arXiv subjects

M. Cukr

Publications and source records attributed to M. Cukr.

At least 19 recordsLinked to original sources

arXiv2012

Influence of Magnetic Anisotropy on Laser-induced Precession of Magnetization in Ferromagnetic Semiconductor (Ga,Mn)As

The laser-induced precession of magnetization in (Ga,Mn)As samples with different magnetic anisotropy was studied by the time-resolved magneto-optical method. We observed that the dependence of the precession amplitude on the external magnetic field depends strongly on the magnetic anisotropy of (Ga,Mn)As and we explain this phenomenon in terms of competing cubic and uniaxial anisotropies. We also show that the corresponding anisotropy fields can be deduced from the magnetic field dependence of the precession frequency.

cond-mat.mtrl-sci↗

arXiv2012

Experimental observation of the optical spin transfer torque

The spin transfer torque is a phenomenon in which angular momentum of a spin polarized electrical current entering a ferromagnet is transferred to the magnetization. The effect has opened a new research field of electrically driven magnetization dynamics in magnetic nanostructures and plays an important role in the development of a new generation of memory devices and tunable oscillators. Optical excitations of magnetic systems by laser pulses have been a separate research field whose aim is to explore magnetization dynamics at short time scales and enable ultrafast spintronic devices. We report the experimental observation of the optical spin transfer torque, predicted theoretically several years ago building the bridge between these two fields of spintronics research. In a pump-and-probe optical experiment we measure coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by circularly polarized laser pulses. During the pump pulse, the spin angular momentum of photo-carriers generated by the absorbed light is transferred to the collective magnetization of the ferromagnet. We interpret the observed optical spin transfer torque and the magnetization precession it triggers on a quantitative microscopic level. Bringing the spin transfer physics into optics introduces a fundamentally distinct mechanism from the previously reported thermal and non-thermal laser excitations of magnets. Bringing optics into the field of spin transfer torques decreases by several orders of magnitude the timescales at which these phenomena are explored and utilized.

cond-mat.mtrl-sci↗

arXiv2011

Non-thermal laser induced precession of magnetization in ferromagnetic semiconductor (Ga,Mn)As

Non-thermal laser induced spin excitations, recently discovered in conventional oxide and metal ferromagnets, open unprecedented opportunities for research and applications of ultrafast optical manipulation of magnetic systems. Ferromagnetic semiconductors, and (Ga,Mn)As in particular, should represent ideal systems for exploring this new field. Remarkably, the presence of non-thermal effects has remained one of the outstanding unresolved problems in the research of ferromagnetic semiconductors to date. Here we demonstrate that coherent magnetization dynamics can be excited in (Ga,Mn)As non-thermally by a transfer of angular momentum from circularly polarized femtosecond laser pulses and by a combination of non-thermal and thermal effects due to a transfer of energy from laser pulses. The thermal effects can be completely suppressed in piezo-electrically controlled samples. Our work is based on pump-and-probe measurements in a large set of (Ga,Mn)As epilayers and on systematic analysis of circular and linear magneto-optical coefficients. We provide microscopic theoretical interpretation of the experimental results.

cond-mat.mtrl-sci↗

arXiv2010

Molecular Beam Epitaxy of LiMnAs

We report on the molecular beam epitaxy (MBE) growth of high crystalline quality LiMnAs. The introduction of a group-I alkali metal element Li with flux comparable to fluxes of Mn and As has not caused any apparent damage to the MBE system after as many as fifteen growth cycles performed on the system to date.

cond-mat.mtrl-sci↗

arXiv2010

Antiferromagnetic I-Mn-V semiconductors

After decades of research, the low Curie temperature of ferromagnetic semiconductors remains the key problem in the development of magnetic semiconductor spintronic technologies. Removing this roadblock might require a change of the field's basic materials paradigm by looking beyond ferromagnets. Recent studies of relativistic magnetic and magnetotransport anisotropy effects, which in principle are equally well present in materials with ferromagnetically and antiferromagnetically ordered spins, have inspired our search for antiferromagnetic semiconductors suitable for high-temperature spintronics. Since these are not found among the magnetic counterparts of common III-V or II-VI semi- conductors, we turn the attention in this paper to high N éel temperature I-II-V magnetic compounds whose electronic structure has not been previously identified. Our combined experimental and theoretical work on LiMnAs provides basic prerequisite for the systematic research of this class of materials by demonstrating the feasibility to grow single crystals of group-I alkali metal compounds by molecular beam epitaxy, by demonstrating the semiconducting band structure of the I-Mn-V's, and by analyzing their spin-orbit coupling characteristics favorable for spintronics.

cond-mat.mtrl-sci↗

arXiv2009

Electronic structure of ferromagnetic semiconductor Ga1-xMnxAs probed by sub-gap magneto-optical spectroscopy

We employ Faraday and Kerr effect spectroscopy in the infrared range to investigate the electronic structure of Ga1-xMnxAs near the Fermi energy. The band structure of this archetypical dilute-moment ferromagnetic semiconductor has been a matter of controversy, fueled partly by previous measurements of the unpolarized infrared absorption and their phenomenological impurity-band interpretation. The infrared magneto-optical effects we study arise directly from the spin-splitting of the carrier bands and their chiral asymmetry due to spin-orbit coupling. Unlike the unpolarized absorption, they are intimately related to ferromagnetism and their interpretation is much more microscopically constrained in terms of the orbital character of the relevant band states. We show that the conventional theory of the disordered valence band with dominant As p-orbital character and coupled by kinetic-exchange to Mn local moments accounts semi-quantitatively for the overall characteristics of the measured infrared magneto-optical spectra.

cond-mat.str-el↗

arXiv2008

Coherent control of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As

We report single-color, time resolved magneto-optical measurements in ferromagnetic semiconductor (Ga,Mn)As. We demonstrate coherent optical control of the magnetization precession by applying two successive ultrashort laser pulses. The magnetic field and temperature dependent experiments reveal the collective Mn-moment nature of the oscillatory part of the time-dependent Kerr rotation, as well as contributions to the magneto-optical signal that are not connected with the magnetization dynamics.

cond-mat.mtrl-sci↗

arXiv2008

Damping and magnetic anisotropy of ferromagnetic GaMnAs thin films

The magnetic properties of annealed, epitaxial Ga0.93Mn0.07As layers under tensile and compressive stress have been investigated by X-band (9GHz) and Q-band (35GHz) ferromagnetic resonance (FMR) spectroscopy. From the analysis of the linewidths of the uniform mode spectra the FMR Gilbert damping factor "alpha" has been determined. At T=4K we obtain a minimum damping factor of "alpha" = 0.003 for the compressively stressed layer. Its value is not isotropic. It has a minimum value for the easy axes orientations of the magnetic field and increases with the measuring temperature. Its average value is for both type of films of the order of 0.01 in spite of strong differences in the inhomogeneous linewidth which vary between 20 Oe and 600 Oe for the layers grown on GaAs and GaInAs substrates respectively.

cond-mat.mtrl-sci↗

arXiv2008

Curie point singularity in the temperature derivative of resistivity in (Ga,Mn)As

We observe a singularity in the temperature derivative $dρ/dT$ of resistivity at the Curie point of high-quality (Ga,Mn)As ferromagnetic semiconductors with $T_c$'s ranging from approximately 80 to 185 K. The character of the anomaly is sharply distinct from the critical contribution to transport in conventional dense-moment magnetic semiconductors and is reminiscent of the $dρ/dT$ singularity in transition metal ferromagnets. Within the critical region accessible in our experiments, the temperature dependence on the ferromagnetic side can be explained by dominant scattering from uncorrelated spin fluctuations. The singular behavior of $dρ/dT$ on the paramagnetic side points to the important role of short-range correlated spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2008

Light-induced magnetization precession in GaMnAs

We report dynamics of the transient polar Kerr rotation (KR) and of the transient reflectivity induced by femtosecond laser pulses in ferromagnetic (Ga,Mn)As with no external magnetic field applied. It is shown that the measured KR signal consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in (Ga,Mn)As. The origin of the light-induced magnetization precession is discussed and the magnetization precession damping (Gilbert damping) is found to be strongly influenced by annealing of the sample.

cond-mat.mtrl-sci↗

arXiv2008

Laser-induced Precession of Magnetization in GaMnAs

We report on the photo-induced precession of the ferromagnetically coupled Mn spins in (Ga,Mn)As, which is observed even with no external magnetic field applied. We concentrate on various experimental aspects of the time-resolved magneto-optical Kerr effect (TR-MOKE) technique that can be used to clarify the origin of the detected signals. We show that the measured data typically consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in the sample.

cond-mat.mtrl-sci↗

arXiv2007

Substrate temperature changes during MBE growth of GaMnAs

Remarkably big increase of the substrate temperature during the low-temperature MBE growth of GaMnAs layers is observed by means of band gap spectroscopy. It is explained and simulated in terms of changes in the absorption/emission characteristics of the growing layer. Options for the temperature variation damping are discussed.

cond-mat.mtrl-sci↗

arXiv2006

Mn incorporation in as-grown and annealed (Ga,Mn)As layers studied by x-ray diffraction and standing-wave uorescence

A combination of high-resolution x-ray diffraction and a new technique of x-ray standing wave uorescence at grazing incidence is employed to study the structure of (Ga,Mn)As diluted magnetic semiconductor and its changes during post-growth annealing steps. We find that the film is formed by a uniform, single crystallographic phase epilayer covered by a thin surface layer with enhanced Mn concentration due to Mn atoms at random non-crystallographic positions. In the epilayer, Mn incorporated at interstitial position has a dominant effect on lattice expansion as compared to substitutional Mn. The expansion coeffcient of interstitial Mn estimated from our data is consistent with theory predictions. The concentration of interstitial Mn and the corresponding lattice expansion of the epilayer are reduced by annealing, accompanied by an increase of the density of randomly distributed Mn atoms in the disordered surface layer. Substitutional Mn atoms remain stable during the low-temperature annealing.

cond-mat.mtrl-sci↗

arXiv2004

Photoluminescence rings in Corbino disk at quantizing magnetic fields

Spatially resolved photoluminescence of modulation doped AlGaAs/GaAs heterojunction was investigated in a sample of Corbino disk geometry subject to strong perpendicular magnetic fields. Significant spatial modulation of the photoluminescence was observed in form of one or more concentric rings which travelled across the sample when the magnetic field strength was varied. A topology of the observed structure excludes the possibility of being a trace of an external current. The effect is attributed to formation of compressible and incompressible stripes in a 2DEG density gradient across the sample.

cond-mat.mes-hall↗

arXiv2003

In-plane Magnetic Field Dependent Magnetoresistance of Gated Asymmetric Double Quantum Wells

We have investigated experimentally the magnetoresistance of strongly asymmetric double-wells. The structures were prepared by inserting a thin Al$_{0.3}$Ga$_{0.7}$As barrier into the GaAs buffer layer of a standard modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure. The resulting double-well system consists of a nearly rectangular well and of a triangular well coupled by tunneling through the thin barrier. With a proper choice of the barrier parameters one can control the occupancy of the two wells and of the two lowest (bonding and antibonding) subbands. The electron properties can be further influenced by applying front- or back-gate voltage.

cond-mat.mes-hall↗

arXiv2001

Novel critical field in magneto-resistance oscillation of 2DEG in asymmetric GaAs/AlGaAs double wells measured as a function of the in-plane magnetic field

We have investigated the magnetoresistance of strongly asymmetric double-well structures formed by a thin AlGaAs barrier grown far from the interface in the GaAs buffer of standard heterostructures. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and with the field-induced transition into a decoupled bilayer. In addition, the increasing field transfers electrons from the triangular to rectangular well and, at high enough field value, the triangular well is emptied. Consequently, the electronic system becomes a single layer which leads to a sharp step in the density of electron states and to an additional minimum in the magnetoresistance curve.

cond-mat.mes-hall↗

arXiv2000

Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field

Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent-field calculations of the electronic structure.

cond-mat.mes-hall↗

arXiv1996

Electron conduction within Landau level tails of medium-mobility GaAs / AlGaAs heterostructures

The temperature dependence of both components of the resistivity tensor $\varrho_{xx}(T)$ and $\varrho_{xy}(T)$ has been studied at T $\geq$ 4.2 K within IQHE plateaux around filling factors ν=2 and ν=4 of medium- -mobility GaAs/AlGaAs heterostructures. In the middle of the mobility gap standard activated conductivity has been found with activation energies $Δ$ scaling well with $\hbarω_{c} / 2$ . At filling factors slightly below $ν$=2 another contribution adds to the activated conductivity at T $\leq$ 12 K. This additional contribution can be further enhanced at higher mesuring d.c. currents.We suggest, that it arises due to enhanced electric field assisted tunneling across potential barriers separating localized states within the bulk of the sample.This effect contributes to the backscattering across the sample leading to an enhanced longitudinal conductivity. The additional contribution to $σ_{xx}(T)$ can be reasonably well fitted to the formula for the variable range hopping in strong magnetic fields indicating that the hopping can persist even at temperatures well above 4.2K.

cond-mat.mes-hall↗