M. Czapkiewicz

Magnetization reversal and field annealing effects in perpendicular exchange-biased Co∕Pt multilayers and spin valves with perpendicular magnetization

A study on the Co layer thickness dependence of the magnetization reversal process and magnetoresistance of perpendicular exchange-biased Co∕Pt multilayers and spin valves with perpendicular magnetization is presented. The hysteresis of [20APt∕tACo]3∕100A IrMn∕20A Pt multilayers with t⩾5A are found to be asymmetric after deposition. This asymmetry reflects a lateral variation in the perpendicular exchange-bias direction due to the growth of IrMn onto multidomain Co∕Pt multilayers. Magnetic annealing in a perpendicular field removes the switching asymmetry and leads to a maximum exchange-bias field of 16.3mT for t=7A. Perpendicular spin valves with an optimized structure of 50 Ta∕20A Pt∕tA Co∕30A Cu∕tA Co∕20A Pt∕6A Co∕3A Pt∕100 IrMn∕20A Pt are found to exhibit good switching behavior but limited magnetoresistance for small Co layer thickness. The magnetoresistance of these spin valves increases with t up to a maximum of 3.9% for t=10A, beyond which it decreases due to simultaneous magnetization reversal in...

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

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.

Reduced domain wall pinning in ultrathin Pt/Co100−xBx/Pt with perpendicular magnetic anisotropy

We have studied the magnetization reversal process in perpendicularly magnetized ultrathin Pt/Co100−xBx/Pt films by means of magneto-optical magnetometry and microscopy. The addition of boron enhances the effective Barkhausen volume indicating a decrease in domain-wall pinning site density and/or strength. This potentially reduces the field and critical current-density for domain-wall depinning/motion, indicating that perpendicularly magnetized Pt/Co100−xBx/Pt could be an interesting candidate for domain-wall motion studies and applications.

Magnetometer for the measurements of the hysteresis loop of ultrathin magnetic layers

In this paper the resonance-VSM (based on the original Foner solution) designed for measurements of ultrathin films is presented. The measuring head is universal and could be working in the Helmholtz coils as well as in an electromagnet. The resolution of the magnetic field was 0.01 Oe and the sensitivity measurements of magnetic moment 10 -4 emu. The dynamic parameters of the sample vibration are: maximum of amplitude 8 mm and frequency 65 <f < 85 Hz. The test measurements were performed on epitaxial monolayers of Fe.

The study of conductance in magnetic tunnel junctions with a thin MgO barrier: The effect of Ar pressure on tunnel magnetoresistance and resistance area product

The thickness dependence of tunnel magnetoresistance and resistance area product in Co40Fe40B20/MgO wedge/Co40Fe40B20 magnetic tunnel junctions (MTJs) has been studied for multiple Ar partial pressure (PAr) values during MgO sputtering. The extension of the simple equivalent circuit model [B. Oliver et al., J. Appl. Phys. 91, 4348 (2002)] has been suggested in order to include different transport mechanism contributions to the overall conductance of the MTJ as a function of the MgO barrier thickness. Parameters of the model, used for quantitative description of the conductivity of unpatterned MTJ stacks, have been analyzed as a function of PAr.

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

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...

Structure and magnetic properties of Co nanowires electrodeposited into the pores of anodic alumina membranes

Cobalt nanowires were obtained in the process of electrodeposition into pores of an alumina membrane. Structural research (XRD, TEM) of Co revealed the face-centered cubic structure. However, the existence of the hexagonal structure cannot be excluded due to strong texture. The influences of an external magnetic field and Al2O3 membrane geometry on magnetic properties of obtained wires were examined. It was found that cobalt nanowires exhibit pronounced shape anisotropy in a direction parallel to the wire axis. The highest influence on the magnetic properties is ascribed to the nanowires geometry i.e., height, diameter, and distances between single wires. Application of an external magnetic field in a perpendicular direction to the sample surface during cobalt electrodeposition increases magnetic anisotropy with a privileged direction along the wire axis. Application of the magnetic field in a parallel direction to the sample surface changes the direction of magnetization.

Correlation between exchange bias dynamics and magnetization reversal asymmetry in [Pt/Co]3/Pt/IrMn multilayers

The dependence of exchange bias field on magnetic field sweep rate was investigated for [Pt∕Co]3∕t Pt∕IrMn multilayers. For structures with t=0nm and t=0.3nm, the bias field decreases above a field sweep rate of a few T/s. The observed reduction of exchange bias at high frequencies correlates with a magnetization reversal asymmetry at low field sweep rates. Multilayers with t=0.1nm exhibit symmetric reversal and constant bias. The correlation between exchange bias dynamics and magnetization reversal asymmetry is explained by the frequency dependence of the domain wall motion to domain nucleation ratio.

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

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.