Tomasz Stobiecki

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.

Influence of buffer layer texture on magnetic and electrical properties of IrMn spin valve magnetic tunnel junctions

Spin valve magnetic tunnel junctions (SV-MTJs) with the structure Si(100)∕SiOx∕buffer(A,B)∕IrMn∕CoFe∕AlOx∕NiFe∕Ta have been deposited on two buffers: Cu (group A) and Ta∕Cu (group B). The A junctions were characterized by a low degree of texture and a small amplitude of roughness, and B junctions by a high degree of texture and a high amplitude of roughness. The strongly textured buffer Ta∕Cu (group B) grew in a columnarlike fashion and induced interfacial roughness. The texture and the roughness modified strongly the interlayer and the exchange bias coupling fields in the SV-MTJs. A substantial influence of the roughness, due to barrier thickness fluctuation, on the resistance area product (R×A) of junctions was also observed. The influence on the temperature dependence of conductance and tunnel magnetoresistance (TMR) was, however, small and only observable at low temperature for the two groups of junctions. A significantly larger increase of the conductance and decrease of the TMR with increasing bias ...

Magnetic Tunnel Junctions Based on Out-of-Plane Anisotropy Free and In-Plane Pinned Layer Structures for Magnetic Field Sensors

We have studied hysteresis, nonlinearity and sensitivity of CoFeB-MgO based magnetic tunnel junctions with out-of-plane and in-plane magnetizations configuration in the CoFeB free and reference layers. The configuration was achieved by using perpendicular interface anisotropy in the free and exchange bias structure in the pinned layers. For the CoFeB thickness range from 1.05 to 1.4 nm the junctions show linear transfer curves. In this CoFeB thickness range devices show linear range from ± 3 Oe to ± 1 kOe, hysteresis better than 0.6 Oe, nonlinearity 1% of the full scale and sensitivity in the range from 0.0045%/Oe to 3.8%/Oe.

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...

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.

MOSSBAUER STUDY OF ION-BEAM MIXING OF FE/ZR MULTILAYERS

The Ar‐ion‐beam mixing of Fe/Zr multilayers is studied in detail by conversion electron Mossbauer spectroscopy (CEMS) and x‐ray diffraction (XRD). The dependence of the ion‐beam induced amorphization and interfacial mixing on the sublayer thickness and ion dose (1×1013–2×1016 Ar/cm2) is studied systematically for samples with Fe to Zr thickness ratios dFe/dZr=1 and 0.5 and modulation wavelengths Λ=dFe+dZr of 5–80 nm and 7.5–90 nm, respectively. The CEMS results allowed the evaluation of the mixing efficiency from the thickness of the mixed layers. The experimentally determined mixing efficiency was compared with theoretical estimates based on the ballistic collision and thermal spike models, showing good agreement with the predictions of the modified ballistic collision model. For high degrees of amorphization the composition of the amorphous phase formed due to ion‐beam mixing is close to the nominal composition of the sample, as revealed by CEMS measurements. These results were compared with those obtai...

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

Microstructure and magnetic properties of Fe/Ti multilayers

The microstructure and magnetic properties of Fe/Ti multilayer films prepared by rf-sputtering deposition are studied via conversion electron Mossbauer spectroscopy (CEMS), x-ray diffraction (XRD), and magnetic techniques. Samples with the two Ti-to-Fe thickness ratios and , and with modulation wavelengths of 5 to 80 nm were studied. The XRD, electrical conductivity and coercive-field measurements revealed that the amorphous phase is formed during deposition and is distributed in the plane between the crystalline sublayers as well as in the grain boundaries. From CEMS measurements the relative fractions of the various phases (-Fe, the interfacial FeTi crystalline phase and the amorphous phase) were determined. The thickness of the mixed interfacial region was estimated for various values of and . The CEM spectra revealed that the spins in the Fe sublayers are aligned in the plane of the film, while in the interfacial regions they are randomly oriented. It is shown that the films with are almost entirely amorphous. The relative fraction of the amorphous phase decreases dramatically with increasing .

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.

Low resistance magnetic tunnel junctions with MgO wedge barrier

We investigated the dependence of tunnel magnetoresistance (TMR), the resistance area (RA) product, crystallographic structure, the interlayer exchange coupling (IEC) and the coercive (Hc) fields in Co40Fe40B20/MgO/Co40Fe40B20 exchange biased spin valves on the Ar partial pressure, in the range from 1 mTorr to 15 mTorr, during the MgO sputtering. For all the metallic layers a linear dynamic deposition (LDD) with a constant wafer velocity was used, whereas for the MgO deposition a LDD wedge technology with gradient velocity. The crystallographic texture of MgO(001), IEC and Hc fields, TMR and RA significantly depend on the Ar partial pressure. We found that high Ar partial pressure (above 5.6 mTorr) resulted in weaker (001) texture, lower interplanar distances of the MgO barrier and higher IEC field. Low Ar partial pressure gives smoother MgO surfaces and provides good barrier quality which results in higher TMR and RA values