of MRFM cantilevers is ∼ 10-16 N/√Hz, which is comparable to the room temperature sensitivities of similar devices of its type. Finite element modeling was used to improve design parameters, ensure that the devices meet experimental demands, and correlate mode shape with observed results. The photolithographic fabrication process was optimized, yielding an average of ∼ 85% and alignment better than 1 μm. Postfabrication-focused ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required. [ABSTRACT FROM AUTHOR]

Statistical variance in the switching probability of spin-torque switching was studied on conventional in-plane MgO-MTJs. Spin-torque switching events by a current pulse longer than inverse of the attempt frequency (typically \sim1 nsec) are well described by the thermal activation model. This model provides the relationship between the switching probability and the current amplitude. If the switching probability is unchanged for a large number of switching events, the statistical variance in the switching probability can be obtained analytically. However, we found the statistical variance in the switching probability varies widely on measured in-plane type CoFeB/MgO/CoFeB-MTJs. Contrary to a simple expectation, the slightly negative correlation was observed between the thermal stability parameter \Delta and the statistical variance in the switching probability. We also measured the external magnetic field dependence along the easy axis, and found that the external field which assists the spin-torque switching dramatically increases the statistical variance of the switching probability. This behavior is considered one of the evidence of the effect of sub-volume excitation. [ABSTRACT FROM PUBLISHER]

This paper reports the results obtained using TbFe and FeCo alloys as the electrodes of MgO-based perpendicular magnetic tunnel junctions (pMTJs). Without annealing treatment, the as-deposited structure showed MgO (001) crystalline texture. The tunnel magnetoresistance (TMR) ratio measured from the patterned cell of size 3\,\times\,6 \mum^2 was 9.5% at room temperature. However, the coercivities of both free and fixed layers were reduced considerably due to the damage caused from the patterning process. The hysteresis loop measurements showed that the magnetic properties of the structures can still be maintained after annealing up to 350 ^\circC. [ABSTRACT FROM AUTHOR]

In this paper, we present experimental results of the ferromagnetic interlayer coupling effect in magnetic tunnel junction devices. This investigation is focused on the sensor size dependency of the ferromagnetic interlayer coupling. The geometry-dependent magnetostatic edge-coupling contribution is estimated by micromagnetic simulation and then subtracted from the net coupling field in order to obtain the correlation between ferromagnetic interlayer coupling and the sensor size. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers. [ABSTRACT FROM AUTHOR]

Microstructured magnetic tunnel junction rings have been fabricated by a top-down technique combining electron beam lithography and ion milling process. Four-terminal magnetoresistance measurements and magnetic force microscopy were used to successfully explore a four-transition process within the free layer throughout the magnetization reversal. Various magnetization configurations were identified to be the onion state, vortex-pair state, vortex state, vortex-core state, and reverse onion state. In addition, the various durations of each magnetic state observed in the magnetoresistance curve can be utilized for the study of a coupling effect between the pinned layer and the free layer. [ABSTRACT FROM AUTHOR]

The read-cycle endurance of a magnetic tunneling junction (MTJ) has been investigated, focusing on the spin-dependent tunneling current at high temperatures. The MTJ structure used in this study consisted of Ta-NiFe-CoFe-AlOx-CoFe-IrMn-NiFe-Ta prepared on a thermally oxidized Si wafer. Both the tunneling current and tunneling magnetoresistance ratio showed no significant degradation during the 1E9 read-cycle test using unipolar voltage pulses of 0.5 V at room temperature. Temperature dependence of the MTJs resistance calculated from the measured tunneling current has also been examined in a temperature range from room temperature to 175 °C. Furthermore in the case of read-cycle tests for thermally stressed MTJs, the variation of the tunneling current was less than 2% after 1E9 cycles at a stress temperature of 175 °C. This indicates that MTJs have sufficient read-cycle endurance under high-temperature operation. [ABSTRACT FROM AUTHOR]

We study current-perpendicular-to-plane (CPP) geometrical giant magnetoresistance (GMR) and magnetic switching property for single spin-valves (SVs). Applying a synthetic antiferromagnet (SyAF) instead of a single ferromagnet film as a free layer, the resistance-change area product ΔRA of the SVs is strongly enhanced from ∼ 1.84 mΩ μm² to ∼ 16.8 mΩ μm² and the CPP-GMR ratio changes from 0.8% to 3.6%. The study of the magnetic switching behavior by both experiment and single-domain thermal activation modeling shows that the SVs with SyAF as a free layer approaches a single-domain magnet even with a low aspect ratio 1 as long as its size diminishes to ∼0.18 μm². Because of the single-domain structure, the SVs with SyAF give a size-independent magnetic switching field when the aspect ratio is 1. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) with the layer structures of Ta (5 nm)/Al(20 nm)/Ni[sub79]Fe[sub21] (5 nm)/Ir[sub22]Mn[sub78](10 nm)/Co[sub75]Fe[sub25] (4 nm)/Al(0.8 nm)-oxide/Co[sub75]Fe[sub25] (4 nm)/Ni[sub79]Fe[sub21] (20 nm)/Ta(5 nm) were fabricated using Al as a conduction layer/electrode and lithographic methods. A high magneto-resistance ratio of 16% and 45% and resistance-area product RS of 11.8 kΩµm² and 11.6 kΩµm² in the as-deposited state and after annealing were attained at room temperature. After annealing, the coercivity of the free layer is about 23.4 Oe. Such MTJs can be used to fabricate the cell of magnetoresistive random access memory and other magnetic field sensors after further optimization. [ABSTRACT FROM AUTHOR]

Presents a study that measured the stray magnetic field of permalloy micromagnets using a Hall magnetometer. Analysis of the Hall traces of the micromagnets; Determination of the bend resistance of the micromagnets; Examination of the magnetostatic interaction of the micromagnets.