Tailoring the switching efficiency of magnetic tunnel junctions by the fieldlike spin-orbit torque

Current-induced spin-orbit torques provide a versatile tool for switching magnetic devices. In perpendicular magnets, the dampinglike component of the torque is the main driver of magnetization reversal. The degree to which the fieldlike torque assists the switching is a matter of debate. Here we study the switching of magnetic tunnel junctions with a CoFeB free layer and either W or Ta underlayers, which have a ratio of fieldlike to dampinglike torque of 0.3 and 1, respectively. We show that the fieldlike torque can either assist or hinder the switching of CoFeB when the static in-plane magnetic field required to define the polarity of spin-orbit torque switching has a component transverse to the current. In particular, the non-collinear alignment of the field and current can be exploited to increase the switching efficiency and reliability compared to the standard collinear alignment. By probing individual switching events in real-time, we also show that the combination of transverse magnetic field and fieldlike torque can accelerate or decelerate the reversal onset. We validate our observations using micromagnetic simulations and extrapolate the results to materials with different torque ratios. Finally, we propose device geometries that leverage the fieldlike torque for density increase in memory applications and synaptic weight generation

Perpendicular magnetic anisotropy of CoFeB\Ta bilayers on ALD HfO2

Perpendicular magnetic anisotropy (PMA) is an essential condition for CoFe thin films used in magnetic random access memories. Until recently, interfacial PMA was mainly known to occur in materials stacks with MgO\CoFe(B) interfaces or using an adjacent crystalline heavy metal film. Here, PMA is reported in a CoFeB\Ta bilayer deposited on amorphous high-kappa dielectric (relative permittivity kappa=20) HfO2, grown by atomic layer deposition (ALD). PMA with interfacial anisotropy energy K-i up to 0.49 mJ/m(2) appears after annealing the stacks between 200 degrees C and 350 degrees C, as shown with vibrating sample magnetometry. Transmission electron microscopy shows that the decrease of PMA starting from 350 degrees C coincides with the onset of interdiffusion in the materials. High-kappa dielectrics are potential enablers for giant voltage control of magnetic anisotropy (VCMA). The absence of VCMA in these experiments is ascribed to a 0.6 nm thick magnetic dead layer between HfO2 and CoFeB. The results show PMA can be easily obtained on ALD high-kappa dielectrics

Effect of Tantalum spacer thickness and deposition conditions on the properties of MgO/CoFeB/Ta/CoFeB/MgO free layers

International audienceTo get stable perpendicularly magnetized tunnel junctions at small device dimensions, composite free layers that comprise two MgO/FeCoB interfaces as sources of interface anisotropy are generally used. Proper cristallisation and an-nealing robustness is typically ensured by the insertion of a spacer layer of the early transition metal series within the FeCoB layer. We study the influence of the spacer thickness and growth condition on the switching metrics of tunnel junctions thermally annealed at 400 • C for the case of 1-4Å Ta spacers. Thick Ta spacer results in a large anisotropies indicative of a better defined top FeCoB/MgO interface, but this is achieved at the systematic expense of a stronger damping. For the best anisotropy-damping compromise, junctions of diameter 22 nm can still be stable and spin-torque switched. Coercivity and inhomogeneous linewidth broadening, likely arising from roughness at the FeCoB/Ta interface, can be reduced if a sacrificial Mg layer is inserted before the Ta spacer deposition. The design of magnetic tunnel junctions (MTJ) suitable for spintronic applications involves material science and nanomagnetism aspects. An emblematic example is the spin-transfer-torque magnetic random access memory (STT-MRAM) technology 1 which requires to include in a single nanodevice the physics of perpendicular magnetic anisotropy (PMA), tunnel magneto-resistance (TMR), interlayer exchange coupling, microwave magnetization dynamics, alloy metallurgy, as well as crystallization and elemental diffusion kinetics 2-4. While when studied on model systems, these phenomena are understood and the corresponding technique are well mastered, their quantitative prediction in complex systems remains challenging. This is especially true when the layer thicknesses are in the near-monolayer regime when the concepts based on diffusive transport (such as spin pumping induced damping) are out of their range of validity while ab-initio approaches can't be applied because of a insufficient knowledge on the sample structure. For instance, it still difficult to predict or understand quantitatively the consequences of the insertion of a metallic spacer at the middle of a magnetic layer despite the fact that this material science trick is commonly practiced 5-8 in the free layers of STT-MRAMs. In this paper, we study experimentally the influence of a Tantalum spacer layer inserted within the dual MgO FeCoB-based free layer of optimized MTJs with PMA. We measure the transport properties as well as the magnetic anisotropy and the Gilbert damping of samples having faced the standard 400 • C annealing of a CMOS back end of line process. We show that both the damping and the magneto-crystalline anisotropy increase with the Ta thickness. We also show that the deposition condition (with or without a Mg sacrificial layer before the deposition of the spacer) influences the homogene-ity of the magnetic properties. Our objective is to understand the influence of the Tanta-lum spacer onto the properties of free layers of perpendicularly magnetized MTJ that have undergone 400 • C annealing a) Electronic mail: [email protected] and in which element interdiffusion is minimized by a proper engineering of the Boron content 4 within the FeCoB parts of the free layer. We consider free layers embodied in state-of-the-art bottom-pinned MTJs. For this study, we consider the generic stack sketched in Fig. 1(a). It is deposited by physical vapor deposition and then annealed at 400 • C for 30 min in a 1 T perpendicular magnetic field. The layer compositions are described from bottom to top with the numbers denoting the thickness inÅ. The stack organisation is: Hard Layer / Ir (5.2) / Reference Layer / MgO (rf) / Free layer / MgO (rf) / cap. Following previous optimizations 4,9 , the Hard Layer is the conventional [Co (5) / Pt (3) ] ×5 / Co (6). Antiferromagnetic coupling with the reference layer is supplied by the iridium spacer. Note that the hysteresis loops comprise 4 steps for each sweeping direction [see Fig. 1(c)]. This contrasts with anterior configurations using the weaker antiferro-couplers (e.g. Ru) which allowed the RL and HL to switch separately, resulting in loops comprising only 3 successive steps (RL, FL and then finally HL switching) 9-12. The present reference layer is Co (6) / WFeCoB (8) / FeB (9). We have chosen WFeCoB to benefit from a strong interlayer exchange coupling with the cobalt layer and because it induces a large anisotropy 9 in the neighboring Fe(Co)B layers. We use FeB to benefit from strong interface anisotropy with MgO 13,14. As clear from Fig. 1b, these two features prevent the dynamical back-hopping of the antiparallel-to-Parallel transition observed formerly with less stiff reference layers 11,15. The properties of a very similar reference layer are detailed in ref. 16. While the hard and the reference layer were deposited at room temperature, the deposition of the tunnel oxide is followed by a sample refrigeration at cryogenic temperatures (80 K) which allows a better wetting of the subsequently deposited FeCoB on the MgO tunnel oxide, with benefits 17 (+20%) in TMR and RA. The free layer is then Fe 52.5 Co 17.5 B 30 (15) / spacer / Fe 52.5 Co 17.5 B 30 (9). 8 variants of the spacer were used, consisting of Tantalum thickness of 1, 2, 3 or 4Å deposited on an optional sacrificial Mg (6.5) layer 18. Note that thicker Ta spacers lead generally to a loss of arXiv:1909.02741v1 [cond-mat.mtrl-sci

Spin-torque induced wall motion in perpendicularly magnetized discs: Ballistic versus oscillatory behavior

International audienceWe use time-resolved measurement and modeling to study the spin-torque induced motion of a domain wall in perpendicular anisotropy magnets. In disc of diameters between 70 and 100 nm, the wall drifts across the disc with pronounced back-and-forth oscillations that arise because the wall moves in the Walker regime. Several switching paths occur stochastically and lead to distinct switching durations. The wall can cross the disc center either in a ballistic manner or with variably marked oscillations before and after the crossing. The crossing of the center can even occur multiple times if a vertical Bloch line nucleates within the wall. The wall motion is analyzed using a collective coordinate model parametrized by the wall position q and the tilt φ of its in-plane magnetization projection. The dynamics results from the stretch field, which describes the affinity of the wall to reduce its length and the wall stiffness field describing the wall tendency to reduce dipolar energy by rotating its tilt. The wall oscillations result from the continuous exchange of energy between to the two degrees of freedom q and φ. The stochasticity of the wall dynamics can be understood from the concept of the retention pond: a region in the q − φ space in which walls are transiently bound to the disc center. Walls having trajectories close to the pond must circumvent it and therefore have longer propagation times. The retention pond disappears for a disc diameter of typically 40 nm: the wall then moves in a ballistic manner irrespective of the dynamics of its tilt. The propagation time is then robust against fluctuations hence reproducible

Dynamical properties of ordered Fe-Pt alloys

© 2015 Elsevier B.V. All rights reserved. The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoichiometric compositions, Fe3Pt, FePt and FePt3, have been investigated using the density functional theory. Additionally, the existing experimental data have been complemented by new measurements of the Fe projected phonon density of states performed for the Fe3Pt and FePt3 thin films using the nuclear inelastic scattering technique. The calculated phonon dispersion relations and phonon density of states have been compared with the experimental data. The dispersion curves are very well reproduced by the calculations, although, the softening of the transversal acoustic mode TA1 leads to some discrepancy between the theory and experiment in Fe3Pt. A very good agreement between the measured spectra and calculations performed for the tetragonal structure derived from the soft mode may signal that the tetragonal phase with the space group P4/mbm plays an important role in the martensitic transformation observed in Fe3Pt. For FePt3, the antiferromagnetic order appearing with decreasing temperature has been also investigated. The studies showed that the phonon density of states of FePt3 very weakly depends on the magnetic configuration.publisher: Elsevier articletitle: Dynamical properties of ordered Fe–Pt alloys journaltitle: Journal of Alloys and Compounds articlelink: http://dx.doi.org/10.1016/j.jallcom.2015.08.097 content_type: article copyright: Copyright © 2015 Elsevier B.V. All rights reserved.status: publishe

Probing the magnetization inside a superconducting Nb film by nuclear resonant scattering

We present an approach to probe the magnetization inside superconducting films using ultrathin Fe-57 probe layers excited by synchrotron radiation. We investigate the evolution of the Fe-57 hyperfine field orientation as a function of magnetic field above and below the superconducting transition temperature T-c for a Nb(50 nm)/Fe-57(0.6 nm)/Nb(50 nm) trilayer. It is found that significant screening of the external field in the superconductor occurs only at low field, leading to a change in the hyperfine field angle below T-c. The presented approach allows to study the influence of magnetic fields and vortex induced electron correlations in complex layered structures incorporating superconductors. (C) 2011 American Institute of Physics. [doi:10.1063/1.3625941]status: publishe

A Systematic Assessment of W-Doped CoFeB Single Free Layers for Low Power STT-MRAM Applications

Spin-transfer torque magnetoresistive random access memory (STT-MRAM) technology is considered to be the most promising nonvolatile memory (NVM) solution for high-speed and low power applications. Dual MgO-based composite free layers (FL) have driven the development of STT-MRAMs over the past decade, achieving data retention of 10 years at the cost of higher write power consumption. In addition, the need for tunnel magnetoresistance (TMR)-based read schemes limits the flexibility in materials beyond the typical CoFeB/MgO interfaces. In this study, we propose a novel spacerless FL stack comprised of CoFeB alloyed with heavy metals such as tungsten (W) which allows effective modulation of the magnet properties (Ms, Hk) while retaining compatibility with MgO layers. The addition of W results favours a delayed crystallization process, in turn enabling higher thermal budgets up to 180 min at 400 °C. The presence of tungsten reduces the total FL magnetization (Ms) but simultaneously increasing its temperature dependence, thus, enabling a dynamic write current reduction of ~15% at 2 ns pulse widths. Reliable operation is demonstrated with a WER of 1 ppm and endurance >1010 cycles. These results pave the way for alternative designs of STT-MRAMs for low power electronics