Marek Frankowski

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.

Rectification of radio-frequency current in a giant-magnetoresistance spin valve

We report on a highly efficient spin diode effect in an exchange-biased spin-valve giant magnetoresistance (GMR) strips. In such multilayer structures, symmetry of the current distribution along the vertical direction is broken and, as a result, a non-compensated Oersted field acting on the magnetic free layer appears. This field, in turn, is a driving force of magnetization precessions. Due to the GMR effect, resistance of the strip oscillates following the magnetization dynamics. This leads to rectification of the applied radio frequency current and induces a direct current voltage

Buffer influence on magnetic dead layer, critical current, and thermal stability in magnetic tunnel junctions with perpendicular magnetic anisotropy

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Spatial Spectrum Analyzer (SSA): A tool for calculations of spatial distribution of fast Fourier transform spectrum from Object Oriented Micromagnetic Framework output data☆

. We present a theoretical description of this phenomenon and calculate the spin diode signal,

Understanding stability diagram of perpendicular magnetic tunnel junctions

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Antiferromagnetic nano-oscillator in external magnetic fields

, as a function of frequency, external magnetic field, and angle at which the external field is applied. A satisfactory quantitative agreement between theoretical predictions and experimental data has been achieved. Finally, we show that the spin diode signal in GMR devices is significantly stronger than in the anisotropic magnetoresistance permalloy-based devices.

Electric-field tunable spin waves in PMN-PT/NiFe heterostructure: Experiment and micromagnetic simulations

We present a detailed study of Ta/Ru-based buffers and their influence on features crucial from the point of view of applications of Magnetic Tunnel Junctions (MTJs) such as critical switching current and thermal stability. We study buffer/FeCoB/MgO/Ta/Ru and buffer/MgO/FeCoB/Ta/Ru layers, investigating the crystallographic texture, the roughness of the buffers, the magnetic domain pattern, the magnetic dead layer thickness, and the perpendicular magnetic anisotropy fields for each sample. Additionally, we examine the effect of the current induced magnetization switching for complete nanopillar MTJs with lateral dimensions of 270 × 180 nm. Buffer Ta 5/Ru 10/Ta 3 (thicknesses in nm), which has the thickest dead layer, exhibits a much larger thermal stability factor (63 compared to 32.5) while featuring a slightly lower critical current density value (1.25 MA/cm2 compared to 1.5 MA/cm2) than the buffer with the thinnest dead layer Ta 5/Ru 20/Ta 5. We can account for these results by considering the differen...

Spin-Torque Oscillator Read Head in Inhomogeneous Magnetic Fields

Abstract We present a tool for calculations of Fourier transform spatial distribution taken from magnetization dynamics simulated in Object Oriented Micromagnetic Framework (OOMMF). In OOMMF, as well as in other popular micromagnetic software, output data is organized as magnetization vectors from each simulation cell written down to separate file for each simulation step. Therefore, we use parallel computations to reorganize data in files containing time evolution for each cell. Fast Fourier transform is obtained for selected time period by parallel computations using Matlab. The output is a spatial distribution of the magnitude for the selected frequency in the sample cross-section. It allows for analysis of spin waves localization and therefore helps to understand their origin in investigated sample. Program summary Program title: Spatial Spectrum Analyzer (SSA) Catalogue identifier: AEUU_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEUU_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 6459509 No. of bytes in distributed program, including test data, etc.: 134249057 Distribution format: tar.gz Programming language: Bourne Again SHell (Bash), MATLAB. Computer: Any computer with MATLAB and Bourne Again SHell (Bash) installed. Operating system: Any system with MATLAB and Bourne Again SHell (Bash) installed. Classification: 9. External routines: MATLAB Parallel Computing Toolbox Nature of problem: Numerous dynamic problems of ferromagnetic structures can be investigated by micromagnetic simulations using The Object Oriented MicroMagnetic Framework (OOMMF). However, large amounts of OOMMF output data (typically magnetization configuration files take up several gigabytes for few nanoseconds simulation, depending on the structure size) are difficult to process because of being stored in separate files for each simulation step. In particular, for the Fourier Transform (FT) purposes data from all time steps in a single point is needed, but instead the standard output provided by OOMMF and other popular micromagnetic software contains data from all points in a single time step. What is more, calculations of FT for each simulation cell are usually a demanding task. The total time for sequential analysis can exceed the simulation time itself up to several times. Solution method: The SSA tool reorganizes the simulation data into separate files, which describe a single simulation point each and contain values from all time steps. Afterwards, reorganized files are used to compute Fast Fourier Transform for a chosen frequency. The final result is a spectral density map of a sample at a given frequency. Both parts of the SSA tool make use of parallel computing, greatly decreasing the total time needed to process the data. Additional comments: The distribution file for this program is over 134 Mbytes and therefore is not delivered directly when Download or E-mail is requested. Instead a html file, giving details of how the program can be obtained, is sent. Running time: Strongly depends on sample size and time span. Time for the example described in this paper varies from single hours to few days depending on the number of involved processes.

Influence of different buffers on magnetic dead layer, critical current and thermal stability in magnetic tunnel junctions with perpendicular magnetic anisotropy

Perpendicular magnetic tunnel junctions (MTJ) with a bottom pinned reference layer and a composite free layer (FL) are investigated. Different thicknesses of the FL were tested to obtain an optimal balance between tunneling magnetoresistance (TMR) ratio and perpendicular magnetic anisotropy. After annealing at 400 °C, the TMR ratio for 1.5 nm thick CoFeB sublayer reached 180% at room temperature and 280% at 20 K with an MgO tunnel barrier thickness corresponding to the resistance area product RA = 10 Ohmμm2. The voltage vs. magnetic field stability diagrams measured in pillar-shaped MTJs with 130 nm diameter indicate the competition between spin transfer torque (STT), voltage controlled magnetic anisotropy (VCMA) and temperature effects in the switching process. An extended stability phase diagram model that takes into account all three effects and the effective damping measured independently using broadband ferromagnetic resonance technique enabled the determination of both STT and VCMA coefficients that are responsible for the FL magnetization switching.