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Kilian Lenz

Publications and source records attributed to Kilian Lenz.

27 records · Page 2Linked to original sources
arXiv2019

Experimental evidence of inertial dynamics in ferromagnets

The understanding of how spins move at pico- and femtosecond time scales is the goal of much of modern research in condensed matter physics, with implications for ultrafast and more energy-efficient data storage. However, the limited comprehension of the physics behind this phenomenon has hampered the possibility of realising a commercial technology based on it. Recently, it has been suggested that inertial effects should be considered in the full description of the spin dynamics at these ultrafast time scales, but a clear observation of such effects in ferromagnets is still lacking. Here, we report the first direct experimental evidence of inertial spin dynamics in ferromagnetic thin films in the form of a nutation of the magnetisation at a frequency of approximately 0.6 THz. This allows us to evince that the angular momentum relaxation time in ferromagnets is on the order of 10 ps.

cond-mat.mes-hall↗
arXiv2019

Thermally driven two-magnet nano-oscillator with large spin-charge conversion

Next-generation spintronic applications require material properties that can be hardly met by one material candidate. Here we demonstrate that by combining insulating and metallic magnets, enhanced spin-charge conversion and energy-efficient thermal spin currents can be realized. We develop a nanowire device consisting of an yttrium iron garnet and permalloy bi-layer. An interfacial temperature gradient drives the nanowire magnetization into auto-oscillations at gigahertz frequencies. Interfacial spin coupling and magnetoresistance of the permalloy layer translate spin dynamics into sizable microwave signals. The results show prospect for energy-efficient spintronic devices and present an experimental realization of magnon condensation in a heterogeneous magnetic system.

cond-mat.mes-hall↗
arXiv2018

Spin-Wave Modes in Transition from a Thin Film to a Full Magnonic Crystal

Spin-wave modes are studied under the gradual transition from a flat thin film to a 'full' (one-dimensional) magnonic crystal. For this purpose, the surface of a pre-patterned 36.8 nm thin permalloy film was sequentially ion milled resulting in magnonic hybrid structures, referred to as surface-modulated magnonic crystals, with increasing modulation depth. After each etching step, ferromagnetic resonance measurements were performed yielding the spin-wave resonance modes in backward-volume and Damon-Eshbach geometry. The spin-wave spectra of these hybrid systems reveal an even larger variety of spin-wave states compared to the 'full' magnonic crystal. The measurements are corroborated by quasi-analytical theory and micromagnetic simulations in order to study the changing spin-wave mode character employing spin-wave mode profiles. In backward-volume geometry, a gradual transition from the uniform mode in the film limit to a fundamental mode in the thin part of the magnonic crystal was observed. Equivalently, the first and the second film modes are transform into a center and an edge mode of the thick part of the magnonic crystal. Simple transition rules from the $n^{\mathrm{th}}$ film mode to the $m^{\mathrm{th}}$ mode in the 'full' magnonic crystal are formulated unraveling the complex mode structure particularly in the backward-volume geometry. An analogous analysis was performed in the Damon-Eshbach geometry.

cond-mat.mes-hall↗
arXiv2018

Symmetry and localization properties of defect modes in magnonic superlattices

Symmetry and localization properties of defect modes of a one-dimensional bi-component magnonic superlattice are theoretically studied. The magnonic superlattice can be seen as a periodic array of nanostripes, where stripes with different width, termed as defect stripes, are periodically introduced. By controlling the geometry of the defect stripes, a transition from dispersive to practically flat spin-wave defect modes can be observed inside the magnonic band gaps. It is shown that the spin-wave profile of the defect modes can be either symmetric or antisymmetric, depending on the geometry of the defect. Due to the localized character of the defect modes, a particular magnonic superlattice is proposed, wherein the excitation of either symmetric or antisymmetric flat magnonic modes is enabled at the same time. Also, it is demonstrated that the relative frequency position of the antisymmetric mode inside the band gap does not significantly change with the application of an external field, while the symmetric modes move to the edges of the frequency band gaps. The results are complemented by numerical simulations, where an excellent agreement is observed between both methods. The proposed theory allows exploring different ways to control the dynamic properties of the defect modes in metamaterial magnonic superlattices, which can be useful for applications on multifunctional microwave devices operating over a broad frequency range.

cond-mat.mes-hall↗
arXiv2017

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 $\mathrm{Y_{3}Fe_{5}O_{12}/Pt}$ bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the $\mathrm{Y_{3}Fe_{5}O_{12}}$ layer. This leads to excitation of auto-oscillations of the $\mathrm{Y_{3}Fe_{5}O_{12}}$ magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.

cond-mat.mes-hall↗
arXiv2016

The role of the internal demagnetizing field in a surface-modulated magnonic crystal

Magnonic crystals with locally alternating properties and specific periodicities exhibit interesting effects, such as a multitude of different spin-wave states and large band gaps. This work aims for demonstrating and understanding the key role of local demagnetizing fields in such systems. To achieve this, hybrid structures are investigated consisting of a continuous thin film with a stripe modulation on top favorable due to the adjustability of the magnonic effects with the modulation size. For a direct access to the spin dynamics, a magnonic crystal was reconstructed from `bottom-up', i.e., the structural shape as well as the internal field landscape of the structure were experimentally obtained on the nanoscale using electron holography. Subsequently, both properties were utilized to perform dynamic response calculations. The simulations yield the frequency-field dependence as well as the angular dependence of spin waves in a magnonic crystal and reveal the governing role of the internal field landscape around the backward-volume geometry. The complex angle-dependent spin-wave behavior is described for a 360 degree in-plane rotation of an external field by connecting the internal field landscape with the individual spin-wave localization.

cond-mat.mtrl-sci↗
arXiv2015

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 Mn$_{1.6}$Ga film, the saturation magnetisation is 0.36~MA/m and the coercivity is 0.29~T. Partial substitution of Mn by Co results in Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$. The saturation magnetisation of those films drops to 0.2~MA/m and the coercivity is increased to 1~T. 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 Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ 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 Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ is determined to be 1.23~MJ/m$^3$.

cond-mat.mtrl-sci↗
arXiv2014

Tuning Perpendicular Anisotropy Gradient in Co/Pd Multilayers by Ion Irradiation

The tunability of Ar+ ion irradiation of Co/Pd multilayers has been employed to create depth-dependent perpendicular anisotropy gradients. By adjusting the Ar+ kinetic energy and fluence, the depth and lateral density of the local structural modification are controlled. First-order reversal curve analysis through X-ray magnetic circular dichroism and conventional magnetometry studies show that the local structural damage weakens the perpendicular anisotropy near the surface, leading to a magnetization tilting towards the in-plane direction. The ion irradiation method is complementary to, and may be used in conjunction with, other synthesis approaches to maximize the anisotropy gradient.

cond-mat.mtrl-sci↗