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We report magnetization changes generated by an electric field in ferromagnetic Ga$_{1-x}$Mn$_x$N grown by molecular beam epitaxy. Two classes of phenomena have been revealed. First, over a wide range of magnetic fields, the magnetoelectric signal is odd in the electric field and reversible. Employing a macroscopic spin model and atomistic Landau-Lifshitz-Gilbert theory with Langevin dynamics, we demonstrate that the magnetoelectric response results from the inverse piezoelectric effect that changes the trigonal single-ion magnetocrystalline anisotropy. Second, in the metastable regime of ferromagnetic hystereses, the magnetoelectric effect becomes non-linear and irreversible in response to a time-dependent electric field, which can reorient the magnetization direction. Interestingly, our observations are similar to those reported for another dilute ferromagnetic semiconductor Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{1-x}$Te$_3$, in which magnetization was monitored as a function of the gate electric field. Those results constitute experimental support for theories describing the effects of time-dependent perturbation upon glasses far from thermal equilibrium in terms of an enhanced effective temperature., Comment: 18 pages, 10 Figures

Strain relaxation of thick InGaN layers was studied in order to develop technology of InGaN templates for deposition of InGaN Quantum Wells (QWs) and InGaN layers of high-In-content. In this paper, we show that InGaN layers grown on misoriented (0001)-GaN substrates relax by preferential activation of certain glide planes for misfit dislocation formation. Substrate misorientation changes resolved shear stresses, affecting the distribution of misfit dislocations within each dislocation set. We demonstrate that this mechanism leads to an anisotropic strain as well as a tilt of the InGaN layer with respect to the GaN substrate. It appears that these phenomena are more pronounced in structures grown on substrates misoriented toward <11-20> direction than corresponding structures with <-1100> misorientation. These features would influence the properties of the overgrown InGaN QWs and should be taken into consideration during designing structures grown on relaxed InGaN templates. We reveal that the lattice of partially relaxed InGaN has a triclinic deformation, thus requiring advanced XRD analysis. The presentation of just a single asymmetric reciprocal space map commonly practiced in the literature can lead to misleading information regarding the relaxation state of partially relaxed wurtzite structures., Comment: 29 pages, 6 figures, 3 tables

Anomalous Hall effect (AHE) plays important role in the rapidly developing field of antiferromagnetic spintronics. It has been recently discussed that it can be a feature of not only uncompensated magnetic systems but also in altermagnetic materials. Hexagonal MnTe belongs to this appealing group of compounds exhibiting AHE and is commonly perceived as magnetically compensated. Here, we demonstrate that bulk form of MnTe exhibits small but detectable magnetic moment correlating with hysteretic behaviour of the AHE. We formulate a phenomenological model which explains how this feature allows to create a disbalance between states with opposite N\'eel vector and prevent the AHE signal from averaging out to zero. Moreover, we show how the dependence of AHE on the N\'eel vector arises on microscopical level and highlight the differences in Berry curvature between magnetically compensated and uncompensated systems.

In this paper, we apply the angle-resolved Optically Detected Magnetic Resonance (ODMR) technique to study series of strained (Cd, Mn)Te/(Cd, Mg)Te quantum wells (QWs) produced by molecular beam epitaxy. By analyzing characteristic features of ODMR angular scans, we determine strain-induced axial-symmetry spin Hamiltonian parameter D with neV precision. Furthermore, we use low-temperature optical reflectivity measurements and X-ray diffraction scans to evaluate the local strain present in QW material. In our analysis, we take into account different thermal expansion coefficients of GaAs substrate and CdTe buffer. The additional deformation due to the thermal expansion effects has the same magnitude as deformation origination from the different compositions of the samples. Based on the evaluated deformations and values of strain-induced axial-symmetry spin Hamiltonian parameter D, we find strain spin-lattice coefficient G11 = (72.2 +- 1.9) neV for Mn2+ in CdTe and shear deformation potential b = (-0.94 +- 0.11) eV for CdTe.

A ferromagnetic coupling between localized Mn spins was predicted in a series of \textit{ab initio} and tight binding calculations and experimentally verified for the dilute magnetic semiconductor Ga$_{1-x}$Mn$_x$N. In the limit of small Mn concentrations, $x \lesssim 0.01$, the paramagnetic properties of this material were successfully described using a single ion crystal field model approach. In order to obtain the description of magnetization in (Ga,Mn)N in the presence of interacting magnetic centers, we extend the previous model of a single substitutional Mn$^{3+}$ ion in GaN by considering pairs, triplets and quartets of Mn$^{3+}$ ions coupled by a ferromagnetic superexchange interaction. Using this approach we investigate how the magnetic properties, particularly the magnitude of the uniaxial anisotropy field, change as the number of magnetic Mn$^{3+}$ ions in a given cluster increases from 1 to 4. Our simulations are then exploited in explaining experimental magnetic properties of Ga$_{1-x}$Mn$_x$N with $x \cong 0.03$, where the presence of small magnetic clusters gains in significance. As a result the approximate lower and upper limits for the values of exchange couplings between Mn$^{3+}$ ions in GaN, being in nearest neighbors $J_{\mathrm{nn}}$ and next nearest neighbors $J_{\mathrm{nnn}}$ positions, respectively, are established., Comment: 14 pages, 6 figures

Weak antilocalization measurements has become a standard tool for studying quantum coherent transport in topological materials. It is often used to extract information about number of conducting channels and dephasing length of topological surface states. We study thin films of prototypical topological crystalline insulator SnTe. To access microscopic characteristic of these states we employ a model developed by Tkachov and Hankiewicz, [Physical Review B 84, 035444]. Using this model the spatial decay of the topological states is obtained from measurements of quantum corrections to the conductivity in perpendicular and parallel configurations of the magnetic field. Within this model we find interaction between two topological boundaries which results in scaling of the spatial decay with the film thickness. We attribute this behavior to bulk reservoir which mediates interactions by scattering events without phase breaking of topological carriers., Comment: 6 pages , 5 figures

Historically, comprehensive studies of dilute ferromagnetic semiconductors, e.g., $p$-type (Cd,Mn)Te and (Ga,Mn)As, paved the way for a quantitative theoretical description of effects associated with spin-orbit interactions in solids, such as crystalline magnetic anisotropy. In particular, the theory was successful in explaining {\em uniaxial} magnetic anisotropies associated with biaxial strain and non-random formation of magnetic dimers in epitaxial (Ga,Mn)As layers. However, the situation appears much less settled in the case of the {\em cubic} term: the theory predicts switchings of the easy axis between in-plane $\langle 100\rangle$ and $\langle 110\rangle$ directions as a function of the hole concentration, whereas only the $\langle 100\rangle$ orientation has been found experimentally. Here, we report on the observation of such switchings by magnetization and ferromagnetic resonance studies on a series of high-crystalline quality (Ga,Mn)As films. We describe our findings by the mean-field $p$-$d$ Zener model augmented with three new ingredients. The first one is a scattering broadening of the hole density of states, which reduces significantly the amplitude of the alternating carrier-induced contribution. This opens the way for the two other ingredients, namely the so-far disregarded single-ion magnetic anisotropy and disorder-driven non-uniformities of the carrier density, both favoring the $\langle 100\rangle$ direction of the apparent easy axis. However, according to our results, when the disorder gets reduced a switching to the $\langle 110\rangle$ orientation is possible in a certain temperature and hole concentration range., Comment: 12 pages, 9 figures

Point-contact spectroscopy of several non-superconducting topological materials reveals a low temperature phase transition that is characterized by a Bardeen-Cooper-Schrieffer-type of criticality. We find such a behavior of differential conductance for topological surfaces of non-magnetic and magnetic Pb$_{1-y-x}$Sn$_y$Mn$_x$Te. We examine a possible contribution from superconducting nanoparticles, and show to what extent our data are consistent with Brzezicki's et al. theory [arXiv:1812.02168], assigning the observations to a collective state adjacent to atomic steps at topological surfaces., Comment: 15 pages, 27 figures (with Supplemental Material)

The high-spectral-resolution spectroscopic studies of the energy gap evolution, supplemented with electronic, magnetic and structural characterization, show that the modification of the GaAs valence band caused by Mn incorporation occurs already for a very low Mn content, much lower than that required to support ferromagnetic spin - spin coupling in (Ga,Mn)As. Only for n-type (Ga,Mn)As with the Mn content below about 0.3% the Mn-related extended states are visible as a feature detached from the valence-band edge and partly occupied with electrons. The combined magnetic and low-temperature photoreflectance studies presented here indicate that the paramagnetic - ferromagnetic transformation in p-type (Ga,Mn)As takes place without imposing changes of the unitary character of the valence band with the Fermi level located therein. The whole process is rooted in the nanoscale fluctuations of the local (hole) density of states and the formation of a superparamagnetic-like state. The Fermi level in (Ga,Mn)As is coarsened by the carrier concentration of the itinerant valence band holes and further fine-tuned by the many-body interactions.

We demonstrate that the metallic topological surface states wrap on all sides the 3D topological crystalline insulator SnTe. This is achieved by studying oscillatory quantum magneto-transport and magnetization at tilted magnetic fields which enables us to observe simultaneous contributions from neighbouring sample sides. Taking into account pinning of the Fermi energy by the SnTe reservoir we successfully describe theoretically the de Haas-van Alphen oscillations of magnetization. The determined \pi-Berry phase of surface states confirms their Dirac fermion character. We independently observe oscillatory contributions of magneto-transport and magnetization originating from the bulk SnTe reservoir of high hole density. It is concluded that the bulk and surface Landau states exist in parallel. Our main result that the bulk reservoir is surrounded on all sides by the topological surface states has an universal character., Comment: 5 pages, 2 figures, 12 pages supplemental material

Magnetic and magneto-transport properties of thin layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Postgrowth annealing treatment has been shown to enhance the hole concentration and Curie temperature in the layers. Significant increase in the magnitude of magnetotransport effects caused by incorporation of a small amount of Bi into the (Ga,Mn)As layers revealed in the planar Hall effect (PHE) measurements, is interpreted as a result of enhanced spin-orbit coupling in the (Ga,Mn)(Bi,As) layers. Two-state behaviour of the planar Hall resistance at zero magnetic field provides its usefulness for applications in nonvolatile memory devices., Comment: 10 pages, 3 figures, to be published in the Proceedings of ICSM-2016 conference

Ferromagnetic resonance (FMR) study of magnetic anisotropy is presented for thin layers of IV-VI diluted magnetic semiconductor Ge1-xMn xTe with x=0.14 grown by molecular beam epitaxy (MBE) on KCl (001) substrate with a thin PbTe buffer. Analysis of the angular dependence of the FMR resonant field reveals that an easy magnetization axis is located near to the normal to the layer plane and is controlled by two crystal distortions present in these rhombohedral Ge1-xMnxTe layers: the ferroelectric distortion with the relative shift of cation and anion sub-lattices along the [111] crystal direction and the biaxial in-plane, compressive strain due to thermal mismatch.

Effect of misfit strain in the layers of (Ga,Mn)(Bi,As) quaternary diluted magnetic semiconductor, epitaxially grown on either GaAs substrate or (In,Ga)As buffer, on their magnetic and magneto-transport properties has been investigated. High-resolution X-ray diffraction, applied to characterize the structural quality and misfit strain in the layers, proved that the layers were fully strained to the GaAs substrate or (In,Ga)As buffer under compressive or tensile strain, respectively. Ferromagnetic Curie temperature and magnetocrystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the layers has been shown to enhance the hole concentration and Curie temperature in the layers., Comment: 8 pages, 3 figures

High-quality layers of the (Ga,Mn)(Bi,As) quaternary compound semiconductor have been grown by the low-temperature molecular-beam epitaxy technique. An effect of Bi incorporation into the (Ga,Mn)As ferromagnetic semiconductor and the post-growth annealing treatment of the layers have been investigated through examination of their magnetic and magneto-transport properties. Significant enhancement of the planar Hall effect magnitude upon addition of Bi into the layers is interpreted as a result of increased spin-orbit coupling in the (Ga,Mn)(Bi,As) layers., Comment: 9 pages, 3 figures, to be published in the proceedings of the "43rd "Jaszowiec" International School and Conference on the Physics of Semiconductors", June 7-12, 2014, Wisla, Poland

We present angle resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb$_{1-x}$Sn$_{x}$Se, a three dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at $\bar{\Gamma}$ and $\bar{M}$ in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality and alternative surface orientations in (Pb,Sn)Se compounds., Comment: 5 pages, 4 figures

The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results revealing a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy in the annealed (Ga,Mn)As layers is interpreted as a result of the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the enhanced free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors., Comment: 17 pages, 4 figures. arXiv admin note: substantial text overlap with arXiv:1305.4056

Modulation photoreflectance spectroscopy and Raman spectroscopy have been applied to study the electronic- and band-structure evolution in (Ga,Mn)As epitaxial layers with increasing Mn doping in the range of low Mn content, up to 1.2%. Structural and magnetic properties of the layers were characterized with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. The revealed results of decrease in the band-gap transition energy with increasing Mn content in very low-doped (Ga,Mn)As layers with n-type conductivity are interpreted as a result of merging the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy with increasing Mn content in (Ga,Mn)As layers with higher Mn content and p-type conductivity indicates the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediated ferromagnetic ordering in the (Ga,Mn)As diluted ferromagnetic semiconductor., Comment: 20 pages, 5 figures. arXiv admin note: text overlap with arXiv:1012.4760

The annealing-induced formation of (Mn,Ga)As nanocrystals in (Ga,Mn)As/GaAs superlattices was studied by X-ray diffraction, transmission electron microscopy and magnetometry. The superlattice structures with 50 A thick (Ga,Mn)As layers separated by 25, 50 and 100 A thick GaAs spacers were grown by molecular beam epitaxy at low temperature (250 C), and then annealed at high temperatures of: 400, 560 and 630 C. The high temperature annealing causes decomposition to GaMnAs ternary alloy and formation of (Mn,Ga)As nanocrystals inside the GaAs matrix. The nanocrystals are confined in the planes that were formerly occupied by (Ga,Mn)As layers for up to the 560 C of annealing and diffuse throughout the GaAs spacer layers at 630 C annealing. The corresponding magnetization measurements show the evolution of the magnetic properties of as-grown and annealed samples from ferromagnetic, through superparamagnetic to the combination of both., Comment: 14 pages, 3 figures

Voids adjacent to both cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in annealed Ga0.995Mn0.005As magnetic semiconductor specimen grown by molecular beam epitaxy. Nano-beam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter from that of bulk MnAs. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals. After annealing at 903 K, the magnetic transition temperature of the specimen likely to be dominated by the presence of cubic ferromagnetic nanocrystals.

Fundamental optical properties of thin films of (Ga,Mn)As diluted ferromagnetic semiconductor with a low (1%) and high (6%) Mn content and of a reference GaAs film, grown by low-temperature molecular-beam epitaxy, have been investigated by photoreflectance (PR) spectroscopy. In addition, the films were subjected to complementary characterization by means of superconducting quantum interference device (SQUID) magnetometry, Raman spectroscopy, and high resolution X-ray diffractometry. Thorough full-line-shape analysis of the PR spectra, which enabled determination of the E0 electronic transition in (Ga,Mn)As, revealed significant differences between the energy band structures in vicinity of the {\Gamma} point of the Brillouin zone for the two (Ga,Mn)As films. In view of the obtained experimental results the evolution of the valence band structure in (Ga,Mn)As with increasing Mn content is discussed, pointing to a merging the Mn-related impurity band with the host GaAs valence band for high Mn content., Comment: 21 pages, 6 figures

Structural and magnetic properties of GaAs thin films with embedded MnAs nanoclusters were investigated as function of the annealing temperature and layers composition. The presence of two kinds of nanoclusters with different dimensions and structure were detected. The fraction of Mn atoms in each kind of cluster was estimated from the extended X-ray absorption fine structure analysis. This analysis ruled out the possibility of the existence of nanoclusters containing a hypothetic MnAs cubic compound - only (Mn,Ga)As cubic clusters were detected. Change of the layer strain from the compressive to tensile was related to the fraction of zinc blende and hexagonal inclusions. Thus the zinc blende inclusions introduce much larger strain than hexagonal ones. The explanation of observed thermal induced strain changes of the layers from the compressive to tensile is proposed. The magnetic properties of the samples were consistent with structural study results. Their showed that in sample containing solely cubic (Mn,Ga)As inclusions Mn ions inside the inclusions are still ferromagnetically coupled, even at room temperature. This fact can be explained by existence in these clusters of GaMnAs solid solution with content of Mn higher than 15 % as was found in theoretical calculations., Comment: 12 pages, 16 figures, 4 tables

Thin GaMnAs layers grown by molecular beam epitaxy were subjected to low-temperature post growth annealing, with an amorphous arsenic capping layer deposited on the GaMnAs surface directly after the epitaxial growth. It is shown that the presence of arsenic capping at the GaMnAs surface significantly shortens the post-growth annealing times and facilitates a complete out-diffusion of Mn interstitials from GaMnAs volume., Comment: 17 pages including 6 figures

Formation of MnAs quantum dots in a regular ring-like distribution has been found on MBE-grown (GaMn)As surfaces after low-temperature annealing under As capping. The Mn was supplied by out-diffusing Mn interstitials from (GaMn)As. With 5 at% substitutional Mn the quantum dots appeared for (GaMn)As layers thicker than 500 A. For thinner layers the Mn-rich surfaces, presumably monolayer thick MnAs, are smooth and well-ordered (1x2), and are well suited for continued epitaxial growth., Comment: 12 pages, 4 figures

We demonstrate that in situ post-growth annealing of GaMnAs layers under As capping is adequate for achieving high Curie temperatures (Tc) in a similar way as ex situ annealing in air or in N2 atmosphere practiced earlier., Comment: 13 pages, 4 figures

We study the influence of main compensating defects: As antisites and Mn interstitials, known to occur in GaMnAs ferromagnetic semiconductor, on its structural properties. Our experimental results show that there is a balance between Mn interstitial and As antisite defects, leading to the reduced density of one type of defect upon increased density of another defect. The significant differences in the lattice parameters of GaMnAs with the different balance between these two types of defects were observed. The annealing induced reduction of GaMnAs lattice constant is inhibited in the samples with large density of As antisites., Comment: 17 pages, 4 figures

Structural investigation of as-grown as well as annealed Ga(1-x)Mn(x)As epilayers was carried out using high resolution X-ray diffraction (XRD) measurements for a wide range of Mn concentrations (0.027<=x<=0.083), with special attention on how the interstitial Mn atoms (Mn_I) influence the lattice parameter of this material. We observe a distinct decrease of the lattice parameter after low temperature annealing of Ga(1-x)Mn(x)As, which is known to reduce the Mn_I concentration. The reciprocal space maps measured for all the investigated samples showed that the Ga(1-x)Mn(x)As layers are fully strained - i.e., they remain pseudomorphic to the GaAs (001) substrate - for the entire thickness of the samples used (in the present case over 100 nm). In all cases studied the XRD measurements revealed high crystalline perfection of both as-grown as well as annealed Ga(1-x)Mn(x)As epilayers, Comment: 26 pages

High-field magnetic measurements performed with the use of magnetooptical Kerr effect (MOKE) in the polar configuration as well as high-field and low-field magnetotransport studies were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. The structural investigations by means of high resolution XRD were also performed. We observe significant changes in magnetoresistivity curves, magnetization and strain introduced by the annealing., Comment: International Conference on Magnetism, ICM 2003, Roma; to be published on Journal of Magnetism and Magnetic Materials

We have observed a paramagnetic-to-ferromagnetic phase transition in short period InGaMnAs/InGaAs superlattices. The thicknesses of magnetic InGaMnAs layers in the structures studied was chosen to be 4 or 8 molecular layers (12 \AA or 24 \AA). The nonmagnetic InGaAs spacer layers are 12 \AA thick. The In content of InGaMnAs and InGaAs layers was chosen in such a way that magnetic layers were: deep potential wells, high potential barriers or shallow potential wells. For superlattices with 8 monolayer thick InGaMnAs magnetic layers and 4 monolayer thick InGaAs nonmagnetic spacers the temperatures of paramagnetic-to-ferromagnetic phase transition do not depend on the band offsets between InGaMnAs and InGaAs adjusted by the In content., Comment: Proceedings of the XXXI International School on the Physics of Semiconducting Compounds, Jaszowiec, Poland, June 7-14 (2002)

We have grown the ferromagnetic semiconductor GaMnAs containing up to 10% Mn by migration enhanced epitaxy at a substrate temperature of 150^oC. The alternate supply of As2 molecules and Ga and Mn atoms made it possible to grow single crystalline GaMnAs layers at very low substrate temperature, at which conventional molecular beam epitaxial growth under excess As supply is not possible due to As condensation. Secondary ion mass spectroscopy and X-ray diffraction measurements confirmed a higher Mn content in the films grown by this method in comparison to the GaMnAs layers grown by low temperature molecular beam epitaxy. The lattice constant of hypothetical zinc-blende structure MnAs is determined to be 5.9 \AA, which deviates somewhat from previously reported values. This deviation is ascribed to growth-condition dependent density of point defects. It is stressed that this effect must be taken into account for any assessment of Mn content from X-ray diffraction data. Magnetization measurements showed an onset of ferromagnetic ordering around 75 K for the GaMnAs layer with 10% Mn. This means that the trend of falling Curie temperatures with increasing Mn concentrations above 5.5% is broken. We tentatively assign this to the variation of the carrier concentration, including contributions from donor and acceptor centers formed by antisite defects and Mn doping, and increased density of magnetically active Mn ions., Comment: No LaTeX source; gzipped postscript text + 3 gzipped postscript figures