Stable Magnetic Configurations Research Articles

Zero-Field Spin Waves in YIG Nanowaveguides.

Spin-wave-based transmission and processing of information is a promising emerging nanotechnology that can help overcome limitations of traditional electronics based on the transfer of electrical charge. Among the most important challenges for this technology is the implementation of spin-wave devices that can operate without the need for an external bias magnetic field. Here we experimentally demonstrate that this can be achieved using submicrometer wide spin-wave waveguides fabricated from ultrathin films of a low-loss magnetic insulator, yttrium iron garnet (YIG). We show that these waveguides exhibit a highly stable single-domain static magnetic configuration at zero field and support long-range propagation of spin waves with gigahertz frequencies. The experimental results are supported by micromagnetic simulations, which additionally provide information for the optimization of zero-field guiding structures. Our findings create the basis for the development of energy-efficient zero-field spin-wave devices and circuits.

Modelling of chemical and magnetic order in Ni-Mn-Al shape memory alloys using Monte Carlo simulations

A model for the description of atomic and magnetic ordering in NiMnAl shape memory alloys is presented. The parameters characterizing the energies of the chemical and magnetic interactions were obtained from fitting to experimental data. Monte Carlo simulations based on this model correctly reproduces the critical B2↔L21 ordering temperatures as a function of composition for three pseudobinary composition lines, as well as the nature of the low temperature magnetic transition. A ferromagnetic configuration is predicted for alloys in the stable L21 structure, whereas an antiferromagnetic state is obtained for metastably retained B2 order; this is in agreement with experimental observations. It is verified that atomic ordering is strongly dominated by the interactions between Ni and Al atoms in nearest neighbours. As a consequence, the formation of B2 order can delay the stabilization of the L21 configuration at low temperatures. Since the magnetic interactions change sign with the Mn–Mn distance, the type of stable magnetic configuration depends on atomic ordering.

Theory of the collective behavior of two-dimensional periodic ensembles of dipole-coupled magnetic nanoparticles

The magnetic behavior of two-dimensional (2D) periodic ensembles of dipole-coupled nanoparticles (NPs) is investigated theoretically by considering all possible structural organizations in 2D Bravais lattices, namely, square, triangular, rectangular, rhombic, and oblique lattices. The different interaction-energy landscapes (ELs) are characterized by determining the local minima that define the stable and metastable magnetic configurations as well as the transition states connecting them. The topology of the resulting ergodic ensemble of stationary states is analyzed from both local and energy perspectives by calculating the corresponding kinetic networks and disconnectivity graphs. For all lattices, the magnetic orders of the ground-state and low-lying metastable configurations are identified, including the elementary relaxation processes between them. A remarkable profound dependence of the collective magnetic behavior on the structural arrangement of the NPs is revealed. Square and triangular nanostructures are extremely good structure seekers, showing starlike kinetic networks and disconnectivity graphs with palm-tree form. Their continuously degenerate ground states are throughout-reaching hubs connected to nearly all excited magnetic configurations over a single first-order saddle point. Rhombic ensembles have double-funnel ELs in which the time-inversion-related ground states play the role of hubs with exceedingly high connectivity densities. These systems need to undergo very few elementary transitions with small downward energy barriers to relax from any configuration towards one of the ground states. However, the energy barriers between the two funnels are quite large and, furthermore, they increase as the number of particles in the unit cell is increased. Therefore, ergodicity breaking is expected in the thermodynamic limit. Finally, the rectangular and oblique lattices show contrasting ground-state orders, the former consists of an antiferromagnetic alternation of head-to-tail chains of spins while the latter is ferromagnetic. Nevertheless, the ELs of these nanostructures are qualitatively very similar. In both cases, all excited magnetic configurations consist of independent flipping of the chains of spins that are formed along the direction of the shorter Bravais vector. The whole energy spectrum of metastable magnetic configurations can thus be mapped to a one-dimensional Ising model. As a result, the kinetic network of stationary points is latticelike and the disconnectivity graphs have a willow-tree form. In conclusion, the collective magnetic behaviors of nanostructures having different point-group symmetries are contrasted and related by varying the parameters of the lattices that interpolate between them.

Electronic structure of YbFe4Al8 antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe4Al8 using X-ray photoelectron spectroscopy (XPS) and first-principles calculations. The XPS valence band of YbFe4Al8 consists of two contributions coming from divalent (Yb2+) and trivalent (Yb3+) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4d XPS spectra. We study several collinear antiferromagnetic models of YbFe4Al8 from the first-principles and for comparison we also consider LuFe4Al8 with a fully filled 4f shell. We predict that only Fe sublattices of YbFe4Al8 carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mulliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4f orbitals on Yb valence and Fe magnetic moments.

Equiatomic quaternary CoXCrAl (X = V, Nb, and Ta) Heusler compounds: Insights from DFT calculations

The half-metallic ferromagnets are likely materials to the spintronics systems for the next generation electronic devices. Among the half-metallic ferromagnetic materials, the equiatomic quaternary Heusler (EQH) compounds get great attention in the recent times. In this study, the EQH CoXCrAl (X = V, Nb, and Ta) compounds are investigated using Density Functional Theory (DFT). The three structural types as type-I, type-II, and type-III, are considered to determine the most stable structural types of CoXCrAl compounds. Furthermore, the three magnetic configurations as paramagnetic (PM), ferromagnetic (FM), and antiferromagnetic (AFM), are studied to find out the most stable magnetic configuration. The type-I FM configuration is the most stable configuration for the CoXCrAl compounds. The calculated electronic band structures reveal that the CoXCrAl compounds are half-metals with metallic behavior in spin-up channel and semiconducting behavior in spin-down channel. Also, the possible d-d hybridizations between the Co, X, and Cr atoms are investigated to show the half-metallic character in detail. Furthermore, the calculated elastic constants satisfy the Born stability criteria, then the CoXCrAl compounds have mechanical stability. Using the determined constants, the mechanical properties are obtained for CoXCrAl compounds. In addition, the dynamical properties are studied and it is found that CoXCrAl compounds are dynamically stable. The structural, magnetic, electronic, mechanic and dynamical properties of CoXCrAl compounds that are the potential candidates for the spintronics applications are presented in this study.

Structural, magnetic, electronic, and phonon properties of NdNiO2 under pressure from first-principles

In this paper, the structural, electronic, magnetic, and phonon properties of NdNiO2 under the pressure range between 0 and 20 ​GPa are studied. The ground state of NdNiO2 is the C-AFM phase within the pressure range of 0–20 ​GPa. Another surprising finding is that no matter what the pressure is, no matter what kind of magnetic structure, the local magnetic moment of Nd is almost 3.0 μB. The results suggest that assuming the spin value of Nd moment is always 3.0 μB, which is a characteristic of an f3 Hund's rule. The electronic structures for the stable C-AFM magnetic configuration of NdNiO2 under the pressure range between 0 and 20 ​GPa show metallic properties. Both the Nd-f bands and Ni-3dxy bands across the Fermi level. The phonon spectra show no imaginary modes under the pressure range between 0 and 20 ​GPa, so NdNiO2 is dynamically stable within the pressure scope between 0 and 20 ​GPa. The vibration frequency of the O atom is nearly higher than that of Ni and Nd atoms, the main reason is that the mass of the O atom is much smaller than that of Ni and Nd atoms.

A DFT study of electronic, magnetic, optical and transport properties of rare earth element (Gd, Sm)-doped GaN material

The main objective of this article is to deal with and establish a new type of transparent conducting material by doping Gallium Nitride (GaN) with rare-earth RE (Gd, Sm) elements. A theoretical study was performed by using the density functional theory (DFT) implemented in WIEN2k code and BoltzTraP package based on semi-classical Boltzmann transport equation (BTE) within constant relaxation time and rigid band approximation (RBA). The application of the modified Becke–Johnson (TB-mBJ) potential has improved the electronic structure, especially the electronic bandgap. Our simulation result matches with other computational results as well. We have compared the total energy of ferromagnetic (FM) and Anti-ferromagnetic (AFM) configurations for the doped system to analyze the magnetic ground state stability. Preferentially the most stable magnetic configuration is found to be ferromagnetic. The low value of formation energy indicates the possibility of the preparation of these materials in the lab. To further compare the results, we have also used the VNL-ATK quantumwise code based on the Linear Combination of Atomic Orbital (LCAO) approach. VNL-ATK incorporate a new functional DFT-1/2 to correct the Kohn–Sham KS eigenvalues around the minima and maxima of the conduction band, and the valence band, respectively, which enhances the bandgap. It shows the high optical absorption and the high electrical conductivity. These characteristics offer that GaN doped with rare earth elements are potential candidates for optoelectronic, solar cell, and spin-based magnetic devices.

Structural Disorder and Collective Behavior of Two-Dimensional Magnetic Nanostructures.

Structural disorder has been shown to be responsible for profound changes of the interaction-energy landscapes and collective dynamics of two-dimensional (2D) magnetic nanostructures. Weakly-disordered 2D ensembles have a few particularly stable magnetic configurations with large basins of attraction from which the higher-energy metastable configurations are separated by only small downward barriers. In contrast, strongly-disordered ensembles have rough energy landscapes with a large number of low-energy local minima separated by relatively large energy barriers. Consequently, the former show good-structure-seeker behavior with an unhindered relaxation dynamics that is funnelled towards the global minimum, whereas the latter show a time evolution involving multiple time scales and trapping which is reminiscent of glasses. Although these general trends have been clearly established, a detailed assessment of the extent of these effects in specific nanostructure realizations remains elusive. The present study quantifies the disorder-induced changes in the interaction-energy landscape of two-dimensional dipole-coupled magnetic nanoparticles as a function of the magnetic configuration of the ensembles. Representative examples of weakly-disordered square-lattice arrangements, showing good structure-seeker behavior, and of strongly-disordered arrangements, showing spin-glass-like behavior, are considered. The topology of the kinetic networks of metastable magnetic configurations is analyzed. The consequences of disorder on the morphology of the interaction-energy landscapes are revealed by contrasting the corresponding disconnectivity graphs. The correlations between the characteristics of the energy landscapes and the Markovian dynamics of the various magnetic nanostructures are quantified by calculating the field-free relaxation time evolution after either magnetic saturation or thermal quenching and by comparing them with the corresponding averages over a large number of structural arrangements. Common trends and system-specific features are identified and discussed.

Benchmark for Ab Initio Prediction of Magnetic Structures Based on Cluster-Multipole Theory

The cluster multipole (CMP) expansion for magnetic structures provides a scheme to systematically generate candidate magnetic structures specifically including noncollinear magnetic configurations adapted to the crystal symmetry of a given material. A comparison with the experimental data collected on MAGNDATA shows that the most stable magnetic configurations in nature are linear combinations of only few CMPs. Furthermore, a high-throughput calculation for all candidate magnetic structures is performed in the framework of spin-density functional theory (SDFT). We benchmark the predictive power of CMP+SDFT with $2935$ calculations, which show that (i) the CMP expansion administers an exhaustive list of candidate magnetic structures, (ii) CMP+SDFT can narrow down the possible magnetic configurations to a handful of computed configurations, and (iii) SDFT reproduces the experimental magnetic configurations with an accuracy of $\pm0.5\,\mu_\text{B}$. For a subset the impact of on-site Coulomb repulsion $U$ is investigated by means of $1545$ CMP+SDFT+U calculations revealing no further improvement on the predictive power.

On the electronic properties of defective graphene buffer layer on 6H–SiC(0001)

Using first-principles calculations we predict that the electronic properties of graphene buffer layer (BL) on silicon carbide (SiC) can be modified by defect-induced itinerant states. Band structures and effective masses of single vacancy (SV), divacancy (DV) and Stone-Wales (SW) defective BL were calculated. Structural reconstruction at the vicinity of defects as well as spin polarization of silicon dangling bonds in the top bilayer SiC and nearby carbons from the BL, give rise to energetically degenerate magnetic states displaying electronic properties completely apart. This is particularly true for the SW and SV defective models where as many as three different magnetic states, turned out to be degenerate, either displaying a semiconductor nature or becoming half-metallic or even metallic. This is in contrast with previously reported results for the perfect BL (PBL) which suggested that most stable degenerate magnetic configurations share a semiconductor nature. On the other hand, for the DV system, the introduction of two vacancies in the BL causes loss of magnetism whereas a band gap of 0.54 eV is opened. Hole effective masses decay upon removal of one and two carbon atoms in the SV and DV systems where an increase in mobility is conceivable, provided current direction is carefully selected.

Depth selective magnetic phase coexistence in FeRh thin films

We demonstrate the manipulation of magnetic phases in FeRh thin films through atomic displacements and the distribution of structural defects. Atomic scale disorder can be controlled via irradiation with light noble gas ions, producing depth-varying nanoscale phase configurations of distinct antiferromagnetic, ferromagnetic, and paramagnetic regions. Here, we perform a spatial characterization of the magnetic phases and the local magnetic environment around the Fe atoms, as well as the variation of the open-volumes around atomic sites. Thus, a direct correspondence between the existence of the three magnetic phases and lattice defects is revealed. By careful selection of the irradiating fluence, we show that it is possible to produce simple and thermally stable magnetic configurations, such as uniform magnetization or a bilayer phase structure. Furthermore, the thin film surface and interfaces are observed as the nucleation sites for the transitions between the phases. These results demonstrate a sensitive nanoscale manipulation of magnetic properties, shedding light on magnetic ordering in alloy lattices and broadening the scope for applications.

Ar+ ion irradiation of magnetic tunnel junction multilayers: impact on the magnetic and electrical properties

The impact of 400 keV Ar+ irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The ion fluences ranged from to . Below , the anisotropy of the Ta-capped FeCoB free layer was weakly modulated, following a decrease in the saturation magnetization. The tunnel magnetoresistance (TMR), along with the exchange-bias and the interlayer exchange coupling providing a stable magnetic configuration to the reference layer, decreased continuously. Above , a strong decrease in the saturation magnetization was accompanied by a loss of the magnetic coupling and of the TMR. We show there is an ion-fluence window where the modulation of magnetic anisotropy can occur while preserving a large TMR and stable magnetic configuration of the MTJ, and demonstrate that the layers surrounding the free layer play a decisive role in determining the trend of the magnetic anisotropy modulation resulting from the irradiation. Our results provide guidance for the tailoring of MTJ parameters via ion irradiation, which we propose as a potentially suitable technique for setting the magnetic easy-cone state in MTJ for attaining field-free, fast, and non-stochastic magnetization switching.

Target Bubbles in Fe3Sn2 Nanodisks at Zero Magnetic Field.

We report a vortex-like magnetic configuration in uniaxial ferromagnet Fe3Sn2 nanodisks using differential phase contrast scanning transmission electron microscopy. This magnetic configuration is transferred from a conventional magnetic vortex using a zero-magnetic-field warming process and is characterized by a series of concentric cylinder domains. We termed them as "target bubbles" that are identified as three-dimensional depth-modulated magnetic objects in combination with numerical simulations. Target bubbles have room-temperature stability even at zero magnetic field and multiple stable magnetic configurations. These advantages render the target bubble an ideal bit to be an information carrier and can advance magnetic target bubbles toward functionalities in the long term by incorporating emergent degrees of freedom and purely electrically controllable magnetism.

Structural, strong magnetic exchange, and room temperature multiferroic properties of single-phase 0.75BiFeO3–BaFe1/2Nb1/2O3 bulk ceramics

The ceramic 0.75BiFeO3–BaFe1/2Nb1/2O3 (0.75BF–BaFN) exhibits enhanced magnetisation and polar order regions at room temperature. This study investigated the microscopic structure and the magnetic, ferroelectric, magneto-ferroelectric, and optical properties of 0.75BF–BaFN by conducting experiments and performing density functional theory (DFT) calculations. Evidence of weak amplitude R-type octahedral rotations was observed using transmission electron microscopy (TEM). X-ray diffraction (XRD) refinement revealed that the P1 space group more appropriately described the structure of 0.75BF–BaFN. The weak influence of the magnetic field H on the polarisation of 0.7BF–BaFN and 0.75BF–BaFN ceramics was investigated. A notable magnetic enhancement was observed in 0.75BF–BaFN ceramics with a saturation magnetisation of 0.19 μB per Fe. DFT calculations predicted that the stable magnetic configuration of 0.75BF–BaFN is a G-type antiferromagnetic (AFM) structure. This study found stronger magnetic interactions between first-neighbour Fe ions (Ja = 12 meV) compared to pure BiFeO3. With a significant remanent moment, strong exchange coupling, and magnetic field control of polarisation, 0.75BF–BaFN may facilitate the development of room temperature single-phase magnetoelectric components.

Magnetic Domains without Domain Walls: A Unique Effect of He^{+} Ion Bombardment in Ferrimagnetic Tb/Co Films.

We show that it is possible to engineer magnetic multidomain configurations without domain walls in a prototypical rare-earth-transition-metal ferrimagnet using keV He^{+} ion bombardment. We additionally show that these patterns display a particularly stable magnetic configuration due to a deep minimum in the energy of the system caused by flux closure and a corresponding reduction of the magnetostatic energy without an increase in energy by exchange and anisotropy terms across the walls. This occurs because light-ion bombardment affects an element's relative contribution to the properties of the ferrimagnet differently. Therefore, it is possible to control the relative contribution from each magnetic subsystem. The selection of material and the use of light-ion bombardment allow us to engineer domain patterns in continuous magnetic films, which open a way to fabricate them in a much smaller scale than currently possible. Our Letter emphasizes that the right criterion to determine the presence or absence of a domain wall is whether there is a rotation of the spin for each sublattice and that changes of the direction of effective magnetization alone do not constitute an appropriate criterion.

Hydrogen induced AFM to FM magnetic transition in [formula omitted]-[formula omitted

We study the magnetic behavior of the metallic system ∊-FeHx as a function hydrogen concentration (x). Using ab-initio density functional theory, we investigate the effect of hydrogen on the stability of magnetic configurations considering two antiferromagnetic (AFM) phases and a ferromagnetic (FM) one present in ∊-Fe (AFM-I and AFM-II). Our results show a magnetic transition induced by H between AFM and FM around x = 0.44. In this way, two main effects can determine the stability and the magnetic phase transition: a volume increase that produces an AFM to FM transition and the charge transfer effect from H to Fe energetically favors the FM phase. We discuss these behaviors in detail and physically explain the magnetic transition and the overall behavior using a phenomenological Stoner model for these systems.

Electronic and Magnetic Structure and Elastic and Thermal Properties of Mn2-Based Full Heusler Alloys

Magnetism, electronic structure, elastic and thermal properties of Mn2YAl (with Y = Cr, V) have been investigated. The optimized lattice parameters, bulk modulus, and cohesive energy have been obtained. These alloys have the ferrimagnetic state as the most stable magnetic configuration, since the calculations showed a strong Mn-V antiferromagnetic coupling leading to the ferromagnetism of the Mn sublattices. A small and itinerant magnetic moment of Mn at the A site is found, which is antiparallel to the moment of Y at the B position in Mn2YAl (with Y = Cr, V) compounds. The calculated total spin moments are integral values and increase from − 2 μB/f.u. for Mn2VAl to – 1 μB/f.u. for Mn2CrAl with increasing the number of valence electrons. Band structure and total and partial density of states could be calculated via applying the modified Becke Johnson approximation (mBJ). Based on these results, Mn2YAl (with Y = Cr, V) are half-metallic ferrimagnets with the energy gap lies in the majority spin direction and a high-spin polarization (100%). The main difference between these two compounds is that the band gap is increased by 48% (0.210 eV for Mn2CrAl and 0.401 eV Mn2VAl). Elastic anisotropies, brittleness, and thermodynamic properties are determined for the Mn2YAl (with Y = Cr, V). The slight difference in the spatial distributions of Young’s moduli of Mn2YAl (with Y = Cr, V) reflects the small differences for the elastic anisotropies of the alloys under consideration. The mechanical stability of Mn2YAl (with Y = Cr, V) alloys are studied based on the elastic constants. The thermal properties are studied and investigated using the quasi-harmonic model, in addition, the temperature effect on heat capacities at constant pressure and volume, entropy, and thermal expansion are analyzed and discussed.

Effect of Ca doping on structural, magnetic and electronic properties of TbMnO3

First principles calculations have been carried out to investigate the structural, magnetic and electronic properties of the multiferroic manganite TbMnO3 with and without Ca doping. For pure TbMnO3, results show that spiral-spin antiferromagnetic ordering is energetically found favorable. The presence of Ca doping at the manganite Tb1−xCaxMnO3 with x = 0.25, 0.5 and 0.75 directly affects the Mn ordering. For x ≤ 0.5, the ferromagnetic ordering is most favored. While for x = 0.75, the most stable magnetic configuration is antiferromagnetic arrangement. It is found that the decrease in the intensity of the ferromagnetic double-exchange interaction has been correlated with the increase in the Ca content and the Mn-O-Mn bond angle.

Sharp-pointed susceptibility of ferromagnetic films with magnetic anisotropy inhomogeneous in thickness

It is shown that in ferromagnetic films with anisotropy inhomogeneous in thickness, an interphase boundary can be formed for certain values and directions of the magnetic field. This boundary has a stable magnetic configuration. It is oriented parallel to the film plane and separates the regions with different orientations of the magnetization. In an alternating magnetic field, the interphase boundary oscillates, which may be accompanied by a resonance. The field and frequency dependences of the components of the magnetic susceptibility tensor are determined. It is shown that the susceptibility coefficients at a resonance are extremely sensitive to the direction of the external magnetic field which can underlie the development of a highly sensitive sensor.

Effect of tensile and compressive strain on the magnetic ordering of the ( $$\hbox {LaMnO}_{3})_{1}/\hbox {(SrTiO}_{3})_{1}$$ LaMnO 3 ) 1 / (SrTiO 3 ) 1 superlattice

We study the effects of tensile and compressive strain on the $$(\hbox {LaMnO}_{3})_{1}/(\hbox {SrTiO}_{3})_{1}$$ superlattice from density functional theory using Quantum-Espresso open source code. In the unstrained superlattice, electron interactions in out-of-plane Mn–O–Ti chains are dominated by superexchange interactions, giving rise to ferromagnetic and half-metallic conducting characters. We found that the most stable magnetic configuration is G-type antiferromagnetic configuration for strong compressive strain and for strong tensile strain it is A-type antiferromagnetic configuration. The results are in accordance with the experimental observations which show that the superlattices can be grown on different substrates, and due to the difference in lattice parameters of the substrate and the main layer, there are also changes in the amount of strain applied to the superlattice.