arXiv subjects

M. Münzenberg

Publications and source records attributed to M. Münzenberg.

25 records · Page 2Linked to original sources

arXiv2013

Speed limit of FePt spin dynamics on femtosecond timescales

Magnetization manipulation is becoming an indispensable tool for both basic and applied research. Theory predicts two types of ultrafast demagnetization dynamics classified as type I and type II. In type II materials, a second slower process takes place after the initial fast drop of magnetization. In this letter we investigate this behavior for FePt recording materials with perpendicular anisotropy. The magnetization dynamics have been simulated using a thermal micromagnetic model based on the Landau-Lifshitz-Bloch equation. We identify a transition to type II behavior and relate it to the electron temperatures reached by the laser heating. This slowing down is a fundamental limit to reconding speeds in heat assisted reversal.

cond-mat.mtrl-sci↗

arXiv2012

Engineering ultrafast spin currents and terahertz transients by magnetic heterostructures

In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz (THz) regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin-current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is employed to drive spins from a ferromagnetic Fe thin film into a nonmagnetic cap layer that has either low (Ru) or high (Au) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter based on the inverse spin Hall effect that converts the spin flow into a THz electromagnetic pulse. We find that the Ru cap layer yields a considerably longer spin-current pulse because electrons are injected in Ru d states that have a much smaller mobility than Au sp states. Thus, spin current pulses and the resulting THz transients can be shaped by tailoring magnetic heterostructures, which opens the door for engineering high-speed spintronic devices as well as broadband THz emitters in particular covering the elusive range from 5 to 10THz.

cond-mat.mes-hall↗

arXiv2009

Electric breakdown in ultra-thin MgO tunnel barrier junctions for spin-transfer torque switching

Magnetic tunnel junctions for spin-transfer torque switching were prepared to investigate the dielectric breakdown. The breakdown occurs typically at voltages not much higher than the switching voltages, a bottleneck for the implementation of spin-transfer torque Magnetic Random Access Memory. Intact and broken tunnel junctions are characterized by transport measurements and then prepared for transmission electron microscopy and energy dispersive x-ray spectrometry by focussed ion beam. The comparison to our previous model of the electric breakdown for thicker MgO tunnel barriers reveals significant differences arising from the high current densities.

cond-mat.mtrl-sci↗

arXiv2008

Intrinsic and non-local Gilbert damping in polycrystalline nickel studied by Ti:Sapphire laser fs spectroscopy

The use of femtosecond laser pulses generated by a Ti:Sapphire laser system allows us to gain an insight into the magnetization dynamics on time scales from sub-picosecond up to 1 ns directly in the time domain. This experimental technique is used to excite a polycrystalline nickel (Ni) film optically and probe the dynamics afterwards. Different spin wave modes (the Kittel mode, perpendicular standing spin-wave modes (PSSW) and dipolar spin-wave modes (Damon-Eshbach modes)) are identified as the Ni thickness is increased. The Kittel mode allows determination of the Gilbert damping parameter alpha extracted from the magnetization relaxation time tau_alpha. The non-local damping by spin currents emitted into a non-magnetic metallic layer of vanadium (V), palladium (Pd) and the rare earth dysprosium (Dy) are studied for wedge-shaped Ni films 1 nm-30 nm. The damping parameter increases from alpha=0.045 intrinsic for nickel to alpha>0.10 for the heavy materials, such as Pd and Dy, for the thinnest Ni films below 10 nm thickness. Also, for the thinnest reference Ni film thickness, an increased magnetic damping below 4 nm is observed. The origin of this increase is discussed within the framework of line broadening by locally different precessional frequencies within the laser spot region.

cond-mat.other↗

arXiv2008

Energy equilibriation processes of electrons, magnons and phonons on the femtosecond timescale

By means of time-resolved Kerr spectroscopy experiments we relate the energy dissipation processes on the femtosecond (electron-spin relaxation time $τ_{el-sp}$) and nanosecond timescale (Gilbert relaxation $τ_α$) and compare the results to the first microscopic model, which was proposed by Koopmans. For both energy dissipation processes, Elliot-Yafet scattering is proposed as the dominant contributor. We controllably manipulate the energy dissipation processes by transition metal doping (Pd) and rare earth doping (Dy) of a Permalloy film and find that while a change of $τ_α$ of more than a factor two is observed, τ_{el-sp}$ remains constant, contrary to the predictions of the model. We explain the discrepancies by relaxation channels not considered in the original microscopic model and identify thereby the applicability of the model and possible necessary extensions to the model.

cond-mat.other↗

arXiv2007

Activation of additional energy dissipation processes in the magnetization dynamics of epitaxial chromium dioxide films

The precessional magnetization dynamics of a chromium dioxide$(100)$ film is examined in an all-optical pump-probe setup. The frequency dependence on the external field is used to extract the uniaxial in-plane anisotropy constant. The damping shows a strong dependence on the frequency, but also on the laser pump fluency, which is revealed as an important experiment parameter in this work: above a certain threshold further channels of energy dissipation open and the damping increases discontinuously. This behavior might stem from spin-wave instabilities.

cond-mat.other↗

arXiv2006

Tuning the 4f-state occupancy of cerium in highly correlated CeSi/ Fe multilayers: a study by x-ray absorption spectroscopy

Spectra of x-ray absorption and magnetic circular dichroism were measured at M4,5(3d) and L2,3(2p) edges of Ce in multilayers [Ce(1-x)Six/Fe]xn, with x between 0.1 and 0.65. The study uncovers the highly correlated nature of this layered system. An alpha-phase like electronic configuration of Ce is observed, with ordered magnetic moments on the 4f and 5d electrons induced by the interaction with Fe. Increasing the Si content reduces the strength of the hy-bridization between the 4f and conduction-band states which is reflected in a growing occupation and magnetic polarization of the 4f states. Variations of the shape and intensity of the L2,3-edge dichroism spectra, discussed in a simple phenomenological model, show the importance of the exchange interaction between the Ce-4f and 5d electrons, spin polarized by the interaction with Fe at the interfaces, for the electronic structure of Ce at high Si concentration and low temperature. A model of the band structure of rare-earth transition-metal compounds permits to argue that magnetic order on the Ce 4f electrons in the multilayers is due to different mechanisms: to hybridization of the Ce-4f with the Fe-3d states at low Si concentration and to intra-atomic 4f-5d exchange at high Si concentration. This is at variance with magnetic order in the intermetallics CeSi2-delta and CeSi which results from interaction between the localized 4f magnetic moments mediated by the Si-derived (s,p) conduction electrons, in competition with the Kondo effect.

cond-mat.str-el↗