Ferromagnetic System Research Articles

Renormalization group study of marginal ferromagnetism.

When studying the collective motion of biological groups, a useful theoretical framework is that of ferromagnetic systems, in which the alignment interactions are a surrogate of the effective imitation among the individuals. In this context, the experimental discovery of scale-free correlations of speed fluctuations in starling flocks poses a challenge to common statistical physics wisdom, as in the ordered phase of standard ferromagnetic models with O(n) symmetry, the modulus of the order parameter has finite correlation length. To make sense of this anomaly, a ferromagnetic theory has been proposed, where the bare confining potential has zero second derivative (i.e., it is marginal) along the modulus of the order parameter. The marginal model exhibits a zero-temperature critical point, where the modulus correlation length diverges, hence allowing us to boost both correlation and collective order by simply reducing the temperature. Here, we derive an effective field theory describing the marginal model close to the T=0 critical point and calculate the renormalization group equationsat one loop within a momentum shell approach. We discover a nontrivial scenario, as the cubic and quartic vertices do not vanish in the infrared limit, while the coupling constants effectively regulating the exponents ν and η have upper critical dimension d_{c}=2, so in three dimensions the critical exponents acquire their free values, ν=1/2 and η=0. This theoretical scenario is verified by a Monte Carlo study of the modulus susceptibility in three dimensions, where the standard finite-size scaling relations have to be adapted to the case of d>d_{c}. The numerical data fully confirm our theoretical results.

Single-ion anisotropy as the source of anomalies in thermodynamic properties of magnetic spin-1/2 − 1 mixed systems on square and simple cubic lattices

The influence of the single-ion anisotropy on the thermodynamic properties of ferromagnetic as well as ferrimagnetic spin-1/2 − 1 mixed systems on the square and simple cubic lattice is investigated in the framework of the exactly solvable Ising-like models on the corresponding recursive lattices. The exact solutions of the models are present in the form of the explicit expressions for the free energy per site as the functions of the coordinates of the fixed points of the corresponding systems of recursion relations. The phase diagrams of the models are determined and the existence of compensation temperatures in the ferrimagnetic cases are discussed. The equations that drive the positions of the critical points are derived. The existence of the tricritical behavior in the three-dimensional system on the simple cubic lattice and its nonexistence in the case of the two-dimensional system on the square lattice is demonstrated. It is shown that the presence of the single-ion anisotropy can naturally lead to the emergence of the thermodynamic anomalies typical for frustrated magnetic systems even in unfrustrated ferromagnetic systems on studied bipartite lattices. First of all, the studied models exhibit the formation of strict residual-entropy hierarchies between the neighboring ground states of the system of three different ground states. It is also shown that due to this behavior of the entropy the specific heat capacity of the models exhibit anomalous (Schottky) behavior at low temperatures in the vicinity of the values of the anisotropy parameter, for which highly macroscopically degenerated single-point-like ground states are formed.

Orbital Angular Momentum of Magnons in Collinear Magnets.

We study the orbital angular momentum of magnons for collinear ferromagnet (FM) and antiferromagnetic (AF) systems with nontrivial networks of exchange interactions. The orbital angular momentum of magnons for AF and FM zigzag and honeycomb lattices becomes nonzero when the lattice contains two inequivalent sites and is largest at the avoided-crossing points or extremum of the frequency bands. Hence, the arrangement of exchange interactions may play a more important role at producing the orbital angular momentum of magnons than the spin-orbit coupling energy and the resulting noncollinear arrangement of spins.

Interplay of disorder and type of driving in disordered ferromagnetic systems.

We investigate the effects of adiabatic, quasistatic, and finite-rate types of driving on the evolution of disordered three-dimensional ferromagnetic systems, studied within the frame of the nonequilibrium athermal random field Ising model. The effects were examined in all three domains of disorder (low, high, and transitional) for all types of driving, and in a wide range of driving rates for quasistatic and finite-rate driving, providing an extensive overview and comparison of the joint effects that the disorder, type of driving, and rate regime have on the system's behavior.

PANI-Based Stacked Ferromagnetic Systems: Electrochemical Preparation and Characterization

In this work, the electropolymerization of polyaniline (PANI) is explored for its action as either a suitable coating or as a substrate for Nickel (Ni) and Magnetite (Fe3O4) surfaces. PANI electropolymerization has been achieved through cyclic voltammetry (CV), potentiostatic and galvanostatic electrochemical methods. The interaction between the obtained surfaces and the ferromagnetic layers (Ni and Fe3O4) has been investigated as a function of the pH of the electrolytic PANI solution, and also a variety of experimental parameters have been optimized in order to achieve the synthesis of PANI coatings (solvent, substrate, concentrations, and cell set-up). Thus, we obtained stable and consistent PANI thick films at the interface of both the nickel and the magnetite ferromagnetic materials.

Topological phase transition in magnon bands in a honeycomb ferromagnet driven by sublattice symmetry breaking

Ferromagnetic honeycomb systems are known to exhibit a magnonic topological phase under the existence of the next-nearest neighbor Dzyaloshinskii-Moriya interaction (DMI). Motivated by the recent progress in the sublattice-specific control of magnetic anisotropy, we study the topological phase of magnon bands of honeycomb ferromagnetic monolayer and bilayer with the sublattice symmetry breaking due to the different anisotropy energy in the presence of the DMI. We show that there is a topological phase transition between the topological magnon insulator and the topologically trivial magnon phase driven by the change of the relative size of the DMI and the anisotropy differences between the sublattices. The magnon thermal Hall conductivity is proposed as an experimental probe of the magnon topology.

Explorations on properties of ε′ phase and new crystalline phase of Fe3Ge alloy under high pressures

We calculated the lattice dynamics, elasticity, electron spin polarizability P(EF), and electron specific heat coefficient γ of the L12-Fe3Ge crystalline alloy under high pressure using first-principles method based on density functional theory. By comparing the lattice dynamics and elasticity of the ferromagnetic and non-magnetic systems, it can be found that the spontaneous magnetization of the system induces the dynamic stability of the system at low pressure (less than 6 GPa). Furthermore, the magnetostriction induced by the spontaneous magnetization enhances the elastic modulus of the system. With the increase of pressure, in the pressure range of 130-290 GPa where the spin magnetic moment of the system decreases sharply, the elastic modulus exhibits an abnormal behavior, which leads to double peaks in Poisson's ratio v and Pugh's ratio B/G, correspondingly, double valleys appear in shear modulus G, Young's modulus E and Debye temperature ΘD, which are associated with dynamic instability in the 130-290 GPa. Finally, for the phonon spectrum at 260 GPa with a significant imaginary frequency, the metastable orthogonal Fmmm structure is obtained using the soft mode phase transition theory. After comparing the enthalpy differences with different structures, it is found that the ferromagnetic Fmmm structure is a metastable phase, which is dynamic stable at 0 GPa and 260 GPa.

Large Room Temperature Anomalous Transverse Thermoelectric Effect in Kagome Antiferromagnet YMn6 Sn6.

Kagome magnets possess several novel nontrivial topological features owing to the strong correlation between topology and magnetism that extends to their applications in the field of thermoelectricity. Conventional thermoelectric (TE) devices use the Seebeck effect to convert heat into electrical energy. In contrast, transverse thermoelectric devices based on the Nernst effect are attracting recent attention due to their unique transverse geometry, which uses a single material to eliminate the need for a multitude of electrical connections compared to conventional TE devices. Here, a large anomalous transverse thermoelectric effect of ≈2 µV K-1 at room temperature in a kagome antiferromagnet YMn6 Sn6 single crystal is obtained. The obtained value is larger than that of state-of-the-art canted antiferromagnetic (AFM) materials and comparable with ferromagnetic systems. The large anomalous Nernst effect (ANE) can be attributed to the net Berry curvature near the Fermi level. Furthermore, the ANE of the AFM YMn6 Sn6 exceeds the magnetization scaling relationship of conventional ferromagnets. The results clearly illustrate that AFM material YMn6 Sn6 is an ideal topological material for room-temperature transverse thermoelectric applications.

On multi-soliton solutions to a generalized inhomogeneous nonlinear Schrödinger equation for the Heisenberg ferromagnetic spin chain

A generalized inhomogeneous higher-order nonlinear Schrödinger (GIHNLS) equation for the Heisenberg ferromagnetic spin chain system in (1+1)-dimensions under zero boundary condition at infinity is taken into account. The spectral analysis is first performed to generate a related matrix Riemann–Hilbert problem on the real axis. Then, through solving the resulting matrix Riemann–Hilbert problem by taking the jump matrix to be the identity matrix, the general bright multi-soliton solutions to the GIHNLS equation are attained. Furthermore, the one-, two-, and three-soliton solutions are written out and analyzed by figures.

Large Dzyaloshinskii-Moriya interaction and field-free topological chiral spin states in two-dimensional alkali-based chromium chalcogenides

Noncentrosymmetric ferromagnetic systems with large Dzylochinskii-Moriya interaction (DMI) have gained intensive attention for hosting topologically protected solitons such as skyrmions and bimerons. Recently, a series of two-dimensional (2D) Li- and Na-based chromium chalcogenides has been theoretically predicted to be a new family of intrinsic inversion asymmetric ferromagnets. Here, by combining first-principles calculations and atomistic spin model simulations, we unveil that there is very large DMI in these series of 2D alkali-based chromium chalcogenides, which enables a variety of filed-free topological spin textures in these structures. Furthermore, we find robust hole-doping tunable DMI in the ${\mathrm{LiCrSe}}_{2}$ and ${\mathrm{NaCrSe}}_{2}$ monolayers. Our finding opens up further possibilities toward 2D materials in spintronic devices.

Chemically Induced Magnetic Dead Shells in Superparamagnetic Ni Nanoparticles Deduced from Polarized Small-Angle Neutron Scattering.

Advances in the synthesis and characterization of colloidal magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behavior, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic system, for example, achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a non-ferromagnetic dead shell indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structures of Ni NP ensembles via unpolarized, half-polarized, and fully polarized small-angle neutron scattering (SANS) measurements and analyses coupled with X-ray diffraction and magnetometry. The postulated nanometric magnetic dead shell is not only detected but conclusively identified as a non-ferromagnetic Ni phosphide derived from the trioctylphosphine commonly used in hot-injection colloidal NP syntheses. The phosphide shell thickness is tunable via synthesis temperature, falling to as little as 0.5 nm at 170 °C. Temperature- and magnetic field-dependent polarized SANS measurements additionally reveal essentially bulk-like ferromagnetism in the Ni core and negligible interparticle magnetic interactions, quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis-structure-property relationships in metallic magnetic NPs, point to a simple potential route to ligand-free stabilization, and highlight the power of the currently available suite of polarized SANS measurement and analysis capabilities for magnetic NP science and technology.

Critical and hysteresis phenomena in the mixed spin-[formula omitted] anisotropic two-dimensional Heisenberg model: A Monte Carlo study

The critical and hysteresis behaviors of the ferromagnetic mixed-spin system consisting of two sublattices A and B with two types of spins SA=32 and SB=12 described by anisotropic Classical-Heisenberg Model on a square lattice are examined by using Monte Carlo simulations based on metropolis algorithm. By exploiting the thermal variations of the order-parameters and the magnetic susceptibility, the phase diagrams are calculated on the (Dz/J,kBT/J), (Dx/J,kBT/J) and (Δ,kBT/J) planes and the effects of various interaction parameters in the Hamiltonian are explored. It is found that the model yields only second-order phase transitions. We have also estimated the critical exponents ν, β and γ of the model as functions of Dz and Dx by using the finite-size scaling arguments and found that they depend on the details of the model. Finally, the hysteresis behavior of the model is also investigated and it is revealed that the width of hysteresis loop decreases with temperature while it increases for positive Dz values.

Theoretical approach and experimental investigation of spin current tunnelling in Ni80Fe20/Si3N4/Fe40Co37Zr11N12 magnetically collinear trilayer films with defined in-plane uniaxial anisotropy

Ferromagnetic trilayer film systems with nonferromagnetic Si3N4 interface layers were fabricated by using magnetron sputtering. A post-annealing process was carried out at 400 °C for 2 h in a static magnetic field, in order to induce an in-plane uniaxial anisotropy. The present study introduces the exchange interaction between ferromagnetic layers which influences the static magnetisation properties as well as the natural resonance frequency behaviour and its damping features, i.e., damping and anti-damping effects. This is expressed by the total damping parameter αitot = αGi ± αspi, which is enhanced or reduced by a spin transfer torque damping parameter ± αspi whereupon Gilbert bulk damping αGi is considered to be independent. The static polarisation loops are slightly but noticeably different, caused by an interface with thickness up to 100 nm, which reflects a decreasing exchange interaction. With varying the interface thickness, the films also show a distinct variation in their frequency spectra. This leads to the conclusion that spin currents caused by spin pumping arise, and tunnelling within a Si3N4 insulating barrier generates spin transfer torque. This can be assumed because the observed ferromagnetic resonance dual lines show remarkable changes in their Full Width at Half Maxima (FWHM) at frequencies of approximately 600 MHz caused by the Ni80Fe20 layers and 2.32 GHz caused by the Fe40Co37Zr11N12 layers. Regarding the FWHM of the resonance frequency spectra in association with the damping constant αeff on the dependence of the interface layer thickness, one can observe an exponential curve shape for both ferromagnetic layers, which is obviously a sign for the interaction between the layers through a spin current tunnelling mechanism.

Phase diagram of chiral magnets via Green’s function method

In this paper, Green’s function method is applied to study the ferromagnetic system with Dzyaloshinkii–Moriya (DM) interaction in both two-dimension (2D) and three-dimension chiral magnets. Relevant properties in 2D magnets are calculated, such as the susceptibility, correlation function and analytical expressions of phase boundary. Based on the theoretical results, a new phase is predicted in the window of strong DM interaction characterized by a negative winding number. In addition, helical state in pure 2D material only appears at zero temperature. The analysis on correlation function shows a special symmetry of transverse spin correlation, which corresponds to the skyrmion phase. The results also prove the instability of helical state and its lifetime is numerically computed. Different from 2D magnets, helical state in 3D exists in the window of a lower Zeeman energy and has a long lifetime. DM interaction also reduces Curie temperature because of the spatial symmetry breaking.

Characterizing quantum nonlocalities under the Heisenberg XYZ spin model with Dzyaloshinskii–Moriya interaction

In quantum information science, the nontrivial applications of the Heisenberg spin model are to realize quantum communication and quantum computing using the quantum nonlocalities between particles in the spin chain. Here, considering Heisenberg XYZ spin model with Dzyaloshinskii–Moriya (DM) interaction, our attentions are directed to ascertain the nonlocal advantage of quantum coherence (NAQC), and also characterize the Bell nonlocality (BN). As revealed from the results, one can use low temperature to realize situations in which the NAQC and BN are invariant. The NAQC and BN cannot be detected if temperature is high. External temperature strongly influences the two quantum nonlocalities in ferromagnetic systems. The strong coupling parameter brings on the fact that the two quantum nonlocalities are invariant. It is very difficult to capture the NAQC and BN if is weak. Considering , the strong is responsible for freezing quantum nonlocalities, and one cannot witness the NAQC when is low. Moreover, the freezing of quantum nonlocalities can be achieved via enhancing , and the detection of NAQC is difficult if is weak. Of particular note, under the influence of DM interactions, NAQC (BN) cannot (can) be frozen both in antiferromagnetic and ferromagnetic systems. The strong and give rise to the difficulty of capturing the NAQC.

The N-Clock Model: Variational Analysis for Fast and Slow Divergence Rates of N

We study a nearest neighbors ferromagnetic classical spin system on the square lattice in which the spin field is constrained to take values in a discretization of the unit circle consisting of N equi-spaced vectors, also known as the N-clock model. We find a fast rate of divergence of N with respect to the lattice spacing for which the N-clock model has the same discrete-to-continuum variational limit as the classical XY model (also known as planar rotator model), in particular concentrating energy on topological defects of dimension 0. We prove the existence of a slow rate of divergence of N at which the coarse-grain limit does not detect topological defects, but it is instead a BV-total variation. Finally, the two different types of limit behaviors are coupled in a critical regime for N, whose analysis requires the aid of Cartesian currents.

Observation of magnetoconductivity with terahertz probes for ferromagnetic Fe films

Ultrafast characterization of spin-dependent transport behavior in ferromagnetic systems without contact probes has been of strong demand, recently. We have experimentally investigated spin-dependent complex conductivity for ferromagnetic Fe films of various thickness by means of terahertz time-domain transmission spectroscopy. Comparison of the transmitted terahertz wave amplitude and the spin-dependent conductivity reveals that the magnetization state of films effectively determines the complex conductivity. Non-invasive observation of spin-dependent conductivity by contact-free terahertz probe method is proven to be promising in further investigating spintronic materials, particularly on an ultrafast timescale.

Magnetic properties of B-site mixed of quantum spin liquid in the triangular ferromagnetic Sr3CuSb2O9 system

Magnetic properties of B-site mixed of quantum spin liquid in the triangular ferromagnetic Sr3CuSb2O9 system

Effective decoupling of ferromagnetic sublattices by frustration in Heusler alloys

Magnetic frustration in ferromagnetic metallic systems is unusual due to the long-range and symmetric nature of the exchange interactions. In this work we prove that it is possible to obtain a highly frustrated ferromagnetic phase in a multi-sublattices cubic structure through a fine tuning of the magnetic interactions. This peculiar state is achieved in Ni-Mn-(In, Sn) Heusler alloys and results in the effective decoupling of their two intertwined ferromagnetic sublattices. One sublattice is ferromagnetic long range ordered below the macroscopic Curie temperature (TC ) whereas the second one remains disordered until a crossover to a polarized state occurs at T << TC . This result points out that a fine engineering of the magnetic interactions in metallic systems can lead to interesting novel and emergent phenomena.

Multi-task graph neural networks for simultaneous prediction of global and atomic properties in ferromagnetic systems * * This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable,

We introduce a multi-tasking graph convolutional neural network, HydraGNN, to simultaneously predict both global and atomic physical properties and demonstrate with ferromagnetic materials. We train HydraGNN on an open-source ab initio density functional theory (DFT) dataset for iron-platinum with a fixed body centered tetragonal lattice structure and fixed volume to simultaneously predict the mixing enthalpy (a global feature of the system), the atomic charge transfer, and the atomic magnetic moment across configurations that span the entire compositional range. By taking advantage of underlying physical correlations between material properties, multi-task learning (MTL) with HydraGNN provides effective training even with modest amounts of data. Moreover, this is achieved with just one architecture instead of three, as required by single-task learning (STL). The first convolutional layers of the HydraGNN architecture are shared by all learning tasks and extract features common to all material properties. The following layers discriminate the features of the different properties, the results of which are fed to the separate heads of the final layer to produce predictions. Numerical results show that HydraGNN effectively captures the relation between the configurational entropy and the material properties over the entire compositional range. Overall, the accuracy of simultaneous MTL predictions is comparable to the accuracy of the STL predictions. In addition, the computational cost of training HydraGNN for MTL is much lower than the original DFT calculations and also lower than training separate STL models for each property.