arXiv subjects

G. Schmidt

Publications and source records attributed to G. Schmidt.

At least 37 records · Page 2Linked to original sources

arXiv2017

Temperature dependent giant resistance anomaly in LaAlO3/SrTiO3 nanostructures

The resistance of the electron gas at the interface between the two band insulators LaAlO3 (LAO) and SrTiO3 (STO) typically drops monotonically with temperature and R/T curves during cooling and warm-up look identical for large area structures. Here we show that if the LAO/STO is laterally restricted by nanopatterning the resistance exhibits a temperature anomaly. Warming up nanostructures from low temperatures leads to one or two pronounced resistance peaks between 50 and 100 K not observed for larger dimensions. During cool-down current filaments emerge at the domain walls that form during the well-known structural phase transition of the STO substrate. During warm-up the reverse phase transition can interrupt filaments before the sheet conductivity which dominates at higher temperature is reestablished. Due to the limited number of filaments in a nanostructure this process can result in a complete loss of conductance. As a consequence of these findings the transport physics extracted from experiments in small and large area LAO/STO structures may need to be reconsidered.

cond-mat.mes-hall↗

arXiv2016

Investigation of the unidirectional spin heat conveyer effect in a 200nm thin Yttrium Iron Garnet film

We have investigated the unidirectional spin wave heat conveyer effect in sub-micron thick yttrium iron garnet (YIG) films using lock-in thermography (LIT). Although the effect is small in thin layers this technique allows us to observe asymmetric heat transport by magnons which leads to asymmetric temperature profiles differing by several mK on both sides of the exciting antenna, respectively. Comparison of Damon-Eshbach and backward volume modes shows that the unidirectional heat flow is indeed due to non-reciprocal spin-waves. Because of the finite linewidth, small asymmetries can still be observed when only the uniform mode of ferromagnetic resonance is excited. The latter is of extreme importance for example when measuring the inverse spin-Hall effect because the temperature differences can result in thermovoltages at the contacts. Because of the non-reciprocity these thermovoltages reverse their sign with a reversal of the magnetic field which is typically deemed the signature of the inverse spin-Hall voltage.

cond-mat.mtrl-sci↗

arXiv2015

Inverse spin Hall effect in a complex ferromagnetic oxide heterostructure

Complex oxide heterostructures are hot candidates for post CMOS multi-functional devices. Especially in spintronics applications ferromagnetic oxides may play a key role because they can exhibit extraordinary high spin polarization. Indeed, there are already plenty of examples in spintronics, notably in the area of spin pumping and inverse spin Hall effect (ISHE) \cite{Azevedo2011, Czeschka2011, Hahn2013, Obstbaum2014}. Although complex oxides have been used in these experiments as a source of spin currents, they have never been demonstrated to act as a spin sink that exhibits ISHE. Here we show that in a heterostructure consisting of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and SrRuO$_{3}$ (SRO) the low temperature ferromagnet SRO can act as a spin sink and exhibit a sizeable ISHE which persists even below its Curie temperature. This result opens up new possibilities for application of all oxide heterostructures in spintronics and may significantly extend the research on spin Hall effect and related phenomena.

cond-mat.mtrl-sci↗

arXiv2015

All electrical coherent control of the magnetization in thin Yittrium Iron Garnet film

We demonstrate coherent control of time domain ferromagnetic resonance by all electrical excitation and detection. Using two ultrashort magnetic field steps with variable time delay we control the induction decay in yttrium iron garnet (YIG). By setting suitable delay times between the two steps the precession of the magnetization can either be enhanced or completely stopped. The method allows for a determination of the precession frequency within a few precession periods and with an accuracy much higher than can be achieved using fast fourier transformation. Moreover it holds the promise to massively increase precession amplitudes in pulsed inductive microwave magnetometry (PIMM) using low amplitude finite pulse trains. Our experiments are supported by micromagnetic simulations which nicely confirm the experimental results.

cond-mat.mes-hall↗

arXiv2015

High Quality Yttrium Iron Garnet Grown by Room Temperature Pulsed Laser Deposition and Subsequent Annealing

We have investigated recrystallization of amorphous Yttrium Iron Garnet (YIG) by annealing in oxygen atmosphere. Our findings show that well below the melting temperature the material transforms into a fully epitaxial layer with exceptional quality, both structural and magnetic.\\ In ferromagnetic resonance (FMR) ultra low damping and extremely narrow linewidth can be observed. For a 56 nm thick layer a damping constant of $α$=(6.63$\pm$1.50)$\cdot$10$^{-5}$ is found and the linewidth at 9.6 GHz is as small as 1.30$\pm$0.05 Oe which are the lowest values for PLD grown thin films reported so far. Even for a 20 nm thick layer a damping constant of $α$=(7.51$\pm$1.40)$\cdot$10$^{-5}$ is found which is the lowest value for ultrathin films published so far. The FMR linewidth in this case is 3.49$\pm$0.10 Oe at 9.6 GHz. Our results not only present a method of depositing thin film YIG of unprecedented quality but also open up new options for the fabrication of thin film complex oxides or even other crystalline materials.

cond-mat.mtrl-sci↗

arXiv2015

Current-induced magnetization reversal in pseudo spin valves in current-in-plane configuration

We demonstrate the current-induced complete magnetization reversal of the free layer in lateral nanowires patterned from metallic pseudo spin valve stacks. The reversal is induced by Oersted fields in conjunction with a dipolar coupling via the lateral stray fields of the two magnetic layers. Starting from the parallel state the Oersted field rotates the magnetization vectors of both magnetic layers in opposite directions so far off the wire axis that the dipolar coupling becomes strong enough to stabilize an antiparallel configuration. As simulations and experiments show this coupling is only achieved for a narrow range of layer thicknesses and for suitable wire geometries. The reversal process is demonstrated as well for straight wires as for wire segments of L-shaped wires with inhomogeneous magnetization in which domain walls are present before and after the switching process.

cond-mat.mes-hall↗

arXiv2015

Sidewall depletion in nano-patterned LAO/STO heterostructures

We report the fabrication of nanostructures from the quasi-two-dimensional electron gas (q2DEG) formed at the LaAlO$_{3}$/ SrTiO$_{3}$ (LAO/STO) interface. The process uses electron beam lithography in combination with reactive ion etching. This technique allows to pattern high-quality structures down to lateral dimensions as small as $100$nm while maintaining the conducting properties without inducing conductivity in the STO substrate. Temperature dependent transport properties of patterned Hall bars of various widths show only a small size dependence of conductivity at low temperature as well as at room temperature. The deviation can be explained by a narrow lateral depletion region. All steps of the patterning process are fully industry compatible.

cond-mat.mes-hall↗

arXiv2015

Nanosized Vertical Organic Spin-Valves

A fabrication process for vertical organic spin-valve devices has been developed which offers the possibility to achieve active device areas of less than 500x500 nm^2 and is flexible in terms of material choice for the active layers. Characterization of the resulting devices shows a large magnetoresistance of sometimes more than 100%, however with equally large variation from device to device. Comparison with large-area spin-valves indicates that the magnetoresistance of both, large and small devices most likely originates from tunneling through pinholes and tunneling magnetoresistance.

cond-mat.mes-hall↗

arXiv2013

Large room-temperature magnetoresistance in lateral organic spin valves fabricated by in-situ shadow evaporation

We report the successful fabrication of lateral organic spin valves with a channel length in the sub $100\,nm$ regime. The fabication process is based on in-situ shadow evaporation under UHV conditions and therefore yields clean and oxygen-free interfaces between the ferromagnetic metallic electrodes and the organic semiconductor. The spin valve devices consist of Nickel and Cobalt-iron electrodes and the high mobility \emph{n}-type organic semiconductor $N,N'$-bis(heptafluorobutyl)-$3,4:9,10$-perylene diimide. Our studies comprise fundamental investigations of the process' and materials' suitability for the fabrication of lateral spin valve devices as well as magnetotransport measurements at room temperature. The best devices exhibit a magnetoresistance of up to $50\,%$, the largest value for room temperature reported so far.

cond-mat.mtrl-sci↗

arXiv2013

Vertical organic spin valves in perpendicular magnetic fields

We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors the transit time of spin polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance. However, in none of the experiments we do observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle type effects indicates that the magnetoresistance is not based on the injection of spin polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance.

cond-mat.mtrl-sci↗

arXiv2012

Computation of volume potentials over bounded domains via approximate approximations

We obtain cubature formulas of volume potentials over bounded domains combining the basis functions introduced in the theory of approximate approximations with their integration over the tangential-halfspace. Then the computation is reduced to the quadrature of one dimensional integrals over the halfline. We conclude the paper providing numerical tests which show that these formulas give very accurate approximations and confirm the predicted order of convergence.

math.NA↗

arXiv2011

Tunneling anisotropic magnetoresistance in organic spin valves

We report the observation of tunneling anisotropic magnetoresistance (TAMR) in an organic spin-valve-like structure with only one ferromagnetic electrode. The device is based on a new high mobility perylene diimide-based n-type organic semiconductor. The effect originates from the tunneling injection from the LSMO contact and can thus occur even for organic layers which are too thick to support the assumption of tunneling through the layer. Magnetoresistance measurements show a clear spin-valve signal, with the typical two step switching pattern caused by the magnetocrystalline anisotropy of the epitaxial magnetic electrode.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic field-induced exchange effects between Mn ions and free carriers in ZnSe quantum well through the intermediate nonmagnetic barrier studied by photoluminescence

Photoluminescence (PL) of the 50 nm $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$/ $d$ nm $Zn_{0.943}Be_{0.057}Se$/ 2.5 nm $ZnSe$/ 30 nm $Zn_{0.943}Be_{0.057}Se$ structures is investigated as a function of magnetic field ($B$) and thickness ($d$) of intermediate $Zn_{0.943}Be_{0.057}Se$ nonmagnetic barrier between the $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ semimagnetic barrier and $ZnSe$ quantum well at the temperature 1.2 K. The rate of the shift of different PL bands of the structures under study is estimated in low and high magnetic fields. The causes of the shift rate increase under pass from low to high magnetic fields are interpreted. The peculiarities of the effect of the intermediate barrier on the luminescence properties of the structures are presented. It is shown that deformation of adjacent layers by the barrier plays a crucial role in the formation of these properties, especially in forming the $Mn$ complexes in the $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ layer. The change of the band gap as well as of the donor and acceptor levels energies under the effect of biaxial compression of the $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ layer by the $Zn_{0.943}Be_{0.057}Se$ are estimated. It is concluded that the $Zn_{0.943}Be_{0.057}Se$ intermediate barrier also appreciably changes the effect of giant Zeeman splitting of the semimagnetic $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ barrier energy levels on the movement of the energy levels of $ZnSe$ quantum well in a magnetic field and on polarization of the quantum well exciton emission.

cond-mat.mes-hall↗

arXiv2010

Interaction between Mn Ions and Free Carriers in Quantum Wells with Asymmetrical Semimagnetic Barriers

Investigations of photoluminescence (PL) in the magnetic field of quantum structures based on the ZnSe quantum well with asymmetrical ZnBeMnSe and ZnBeSe barriers reveal that the introduction of Be into semimagnetic ZnMnSe causes a decrease of the exchange integrals for conductive and valence bands as well as the forming of a complex based on Mn, degeneration of an energy level of which with the energy levels of the V band of ZnBeMnSe or ZnSe results in spin-flip electron transitions.

cond-mat.mtrl-sci↗

arXiv2010

A Frequency-Controlled Magnetic Vortex Memory

Using the ultra low damping NiMnSb half-Heusler alloy patterned into vortex-state magnetic nano-dots, we demonstrate a new concept of non-volatile memory controlled by the frequency. A perpendicular bias magnetic field is used to split the frequency of the vortex core gyrotropic rotation into two distinct frequencies, depending on the sign of the vortex core polarity $p=\pm1$ inside the dot. A magnetic resonance force microscope and microwave pulses applied at one of these two resonant frequencies allow for local and deterministic addressing of binary information (core polarity).

cond-mat.mtrl-sci↗

arXiv2009

Bistability of vortex core dynamics in a single perpendicularly magnetized nano-disk

Microwave spectroscopy of individual vortex-state magnetic nano-disks in a perpendicular bias magnetic field, $H$, is performed using a magnetic resonance force microscope (MRFM). It reveals the splitting induced by $H$ on the gyrotropic frequency of the vortex core rotation related to the existence of the two stable polarities of the core. This splitting enables spectroscopic detection of the core polarity. The bistability extends up to a large negative (antiparallel to the core) value of the bias magnetic field $H_r$, at which the core polarity is reversed. The difference between the frequencies of the two stable rotational modes corresponding to each core polarity is proportional to $H$ and to the ratio of the disk thickness to its radius. Simple analytic theory in combination with micromagnetic simulations give quantitative description of the observed bistable dynamics.

cond-mat.mtrl-sci↗

arXiv2009

Lateral magnetic anisotropy superlattice out of a single (Ga,Mn)As layer

We use lithographically induced strain relaxation to periodically modulate the magnetic anisotropy in a single (Ga,Mn)As layer. This results in a lateral magnetoresistance device where two non-volatile magnetic states exist at zero external magnetic field with resistances resulting from the orientation of two lithographically defined regions in a single and contiguous layer.

cond-mat.mes-hall↗

arXiv2009

Fermi-edge Singularity in II-VI Semiconductor Resonant Tunneling Structures

We report on the observation of Fermi edge enhanced resonant tunneling transport in a II-VI semiconductor heterostructure. The resonant transport through a self assembled CdSe quantum dot survives up to 45 K and probes a disordered two dimensional (2D) like emitter which dominates the magnetic field dependence of the transport. An enhancement of the tunnel current through many particle effects is clearly observable, even without an applied magnetic field. Additional fine structure in the tunneling current suggests that while conventional Fermi edge singularity theory successfull reproduces the general features of the increased transmission, it is not adequate to describe all details of the current enhancement.

cond-mat.mes-hall↗