Strength Of Magnetic Interactions Research Articles

Strong In-Plane Magnetization and Spin Polarization in (Co0.15Fe0.85)5GeTe2/Graphene van der Waals Heterostructure Spin-Valve at Room Temperature.

Van der Waals (vdW) magnets are promising, because of their tunable magnetic properties with doping or alloy composition, where the strength of magnetic interactions, their symmetry, and magnetic anisotropy can be tuned according to the desired application. However, so far, most of the vdW magnet-based spintronic devices have been limited to cryogenic temperatures with magnetic anisotropies favoring out-of-plane or canted orientation of the magnetization. Here, we report beyond room-temperature lateral spin-valve devices with strong in-plane magnetization and spin polarization of the vdW ferromagnet (Co0.15Fe0.85)5GeTe2 (CFGT) in heterostructures with graphene. Density functional theory (DFT) calculations show that the magnitude of the anisotropy depends on the Co concentration and is caused by the substitution of Co in the outermost Fe layer. Magnetization measurements reveal the above room-temperature ferromagnetism in CFGT and clear remanence at room temperature. Heterostructures consisting of CFGT nanolayers and graphene were used to experimentally realize basic building blocks for spin valve devices, such as efficient spin injection and detection. Further analysis of spin transport and Hanle spin precession measurements reveals a strong in-plane magnetization with negative spin polarization at the interface with graphene, which is supported by the calculated spin-polarized density of states of CFGT. The in-plane magnetization of CFGT at room temperature proves its usefulness in graphene lateral spin-valve devices, thus revealing its potential application in spintronic technologies.

Polymer-nanoparticle thin scaffolds with any-shape magnetic field gradients

This study investigates the self-assembly of magnetic nanoparticles into a flat nanocomposite surface with concealed magnetic patterns. The application of a multilayer stamping medium and the presence of a magnetic field gradient would enable the attraction and organization of superparamagnetic iron oxide nanoparticles (SPIONs) onto predefined areas, corresponding to a pre-fabricated magnetic layer. The experimental approach involved preparing patterned surfaces by applying a multilayer of poly(ethyleneimine) (PEI) and poly(styrene sulfonate) (PSS) as a stamping medium. The SPIONs selectively adhered to the surface, mirroring the underlying magnetic layer shape. Subsequently, these SPION were subjected to an 80 °C treatment in the presence of an external magnetic field to enhance their magnetic properties, and then covered with a polystyrene layer. Using atomic force microscopy, magnetic force microscopy and dark field mode optical microscopy we demonstrated that obtained patterned magnetic field gradients are strong enough to attract and organize magnetic material on top of them, even when being covered with polymeric layers. Moreover, our findings show that the oriented deposition of magnetic nanoparticles is primarily determined by the strength of magnetic interactions between the pattern and nanoparticles, independently of surface topography (with roughness of 0.21 ± 0.06 nm) or Brownian motions.

Regime change in the aggregate structures of spherical magnetic Janus particles (quasi-2D Monte Carlo simulations)

We have investigated a suspension composed of spherical magnetic Janus particles in thermodynamic equilibrium. The magnetic moment of Janus particles treated here is located at a position shifted from the particle centre, which is on the line perpendicular to the magnetic moment direction. We conducted quasi-2D Monte Carlo simulations to investigate the change in the regime of aggregate structures and the dependence of the regime on various factors, such as the strength of the magnetic particle-particle interaction, magnetic particle-field interaction, and the distance of the magnetic moment from the particle centre. In the absence of an applied magnetic field, the Janus particles aggregate to form stable clusters composed of a few particles with increased values of the magnetic particle-particle interaction strength. As the strength of the magnetic field is increased, these clusters collapse, and chain-like clusters are formed aligned in the direction of the magnetic field. The appearance of this regime change requires a stronger magnetic field as the position of the magnetic moment approaches the particle surface. This can be attributed to the fact that the magnetic interaction between neighbouring particles becomes more dominant as the magnetic moments of the constituent particles in the cluster units approach each other. Highlights We have addressed the aggregation phenomena in a suspension composed of spherical magnetic Janus particles. We have investigated the regime change in the aggregate structures of the Janus particles under various conditions. In a strong magnetic field, thick chain-like clusters are formed as the magnetic moment is shifted away from the particle centre. The present results are useful for elucidating the internal structure of complex aggregates in 3D systems.

Unexpected coexisting solid solutions in the quasi-binaryAg(II)F2/Cu(II)F2 phase diagram.

High-temperature solid-state reaction between orthorhombic AgF2 and monoclinic CuF2 (y = 0.15, 0.3, 0.4, 0.5) in a fluorine atmosphere resulted in coexisting solid solutions of Cu-poor orthorhombic and Cu-rich monoclinic phases with stoichiometry Ag1-xCuxF2. Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF2:Cu) appears to be at an upper limit of Cu concentration of 30 mol % (Ag0.7Cu0.3F2), while the monoclinic phase (CuF2:Ag) can form a nearly stoichiometric Cu: Ag = 1: 1 solid solution (Cu0.56Ag0.44F2), preserving the CuF2 crystal structure. Experimental data and DFT calculations showed that AgF2:Cu and CuF2:Ag solid solutions deviate from the classical Vegard's law. Magnetic measurements of Ag1-xCuxF2 showed that the Néel temperature (TN) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag1-xCuxF2 showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J2D| in both structures. Electrical conductivity measurements of Ag0.85Cu0.15F2 showed a ca. 2-fold increase in specific ionic conductivity (3.71·10-13 ± 2.6·10-15 S/cm) as compared to pure AgF2 (1.85·10-13 ± 1.2·10-15 S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample.

Memristive field effect in a single and multilayer neural network with different connection topologies

The network connectivities play an important role in displaying the vastly different collective dynamics in many complex systems. To demonstrate the presence of different dynamical states and their transition, we consider Hindmarsh-Rose neurons, which are linked up by electromagnetic interactions. The dynamical behaviors of the proposed system are specifically explored under three distinct network connectivities: Regular(RG), small-world (SW), and random (RAND) interactions. We discover that increasing coupling intensity illustrates the dynamical transition from the desynchronization state to the traveling wave state for all the considered network interactions. Particularly, the regular network shows the emergence of spatial symmetric patterns that transit to a homogeneous steady state while increasing the strength of magnetic field interaction. In addition, the transition from an asymmetric pattern to an in-homogeneous steady state is noticed for small-world and random networks. The investigation is then extended to a three-layer multiplex network in which nodes interact electromagnetically within (intra-) layers and magnetically between (inter-) layers, since many complex networks in the real world have large numbers of nodes organized (or located) in different subnetworks. We show that increasing the coupling intensity leads to synchronization in all layers and, eventually, a rest state at high coupling strength.

Interparticle magnetic interactions and magnetic field dependence of superparamagnetic blocking temperature in ferrihydrite nanoparticle powder systems

In this study, nanoparticles of initial synthetic ferrihydrite have been coated with arabinogalactan. The synthesized series of samples with different degrees of coverage of particles has been characterized by X-ray photoelectron spectroscopy, Mössbauer spectroscopy, transmission electron microscopy and magnetometry. The superparamagnetic blocking temperature decreases monotonically with an increase in the degree of coverage of ferrihydrite particles, which is unambiguously related to the different role of the interparticle magnetic interactions in the investigated powder systems. Analysis of the field dependence of the blocking temperature within the random anisotropy model has shown that an increase in the degree of coverage of ferrihydrite particles leads to a decrease in the size of a cluster in which the behaviors of the nanoparticle magnetic moments are correlated. The results obtained have shown the possibility of effective control of the strength of magnetic interparticle interactions in powder ferrihydrite systems by coating nanoparticles with arabinogalactan.

球状磁性ヤヌス粒子からなる分散系を対象にした擬2次元モンテカルロ・シミュレーション（磁気モーメントが粒子中心から横方向にずれた粒子モデル）

I have addressed a magnetic Janus particle suspension in the thermodynamic equilibrium situation. The magnetic moment of Janus particles treated in the present study is shifted from the particle center, which is perpendicular to the magnetic moment direction. From the viewpoint of elucidating the internal structure of particle aggregates in a three-dimensional (3D) system, I have treated a dispersion composed of magnetic Janus particles with a full 3D rotational ability in a quasi-two-dimensional (2D) system. The quasi-2D Monte Carlo simulations have been performed in order to investigate the dependence of a regime change in the aggregate structures on various factors such as the magnetic particle-particle interaction strength, magnetic particle-field interaction strength and the distance of the magnetic moment from the particle center. In no applied magnetic field, the Janus particles aggregate to form a stable cluster unit composed of 2-4 particles if the magnetic particle-particle interaction strength is increased. As the magnetic field is increased, the cluster units collapse and chain-like clusters are formed aligned in the magnetic field direction. In the situation of a strong magnetic field, the chain-like clusters composed of Janus particles tend to be thicker as the magnetic moment is shifted away from the particle center. This is because the magnetic interaction between neighboring particles increases due to the magnetic moments of constituent particles in chain-like clusters being closer to each other. The present results are useful for elucidating the internal structure of complex aggregates in a 3D system.

Effect of doping concentration on the antiferromagnetic transition temperature of multiferroic YMnO3: A first-principles study

Hexagonal YMnO3 is an attractive multiferroic material whose polarization and magnetic orders are strongly coupled with each other. Doping different cations in hexagonal YMnO3 would influence its properties, especially the magnetic transition temperature (the Néel temperature). In this work, by using first-principles calculations, we not only studied the effects of doping Fe and Cr ions at the Mn-site on the Néel temperature, but also revealed that the variations of electronic structures and the extents of orbital coupling overlaps are both factors to determine the magnetic interaction strength and Néel temperature for YMn1-x(Fe,Cr)xO3 (0 ≤ x < 0.3). Besides the Mn-site, doping rare earth element (La) at the non-magnetic Y-site also influences the Néel temperature. These results are important to provide more understanding about the performance of multiferroic materials.

Spectroscopic and magnetic investigations of the dilute magnetically doped semiconductors BaSn1-xMnxO3 (0.02 ≤ x ≤ 0.1)

Dilute magnetically doped semiconductors (DMSs) are key to fabricating novel spintronic devices with high functionalities, which could be utilized for the next-generation technological development. BaSnO3 is a transparent wide band gap (∼3.1 eV) semiconductor, in which magnetism can be induced through a subtle amount of magnetic impurity doping without deterring its optical transparency. In this work, we report an experimental investigation on the optical spectroscopic and magnetic properties of the DMSs BaSn1-xMnxO3 (0 ≤ x ≤ 0.1), synthesized using standard solid-state-reaction method at high-temperature and ambient pressure, through various spectroscopic, electron paramagnetic resonance (EPR) and temperature- and field-dependent magnetization measurements. All samples are found to be crystallize in the cubic structure with space group Pm3‾m, in which Mn ions are stabilized in the 4+ valence state. We find that the optical band gap monotonically decreases (from 3.1 eV to 2.08 eV) with increase in Mn doping concentration, which in turn leads to an increase in absorption efficiency in the visible region. An enhanced sp-d hybridization between localized d electrons of Mn ions and band electrons is likely the main reason for such band gap reduction. Interestingly, theoretical simulation of EPR spectra suggests that the signal is primarily arising from a combination of two spin systems, i.e., Mn4+ ions and singly ionized oxygen vacancies. The strength of magnetic interaction between Mn4+ ions is also increased for higher doped samples. Moreover, the nature of magnetic interaction is predominantly antiferromagnetic in higher doped samples, as corroborated by the negative value of Curie-Weiss temperature from the inverse susceptibility fitting.

Aggregation phenomena and regime change in a magnetic cubic particle suspension in an alternating magnetic field via quasi-two-dimensional Brownian dynamics

We have addressed the aggregation phenomena of a cubic haematite particle suspension in an alternating magnetic field in order to investigate the regime change in the aggregate structure. Brownian dynamics simulations have been performed for a quasi-two-dimensional system with an alternating magnetic field applied in the plane of the system. In a weak alternating field, the cubic particles aggregate to form closely-packed structures tending to an aligned face-to-face configuration with increasing magnetic particle-particle interaction strength. In a closely-packed cluster, the orientation of the magnetic moment of a constituent particle tends to be more strongly influenced by the magnetic interaction between neighbouring particles and less influenced by the direction of the alternating magnetic field. As the strength of the applied field is increased, the particle moments become more restricted to the direction of the alternating field and the closely-packed structures transform into elongated aggregates with an offset face-to-face configuration. As the direction of the alternating magnetic field switches from the positive to the negative x-direction, the elongated aggregates tend to collapse temporarily and when the orientation of the moments is again strongly restricted to the switched field direction, elongated aggregate structures with offset face-to-face contact are re-formed in the system. Highlights of the present paper We have investigated a regime change in the internal structure of cubic particle aggregates in an alternating magnetic field. As the magnetic field strength is increased, closely-packed aggregates with an aligned face-to-face configuration transform into elongated aggregates with an offset face-to-face configuration. We have clarified that the elongated aggregates collapse temporarily and then re-form when the direction of the alternating magnetic field switches. We have suggested that the frequency of the magnetic field may be used as a means for controlling the local internal structure of particle aggregates.

Improvement of trapping performance of magnetic particles by magnetic multi-poles via Brownian dynamics simulations of magnetic rod-like particles in a Hagen-Poiseuille flow

In the present study we have implemented a Brownian dynamics simulation of a Hagen-Poiseuille flow in a cylindrical vessel and have investigated the feasibility of multi-pairs of magnetic poles for exhibiting a good particle trapping performance. We have focused on two representative types of configuration consisting of two pairs of magnetic poles. The first is a vertical configuration of two pairs of poles arranged perpendicular to the direction of the flow field. The second is a parallel configuration, where each pair of magnetic poles is arranged along the surface of the vessel wall. In the case of the vertical pole arrangement, although densely-packed aggregates were formed between the poles, their location was unstable in the flow field and they tended to be carried away downstream. In contrast, a densely-packed cluster formed in the parallel arrangement tended locate in a more stable manner along the surface of the wall where the flow field is weaker. We conclude that under appropriate conditions of the magnetic particle-particle interaction strength, the applied magnetic field strength and the pole separation distance, the parallel arrangement of magnetic poles is the configuration of choice for giving rise to a superior trapping performance.

Geometric Frustration on the Trillium Lattice in a Magnetic Metal-Organic Framework.

In the dense metal-organic framework Na[Mn(HCOO)_{3}], Mn^{2+} ions (S=5/2) occupy the nodes of a "trillium" net. We show that the system is strongly magnetically frustrated: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at 1/3-saturation magnetization. A simple model of nearest-neighbor Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions.

Angle dependent field-driven reorientation transitions in uniaxial antiferromagnet MnBi2Te4 single crystal

MnBi2Te4, a two-dimensional magnetic topological insulator with a uniaxial antiferromagnetic structure, is an ideal platform to realize quantum anomalous Hall effects. However, the strength of magnetic interactions is not clear yet. We performed systematic studies on the magnetization and angle dependent magnetotransport on MnBi2Te4 single crystals. The results show that the direction of the magnetic field has significant effects on the critical field values and the magnetic structure of this compound, which lead to different magnetotransport behaviors. The field-driven reorientation transitions can be utilized to estimate the antiferromagnetic interlayer exchange interaction coupling Jc and uniaxial magnetic anisotropy D. The obtained Hamiltonian can well explain the experimental data by Monte Carlo simulations. Our comprehensive studies on the field-driven magnetic transition phenomenon in MnBi2Te4 provide a general approach for other topological systems with antiferromagnetism.

Milli-magnetic monopole dark matter and the survival of galactic magnetic fields

Dark sectors with Abelian gauge symmetries can interact with ordinary matter via kinetic mixing. In such scenarios, magnetic monopoles of a broken dark U(1) will appear in our sector as confined milli-magnetically charged objects under ordinary electromagnetism. Halo ellipticity constraints are shown to significantly bound the strength of dark magnetic Coulomb monopole interactions. The bound magnetic monopole ground state, which in vacuum is stable and has no magnetic charge or moment, is shown to become quantum mechanically unstable in the presence of an external, ordinary magnetic field. If these states contribute sizably to the local dark matter density, they extract significant energy from the galactic magnetic field. We revise and extend this “Parker Bound” on galactic magnetic energy loss to milli-magnetic monopoles which leads to the strongest existing constraints on these states, satisfying our halo ellipticity bounds, over a wide range of magnetic monopole masses.

Geometric frustration on the trillium lattice in a magnetic metal–organic framework

In the dense metal-organic framework Na[Mn(HCOO)$_3$], Mn$^{2+}$ ions ($S=\frac{5}{2}$) occupy the nodes of a `trillium' hyperkagome net. We show that this material exhibits a variety of behaviour characteristic of geometric frustration: the N\'eel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the N\'eel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at $\frac{1}{3}$-saturation magnetisation. We demonstrate that a simple nearest-neighbour Heisenberg antiferromagnet model accounts quantitatively for each observation, and hence Na[Mn(HCOO)$_3$] is the first experimental realisation of this model on the trillium net. We develop a mapping between this trillium model and that on the two-dimensional Shastry-Sutherland lattice, and demonstrate how both link geometric frustration within the classical spin liquid regime to a strong magnetocaloric response at low fields.

Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, a Canted Antiferromagnet with a Hard Ferromagnetic Component.

Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores AM2+M3+F6 offer a convenient platform for probing the interplay between electron distribution over M2+ and M3+ sites and structural distortions; however, they are limited to compounds with M2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl- anions under hydrothermal conditions for the mild reduction of Mn2O3 in concentrated HF to obtain the CsMn2+Mn3+F6 composition as a phase pure sample and study its properties. The magnetism of CsMn2F6 was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn2F6 using powder neutron diffraction, which revealed successive long-range ordering of the Mn2+ and Mn3+ sites that is accompanied by a second transition. The role and strength of magnetic exchange interactions were characterized using DFT calculations.

Effect of nanoscale size reduction on the magnetic properties of Pr0.6Sr0.4MnO3

The ferromagnetic Pr0.6Sr0.4MnO3 sample has been subjected to time bound high energy planetary ball milling with an aim to reduce the particle size. When the particle size is analyzed as a function of milling time, it is found that particle size decreases with increasing milling time and then saturates at a certain value. The implications of the decrease in particle size on magnetic properties have been carefully studied by analyzing the magnetization data in detail. With a decrease in particle size, the Curie temperature, TC and the magnitude of magnetization drops. A deviation in inverse magnetic susceptibility from the Curie-Weiss behaviour has been noticed for nanoparticles which suggest the presence of quenched disorder due to particle size reduction. The strength of magnetic interactions switches from long range to short range with a systematic decrease in particle size. The observed properties can be explained considering the core–shell structure of the nanoparticle where the surface defects strongly influence the degree of disorder in the system.

Magnetic excitations of the hybrid multiferroic (ND4)2FeCl5·D2O

We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound (ND4)2FeCl5D2O in the zero-field incommensurate cycloidal phase and the high-field quasi-collinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic a-axis to the c-axis. To account for such change of polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyalloshinskii-Moriya interaction to the p-d hybridization model. We perform a detailed analysis of the inelastic neutron data of (ND4)2FeCl5D2O using linear spin-wave theory to quantify magnetic interaction strengths and investigate possible impact of different multiferroic mechanisms on the magnetic couplings. Our result reveals that the spin dynamics of both multiferroic phases can be well-described by a Heisenberg Hamiltonian with an easy-plane anisotropy. We do not find notable differences between the optimal model parameters of the two phases. The hierarchy of exchange couplings and the balance among frustrated interactions remain the same between two phases, suggesting that magnetic interactions in (ND4)2FeCl5D2O are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field.

Spin-ice physics in cadmium cyanide

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1–5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.

The complementary effects of Fe and metalloids on the saturation magnetization of Fe-based amorphous alloys

Developing Fe-based amorphous alloys with high saturation flux density (Bs) have been challenging topics for a long time. Metalloid elements like B, Si, C, and P are key elements for Fe-based amorphous alloys, but the addition of multiple metalloids would significantly complicate the effects of each metalloid on saturation flux density. Herein, Fe83-85(B, C, Si)14–16P1 amorphous alloys with high Bs of 1.61–1.71 T and low coercivity (Hc) of 2.2–8.7 A/m were successfully developed. Fe and metalloids were found to have strong interrelated effects on the total magnetic moment and the strength of magnetic exchange interaction, which largely determine the Bs of Fe-based amorphous alloys. It was revealed that for Fe-rich amorphous alloys, the effects of metalloids on Bs are much more significant than that of Fe, and the effects are largely dependent on Fe content. The complementary effects of metalloids on Bs and the Fe-sensitive behavior of these effects, as well as the underlying mechanisms, have been revealed. These results pave a new way to understand the composition-property relationship, and would help design and develop high-performance Fe-based soft magnetic amorphous alloys.