Volker Sluka

Direct measurement of the magnetic anisotropy field in Mn-Ga and Mn-Co-Ga Heusler films

The static and dynamic magnetic properties of tetragonally distorted Mn–Ga based alloys were investigated. Static properties are determined in magnetic fields up to 6.5 T using SQUID magnetometry. For the pure Mn1.6Ga film, the saturation magnetisation is 0.36 MA m−1 and the coercivity is 0.29 T. Partial substitution of Mn by Co results in Mn2.6Co0.3Ga1.1. The saturation magnetisation of those films drops to 0.2 MA m−1 and the coercivity is increased to 1 T.The time-resolved magneto-optical Kerr effect (TR-MOKE) is used to probe the high-frequency dynamics of Mn–Ga. The ferromagnetic resonance frequency extrapolated to zero-field is found to be 125 GHz with a Gilbert damping, α, of 0.019. The anisotropy field is determined from both SQUID and TR-MOKE to be 4.5 T, corresponding to an effective anisotropy density of 0.81 MJ m−3.Given the large anisotropy field of the Mn2.6Co0.3Ga1.1 film, pulsed magnetic fields up to 60 T are used to determine the field strength required to saturate the film in the plane. For this, the extraordinary Hall effect was employed as a probe of the local magnetisation. By integrating the reconstructed in-plane magnetisation curve, the effective anisotropy energy density for Mn2.6Co0.3Ga1.1 is determined to be 1.23 MJ m−3.

Spin-transfer torque induced vortex dynamics in Fe/Ag/Fe nanopillars

We report on the experimental and analytical work on spin-transfer torque induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. We study nanopillars with a diameter of 150?nm, containing two Fe layers with a thickness of 15?nm and 30?nm, respectively, separated by a 6?nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe discs. As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We experimentally show that in this configuration spin-transfer torque can excite vortex dynamics and analyse their dependence on a magnetic field applied in the sample plane. The centre of gyration is continuously dislocated from the disc centre, and the potential changes its shape with field strength. The latter is reflected in the field dependence of the excitation frequency. In the second part we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We analytically show that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations.

Zero-field spin-transfer oscillators combining in-plane and out-of-plane magnetized layers

Excited magnetization dynamics in a spin-valve device consisting of an in-plane polarizer and an out-of-plane free layer were studied numerically. In the case where the free layer is assumed to lack any in-plane anisotropy components, a finite external field is required to generate steady-state dynamics, in agreement with previous reports. We demonstrate that this constraint can be removed and precession can be stabilized in zero applied field by introducing an additional in-plane anisotropy axis. Moreover, the in-plane anisotropy offers an additional degree of freedom for tuning the frequency response of the device.

Stacked topological spin textures as emitters for multidimensional spin wave modes

The investigation of propagating spin waves is a key topic of contemporary magnetism research. For the excitation of spin waves with short wavelengths, it was typically necessary to either use transducers with sizes on the order of the desired wavelengths (striplines or point-contacts) or to generate those spin waves parametrically by a double-frequency spatially uniform microwave signal. Only recently, a novel mechanism for the local excitation of spin waves, which overcomes the wavelength limit given by the minimum patterning size has been discovered. This method utilizes the translation of natural topological defects, namely the gyration of spin vortex cores. A spin vortex is characterized by a planar, flux-closing magnetization curl, which tilts out of the plane in the central nanoscopic core region [cf. Fig. 1(a)]. Both, the in-plane rotation sense of the curl (circulation) and the orientation of the perpendicular core (polarity), are independently either positive or negative. The initial study was carried out on a vortex pair system with opposite circulations and equal polarities, in which the two vortices were stacked via a nonmagnetic inter-layer [cf. Fig. 1(b) and 1(c)]. In such a system, spin waves can be generated by lateral magnetic field excitation at the vortex cores. Scanning transmission x-ray microscopy (STXM) was used to directly image these spin waves propagating to the rim of the sample in a spiraling manner [cf. Fig. 1(d)]. Thereby, the resulting spin wave length was found to be directly tunable by the excitation frequency. Moreover, the resulting spin waves were analytically calculated to exhibit a gapless, linear, and non-reciprocal dispersion relation with much shorter wave lengths compared to spin waves of the same frequency in corresponding single layer films.

Evolution of the interfacial magnetic anisotropy in MgO/CoFeB/Ta/Ru based multilayers as a function of annealing temperature

We report the effect of annealing temperature on the dynamic and static magnetic properties of MgO/CoFeB/Ta/Ru multilayers. Angular resolved ferromagnetic resonance measurement results show that the as-deposited film exhibits in-plane magnetic anisotropy, whereas in the annealed films the magnetic easy-axis is almost along the direction perpendicular to the plane of the layers. The extracted interfacial anisotropy energy, Ki, is maximized at an annealing temperature 225∘C, in agreement with the vibrating sample magnetometry results. Although the magnetization is not fully out-of-plane, controlling the degree of the magnetization obliqueness may be advantageous for specific applications such as spin-transfer oscillators.

Spin-transfer effects in MgO-based tunnel junctions with an out-of-plane free layer and an in-plane polarizer: Static states and steady-state precession

This paper aims to explore potential mechanisms for sustaining steady-state precession in MgO-based magnetic tunnel junctions (MTJ) with an in-plane polarizer and an out-of-plane free layer. The Landau-Lifshitz-Gilbert-Slonczewski equation is analytically and numerically solved for a nano-pillar MTJ with circular cross-section under constant perpendicular applied current and field. It is demonstrated that the spin torque angular asymmetry is sufficient to sustain the spin transfer torque-driven dynamics of spin-torque nano-oscillators.

Zero-field spin transfer oscillators combining in-plane and out-of-plane magnetized free layers

Spin-transfer-torque driven magnetization dynamics in a spin-valve device consisting of an in-plane magnetized polarizer and an out-of-plane magnetized free layer were studied numerically. Such devices hold promise for nanoscale wireless transmitters operating at gigahertz frequencies, compatible with current mobile telephone and wireless local area network technologies [1]. In traditional spin-transfer-torque devices, with applications as memory elements (spin-transfer-torque MRAM), the magnetic easy axes of both the free and reference layers are co-linear (either in-plane magnetized or perpendicularly magnetized) in order to give the maximum difference in magnetoresistance between the two available storage states i .e . fully parallel or fully anti-parallel alignment. For spin-transfer-oscillators the situation is somewhat different. The criterion for having two stable static states with well separated resistance values is no longer an important factor. What is desired is a precessional orbit that passes through both the fully parallel and fully anti-parallel state as well as the maximisation of the torque in the initial state. For this, the most efficient geometry is one in which the free layer is magnetized out-of-plane and the polarizing layer is magnetized in-plane. For the ground state, the spin-transfer-torque efficiency is close to maximum as the angle between the two layers is 90°. The amplitude of oscillation is maximised as precession around the film normal allows passage through the parallel and anti-parallel states in one precession cycle [2,3].

Ferromagnetic resonance study of the perpendicular magnetic anisotropy in MgO/CoFeB/ Ta multilayers as a function of annealing temperature

MgO-based magnetic tunnel junctions (MTJs) are currently the structures of choice for magnetic random access memories (MRAMs), as they exhibit extremely high tunnel magnetoresistance (TMR) values due to highly effective spin-dependent tunneling [1, 2]. Initial studies focused on devices with both free and reference layers exhibiting in-plane remnant states [3, 4]. On the other hand, it has been reported that devices having the magnetic layers magnetized perpendicular to the layer interface offer a better trade-off between reducing the writing power and maintaining a thermal stability sufficient for data retention [5, 6]. It has also been recently demonstrated that CoFeB-based MgO-MTJs can exhibit perpendicular magnetic anisotropy (PMA), while maintaining the crystalline quality of the barrier required for achieving high TMR ratios, thus making them good candidates for next generation spin-transfer-torque (STT) MRAM [7].

Spin-Transfer Torque Effects in Single-Crystalline Nanopillars

We review our recent work on spin-transfer torque (STT) effects in single-crystalline, all-metal nanopillars. The experiments deal with current-driven magnetization switching and excitation of steady-state high-frequency magnetic oscillatory modes. The interplay between the magnetocrystalline anisotropy and STT gives rise to a two-step switching mechanism and to zero-field magnetic precession. Both are manifestations of the angular STT asymmetry and are explained within Slonczewski’s theory for currents and torques in metallic multilayers. The normal and inverse torques observed in a double spin-valve nanopillar are related to spin-dependent interface resistances and confirm ab initio calculations by Stiles and Penn. The magnetization of a nanodisk of suitable aspect ratio can be switched by STT between the quasi-uniform and the vortex configuration. The STT-excited gyrotropic mode of the vortex emits more microwave power than the standing-wave mode and can be locked to an external high-frequency signal in a wide frequency range.