Search

Antiferromagnets with broken time-reversal symmetry, such as Mn3Sn, have emerged as promising platforms for exploring topological and correlated electron physics. Mn3Sn is known to show two magnetic phase transitions: a non-collinear inverse triangular antiferromagnetic (IT-AFM) spin configuration is formed below its Neel temperature, whereas at T1 that usually locates below room temperature, it transits to an incommensurate spin state. Accordingly, intriguing properties such as a strong anomalous Hall effect, observed from TN to T1, disappear below T1, limiting its utility at low temperatures. While bulk Mn3Sn has been extensively studied, the magnetic phase transitions and their tunability in thin films remain largely unexplored. Here, we investigate the magnetic and magneto-transport properties of Mn3+xSn1-x epitaxial thin films prepared by magnetron sputtering, systematically varying the Mn-Sn composition. Our results reveal that intrinsic alloying with Mn provides us with a handle to tune T1, with the IT-AFM phase stabilized down to liquid helium temperatures for x > 0.15. From a magnetic phase diagram for epitaxial thin films, we also find a consistent magnetic anomaly ~55 K below TN, accompanied by thermal hysteresis. Furthermore, the reduction of TN in thin films relative to bulk values is shown to correlate with lattice parameter changes. These findings extend the accessible temperature range for Mn3Sn's topological properties, paving the way for novel applications and further investigations into the interplay of spin, lattice, and electronic degrees of freedom in thin-film geometries., Comment: 16 pages, 5 figures, supplementary information

The recent discovery of superconductivity in infinite layer thin films and bulk Ruddlesden-Popper nickelates has stimulated the investigation of other predicted properties of these materials. Among them, the existence of magnetism-driven ferroelectricity in the parent compounds RNiO3 (R = 4f lanthanide and Y) at the onset of the N\'eel order, TN, has remained particularly elusive. Using diffraction techniques, we reveal here the existence of magnetostriction at TN in bulk YNiO3 single crystals. Interestingly, the associated lattice anomalies are much more pronounced along the b crystal axis, which coincides with the electric polarization direction expected from symmetry arguments. This axis undergoes an abrupt contraction below TN that reaches deltab/b ~ - 0.01 %, a value comparable to those found in some magnetoresistive manganites and much larger than those reported for magnetism-driven multiferroics. This observation suggests a strong spin-lattice coupling in these materials, consistent with theoretical predictions. Using the symmetry-adapted distortion mode formalism we identify the main ionic displacements contributing to the lattice anomalies and discuss the most likely polar displacements below TN. Furthermore, our data support symmetric superexchange as the most likely mechanism responsible for the magnetoelastic coupling. These results, that may be common to the full RNiO3 family, provide new experimental evidence supporting the predicted existence of magnetism-driven ferroelectricity in RNiO3 perovskites, Comment: 36 pages, 6 figures

High-resolution transmission electron microscopy and superconducting quantum interference device magnetometry shows that epitaxial CuMnSb films exhibit a coexistence of two magnetic phases, coherently intertwined in nanometric scales. The dominant $\alpha$~phase is half-Heusler cubic antiferromagnet with the N\'{e}el temperature of 62~K, the equilibrium structure of bulk CuMnSb. The secondary phase is its ferromagnetic tetragonal $\beta$ polymorph with the Curie temperature of about 100~K. First principles calculations provide a consistent interpretation of experiment, since (i) total energy of $\beta$--CuMnSb is higher than that of $\alpha$--CuMnSb only by 0.12~eV per formula unit, which allows for epitaxial stabilization of this phase, (ii) the metallic character of $\beta$--CuMnSb favors the Ruderman-Kittel-Kasuya-Yoshida ferromagnetic coupling, and (iii) the calculated effective Curie-Weiss magnetic moment of Mn ions in both phases is about $5.5~\mu_\mathrm{B}$, favorably close to the measured value. Calculated properties of all point native defects indicate that the most likely to occur are $\mathrm{Mn}_\mathrm{Cu}$ antisites. They affect magnetic properties of epilayers, but they cannot induce the ferromagnetic order in CuMnSb. Combined, the findings highlight a practical route towards fabrication of functional materials in which coexisting polymorphs provide complementing functionalities in one host., Comment: 14 pages, 8 figures, 5 tables

Semiconductor superlattices have found widespread applications in electronic industries. In this work, short-period superlattice structure (SLs) composed of CdO and MgO layers was grown using the plasma-assisted molecular beam epitaxy (PA-MBE) technique. The optical property of the SLs was investigated by absorption measurement at room temperature. The ambient-pressure direct bandgap was found to be 2.76 eV. The pressure dependence of fundamental bandgap has been studied using a diamond anvil cell (DAC) technique. It has been found that the band-to-band transition shifts toward higher energy with applied pressure. The bandgap of SLs was varied from 2.76 eV to 2.87 eV with applied pressure varied from 0 to 5.9 GPa. The pressure coefficient for the direct bandgap of SLs was found to be 26 meV/GPa. The obtained experimental result was supported by theoretical results obtained using DFT calculations. The volume deformation potential was estimated using the empirical rule. We believe our findings may provide valuable insight for a better understanding of {CdO/MgO} SLs towards their future applications in optoelectronics., Comment: 21 pages, 5 figures, 2 tables

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state., Comment: main manuscript 33 pages, 9 figures; supplemental material 19 pages, 13 figures - all in one file

$\alpha$-Sn is an elemental topological material, whose topological phases can be tuned by strain and magnetic field. Such tunability offers a substantial potential for topological electronics. However, InSb substrates, commonly used to stabilize $\alpha$-Sn allotrope, suffer from parallel conduction, restricting transport investigations and potential applications. Here, the successful MBE growth of high-quality $\alpha$-Sn layers on insulating, hybrid CdTe/GaAs(001) substrates, with bulk electron mobility approaching 20000 cm$^2$V$^{-1}$s$^{-1}$ is reported. The electronic properties of the samples are systematically investigated by independent complementary techniques, enabling thorough characterization of the 3D Dirac (DSM) and Weyl (WSM) semimetal phases induced by the strains and magnetic field, respectively. Magneto-optical experiments, corroborated with band structure modeling, provide an exhaustive description of the bulk states in the DSM phase. The modeled electronic structure is directly observed in angle-resolved photoemission spectroscopy, which reveals linearly dispersing bands near the Fermi level. The first detailed study of negative longitudinal magnetoresistance relates this effect to the chiral anomaly and, consequently, to the presence of WSM. Observation of the $\pi$ Berry phase in Shubnikov-de Haas oscillations agrees with the topologically non-trivial nature of the investigated samples. Our findings establish $\alpha$-Sn as an attractive topological material for exploring relativistic physics and future applications., Comment: Accepted version; Main text: 37 pages, 7 figures; Supplementary Materials: 29 pages, 13 figures

Nominally 45 nm GaN:Mg/ 5 nm (Ga,Mn)N / 45 nm GaN:Mg trilayers structures prepared by molecular beam epitaxy on GaN-buffered Al2O3 substrates are investigated to verify whether the indirect co-doping by holes from the cladding layers can alter the spin-spin interaction in (Ga,Mn)N. The four investigated structures, differing with the Mg doping level, are carefully characterized at the nanoscale by HRTEM, EDX, and by SIMS. HRTEM decisively excluded a presence of foreign Mn-rich phases. The structures, up to medium Mg doping, show no Mg over-doping effects. Magnetic studies of these structures are aided by the employment of a dedicated experimental approach of the in situ compensation of the magnetic contribution from the substrate, allowing up to about fifty-fold reduction of this contribution. This technique, dedicated to these structures, simultaneously provides a tenfold reduction of temporal instabilities of the magnetometric unit and lowers the experimental jitter to merely $5 \times 10^{-7}$~emu at 70~kOe, vastly increasing the precision and the credibility of the results of the standard integral SQUID magnetometry in high magnetic fields. The magnetic characteristics of the trilayers structures established here prove identical with the already known properties of the thick (Ga,Mn)N single layers, namely (i) the low temperature ferromagnetism among Mn$^{3+}$ ions driven by superexchange and (ii) purely paramagnetic response at higher temperatures. The possible cause of the lack of any effects brought about by the adjacent Mg-doping is a presence of residual Mn in the cladding layers, resulting in the deactivation of the p-type doping intended there. This finding points out that a more intensive technological effort has to be exerted to promote the co-doping-driven carrier-mediated ferromagnetic coupling in Mn-enriched GaN, especially at elevated temperatures., Comment: 16 pages, 6 figures

Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni L3 x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test. This rigorous statistical approach provides an unambiguous proof of the existence of interlayer coupling mediated by spin pumping., Comment: 11 pages, 7 figures

In this work, we comprehensively investigate and discuss the structural, magnetostatic, dynamic, and magnetoresistive properties of epitaxial Co/Mo superlattices. The magnetization of the Co sublayers is coupled antiferromagnetically with a strength that depends on the thickness of the nonmagnetic Mo spacer. The magnetization and magnetoresistance hysteresis loops clearly reflect interlayer exchange coupling and the occurrence of uniaxial magnetic anisotropy induced by the strained Co sublayers. Upon accounting for a deviation of the sublayer thicknesses from the nominal value, theoretical modeling, including both micromagnetic and macrospin approaches, precisely reproduces experimental magnetic hysteresis loops, magnetoresistance curves, and ferromagnetic resonance dispersion relations. The Mo spacer thickness as a function of the interlayer magnetic coupling is determined as a fitting parameter by modeling the experimental results.

Third-order elastic coefficients (TOECs) have been measured experimentally and tabulated with pretty good accuracy since the middle of the previous century. In the classical acoustic measurement method the recalculation of instantaneous stiffness change onto TOECs is based on the use of Green strain. In recent calculations performed by means of atomistic and quantum methods many different strain measures are employed. In result, quite different sets of TOECs can be obtained for the same material. In this paper, it is shown how dramatically the coefficients obtained depend on the choice of strain measure. The known formulas for calculation of the second derivative of a tensor-valued function of tensor variable are corrected. The formulas are essential for the correct analytic calculation of the tangent stiffness matrix in finite element method, among others.

A range of high quality Ga1-xMnxN layers have been grown by molecular beam epitaxy with manganese concentration 0.2 < x < 10%, having the x value tuned by changing the growth temperature (Tg) between 700 and 590 {\deg}C, respectively. We present a systematic structural and microstructure characterization by atomic force microscopy, secondary ion mass spectrometry, transmission electron microscopy, powder-like and high resolution X-ray diffraction, which do not reveal any crystallographic phase separation, clusters or nanocrystals, even at the lowest Tg. Our synchrotron based X-ray absorption near-edge spectroscopy supported by density functional theory modelling and superconducting quantum interference device magnetometry results point to the predominantly +3 configuration of Mn in GaN and thus the ferromagnetic phase has been observed in layers with x > 5% at 3 < T < 10 K. The main detrimental effect of Tg reduced to 590 {\deg}C is formation of flat hillocks, which increase the surface root-mean-square roughness, but only to mere 3.3 nm. Fine substrates surface temperature mapping has shown that the magnitudes of both x and Curie temperature (Tc) correlate with local Tg. It has been found that a typical 10 {\deg}C variation of Tg across 1 inch substrate can lead to 40% dispersion of Tc. The established here strong sensitivity of Tc on Tg turns magnetic measurements into a very efficient tool providing additional information on local Tg, an indispensable piece of information for growth mastering of ternary compounds in which metal species differ in almost every aspect of their growth related parameters determining the kinetics of the growth. We also show that the precise determination of Tc by two different methods, each sensitive to different moments of Tc distribution, may serve as a tool for quantification of spin homogeneity within the material., Comment: 29 pages, 18 figures

The Lorentz off-axis electron holography technique is applied to study the magnetic nature of Mn rich nanoprecipitates in (Mn,Ga)As system. The effectiveness of this technique is demonstrated in detection of the magnetic field even for small nanocrystals having an average size down to 20 nm., Comment: 11 pages, 3 figures

Epitaxy of semiconductors is a process of tremendous importance in applied science and optoelectronic industry. Controlling of defects introduced during epitaxial growth is a key point in manufacturing devices of high efficiency and durability. In this work, we demonstrate how useful hybrid reflections are on the study of epitaxial structures with anisotropic strain gradients due to patterned substrates. High accuracy to detect and distinguish elastic and plastic relaxations are one of the greatest advantages of measuring this type of reflection, as well as the fact that it can be exploited in symmetrical reflection geometry on a commercial high-resolution diffractometer., Comment: 23 pages, double spaced, 9 color figures, and 1 table

The paper presents a study of two full-core, edge dislocations of opposite Burgers vectors in 4H-SiC, conducted using the first-principles density functional theory methods. We have determined the creation energy of the dislocations as a function of distance between their cores. The radial distribution function has been applied to examine strong impact of the dislocations on the local crystal structure. The analysis of the electronic structure reveals mid-gap levels induced by broken atomic bonds in the dislocation core. The maps of charge distribution and electrostatic potential have been calculated and the significant decrease of the electrostatic barriers in the vicinity of the dislocation cores has been quantified. The obtained results have been discussed in the light of available experimental data., Comment: 10 pages, 4 figures

Despite many efforts the origin of a ferromagnetic (FM) response in ZnMnO and ZnCoO is still not clear. Magnetic investigations of our samples, not discussed here, show that the room temperature FM response is observed only in alloys with a non-uniform Mn or Co distribution. Thus, the control of their distribution is crucial for explanation of contradicted magnetic properties of ZnCoO and ZnMnO reported till now. In the present review we discuss advantages of the Atomic Layer Deposition (ALD) growth method, which enables us to control uniformity of ZnMnO and ZnCoO alloys. Properties of ZnO, ZnMnO and ZnCoO films grown by the ALD are discussed., Comment: 41 pages, 19 figures, 48 references, review paper

The magnetic anisotropy of La0.7Sr0.3MnO3 nanopowders was measured as a function of temperature by the modified singular point detection technique. In this method singularities indicating the anisotropy field were determined analyzing ac susceptibility data. The observed relationship between temperature dependence of anisotropy constant and temperature dependence of magnetization was used to deduce the origin of magnetic anisotropy in the nanopowders. It was shown that magnetic anisotropy of La0.7Sr0.3MnO3 nanopowder is determined by two-ion (dipolar or pseudodipolar) and single-ion mechanisms.

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed RbxFe2-ySe2 single crystals is presented. Magnetization experiments evidence that annealing of RbxFe2-ySe2 at 413 K, well below the onset of phase separation Tp=489 K, neither changes the magnetic nor the superconducting properties of the crystals. In addition, annealing at 563 K, well above Tp, suppresses the superconducting transition temperature Tc and leads to an increase of the antiferromagnetic susceptibility accompanied by the creation of ferromagnetic impurity phases, which are developing with annealing time. However, annealing at T=488K=Tp increases Tc up to 33.3 K, sharpens the superconducting transition, increases the lower critical field, and strengthens the screening efficiency of the applied magnetic field. Resistivity measurements of the as-grown and optimally annealed samples reveal an increase of the upper critical field along both crystallographic directions as well as its anisotropy. Muon spin rotation and scanning transmission electron microscopy experiments suggest the coexistence of two phases below Tp: a magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens just at Tp does not affect the volume fraction of the two phases, although the magnetic field distribution in the samples changes substantially. This suggests that the microstructure of the sample, caused by mesoscopic phase separation, is modified by annealing just at Tp, leading to an improvement of the superconducting properties of RbxFe2-ySe2 and an enhancement of Tc., Comment: 13 pages, 12 figures

A series of (ZnO)m(CoO)n digital alloys and superlattices grown by atomic layer deposition has been investigated by a range of experimental methods. The data provide evidences that the Co interdiffusion in the digital alloy structures is sufficient to produce truly random Zn1-xCoxO mixed crystals with x up to 40%. Conversely, in the superlattice structures the interdiffusion is not strong enough to homogenize the Co content along the growth direction results in the formation of (Zn,Co)O films with spatially modulated Co concentrations. All structures deposited at 160\circC show magnetic properties specific to dilute magnetic semiconductors with localized spins S = 3/2 coupled by strong but short range antiferromagnetic interactions that lead to low temperature spin-glass freezing. It is demonstrated that ferromagnetic-like features, visible exclusively in layers grown at 200\circC and above, are associated with an interfacial mesh of metallic Co granules residing between the substrate and the (Zn,Co)O layer. This explains why the magnitude of ferromagnetic signal is virtually independent of the film thickness as well as elucidates the origin of magnetic anisotropy. Our conclusions have been derived for layers in which the Co concentration, distribution, and aggregation have been determined by: secondary-ion mass spectroscopy, electron probe micro-analysis, high-resolution transmission electron microscopy with capabilities allowing for chemical analysis; x-ray absorption near-edge structure; extended x-ray absorption fine-structure; x-ray photoemission spectroscopy, and x-ray circular magnetic dichroism. Macroscopic properties of these layers have been investigated by superconducting quantum interference device magnetometery and microwave dielectric losses allowing to confirm the important role of metallic inclusions., Comment: 23 pages, 22 figures

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

In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogenities in the iron chalcogenide system FeTe1-xSex and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe0.65Se0.35 studied with transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate a presence of nanometre scale hexagonal regions coexisting with tetragonal host lattice, a chemical disorder demonstrating non homogeneous distribution of host atoms in the crystal lattice, as well as hundreds-of-nanometres-long iron-deficient bands. From magnetic data and ferromagnetic resonance temperature dependence, we attribute magnetic phases in Fe-Te-Se to Fe3O4 inclusions and to hexagonal symmetry nanometre scale regions with structure of Fe7Se8 type. Our results suggest that nonhomogeneous distribution of host atoms might be an intrinsic feature of superconducting Fe-Te-Se chalcogenides and we find a surprising correlation indicating that faster grown crystal of inferior crystallographic properties is a better superconductor., Comment: 16 pages, 8 figures, 2 tables

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

GaAs nanowires were grown by molecular beam epitaxy on Si(100) substrates covered with 5 nm SiO2. The growth was performed with As4 at low, close to stoichiometric, As4/Ga flux ratio, using Ga nanodroplets as catalyst. The nanowires are found to be in epitaxial relation to the Si substrate due to contact via pinholes through the SiO2 layer. The nanowires are up to 15 micrometers long and have diameters of about 100 nm, the catalyzing Ga nanodroplets are observed at their tops., Comment: 10 pages, 4 figures

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self assembled structures for nanospintronics., Comment: 13 pages, 6 figures