Magnetic Subsystem Research Articles

Mössbauer study of non-stoichiometric FeCr2S4 system

A partial spin transition stimulated by spontaneous symmetry breaking has been observed, for the first time, in the non-stoichiometric chalcogenide spinel FeCr2S4. A Fe2+ high spin - low spin transition HS-LS: S = 2 → S = 0 is detected below the Néel temperature TN in the magnetic subsystem corresponding to a part of the sample with deviations from stoichiometry. The fraction of diamagnetic Fe atoms increases with decreasing temperature.

Peripheral nerve stimulation limits of a high amplitude and slew rate magnetic field gradient coil for neuroimaging.

To establish peripheral nerve stimulation (PNS) thresholds for an ultra-high performance magnetic field gradient subsystem (simultaneous 200-mT/m gradient amplitude and 500-T/m/s gradient slew rate; 1 MVA per axis [MAGNUS]) designed for neuroimaging with asymmetric transverse gradients and 42-cm inner diameter, and to determine PNS threshold dependencies on gender, age, patient positioning within the gradient subsystem, and anatomical landmarks. The MAGNUS head gradient was installed in a whole-body 3T scanner with a custom 16-rung bird-cage transmit/receive RF coil compatible with phased-array receiver brain coils. Twenty adult subjects (10 male, mean ± SD age = 40.4 ± 11.1 years) underwent the imaging and PNS study. The tests were repeated by displacing subject positions by 2-4 cm in the superior-inferior and anterior-posterior directions. The x-axis (left-right) yielded mostly facial stimulation, with mean ΔGmin = 111 ± 6 mT/m, chronaxie = 766 ± 76 µsec. The z-axis (superior-inferior) yielded mostly chest/shoulder stimulation (123 ± 7 mT/m, 620 ± 62 µsec). Y-axis (anterior-posterior) stimulation was negligible. X-axis and z-axis thresholds tended to increase with age, and there was negligible dependency with gender. Translation in the inferior and posterior directions tended to increase the x-axis and z-axis thresholds, respectively. Electric field simulations showed good agreement with the PNS results. Imaging at MAGNUS gradient performance with increased PNS threshold provided a 35% reduction in noise-to-diffusion contrast as compared with whole-body performance (80 mT/m gradient amplitude, 200 T/m/sec gradient slew rate). The PNS threshold of MAGNUS is significantly higher than that for whole-body gradients, which allows for diffusion gradients with short rise times (under 1 msec), important for interrogating brain microstructure length scales.

Electromagnetic worm-like locomotion system for in-pipe robots: novel design of magnetic subsystem

This paper describes a magnetic subsystem of an electromagnetic worm-like locomotion system (WLLS) with coupled electromagnets. WLLS simple-design consists of two elastically connected ring-like segments that form two magnetically coupled electromagnetic actuators. The actuators generate longitudinal and transverse displacements which lead to locomotion due to synchronized changes of inertial, normal pressure, and friction forces. In present paper, analysis of the magnetic circuit of the WLLS had been performed. From symmetry and general consideration of magnetic system effectiveness, a novel design of the coupled magnetic circuits had been developed. The novel WLLS has two-force component electromagnetic actuators that produce the transverse and longitudinal forces. The novel design of the electromagnetic WLLS is able to move an in-pipe robots and should have better specific pulling force.

Features of the Magnetic and Elastic Subsystem Dynamics in a Thin Ferrite Film upon the Conversion of Magnetostrictive Frequencies

Features of the nonlinear dynamics of magnetic and elastic subsystems are studied upon the generation of hypersonic oscillations excited by an alternating magnetic field oriented in the film plane.

Dynamics of Magnetization in Multilayer TbCo / FeCo Structures under the Influence of Femtosecond Optical Excitation

The need to study ultrafast processes in magnetism is due to the prospects for creating ultrafast magnetic recording and ultrafast spintronic devices. In order to excite the magnetic subsystem femtosecond optical pulses are used. The excitement is manifested as in spin precession. In metals, the material is heated first due to significant optical absorption, and significant Joule losses occur. The most important task is to search for materials in which spin processes are excited without heating. Obvious candidates are weakly absorbing materials, such as ferrite garnets. However, the range of such materials and the range of their functionality are limited.The purpose of this work is to study the dynamics of systems with nonthermal mechanisms of spin precession excitation. Such excitation is possible in ferromagnetic / antiferromagnetic heterostructures with exchange interaction, provided that the recombination time of photocarriers is shorter than the time of heat diffusion. Multilayer TbCo / FeCo structures of the near IR range were investigated for a femtosecond optical pulse. The spin dynamics are compared with the direction of the wave vector of the exciting pulse along and perpendicular to the axis of easy magnetization of the structures (“easy axis” and “hard axis” geometry, respectively). It is shown that in case of “easy axis” geometry the determinative mechanism is the thermal interaction. When the system is exposed to an excitation pulse, this mechanism leads to a decrease in the projection of magnetization on the direction of propagation of the test beam. In case of “hard axis” geometry, the magnetization turns to the magnetic field at the initial stage. Then it precesses and relaxes to an equilibrium angular orientation. Such dynamics indicate a rapid recovery of the uniaxial anisotropy field after laser irradiation. The presented results demonstrate an ultrafast change in the magnetic anisotropy induced during the fabrication of the heterostructure under study, which may be of interest for optical control of the orientation of the magnetization.

On the Effect of Magnetostatic Interaction on the Collective Motion of Vortex Domain Walls in a Pair of Nanostripes

The periodic motion of interacting vortex domain walls in a pair of nanostripes has been analytically and numerically investigated. A model consisting of two parallel nanostripes with the domain magnetization structure has been proposed, where domains are separated by vortex walls. The magnetic subsystems of the stripes magnetostatically interact, which causes the existence of normal magnetic vortex motion modes in the latter. Frequencies of the collective magnetization modes have been calculated using empirical expressions for the magnetic energy of interaction between vortex walls. It is shown that not any combinations of the polarity and chirality lead to the resonance magnetization behavior in ac fields.

Crystal-field levels of Nd3+ in a new langasite compound Nd3CrGe3Be2O14

We report on the crystal-field (CF) levels of Nd3+ in Nd3CrGe3Be2O14, a representative of recently synthesized Be-containing langasites. They are the first in the family to contain two magnetic subsystems, 1D chromium and Kagome-like rare-earth ones. High-resolution broad-band temperature-dependent spectra allow us to find all five CF Kramers doublets of the ground 4I9/2 CF multiplet and 38 levels of the excited 4I11/2, 13/2, 15/2, 4F3/2, 4F5/2 + 2H9/2, 4F7/2 + 4S3/2 and 4F9/2 CF multiplets. These data form a basis for a future exploration of this interesting two-sublattice magnet with frustrated interactions. Optical transitions between Kramers doublets of Nd3+ indicate magnetic activity of 1D-chromium chains at T < 20 K.

Orbital Ordering Effect on Magnetic Resonance Spectra in Charge-Ordered and Frustrated Manganites

A theoretical study of the interrelation of the crystal structure, charge, orbital, and magnetic subsystems in R1 – xAxMnO3 charge-ordered manganites has been carried out (where R3+ is the rare earth ion, A2+ is the alkaline earth metal ion, x = 0.5, 2/3). The model of orbital-dependent exchange interactions and single-ion anisotropy is used. The presence of quasi-low-dimensional magnetic structures is exhibited. The spin waves spectra and antiferromagnetic resonance spectra are calculated.

Multiplicative Frequency Conversion in a Scheme of Magnetostrictive Transducer. Frequency Multiplication in the Relaxation Regime

Stimulated oscillations of magnetization and elastic displacement in a normally magnetized ferrite plate with magnetoelastic properties are considered. Frequency multiplication at the ratio determined by the multiplicity of the resonance frequencies of the magnetic and elastic subsystems is analyzed. Time and amplitude characteristics of the effect are studied using a model of the impact excitation of the elastic system due to short-term effect of the magnetic system. Frequency division in the same scheme of the magnetostrictive transducer is employed for comparison.

Attitude Control of a 2U Cubesat by Magnetic and Air Drag Torques

This paper describes the development of a magnetic attitude control subsystem for a 2U cubesat. Due to the presence of gravity-gradient torques, the satellite dynamics are open-loop unstable near the desired pointing configuration. Nevertheless, the linearized time-varying system is completely controllable, under easily verifiable conditions, and the system’s disturbance rejection capabilities can be enhanced by adding air drag panels exemplifying a beneficial interplay between hardware design and control. In this paper, conditions for the complete controllability for the case of a magnetically controlled satellite with passive air drag panels are developed, and simulation case studies with the linear quadratic regulator and model predictive control designs applied in combination with a nonlinear time-varying input transformation are presented to demonstrate the ability of the closed-loop system to satisfy mission objectives despite disturbance torques.

An Influence of Homogeneity of Magnetic Field on Stability of a Rectangular Plate

The main objective of presented work is a rectangular plate subjected to dynamic in-plane load generated by magnetic field. The plate is made of polyethylene (PE). There are two pockets on each of the two opposite edges of the plate. These pockets with porous structure are filled in with ferrofluid. The coil system consists of two magnetic field coil subsystems. These systems are built of Helmholtz (MC) and Golay coils (GC) and generate nonhomogeneous magnetic field. If the magnetic field is more homogeneous, the compression load is induced. In other cases, local tensile load occurs (compression load dominates). For presented coil systems, the intensity of load was examined for two variants. For the first of them, the intensity of load was dependent on the radius of Golay coil arcs. The change of the radius of saddle coils also influences on the strength of the gradient of the magnetic field. The second one describes the intensity of load which depends on the change of GC radius without changing the strength of magnetic field (the strength of magnetic field is compensated with changing the current flowing through the coils wires or with changing the number of wires). In this paper, the analytical model of the plate is presented. The model of the plate was formulated with the use of classical Kirchhoff–Love hypothesis. Elastic strain energy, kinetic energy as well as work of load were formulated. The equation of motion was derived based on the Hamilton’s principle. The numerical studies were related to the analysis of the intensity of load distribution.

Converse magnetoelectric effects in composites of liquid phase epitaxy grown nickel zinc ferrite films and lead zirconate titanate: Studies on the influence of ferrite film parameters

The interactions between electric and magnetic subsystems in a ferroelectric-ferromagnetic composite occur through mechanical forces. Here we discuss results of a systematic investigation on the strength of the magnetic response of the composite to an applied electric field, known as the converse magnetoelectric (CME) effect, and its dependence on the ferroic order parameters and volume fraction for the two phases. Studies were carried out on composites of lead zirconate titanate and 2--30-\ensuremath{\mu}m-thick nickel zinc ferrite (NZFO) films grown by liquid phase epitaxy on lattice matched (100) and (111) MgO substrates. Ferromagnetic resonance was utilized to determine the strength of CME from data on electric field E induced shift in the resonance frequency and its dependence on ferrite film orientation and thickness as well as MgO substrate thickness. The CME coupling coefficient A was found to be a factor of 2 to 4 higher in samples with NZFO films with (100) orientation than for (111) films. A decrease in A was measured with increasing ferrite film thickness and a very significant enhancement in the strength of CME was measured for decreasing MgO thickness. A model for CME that takes into consideration the influence of nonferroic MgO substrate was developed, and estimated A values are in very good agreement with the data. The results presented here are also of importance for a new class of electric field tunable ferrite microwave devices.

Self-induced magnetic flux structure in the magnetic superconductor RbEuFe4As4

We report an unusual enhancement of the magnetic induction in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ , highlighting the interplay between superconducting and magnetic subsystems in this material. Contrary to the conventional Meissner expulsion of magnetic flux below the superconducting transition temperature, we observe a substantial boost of the magnetic flux density upon approaching the magnetic transition temperature, Tm. Direct imaging of the flux evolution with a magneto-optical technique, shows that the magnetic subsystem serves as an internal magnetic flux pump, drawing Abrikosov vortices from the surface, while the superconducting subsystem controls their conveyance into the bulk of the magnetic superconductor via a peculiar self-organized critical state.

Highly tunable magnetic spirals and electric polarization in Gd0.5Dy0.5MnO3

Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic ac-plane, which is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e. a rotation as well as a strong enhancement of the polarization depending on the magnetic field direction. We show that this unusual behavior originates from the coupling between the transition metal and rare-earth magnetic subsystems.

Formation of the magnetic subsystems in antiferromagnetic NiO nanoparticles using the data of magnetic measurements in fields up to 250 kOe

Abstract It is well-known that the fraction of surface atoms and the number of defects in an antiferromagnetic particle increase with a decrease in the particle size to tens of nanometers, which qualitatively changes the properties of the particle. Specifically, in antiferromagnetic nanoparticles, spins in the ferromagnetically ordered planes can partially decompensate; as a result, an antiferromagnetic particle acquires a magnetic moment. As a rule, uncompensated chemical bonds of the surface atoms significantly weaken the exchange coupling with the antiferromagnetic particle core, which can lead to the formation of an additional magnetic subsystem paramagnetic at high temperatures and spin-glass-like in the low-temperature region. The existence of several magnetic subsystems makes it difficult to interpret the magnetic properties of antiferromagnetic nanoparticles. It is shown by the example of NiO nanoparticles with an average size of 8 nm that the correct determination of the contributions of the magnetic subsystems forming in antiferromagnetic nanoparticles requires magnetic measurements in much stronger external magnetic fields than those commonly used in standard experiments (up to 60–90 kOe). An analysis of the magnetization curves obtained in pulsed magnetic fields up to 250 kOe allows one to establish the contributions of the uncompensated particle magnetic moment μun, paramagnetic subsystem, and antiferromagnetic particle core. The μun value obtained for the investigated NiO particles is consistent with the Neel model, in which μun ∼ N1/2 (N is the number of magnetically active atoms in a particle), and thereby points out the existence of defects on the surface and in the bulk of a particle. It is demonstrated that the anomalous behavior of the high-field susceptibility dM/dH of antiferromagnetic NiO nanoparticles, which was observed by many authors, is caused by the existence of a paramagnetic subsystem, rather than by the superantiferromagnetism effect.

Motion of Coupled Magnetic Vortices in Parallel Nanostripes

The periodic motion of the interacting vortex domain walls in a pair of nanostripes has been theoretically investigated. As a model, two parallel nanostripes with magnetization inhomogeneities in the form of magnetic vortices have been examined. The magnetic subsystems of the stripes are magnetostatically coupled. The quasi-elastic coupling between vortices ensures the existence of normal modes of the periodic magnetization motion. The frequencies of these modes have been calculated. It is shown that not any combination of the vortex topological charge leads to the resonant behavior of magnetization in ac fields. The effect of the static component of a magnetic field on the frequency of the periodic motion of vortex domain walls is discussed.

Structural, magnetic, and thermal properties of Ce1−xEuxCrO3 orthochromite solid solutions

The interplay of slight changes of the rare-earth composition and antiferromagnetic ordering of Cr in a series of high purity solid solutions of orthochromites with composition ${\mathrm{Ce}}_{1\ensuremath{-}x}\mathrm{Eu}{}_{x}{\mathrm{CrO}}_{3}\phantom{\rule{4pt}{0ex}}(1\ensuremath{\leqq}x\ensuremath{\leqq}0)$ was investigated by neutron and x-ray powder diffraction, magnetization, and heat capacity measurements. A careful study of the crystal structure by x-ray powder diffraction data across the whole range of $x$ enables us to correlate the magnetic properties of the Cr magnetic subsystem with the size of the lattice and minute changes of the bond and torsion angles within and between the ${\mathrm{CrO}}_{6}$ octahedra. We find that the sizes and the shapes of the ${\mathrm{CrO}}_{6}$ octahedra remain essentially unchanged as the size of the rare-earth cations and the cell volumes are gradually reduced by Eu substitution, whereas decreasing Cr-O-Cr bond angles and increasing inclination of neighboring octahedra compensate for the decreasing lattice size. Adverse to external hydrostatic pressure that reduces the cell volume but raises the ordering temperature, lowering the cell volume by replacing Ce by Eu decreases the N\'eel temperature linearly with the Eu concentration from 260 K for ${\mathrm{CeCrO}}_{3}$ to 178 K for ${\mathrm{EuCrO}}_{3}$.

Magnetic and transport properties of Mn2FeAl

Experimental studies of polycrystalline Mn2FeAl revealed that it crystallizes in a primitive cubic structure of β-Mn type (space group P4132) and is an antiferromagnet with Néel temperature TN = 40 K. The alloy exhibits temperature dependence of transport properties typical for semiconductors and has abnormally high electrical resistivity (ρ ∼ 238 μΩ·cm at room temperature). High Curie-Weiss temperature ΘCW ∼ –905 K and a large value of the frustration parameter ƒ ≈ 23 points to a strong frustration of the magnetic subsystem. These experimental findings are in contrast with reported results of first-principles calculations which predicted Mn2FeAl to be half metallic ferrimagnet crystallizing in the Heusler crystal structure.

Magnetization-induced shape transformations in flexible ferromagnetic rings

Flexible ferromagnetic rings are spin-chain magnets, in which the magnetic and mechanical subsystems are coupled. The coupling is achieved through the tangentially oriented anisotropy axis. The possibility to operate the mechanics of the nanomagnets by controlling their magnetization is an important issue for the nanorobotics applications. A minimal model for the deformable curved anisotropic Heisenberg ferromagnetic wire is proposed. An equilibrium phase diagram is constructed for the closed loop geometry: (i) A vortex state with vanishing total magnetic moment is typical for relatively large systems; in this case the wire has the form of a regular circle. (ii) A topologically trivial onion state with the planar magnetization distribution is realized in small enough systems; magnetic loop is elliptically deformed. By varying geometrical and elastic parameters a phase transition between the vortex and onion states takes place. The detailed analytical description of the phase diagram is well confirmed by numerical simulations.

Влияние орбитального упорядочения на спектры магнитного резонанса в зарядово-упорядоченных и фрустрированных манганитах

A theoretical study of the interrelation of the crystal structure, charge, orbital, and magnetic subsystems in R1–xAxMnO3 charge-ordered manganites has been carried out (where R3+ is the rare earth ion, A2+ is the alkaline earth metal ion, x = 0.5, 2/3). The model of orbital-dependent exchange interactions and single-ion anisotropy is used. The presence of quasi-low-dimensional magnetic structures is exhibited. The spin waves spectra and antiferromagnetic resonance spectra are calculated.