Magnetic Subsystem Research Articles

Entropy and magnetocaloric effect in ferrimagnets RCo2

The equations of state for the magnetic and elastic subsystems of a ferrimagnet are obtained in terms of the exchange–striction model. In the formulas derived for magnetic entropy, it is represented as the sum of the contributions of two magnetic sublattices of the ferrimagnet. One of the main characteristics of the magnetocaloric effect, viz., isothermal variation ΔS iso of the entropy in a magnetic field, is calculated for compounds RCo2 (R = Er, Ho, Dy) that experience a first-order magnetic phase transition and TbCo2 that experiences a second-order magnetic phase transition. It is shown that the calculated values of ΔS iso for these compounds are in satisfactory quantitative agreement with experimental results. The change in the entropy of a ferrimagnet in a strong magnetic field upon a transition from the ferrimagnetic to ferromagnetic ordering is calculated. The peculiarities in the magnetic field dependences of the magnetic entropy of a two-sublattice ferrimagnet are analyzed.

Magnetoelectric write and read operations in a stress-mediated multiferroic memory cell

Magnetic memory cells associated with the stress-mediated magnetoelectric effect promise extremely low bit-writing energies. Most investigations have focused on the process of writing information in memory cells, and very few on readout schemes. The usual assumption is that the readout will be achieved using magnetoresistive structures such as Giant Magneto-Resistive stacks or Magnetic Tunnel Junctions. Since the writing energy is very low in the magnetoelectric systems, the readout energy using magnetoresistive approaches becomes non negligible. Incidentally, the magneto-electric interaction itself contains the potentiality of the readout of the information encoded in the magnetic subsystem. In this letter, the principle of magnetoelectric readout of the information by an electric field in a composite multiferroic heterostructure is considered theoretically and demonstrated experimentally using [N×(TbCo2/FeCo)]/[Pb(Mg1/3Nb2/3)O3](1−x)−[PbTiO3]x stress-mediated ME heterostructures.

Features of the intensity behavior of Kramers doublet components in NdFe3(BO3)4in the transverse Zeeman geometry

Nontrivial magnetic field dependence of the polarization of the Kramers doublet absorption lines (15 971–15 978 cm−1) was observed in the range of the 4I9/2 → 2H11/2 optical transition of Nd3+ ion in an easy-plane antiferromagnet NdFe3(BO3)4 in the transversal Zeeman effect geometry. Values of the transversal g factor and the effective exchange field with Fe3+ magnetic subsystem were determined for the excited state of Nd3+ from the field dependence of the doublet splitting. A semiempirical equation was proposed for the description of the field dependence of the intensities of the polarized components of the absorption lines corresponding to the Kramers doublet 15 971–15 978 cm−1. Besides the scaling factors, the equation contains only the independently determined magnitudes of exchange fields for the ground and excited states as parameters.

Numerical simulation of the lattice properties of Fe1–xCoxSi – strongly correlated electron systems

Within the framework of a thermodynamic model, the phonon anharmonicity associated with both acoustic and optical vibrations of the crystal lattice is investigated. On this basis we have performed self‐consistent simulations of thermal and elastic properties of FeSi and CoSi compounds and their solid solutions Fe1–xCoxSi (x = 0.1, 0.3, 0.5), which are promising spintronic materials. The non‐lattice contributions to the heat capacity and coefficient of thermal expansion of the considered systems have been established. It is shown that the Invar effect observed in Fe0.7Co0.3Si and Fe0.5Co0.5Si is associated with anomalies of their electronic and magnetic subsystems.

Magnetoelectric and electric measurements of the (1−x)BiFeO3–(x)Pb(Fe1/2Nb1/2)O3 solid solutions

In the paper ferro–electro–magnetic (1-x)BiFeO3−(x)Pb(Fe1/2Nb1/2)O3 (BF-PFN) solid solutions were obtained (containing the percentage BF/PFN: 60/40 and 70/30). Individual components of the solid solution were prepared by follows methods: synthesizing a powder BF was performed by calcining the simple oxides (Bi2O3, Fe2O3), and synthesizing a powder PFN was carried out by calcining a mixture of complex oxides (FeNbO4, PbO). Compaction of synthesized and mixed BiFeO3, PbFe1/2Nb1/2O3 powders was carried out by free sintering methods.X–ray, microstructure, dielectric, magnetic and magnetoelectric studies, DC electrical conductivity and electrical hysteresis loop were carried out. Magnetoelectric effect measurements performed at room temperature showed coupling between electric and magnetic subsystem of the BF–PFN solid solutions.

Magnon-polaron transport in magnetic insulators

We theoretically study the effects of strong magnetoelastic coupling on the transport properties of magnetic insulators. We develop a Boltzmann transport theory for the mixed magnon-phonon modes (magnon polarons) and determine transport coefficients and spin diffusion length. Magnon-polaron formation causes anomalous features in the magnetic field and temperature dependence of the spin Seebeck effect when the disorder scattering in the magnetic and elastic subsystems is sufficiently different. Experimental data by Kikkawa et al. [PRL 117, 207203 (2016)] on yttrium iron garnet films can be explained by an acoustic quality that is much better than the magnetic quality of the material. We predict similar anomalous features in the spin and heat conductivity and non-local spin transport experiments.

Ludwigites: From natural mineral to modern solid solutions

Ludwigites is a type of oxyborates compounds with BO3 groups, which have low‐dimensional elements: stairs, zigzag walls, tapes. Such structural features influence on the physical properties of the compounds and cause charge ordering, the existence of two magnetic subsystems, ordered at different temperatures in the perpendicular direction, the quasi‐one‐dimensional magnetism, spin‐glass state. In this survey, we have summarized all known experimental and theoretical studies and analyzed the proposed by the authors mechanisms of phase transitions in the ludwigite structure oxyborates.

Multiferroic properties and structural features of M-type Al-substituted barium hexaferrites

Precise studies of the crystal and magnetic structures of M-type substituted barium hexaferrites BaFe12–x Al x O19 (0.1 ≤ x ≤ 1.2) have been performed by powder neutron diffraction in the temperature range 300–730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in M-type substituted barium hexaferrites BaFe12–x Al x O19 (x ≤ 1.2) with a collinear ferromagnetic structure.

Self-assembly of nanostructures with multiferroic components using nucleic acid linkers

Abstract

Crossing and anti-crossing effects of polaritons in a magnetic-semiconductor superlattice influenced by an external magnetic field

Crossing and anti-crossing effects in dispersion characteristics of both bulk and surface polaritons in a magnetic-semiconductor superlattice influenced by an external static magnetic field being in the Faraday geometry are discussed. The bulk polaritons are classified as eigenwaves with right-handed and left-handed elliptically polarized states, whereas the surface polaritons are considered as hybrid modes having a predominant effect of either magnetic or semiconductor subsystem, and distinctions in dispersion characteristics of such polaritons are revealed involving the concept of critical points.

Strong corelation between magnetic and electrical subsystems in diamagnetically substituted hexaferrites ceramics

The BaFe11.9Al0.1O19 M-type hexagonal barium ferrite was obtained by a standard ceramic method (powder synthesis in a solid state reaction method). at 4.2, 50, 100, 200, 300, 400, 630 and 730K via neutron powder diffraction (NPD) on a high-resolution Fourier diffractometer. An anomal microstructural transition on atomic level has been observed in BaFe11.9Al0.1O19 due to transition from a centrosymmetric to a non-centrosymmetric Space Group (SG). The NPD data were successfully refined for both space groups (SG): a centrosymmetric SG #194 (non-polar phase) and a non-centrosymmetric SG #186 (polar phase). The Goodness-offit quality factor (χ2) demonstrated good agreement between experimental data and theoretical model for both SG #194 and for #186. Goodness-offit quality factor (χ2) was better at low temperatures (4.2–150K) for SG #186, while at higher temperatures (630–730K) χ2 was better for SG #194. This might be because of the two different phases in BaFe11.9Al0.1O19 and there are mutual phase transitions at the atomic level. Dual ferroic properties (coexistence of magnetization and polarization) were demonstrated at room temperature in BaFe11.9Al0.1O19. Strong correlation between magnetic and electrical subsystems (magnetoelectrical effect – ME) has been shown. Temperatures dependences of the polarization, magnetization and ME effect were studied.

Role of Fe magnetic subsystems to form a magnetic spin glass state in RFeTi2O7

The experimental studies on R3+Fe3+Ti2O7 (R=Sm, Gd, Tb, Tm, Dy) magnetic properties evidence the low temperature spin glass state in all compounds. The possibility of rare-earth cation substitution allows the investigation of the role of magnetic iron Fe3+ ions and rare earth R3+ ions subsystems in a ground state formation in these oxide compounds.

Synthesis, thermal and magnetic properties of RE-diborides

Techniques of synthesis of RE diborides (RB2) are developed (R=Tb, Dy, Ho, Er, Lu). Temperature dependence of magnetisation, a heat capacity, a lattice parameters of diborides in the range of 2–300K are measured. According to joint calorimetric and X-ray research the analysis of a phonon component of a heat capacity and thermal expansion of RE-diborides is carried out by Debye-Einstein's models, the parameters of the model are determined.The change of magnetisation of the ferromagnetic RB2 compounds with growth of temperature caused by violation of ordering in the system of the atomic magnetic moments is compared with the change of entropy of a magnetic subsystem calculated from calorimetric data. Analytical expansion for calculation of a magnetic component of a heat capacity by RB2 magnetisation data at the temperatures of 2–300K is obtained.

Specific features of the states of cobalt fluoride in the vicinity of the critical field

The state of cobalt fluoride in the vicinity of the critical value H c of a longitudinal magnetic field H, in which the magnetic subsystem of a CoF2 crystal with a strong Dzyaloshinskii interaction is transformed from the antiferromagnetic phase into the canted phase, has been investigated taking into account the increasing number of experimental studies related to the use of cobalt fluoride. It has been found that, despite the unusually high magnetic anisotropy of the crystal, the state of the magnetic subsystem at H = H c is extremely sensitive to a small deviation of the vector H from the C 4 axis. Another feature is that the high sensitivity disappears with an increase or decrease in the magnetic field by only a few thousandths of H c . The results of the investigations performed in this work are applicable to magnetically ordered crystals FeF3 and Cu2OSeO3, which, as well as the CoF2 crystals, are characterized by a strong Dzyaloshinskii interaction and a significant magnetic anisotropy. The revealed anomaly in the reduction of the effective magnetic anisotropy is of interest in connection with numerous attempts to decrease the magnetic anisotropy in crystals with giant magnetostriction, which are necessary for the use as sensors and vibrators.

Self-consistent model of a solid for the description of lattice and magnetic properties

In the paper a self-consistent theoretical description of the lattice and magnetic properties of a model system with magnetoelastic interaction is presented. The dependence of magnetic exchange integrals on the distance between interacting spins is assumed, which couples the magnetic and the lattice subsystem. The framework is based on summation of the Gibbs free energies for the lattice subsystem and magnetic subsystem. On the basis of minimization principle for the Gibbs energy, a set of equations of state for the system is derived. These equations of state combine the parameters describing the elastic properties (relative volume deformation) and the magnetic properties (magnetization changes).The formalism is extensively illustrated with the numerical calculations performed for a system of ferromagnetically coupled spins S=1/2 localized at the sites of simple cubic lattice. In particular, the significant influence of the magnetic subsystem on the elastic properties is demonstrated. It manifests itself in significant modification of such quantities as the relative volume deformation, thermal expansion coefficient or isothermal compressibility, in particular, in the vicinity of the magnetic phase transition. On the other hand, the influence of lattice subsystem on the magnetic one is also evident. It takes, for example, the form of dependence of the critical (Curie) temperature and magnetization itself on the external pressure, which is thoroughly investigated.

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.

Lattice and magnetic dynamics of a quasi-one-dimensional chain antiferromagnet PbFeBO4

A group of recently synthesized orthorhombic Pnma crystals PbMBO4 (M = Cr, Mn, Fe) demonstrates a number of unusual structural and magnetic properties. We report on polarized Raman scattering study of the lattice and magnetic dynamics in single crystals of an antiferromagnet PbFeBO4 below and above K. Polarization properties of the observed magnetic excitations below TN as well as intense quasi-elastic scattering support the quasi-one-dimensional character of the magnetic structure of PbFeBO4. Frequency overlapping of magnetic excitations and low-frequency phonons in the range of 90–200 cm−1 leads to pronounced asymmetric anomalies thus confirming intrinsic coupling of magnetic and lattice subsystems. This conclusion is also supported by observation of anomalous temperature behaviour of higher frequency phonons in the vicinity of TN. Experimental investigations are supported by relevant magnetic symmetry analysis which allows us to explain previously observed anomalous results.

Optimal temperature range for determining magnetocaloric magnitudes from heat capacity

The determination of the magnetocaloric magnitudes (specific magnetic entropy change, ΔsM, and adiabatic temperature change, ΔTad) from heat capacity (cH) measurements requires measurements performed at very low temperatures (~0 K) or data extrapolation when the low temperature range is unavailable. In this work we analyze the influence on the calculated ΔsM and ΔTad of the usually employed linear extrapolation of cH from the initial measured temperature down to 0 K. Numerical simulations have been performed using the Brillouin equation of state, the Debye model and the Fermi electron statistics to reproduce the magnetic, lattice and electronic subsystems, respectively. It is demonstrated that it is not necessary to reach experimentally temperatures very close to 0 K due to the existence of certain starting temperatures of the experiments, the same for ΔsM and ΔTad, that minimize the error of the results. A procedure is proposed to obtain the experimental magnitudes of ΔsM and ΔTad with a minimum error from cH data limited in temperature. It has been successfully applied to a GdZn alloy and results are compared to those derived from magnetization measurements.

Magnetic-field induced orientational transition in a helicoidal liquid-crystalline antiferromagnet

The magnetic-field induced orientational transition in helicoidal liquid-crystalline antiferromagnets representing compensated suspensions of magnetic nanoparticles in cholesteric liquid crystals is theoretically studied. The untwisting of a helicoidal structure and the behavior of mean magnetization as a function of the field strength and material parameters are investigated. It is shown that the magnetic subsystems in the field-untwisted ferronematic phase are not completely compensated, and the ferronematic phase is ferrimagnetic.

Static and dynamic magnetic properties of two synthetic francisites Cu3La(SeO3)2O2X (X = Br and Cl)

The formation of long-range magnetic order at low temperatures was established in francisite—type compounds Cu3La(SeO3)2O2X (X = Br and Cl) through measurements of magnetic susceptibility, magnetization, specific heat and X-band electron spin resonance. The significantly enhanced critical index p = 1.0 ± 0.1 in Cu3La(SeO3)2Br and p = 0.8 ± 0.1 in Cu3La(SeO3)2Cl in the temperature dependence of the width of ESR signal evidence the reduced dimensionality of the kagome-type francisite’s magnetic subsystem. Under action of external magnetic field, the presumably non-collinear six-sublattices antiferromagnetic structure of these compounds experiences the first-order metamagnetic transformation. The B–T magnetic phase diagrams were established from the positions of singularities in temperature and field dependences of thermodynamic properties. Contrary to pristine mineral Cu3Bi(SeO3)2Cl, no signature of structural phase transition was detected.