Spontaneous Magnetization Reversal Research Articles

Energy Barriers for the Spontaneous Magnetization Reversal of the Atomic Co Chains on Pt(664) Surface in the Model with Dzyaloshinskii–Moriya Interaction

The analytical approach has been developed in the framework of the continuous XY-model. This approach allows calculating the spontaneous magnetization reversal time of finite-length atomic chains on the metallic surface. The interaction of the magnetic moments of atoms is described by the classical Hamiltonian, which includes the Heisenberg exchange interaction, the Dzyaloshinskii–Moriya interaction, and the magnetic anisotropy energy. Using the Co/Pt(664) system as an example, it has been shown that the proposed method is in a good agreement with the results of the numerical simulation in the limit of short and long atomic chains. And for atomic chains of intermediate length, it can be used to estimate an upper bound on the spontaneous magnetization reversal time. We obtained the dependences of the spontaneous magnetization reversal time of finite-length Co chains the value of the exchange integral, parameters of the magnetic anisotropy, and also on the value of the projection of the Dzyaloshinskii vector onto the axis perpendicular to the plane containing the magnetic moments of the atoms. It is shown that the proposed method has a wide range of applicability both in terms of temperature and the values of the physical parameters characterizing the magnetic properties of the atomic chains.

Energy Barriers for the Spontaneous Magnetization Reversal of Atomic Co Chains on the Surface Pt(664) in the Model of Dzyaloshinskii–Moriya Interaction

Energy Barriers for the Spontaneous Magnetization Reversal of Atomic Co Chains on the Surface Pt(664) in the Model of Dzyaloshinskii–Moriya Interaction

Influence of Dzyaloshinskii–Moriya and Dipole–Dipole Interactions on Spontaneous Magnetization Reversal Time of Finite-Length Co Chains on Pt(664) Surfaces

The spontaneous magnetization reversal of the finite-length Co chains on Pt(664) surface is investigated in the framework of the classical effective theory. The effective theory includes the Heisenberg exchange interaction, the magnetic anisotropy energy, the Dzyaloshinskii-Moriya interaction, and the dipole-dipole interaction. The geodesic nudged elastic band method is employed for calculations of the energy barriers for magnetization reversal of the finite-length Co chains. The calculation of the spontaneous magnetization reversal time shows that the dipole-dipole interaction can be neglected at the temperatures higher than 10.9 K. DMI can be neglected at the temperatures higher than 60.2 K. This means that Dzyaloshinskii-Moriya interaction can significantly influence on the magnetization reversal process at low temperatures and should be taken into account.

Behavior of thermally quenched topological defects in quasicrystal artificial spin ices

Understanding the formation of topological defects in connected magnetic nanowires such as in artificial spin ices (ASIs) is critical to controlling their behavior. Moreover, emergent frustration behavior of these defects can be expected due to complex underlying lattice geometry of the ASIs. We have explored in situ thermalization and quenching of quasicrystalline ASIs, which exhibit additional frustration due to aperiodicity from near the Curie temperature, and we observe the formation of two sets of topological vortex defects: within the magnetic bars and within the vertices at which the bars meet. By varying the laser fluence, we show that the number of defects in the vertices varies as the quenching rate changes following a power law relation, which we interpret as being related to the ferromagnetic-to-paramagnetic phase transition. Conversely, the defects formed within the bars are metastable states related to the spontaneous magnetization reversal of the magnetic bars and are therefore strongly dependent on underlying lattice geometry. These results provide insight into the emergence and control of topological defects in confined frustrated magnetic systems.

Spontaneous magnetization reversal and positive exchange bias effect in CoFeSiB/FeCrO3 heterostructures

The exchange bias (EB) phenomenon is an interface-driven effect; it depends on the interface spin configuration, which in turn is immensely related to antiferromagnetic spin configuration. Here, we chose an antiferromagnet, FeCrO3, with a corundum structure with randomly occupied cations. We report EB in the bilayer heterostructures that are composed of an amorphous CoFeSiB ferromagnet interfaced with an epitaxial FeCrO3 layer fabricated in-situ by pulsed laser deposition. The bilayers exhibit crossover from conventional negative EB to positive EB from room temperature down to 5 K. The crossover occurs for a cooling field that exceeds the interfacial coupling strength at the interface. Systems that show positive EB effects sometimes also display spontaneous magnetization reversal; the present bilayers showed spontaneous EB below 50 K. Temperature and field-dependent magnetisation studies reveal that the prepared heterostructure consists of uncompensated interface spins due to the disorder in the FeCrO3 layer that gets frozen at low temperatures, pins the spins in the ferromagnetic layer and causes the spontaneous EB effect.

First detection of low field microwave absorption in the disordered multiferroic double perovskite BiFe0.5Mn0.5O3

BiFe0.5Mn0.5O3 (BFMO) is an intriguing magnetic double perovskite, only obtainable through high pressure-high temperature synthesis. It shows bulk multiferroic properties, namely the coexistence between a spin canted antiferromagnetic structure superimposed to an externally induced electric polarization at least from 77 K. In particular, the system is characterized by a significant weak ferromagnetic hysteresis loop and by a very rare phenomenon: the spontaneous magnetization reversal (MRV) versus temperature in the low field regime. To clarify the BFMO exotic magnetic phase in the low field regime, the Electron Spin Resonance (ESR) and the low field microwave absorption (LFMA) techniques were used, providing the first observation of LFMA in the bulk BFMO as an additional functionality of this material. A striking feature is that the hysteresis in LFMA signals vanishes above 45 K, while the bulk M-H loop hysteresis, measured in the same field range of LFMA, persists till room temperature. The temperature at which LFMA hysteresis vanishes qualitatively matches the position of the magnetic susceptibility’s second derivative peak, corresponding to the temperature at which the local second order mechanism responsible for MRV is maximum. The line shape of LFMA completely changes above 45 K and the ESR linewidth starts decreasing above this temperature, indicating the role of defect/disorder induced inhomogeneity. The temperature evolution of LFMA hysteresis and line shapes as a measure of the competition between Fe- and Mn-rich clusters suggests a sort of local frustration at the microscopic scale, responsible for the peculiar magnetization reversal of this system.

Thermal and Magnetic-Field Stability of Holmium Single-Atom Magnets.

We use spin-polarized scanning tunneling microscopy to demonstrate that Ho atoms on magnesium oxide exhibit a coercive field of more than 8T and magnetic bistability for many minutes, both at 35K. The first spontaneous magnetization reversal events are recorded at 45K, for which the metastable state relaxes in an external field of 8T. The transverse magnetic anisotropy energy is estimated from magnetic field and bias voltage dependent switching rates at 4.3K. Our measurements constrain the possible ground state of Ho single-atom magnets to either J_{z}=7 or 8, both compatible with magnetic bistability at fields larger than 10mT.

Spontaneous and Induced Magnetization Reversal in Thin GaMnSb Films

The time, temperature, and magnetic field dependences of the magnetic moment of thin GaMnSb films containing MnSb clusters are measured using a SQUID (superconducting quantum interference device) magnetometer. It is found that magnetic field-induced magnetization reversal and thermally activated spontaneous magnetization reversal in thin GaMnSb films are interrelated as follows: the maximum of the magnetic-field dependence of the magnetic viscosity coincides with the coercive force for the samples.

An estimate for the magnetization reversal time of antiferromagnetic chains within the Heisenberg model

Within the Heisenberg model in the presence of uniaxial magnetic anisotropy, formulas are obtained that allow one to estimate both the spontaneous magnetization reversal time of an antiferromagnetic chain and the magnetization reversal time due to interaction with the conducting tip of a scanning tunneling microscope (STM). Corrections due to a possible difference between the properties of the end and inner atoms of the chain are calculated. Numerical estimates are obtained for typical parameter values of the Heisenberg Hamiltonian.

Spontaneous magnetization reversal caused by magnetic fluctuation in GaMnSb thin films

Time dependences of the magnetic moment m(t) in GaMnSb thin films with MnSb clusters are measured. The m(t) dependences are straightened in semi-logarithmic coordinates m(lnt). The field dependences of magnetic viscosity S(H) corresponding to the slope of straight lines m(lnt) are studied. It is demonstrated that the behaviour of dependences S(H) is governed by the magnetic moment fluctuations of MnSb clusters with lognormal distribution of the magnetic anisotropy energy.

Effect of the magnetic anisotropy energy distribution of MnSb clusters on spontaneous magnetization reversal of GaMnSb thin films

Temperature m(T) and time m(t) dependences of the magnetic moment of GaMnSb thin films with MnSb clusters have been measured. The m(t) dependences are straightened in semilogarithmic coordinates m(lnt). The temperature dependences of magnetic viscosity S(T) corresponding to the slope of straight lines m(lnt) have been studied. It have been demonstrated that the behavior of dependences S(T) is governed by the lognormal distribution of the magnetic anisotropy energy of MnSb clusters. It have been found that the behavior of dependences m(T) measured after the films were cooled in zero magnetic field and in magnetic field H = 10 kOe is also governed by the lognormal distribution of the magnetic anisotropy energy of MnSb clusters.

Magnetic noise as the cause of the spontaneous magnetization reversal of RE–TM–B permanent magnets

The relation between the macroscopic spontaneous magnetization reversal (magnetic viscosity) of (NdDySm)(FeCo)B alloys and the spectral characteristics of magnetic noise, which is caused by the random microscopic processes of thermally activated domain wall motion in a potential landscape with uniformly distributed potential barrier heights, is found.

Engineering and Scaling the Spontaneous Magnetization Reversal of Faraday Induced Magnetic Relaxation in Nano-Sized Amorphous Ni Coated on Crystalline Au.

We report on the generation of large inverse remanent magnetizations in nano-sized core/shell structure of Au/Ni by turning off the applied magnetic field. The remanent magnetization is very sensitive to the field reduction rate as well as to the thermal and field processes before the switching off of the magnetic field. Spontaneous reversal in direction and increase in magnitude of the remanent magnetization in subsequent relaxations over time were found. All of the various types of temporal relaxation curves of the remanent magnetizations are successfully scaled by a stretched exponential decay profile, characterized by two pairs of relaxation times and dynamic exponents. The relaxation time is used to describe the reduction rate, while the dynamic exponent describes the dynamical slowing down of the relaxation through time evolution. The key to these effects is to have the induced eddy current running beneath the amorphous Ni shells through Faraday induction.

Crystal growth and spin reorientation transition in Sm0.4Er0.6FeO3 orthoferrite

Abstract High quality Sm0.4Er0.6FeO3 single crystal has been successfully grown by the floating zone method. Temperature dependence of the magnetizations of Sm0.4Er0.6FeO3 under ZFC process is studied in the temperature range of 4–300 K. Spin reorientation transition between Γ2 (Gz, Fx) and Γ4 (Gx, Fz) is observed in the temperature range of 170–210 K, which is significantly lower than that of SmFeO3, while much higher than that of ErFeO3. A compensation point (35.8 K) corresponding to zero magnetization and a spontaneous magnetization reversal transition at 49.5 K are observed in Sm0.4Er0.6FeO3. Temperature-induced SRT of Sm0.4Er0.6FeO3 is systematically studied by THz-TDS range from 40 K to 300 K. The temperature dependence of amplitude of AFM mode coincides well with the magnetization measurement which demonstrate that SRT in Sm0.4Er0.6FeO3 can be studied through the amplitude of AFM mode FID emission. The AF mode frequency is almost invariant, while the F mode frequency significantly decreases with increasing temperature, which could be explained by the temperature dependence of anisotropy energy.

Field effects on spontaneous magnetization reversal of bulk BiFe0.5Mn0.5O3, an effective strategy for the study of magnetic disordered systems

We report a comprehensive study of the spontaneous magnetization reversal (MRV) performed on the disordered polycrystalline perovskite BiFe0.5Mn0.5O3, an intriguing compound synthesized in high pressure–high temperature conditions. In disordered systems, the origin of MRV is not completely clarified, yet. In BiFe0.5Mn0.5O3, compositional disorder involves the ions on the B-site of the perovskite determining the presence of mesoscopic clusters, characterized by high concentrations of iron or manganese and thus by different resultant magnetization. This leads to the observation of two singular fields H1 and H2 dependent on the degree of inhomogeneity, unpredictably changing from sample to sample due to synthesis effects. These fields separate different magnetic responses of the system; for applied fields H < H1, the Fe and Mn clusters weakly interact in a competitive way, giving rise to MRV, while for an intermediate field regime the energy of this weak interaction becomes comparable to the energy of the system under field application. As a consequence, the zero field cooled magnetization thermal evolution depends on the sample degree of inhomogeneity. In this field regime, applied field Mössbauer spectroscopy indicates that the iron rich clusters are highly polarized by the field, while the largest part of the material, consisting of AFM clusters characterized by axial anisotropy and uncompensated moments, shows soft or hard magnetism depending on T. Above the higher singular field, the M(T) curves show the trend expected for a classical antiferromagnetic material and the competitive character is suppressed. The MRV phenomenon results to be highly sensitive on both the thermal and magnetic measurement conditions; for this reason the present work proposes a characterization strategy that in principle has a large applicability in the study of disordered perovskites showing similar phenomenology.

Improving information storage by means of segmented magnetic nanowires

A set of magnetic nanowires trapped in the membrane used to produce them can be employed to inscribe information in the form of sectors of wires with the same ferromagnetic orientation (Cisternas and Vogel, 2013 [1]). However, such a system relays on the shape anisotropy of each nanowire as the stabilizing mechanism avoiding magnetization reversal. Such stabilization mechanism weakens as the size of the nanowires decrease. In the present paper we introduce a way of using segmented nanowires to produce a self-stabilization mechanism based on the fact that interactions among segments of different layers can contribute with negative energies. Then, for some particular geometries it is possible to make this interaction the most important one producing a more stable system with respect to spontaneous magnetization reversal. Such inscribed ferromagnetic sector will then last longer than other ferromagnetic sectors formed by exclusively repelling elements. We make use of available algebraic expressions to calculate the energy contribution of noncoaxial segments. For the coaxial segments a similar expression is developed here and it is applied to real systems. The total interaction energy for all segments in the system is calculated for different geometrical possibilities. Application to two particular symbols (letters T and O) is fully discussed bringing out general aspects that could be applied to other symbols. Projections of this work are finally mentioned.

Magnetization reversal phenomena and bipolar switching in La1.9Bi0.1FeCrO6

A magnetization reversal phenomenon in double perovskite La1.9Bi0.1FeCrO6 has been investigated. Polycrystalline sample was prepared by conventional solid state reaction method and it was found to stabilize in orthorhombic crystal structure with Pnma space group. Temperature dependent magnetization study at low fields has shown the spontaneous magnetization reversal phenomena below the compensation temperature (Tcomp~186K) and it is attributed to the antiferromagnetic arrangement of uncompensated weak ferromagnetic clusters of Fe and Cr. The field dependent tunable bipolar magnetic switching and observation of normal and inverse magnetocaloric effects evidence the magnetization reversal phenomena in La1.9Bi0.1FeCrO6.

Structurally Tailored Hexagonal Ferroelectricity and Multiferroism in Epitaxial YbFeO3 Thin-Film Heterostructures

Multiferroics have received a great deal of attention because of their fascinating physics of order-parameter cross-couplings and their potential for enabling new device paradigms. Considering the rareness of multiferroic materials, we have been exploring the possibility of artificially imposing ferroelectricity by structurally tailoring antiferromagnets in thin-film forms. YbFeO(3) (YbFO hereafter), a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric (space group Pnma). Here we report that a YbFO thin-film heterostructure fabricated by adopting a hexagonal template surprisingly exhibits nonferroelastic ferroelectricity with the Curie temperature of 470 K. The observed ferroelectricity is further characterized by an extraordinary two-step polarization decay, accompanied by a pronounced magnetocapacitance effect near the lower decay temperature, ~225 K. According to first-principles calculations, the hexagonal P6(3)/mmc-P6(3)mc-P6(3)cm consecutive transitions are primarily responsible for the observed two-step polarization decay, and the ferroelectricity originates from the c-axis-oriented asymmetric Yb 5d(z(2))-O 2p(z) orbital hybridization. Temperature-dependent magnetization curves further reveal an interesting phenomenon of spontaneous magnetization reversal at 83 K, which is attributed to the competition between two distinct magnetocrystalline anisotropy terms, Fe 3d and Yb 4f moments.

Thermally activated magnetization reversal in monatomic magnetic chains on surfacesstudied by classical atomistic spin-dynamics simulations

We analyse the spontaneous magnetization reversal of supported monatomic chains of finitelength due to thermal fluctuations via atomistic spin-dynamics simulations. Our approachis based on the integration of the Landau–Lifshitz equation of motion of a classical spinHamiltonian in the presence of stochastic forces. The associated magnetizationlifetime is found to obey an Arrhenius law with an activation barrier equal to thedomain wall energy in the chain. For chains longer than one domain wall width, thereversal is initiated by nucleation of a reversed magnetization domain primarilyat the chain edge followed by a subsequent propagation of the domain wall tothe other edge in a random-walk fashion. This results in a linear dependence ofthe lifetime on the chain length, if the magnetization correlation length is notexceeded. We studied chains of uniaxial and triaxial anisotropy and found that atriaxial anisotropy leads to a reduction of the magnetization lifetime due to ahigher reversal attempt rate, even though the activation barrier is not changed.

Spin-Canting-Induced Improper Ferroelectricity and Spontaneous Magnetization Reversal inSmFeO3

SmFeO3, a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric. However, we have found that SmFeO3 is surprisingly ferroelectric at room temperature with a small polarization along the b axis of Pbnm. First-principles calculations indicate that the canted antiferromagnetic ordering with two nonequivalent spin pairs is responsible for this extraordinary polarization and that the reverse Dzyaloshinskii-Moriya interaction dominates over the exchange-striction mechanism in the manifestation of the improper ferroelectricity. SmFeO3 further exhibits an interesting phenomenon of spontaneous magnetization reversal at cryogenic temperatures. This reversal is attributed to the activation of the Sm-spin moment which is antiparallel to the Fe-spin moment below ∼5 K.