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We examine the thermal origin of the detected "saw-tooth" shaped Hall resistance (R xy ) signals in the spin–orbit torque switching experiment for antiferromagnetic MnN. Compared with the results of the heavy metal/antiferromagnet bilayers (MnN/Ta), the qualitatively same "saw-tooth" shaped signals also appear in the samples with the heavy metal layer alone (either Ta or Pt) without the MnN layer. In addition, the R xy signal changes oppositely in the devices with Ta and Pt, due to the opposite temperature coefficient of resistivity of the two materials. All those results are consistent with the "localized Joule heating" mechanism in devices with Hall crosses geometry. Moreover, by utilizing a structure with separated writing current paths and Hall cross area, the quadratic relationship between Δ R xy and the writing current's amplitude is observed, which provides quantitative evidence of the thermal contribution. These results reveal the dominant thermal artifact in the widely used Hall crosses geometry for Néel vector probing and also provide a strategy to semi-quantitatively evaluate the thermal effect, which can shed light on a more conclusive experiment design. [ABSTRACT FROM AUTHOR]

The use of voltage induced strain to switch magnetic tunnel junctions (MTJs) is a promising solution for reducing the switching energy in MRAM technologies. The MTJ is integrated with a piezoelectric layer to generate the strain. A very thin layer is needed to switch with small voltages and small energy dissipation. It is challenging to synthesize ultrathin piezoelectric layers that retain a high degree of piezoelectricity. An alternate approach is to use time-varying strain generated by a surface acoustic wave (SAW). This approach does not require a thin piezoelectric layer since the SAW is confined to the surface of the layer. In this study, we fabricated in-plane MTJs on piezoelectric LiNbO3 substrates and used IDTs to generate the SAW signal within the substrate. Our results showed that the SAW signal had a significant influence on the resistance and the tunneling magnetoresistance (TMR) ratio of the MTJs. The influence was much less significant in nanometer size MTJs than in micrometer sized ones. Most surprisingly, the SAW signal caused the tunneling magnetoresistance ratio (TMR) to drop below zero for the micrometer size MTJ, meaning that the antiparallel resistance RAP is temporarily less than the parallel resistance RP under SAW excitation. Our results provide insight into the dynamic behavior of MTJs under periodic strain and the dependence of this behavior on the device dimensions as they are scaled down to nanometer sizes. [ABSTRACT FROM AUTHOR]

Recent advancement in the switching of perpendicular magnetic tunnel junctions with an electric field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current spin-transfer torque-magnetic tunnel junction and spin–orbit torque-magnetic tunnel junction devices, the typical linear fJ/V m range voltage controlled magnetic anisotropy (VCMA) needs to be significantly enhanced with approaches that include new materials or stack engineering. A possible bidirectional and 1.1 pJ/V m VCMA effect has been predicted by using heavily electron-depleted Fe/MgO interfaces. To improve upon existing VCMA technology, we have proposed inserting high work function materials underneath the magnetic layer. This will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime, where the pJ/V m and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the Ta/Pd(x)/Ta underlayer's work function. By varying the Pd thickness (x) from 0 to 10 nm, we have observed a tunable change in the Ta layer's work function from 4.32 to 4.90 eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the underlayer, we have performed DFT calculations on supercells of Ta/Pd(x)/Ta/CoFe/MgO, which demonstrate that electron depletion will not be fully screened at the CoFe/MgO interface. Gated pillar devices with Hall cross geometries were fabricated and tested to extract the anisotropy change as a function of applied gate voltage for samples with various Pd thicknesses. The electron-depleted Pd samples show three to six times VCMA improvement compared to the electron accumulated Ta control sample. [ABSTRACT FROM AUTHOR]

Unidirectional spin Hall magnetoresistance (USMR) is a magnetoresistance effect with potential applications to read two-terminal spin–orbit-torque (SOT) devices directly. In this work, we observed a large USMR value (up to 0.7 × 10−11 per A/cm2, 50% larger than reported values from heavy metals) in sputtered amorphous PtSn4/CoFeB bilayers. Ta/CoFeB bilayers with interfacial MgO insertion layers are deposited as control samples. The control experiments show that increasing the interfacial resistance can increase the USMR value, which is the case in PtSn4/CoFeB bilayers. The observation of a large USMR value in an amorphous spin–orbit-torque material has provided an alternative pathway for USMR application in two-terminal SOT devices. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic tunnel junctions (pMTJs) with synthetic antiferromagnetic (SAF) free layers have attracted much interest for applications on spintronic memory devices with ultrafast speed and ultralow energy. In this work, SAF free layer pMTJs (SAF-pMTJs) were designed and fabricated, in which a Ru/Ta bilayer spacer is used to form the SAF structure. We first characterized the magnetization dynamics of the SAF free layer by using ferromagnetic resonance and found that the Gilbert damping constant of the SAF free layer is around 0.019. Then, in device level studies that span from 900 nm down to 200 nm lateral size, we observed a transition of the SAF free layer from a preferred antiparallel to parallel magnetic configuration at small device sizes, which can be explained by the increased dipole interaction. The impact of the operating current was also investigated. We report an extraordinarily strong dependence, up to 144.1 kOe per A/ μ m2, of the offset field on the applied current, suggesting an electric-field modulation on the interlayer exchange coupling of the SAF free layer. These results will be instructive to improve the understanding of material properties and device performance of SAF-pMTJs for ultrafast, ultralow-power consumption spintronic devices. [ABSTRACT FROM AUTHOR]

Development of L10 FePd thin films with large bulk perpendicular magnetic anisotropy and a low damping constant may permit superior scaling of next-generation ultra-high density magnetic memory elements. The buffer layer influences the L10-order parameter, static and dynamic magnetic properties of FePd and demands consideration for the design of high anisotropy strength and low damping films. In this report, we systematically investigate the perpendicular magnetic anisotropy and damping constant of the FePd thin films engineered through the Cr/(Pt, Ru, Ir, Rh), Mo/Ir, and Ir buffer layers. We observed that the Ir(001), Cr(001)/Ir(001), Cr(001)/Pt(001), Cr(001)/Rh(001), and Cr(001)/Ru(001) buffer layers can induce highly oriented (001) FePd films while the Mo/Ir buffer layer does not. Of all the buffer layers, the largest perpendicular magnetic anisotropy Ku ∼ 1.2 MJ/m3 and damping constant α ∼ 0.005 were achieved for the Cr/Pt buffered FePd sample, consistent with a high ordering parameter S ∼ 0.82. The Cr/Ru buffered FePd sample shows the lowest α ∼ 0.008, despite having a lower S ∼ 0.64 and a lower Ku ∼ 0.9 MJ/m3. These film-level properties would be sufficient for the engineering of devices that require thermally stable, sub-10 nm lateral size elements with low damping for applications of low energy-delay magnetic memory devices. [ABSTRACT FROM AUTHOR]

In this work, we study the effects of mobile oxygen ions in a synthetic antiferromagnet structure gated by a sputtered SiO2 dielectric layer for memory and logic applications. Our devices utilize electrochemical reactions between dielectric reactive species and magnetic elements to create irreversible changes in magnetization. We analyzed the dependence of ion velocity on the gate dielectric properties such as the lattice parameter, oxygen migration energy barrier, and electric field (E-field). Hall bar devices were patterned and tested to determine the interlayer exchange coupling between the CoFeB and [Co/Pd]n layers. The anomalous Hall effect (AHE) of the CoFeB layer at different gate voltages (Vg) was measured to determine the Vg dependence. A sharp change in the behavior of the CoFeB layer with respect to negative Vg results in a non-reciprocal decrease in the coercivity and magnetization and an increase in exchange bias. The observed change in exchange bias field and magnetization allows us to measure the change in the effective thickness of the CoFeB layer. This led us to conclude that the source of such behavior is the negatively charged mobile oxygen ions from the SiO2 gate. [ABSTRACT FROM AUTHOR]

Bulk perpendicular magnetic anisotropy materials are proposed to be a promising candidate for next-generation ultrahigh density and ultralow energy-consumption spintronic devices. In this work, we experimentally investigate the structure, thermal stability, and magnetic properties of FePd thin films seeded by an Ru layer. An fcc-phase Ru layer induces the highly-ordered L10-phase FePd thin films with perpendicular magnetic anisotropy (Ku ∼10.1 Merg/cm3). The thermal stability of FePd samples is then studied through the annealing process. It is found that a Ku ∼6.8 Merg/cm3 can be obtained with an annealing temperature of 500 °C. In addition, the Gilbert damping constant α, an important parameter for switching current density, is determined as a function of the testing temperature. We observe that α increases from 0.006 to 0.009 for the as-deposited FePd sample and from 0.006 to 0.012 for the 400 °C-annealed FePd sample as the testing temperature changes from 25 °C to 150 °C. These results suggest that Ru-seeded FePd provides great potential in scaling perpendicular magnetic tunnel junctions below 10 nm for applications in ultralow energy-consumption spintronic devices. [ABSTRACT FROM AUTHOR]

We studied the spin-to-charge and charge-to-spin conversion at room temperature in sputtered WTe2−x (x = 0.8) (t)/Co20Fe60B20(6 nm) heterostructures. Spin pumping measurements were used to characterize the spin-to-charge efficiency, and the spin efficiency was calculated to be larger than ∼0.035. Second harmonic Hall measurements were carried out to estimate the charge-to-spin conversion ratio. We found that the system exhibits a large field-like torque (spin torque efficiency ∼0.1) and small damping-like torque (spin torque efficiency ∼0.001) compared to those reported for heavy metals. High-resolution transmission electron microscopy images show that the WTe2−x layer is amorphous, which may enhance the spin swapping effect by inducing large interfacial spin–orbit scattering, thus contributing to a large field-like torque. [ABSTRACT FROM AUTHOR]

Recently, two-dimensional (2D) materials have attracted considerable interest for use in spintronic applications, especially hexagonal close-packed (hcp)-phase boron nitride (BN) as a tunnel barrier. In this paper, we experimentally investigated the structural properties of a sputtered hcp-BN thin film. By optimizing the experimental conditions, we obtained the stoichiometric BN thin film with a ratio of 1:1 of the Ar/N2 sputtering gas. Then the Co/BN/Co magnetic tunnel junction (MTJ) stacks were prepared to study the crystalline structure of the BN tunnel barrier and their epitaxial relationship. We found that the as-deposited BN tunnel barrier layer follows the texture of the bottom Co layer and forms a polycrystalline structure. After the high-temperature treatment of the MTJ stack, texturing of the BN tunnel barrier layer is observed, however, this annealing process makes the BN tunnel barrier noncontinuous and induces serious interdiffusion between layers. These results will open the door for development of spintronic devices based on MTJs with hcp-phase BN tunnel barrier and hcp-phase perpendicular magnetic anisotropy ferromagnetic layer. [ABSTRACT FROM AUTHOR]

Magnetic storage and magnetic memory have recently shifted towards the use of magnetic thin films with large perpendicular magnetic anisotropy (PMA) to simultaneously satisfy the requirements in storage density and thermal stability. Understanding the magnetic switching process and its dependence on the Gilbert damping (α) of materials with large PMA is crucial for developing low-power consumption, fast-switching, and high-thermal stability devices. The need to quantify α of materials with large PMA has resulted in the development of the all-optical ultrafast Time-Resolved Magneto-optical Kerr Effect (TR-MOKE) technique. While TR-MOKE has demonstrated its capability of capturing magnetization dynamics of materials with large PMA, a quantitative analysis regarding the operational optimization of this emerging technique is still lacking. In this paper, we discuss the dependence of the TR-MOKE signal on the magnitude and angle of the applied field, by utilizing a numerical algorithm based on the Landau-Lifshitz-Gilbert equation. The optimized operational conditions that produce the largest TR-MOKE signals are predicted. As an experimental verification, we conduct TR-MOKE measurements on a representative sample of a tungsten-seeded CoFeB PMA thin film to show the excellent agreement of the model prediction with measurements. Our analysis results in a better understanding of the external field influence on the magnetization precession processes. The results of this work can also provide guidance on selecting operational conditions of the TR-MOKE technique to achieve optimal signal-to-noise ratios and thus more accurate measurements of magnetization dynamics. [ABSTRACT FROM AUTHOR]

Fe4N thin films with (001) texture were prepared by reactive sputtering on MgO substrates, utilizing either a Cr or Ag buffer layer to facilitate the epitaxial growth. X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry measurements show that the Fe4N thin film grown on the Ag buffer layer is superior to that grown on the Cr buffer layer. The point contact Andreev reflection measurement was then conducted, and the spin polarizations were determined to be 61.1% and 81.3% for Fe4N thin films with Cr and Ag buffer layers, respectively. The 81.3% spin polarization is significantly higher than the ratio reported previously for Fe4N and is comparable with that of state-of-the-art Heusler alloys. This result is in agreement with the theoretical prediction on the discrepancy between the two differently defined spin polarizations for Fe4N. Moreover, our study indicates that an optimized growth process for Fe4N thin films is crucial for achieving a high spin polarization and that true half-metallicity could potentially be realized with Fe4N. The high spin polarization of Fe4N combined with its low fabrication temperature and simple composition makes Fe4N a competitive candidate to be a half-metallic ferromagnet in spintronic devices. [ABSTRACT FROM AUTHOR]

We have studied the interlayer exchange coupling in Co2FeAl (CFA)/Cr/CFA/MgO multilayers via both experiments and numerical simulation. Magnetization measurement on the films shows a clear oscillation attenuation behavior with the thickness (0.6 nm < t < 10 nm) of the Cr spacer layer, and the oscillation period is about 2.1 nm. The numerical simulation demonstrates that the interlayer exchange coupling between CFA layers is 90° coupling having an oscillation behavior, which is in good agreement with the experiments. MgO capping layer is supposed to be a key factor for the clear periodic oscillation behavior in CFA/Cr/CFA trilayers. [ABSTRACT FROM AUTHOR]