We report measurements of the superconducting transition temperature, T c , in CoO/Co/Cu/Co/Nb multilayers as a function of the angle α between the magnetic moments of the Co layers. Our measurements reveal that T c (α) is a nonmonotonic function, with a minimum near α = π/2. Numerical self-consistent solutions of the Bogoliubov-de Gennes equations quantitatively and accurately describe the behavior of T c as a function of α and layer thicknesses in these superconductor / spin-valve heterostructures. We show that experimental data and theoretical evidence agree in relating T c (α) to enhanced penetration of the triplet component of the condensate into the Co/Cu/Co spin valve in the maximally noncollinear magnetic configuration.

Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here, we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in Y3Fe5O12/Pt bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the Y3Fe5O12 layer. This leads to excitation of auto-oscillations of the Y3Fe5O12 magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.

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We experimentally study nanowire-shaped spin-Hall nano-oscillators based on nanometer-thick epitaxial films of Yttrium Iron Garnet grown on top of a layer of Pt. We show that, although these films are characterized by significantly larger magnetic damping in comparison with the films grown directly on Gadolinium Gallium Garnet, they allow one to achieve spin current-driven auto-oscillations at comparable current densities, which can be an indication of the better transparency of the interface to the spin current. These observations suggest a route for improvement of the flexibility of insulator-based spintronic devices and their compatibility with semiconductor technology.

Magnetic tunnel junctions operating in the superparamagnetic regime are promising devices in the field of probabilistic computing, which is suitable for applications like high-dimensional optimization or sampling problems. Further, random number generation is of interest in the field of cryptography. For such applications, a device’s uncorrelated fluctuation time-scale can determine the effective system speed. It has been theoretically proposed that a magnetic tunnel junction designed to have only easy-plane anisotropy provides fluctuation rates determined by its easy-plane anisotropy field and can perform on a nanosecond or faster time-scale as measured by its magnetoresistance’s autocorrelation in time. Here, we provide experimental evidence of nanosecond scale fluctuations in a circular-shaped easy-plane magnetic tunnel junction, consistent with finite-temperature coupled macrospin simulation results and prior theoretical expectations. We further assess the degree of stochasticity of such a signal.

The exchange-stiffness and saturation magnetization for the CoFeB based free layer of perpendicularly magnetized tunnel junctions (MTJs) were determined by performing spin torque ferromagnetic resonance measurements over a range of different sized devices. The field dispersion of several low-frequency spin wave modes shows a size dependent shift in the resonance frequencies due to the change in the lateral confinement and demagnetization field. From the effect of the demagnetizing field, the free layer saturation magnetization is estimated to be ∼800 emu/cm3 and its total perpendicular anisotropy field ∼12 kOe. From the separation of spin wave dispersion relations, an exchange stiffness value of 0.35 eV Å2 is extracted.

A practical problem for memory applications involving perpendicularly magnetized magnetic tunnel junctions is the reliability of switching characteristics at high-bias voltage. Often it has been observed that at high-bias, additional error processes are present that cause a decrease in switching probability upon further increase of bias voltage. We identify the main cause of such error-rise process through examination of switching statistics as a function of bias voltage and applied field, and the junction switching dynamics in real time. These experiments show a coincidental onset of error-rise and the presence of a new low-frequency microwave emission well below that dictated by the anisotropy field. We show that in a few-macrospin coupled numerical model, this is consistent with an interface region with concentrated perpendicular anisotropy, and where the magnetic moment has limited exchange coupling to the rest of the layers. These results point to the important role high-frequency interface magnetic moment dynamics play in determining the switching characteristics of these tunnel junction devices.

We study thin films and magnetic tunnel junction nanopillars based on Ta/Co20Fe60B20/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co20Fe60B20/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories.The ferromagnet/oxide (FM/Ox) interfaces play an important role in modern spintronics. Insulating oxide layers sandwiched between two metallic ferromagnets form magnetic tunnel junctions (MTJs) [1][2][3] which find applications in magnetic sensors [4][5][6][7], spin torque oscillators [8][9][10][11], and non-volatile spin torque memory (STT-RAM) [12]. A number of FM/Ox interfaces exhibit large perpendicular magnetic anisotropy (PMA) [13][14][15][16] needed for enhancing thermal stability of . Furthermore, some FM/Ox interfaces exhibit magneto-electric coupling that allows control of the interfacial PMA with an electric field applied perpendicular to the interface. This effect, known as voltagecontrolled magnetic anisotropy (VCMA) [20][21][22][23], can be used for energy-efficient voltage-driven switching of magnetization at low current densities [15,24,25].The interface between Co x Fe y B z (CoFeB) ferromagnet and MgO insulator is one of the most important interfaces in spintronics because the STT-RAM technology is based on CoFeB/MgO/CoFeB MTJs [26]. Comprehensive understanding of structural, magnetic and electronic properties of CoFeB/MgO-based multilayers is at the forefront of applied spintronics research. In this Letter, we report magnetometry and electrical transport studies of (i) CoFeB/MgO/CoFeB nanoscale magnetic tunnel junctions and (ii) CoFeB/MgO based multilayer films. These studies reveal surprising anomalies in the temperature dependence of resistance and magnetization of the CoFeB films interfaced with MgO. Our data suggest that ultrathin magnetic oxide layers are formed at the CoFeB/MgO interfaces prepared under typical deposition and annealing conditions employed in fabrication of CoFeB/MgO/CoFeB MTJs with high tunneling magnetoresistance (TMR) [1,2]. Interestingly, the high TMR observed in these MTJs seems to be largely unaffected by the interfacial oxide formation.We Prior to patterning, the multilayers are annealed for 2 hours at 300• C in a 1 Tesla in-plane magnetic field that sets the exchange bias direction for the SAF bottom layer parallel to the long (easy) axis of the nanopillar.The CoFeB/MgO based multilayer film composition is Si/ SiOx/ Ta(5 nm)/ Co 20 Fe 60 B 20 (d)/ MgO(1.1 nm) /Ta(1 nm)/ Ru(2 nm), where the CoFeB layer thickness d ranges from 0.9 nm to 2.5 nm. In order to minimize the sample-to-sample variations, the films were grown in a single run without changing the deposition parameters. The films w...