Ferromagnetic Coupling Research Articles

Layer-resolved vector magnetometry using generalized magneto-optical ellipsometry

We demonstrate the ability of a single magneto-optical reflection experiment to achieve layer-resolved vector magnetometry in multilayer films. For this purpose, we designed, fabricated, and measured a set of epitaxial ferromagnetic/non-magnetic/ferromagnetic heterostructure multilayer samples that exhibit in-plane uniaxial anisotropy and a tunable ferromagnetic interlayer coupling strength through the non-magnetic interlayer. By means of generalized magneto-optical ellipsometry measurements, we obtain the magnetization angles of the two different ferromagnetic layers independently as a function of the applied field. Hereby, we observe that the magnetization switching of one layer can trigger a discontinuous shift of the magnetization angle in the second layer if ferromagnetic interlayer coupling is present. Moreover, we reproduce the obtained behavior using a model of two coupled macrospins, which corroborates even the unexpected aspects of our experimental results and thus reinforces the sensitivity and reliability of our experimental layer-resolved vector magnetometry.

Ferrovalley and Quantum Anomalous Hall Effect in Janus TiTeCl Monolayer.

Ferrovalley materials are garnering significant interest for their potential roles in advancing information processing and enhancing data storage capabilities. This study utilizes first-principles calculations to determine that the Janus monolayer TiTeCl exhibits the properties of a ferrovalley semiconductor. This material demonstrates valley polarization with a notable valley splitting of 80 meV. Additionally, the Berry curvature has been computed across the first Brillouin zone of the monolayer TiTeCl. The research also highlights that topological phase transitions ranging from ferrovalley and half-valley metals to quantum anomalous Hall effect states can occur in monolayer TiTeCl under compressive strains ranging from -1% to 0%. Throughout these strain changes, monolayer TiTeCl maintains its ferromagnetic coupling. These characteristics make monolayer TiTeCl a promising candidate for the development of new valleytronic and topological devices.

Ferromagnetic InMoTe3 monolayer with strain-modulated magnetic interactions and Curie temperature

Two-dimensional (2D) ferromagnetic semiconductors with high Curie temperature (TC) and magnetic tunability have garnered significant research interest owing to their immense potential in the realm of spintronic devices. Herein, 2D Ising ferromagnetic semiconductor InMoTe3 monolayer with robust ferromagnetic coupling and TC above room temperature is predicted. Additionally, it has been shown that biaxial strain can notably affect the magnetic interactions and TC of InMoTe3 monolayer. The findings in this study suggest that InMoTe3 monolayer holds promise as a candidate for spintronic device applications, thereby encouraging further theoretical and experimental investigations in this field.

Enhanced Electron Delocalization Induced by Ferromagnetic Sulfur doped C3N4 Triggers Selective H2O2 Production.

For the 2D metal-free carbon catalysts, the atomic coplanar architecture enables a large number of pz orbitals to overlap laterally, thus forming π-electron delocalization, and the delocalization degree of the central atom dominates the catalytic activity. Herein, designing sulfur-doped defect-rich graphitic carbon nitride (S-Nv-C3N4) materials as a model, we propose a strategy to promote localized electron polarization by enhancing the ferromagnetism of ultra-thin 2D carbon nitride nanosheets. The introduction of sulfur (S) further promotes localized ferromagnetic coupling, thereby inducing long-range ferromagnetic ordering and accelerating the electron interface transport. Meanwhile, the hybridization of sulfur atoms breaks the symmetry and integrity of the unit structure, promotes electron enrichment and stimulating electron delocalization at the active site. This optimization enhances the *OOH desorption, providing a favorable kinetic pathway for the production of hydrogen peroxide (H2O2). Consequently, S-Nv-C3N4 exhibits high selectivity (>95 %) and achieves a superb H2O2 production rate, approaching 4374.8 ppm during continuous electrolysis over 300 hour. According to theoretical calculation and in situ spectroscopy, the ortho-S configuration can provide ferromagnetic perturbation in carbon active centers, leading to the electron delocalization, which optimizes the OOH* adsorption during the catalytic process.

Control of magnetism on the topological SnTe(001) surface by doping, strain, and gap opening

The spin–spin exchange interaction is an important observable that helps in understanding the magnetism of materials. Topological materials are of particular interest due to their strong spin–orbit coupling, which makes them promising for applications in valleytronics and spintronics. In this study, we have theoretically demonstrated the ability to control the switching of ferromagnetic (FM) and antiferromagnetic (AFM) couplings between two magnetic impurities on the (001) surface of the topological crystalline insulator SnTe. By considering experimental parameters such as doping, strain, and gap opening at surface Dirac cones, we provide a generic way to build the FM-AFM and clockwise–counterclockwise phase diagrams of SnTe(001), which are still missing in the literature, especially for the spintronics community.

Thermally induced localization of dopants in a magnetic spin ladder

I unveil an emergent localization phenomenon pertaining to the motion of a dopant in a thermal spin lattice. This is rendered localized by thermal spin fluctuations, whereby it is in stark constrast to the intrinsic origin of Anderson localization for quenched disorder. The system of interest consists of spin-1/2 particles organized in a two-leg ladder with nearest-neighbor Ising interactions J. The motion of a hole—the dopant—is initialized by suddenly removing a spin from the thermal spin ensemble, which then moves along the ladder via nearest-neighbor hopping t. I find that the hole remains for all values of J/t and for nonzero temperatures. The origin is an effective disorder potential seen by the hole and induced by thermal spin fluctuations. Its length scale is found to match with the underlying spin-spin correlation length at low temperatures. For ferromagnetic couplings (J<0), the associated localization length of the hole increases with decreasing temperature and becomes proportional to the correlation length at low temperatures, asymptotically delocalizing at low temperatures. For antiferromagnetic couplings (J>0), there is a smooth crossover between thermal localization at high temperatures to localization driven by the antiferromagnetic order at low temperatures. At infinite temperatures, the dynamics becomes independent of the sign of the spin coupling, whereby the localization length is a universal function of |J|/t, diverging as (t/J)2 for |J|≪t. Finally, I analyze a setup with Rydberg-dressed atoms, which naturally realizes finite-range Ising interactions, accessible in current experimental setups. I show that the discovered localization phenomenon can be probed on experimentally accessible length- and timescales, providing a strong testing ground for my predictions. Published by the American Physical Society 2024

Quantum magnetism in Fe2Cu2 polymeric branched chains: insights from exactly solved Ising-Heisenberg model

The spin-1/2 Ising-Heisenberg branched chain, inspired by the magnetic structure of three isostructural polymeric coordination compounds [(Tp)2Fe2(CN)6X (bdmap)Cu2(H2O)] ⋅ H2O to be further denoted as Fe2Cu2 (Tp = tris(pyrazolyl)hydroborate, bdmapH = 1,3-bis(dimethylamino)-2-propanol, HX = acetic acid, propionic acid or trifluoroacetic acid), is rigorously studied using the transfer-matrix method. The overall ground-state phase diagram reveals three distinct phases: a quantum antiferromagnetic phase, a quantum ferrimagnetic phase and a classical ferromagnetic phase. In the zero-temperature magnetization curve, two quantum ground states are manifested as intermediate plateaus at zero and half of the saturation magnetization, while the magnetization reaches its saturated value within the classical ferromagnetic phase. The bipartite entanglement between nearest-neighbor Heisenberg spins is more pronounced in the quantum ferrimagnetic phase compared to the quantum antiferromagnetic phase due to a fully polarized nature of the Ising spins. A reasonable agreement between theoretical predictions for the spin-1/2 Ising-Heisenberg branched chain and experimental data measured for a temperature dependence of the magnetic susceptibility and a low-temperature magnetization curve suggests strong antiferromagnetic coupling between nearest-neighbor Cu2+-Cu2+ magnetic ions and moderately strong ferromagnetic coupling between nearest-neighbor Cu2+-Fe3+ magnetic ions in the polymeric compounds Fe2Cu2. A thermal entanglement between nearest-neighbor Cu2+-Cu2+ magnetic ions persists up to a relatively high threshold temperature T ≈ 224 K and undergoes a transient magnetic-field-driven strengthening.

Investigation of the structural and magnetic properties of the GdCoC compound featuring excellent cryogenic magnetocaloric performance

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) has been recognized as an environmentally benign and energy-efficient cooling technology. Exploring suitable magnetocaloric materials is a crucial prerequisite for practical MR applications. We have herein provided a systematic investigation of the crystal structure, microstructure, electronic structure, magnetic phase transition, critical behavior, and MCE of the GdCoC compound featuring excellent cryogenic magnetocaloric performance by means of experimental determination and theoretical calculation. The GdCoC compound is crystallized in a simple layered tetragonal crystal structure with a P42/mmc space group and undergoes two successive ferromagnetic (FM) transitions along with a low-temperature weak antiferromagnetic (AFM) transition under low magnetic fields. Density functional theory calculations confirms the FM coupling of the Gd and Co intra-sublattice interactions, whereas AFM coupling for their inter-sublattice interaction. The magnetic transitions are merged in to one under high magnetic fields which has been confirmed to be second-order type and its critical behavior can be understood in the framework of tri-critical mean-field model, whereas the low-temperature weak AFM transition is belonging to the first-order type. The excellent magnetocaloric performance of the GdCoC compound was identified by the parameters of magnetic entropy change, adiabatic temperature change, temperature-averaged entropy change, relative cooling power, and refrigerant capacity, which are superior to most of the well-known magnetocaloric materials with similar working temperatures, making it attractive for practical cryogenic MR applications.

Magnetic properties of CrMnGen (n = 3-20) clusters.

Due to the potential applications in next-generation micro/nano electronic devices and functional materials, magnetic germanium (Ge)-based clusters are receiving increasing attention. In this work, we reported the structures, electronic and magnetic properties of CrMnGen with sizes n = 3-20. Transition metals (TMs) of Cr and Mn tend to stay together and be surrounded by Ge atoms. Small sized clusters with n ≤ 8 prefer to adopt bipyramid-based structures as the motifs with the excess Ge atoms absorbed on the surface. Starting from n = 9, the structure with one TM atom interior appears and persists until n = 16, and for larger sizes n = 17-20, the two TM atoms are full-encapsulated by Ge atoms to form endohedral structures. The Hirshfeld population analyses show that Cr atom always acts as the electron donor, while Mn atom is always the acceptor except for sizes 3 and 6. The average binding energies of these clusters increase with cluster size n, sharing a very similar trend as that of CrMnSin (n = 4-20) clusters, which indicates that it is favorable to form large-sized clusters. CrMnGen (n = 6, 13, 16, 19, and 20) clusters prefer to exhibit ferromagnetic Cr-Mn coupling, while the remaining clusters are ferrimagnetic.

Adsorption of dihalogen molecules X2 (X = F, Cl, Br and I) on the Fe/W(110) substrate

We report on spin-polarized density-functional theory study of adsorption of dihalogen molecules X2 (X = F, Cl, Br and I) on the Fe/W(110) substrate, i.e., X2/Fe/W(110) systems. We considered different molecular orientations and adsorption sites of the halogens and obtained their corresponding ground-state structures. We obtained initial molecular orientation (IMO) and initial adsorption site (IAS), i.e., IMO-IAS combinations that give the minimum energy configurations for each of the X2/Fe/W(110) systems. Our results shows that all the molecules studied in this work are chemisorbed on the Fe surface. Also, the halogen atoms may be adsorbed dissociatively on the hollow sites in such a way that an X2 separates into two X atoms with each of the atoms located at two nearby hollow sites. Similarly, we found IMO-IAS combinations which resulted in a non-dissociative adsorption. In the latter, the pre-relaxed IMO-IAS is maintained even after the structural relaxation. The most stable configuration for the X2 dihalogen molecule in this case is either the top or bridge site while the halogen is in perpendicular orientation to the Fe surface. We conclude therefore that, the final relaxed configurations of the X2 halogen depends on the IMO through which is deposited on the Fe/W(110) substrate. The trend in the adsorption energy EA for the most stable configurations for the dissociative adsorption is EA (F) > EA (Cl) > EA (Br) > EA (I). The trend of EA for non-dissociative adsorption is similar to that of dissociative adsorption, however, the latter is the more energetically favorable. Electronic structure calculations show hybridization between the p and d orbitals of X and Fe atoms respectively. Furthermore, we have found antiferromagnetic coupling between the interfacial W atoms and the Fe overlayer atoms while ferromagnetic coupling is found between the halogens and the Fe atoms. Our work represents a detailed study of adsorption properties of highly reactive halogens in contact with the Fe/W(100) surface.

Regulating interfacial exchange coupling in perpendicular magnetized Fe/DO22-Mn3Ga bilayer films

Manganese gallium alloy with DO22-type chemical ordering (DO22-Mn3Ga) thin film represents an intriguing candidate for fabricating the fundamental composition of magnetic devices. In this paper, we prepare the Fe/DO22-Mn3Ga composite films on thermal MgO (100) substrates with different annealing temperatures and report the interfacial exchange coupling between Fe and DO22-Mn3Ga bilayers. Fe layer thickness and annealing temperature are predominant in regulating the nature and strength of interfacial exchange coupling. It is found that there is a ferromagnetic coupling between Fe and DO22-Mn3Ga layers. Their exchange coupling strength can be regulated by changing the Fe soft magnetic layer thickness in a reasonable range. Additionally, the interface diffusion generated from heat treatment induces to change of the nature of exchange coupling. It is possible that Fe replaces Mn site in the DO22-Mn3Ga unit cell due to smaller atomic radius and higher electronegativity compared with Mn. Antiferromagnetic coupling from annealed Fe/DO22-Mn3Ga film with high perpendicular magnetic anisotropy is technologically important to research synthetic antiferromagnetic structure for spin-transfer-torque magnetoresistive random access memory. Our works is helpful for the exploration of interfacial science and development of magnetic devices.

Sublattice Structure and Topology in Spontaneously Crystallized Electronic States.

The prediction and realization of the quantum anomalous Hall effect are often intimately connected to honeycomb lattices in which the sublattice degree of freedom plays a central role in the nontrivial topology. Two-dimensional Wigner crystals, on the other hand, form triangular lattices without sublattice degrees of freedom, resulting in a topologically trivial state. Here, we discuss the possibility of spontaneously formed honeycomb-lattice crystals that exhibit the quantum anomalous Hall effect. Starting from a single-band system with nontrivial quantum geometry, we derive the mean-field energy functional of a class of crystal states and express it as a model of sublattice pseudospins in momentum space. We find that nontrivial quantum geometry leads to extra terms in the pseudospin model that break an effective "time-reversal symmetry" and favor a topologically nontrivial pseudospin texture. When the effects of these extra terms dominate over the ferromagnetic exchange coupling between pseudospins, the anomalous Hall crystal state becomes energetically favorable over the trivial Wigner crystal state.

On the Origin of the Above-Room-Temperature Magnetism in the 2D van der Waals Ferromagnet Fe3GaTe2.

2D magnetic materials have attracted growing interest driven by their unique properties and potential applications. However, the scarcity of systems exhibiting magnetism at room temperature has limited their practical implementation into functional devices. Here we focus on the van der Waals ferromagnet Fe3GaTe2, which exhibits above-room-temperature magnetism (Tc = 350-380 K) and strong perpendicular anisotropy. Through first-principles calculations, we examine the magnetic properties of Fe3GaTe2 and compare them with those of Fe3GeTe2. Our calculations unveil the microscopic mechanisms governing their magnetic behavior, emphasizing the pivotal role of ferromagnetic in-plane couplings in the stabilization of the elevated Tc in Fe3GaTe2. Additionally, we predict the stability, substantial perpendicular anisotropy, and high Tc of the single-layer Fe3GaTe2. We also demonstrate the potential of strain engineering and electrostatic doping to modulate its magnetic properties. Our results incentivize the isolation of the monolayer and pave the way for the future optimization of Fe3GaTe2 in magnetic and spintronic nanodevices.

Copycat perceptron: Smashing barriers through collective learning.

We characterize the equilibrium properties of a model of y coupled binary perceptrons in the teacher-student scenario, subject to a suitable cost function, with an explicit ferromagnetic coupling proportional to the Hamming distance between the students' weights. In contrast to recent works, we analyze a more general setting in which thermal noise is present that affects each student's generalization performance. In the nonzero temperature regime, we find that the coupling of replicas leads to a bend of the phase diagram towards smaller values of α: This suggests that the free entropy landscape gets smoother around the solution with perfect generalization (i.e., the teacher) at a fixed fraction of examples, allowing standard thermal updating algorithms such as Simulated Annealing to easily reach the teacher solution and avoid getting trapped in metastable states as happens in the unreplicated case, even in the computationally easy regime of the inference phase diagram. These results provide additional analytic and numerical evidence for the recently conjectured Bayes-optimal property of Replicated Simulated Annealing for a sufficient number of replicas. From a learning perspective, these results also suggest that multiple students working together (in this case reviewing the same data) are able to learn the same rule both significantly faster and with fewer examples, a property that could be exploited in the context of cooperative and federated learning.

Magnetic field analysis and modeling of gradient coils based on ferromagnetic coupling inside magnetically shielded cylinder

To accurately analyze the magnetic field of gradient coils inside a magnetically shielded cylinder (MSC) and enhance the precision of remanence compensation, analytical models for calculating the magnetic field of the gradient coils are proposed. First, the coupling between the high-permeability MSC and the coils is accurately analyzed based on Green’ s function method, magnetic field boundary conditions and image method. Secondly, the current density of four commonly used gradient coils is analyzed, and analytical models for the magnetic fields of these four types of gradient coils are established. Finally, the validity of the analytical models is proved through finite element simulations and experimental tests. The experimental results indicate that within the range of x,z∈−80,80mm (± 0.5R), the analytical values have good consistency with experimental values, and the maximum relative error between the two is only 0.91%. The analytical models will be beneficial to the research on the coil optimization design inside the MSC.

CrI3/Arsenene vdW heterstructure: Outstanding physical properties and substantially enhanced magnetic stability

CrI3 monolayer has attracted much attention, but it holds a very low magnetic stability, with Curie temperature (TC) of 45 K, seriously limiting the practical applications. To find solutions for this issue, we construct CrI3/As vdW heterstructures (vdWHs). Calculations show that such vdWHs are half-semiconductor with excellent mechanical property and TC of 75.9 K, improved by 73.3 % compared with CrI3 monolayer. Due to a weak orbital hybridization and coupling occurring between As layer and Iin atom-layer upon vdW interactions, the potential well of interfacial Iin atom-layer would become shallower, which make electron flow out from Iin atoms and magnetic moments increased for Cr and Iin atoms, in turn, the ferromagnetic (FM) coupling in Cr-Iin-Cr superexchange magnetic coupling channel is strengthened. Particularly, the magnetic stability can be significantly raised by compression strain, leading TC up to 111.3 K, which is caused by super-exchange interactions of both Cr-Iin-Cr and Cr-As-Cr channels, and magnetic proximity effects increased further. For light absorption, the red-shifted absorption edge relative to CrI3 monolayer and high absorption peak appearing in visible and infrared region allow heterstructures absorbing solar energy in a wider range. Such a phenomenon can be further strengthened by compression strain.

Cu(II)-Ln(III) (Ln = Gd, Tb and Dy) complexes of an unsymmetrical N2O3 donor ligand: field induced SMM behaviour of Cu(II)-Tb(III) complexes.

Three new hetero-metallic CuII-LnIII complexes [(CuL)Gd(NO3)3(CH3OH)]n (1), [(CuL)Tb(NO3)3(H2O)]·[CuL] (2) and [(CuL)Dy(NO3)3(H2O)]·[CuL] (3) have been synthesized using a mono-nuclear Cu(II) complex, [CuL], of an unsymmetrically di-condensed N2O3 donor Schiff base ligand, N-(3-methoxysalicylidene)-N-(salicylidene)-1,2-ethylenediamine (H2L). Single crystal X-ray crystallography revealed that complex 1 is a nitrate bridged 1D chain of dinuclear Cu(II)-Gd(III) units whereas in 2 and 3, the dinuclear Cu(II)-Ln(III) units are co-crystallized with a [CuL] unit. The Ln(III) centers are nine coordinated with the geometry of a spherical capped square antiprism for Gd and spherical tricapped trigonal prism for Tb and Dy. The geometry of the Cu(II) center is distorted octahedral for complex 1 and distorted square planar for complexes 2 and 3. Temperature-dependent molar magnetic susceptibility measurements in 1-3 revealed the presence of overall ferromagnetic coupling between the Cu(II) and Ln(III) centers. Notably, field induced single-molecule magnet behavior was witnessed in the Tb(III) derivative (2). The ab initio calculations indicated that upon application of an external magnetic field, the tunneling in the ground state of complex 2 gets reduced and thereby field-induced SMM behaviour is observed. Besides, in the case of complex 1, BS-DFT calculations were carried out to gain further insights into the magnetic exchange coupling interactions between the Cu(II) and Gd(III) centers.

High-performance quantum anomalous Hall effect in monolayer Ti2Sb2KRb and Ti2Bi2NaK

Quantum anomalous Hall (QAH) insulators are an ideal platform for developing topological electronic devices, but their low observation temperature limits the applications. In this study, based on first-principles calculations, monolayer Ti2Sb2KRb and Ti2Bi2NaK are demonstrated to be QAH insulators with topological gaps 43 and 57 meV, respectively. Their Chern numbers are calculated to be C=−2. The study of electronic structures indicates that the ferromagnetic topological property is induced by the energy band inversion of dxy and dx2−y2 orbitals for Ti atoms near the Dirac cone. Both monolayer Ti2Sb2KRb and Ti2Bi2NaK exhibit a perpendicular magnetic anisotropy, and their Curie temperatures are estimated to be 480 and 478 K, respectively. The ferromagnetic coupling is induced by the small crystal-field splitting energy caused by Sb and Bi atom's large radius. Our study suggests that monolayer Ti2Sb2KRb and Ti2Bi2NaK are promising candidates for room temperature QAH insulators.

Enhanced Electron Delocalization Induced by Ferromagnetic Sulfur doped C 3 N 4 Triggers Selective H 2 O 2 Production

For the 2D metal‐free carbon catalysts, the atomic coplanar architecture enables a large number of pz orbitals to overlap laterally, thus forming π‐electron delocalization, and the delocalization degree of the central atom dominates the catalytic activity. Herein, designing sulfur‐doped defect‐rich graphitic carbon nitride (S‐Nv‐C3N4) materials as a model, we propose a strategy to promote localized electron polarization by enhancing the ferromagnetism of ultra‐thin 2D carbon nitride nanosheets. The introduction of sulfur (S) further promotes localized ferromagnetic coupling, thereby inducing long‐range ferromagnetic ordering and accelerating the electron interface transport. Meanwhile, the hybridization of sulfur atoms breaks the symmetry and integrity of the unit structure, promotes electron enrichment and stimulating electron delocalization at the active site. This optimization enhances the *OOH desorption, providing a favorable kinetic pathway for the production of hydrogen peroxide (H2O2). Consequently, S‐Nv‐C3N4 exhibits high selectivity (>95%) and achieves a superb H2O2 production rate, approaching 4374.8 ppm during continuous electrolysis over 300‐hour. According to theoretical calculation and in‐situ spectroscopy, the ortho‐S configuration can provide ferromagnetic perturbation in carbon active centers, leading to the electron delocalization, which optimizes the OOH* adsorption during the catalytic process.

Evidence for multiferroicity in single-layer CuCrSe2

Multiferroic materials, which simultaneously exhibit ferroelectricity and magnetism, have attracted substantial attention due to their fascinating physical properties and potential technological applications. With the trends towards device miniaturization, there is an increasing demand for the persistence of multiferroicity in single-layer materials at elevated temperatures. Here, we report high-temperature multiferroicity in single-layer CuCrSe2, which hosts room-temperature ferroelectricity and 120 K ferromagnetism. Notably, the ferromagnetic coupling in single-layer CuCrSe2 is enhanced by the ferroelectricity-induced orbital shift of Cr atoms, which is distinct from both types I and II multiferroicity. These findings are supported by a combination of second-harmonic generation, piezo-response force microscopy, scanning transmission electron microscopy, magnetic, and Hall measurements. Our research provides not only an exemplary platform for delving into intrinsic magnetoelectric interactions at the single-layer limit but also sheds light on potential development of electronic and spintronic devices utilizing two-dimensional multiferroics.