Easy-axis Anisotropy Research Articles

Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3

We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe3. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe3 is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across TN, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above TN. These results suggest that the first-order AF phase transition in FePSe3 may arise from the competition between C3 symmetric AF and C1 symmetric FM spin fluctuations around TN, in place of a conventional second-order AF phase transition.

Field-induced slow magnetic relaxation in a distorted trigonal prismatic cobalt(II) complex

Two mononuclear complexes [M(DQ)(OTf)](OTf)·H2O (M = Co for 1 and Fe for 2, DQ = 6,6′′-di(quinolin-8-yl)-4′-phenyl-2,2′:6′,2′′-terpyridine) with distorted trigonal prismatic geometry have been synthesized and characterized. Crystal structures show that the 3d ions are helically coordinated by a pentadentate DQ and encapsulated by a terminal OTf− anion, resulting in [MN5O] coordination environments. Magnetic measurements and ab initio calculation indicate that complex 1 displays easy axis anisotropy and exhibits field-induced slow magnetic relaxation which is dominated by Raman and direct processes. For 2, no slow magnetic relaxation had been recorded owing to the significant fast quantum tunneling of magnetization.

Magnetic Anisotropy and Relaxation in Four-Coordinate Cobalt(II) Single-Ion Magnets with a [CoIIO4] Core.

A mononuclear four-coordinate Co(II) complex with a [CoIIO4] core, namely, PPN[Li(MeOH)4][Co(L)2] (1) (PPN = bis(phosphoranediyl)iminium; H2L = perfluoropinacol), has been studied by X-ray crystallography, magnetic characterization, and theoretical calculations. This complex presents a severely distorted coordination geometry. The O-Co-O bite angle is 83.42°/83.65°, and the dihedral twist angle between the O-Co-O chelate planes is 55.6°. The structural distortion results in a large easy-axis magnetic anisotropy with D = -104(1) cm-1 and a transverse component with |E| = +4(2) cm-1. Alternating current (ac) susceptibility measurements demonstrate that 1 exhibits slow relaxation of magnetization at zero static field. However, the frequency-dependent out-of-phase (χ"M) susceptibilities of 1 at 0 Oe do not show a characteristic maximum. Upon the application of a dc field or the dilution with a diamagnetic Zn matrix, the quantum tunneling of magnetization (QTM) process can be successfully suppressed. Notably, after dilution with the Zn matrix, the obtained sample exhibits a structure different from that of the pristine complex. In this altered sample, the asymmetric unit does not contain the Li(MeOH)4+ cation, resulting in an O-Co-O bite angle of 86.05° and a dihedral twist angle of 75.84°, thereby leading to an approximate D2d symmetry. Although such differences are not desirable for magnetic studies, this study still gives some insights. Theoretical calculations reveal that the D parameter is governed by the O-Co-O bite angle, in line with our previous report for other tetrahedral Co(II) complex with a [CoIIN4] core. On the other hand, the rhombic component is found to increase as the dihedral angle deviates from 90°. These findings provide valuable guidelines for fine-tuning the magnetic properties of Co(II) complexes.

High easy-plane anisotropy Y-Co intermetallic nanoparticles for boosting gigahertz magnetic loss ability

Soft magnetic materials with ferromagnetic loss abilities have been proven effective in solving gigahertz electromagnetic problems. However, it remains a challenge to synergistically improve their loss capacities and working frequencies due to the Snoek limit. Here, we proposed Y-Co nanoparticles with optimized magnetic anisotropy to break this limit and boost microwave absorption properties. As a result, the soft magnetic phases have manipulated the ferromagnetic loss ability, resulting in the improvement of the easy-plane anisotropy, thus increasing the resonance frequency. A loss peak at 6.2 GHz with a loss capacity (μ") of 0.74 can be found in soft magnetic phase-rich nanoparticles with easy-plane anisotropy, which is higher than that of hard magnetic phase-rich with easy-axis anisotropy (0.76, 4.3 GHz) and Co phase (0.4, 3.95 GHz) nanoparticles. The minimum reflection loss of it can reach −26.4 dB (6.85 GHz) with a broad effective absorption bandwidth of 4.35 GHz (5.85 ∼ 10.20 GHz). The present study provides intrinsic insight into the relationship between easy-plane anisotropy and magnetic loss ability, inspiring the design of high-frequency electromagnetic compatibility materials.

Low-energy spin dynamics in a Kitaev material Na3Ni2BiO6 investigated by nuclear magnetic resonance

Abstract We perform 23Na nuclear magnetic resonance (NMR) and magnetization measurements on an S = 1, quasi-2D honeycomb lattice antiferromagnet Na3Ni2BiO6. A large positive Curie–Weiss constant of 22.9 K is observed. The NMR spectra at low fields are consistent with a zigzag magnetic order, indicating a large easy-axis anisotropy. With the field applied along the c * axis, the NMR spectra confirm the existence of a 1/3-magnetization plateau phase between 5.1 T and 7.1 T. The transition from the zigzag order to the 1/3-magnetization plateau phase is also found to be a first-order type. A monotonic decrease of the spin gap is revealed in the 1/3-magnetization plateau phase, which reaches zero at a quantum critical field H C ≈ 8.35 T before entering the fully polarized phase. These data suggest the existence of exchange frustration in the system along with strong ferromagnetic interactions, hosting the possibility for Kitaev physics. Besides, well below the ordered phase, the 1/T 1 at high fields shows either a level off or an enhancement upon cooling below 3 K, which suggests the existence of low-energy fluctuations.

Two-Dimensional Asymmetric Multiferroics: Unique Way toward Strong Magnetoelectric Coupling and Multistate Memory.

Two-dimensional (2D) materials have provided a fascinating platform for exploring novel multiferroics and emergent magnetoelectric coupling mechanisms. Here, a novel 2D asymmetric multiferroic based on Janus 2D multiferroic MXene-analogous oxynitrides (InTlNO2) is presented by using first-principles calculations. We find three inequivalent phases for InTlNO2, including two metallic phases (p1 and p2) and one semiconducting phase (p3) with a band gap of 0.88 eV. All phases are room-temperature multiferroics with different Curie temperatures, leading to tunability by phase transitions. We show that there is a 90° rotation of the magnetic anisotropy easy axis between p1 and p2, where p1 favors the in-plane and p2 the out-of-plane easy axis. Therefore, the magnetic anisotropy can be tuned by reversing the out-of-plane polarization. Our strategy provides a unique way toward strong magnetoelectric coupling and multistate memory.

Magnetic exchange, anisotropy and excitonic fluctuations in a [NiII7] Anderson wheel

Susceptibility and magnetisation measurements, magnetic neutron spectroscopy and first principles calculations reveal the presence of competing ferro- and antiferromagnetic exchange interactions and both easy plane and easy axis anisotropy in a [Ni7] Anderson wheel.

Control of the geometry and anisotropy driven by the combination of steric and anion coordination effects in CoII complexes with N6-tripodal ligands: the impact of the size of the ligand on the magnetization relaxation time.

Four mononuclear CoII complexes of formula [Co(L)(SCN)2(CH3OH)0.5(H2O)0.5]·1.5H2O·0.75CH3OH (1), [Co(L1)Cl2]·H2O·2CH3CN (2), [Co(L1)(SCN)2]·1.5H2O·CH3OH (3) and [Co(L1)]ClO4·2CH3OH (4) were prepared from the N6-tripodal Schiff base ligands (S)P[N(Me)NC(H)2-Q]3 (L) and (S)P[N(Me)NC(H)1-ISOQ]3 (L1), where Q and ISOQ represent quinolyl and isoquinolyl moieties, respectively. In 1, the L ligand does not coordinate to the CoII ion in a tripodal manner but using a new N,N,S tridentate mode, which is due to the fact that the N6-tripodal coordination promotes a strong steric hindrance between the quinolyl moieties. However, L1 can coordinate to the CoII ions either in a tripodal manner using CoII salts with poorly coordinating anions to give 4 or in a bisbidentate fashion using CoII salt-containing medium to strongly coordinating anions to afford 2 and 3. In the case of L1, there is no steric hindrance between ISOQ moieties after coordination to the CoII ion. The CoII ion exhibits a distorted octahedral geometry for compounds 1-3, with the anions in cis positions for the former and in trans positions for the two latter compounds. Compound 4 shows an intermediate geometry between an octahedral and trigonal prism but closer to the latter one. DC magnetic properties, HFEPR and FIRMS measurements and ab initio calculations demonstrate that distorted octahedral complexes 1-3 exhibit easy-plane magnetic anisotropy (D > 0), whereas compound 4 shows large easy-axis magnetic anisotropy (D < 0). Comparative analysis of the magneto-structural data underlines the important role that is played not only by the coordination geometry but also the electronic effects in determining the anisotropy of the CoII ions. Compounds 2-3 show a field-induced slow relaxation of magnetization. Despite its large easy-axis magnetic anisotropy, compound 4 does not show significant slow relaxation (SMR) above 2 K under zero applied magnetic fields, but its magnetic dilution with ZnII triggers SMR at zero field. Finally, it is worth remarking that compounds 2-4 show smaller relaxation times than the analogous complexes with the tripodal ligand bearing in its arms pyridine instead of isoquinoline moieties, which is most likely due to the increase of the molecular size in the former one.

Tailoring single-ion magnet properties of coordination polymer C11H18DyN3O9 (Dy-CP) using the radial effective charge model (RECM) and superposition model (SPM).

We investigate Dy-based coordination polymer C11H18DyN3O9 (Dy-CP) exhibiting single-ion magnet (SIM) properties, e.g., quantum tunnelling of magnetization (QTM), magnetic anisotropy, magnetic relaxation, and effective energy barrier (Ueff). To elucidate the underlying mechanisms, crystal field parameters (CFPs) for Dy3+ ions were modelled using the radial effective charge model (RECM) and superposition model (SPM), and the computational packages SIMPRE and SPECTRE. The modelled CFPs enable the prediction of the energy levels and associated wave functions, which successfully explain the field-induced Dy-CP SIM properties. The so-calculated magnetic susceptibility and isothermal magnetization match the experimental data reasonably well. The smaller energy separations of the first (Δ0-1 ∼ 31 cm-1) and the second (Δ0-2 = 74 cm-1) excited Kramers doublets suggest small Ueff = 65 cm-1 for Dy-CP. The magnetic moments of Dy3+ ions exhibit an easy-axis type magnetic anisotropy in the ground state, but change orientation in the excited states due to mixing of states from different Kramers doublets. Low-symmetry CF components play a crucial role in connecting different |±MJ〉 states within the ground multiplet, resulting in QTM and magnetic relaxation to the ground state occurring via the excited states. The RECM and SPM calculated CFP sets are standardized employing the 3DD package to enable meaningful comparison and assessing their mutual equivalence. The results demonstrate the correlation between structural and electronic features of the molecule and site symmetry and distortion of the local coordination polyhedra with SIM properties, offering insights for rational design of new SIMs. The importance of considering low-symmetry aspects in CFP modelling for accurate predictions of magnetic properties is highlighted. This study provides deeper understanding of field-induced behaviour in rare-earth-based SIMs and approaches for rationalization of experimentally measured SIMs' properties.

Effect of interlayer exchange coupling interaction on topological phase of a bilayer honeycomb Heisenberg ferromagnet

Layered magnetic topological materials are material systems that exhibit both magnetic ordering and topological properties in their smallest two-dimensional units. Studying these systems may lead to the observation of new physical properties and phenomena, which has attracted considerable attention from researchers. The effect of interlayer exchange coupling interactions on bilayer honeycomb Heisenberg ferromagnets with interlayer coupled topological phase is investigated by using linear spin wave theory. The influence of introducing two additional types of interactions, i.e. interlayer exchange coupling interaction and interlayer easy-axis anisotropy interaction, on the topological phase transition are also explored in this work. By calculating the magnon dispersion relations at various interlayer exchange coupling interaction intensities, it is found that the band gaps of high energy band and low energy band both close and reopen at the Dirac points when the system reaches the critical value of interlayer exchange coupling interaction. In magnon systems, such physical phenomena typically relate to topological phase transitions. When calculating the Berry curvature and Chern numbers for the bands in the aforementioned process, it is found that the sign of the Berry curvature reverses and the Chern numbers change when the critical value of interlayer exchange coupling interaction strength is reached, confirming that a topological phase transition occurs indeed. Introducing two other types of interlayer exchange coupling interactions in this process can lead various novel topological phases to occur in the system. The enhancement of interlayer easy-axis anisotropy interactions is likely to impede the topological phase transitions occurring in the system. We find that a major distinction between bilayer honeycomb ferromagnets and their single-layer counterparts lies in the fact that during a topological phase transition, the sign of the magnon thermal Hall coefficient does not change; on the contrary, abrupt shift in the thermal Hall coefficient curve occurs which can be seen as an indicator of topological phase transition of bilayer honeycomb ferromagnets, and is also reflected in the change in magnon Nernst coefficient. The research results of this work can provide theoretical support for developing novel spintronic devices with enhanced information transmission capabilities by using bilayer honeycomb ferromagnetic materials, and can also provide theoretical reference for studing other bilayer ferromagnetic systems.

Structural, optical and magnetic properties of a new metal-organic CoII-based complex.

A mononuclear cobalt(ii) complex [C5H8N3]2[CoCl4(C5H7N3)2] (I) was synthesized and structurally characterized. Single crystal X-ray diffraction analysis indicates that monometallic Co(ii) ions acted as coordination nodes in a distorted octahedral geometry, giving rise to a supramolecular architecture. The latter is made up of a ½ unit form composed of an anionic element [Co0.5Cl2(C5H7N3)]- and one 2-amino-4-methylpyrimidinium cation [C5H8N3]+. The crystalline arrangement of this compound adopts the sandwich form where inorganic parts are sandwiched between the organic sheets following the [100] direction. More information regarding the structure hierarchy has been supplied based on Hirshfeld surface analysis; the X⋯H (X = N, Cl) interactions play a crucial role in stabilizing the self-assembly process of I, complemented by the intervention of π⋯π electrostatic interaction created between organic entities. Thermal analyses were carried out to study the thermal behavior process. Static magnetic measurements and ab initio calculations of compound I revealed the easy-axis anisotropy character of the central Co(ii) ion. Two-channel field-induced slow-magnetic relaxation was observed; the high-frequency channel is characterized by underbarrier relaxation with U eff = 16.5 cm-1, and the low-frequency channel involves a direct relaxation process affected by the phonon-bottleneck effect.

Releasing antiferromagnetic skyrmions from local magnetic-anisotropy defects

Lattice defects may work as a kind of apparatus for catching topological excitations, preventing their escape. So, the problem of removing skyrmions from eventual local defects in magnetic materials must be closely related to new technologies such as skyrmionic. Here, we examine the conditions for drawing a skyrmion from a magnetic impurity in a two-dimensional antiferromagnetic system by applying spin-polarized currents (SPC). Two types of impurities are investigated (local easy-axis and easy-plane anisotropy defects). Also, two methods to release the skyrmion with SPC are explored. In principle, our results could be qualitatively relevant to any other type of lattice defect.

Leveraging available data for efficient exploration of materials space using Machine Learning: A case study for identifying rare earth-free permanent magnets

It is shown that even the relatively small amount of available materials data can be innovatively utilized to explore the materials space in order to identify materials with desired target properties. As an example of this, data from the novomag and Novamag databases are used to train random forest and neural network models which can predict thermodynamic stability, and magnetic properties of materials. Performance of these models are tested thoroughly and are found satisfactory. These models are subsequently used to interpolate within the above databases, and to extrapolate to parts of the materials composition and structure space not covered in these databases, to identify stable, magnetic materials that have large saturation magnetization and large easy-axis anisotropy. Screening 686 materials via the trained models, and subsequently performing first principles calculations, 21 new candidate materials for rare earth free permanent magnet are identified. Some of these materials have anisotropy constants as large as 5 and 6 MJ m−3, larger than that of the most widely used permanent magnet Nd2Fe14B. This simple approach can be used to screen materials with other functionalities in future.

Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers.

The advent of twist engineering in two-dimensional crystals enables the design of van der Waals heterostructures with emergent properties. In the case of magnets, this approach can afford artificial antiferromagnets with tailored spin arrangements. Here we fabricate an orthogonally twisted bilayer by twisting two CrSBr ferromagnetic monolayers with an easy-axis in-plane spin anisotropy by 90°. The magnetotransport properties reveal multistep magnetization switching with a magnetic hysteresis opening, which is absent in the pristine case. By tuning the magnetic field, we modulate the remanent state and coercivity and select between hysteretic and non-hysteretic magnetoresistance scenarios. This complexity pinpoints spin anisotropy as a key aspect in twisted magnetic superlattices. Our results highlight control over the magnetic properties in van der Waals heterostructures, leading to a variety of field-induced phenomena and opening a fruitful playground for creating desired magnetic symmetries and manipulating non-collinear magnetic configurations.

First-principles study of magnetic interactions and excitations in antiferromagnetic van der Waals material MPX3 (M=Mn, Fe, Co, Ni; X=S, Se)

Transition metal phosphorus trichalcogenides MPX3 (M = Mn, Fe, Co, Ni; X = S, Se), as layered van der Waals antiferromagnetic (AFM) materials, have emerged as a promising platform for exploring two-dimensional (2D) magnetism. Based on density functional theory, we present a comprehensive investigation of the electronic and magnetic properties of MPX3. We calculated the spin exchange interactions as well as magnetocrystalline anisotropy energy. The numerical results reveal that J 3 is AFM in all cases, and J 2 is significantly smaller compared to both J 3 and J 1. This behavior can be understood with regard to exchange paths and electron filling. Compared to other materials within this family, FePS3 and CoPS3 demonstrate significant easy-axis anisotropy. Using the obtained parameters, we estimated the Néel temperature T N and Curie-Weiss temperature , and the results are in good agreement with the experimental observations. We further calculated the magnon spectra and successfully reproduce several typical features observed experimentally. Finally, we give helpful suggestions for the strong constraints about the range of non-negligible magnetic interactions based on the relations between magnon eigenvalues at high-symmetry k points in honeycomb lattices.

Formation of a single-ion magnet in the lanthanum calcium silicate apatite structure by the cobalt oxide doping

A small amount of cobalt was incorporated into the lanthanum calcium silicate apatite structure by annealing at 1500 °C in argon. The compound exhibits easy-axis magnetic anisotropy with a zero-field splitting parameter 2D of −60 cm−1 and field- induced slow relaxation of magnetization with a remagnetization energy barrier of 58–63 cm−1. Thereby, for the first time, a cobalt-based single-ion magnet was created in silicate.

Non uniform micromagnetic states in spheroidal magnetite nanoparticles

Non uniform micromagnetic configurations are calculated in spheroidal magnetite nanoparticles with different semiaxis ratios, a/b = 0.5 – 2.0, using Landau-Lifshitz-Gilbert equation. We take into account a complex, combined type of particle magnetic anisotropy, associated with the presence of both cubic magneto-crystalline anisotropy and shape anisotropy of spheroidal magnetite nanoparticle. It is shown that, depending on the particle aspect ratio, there are different types of vortex states, which compete in energy with the uniform magnetization. It is also found that the single-domain diameters of magnetite nanoparticles significantly depend on the orientation of the cubic easy anisotropy axes with respect to the symmetry axis of a spheroid. The calculated single-domain diameters of spheroidal nanoparticles are in satisfactory agreement with analytical estimates.

Syntheses, Structures, and Magnetic Properties of Three Cyano-Bridged FeII-MoIII Single-Molecule Magnets.

Three new cyano-bridged FeII-MoIII complexes assembled from the [MoIII(CN)7]4- unit, FeII ions, and three pentadentate N3O2 ligands, namely {[Fe2H3(dapab)2][Mo(CN)6]}n·2H2O·3.5MeCN (1), [Fe(H2dapb)(H2O)][Fe(Hdapb)(H2O)][Mo(CN)6]·4H2O·3MeCN (2), and [Fe(H2dapba)(H2O)]2[Mo(CN)7]·6H2O (3) (H2dapab = 2,6-diacetylpyridine bis(2-aminobenzoylhydrazone), H2dapb = 2,6-diacetylpyridine bis(benzoylhydrazone), H2dapba = 2,6-diacetylpyridine bis(4-aminobenzoylhydrazone)), have been synthesized and characterized. Single-crystal structure analyses suggest that complex 1 contains a one-dimensional (1D) chain structure where two FeII ions are bridged by the in situ generated [MoIII(CN)6]3- unit through two trans-cyanide groups into trinuclear Fe2IIMoIII clusters that are further linked by the amino of the ligand into an infinite chain. Complexes 2 and 3 are cyano-bridged Fe2IIMoIII trinuclear clusters with two FeII ions connected by the [MoIII(CN)6]3- and [MoIII(CN)7]4- units, respectively. Direct current magnetic studies confirmed the ferromagnetic interactions between the cyano-bridged FeII and MoIII centers and significant easy-axis magnetic anisotropy for all three complexes. Furthermore, complexes 1-3 exhibit slow magnetic relaxation under a zero dc field, with relaxation barriers of 42.3, 21.6, and 14.4 K, respectively, making them the first examples of cyano-bridged FeII-MoIII single-molecule magnets.

Ferromagnetic Interlayer Coupling in CrSBr Crystals Irradiated by Ions.

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly, the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggests that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

Spatial Spin-Modulated Structure of Bi1 – xSrxFeO3 – y (x = 0, 0.05, and 0.10) Multiferroics

The room-temperature X-ray and Mössbauer data on the BiFeO3, Bi0.95Sr0.05FeO3 – y, and Bi0.90Sr0.10FeO3 – y multiferroics obtained by solid-phase synthesis are presented. The samples have a rhombohedral crystal structure with the sp. gr. R3c. The lattice parameter ah is invariable, while the parameter ch decreases with increasing strontium content. The 57Fe Mössbauer spectra recorded at a temperature of 295 K have been interpreted within the cycloid spatial spin-modulated structure model. It has been established that the easy-axis magnetic anisotropy is implemented in the investigated multiferroics. The anharmonicity parameter m of the spin-modulated structure has been measured. Upon replacement of trivalent Bi3+ ions in small amounts (x = 0–0.10) with divalent Sr2+ ions, the parameter m increases by a factor of more than 3: from 0.10(4) at x = 0.00 to 0.36(10) at x = 0.10.