J. Lindner

Spin caloritronic nano-oscillator

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.Harvesting ohmic heat for signal processing is one of major challenges in modern electronics and spin caloritronics, but not yet well accomplished. Here the authors demonstrate a spin torque oscillator device driven by pure spin current arising from thermal gradient across an Y3Fe5O12/Pt interface.

Tunnelling magnetoresistance of the half-metallic compensated ferrimagnet Mn2RuxGa

Tunnel magnetoresistance ratios of up to 40% are measured between 10 K and 300 K when the highly spin-polarized compensated ferrimagnet, Mn2RuxGa, is integrated into MgO-based perpendicular magnetic tunnel junctions. Temperature and bias dependences of the tunnel magnetoresistance effect, with a sign change near −0.2 V, reflect the structure of the Mn2RuxGa interface density of states. Despite magnetic moment vanishing at a compensation temperature of 200 K for x≈0.8, the tunnel magnetoresistance ratio remains non-zero throughout the compensation region, demonstrating that the spin-transport is governed by one of the Mn sub-lattices only. Broad temperature range magnetic field immunity of at least 0.5 T is demonstrated in the same sample. The high spin polarization and perpendicular magnetic anisotropy make Mn2RuxGa suitable for applications in both non-volatile magnetic random access memory cells and terahertz spin-transfer oscillators.

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.

Parameter-free determination of the exchange constant in thin films using magnonic patterning

An all-electrical method is presented to determine the exchange constant of magnetic thin films using ferromagnetic resonance. For films of 20 nm thickness and below, the determination of the exchange constant A, a fundamental magnetic quantity, is anything but straightforward. Among others, the most common methods are based on the characterization of perpendicular standing spin-waves. These approaches are however challenging, due to (i) very high energies and (ii) rather small intensities in this thickness regime. In the presented approach, surface patterning is applied to a permalloy (Ni80Fe20) film and a Co2Fe0.4Mn0.6Si Heusler compound. Acting as a magnonic crystal, such structures enable the coupling of backward volume spin-waves to the uniform mode. Subsequent ferromagnetic resonance measurements give access to the spin-wave spectra free of unquantifiable parameters and, thus, to the exchange constant A with high accuracy.

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].