Local Magnetization Research Articles

Out-of-plane polarization reversal and changes in in-plane ferroelectric and ferromagnetic domains of multiferroic BiFe0.9Co0.1O3 thin films by water printing

BiFe0.9Co0.1O3 is a promising material for an ultra-low-power-consumption nonvolatile magnetic memory device because local magnetization reversal is possible through application of an electric field. Here, changes in ferroelectric and ferromagnetic domain structures in a multiferroic BiFe0.9Co0.1O3 thin film induced by “water printing”, which is a polarization reversal method involving chemical bonding and charge accumulation at the interface between the liquid and the film, was investigated. Water printing using pure water with pH = 6.2 resulted in an out-of-plane polarization reversal from upward to downward. The in-plane domain structure remained unchanged after the water printing process, indicating that 71° switching was achieved in 88.4% of the observation area. However, magnetization reversal was observed in only 50.1% of the area, indicating a loss of correlation between the ferroelectric and magnetic domains because of the slow polarization reversal due to nucleation growth.

Spatial antiferromagnetic spin texture as a nano-oscillator

We report a theoretical study of the localized spatial magnetization configuration, which is a confined spin configuration of the target skyrmion/hopfion type in an antiferromagnet with perpendicular magnetic anisotropy, and then we solve the particular problem of self-oscillations of such a topological spin texture. Using the energy approach, a self-consistent account of inhomogeneity of the characteristics of the topological magnetic spin texture was carried out. On this basis, the equation of free oscillations of the confined spin configuration magnetization was derived and its quasi-classical solution was found. For a thin ring spin texture, the frequency, period of oscillations and relative amplitude of the main tone of oscillations are found. For the first time, we determined the topological mass, inertial mass and total energy of the main tone of oscillations of such spatial spin texture. The self-oscillatory process of a spatial spin texture is interpreted as a magnetic nano-oscillator.

Local and global magnetization on the Sierpiński carpet.

The phase transition of the classical Ising model on the Sierpiński carpet, which has the fractal dimension log_{3}^{}8≈1.8927, is studied by an adapted variant of the higher-order tensor renormalization group method. The second-order phase transition is observed at the critical temperature T_{c}^{}≈1.478. Position dependence of local functions is studied through impurity tensors inserted at different locations on the fractal lattice. The critical exponent β associated with the local magnetization varies by two orders of magnitude, depending on lattice locations, whereas T_{c}^{} is not affected. Furthermore, we employ automatic differentiation to accurately and efficiently compute the average spontaneous magnetization per site as a first derivative of free energy with respect to the external field, yielding the global critical exponent of β≈0.135.

Mechanism Analysis of Electrochemical Micro-machining Behavior Assisted by Magnetic Field with Local Magnetic Induction Lines Deformation

Mechanism Analysis of Electrochemical Micro-machining Behavior Assisted by Magnetic Field with Local Magnetic Induction Lines Deformation

Crystal Structure and Ferromagnetism of the CeFe9Si4Intermetallic Compound

We have determined the crystal structure and the magnetic state of the CeFe9Si4 intermetallic compound. Our revised structural model (fully ordered tetragonal unit cell, I4/mcm) agrees with the previous literature report, except for some minor quantitative differences. Magnetically, the CeFe9Si4 undergoes a ferromagnetic transition at the temperature TC ≈ 94 K. Ferromagnetism in the combined Ce-Fe spin system is a result of interplay between the localized magnetism of the Ce sublattice and the Fe band (itinerant) magnetism. Ferromagnetic ordering obeys the rather general rule that the exchange spin coupling between atoms possessing more than half-full d shells with atoms possessing less than half-full d shells is antiferromagnetic (where the Ce atoms are considered as light d elements). Since in rare-earth metals from the light half of the lanthanide series, the magnetic moment is directed opposite to the spin, this results in ferromagnetism. The magnetoresistance and the magnetic specific heat show an additional temperature-dependent feature (a shoulder) deep inside the ferromagnetic phase that is considered to originate from the influence of the magnetization on the electronic band structure via the magnetoelastic coupling, which alters the Fe band magnetism below TC. The ferromagnetic phase of CeFe9Si4 is magnetically soft.

Simulated FMR to study in plane magnetic anisotropy of 3x3 array constituted by square base nickel nanopillars

A study about the magnetic anisotropy in plane of 3x3 arrays constituted by square base nickel nanopillars (NPs), with D = 30 nm width and L = 120 nm long, is presented. Variations in the effective anisotropy, due to the hard dipolar interactions, were provoked by modification in the interpillar distance, a-D = 5, 10, 20 and 50 nm. The data to study the anisotropy field, HA, was collected from micromagnetic simulation performed by means of a ferromagnetic resonance (FMR) standard problem using the OOMMF package. The interpillar distance (face to face) was varied from the lower value, a-D = 5 nm where the dipolar interactions are very strong, to the maximum value, a-D = 50 nm were no longer significant are the dipole effects (to rescue the behavior of a single NP). Based on the isolated nanopillar symmetry, the initial proposed model for the collected data was the tetragonal anisotropy. The data showed a breaking of this symmetry for a-D ≤ 20 nm due to strong dipole interpillars interactions. Taking into account the array characteristics and the effects of local magnetization configuration at the corners of NPs, it was possible to propose a new model. The results are well described by the proposed model for all interpillar distance and, for a-D = 50 nm, the model coincides with the tetragonal symmetry compatible with the single pillar.

Spin diffusion in spin glasses requires two magnetic variables, M⃗ and m⃗

Experiment has established that spin-glasses can support a steady-state spin current $\vec{j}_{i}$. However, the accepted theory of spin glass dynamics permits oscillations but no steady-state spin current. Onsager's irreversible thermodynamics implies that the spin current is proportional to the gradient of a magnetization. We argue, however, that the magnon distribution function associated with the local equilibrium magnetization $\vec{M}$ cannot diffuse because it represents $10^{23}$ variables. We therefore invoke the non-equilibrium magnetization $\vec{m}$, which in spintronics is called the {\it spin accumulation}. Applying the theory of irreversible thermodynamics we indeed find that it predicts spin diffusion, and we consider other experimental consequences of the theory, including a wavelength-dependent coupling between the reactive and the diffusive degrees of freedom.

Nonequilibrium dynamics of the Ising model on heterogeneous networks with an arbitrary distribution of threshold noise.

The Ising model on networks plays a fundamental role as a testing ground for understanding cooperative phenomena in complex systems. Here we solve the synchronous dynamics of the Ising model on random graphs with an arbitrary degree distribution in the high-connectivity limit. Depending on the distribution of the threshold noise that governs the microscopic dynamics, the model evolves to nonequilibrium stationary states. We obtain an exact dynamical equation for the distribution of local magnetizations, from which we find the critical line that separates the paramagnetic from the ferromagnetic phase. For random graphs with a negative binomial degree distribution, we demonstrate that the stationary critical behavior as well as the long-time critical dynamics of the first two moments of the local magnetizations depend on the distribution of the threshold noise. In particular, for an algebraic threshold noise, these critical properties are determined by the power-law tails of the distribution of thresholds. We further show that the relaxation time of the average magnetization inside each phase exhibits the standard mean-field critical scaling. The values of all critical exponents considered here are independent of the variance of the negative binomial degree distribution. Our work highlights the importance of certain details of the microscopic dynamics for the critical behavior of nonequilibrium spin systems.

Quantum phase transitions in non-Hermitian [formula omitted]-symmetric transverse-field Ising spin chains

We present a theoretical study of quantum phases and quantum phase transitions occurring in non-Hermitian PT-symmetric superconducting qubits chains described by a transverse-field Ising spin model. A non-Hermitian part of the Hamiltonian is implemented via imaginary staggered longitudinal magnetic field, which corresponds to a local staggered gain and loss terms. By making use of a direct numerical diagonalization of the Hamiltonian for spin chains of a finite size N, we explore the dependencies of the energy spectrum, including the energy difference between the first excited and the ground states, the spatial correlation function of local polarization (z-component of local magnetization) on the adjacent spins interaction strength J and the local gain (loss) parameter γ. A scaling procedure for the correlation length ξ allows us to establish a complete quantum phase diagram of the system. We obtain two quantum phases for J<0, namely, PT-symmetry broken antiferromagnetic state and PT-symmetry preserved paramagnetic state, and the quantum phase transition line between them is the line of exception points. For J>0 the PT-symmetry of the ground state is retained in a whole region of parameter space of J and γ, and a system shows two intriguing quantum phase transitions between ferromagnetic and paramagnetic states for a fixed parameter γ>1. We also provide the qualitative quantum phase diagram γ−J derived in the framework of the Bethe–Peierls approximation that is in a good accord with numerically obtained results.

Angstrom-scale imaging of magnetization in antiferromagnetic Fe2As via 4D-STEM

We demonstrate a combination of computational tools and experimental 4D-STEM methods to image the local magnetic moment in antiferromagnetic Fe2As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks, common in antiferromagnetic materials, to create imaging modes that directly visualize the magnetic lattice. Using this approach, we show that center-of-mass analysis can determine the local magnetization component in the plane perpendicular to the path of the electron beam. Moreover, we develop Magnstem, a quantum mechanical electron scattering simulation code, to model electron scattering of an angstrom-scale probe from magnetic materials. Using these tools, we identify optimal experimental conditions for separating weak magnetic signals from the much stronger interactions of an angstrom-scale probe with electrostatic potentials. Our techniques should be useful for characterizing the local magnetic order in systems such in thin films, interfaces, and domain boundaries of antiferromagnetic materials, which are difficult to probe with existing methods.

Determining Local Magnetic Susceptibility Tensors in Paramagnetic Lanthanide Crystalline Powders from Solid-State NMR Chemical Shift Anisotropies

Exploring magnetic properties at the molecular level is a challenge that has been met by developing many experimental and theoretical solutions, such as polarized neutron diffraction (PND), muon-spin rotation (μ-SR), electron paramagnetic resonance (EPR), SQUID-based magnetometry measurements, and advanced modeling on open-shell systems and relativistic calculations. These methods are powerful tools that shed light on the local magnetic response in specifically designed magnetic materials such as contrast agents, for MRI, molecular magnets, magnetic tags for biological NMR, etc. All of these methods have their advantages and disadvantages. In order to complement the possibilities offered by these methods, we propose a new tool that implements a new approach combining simulation and fitting for high-resolution solid-state NMR spectra of lanthanide-based paramagnetic species. This method relies on a rigorous acquisition thanks to short high-power adiabatic pulses (SHAP) of high-resolution solid-state NMR isotropic and anisotropic data on a powdered magnetic material. It is also based on an efficient modeling of this data thanks to a semiempirical model based on a parametrization of the local magnetism and the crystal structure provided by diffraction methods. The efficiency of the calculation relies on a thorough simplification of the electron-nucleus interactions (point-dipole interaction, no Fermi contact) which is validated by experimental analysis. By taking advantage of the efficient calculation possibilities offered by our method, we can compare a great number of simulated spectra to experimental data and find the best-matching local magnetic susceptibility tensor. This method was applied to a series of isostructural lanthanide oxalates which are used as a benchmark system for many analytical methods. We present the results of thorough solid-state NMR and extensive modeling of the hyperfine interaction (including up to 400 paramagnetic centers) that yield local magnetic susceptibility tensor measurements that are self-consistent as well as consistent with bulk susceptibility measurements.

Effect of External Pressure and Quantum State on the Local Magnetization of Germanium Layers: Ab Initio Calculation

AbstractDesigning stable quantum computers with the ability to correct errors responsibly requires a detailed understanding of the physical processes that underlie these devices. In this study, detailed quantum mechanical simulations of the atomic structure, charge distribution of electron density, and distribution and orientation of magnetic and quantum states as a function of applied external pressure performed for ultrathin layers of germanium. The results show that, even in the absence of holes, in the negative pressure range, the total spin is characterized by the quantum state |1〉 in the Bloch sphere. It found that the hole states predominantly localized in the volume structure of ultrathin germanium layers. In this structure, there is an ordering of magnetic states with a repetition period of 2a along the elongation axis of the slab.

Tailoring crosstalk between localized 1D spin-wave nanochannels using focused ion beams

1D spin-wave conduits are envisioned as nanoscale components of magnonics-based logic and computing schemes for future generation electronics. À-la-carte methods of versatile control of the local magnetization dynamics in such nanochannels are highly desired for efficient steering of the spin waves in magnonic devices. Here, we present a study of localized dynamical modes in 1-upmum-wide permalloy conduits probed by microresonator ferromagnetic resonance technique. We clearly observe the lowest-energy edge mode in the microstrip after its edges were finely trimmed by means of focused Ne^+ ion irradiation. Furthermore, after milling the microstrip along its long axis by focused ion beams, creating consecutively sim50 and sim100 nm gaps, additional resonances emerge and are attributed to modes localized at the inner edges of the separated strips. To visualize the mode distribution, spatially resolved Brillouin light scattering microscopy was used showing an excellent agreement with the ferromagnetic resonance data and confirming the mode localization at the outer/inner edges of the strips depending on the magnitude of the applied magnetic field. Micromagnetic simulations confirm that the lowest-energy modes are localized within sim15-nm-wide regions at the edges of the strips and their frequencies can be tuned in a wide range (up to 5 GHz) by changing the magnetostatic coupling (i.e., spatial separation) between the microstrips.

Remote control of spin polarization of topological corner states

In two-dimensional higher-order topological insulators, the corner states are separated by a non-negligible distance. The crystalline symmetries protect the robustness of their corner states with long-range entanglement, which are robust against time-reversal-breaking perturbations. Here, we demonstrate the possibility of direct control of the topological corner states by introducing the spin degree of freedom in a rhombus-shaped Kekul\'e nanostructure with local magnetization and local electric potential. By applying a local magnetization on one corner, the other corner can also be strongly spin polarized. By further applying a local electric potential at the same corner, the sign of the spin polarization can be reversed at both corners. We also prove the robustness of the control of the spin polarization under the disorder. Moreover, we demonstrate the material realization in a $\ensuremath{\gamma}$-graphyne nanostructure with Mn adsorption and Si replacement at one corner by using first-principles calculations. Other higher-order lattices and the shape of the nanostructure are also discussed. Our studies give a showcase of the remote correlation of quantum states in higher-order topological materials for spintronic and quantum applications.

Temperature dependent mechanical properties of MAB phase Fe2AlB2

As a new type of laminate materials, the MAB phase Fe2AlB2 is attractive due to its near-room magnetocaloric properties. In this work, we propose the energy-strain approach to the elastic constants of MAB phase Fe2AlB2. The quasi-static approximation in combination with first-principles calculations is used to study the magnetic and elastic properties of Fe2AlB2 at finite temperature (0–1300K). Our temperature dependent polycrystalline moduli have similar tends to experiments, due to the proper approximation to the paramagnetic state above room temperature. Both the lattice parameters and elastic constants show the strong anisotropy in a range of 0–1300K. Although only Fe atoms contribute the magnetic property, their local magnetism plays a key role in the phase stability and the temperature dependent mechanical properties of Fe2AlB2.

First-principles study on small polaron and Li diffusion in layered LiCoO2.

Li-ion conductivity is one of the essential properties that influences the performance of cathode materials for Li-ion batteries. Here, using density functional theory, we investigate the polaron stability and its effect on the Li-ion diffusion in layered LiCoO2 with various magnetic orderings. We show that the local magnetism promotes the localized Co4+ polaron with the Li-diffusion barrier of ∼0.34 eV. While the Li-ion diffuses, the polaron migrates in the opposite direction to the Li movement. In the non-magnetic structure, on the other hand, the polaron does not form, and the Li diffusion barrier is lowered to 0.21 eV. Although the presence of the polaron raises the diffusion barrier, the magnetically ordered structures are energetically more stable during the migration than the non-magnetic case. Thus, our work advocates the hole polaron migration scenario for Li-ion diffusion. We further demonstrate that the strong electron correlation of Co ions plays an essential role in stabilizing the Co4+ polaron.

First Order Phase Transition in Ferromagnetic Semiconductors in External Electric and Magnetic Fields (by the example of La-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-) (on the example of La-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-)

It is shown that during a first-order phase transition in ferromagnetic semiconductors based on lanthanum manganites, along with the semiconductor phase of s-electrons, a phase arises with fluctuation short-range order associated with a change in the sign of the intermode interaction parameter in the system of d-electrons. The phase separation region is characterized by fluctuations of local magnetization depending on temperature and external magnetic field. In this case, the temperature of the first order phase transition, above which phase separation arises, is appreciably affected by double exchange. Due to self-heating, the phase separation is significantly affected by the electric field, which leads to N-shaped current-voltage characteristics. Using La1-xCaxMnO3 as an example, it is shown that the switching on of electric and magnetic fields during a first-order phase transition in ferromagnetic semiconductors is accompanied by the appearance of self-oscillations of the electric current and magnetization. Keywords: double exchange, electron fluctuations of spin and charge density, Anderson localization, self-oscillations.

Фазовый переход первого рода в ферромагнитных полупроводниках во внешних электрическом и магнитном полях (на примере La-=SUB=-1-x-=/SUB=-Ca-=SUB=-x-=/SUB=-MnO-=SUB=-3-=/SUB=-)

It is shown that during a first-order phase transition in ferromagnetic semiconductors based on lanthanum manganites, along with the semiconductor phase of s-electrons, a phase arises with fluctuation short-range order associated with a change in the sign of the intermode interaction parameter in the system of d-electrons. The phase separation region is characterized by fluctuations of local magnetization depending on temperature and external magnetic field. In this case, the temperature of the first order phase transition, above which phase separation arises, is appreciably affected by double exchange. Due to self-heating, the phase separation is significantly affected by the electric field, which leads to N-shaped current-voltage characteristics. Using La1–xCaxMnO3 as an example, it is shown that the switching on of electric and magnetic fields during a first-order phase transition in ferromagnetic semiconductors is accompanied by the appearance of self-oscillations of the electric current and magnetization.

Train Wheel Magnetic Flux Leakage Testing Method Based on Local Magnetization Enhancement

Non-destructive testing (NDT) of train wheels fatigue cracks is a critical technology to ensure railway safety. Early fatigue cracks are small and difficult to detect. In this paper, the magnetic flux leakage (MFL) method is proposed to detect train fatigue cracks. The unsaturated magnetization state near the defect area is revealed through magnetic circuit analysis and finite element methods. As a result, leakage magnetic fields will decay sharply for small fatigue cracks, and they are difficult to detect by the traditional MFL method. To improve the magnetization state near defect areas, a novel MFL method based on local magnetization enhancement is proposed and developed. Experimental verification is also conducted by a train wheel NDT device. Testing results show that this method achieves excellent performance in the detection of defects in train wheels.

Temperature Dependence of the Lower Critical Field of the Noncentrosymmetric Superconductor α‐BiPd

Temperature variation of the lower critical field in the noncentrosymmetric superconductor α‐BiPd is probed by local magnetization measurements using Hall micromagnetometry, performed down to 0.3 K in a magnetic field applied along the crystallographic b axis. Below a critical temperature 3.8 K, a conventional dependence is found, typical for a single‐band s‐wave superconductor. The obtained data imply an absence of spin‐triplet component in the superconducting wavefunction and marginal multiband effects in this material, which contradicts some literature reports.