Session B15: Focus Session: Spins in Metals: Spin Torque Switching and Magnetic Anisotropy Control

Switching Energy Barrier and Current Reduction in MTJs with Composite Free Layer

We investigate the properties of a penta-layer magnetic tunnel junction (MTJ) with a composite soft layer by exhaustive micromagnetic simulations. The structure CoFe/spacer (1nm)/Py (4nm)/spacer (1nm)/ CoFe (Py is Ni$_{81}$Fe$_{19}$) with an elliptical cross-section (major axes 90nm and 35nm, correspondingly) is considered. The system with the composite soft layer is obtained by removing a central stripe of 5nm width from the monolithic free layer. The MTJ with a composite free layer switches two to three times faster than the one with a monolithic layer [1]. We have found that in the MTJ structure with the composite layer the switching energy barrier is decreased and becomes equal to the shape anisotropy energy barrier responsible for thermal stability. This results in a switching current density reduction. The physical reasons for the switching energy barrier reduction are discussed.\\[4pt] [1] A. Makarov {\it et al., Phys. Status Solidi RRL} {\bf 5}, No. 12, 420-422 (2011).   

Switching Distributions in all perpendicular spin-valve nanopillars under thermal activation and spin-transfer torques

The free layer switching field distributions of spin-valve nanopillars with perpendicular magnetization have been studied. The distributions are consistent with expectations of a model based on thermal activation over a single field dependent energy barrier. However, at zero applied current there is a strong asymmetry between the P and AP states and the reverse, with energy barriers more than 50\% higher for P to AP transitions. The inhomogeneous dipolar field from the polarizer is demonstrated to be at the origin of this symmetry breaking\footnote{D. B. Gopman et al, arXiv:1111.1342v1 [cond-mat.mes-hall]}. I will show results on the effects of varying lateral geometry on this symmetry breaking. Also, I will introduce a low-cost method for studying the switching current distributions that applies a continuous waveform to sweep the dc current and simultaneously probe the magnetoresistance. This method permits the acquisition of over $10^{6}$ switching events in six hours, which presents the possibility to obtain deep statistics on the reversal process. The effect of the magnitude and direction of applied dc currents on the thermal stability of a nanomagnet has been investigated and the results will be examined within the acquired statistics. Supported by NSF Grant DMR-1006575.   

Landau-Lifshitz Bloch Macrospin Simulations of Magnetization Switching Dynamics in Perpendicular Anistropy CoNi/Pd Magnetic Multilayer Thin Films

Heating magnetic multilayer thin films close to their Curie temperature (Tc) for brief periods of time, in the presence of a magnetic field, will potentially enable ultra-high density magnetic recording while maintaining thermal stability of perpendicular anisotropy magnetic media. This idea is to be exploited in thermally assisted magnetic recording (TAR) technology, which is currently being pursued by many academic and industrial research labs. In our study, we have performed macrospin simulations of the magnetization switching based on the numerical solution of Landau-Lifshitz-Bloch equation at such elevated temperatures (close to Tc) for a strongly exchange coupled and high perpendicular anisotropy CoNi/Pd magnetic multilayer thin film structure (Tc=448 K). We will discuss the results of a comprehensive model for this material system taking into account temperature dependencies of anisotropy, saturation magnetization and transverse and longitudinal susceptibilities and their effects on the switching process.   

Statistical and Time Resolved Studies of Switching in Magnetic Tunnel Junction based Orthogonal Spin Transfer Devices

We report statistical and single-shot time-resolved studies of spin transfer switching in OST-MRAM devices. These devices consist of a perpendicular polarizer integrated into a layer stack with an in-plane magnetized free and reference layer, which form the electrodes of a magnetic tunnel junction [1]. The perpendicular polarizer provides an initial torque -- designed to reduce the incubation delay in switching. The demagnetization field created during the switching can further accelerate the reversal process [2]. The devices switch reliably at 0.7 V and 500 ps duration for both voltage polarities. We record the change of the device resistance in real time during the pulse to obtain the time needed to initiate the switching $\tau _{start}$ and the time between the initiation and the end of the switching $\tau _{switch}$ for every single switching event. $\tau _{switch}$ is determined to be less than a few hundreds of picoseconds, on the order of the precession time due to the demagnetization field and we find evidence for precession reversal under certain conditions. We further present results on the effects of pulse amplitude and applied field on $\tau _{start}$ and $\tau _{switch}$. This work was supported by Spin Transfer Technologies. [1] H. Liu et al., APL 97, 242510 (2010). [2] A. D. Kent et al., APL 84, 3897 (2004).   

Magnon contribution to the spin torque and magnetoresistance properties of FeCoB/MgO/FeCoB magnetic tunnel junctions

We have studied the spin-torque excited ferromagnetic resonance (ST-FMR) and the tunneling magnetoresistance (TMR) properties of FeCoB/MgO/FeCoB magnetic tunnel junctions as a function of temperature from 300K to 10K. We find that while the TMR increases by $\sim $ 50{\%} upon cooling to 10 K, the in-plane spin torque and the perpendicular or field-like torque both decrease substantially. The results demonstrate that while magnon-assisted tunneling degrades TMR, it acts to significantly enhance ST in MTJs, in accord with theoretical prediction. Moreover, the bias-dependent structure in both the asymmetry of the in-plane ST and the parallel conductance of the MTJ is more pronounced at low temperature, indicative of this asymmetry being due substantially to the interfacial electronic structure of the electrodes.   

Magnetic field dependence of spin torque switching in nanoscale magnetic tunnel junctions

Magnetic random access memory based on spin transfer torque effect in nanoscale magnetic tunnel junctions (STT-RAM) is emerging as a promising candidate for embedded and stand-alone computer memory. An important performance parameter of STT-RAM is stability of its free magnetic layer against thermal fluctuations. Measurements of the free layer switching probability as a function of sub-critical voltage at zero effective magnetic field (read disturb rate or RDR measurements) have been proposed as a method for quantitative evaluation of the free layer thermal stability at zero voltage. In this presentation, we report RDR measurement as a function of external magnetic field, which provide a test of the RDR method self-consistency and reliability.   

Shot noise in double barrier epitaxial magnetic tunnel junctions

Shot noise has been shown to be an effective tool to study statistics of electron tunnelling in single barrier magnetic tunnel junctions [1-2]. Here we report on shot noise and tunnelling magnetoresistance measurements in fully epitaxial Fe/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions. We observed that the shot noise is essentially determined by the barriers' symmetry and relative magnetic configuration. Enhanced barrier asymmetry effectively suppresses electron correlations, and the noise approaches the Poissonian limit. The proposed model of sequential tunnelling (with spin relaxation) through the magnetic layer inside the tunnel barrier satisfactory explains the experimental observations. [1] R.Guerrero, et al., Phys. Rev. Lett. 97, 0266602 (2006). [2] R.Guerrero, et al, Appl. Phys. Lett. 91, 132504 (2007).   

Planar approximation for spin-transfer devices with tilted polarizer

Planar spin-transfer devices with dominating easy-plane anisotropy can be described by an effective one-dimensional equation for the in-plane angle [1-3]. Such a description provides an intuitive qualitative understanding of the magnetic dynamics. We apply the effective planar equation to describe magnetic switching and precession states in devices with a tilted polarizer [4]. The approach allows one to understand the dynamic regimes and sketch the switching diagram without pefroming the detailed calculations. \\[4pt] [1] Ya. B. Bazaliy, Appl. Phys. Lett. 91, 262510 (2007).\\[0pt] [2] Ya. B. Bazaliy, Phys. Rev. B 76, 140402(R) (2007).\\[0pt] [3] Ya. B. Bazaliy and F. Arammash, Phys. Rev. B 84, 132404 (2011).\\[0pt] [4] Ya. B. Bazaliy, arXiv:1109.1331 (2011).   

Modification of the Stoner-Wohlfarth Astroid by a Spin-Polarized Current

Conventional Stoner-Wohlfarth astroid describes the field-induced switching of a nanomagnet with a uniaxial anisotropy. Both theory and experiments show that the spin-transfer torque can change the switching behavior of nanomagnets and therefore modify the astroid. Such a modification was recently analyzed in the limit of small currents [1]. To explore the modification of the astroid by a current of arbitrary magnitude we propose an exact method capable of finding the switching boundaries for a magnet described by an LLG equation with the Slonczewski's spin-torque term. Our approach takes into account both the local destabilization of equilibria and the equilibrium collisions. The self-crossing nature of the modified astroid is explained and a novel region with three stable equilibria is predicted in our result. \\[4pt] [1] Y. Henry, S. Mangin and J. Cucchiara, J. A. Katine, and E. E. Fullerton, Phys. Rev. B 79, 214422(2009).   

Origin of perpendicular magnetic anisotropy in Co/Ni multilayers on Ti layer

Magnetic materials in which their magnetic moment direction is oriented perpendicular to the plane of the magnetic layers in thin film heterostructures have been much studied for their potential application to spintronic devices. In particular, theories of current induced excitation, via the phenomenon of spin torque transfer, show that perpendicularly magnetized layers can be more easily excited or their magnetization direction switched than in-plane magnetized layers. In particular, Co/Ni multilayers are promising due to high spin polarization and small Gilbert damping compared to Co/Pt or Fe/Pt. However, their perpendicular magnetic anisotropy (PMA) is highly sensitive to the underlayer that is critical in device performance because, for instance, the current shunting can substantially reduce the spin transfer torque in magnetic racetrack memory. We observed an excellent PMA in annealed Co/Ni on Ti underlayer whose resistance is significantly greater than those of Co/Ni, thereby minimizing the current shunting. It is found that the PMA does not simply originate from magneto-crystalline effect (spin-orbit interaction) but mainly from magnetoelastic effect caused by compressive strain along (111) direction. We will present systematic results and quantitative analyses.   

Perpendicular magnetization of CoFeGe alloy films induced by MgO interface

The perpendicular magnetization of CoFeGe alloy films was achieved in the structures of CoFeGe/MgO with the perpendicular magnetic anisotropy energy density ($K_{u})$ of $\sim $ 1 x 10$^{6 }$erg/cm$^{3}$. The CoFeGe thickness dependence of $K_{u}$ was investigated, indicating that the perpendicular anisotropy of CoFeGe is contributed by the interfacial anisotropy between CoFeGe and MgO. High-resolution transmission electron microscope images clearly show formation of bcc crystalline structure of CoFeGe well lattice matched with the (100)-oriented MgO barrier. Gilbert damping constant for the films was evaluated by using ferromagnetic resonance measurement.   

Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions

We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.   

Spin Torque in Asymmetric CoFeB/MgO/FeB Magnetic Tunnel Junctions

Recent studies have shown that the use of asymmetric electrodes in MTJs can significantly affect spin torque (ST) behavior. We will report on the measurement via spin torque ferromagnetic resonance (ST-FMR) and switching phase diagram (SPD) studies of the in-plane and field-like (out-of-plane) torkance of low resistance, asymmetric IrMn/FeB/MgO/FeCoB) and IrMn/FeCoB/MgO/FeB MTJ nanopillars in the as-grown state (TMR$\sim $22{\%}) and the annealed state (TMR$\sim $90{\%}), and in comparison to that of symmetric counterparts; IrMn/FeB/MgO/FeB) and Ir/Mn/FeCoB/MgO/FeCoB MTJs. For the asymmetric MTJs only,the ST-FMR data show a strong field-like torkance for low voltage bias V that reverses sign when the free and pinned layers are reversed. At the higher V regime explored by the SPD the equivalent linear term in the field-like torque can dominate over the in-plane torque, resulting in either the parallel or antiparallel alignment being favored for both bias polarities, depending on the composition of the free layer.   

Influence of growth parameters on the spin-filtering properties of epitaxial ferrite tunnel barriers

In spintronics, spin-filtering is a physical phenomenon which has the potential to produce highly spin-polarized currents by the spin-selective transport of electrons across a ferromagnetic tunnel barrier. The insulating ferrites MnFe$_{2}$O$_{4}$, CoFe$_{2}$O$_{4}$ and NiFe$_{2}$O$_{4}$, whose Curie temperatures are above 300K, are promising candidates for spin-filtering at room temperature. In this work, we report on the in-depth study of structural, chemical and physical properties of epitaxial ferrite ultra-thin films and associated spin-filtering measurements as a function of different growth parameters. We analyze the effect of oxidation on the physical properties and the resultant spin-polarization. The influence of structural defects on the spin-filter efficiency is also put on view by tunneling magnetoresistance. Finally, we show the impact of the MgAl$_{2}$O$_{4}$(001) substrates on the magnetic behavior of cobalt ferrite tunnel barriers revealing the important role played by strains in the spin-filter properties.   

Magnetoelectric Control of Magnetic Anisotropy in Ultrathin Fe Films

Magnetoelectric switching of the magnetization vector could enable new low-power logic devices and non-volatile memory cells. Magnetoelectric switching typically requires complex multiferroic oxides or strain coupled magnetostrictive/piezoelectric composites. However, recently it has been demonstrated that surface magnetic anisotropy in ultrathin ferromagnetic metal films can be directly controlled by application of a strong electric field [1]. In this work we apply an electric field across a high-k oxide stack of MgO and ZrO$_{2}$ to induce charge at the surface of an ultrathin Fe film. By using high-k dielectric materials more charge can be induced at the surface of the ferromagnetic film and the efficiency of the magnetoelectric effect can be enhanced. Under application of just a few volts across the oxide stack we observe a strong magnetoelectric effect which results in a shift of the spin reorientation thickness by 0.5 atomic layers and a change in perpendicular surface anisotropy of $\sim $120$\mu $J/m$^{2}$. Moreover, by engineering the high-k oxide stack we realize a novel charge pumping mechanism that permits optical imprinting of the magnetic state in the continuous Fe film. [1] T. Maruyama \textit{et al.} Nature Nanotechnology 4, 158 - 161 (2009)