Witold Skowroński

Zero-Field Spin Torque Oscillator Based on Magnetic Tunnel Junctions with a Tilted CoFeB Free Layer

We present a study of the spin transfer torque oscillator based on CoFeB/MgO/CoFeB asymmetric magnetic tunnel junctions. We observe microwave precession in junctions with different thickness of the free magnetization layer. Taking advantage of the ferromagnetic interlayer exchange coupling between the free and reference layer in the MTJ and perpendicular interface anisotropy in thin CoFeB electrode we demonstrate the nanometer scale device that can generate high frequency signal without external magnetic field applied. The amplitude of the oscillation exceeds 10 nV/ √ Hz at 1.5 GHz.Microwave emission from spin torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions is analyzed with respect to the thickness of the magnetically free electrode. Taking advantage of the ferromagnetic interlayer exchange coupling between the free and reference layers and the perpendicular interface anisotropy of thin CoFeB electrodes on MgO, we demonstrate that large-amplitude oscillations of the tilted CoFeB free layer can be generated in zero applied magnetic field.

Perpendicular magnetic anisotropy of Ir/CoFeB/MgO trilayer system tuned by electric fields

The perpendicular magnetic anisotropy of an Ir/CoFeB/MgO trilayer was investigated after annealing at temperatures ranging from 200 to 350 °C. In the trilayer system annealed at 300 °C, we measured an interface anisotropy energy of 1.9 mJ/m2. Further annealing led to mixing of the buffer and ferromagnet, degrading the properties of the latter. In addition, we show the dependence of the magnetic anisotropy on the bias voltage. The presented system is important for the development of perpendicular magnetic tunnel junctions for storage applications.

Magnetic field sensor with voltage-tunable sensing properties

We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel junctions. By taking advantage of the perpendicular magnetic anisotropy of the MgO/CoFeB interface, the magnetization of the sensing layer is tilted out-of-plane which results in a linear magnetoresistance response to in-plane magnetic fields. The application of a bias voltage across the MgO tunnel barrier of the sensor affects the magnetic anisotropy and thereby its sensing properties. We propose a voltage-tunable magnetic field sensor design that allows for active control of the sensitivity and the operating field range by the strength and polarity of the applied bias voltage.

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier (0.75 nm < t(MgO) < 1.05 nm) is studied using the spin-torque diode effect. The application of an rf current into nanosized MTJs generates a dc mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin-transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different dc bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed. DOI: 10.1103/PhysRevB.87.094419

Spin-torque diode radio-frequency detector with voltage tuned resonance

We report on a voltage-tunable radio-frequency (RF) detector based on a magnetic tunnel junction (MTJ). The spin-torque diode effect is used to excite and/or detect RF oscillations in the magnetic free layer of the MTJ. In order to reduce the overall in-plane magnetic anisotropy of the free layer, we take advantage of the perpendicular magnetic anisotropy at the interface between ferromagnetic and insulating layers. The applied bias voltage is shown to have a significant influence on the magnetic anisotropy, and thus on the resonance frequency of the device. This influence also depends on the voltage polarity. The obtained results are accounted for in terms of the interplay of spin-transfer-torque and voltage-controlled magnetic anisotropy effects.

Reduction of low frequency magnetic noise by voltage-induced magnetic anisotropy modulation in tunneling magnetoresistance sensors

We demonstrate the reduction and control of magnetic noise by voltage-induced perpendicular anisotropy modulation in CoFeB/MgO/CoFeB sensors. The noise decreases with the increase of the perpendicular anisotropy energy induced by the bias voltage polarity reversal. The bias reversal between −1 and +1 V results in a reduction of the normalized 1/f magnetic noise parameters by a factor of 7.3 and the thermal magnetic noise by a factor of 6.8. In the state of the highest field sensitivity, the lowest normalized 1/f magnetic noise parameter reaches 6.45 × 10−14 μm3T. The results indicate that voltage-induced anisotropy modulation can be used to control and suppress magnetization fluctuations in the sensing layer and thus, significantly reduce the magnetic noise.

Inductive determination of the optimum tunnel barrier thickness in magnetic tunneling junction stacks for spin torque memory applications

We use pulsed inductive microwave magnetometry to study the precessional magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic tunneling junction stacks with varying MgO barrier thickness. From the field dependence of the precession frequency we are able to derive the uniaxial anisotropy energy of the free layer and the exchange coupling between the free and the pinned layer. Furthermore the field dependence of the effective damping parameter is derived. Below a certain threshold barrier thickness we observe an increased effective damping for antiparallel orientation of free and pinned layer which would inhibit reversible low current density spin torque magnetization reversal. Such inductive measurements, in combination with wafer probe station based magneto transport experiments, allow a fast determination of the optimum tunnel barrier thickness range for spin torque memory applications in a lithography free process.

Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier

Current induced magnetization switching and interlayer exchange coupling (IEC) in sputtered CoFeB/MgO/CoFeB exchange-biased magnetic tunnel junctions with an extremely thin (0.96–0.62 nm) MgO wedge barrier is investigated. The IEC is found to be ferromagnetic for all samples and the associated energy increases exponentially down to a barrier thickness of 0.7 nm. Nanopillars with resistance area product ranging from 1.8 to 10 Ω μm2 and sizes of 0.13 μm2 down to 0.03 μm2 and tunneling magnetoresistance values of up to 170% were prepared. We found, that the critical current density increases with decreasing MgO barrier thickness. The experimental data and theoretical estimations show that the barrier thickness dependence of the spin transfer torque can largely be explained by a reduction in the tunnel current polarization at very small barrier thickness.

Room-temperature perpendicular magnetic anisotropy of MgO/Fe/MgO ultrathin films

We used the anomalous Hall effect to study the magnetic properties of MgO/Fe(t)/MgO(001) structures in which the Fe thickness t ranged from 4 A to 14 A. For the iron deposited at 140 K, we obtained perpendicular magnetization at room temperature below the critical thickness of tc = (9 ± 1) A. In the vicinity of tc, the easy magnetization axis switched from an out-of-plane orientation to an in-plane orientation, and the observed spin-reorientation transition was considered in terms of the competition among different anisotropies. The perpendicular magnetization direction was attributed to magnetoelastic anisotropy. Finally, the temperature-dependent spin-reorientation transition was analyzed for Fe thicknesses close to tc.

Influence of intermixing at the Ta/CoFeB interface on spin Hall angle in Ta/CoFeB/MgO heterostructures

When a current is passed through a non-magnetic metal with strong spin-orbit coupling, an orthogonal spin current is generated. This spin current can be used to switch the magnetization of an adjacent ferromagnetic layer or drive its magnetization into continuous precession. The interface, which is not necessarily sharp, and the crystallographic structure of the nonmagnetic metal can both affect the strength of current-induced spin-orbit torques. Here, we investigate the effects of interface intermixing and film microstructure on spin-orbit torques in perpendicularly magnetized Ta/Co40Fe40B20/MgO trilayers with different Ta layer thickness (5 nm, 10 nm, 15 nm), greater than the spin diffusion length. Effective spin-orbit torques are determined from harmonic Hall voltage measurements performed at temperatures ranging from 20 K to 300 K. We account for the temperature dependence of damping-like and field-like torques by including an additional contribution from the Ta/CoFeB interface in the spin diffusion model. Using this approach, the temperature variations of the spin Hall angle in the Ta underlayer and at the Ta/CoFeB interface are determined separately. Our results indicate an almost temperature-independent spin Hall angle of