Ferromagnetic Ground State Research Articles

Raman scattering study and lattice-dynamics calculations in YTiO3: Precursor of the magnetic phase transition in the phonon anomalies

The origin of the low-temperature ferromagnetic instability in YTiO3 driven by the interplay between lattice, orbital, spin, and charge degrees of freedom is on the background of lattice dynamics. We present a comprehensive study of temperature-dependent polarized Raman scattering in a nearly stoichiometric orthorhombic YTiO3 single crystal. The lattice-dynamics calculations authenticate the assignment for the observed Ag- and B2g-symmetry modes in terms of the atoms vibrational patterns. The obtained results suggest that trigonal GdFeO3-type distortions become unstable against tetragonal Jahn-Teller distortions, leading to the consequent orbital structure refinement and ferromagnetic ground state in YTiO3.

Metallocyclic CuII-LnIII Single-Molecule Magnets from the Self-Assembly of 1,4-Diformylnaphthalene-2,3-diol.

We report the synthesis and characterization of seven new tetranuclear 3d–4f complexes derived from the 3:3:1 reaction of 1,4-diformylnaphthalene-2,3-diol (H2L) with copper(II) nitrate and a lanthanide salt, Ln = Tb [L3Cu3TbCl2(NO3)2(H2O)2] (C1), Ho [L3Cu3HoCl3(H2O)3(MeOH)](H2O) (C2), Er [L3Cu3ErCl3(H2O)3.5(MeOH)0.5](H2O) (C3), Gd [L3Cu3Gd(NO3)2(H2O)2(MeOH)](NO3) (C4), Dy [L3Cu3Dy(NO3)2(H2O)2(MeOH)](NO3) (C5), Yb [L3Cu3Yb(NO3)2(H2O)2(MeOH)](NO3) (C6), and La [L3Cu3La(NO3)2(H2O)2(MeOH)](NO3) (C7). Structural elucidation showed that the self-assembly using the acyclic ligand system was successful for all seven complexes, which exhibit the same near-planar Cu3LnO12 core. Five complexes (C1, C2, and C4–C6) were magnetically characterized at 300 K and 1.8 K. Complexes C1, C4, and C5 were observed to have ferromagnetic ground states and showed appreciable frequency dependence in their AC magnetic measurements, which yielded effective barriers between 7.82(4) and 13.2(3) K, confirming the presence of single-molecule magnet properties.

Magnetic and magnetocaloric properties of nanocrystalline sample of a glassy ferromagnetic compound: Modification of short range ordering

The modification of magnetic interactions due to the reduction of particle size has been explored in the case of nanocrystalline Gd0.5Sr0.5MnO3 compound. In contrast to the normal ferromagnetic or antiferromagnetic compounds, the studied nanoparticle exhibits quite different magnetic and magnetocaloric properties. The experimental outcomes are also compared with its bulk counterparts having disordered ferromagnetic ground state. Additionally, at the cryogenic temperature range, the significant magnetocaloric effect in both the samples indicate the applicability as a suitable magnetic refrigerant material. Moreover, the desertion nature of the magnetic hysteresis loop due to the field cycling indicates the superiority of the nanocrystalline compound over its bulk counterpart.

A combined first principles study of the structural, magnetic, and phonon properties of monolayer CrI3.

The first magnetic 2D material discovered, monolayer (ML) CrI3, is particularly fascinating due to its ground state ferromagnetism. However, because ML materials are difficult to probe experimentally, much remains unresolved about ML CrI3's structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin-phonon and spin-lattice coupling of ML CrI3. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI3's ML geometry with DMC accuracy, yielding lattice parameters in good agreement with recently published STM measurements-an accomplishment given the ∼10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find that previous DFT predictions of ML CrI3's magnetic spin moments are correct on average across a unit cell but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts that magnetic moments in ML CrI3 are 3.62 μB per chromium and -0.145 μB per iodine, both larger than previous DFT predictions. The large disparate moments together with the large spin-orbit coupling of CrI3's I-p orbital suggest a ligand superexchange-dominated magnetic anisotropy in ML CrI3, corroborating recent observations of magnons in its 2D limit. We also find that ML CrI3 exhibits a substantial spin-phonon coupling of ∼3.32 cm-1. Our work, thus, establishes many of ML CrI3's key properties, while also continuing to demonstrate the pivotal role that DMC can assume in the study of magnetic and other 2D materials.

Ferromagnetism with in-plane magnetization, Dirac spin-gapless semiconducting properties, and tunable topological states in two-dimensional rare-earth metal dinitrides

As the bulk single-crystal MoN2/ReN2 with a layered structure was successfully synthesized in experiment, transition-metal dinitrides have attracted considerable attention in recent years. Here, we focus on rare-earth-metal (Rem) elements and propose seven stable Rem dinitride monolayers with a 1T structure, namely 1T-RemN2. These monolayers have a ferromagnetic ground state with in-plane magnetization. Without spin-orbit coupling (SOC) effect, the band structures are spin-polarized with Dirac points at the Fermi level. Remarkably, the 1T-LuN2 monolayer shows an isotropic magnetic anisotropy energy in the xy-plane with in-plane magnetization, indicating easy tunability of the magnetization direction. When rotating the magnetization vector in the xy-plane, our proposed model can accurately describe the variety of the SOC band gap and two topological states (Weyl-like semimetal and Chern insulator states) appear with tunable properties. The Weyl-like semimetal state is a critical point between the two Chern insulator states with opposite sign of the Chern numbers. The large nontrivial band gap (up to 60.3 meV) and the Weyl-like semimetal state are promising for applications in spintronic devices.

Classification of magnetic ground states and prediction of magnetic moments of inorganic magnetic materials based on machine learning

Magnetic materials are important basic materials in the information age. Different magnetic ground states are the prerequisite for the wide application of magnetic materials, among which the ferromagnetic ground state is a key requirement for future high-performance magnetic materials. In this paper, machine learning is used to study the classification of ferromagnetic, antiferromagnetic, ferrimagnetic and paramagnetic ground states of inorganic magnetic materials and the prediction of magnetic moments of inorganic ferromagnetic materials. We obtain 98888 inorganic magnetic materials data from the Materials Project database, containing material ids, chemical formulae, CIF files, magnetic ground states and magnetic moments, and extract 582 elemental and structural features for the inorganic magnetic materials by using Matminer. We design a two-step feature selection method. In the first step, RFECV is used to evaluate material features one by one to remove redundant features without degrading the model accuracy. In the second step, we rank the material features to further refine and select the most important material features for the model, and 20 material features are selected for the classification of magnetic ground states and the prediction of magnetic moments, respectively. Among the selected material features, it is found that the electronegativity, the atomic own magnetic moment and the number of unfilled electrons in the atomic peripheral orbitals all make important contributions to the classification of magnetic ground states and the prediction of magnetic moments. We build a magnetic ground state classification model and a magnetic moment prediction model by using the random forest, and quantitatively evaluate the machine learning models by using the 10-fold cross-validation approach, and the results show that the constructed machine learning models has sufficient accuracy and generalization capability. In the test set, the magnetic ground state classification model has an accuracy of 85.23%, a precision of 85.18%, a recall of 85.04%, and an F1 score of 85.24%; the magnetic moment prediction model has a goodness-of-fit of 91.58% and an average absolute error of 0.098 μ<sub>B</sub> per atom. This study provides a new method and choice for high-throughput classification and screening of magnetic ground states of inorganic magnetic materials and predicting the magnetic moment of ferromagnetic materials.

The magnetic anisotropy and spin filtering effect in ferromagnetic phosphorene

A two dimensional (2D) semiconductor integrated onto a ferromagnetic substrate presents a promising design for the expansion of new spintronic devices. Recently discovered 2D magnets, such as CrGeTe3 and CrX3 (X = Br, Cl, I) have led to intense research on new physical appearances and many practical applications. Here, we build the van der Waals heterostructure by layering a black phosphorene on top of a single layer of CrI3 substrate. We systematically investigate the electronic behavior of heterostructure through the first principle calculation. In comparison with CrI3 and phosphorene, the bandgap of heterostructure is reduced and it is more magnetic. Afterward, Phosphorene has twisted at an angle of 3.22° and 3.83° on CrI3 substrate to create new stacking configurations of heterostructure. We observe the band gap changes with twist angle. Our proposed heterostructure has a ferromagnetic ground state with out-of-plane magnetic anisotropic nature with perfect spin filtering capability, which offers a novel route for future magneto-optic and spintronic engineering.

First-principles study of structural and electronic properties of cove-edged zigzag ZnO nanoribbons

ZnO is a wide band gap material that exhibits particular applications in opto-electronic devices. Nanowires, nanotubes, and two-layer thick layers of ZnO have already been synthesized. It is known that bare zigzag ZnO (zZnO) nanoribbons are metallic in nature with a ferromagnetic ground state. In contrast, H-passivated zZnO nanoribbons are non-magnetic in nature. In zZnO nanoribbons, one edge is entirely made up of Zn atoms while there are only O atoms on its opposite edge. In the present work, we investigated the effect of defects (cove-edges) on the structural and electronic properties of zZnO nanoribbons. Various possible configurations were considered which include (a) cove-edge at O side (ZnO-cv) (b) cove-edge at Zn side (cv-ZnO) (c) cove-edge at both the sides (cv-ZnO-cv) (d) Zn-edge is H-passivated while O-edge is coved (H-ZnO-cv) (e) O-edge is H-passivated while Zn-edge is coved (cv-ZnO-H). The introduction of cove-defects affects the structural and electronic properties of zZnO nanoribbons in a significant manner. The binding energy analysis shows that all of these structures are exothermic and energetically feasible to be obtained. Moreover, the energetic feasibility is proportional to the ribbon width. Present findings revealed that ZnO-Cv is a zero-band gap semi-metal similar to graphene whereas, Cv-ZnO and Cv-ZnO-Cv exhibit pure metallic character.

Tuning the magnetic properties of a diamagnetic di-Blatter's zwitterion to antiferro- and ferromagnetically coupled diradicals.

In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), a molecule containing two Blatter's moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from the zwitterionic ground state to the diradicalized state. With a larger coupler, ferromagnetic interactions are realized based on density functional theory (DFT) and wave-function theory (WFT) based complete active space self-consistent field (CASSCF)-N-electron valence state perturbation theory (NEVPT2) methods. An analysis based on the extent of spin contamination, diradical character, CASSCF orbital occupation number, Head-Gordon's index, HOMO-LUMO and SOMOs energy gaps is demonstrated that marks the transition of the ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on a TPHA skeleton with diradicaloid state and, in some cases, even a triplet ground state.

Integrating ferromagnetism and ferroelectricity in an iron chalcogenide monolayer: a first-principles study.

Two-dimensional (2D) ferro-type materials have received great attention owing to the remarkable polarization effect in optoelectronics and spintronics. Using the first-principles method, the coupling between ferromagnetism and ferroelectricity is investigated in a multiferroic Janus 1T-FeSSe monolayer, which has a strong Stoner ferromagnetic ground state. The magnetic anisotropy energy (MAE) is apparently impacted by the out-of-plane asymmetry donated ferroelectricity, which is reflected by the asymmetry of the Z-MAE image. The easy magnetization axis of Janus FeSSe is the +y axis with a large MAE of 0.59 meV, rooting in unpaired d electrons of Fe atoms. The transformation of band splitting and Fermi surface can be effectively engineered by different magnetic polarization directions. The ferromagnetic (FM) coupling of the FeSSe monolayer is very robust under external strain within the range of -6% to 6%, while the strength of magnetic moment of Fe atoms and polarization are easily strain-engineered, the intrinsic mechanism of which can be elaborated by the GKA rules that depend on angles and distances. This multiferroic FeSSe monolayer provides a new platform for exploring the coupling of 2D ferromagnetism and ferroelectricity and designing low-dimensional multiferroic electronics.

Lattice Parameters, Electronic, and Magnetic Properties of Cubic Perovskite Oxides ARuO3 (A=Sr, Rb): A First‑Principles Study

Trioxides of rubidium, strontium, and ruthenium belong to the family of alkali and alkaline earth ruthenates. SrRuO3 crystallizes in various symmetry classes—orthorhombic, tetragonal, or cubic—whereas RbRuO3 is perovskite (cubic) structured and crystallizes only in the cubic space group Pm3¯¯¯m(No. 221). In this study, we investigated the structural stability as well as the electronic and magnetic properties of two cubic perovskites SrRuO3 and RbRuO3. We established the corresponding lattice parameters, magnetic moments, density of states (DOS), and band structures using ab‑initio density‑functional theory (DFT). Both compounds exhibited a metallic ferromagnetic ground state with lattice parameter values between 3.83 and 3.96 Å; RbRuO3 had magnetic moments between 0.29 and 0.34 µBwhereas SrRuO3 had magnetic moments between 1.33 and 1.66 µB. This study paves way for further RbRuO3 research.

Giant Biquadratic Exchange in 2D Magnets and Its Role in Stabilizing Ferromagnetism of NiCl2 Monolayers

Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultra-compact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra.

Charge carrier mediation and ferromagnetism induced in MnBi6Te10 magnetic topological insulators by antimony doping

A new kind of intrinsic magnetic topological insulator (MTI) MnBi2Te4 family has shed light on the observation of novel topological quantum effects such as the quantum anomalous Hall effect (QAHE). However, strong anti-ferromagnetic (AFM) coupling and high carrier concentration in the bulk hinder practical applications. In closely related materials MnBi4Te7 and MnBi6Te10, the interlayer magnetic coupling is greatly suppressed by Bi2Te3 layer intercalation. However, AFM is still the ground state in these compounds. Here, by magnetic and transport measurements, we demonstrate that a Sb substitutional dopant plays a dual role in MnBi6Te10, which can not only adjust the charge carrier type and concentration, but also induces the solid into a ferromagnetic (FM) ground state. The AFM ground state region, which is also close to the charge neutral point, can be found in the phase diagram of Mn(Sb x Bi1−x )6Te10 when x ∼ 0.25. An intrinsic FM-MTI candidate is thus demonstrated, which may take us a step closer to realizing a high-quality and high-temperature QAHE and related topological quantum effects in the future.

Co-existence of short- and long-range magnetic order in LaCo2P2

The ferromagnetic (FM) nature of the metallic LaCo2P2 was investigated with the positive muon spin rotation, relaxation and resonance (μ +SR) technique. Transverse and zero field μ + SR measurements revealed that the compound enters a long range FM ground state at K, consistent with previous studies. Based on the reported FM structure, the internal magnetic field was computed at the muon sites, which were predicted with first principles calculations. The computed result agree well with the experimental data. Moreover, although LaCo2P2 is a paramagnet at higher temperatures T > 160 K, it enters a short range ordered (SRO) magnetic phase for K. Measurements below the vicinity of revealed that the SRO phase co-exists with the long range FM order at temperatures 124 K . Such co-existence is an intrinsic property and may be explained by an interplay between spin and lattice degree of freedoms.

Coexisting ferromagnetic-antiferromagnetic state in twisted bilayer CrI3.

Moiré engineering1-3 of van der Waals magnetic materials4-9 can yield new magnetic ground states via competing interactions in moiré superlattices10-13. Theory predicts a suite of interesting phenomena, including multiflavour magnetic states10, non-collinear magnetic states10-13, moiré magnon bands and magnon networks14 in twisted bilayer magnetic crystals, but so far such non-trivial magnetic ground states have not emerged experimentally. Here, by utilizing the stacking-dependent interlayer exchange interactions in two-dimensional magnetic materials15-18, we demonstrate a coexisting ferromagnetic (FM) and antiferromagnetic (AF) ground state in small-twist-angle CrI3 bilayers. The FM-AF state transitions to a collinear FM ground state above a critical twist angle of about 3°. The coexisting FM and AF domains result from a competition between the interlayer AF coupling, which emerges in the monoclinic stacking regions of the moiré superlattice, and the energy cost for forming FM-AF domain walls. Our observations are consistent with the emergence of a non-collinear magnetic ground state with FM and AF domains on the moiré length scale10-13. We further employ the doping dependence of the interlayer AF interaction to control the FM-AF state by electrically gating a bilayer sample. These experiments highlight the potential to create complex magnetic ground states in twisted bilayer magnetic crystals, and may find application in future gate-voltage-controllable high-density magnetic memory storage.

Magnetic order transition in monolayerMoS2induced by strong intervalley correlation

In this work, we study a model for monolayer molybdenum disulfide with including the intravalley and intervalley electron-electron interaction. We solve the model at a self-consistent mean-field level and get three solutions $L_{0}$, $L_{+}$ and $L_{-}$. As for $L_{0}$, the spin polarizations are opposite at $\textbf{K}$ and $\textbf{K}^{\prime}$ valley and the total magnetization is zero. $L_{\pm}$ describe two degenerate spin-polarized states, and the directions of polarization are opposite for the states of $L_{+}$ and $L_{-}$. Based on these results, the ground state can be deduced to be spin polarized in domains in which their particular states can be randomly described by $L_{+}$ or $L_{-}$. Therefore, a zero net magnetization is induced for zero external magnetic field $\mathbf{B}$, but a global ferromagnetic ground state for a nonzero $\mathbf{B}$. We estimate the size of domains as several nanometers. As the increase of the chemical potential, the ground state changes between $L_{0}$ and $L_{\pm}$, indicating first order phase transitions at the borders, which is coincident with the observation of photoluminescence experiments in the absence of the external magnetic field [J. G. Roch $\it{et}$ $\it{al.}$, Phys. Rev. Lett. ${\bf 124}$, 187602 (2020)].

Effects of Fe Deficiency and Co Substitution in Polycrystalline and Single Crystals of Fe3GeTe2

Fe3GeTe2 is a two-dimensional van der Waals material with a ferromagnetic ground state and a maximum transition temperature Tc ∼ 225 K. However, when Fe3GeTe2 is synthesized, lower values of Tc are often reported. This is attributed to a deficiency in the Fe at the 2c site in the crystal structure. Here, we investigate the effect of Fe deficiency and the substitution of Co for Fe on the magnetic properties of this system. We have synthesized both polycrystalline material and single crystals by chemical vapor transport and the flux method, with the largest crystals obtained using the flux method. Cobalt substitution at the Fe site is found to significantly reduce the magnetic transition temperature. Crystals of Fe3GeTe2 grown by chemical vapor transport with ∼ 8% excess Fe in the starting materials display an optimum Fe content and magnetic transition temperature.

Defect-mediated ferromagnetism in correlated two-dimensional transition metal phosphorus trisulfides.

Controlling the magnetic spin states of two-dimensional (2D) van der Waals (vdW) materials with strong electronic or magnetic correlation is important for spintronic applications but challenging. Crystal defects that are often present in 2D materials such as transition metal phosphorus trisulfides (MPS3) could influence their physical properties. Here, we report the effect of sulfur vacancies on the magnetic exchange interactions and spin ordering of few-layered vdW magnetic Ni1−xCoxPS3 nanosheets. Magnetic and structural characterization in corroboration with theoretical calculations reveal that sulfur vacancies effectively suppress the strong intralayer antiferromagnetic correlation, giving rise to a weak ferromagnetic ground state in Ni1−xCoxPS3 nanosheets. Notably, the magnetic field required to tune this ferromagnetic state (<300 Oe) is much lower than the value needed to tune a typical vdW antiferromagnet (> several thousand oersted). These findings provide a previously unexplored route for controlling competing correlated states and magnetic ordering by defect engineering in vdW materials.

First-principles and Monte Carlo investigation of magnetic properties of two-dimensional transition metal alloyed boron-carbide [formula omitted] sheet

We identify a new two-dimensional (2D) tetragonal phase of transition metal alloyed boron-carbide (t-CrFeBC) sheet through combined first-principles calculations and Monte Carlo (MC) simulations. The t-CrFeBC sheet prefers a ferromagnetic ground state with the metallic electronic property. Also, the t-CrFeBC sheet is dynamically and thermally stable. t-CrFeBC exhibits sizable magnetic anisotropy energy (MAE) of 120 μeV per CrFe alloy with an in-plane easy axis (EA) magnetization direction. Moreover, hysteresis loops and other hysteresis related properties (coercivity and remanent magnetization) which are evidence of existence of ferromagnetism in the t-CrFeBC sheet are presented for a wide range of temperature. MC simulation results indicate that t-CrFeBC sheet is soft magnetic material with a small coercieve field and narrow rectangular shaped hysteresis curve near the room temperature. All results show that 2D t-CrFeBC sheet holds a unique promise for advanced magnetic device applications.

First-principles calculation to investigate structural, electronic and optical properties of transition-metals intercalated bilayer SnS2

Electronic, magnetic, and optical properties of AA-SnS2 bilayer doped with transition metals (TMs) were investigated using the density functional theory (DFT). It has been found that some TM-doped atoms (V, Cr, and Ni) prefer to occupy the octahedral site, while Mn, Fe, and Co atoms tend to occupy the tetrahedral sites. The ground state of single V-, Cr-, Mn-, Fe-, and Co-doped systems are magnetic, which comes mainly from 3d orbitals of TM atoms. Based on the charge density distribution, the covalent bonding features are between the TM and S atoms. In the case of 2-TM doping, V, Mn, Fe, and Co atoms evolve the system towards weak antiferromagnetism (AFM). Whereas the Cr-doped system has a weak ferromagnetic (FM) ground state. In addition, TM doping elements significantly modify the optical properties of the AA-SnS2 bilayer. These results show that the TM-doped AA-SnS2 bilayer can be a helpful candidate for spintronic and UV coating applications.