Anisotropy Axis Research Articles

Field and anisotropy driven transformations of spin spirals in cubic skyrmion hosts

We discuss distinctive features of spiral states in bulk chiral magnets such as MnSi and Cu$_2$OSeO$_3$ that stem from the effect of the cubic magneto-crystalline anisotropy. First of all, at both the helical-to-conical and the conical-to-ferromagnetic transitions, taking place at H$_{c1}$ and H$_{c2}$, respectively, the cubic anisotropy leads to reversible or irreversible jump-like reorientations of the spiral wavevectors. The subtle interplay between the easy and hard anisotropy axes gives rise to a phase transition between elliptically distorted conical states almost without any detectable change in the period. We show that the competition between on-site cubic and exchange anisotropy terms can also lead to oblique spiral states. Our work gives clear directions for further experimental studies to reveal theoretically predicted spiral states in cubic helimagnets beyond the aforementioned well-established states thus, can help to understand the magnetic phase diagram of these archetypal skyrmion hosts. In addition, we show that properties of isolated skyrmions such as inter-skyrmion attraction, orientation and/or nucleation are also rooted in the properties of host spirals states, in which skyrmions are stabilized.

Магнитные свойства композита частиц DyMn-=SUB=-2-=/SUB=-O-=SUB=-5-=/SUB=--Mn-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=-

The paper presents the results of the experimental studies of magnetic and resonance properties of the DyMn2O5 – Mn3O4 particles composite. There are two magnetic transitions at temperatures T1  65 K and T2  230 K. T1 differs from the transition temperatures for the initial materials, and this fact is associated with the interparticle interaction. T2 corresponds to the impurity phase DyMnO3 (1 at.%). Three peaks of microwave absorption are observed in the magnetic resonance spectrum, which is explained in the framework of the model of a two-phase system in magnetic relation. One resonance is attributed to Mn3O4 and the other two peaks refer to the ensemble of strongly anisotropic DyMn2O5 particles with a random distribution of anisotropy axes. Patrin Gennady Semionovich1,3 Mataev Mukhametkali Musagalievich2 Abdraimova Moldir Rashidovna2 Tursinova Zhanar Iliyasovna2 Kezdikbaeva Asel Taupykovna4 Shiyan Yaroslav Germanovich1,3 Plekhanov Vasil Granitovich1 1 Siberian Federal University, Svobodny pr. 79, Krasnoyarsk, 660041, Russia 2 Kazakh State Women’s Teacher Training University, Aiteke bi, 99, Almaty, 050000, Kazakhstan 3 L.V. Kirensky Institute of Physics, Federal Research Center KSC, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia 4 E.A. Buketov Karaganda State University, st. University 28, City of Karaganda, 100028, Kazakhstan

Stressed state of two-layer strip when interacting with rigid base

Relevance. In the calculation of multilayer bases, when the material of one or several layers has a pronounced anisotropy, the nature of the distribution of displacements and stresses depends on the direction of the anisotropy axes in each layer. Therefore, it is necessary to have an evaluation of the influence of this factor in the design and analysis of the operation of multilayer media. The aim of the work - to research the stress state in a strip composed of two anisotropic plane-parallel layers with different physical characteristics, lying without friction on a rigid base. Methods. The integration of the equations of the plane problem of the theory of elasticity of an anisotropic body is carried out by the symbolic method in combination with the method of initial functions. The initial functions on the contact line of the strip and the base are determined from the conditions of tight adhesion between the layers, the conditions of tight contact and the absence of friction between the strip and the base, the nature of the load applied to the upper plane of the strip. After transformations, the functions of displacements and stresses in each layer are written through the normal surface load in the form of improper integrals. Results. Plots of changes in stresses in the strip from the values of the characteristics of anisotropic materials, layer thicknesses are given. The maximum stresses on the interface line of the layers and on the line of contact with the base, depending on the direction of the anisotropy axes in each layer, are presented in the tables and shown in graphs. The effect of the elastic modules of materials on the nature of the stress distribution in a strip composed of two isotropic materials is estimated.

Effect of friction in the interaction of an anisotropic strip with a rigid base

Relevance. Different models of contact between bodies are used in determining the stressed and deformed state in the strip lying on the base. It is necessary to evaluate the qualitative and quantitative nature of the change in stress in the strip depending on the coupling of the strip and base. The aim of the work - to analyze the effect of the coefficient of friction on the value of stresses in an anisotropic band when interacting with a rigid base. Methods. The solution is based on the equations of the plane problem of the theory of elasticity of an anisotropic body under the conditions that the band is closely adjacent to the base and the tangent force on the contact plane is proportional to the normal pressure. Displacements and stresses at any point of the strip are written in the form of the method of initial functions through the functions of displacements and forces on the lower plane, which depend on the nature of the load applied on the upper plane and the conditions of contact between the strip and the base. After the transformations, the calculation formulas for displacements and stresses are expressed using the Fourier integral transform through the normal surface load in the form of improper integrals. Results. Formulas for determining displacements and stresses are obtained for the variant of loading a strip with a concentrated force. These formulas are used to construct influence functions for the problem of equilibrium of an anisotropic strip lying on a rigid base, taking into account friction. Graphs of the effect of the coefficient of friction and the direction of the anisotropy axes of the material on the stress state of the strip are presented. The results of stress calculation are compared using anisotropic and isotropic models.

Extending the family of reduced [Mn12O12(O2CR)16(H2O)x]n− complexes, and their sensitivity to environmental factors

The family of reduced [Mn12O12(O2CR)16(H2O)4]n− (n = 1, 2) single-molecule magnets (SMMs) has been expanded with the synthesis of new members using I− as a stoichiometric one-electron reducing agent of the neutral n = 0 complexes. Electrochemical data, 1H NMR spectra of the [Mn12O12(O2CCH2Cl)16(H2O)3]0,−,2− salts, and the crystal structures of two complexes, (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] and (PPh4)2[Mn12O12(O2CCH2Cl)16(H2O)3] have been obtained. For (PPh4)2[Mn12O12(O2CCH2Cl)16(H2O)3], the data confirm an S = 10 ground state, and magnetization vs dc field scans on a pristine crystal exhibit hysteresis loops possessing steps due to quantum tunneling of magnetization (QTM). In addition, two forms of the cluster have been identified in the crystal with distinctly different relaxation barriers, with the faster-relaxing form being the majority one in pristine crystals from mother liquor, and the slower-relaxing one becoming the majority one in vacuum-dried samples. Similar behavior is seen for (PPh4)[Mn12O12(O2CCHCl2)16(H2O)4] with an S = 19/2 ground state, which shows three forms to be present in the hysteresis loops of a pristine crystal, all converting to a slow-relaxing form on vacuum-drying. High-frequency EPR spectra of the latter confirm a high axial anisotropy with D = −0.477 cm−1 and rationalize its effective relaxation barrier Ueff = 57 K. The combined work emphasizes a high sensitivity of such ultra-small nanomagnets to environmental influences, affecting their properties significantly, especially their quantum properties that are so important to many potential new 21st century applications.

Wireless Control of Two- and Three-Dimensional Actuations of Kirigami Patterns Composed of Magnetic-Particles-Polymer Composites.

We demonstrate wireless remote control of two-dimensional (2D) and three-dimensional (3D) shape transformations of specially designed kirigami patterns by application of static magnetic fields. The kirigami patterns consist of hinge-linked periodic unit blocks composed of magnetic-particle-elastomer composites. By designing the axis of magnetic anisotropy in each unit block and determining the placement of the hinges that link the individual unit blocks, 2D and 3D transformations of the patterns were demonstrated under application of uniform magnetic fields with specific field directions. Magnetic nanoparticles in an elastomer matrix within unit blocks were aligned in-plane or out-of-plane with respect to the frame of the individual unit blocks by application of magnetic fields. Such 2D and 3D actuations of kirigami patterns might offer a first step toward the development of spatiotemporal actuation and transformation of more complex 3D shapes using magnetic-particle-elastomer composites.

Magnetic torque-driven deformation of Ni-nanorod/hydrogel nanocomposites

Abstract Nickel (Ni) nanorods were prepared by the anodized aluminum oxide (AAO) template method and dispersed in poly(acrylamide) (PAM) hydrogels. The deformation of the magnetoresponsive composites was studied with particular attention to the consequences of finite magnetic shape anisotropy as compared to rigid dipoles on the field-dependent torque. For comparison with experiments, the composite was described as an elastic continuum with a local magnetic torque density, applied by discrete particles and determined by the local orientation of their magnetic anisotropy axis with respect to the magnetic field. The mean magnetic moment of the single domain particles m and their volume density in the composite φ vol were derived from the static field-dependent optical transmission (SFOT) of linear polarized light. The mechanical coupling between the particles and their viscoelastic environment was retrieved from the rotational dynamics of the nanorods using oscillating field-dependent optical transmission (OFOT) measurements. Field- and orientation-dependent magnetization measurements were analyzed using the Stoner–Wohlfarth (SW) model and a valid parameter range was identified by introducing an effective anisotropy constant K A as a new empirical model parameter. This adapted SW-model for quantitative description of the field- and orientation dependence of the magnetic torque was validated by measuring the local rotation of nanorods in a soft elastic hydrogel. Finally, torsional and bending deformation of thin magnetically textured composite filaments were computed and compared with experiments.

Spectrum evolution of magnetostatic waves excited through ultrafast laser-induced heating

We study experimentally the influence of the laser-induced temperature gradient on the parameters of propagating magnetostatic surface waves in thin film of the ferromagnetic metallic alloy Galfenol Fe0.81Ga0.19. The material has a pronounced magnetocrystalline anisotropy and exhibits the long-distance propagation of magnetostatic surface waves excited with femtosecond laser pulses. The excitation pulse heats up the sample locally, what leads to the spatial-temporal change of magnetization and anisotropy parameters of the film, and thus excites the magnetostatic surface waves. We show experimentally that the spectrum of the excited waves narrows as they propagate in such a gradient medium. By changing the orientation of external magnetic field with respect to anisotropy axes of the sample, we control whether the low- or high-frequency part of the spin waves spectrum is suppressed.

Блочный принцип построения и оценки снижения ЭПР непоглощающих широкополосных 2-битных анизотропных цифровых метапокрытий.

The block principle of constructing of coding matrices for non-absorbing flat digital anisotropic meta-coatings (MC) used to reduce monostatic RCS of objects is considered. The essence of this principle consists in 2-bit coding of the tilt angles of the anisotropy axes of the MC modules so that four arbitrary adjacent modules of the MC form two pairs of antiphase modules. The use of the block principle in the development of a MC provides a reduction of monostatic RCSs on cross polarization due to cancellation of interference waves. The reduction of the monostatic RCS of a MC at matched polarization (co-RCS) is due to the twist-effect. Simultaneously with cancellation and twist-effect, diffuse wave scattering is implemented in the developed digital MCs. The proposed 2-bit anisotropic MCs make it possible to achieve a more effective reduction of monostatic co-RCS (in relation to the traditionally accepted level of minus 10 dB) in a wide frequency band, for different polarization planes of the incident wave. The impedance and full-wave models of the two main blocks of digital anisotropic MCs are developed. An asymptotic representation of the polarization scattering matrix is obtained for the impedance model of the MC block using the method of physical optics. The algorithm for calculating the frequency characteristics (FC) of monostatic co-RCS of impedance models of MC blocks is implemented in the Octave program. Full-wave models of MC blocks are built using the HFSS program. Layouts of two main MC blocks were made. Monostatic co-RCSs of the blocks are measured in the 7÷17.5 GHz frequency band for different polarizations of the incident wave. It is shown that the calculated and measured frequency characteristics of monostatic RCS of the models and layouts of blocks of 2-bit anisotropic MCs are in good agreement. The calculated and measured levels of monostatic RCS reduction of the main blocks are at least 12.5 - 13.5 dB at different co-polarizations in the band from 10.2 to 17.5 GHz and higher (based on the results of calculations).

Antisite disorder in the battery material LiFePO4

We report detailed magnetometry and high-frequency electron spin resonance (HF-ESR) measurements which allow detailed investigation on Li-Fe antisite disorder in single-crystalline LiFePO$_4$, i.e., exchange of Fe$^{2+}$- and Li$^+$-ions. The data imply that magnetic moments of Fe$^{2+}$-ions at Li-positions do not participate in long-range antiferromagnetic order in LiFePO$_4$ but form quasi-free moments. Anisotropy axes of the magnetic moments at antisite defects are attached to the main crystallographic directions. The local character of these moments is confirmed by associated linear resonance branches detected by HF-ESR studies. Magnetic anisotropy shows up in significant zero-field splittings of $\Delta = 220(3)$~GHz, $\Delta'\sim50$~GHz and a highly anisotropic $g$-factor, i.e., $g_\mathrm{a} = 1.4$, $g_\mathrm{b} = 2.0$, and $g_\mathrm{c} = 6.3$. We demonstrate a general method to precisely determine Fe-antisite disorder in LiFePO$_4$ from magnetic studies which implies a density of paramagnetic Fe$^{2+}$-ions at Li-positions of 0.53\%.

Thermal and dipolar interaction effect on the relaxation in a linear chain of magnetic nanoparticles

We perform computer simulations to study the relaxation in a one-dimensional chain of dipolar interacting magnetic nanoparticles (MNPs). Using the two-level approximation of the energy barrier, we perform kinetic Monte Carlo simulations to probe the relaxation mechanism as a function of dipolar interaction strength and temperature. The anisotropy axes of the MNPs are assumed to have random orientations. At high temperatures, the magnetization decay curve is exponential for weak dipolar interactions. It is found that dipolar interactions slow down the magnetic relaxation and increase the effective Néel relaxation time τN, which is affected by thermal fluctuations. In the weak dipolar limit, there is a perfect agreement between simulated and analytically evaluated values of τN for a wide range of temperatures. Microscopic analyses such as magnetic moments correlations and dynamic domain formation also suggest an increase in ferromagnetic coupling with an increase in dipolar interaction strength or decrease in thermal fluctuations. We believe that the concepts presented in this work are relevant in the context of applications such as data storage, digital data processing, and magnetic hyperthermia, in which the linear chain of MNPs are pervasive.

Анизотропия отрицательного магнетосопротивления в эапитаксиальных слоях GaMnAs

Anisotropy of magnetotransport properties of GaMnAs epitaxial layers in ferromagnetic state has been investigated as a function of temperature. The anisotropy of negative magnetoresistance that is not related to uniaxial anisotropy and hard axis orientation has been observed. This anisotropy may be result of arising of spatially oriented structures in the GaMnAs layer during its growth.

Photoinduced metastable dd-exciton-driven metal-insulator transitions in quasi-one-dimensional transition metal oxides

Photoinduced phase transitions in matters have gained tremendous attention over the past few years. However, their ultrashort lifetime makes their study and possible control very challenging. Here, we report on highly anisotropic d-d excitonic excitations yielding photoinduced metal-insulator transitions (MITs) in quasi-one-dimensional metals Sr1-yNbOx using Mueller-Matrix spectroscopic ellipsometry, transient ultraviolet Raman spectroscopy, transient mid-infrared reflectivity and angular-resolved photoemission spectroscopy supported with density functional theory. Interestingly, the MITs are driven by photo-pumping of d-d excitons, causing the metallic a-axis to become insulating while the insulating b- and c-axis concomitantly become a correlated metal. We assign these effects to an interplay between the melting of charge and lattice orderings along the different anisotropic optical axes and Bose-Einstein-like condensation of the photoinduced excitons. The long lifetime in the order of several seconds of the metastable MITs gives greater flexibility to study and manipulate the transient excitonic state for potential applications in exciton-based optoelectronic devices.

Features of structural plywood operation in conditions of flat stress state

The aim of our research work was to study operation features of plywood structures in flat stress state, which takes place in conditions of real operation of sheet structural materials. The method of study the materials in such conditions was developed. It includes determination of optimal form of specimens and equipment, providing tests of structural plywood under various ratio of values and signs of loads, made in mutually perpendicular directions. The research results of plywood operation from larch timber in conditions of biaxial tension and the assessment of its strength criterion under biaxial tension are shown in the research paper. The values of destructive stresses under various ratio of tension stress, directed along the main axes of elastic anisotropy of the tested material, were obtained. The experimental research results testify to possibility of using the test results of plywood specimens in conditions of biaxial tension and show that addition of extra stress almost does not reduce the strength limit of structural plywood.

Apparent thermal conductivity measurement of anisotropic insulating materials at high temperature by the parallel hot-wire method

This article presents a measurement method of the apparent thermal conductivities of anisotropic insulating materials at high temperature by the parallel hot-wire method. In the proposed variant, the temperature is measured simultaneously at two points located on the main axes of anisotropy of the material to be characterized. A comprehensive model taking into account the anisotropy, the thermal capacity of the heating wire and the contact resistance was developed. A method for the estimation of the thermal properties was devised after carrying out a theoretical study of the reduced sensitivities of the temperatures to the different unknown parameters. An experimental study carried out up to 1000 °C on an anisotropic rigid insulating material confirmed that the thermal conductivities and the volumetric heat capacity of a rigid insulating material could be estimated by the proposed method with an accuracy of approximately 10%.

Antiferromagnetic Resonance in a Spin-Gap Magnet with Strong Single-Ion Anisotropy

Quasi-one-dimensional magnet NiCl$_2\cdot$4SC(NH$_2$)$_2$, usually abbreviated as DTN, does not order at zero field down to $T=0$: due to the strong single-ion anisotropy of the "easy plane" type acting on $S=1$ Ni$^{2+}$ ions, the $S_z=0$ ground state is separated from $S_z=\pm 1$ excitations by an energy gap. Once the magnetic field is applied along the main anisotropy axis, the gap closes at $B_{c1}=2.18$ T and the field-induced antiferromagnetic order arises. The low-energy excitations spectrum of this field-induced ordered state includes two branches of excitations, one of them have to be a gapless Goldstone mode. Recent studies of excitations spectrum in a field-induced ordered state of DTN (T.Soldatov et.al, Phys.Rev.B 101, 104410 (2020)) have revealed that Goldstone mode became gapped as magnetic field deviates from the main symmetry axis. This paper proposes simple description of antiferromagnetic resonance modes of quasi-one-dimensional quantum $S=1$ magnet with strong single-ion anisotropy. The approach used is based on a combination of the strong coupling model for the anisotropic spin chain with the conventional mean-field model of antiferromagnetic resonance. The resulting model fits to the known experimental results without additional tuning parameters.

The Magnetoelectric Effect in Ferroelectric/Ferromagnetic Film Hybrid Systems with Easy-Plane and Easy-Axis Anisotropy

The striction magnetoelectric effect in hybrid systems consisting of a magnetic film applied on the surface of a ferroelectric has been investigated. Films with planar (galfenol, nickel) and perpendicular (Co/Pt multilayer structures) magnetic anisotropy have been used. A PMN–PT crystal has served as ferroelectric. The hysteresis loops of magnetic films have been studied as a function of voltage applied to the ferroelectric crystal. A voltage applied to films with easy-plane anisotropy causes the anisotropy axis to turn in the plane of the sample. In structures with easy-axis anisotropy, the hysteresis shape changes, which, according to numerical simulation, may be associated with a change in interfacial Dzyaloshinsky–Moriya interaction at the Co/Pt interface.

Single Ion Anisotropy of CrIII and FeIII in a Series of {Ti7M} Rings

A series of heterometallic [TiIV7MIII] rings are reported which contain a single paramagnetic metal centre, either FeIII or CrIII. The structures contain an octagon of metals bridged by oxides within the ring and pivalate ligands outside the ring. The structures encapsulate a single cation, either a secondary ammonium cation or a cesium. Multi-frequency EPR spectroscopy was used to measure the zero-field splitting parameters for the paramagnetic metal sites. The FeIII sites have rhombic anisotropy, with D around − 0.18 cm−1 and λ (= D/E) approximately 0.25. The CrIII have a more axial anisotropy, with D around − 0.5 cm−1 and λ around 0.05. Attempts to find a magneto-structural correlation are vitiated by the difficulty of refining the challenging crystal structures.

On the magnetocaloric effect in single crystals

In this paper, we theoretically discuss the magnetocaloric effect in single crystals whose magnetization is mainly due to localized electrons. To this end, we use a model of local magnetic moments with anisotropy in the exchange interaction parameters and a term to consider an axial anisotropy. In order to correctly calculate the magnetocaloric quantities, are also included in the model terms to describe the non interacting electron gas and the crystalline lattice vibrations. One of the important findings of this work is the existence of rotating magnetocaloric quantities (ΔSisorot) and (ΔTadrot) larger than the conventional magnetocaloric counterparts (ΔSiso) and (ΔTad) in temperature range around the critical temperature.

Spin reorientation transition in an ultrathin Fe film on W(110) induced by Dzyaloshinsky-Moriya interactions

Controlling the preferred direction of the magnetic moments is essential for the design of spintronic devices based on ultrathin films and heterostructures. As the film thickness or the temperature is increased, the easy anisotropy axis is typically reoriented from an out-of-plane direction preferred by surface and interface energy contributions to an in-plane alignment favored by the volume anisotropy terms. We study the temperature-driven spin reorientation transition in two atomic layers of Fe on W(110) using well-tempered metadynamics simulations based on a spin model parametrized by ab initio calculations and find that the transition only takes place in the presence of the Dzyaloshinsky--Moriya interaction (DMI). This demonstrates that the chiral DMI does not only differentiate between noncollinear spin structures of different rotational senses, but it also influences the magnetic orientation of collinear magnetic configurations.