In-plane Magnetized Free Layer Research Articles

High-frequency ultra-sensitive spin-torque diode based on Co-rich magnetic tunnel junction with in-plane magnetized free layer

Spin-torque diodes (STDs) based on magnetic tunnel junctions (MTJs) are newly developing spintronic devices. However, so far, the highly sensitive STDs reported in experiments have a low detection frequency, generally lower than 2 GHz, which limits the application of STDs in HF fields. Here, we develop Co-rich MTJs to enhance in-plane magnetic anisotropy and demonstrate an in-plane magnetized HF ultra-sensitive STD. By selecting the proper field and using an injection locking mechanism, HF microwave detection close to 5 GHz can be realized with a sensitivity greater than 3700 mV mW−1. This result may pave the way to developing spin-torque microwave detectors that are HF and highly sensitive.

A cryogenic spin-torque memory element with precessional magnetization dynamics

We present a study of precessional magnetization switching in orthogonal spin-torque spin-valve devices at low temperatures. The samples consist of a spin-polarizing layer that is magnetized out-of-the film plane and an in-plane magnetized free and reference magnetic layer separated by non-magnetic metallic layers. We find coherent oscillations in the switching probability, characterized by high speed switching (~200 ps), error rates as low as 10−5 and decoherence effects at longer timescales (~1 ns). Our study, which is conducted over a wide range of parameter space (pulse amplitude and duration) with deep statistics, demonstrates that the switching dynamics are likely dominated by the action of the out-of-plane spin polarization, in contrast to in-plane spin-torque from the reference layer, as has been the case in most previous studies. Our results demonstrate that precessional spin-torque devices are well suited to a cryogenic environment, while at room temperature they have so far not exhibited coherent or reliable switching.

Spin torque oscillator for microwave assisted magnetization reversal

A theoretical study is given for the self-oscillation excited in a spin torque oscillator (STO) consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer in the presence of a perpendicular magnetic field. This type of STO is a potential candidate for a microwave source of microwave assisted magnetization reversal (MAMR). It is, however, found that the self-oscillation applicable to MAMR disappears when the perpendicular field is larger than a critical value, which is much smaller than a demagnetization field. This result provides a condition that the reversal field of a magnetic recording bit by MAMR in nanopillar structure should be smaller than the critical value. The analytical formulas of currents determining the critical field are obtained, which indicate that a material with a small damping is not preferable to acheive a wide range of the self-oscillation applicable to MAMR, although such a material is preferable from the viewpoint of the reduction of the power consumption.

High power and low critical current density spin transfer torque nano-oscillators using MgO barriers with intermediate thickness

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωμm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (Pout) in the range of 1–40 nW. Here, Pout values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωμm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (JSTT) was observed, without any further change in the magnetic configuration.

Nonlinear stability analysis for a spin-torque nano-oscillator with a perpendicular spin polarizer: A pendulum-like model

An analytical pendulum-like model is developed to describe the dynamics of spin torque nano-oscillators that contain a perpendicular spin current polarizer combined with an in-plane magnetized free layer. Both the in-plane localized oscillation mode and out-of-plane (OP) large angle precession mode have been observed in the absence of damping effect, which can be attributed to the competition between the in-plane uniaxial anisotropic and demagnetization energies. These two types of dynamical oscillation modes correspond to the spin-transfer torque effect acting as a conservative or non-conservative driving force, respectively. We theoretically predict the critical currents for the excitation of the OP procession mode, which are well consistent with the macrospin simulations based on the Landau-Lifshitz-Gilbert equation. Moreover, we show the phase portrait of magnetization precession dynamics at different current and magnetic fields.

Critical current density of a spin-torque oscillator with an in-plane magnetized free layer and an out-of-plane magnetized polarizer

Spin-torque induced magnetization dynamics in a spin-torque oscillator with an in-plane (IP) magnetized free layer and an out-of-plane (OP) magnetized polarizer under IP shape-anisotropy field (Hk) and applied IP magnetic field (Ha) was theoretically studied based on the macrospin model. The rigorous analytical expression of the critical current density (Jc1) for the OP precession was obtained. The obtained expression successfully reproduces the experimentally obtained Ha-dependence of Jc1 reported in [D. Houssameddine et al., Nat. Mater. 6, 447 (2007)].

Zero-field spin transfer oscillators based on magnetic tunnel junction having perpendicular polarizer and planar free layer

We experimentally studied spin-transfer-torque induced magnetization oscillations in an asymmetric MgO-based magnetic tunnel junction device consisting of an in-plane magnetized free layer and an out-of-plane magnetized polarizer. A steady auto-oscillation was achieved at zero magnetic field and room temperature, with an oscillation frequency that was strongly dependent on bias currents, with a large frequency tunability of 1.39 GHz/mA. Our results suggest that this new structure has a high potential for new microwave device designs.

Crossover between fast and slow excitation of magnetization by spin torque

A crossover between two mechanisms destabilizing the magnetization in equilibrium by the spin transfer effect is found in a ferromagnetic multilayer consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer, where an in-plane magnetic field is applied, and electric current flows from the pinned to the free layer. A fast transition from the in-plane to the out-of-plane state occurs in the low-field region, whereas a slow transition with small-amplitude oscillation becomes dominant in the high-field region. On the other hand, only the fast transition mechanism appears for the opposite current direction.

Phase locking of spin-torque nano-oscillator pairs with magnetic dipolar coupling

A spin-torque nanopillar oscillator (STNO) that combines a perpendicular-to-plane polarizer (PERP) with an in-plane magnetized free layer is a good candidate for phase locking, which opens a potential approach to enhancement of the output power of STNOs. In this paper, the magnetic dipolar coupling effect is used as the driving force to synchronize two STNOs. We develop an approximation theory for synchronizing two identical and nonidentical pairs of PERP STNOs, by which the critical current of synchronization, dipolar coupling strength, phase-locking transient time, and frequency can be analytically predicted. These predictions are further confirmed by macrospin and micromagnetic simulations. Finally, we show the phase diagrams of the phase locking as a function of applied current and separation between two STNOs.

Instability analysis of spin-torque oscillator with an in-plane magnetized free layer and a perpendicularly magnetized pinned layer

We study the theoretical conditions to excite a stable self-oscillation in a spin torque oscillator with an in-plane magnetized free layer and a perpendicularly magnetized pinned layer in the presence of magnetic field pointing in an arbitrary direction. The linearized Landau-Lifshitz-Gilbert (LLG) equation is found to be inapplicable to evaluate the threshold between the stable and self-oscillation states because the critical current density estimated from the linearized equation is considerably larger than that found in the numerical simulation. We derive a theoretical formula of the threshold current density by focusing on the energy gain of the magnetization from the spin torque during a time shorter than a precession period. A good agreement between the derived formula and the numerical simulation is obtained. The condition to stabilize the out-of-plane self-oscillation above the threshold is also discussed.

Non-linear mode interaction between spin torque driven and damped modes in spin torque nano-oscillators

The influence of dynamic coupling in between magnetic layers of a standard spin torque nano-oscillator composed of a synthetic antiferromagnet (SyF) as a polarizer and an in-plane magnetized free layer has been investigated. Experiments on spin valve nanopillars reveal non-continuous features such as kinks in the frequency field dependence that cannot be explained without such interactions. Comparison of experiments to numerical macrospin simulations shows that this is due to non-linear interaction between the spin torque (STT) driven mode and a damped mode that is mediated via the third harmonics of the STT mode. It only occurs at large applied currents and thus at large excitation amplitudes of the STT mode. Under these conditions, a hybridized mode characterized by a strong reduction of the linewidth appears. The reduced linewidth can be explained by a reduction of the non-linear contribution to the linewidth via an enhanced effective damping. Interestingly, the effect depends also on the exchange interaction within the SyF. An enhancement of the current range of reduced linewidth by a factor of two and a reduction of the minimum linewidth by a factor of two are predicted from simulation when the exchange interaction strength is reduced by 30%. These results open directions to optimize the design and microwave performances of spin torque nano-oscillators taking advantage of the coupling mechanisms.

Magnetization dynamics of an in-plane magnetized synthetic ferrimagnetic free layer submitted to spin-transfer torques and applied field

An equilibrium stability analysis of an in-plane magnetized synthetic ferrimagnetic free layer (SyF) submitted to multiple spin-transfer-torque (STT) influences and to applied field has been carried out. Analytical expressions for the frequency and linewidth of the excitations were derived, in the case of strong coupling between the two layers compared to the anisotropy field. The expression of the critical current for the onset of excitations in the SyF was then calculated. Above the critical current, the destabilized mode, acoustic or optical, is found to depend on the applied field direction, but also on the asymmetry of the two layers composing the SyF. A strong thickness asymmetry reduces the critical current at zero field, and allows to select the destabilized mode. The critical current of an in-plane SyF with mutual spin torque between the layers and without reference layer was computed. For a symmetric SyF, the optical mode is the only mode that can be destabilized by STT.

Influence of Film Thickness on Magnetic Tunneling Junction Sensor with In-Plane/Out-of-Plane Sensing Capabilities

A magnetic tunnel junction (MTJ) sensor with out-of-plane or in-plane magnetized free layer to achieve a linear and hysteresis-free response is demonstrated. We show that two MTJ devices, with different magnetization arrangements in the free and reference layers, have different sensor responses. The sensor with out-of-plane magnetized free layer has in-plane sensing capability and that with the in-plane one has out-of-plane sensing capability. The present theoretical study based on the coherent rotation of the magnet moment in the free layer exhibits linear and nonlinear dependence of the sensitivity and sensing range, respectively, on the thickness of the free layer. These findings indicate that a sensor with high sensitivity can be achieved by precisely controlling the film thickness of the free layer.

Spin-transfer torque and current-induced switching in metallic spin valves with perpendicular polarizers

We present a theoretical description of spin-transfer torque in a spin valve with perpendicularly magnetized polarizer. The polarizer consisting of several ultrathin layers is considered as a single interfacial magnetic scatterer between two nonmagnetic layers, and is included in the theory based on diffusive transport via appropriate boundary conditions. The model has been used to study systematically the spin-transfer torque and current-induced switching in a spin valve with both perpendicular and in-plane polarizers and with in-plane magnetized free layer. The wave-function matching ab initio calculations have been used to determine transport parameters of the perpendicular polarizer. Additionally, the effect of disorder on the spin-transfer torque has been examined.

Reduction of critical current density in a spin valve with a perpendicular polarizer and a planar biaxial free layer

Ultrafast magnetization switching of a spin valve consisting of a perpendicularly magnetized polarizer and an in-plane magnetized free layer with uniaxial and higher order anisotropy is studied. Here we report the analytical results in which a modified asteroid and current–field state diagram are derived from an effective one-dimensional free energy for higher order anisotropy. Because additional crystalline axes provide an additional nutation channel, the critical current density for the biaxial case is smaller than that commonly observed for quadratic anisotropy. This will be of importance for the design of spin-torque-transfer magnetic random access memories and nano-oscillators.

Spin transfer torque devices utilizing the giant spin Hall effect of tungsten

We report a giant spin Hall effect in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip, we determine the spin Hall angle to be |θSHβ-W|=0.30±0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices, we independently determine |θSHβ-W|=0.33±0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.

Characterization of interlayer interactions in magnetic random access memory layer stacks using ferromagnetic resonance

Ferromagnetic resonance spectroscopy is used to determine magnetic interactions in layer stacks designed for orthogonal spin-transfer magnetic random memory devices. The stacks have layers with different anisotropy directions and coupling, consisting of a perpendicularly magnetized polarizer, an in-plane magnetized free layer, and an in-plane magnetized exchange biased synthetic antiferromagnetic layer. The oscillatory exchange coupling strength in the synthetic antiferromagnet was measured along with its exchange bias. The free layer properties were also determined. It is demonstrated that this one integrated measurement technique is able to provide quantitative measurements of key magnetic parameters in a complex layer stack, which is a prerequisite for high turn-around device materials characterization and optimization.

Ultrafast switching in magnetic tunnel junction based orthogonal spin transfer devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

100 ps precessional spin-transfer switching of a planar magnetic random access memory cell with perpendicular spin polarizer

Ultrafast spin-transfer precessional switching between two stable states of a magnetic random access memory device is demonstrated in structures comprising a perpendicularly magnetized polarizing layer (PL⊥), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL) in a PL⊥/spacer/FL/spacer/AL stack. Back and forth switching can be achieved with sub-ns current pulses of the same polarity. The spin-torque influence from the analyzer leads to an asymmetric dependence of the switching properties as a function of the current sign and initial state.

Spin transfer switching in TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy

Spin transfer (ST) switching in the TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junction (MTJ) was studied. The TbCoFe∕CoFeB free layer with a large coercive field of 1.2kOe and a large thermal stability factor of 107 at room temperature was switched by a 100ns pulse current with a current density of 4.7MA∕cm2. This is the first report of ST switching in a MTJ with perpendicular magnetic anisotropy. The temperature dependence of the coercive field was also investigated to estimate the magnetic anisotropy in the case of rising temperature due to the Joule heating effect. The measured coercive field at 87°C, which was the simulated temperature during the switching pulse current, was about 0.34kOe. The ratio of the switching current density to the coercive field under the switching current in the MTJ with the TbCoFe∕CoFeB free layer is smaller than that in a typical MTJ with an in-plane magnetized CoFeB free layer. This result indicates that a MTJ with perpendicular magnetic anisotropy is advantageous for realizing low switching current magnetoresistive random access memory.