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In this study, we investigate the spin Hall effect in heterostructures of Bi0.85Sb0.15 (10 nm) topological insulator/Ru(Ti)/Ta/Co20Fe60B20/MgO with perpendicular magnetic anisotropy. By optimizing the Ru (Ti) interfacial layer thickness as well as deposition condition of BiSb, we achieve a large effective spin Hall angle of 6.0 ± 0.1 and relatively high electrical conductivity of 1.5 × 105 Ω−1 m−1 at room temperature. We, then, demonstrate spin–orbit torque-induced magnetization switching driven by a small threshold current density of 1 × 106 Acm−2. Benchmarking shows that the writing power consumption of our stack is 2–3 orders smaller than that of heavy metals. [ABSTRACT FROM AUTHOR]

It is technically challenging to shrink the size of a tunneling magnetoresistance reader to below 20 nm for magnetic recording technology beyond 4 Tb/in2 due to its complex film stack. Recently, we proposed a reader architecture based on the inverse spin Hall effect to resolve those challenges, referred below as spin–orbit torque (SOT) reader, whose structure consists of a SOT layer and a ferromagnetic layer. However, the heavy metal-based SOT reader has small output voltage and low signal-to-noise ratio (SNR) due to the limited spin Hall angle θSH (< 1) of heavy metals. In this Letter, we demonstrate the integration of BiSb topological insulator with strong inverse spin Hall effect into the SOT reader that can significantly improve the output voltage and SNR. First, we theoretically calculate the noises in a 20 × 20 nm2 BiSb-based SOT reader to establish the relationships between SNR and θSH at various bias currents. We then demonstrate proof-of-concept BiSb-based SOT readers using CoFe/MgO/BiSb stack, which show large output voltages up to 15 mV at an input current of 9.4 kA/cm2 at room temperature. We project a giant θSH = 61 for BiSb. Our work demonstrates the potential of BiSb for SOT reader beyond 4 Tb/in2 magnetic recording technology. [ABSTRACT FROM AUTHOR]

We have investigated the magnetization dynamics of sputtered Co40Fe40B20 thin films in a wide range of thicknesses used as free layers in MgO-based magnetic tunnel junctions, with the technique of broadband ferromagnetic resonance (FMR). We have observed a large interface-induced magnetic perpendicular anisotropy in the thin film limit. The out-of-plane angular dependence of the FMR measurement revealed the contributions of two different damping mechanisms in thick and thin film limits. In thinner films (< 2 nm), two-magnon scattering and inhomogeneous broadening are significant for the FMR linewidth, while the Gilbert damping dominates the linewidth in thicker films (≥ 4 nm). Lastly, we have observed an inverse scaling of Gilbert damping constant with film thickness, and an intrinsic damping constant of 0.004 in the CoFeB alloy film is determined. [ABSTRACT FROM AUTHOR]

We describe new characterization methods that allow an accurate determination of all of the magnetic parameters that govern the behavior of magnetoresistive devices. These characterization methods are explained and used to measure the magnetic properties of MgO-based magnetic tunnel junction (MTJ) devices with magnetoresistance values of over 150%. We will show that the analysis of so-called “circle transfer curves,” which are measurements of the device magnetoresistance in a rotating, constant-magnitude applied field, can accurately determine the magnitude and direction of the free layer anisotropy as well as the pinned layer orientation and exchange bias strength. We also show how a measurement of the MTJ’s remnant resistance curve, obtained by saturating the MTJ at different field angles and then removing the applied field, can provide additional information on the free layer anisotropy characteristics. We will also compare our results with values extracted from traditional Stoner-Wohlfarth asteroid curves. Finally, we show that the extracted parameters can accurately predict the shape of traditional MTJ transfer curves. [ABSTRACT FROM AUTHOR]

Thermal stability, sensitivity, and noise of micron-scale magnetic tunnel junctions based on MgO tunnel barriers have been studied for both the memory and sensing configurations. Junctions show solid high-temperature performance with substantial magnetoresistance observed even at 500 °C. At temperatures above 375 °C, the junctions begin to experience irreversible degradation due to interlayer diffusion. The thermal stability of these devices depends strongly on the exchange bias of the device and hence on the properties of the antiferromagnetic layer. Sensitivities as high as 3.3%/Oe have been obtained at room temperature for junctions configured as low-field sensors. Sensitivity values are constant up to temperatures of 300 °C, above which performance decays due to a loss of exchange bias and overall magnetoresistance. Noise spectra are 1/f at frequencies up to 51 kHz, and sensors have a resultant field noise better than 1 nT/Hz0.5 at 100 kHz. A comparison is made with devices fabricated with alumina tunnel barriers. [ABSTRACT FROM AUTHOR]

We have investigated the microstructures of magnetic tunneling junctions using high-resolution transmission electron microscopy (HRTEM). These junctions exhibit large magnetoresistance of 34% at room temperature and at tens of gauss. HRTEM reveals well-defined layered polycrystalline metallic structures. The important Al[sub 2]O[sub 3] tunneling barrier has an amorphous structure containing a few tiny crystalline inhomogeneties. Though the barrier is flat over large length scales, its interfaces with the neighboring electrodes are rough. We have characterized the roughness amplitude and wavelength, which are consistent with the magnitude of offset bias fields in measured hysteretic magnetoresistance curves. [ABSTRACT FROM AUTHOR]

We have fabricated and studied the magnetic properties of the Ni[SUB81]Fe[SUB19]/Al[SUB2]O[SUB3]/Ni[SUB81]Fe[SUB19] based magnetic tunnel junction sensors. Magnetoresistance (MR) of 35% is achieved with a small applied field (< 10 Oe). The introduction of a hard axis bias field linearizes the MR response. The hysteresis disappears in hard-axis fields greater than 3 Oe, which corresponds to the effective anisotropy field along the easy axis. A sensitivity of 3.5%/Oe has been demonstrated in this linear region. Low-frequency noise measurements indicate that sensor noise is dominated by field-dependent 1/ƒ noise caused by magnetization fluctuations. Finally, a noise level as low as 1 nT/Hz[SUP1/2] has been obtained. [ABSTRACT FROM AUTHOR]

We map the magnetic field sensitivity and low-frequency 1/f voltage noise of high magnetoresistance MgO-based magnetic tunnel junctions in an orthogonal magnetic field arrangement. Large sensitivity values of over 1%/Oe are obtained only when a sufficiently large hard-axis bias field is applied. The low-frequency voltage noise is observed to scale with the field sensitivity. The magnetic field noise map reveals that the signal-to-noise ratios of these devices get gradually better at higher hard-axis bias fields. [ABSTRACT FROM AUTHOR]

The characterization of switching field and its angular dependence has been tremendously useful in understanding the switching mechanisms in magnetic thin-films and patterned structures. However, the study of the distribution in switching fields (SFD) in arrays and its angular dependence is less common. In this work, we investigate this dependency in arrays of (Co/Pd)n multilayer nanostructures. Results from arrays with different element sizes and periodicities (pitches) are presented, and we found that, like the switching field, the SFD varies with applied field angle in a Stoner-Wohlfarth-like fashion. Furthermore, when the SFD is represented as a dependent variable of the switching field, we consistently found a linear relationship between the two, and that the slope depends on both array element size and pitch. In general, the SFD in arrays with the largest structures and the smallest pitch tends to have the strongest dependence on the switching field. For arrays with nanostructures of a fixed size, however, we found that SFD values are virtually identical, regardless of pitch, if the reversal field is applied near 45° with respect to the surface normal. That the minimum SFD depends only on the size of the elements and not the pitch has significant implications for the practical design of densely-packed magnetic nanostructure arrays. [ABSTRACT FROM AUTHOR]

Low frequency voltage noise was measured in highly sensitive magnetic tunnel junctions with MgO tunnel barrier. The voltage noise is observed to scale linearly with the magnetic field sensitivity. Fluctuations in noise, possibly due to local domain nucleation or annihilation inside the free layer, are also observed. Results indicate that an external hard-axis bias field can significantly suppress the magnetization fluctuations of the free layer and lower the magnetic field noise. [ABSTRACT FROM AUTHOR]

We have systematically investigated the dependence of tunnel magnetoresistance in MgO-based magnetic tunnel junctions as a function of Ar pressure during sputtering. The MgO surface roughness, and therefore device magnetoresistance, depends strongly on Ar gas pressure. Magnetoresistance of up to 236% was achieved at room temperature after thermal annealing at 425 °C and with optimal sputtering conditions. The long mean free path of target atoms at low background pressures increases their kinetic energy at the substrate surface, resulting in smooth surface morphology and correspondingly improved device performance. [ABSTRACT FROM AUTHOR]

We have demonstrated the use of highly sensitive magnetic tunnel junction (MTJ) sensors for the detection of individual micron-sized magnetic labels. By integrating the MTJ sensor into a microfluidic channel, we were able to detect the presence of moving superparamagnetic beads (Dynabeads® M-280) in real time by direct measurement of the magnetic dipole fields associated with single beads. The dipolar fields of a single bead were sufficient to obtain a signal of 80 μV with signal to noise ratio of 24 dB in an applied field of 15 Oe. Our data show conclusively that MTJ sensors are very promising candidates for future applications involving the accurate detection and identification of biomolecules with magnetic labels. [ABSTRACT FROM AUTHOR]