Interplane Interactions Research Articles

Tunable ferromagnetism via in situ strain engineering in single-crystal freestanding SrTiO3-δ membrane

Engineering stoichiometry and lattice field in transition metal oxides (TMOs) is recognized as a promising approach for achieving emergent functional properties and exploring fundamental scientific questions. To overcome the constraints of rigid epitaxial TMOs, this study releases and transfers freestanding SrTiO3-δ (STO3-δ) membranes, derived using a water-dissolution method from STO3-δ/Sr3Al2O6/SrTiO3, onto flexible polyimide. In-plane mechanical strains were then applied to investigate the strain evolution-induced ferromagnetism. Continuous strain modulates interplane and intraplane exchange interactions between neighboring atoms in the ferromagnetic STO3-δ membranes, thereby influencing their ferromagnetic properties. STO3-δ initially undergoes in-plane octahedral distortion when strain is less than 1.5 %, followed by a decrease in the out-plane lattice constant. This structural variation leads to complex strain-dependent behaviors in the saturation magnetic moment (Ms) and coercive field (Hc) of STO3-δ, with Ms and Hc exhibiting a nonlinear, volcano-shaped, and step-wise correlation, respectively. Our research demonstrates that freestanding STO3-δ serves as a platform for studying local defects and their impacts on tunable magnetic properties, greatly enhancing our understanding of t2g electron engineering through modulable inter/intra plane exchange coupling with lattice field.

Multiple non-canonical base-stacking interactions as one of the major determinants of RNA tertiary structure organization

Interplane (stacking) interactions of heterocyclic bases of nucleotide residues (n.t.) of RNA are one of the most important factors in the organization of its secondary and tertiary structure. Most of these (canonical) interactions are carried out between neighbors in the polynucleotide chains of RNA. However, with the accumulation of data on the atomic tertiary structures of a wide variety of RNAs and their complexes with proteins, it became clear that RNA nucleotide residues that are not neighbors in their polynucleotide chains and are sometimes separated in the RNA primary structure by tens or hundreds of n.t. can interact with the help of base stacking (non-canonical). This paper presents an exhaustive database of such elements and their environment in the macromolecules of natural and synthetic RNAs. They were called nonadjacent base-stacking elements (NA-BSE). The analysis of these data showed that the NA-BSE forming nucleotides, on average, account for about a quarter of all nucleotides of a particular RNA, therefore, they should be considered as real motifs in their tertiary structure. The classification of NA-BSE by types of localization in RNA macromolecules is carried out. It is shown that the structure-forming role of NA-BSE consists in the compact folding of single-stranded RNA loops, in the transformation of double-stranded bulges into imperfect helices, as well as in the binding of RNA regions removed in their primary and secondary structure.

H-T magnetic phase diagram of CuCrO2: A Monte Carlo study based on a realistic 3D classical Heisenberg model

In the multiferroic geometrically frustrated triangular antiferromagnet CuCrO2, we investigate the magnetic phases as function of temperature under a magnetic field 70 T ≤ B ≤ 120 T along the [001] axis using the importance sampling Monte Carlo method. We consider a realistic three-dimensional classical Heisenberg model, which takes into consideration the distortion and exchange interactions up to the third-nearest neighbors in the ab planes and an interplane interaction, in addition to hard and easy anisotropy axes along the [110] and the [001] directions, respectively. For 70 T ≤ B < 106 T, our calculations show that two phase transitions occur when decreasing the temperature. The first one, which takes place around 30 K, is a “paramagnetic ↔ up-up-down (UUD)” transition and the second one, at lower temperatures, is an “UUD ↔ commensurate Y (CY)” transition. Thus, the colinear UUD phase is stable in an intermediate temperature range. For B = 90 T, our estimates of the critical exponents indicate that the two transitions belong to the universality class of the two-dimensional Ising model. For 106 T ≤ B < 110 T, since the CY phase is identical to the UUD one, only a “paramagnetic ↔ up-up-down (UUD)” transition occurs. For 110 T ≤ B ≤ 120 T, only a “paramagnetic ↔ V” transition is observed at ∼27 K. Our results are in good agreement with the existing experimental data on the stable phases in this magnetic field range. Moreover, we have determined the temperature range where the CY and UUD phases are stable which was not done previously.

Coupling of magnetism and Dirac fermions in YbMnSb2

We report inelastic neutron scattering measurements of magnetic excitations in ${\mathrm{YbMnSb}}_{2}$, a low-carrier-density Dirac semimetal in which the antiferromagnetic Mn layers are interleaved with Sb layers that host Dirac fermions. We observe a measurable broadening of spin waves, which is consistent with substantial spin-fermion coupling. The spin-wave damping $\ensuremath{\gamma}$ in ${\mathrm{YbMnSb}}_{2}$ is roughly twice larger compared to that in a sister material, ${\mathrm{YbMnBi}}_{2}$, where an indication of a small damping consistent with a theoretical analysis of the spin-fermion coupling was reported. The interplane interaction between the Mn layers in ${\mathrm{YbMnSb}}_{2}$ is also much stronger, suggesting that the interaction mechanism is rooted in the same spin-fermion coupling. Our results establish the systematics of spin-fermion interactions in layered magnetic Dirac materials.

Magnetic structure and spin dynamics of the quasi-two-dimensional antiferromagnet Zn-doped copper pyrovanadate

Magnetic properties of the antiferromagnet Zn$_{0.15}$Cu$_{1.85}$V$_2$O$_7$ (ZnCVO) have been thoroughly investigated on powder and single-crystal samples. The crystal structure determination using powder x-ray and neutron diffraction confirms that ZnCVO with Zn = 0.15 is isostructural with $\beta$-Cu$_{2}$V$_2$O$_7$ ($\beta$-CVO) with small deviation in the lattice parameters. Macroscopic magnetic properties measurements also confirm the similarity between the two compounds. The Cu$^{2+}$ spins were found to align along the crystallographic $c$-axis, antiparallel to their nearest neighbors connected by the leading exchange interaction $J_1$. Spin dynamics reveals a typical symmetric spin-wave dispersion with strong interactions in the $bc$-plane and weak interplane coupling. The exchange interaction analysis indicates that the spin network of ZnCVO is topologically consistent with the previous DFT prediction but the values of leading exchange interactions are contradictory. Furthermore, rather than the predicted 2D honeycomb structure, the spin network in ZnCVO could be better described by the anisotropic 2D spin network composing of $J_1$, $J_5$, and $J_6$ interactions, four bonds per one spin site, coupled by weak interplane interactions.

Microscopic origins and stability of ferromagnetism in Co3Sn2S2

Based on the density functional theory, we examine the origin of ferromagnetism in the Weyl semimetal ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ using different types of response theories. We argue that the magnetism of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ has a dual nature and bears certain aspects of both itineracy and localization. On the one hand, the magnetism is soft, where the local magnetic moments strongly depend on temperature and the angles formed by these moments at different Co sites of the kagome lattice, as expected for itinerant magnets. On the other hand, the picture of localized spins still remains adequate for the description of the local stability of the ferromagnetic (FM) order with respect to the transversal spin fluctuations. For the latter purposes, we employ two approaches, which provide quite different pictures for interatomic exchange interactions: the regular magnetic force theorem and a formally exact theory based on the calculation of the inverse response function. The exact theory predicts ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ to be a three-dimensional ferromagnet with the strongest interaction operating between next-nearest neighbors in the adjacent kagome planes. The ligand states are found to play a very important role by additionally stabilizing the FM order. When the local moments decrease, the interplane interactions sharply decrease, first causing the FM order to become quasi-two-dimensional, and then making it unstable with respect to the spin-spiral order propagating perpendicular to the kagome plane. The latter instability is partly contributed by the states at the Fermi surface and may be relevant to the magnetic behavior of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ near the Curie temperature. Peculiarities of the half-metallic ferromagnetism in ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ are also discussed.

A Monte Carlo study on the temperature dependence of hysteresis loops in Ising Spin-1 Square Bilayers

A Metropolis Monte Carlo simulation is used in this paper to investigate the temperature dependency of the hysteresis loops of a spin-1 bilayer with square monolayers. In this system, the atoms interact ferromagnetically in-plane, with either ferromagnetic or antiferromagnetic interplane interactions. The effects of four distinct combinations of the Hamiltonian parameters on the hysteresis behaviours are discussed in detail. The geometry of the hysteresis loops changes depending on how the exchange couplings are combined. With ferromagnetic interlayer coupling, only the central hysteresis loop opens while for the antiferromagnetic case, the hysteresis loop becomes a double loop for the specific combination of coupling strengths. Additionally, in all these cases, the area of the hysteresis loops grows with the gradual lowering of the temperature.

Magnetic phases of the S=52 triangular-lattice antiferromagnet RbFe( MoO4)2 as determined by ultrasound velocity measurements

$\mathrm{Rb}\mathrm{Fe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ is one of the best physical examples of a quasi-two-dimensional easy-plane triangular-lattice antiferromagnet. Although the magnetic phase diagram of this material has been well-studied, there are discrepancies in the literature concerning the number of magnetic phases, as well as the nature of the phase transitions. By performing ultrasound velocity measurements, we have obtained a detailed phase diagram with magnetic fields parallel and perpendicular to the triangular layers. The order of the phase transitions was found to be consistent with previous theoretical studies of generic two-dimensional triangular antiferromagnets. To support our experimental findings, we present a phenomenological model that includes biquadratic and next-nearest-neighbor interplane interactions. This model is sufficient to reproduce the phase diagram at low temperatures, and the next-nearest-neighbor interactions are shown to stabilize the incommensurate phases ($\mathbb{Y}$ and $\mathbb{V}$). Furthermore, the model provides insight regarding the magnetic ordering along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{c}$-axis in the magnetic plateau phase, suggesting that period 3 is established whenever the next-nearest-neighbor superexchange interactions are unequal. In the systems in which these interactions are expected to be equal, the model predicts degeneracy between period 2 and period 3 structures, at the classical level.

Magnetic field induced phases in CuCrO2 : Monte Carlo and analytical investigations

Motivated by the strong magnetoelectric coupling in the multiferroic geometrically frustrated triangular antiferromagnet $\mathrm{Cu}\mathrm{Cr}{\mathrm{O}}_{2}$ and the high sensitivity of its magnetic structure to external fields, we investigate induced magnetic phases at very low temperatures under high magnetic fields ($B$) up to 325 T applied along the [001] direction. Analytical calculations and Monte Carlo (MC) simulations based on a realistic three-dimensional classical Heisenberg model are used to reveal these magnetic phases. Interestingly, our model mimics a real distorted crystal which considers exchange interactions up to third-nearest neighbors in the $ab$ plane and an interplane interaction, in addition to hard and easy axes anisotropies along the [110] and the [001] directions, respectively. For $B\ensuremath{\ge}70\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, both our MC and analytical results are in an excellent agreement and evidence three commensurate phases, namely, the commensurate Y (CY), the up-up-down (UUD), and the V phases as the magnetic field increases. The field dependence of the characteristic angles of the CY and V phases is determined. Moreover, the saturation field is estimated at 325 T, indicating that the previously predicted values obtained by extrapolation of experimental data are too small. Below 70 T, our MC results indicate that the CY phase is no more stable and several incommensurate Y phases appear. Overall, the observed magnetic phases at nearly 0 K are in a good agreement with a recently published experimental phase diagram. It should be noted that our MC data reject the incommensurate umbrella phase at very low temperatures, which was reported in previous studies.

Half-magnetization plateau and the origin of threefold symmetry breaking in an electrically switchable triangular antiferromagnet

We perform high-field magnetization measurements on the triangular lattice antiferromagnet Fe$_{1/3}$NbS$_2$. We observe a plateau in the magnetization centered at approximately half the saturation magnetization over a wide range of temperature and magnetic field. From density functional theory calculations, we determine a likely set of magnetic exchange constants. Incorporating these constants into a minimal Hamiltonian model of our material, we find that the plateau and of the $Z_3$ symmetry breaking ground state both arise from interplane and intraplane antiferromagnetic interactions acting in competition. These findings are pertinent to the magneto-electric properties of Fe$_{1/3}$NbS$_2$, which allow electrical switching of antiferromagnetic textures at relatively low current densities.

Influence of Interplane Interactions on Formation of Magnetization Plateaus in Generalized 3D Ising Models with Magnetically Coupled Shastry-Sutherland Layers

The combination of the standard Metropolis algorithm and the parallel tempering method is used to examine the influence of interplane interactions on the formation of magnetization plateaus in two types of generalized Ising models with magnetically coupled Shastry-Sutherland layers. In the first model, we consider the first (J1) and second (J2) intraplane and first (I1) and second (I2) interplane nearest neighbour (NN) interactions, while in the second one, we consider all intraplane and interplane interactions between spins inside an sphere of radius r < 6, and the interaction between two spins at Ri and Rj lattice sites is modeled by a simple one-parametric formula with exponentially decaying amplitudes $J_{\text {ij}} \sim e^{-\alpha |\textbf {R}_{\textbf {i}}-\textbf {R}_{\textbf {j}}|}$ . It is shown that the NN interplane interaction I1 does not change qualitatively the magnetic phase diagram of the model found for J1 and J2 (I1 = 0), while the next NN interaction I2 influences strongly both the stability regions of magnetic phases found for J1 and J2 (I1 = 0, I2 = 0) as well as the formation of new phases. In particular, we have found that the ferromagnetic interaction I2 suppresses the stability regions of magnetic plateau phases found for nonzero J1 and J2, while the antiferromagnetic I2 interaction forms, in addition to the standard 1/3 and 1/2 plateaus, the following set of new macroscopic magnetization plateaus: m/ms = 1/6,1/4,2/3, and 3/4. Practically the same plateaus are formed by the second model in the limit of strongly decaying spin interactions ( $\alpha \sim 4$ ), while in the opposite limit (α 1/2 and m/ms < 1/3 disappear and the magnetization curves become continuous in these regions.

Discovery of stable and intrinsic antiferromagnetic iron oxyhalide monolayers.

Two-dimensional (2-D) antiferromagnetic (AFM) materials have shown promise over their ferromagnetic (FM) counterparts for developing advanced spintronic devices; however, they have been rarely found with high Néel temperatures to date. Here, by employing first-principles calculations and Monte Carlo simulations, we demonstrate that the family of 2-D iron oxyhalides monolayers, FeOX (X = F, Cl, Br, I), are magnetic Mott insulators with their AFM ground state possessing relatively high Néel temperatures. The structural stabilities of the FeOX monolayers are proved using a set of phonon, molecular dynamics, and elastic constant calculations. The calculated Néel temperature of the FeOCl monolayer is close to that of FeOCl bulk because of the weak van der Waals interaction between the layers. More importantly, the predicted Néel temperatures of FeOX (X = F, Cl, Br, I) monolayers can be increased by biaxial compression strain. The Néel temperature of the strained FeOF and FeOI monolayers can approach 200 K, which suggests that they can be robust antiferromagnets with relatively high Néel temperatures compared with other available 2-D magnets. Our calculations show that both the in-plane and the inter-plane magnetic interactions affect the AFM coupling between Fe atoms in FeOX monolayers. The easy axis of the 2-D FeOX is found to be along the in-plane direction. The FeOX monolayers may provide an excellent platform for building novel spintronic devices at the nanoscale.

Finite field regime for a quantum spin liquid in α−RuCl3

An external magnetic field can induce a transition in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ from an ordered zigzag state to a disordered state that is possibly related to the Kitaev quantum spin liquid. Here, we present field-dependent inelastic neutron scattering and magnetocaloric effect measurements implying the existence of an additional transition out of the quantum spin-liquid phase at an upper field limit ${B}_{u}$. The neutron scattering shows three distinct regimes of magnetic response. In the low-field ordered state the response shows magnon peaks; the intermediate-field regime shows only continuum scattering, and above ${B}_{u}$ the response shows sharp magnon peaks at the lower bound of a strong continuum. Measurable dispersion of magnon modes along the $(0,0,L)$ direction implies non-negligible interplane interactions. Combining the magnetocaloric effect measurements with other data, a $T\text{\ensuremath{-}}B$ phase diagram is constructed. The results constrain the range where one might expect to observe quantum spin-liquid behavior in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$.

Ambipolar ferromagnetism by electrostatic doping of a manganite

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO3, with electron–hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO3 film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron–hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.

Thermodynamic and critical properties of an antiferromagnetically stacked triangular Ising antiferromagnet in a field

We study a stacked triangular lattice Ising model with both intra- and inter-plane antiferromagnetic interactions in a field, by Monte Carlo simulation. We find only one phase transition from a paramagnetic to a partially disordered phase, which is of second order and 3D XY universality class. At low temperatures we identify two highly degenerate phases: at smaller (larger) fields the system shows long-range ordering in the stacking direction (within planes) but not in the planes (stacking direction). Nevertheless, crossovers to these phases do not have a character of conventional phase transitions but rather linear-chain-like excitations.

Formation of artificial flux pinning centers in Bi-2223 cuprate superconductor with Ni impurities and enhanced resistant to thermal fluxon motions of correlated 2D pancake vortices in new matrix

This comprehensive study attempts to find out the evaluation of magnetoresistivity performance and flux pinning centers in the Bi-2223 superconductor with nickel impurities and diffusion annealing temperature interval 650 °C–850 °C by means of magnetotransport measurements (0–5T). It is found that the cooper-pair probabilities as well as pinning of thermal fluxon motions of two-dimensional (2D) pancake vortices and interlayer Josephson coupling between the isolated grains considerably improve with the annealing temperature up to 700 °C as a result of enhancement in the effective flux pinning regions. However, from 700 °C onwards the potential energy barriers decrease significantly due to the increment of permanent structural problems in the crystal system. Namely, the presence of excess Ni impurities develops the recoupling linelike/discrete pancakelike nature. In this respect, the bulk sample exposed to the diffusion annealing temperature of 850 °C exhibits the worst magnetic performance characteristics. The flux lines in the material more easily jump to neighboring states due to the cooper pair-breaking mechanism, and thus the thermal fluxon motions of 2D pancake vortices lead to the energy dissipation. We also discuss the differentiation of vortex lattice elasticity for the materials with the aid of the theoretically calculated quantities such as the thermodynamic critical field (μ0Hc), irreversibility field (μ0Hirr), lower critical field (μ0Hc1), upper critical field (μ0Hc2), penetration depth (λ), coherence length (ξ) and Ginzburg Landau parameter (κ). According to the computational values, the optimum annealing temperature of 700 °C strengthens the pinning of 2D pancake vortices, vortex lattice period and elasticity as a result of the decrease in the inter-plane interaction energy. To sum up, 700 °C favors the applications of bulk Ni surface-layered Bi-2223 compounds in rather higher magnetic fields for a major innovation.

Design of Magnetic Coordination Polymers Built from Polyoxalamide Ligands: A Thirty Year Story

The aim of this review is to pay tribute to the legacy of O. Kahn. Kahn's credo was to synthesize magnetic compounds with predictable structure and magnetic properties. This is illustrated herein with results obtained by Kahn's group during his Orsay period thirty years ago, but also on the basis of our recent results on the synthesis of coordination polymers with oxamate ligands. The first part of this review is devoted to a short description of the necessary knowledge in physics and theoretical chemistry that Kahn and his group have used to select oxamate ligands, the complex‐as‐ligand strategy and the synthesis of heterobimetallic systems. Then, we describe the strategies we have later used to obtain the desired target compounds. The use of complexes as building‐blocks, associated to a control of the metal ions chirality and stoichiometry, allowed us to obtain coordination polymers with predictable dimensionality. For the synthesis of single‐chain magnets (SCMs) we show that the ligand chemical flexibility makes the isolation of the chains in the solid and the occurence of slow magnetic relaxation possible. For 1D and 2D molecule‐based magnets (MBMs), the magnetic ordering strongly depends on the interchain or interplane interactions, which are difficult to control. Again the flexibility of the oxamate ligands allowed their strengthening in the solid sate, yielding MBMs with critical temperatures up to 30 K. We will also present our results on 3D coordination polymers, particularly on the porous magnets displaying large octagonal channels. This family of porous MBMs possess outstanding chemical properties, such as post‐synthetic transformation in the solid state. Finally, we will also show that oxamate ligands allows the design of multifunctional materials, as in the case of the first chiral SCM. Overall, the results presented in this review show the impressive potential the oxamate ligands have for the design of coordination polymers.

A spin rotator model for Heisenberg helimagnet

We study the dynamics of a helimagnetic spin system by proposing a spin rotator model taking into account bilinear, twist interplane and anisotropic interactions in the continuum limit. The dynamical equations of motion are obtained and studied numerically. The influence of different types of inhomogeneities is also analysed. Similar studies are carried out for the system including biquadratic type interactions.

Electronic band structure of ReS2 by high-resolution angle-resolved photoemission spectroscopy

The rhenium-based transition metal dichalcogenides (TMDs) are atypical of the TMD family due to their highly anisotropic crystalline structure and are recognized as promising materials for two dimensional heterostructure devices. The nature of the band gap (direct or indirect) for bulk, few and single layer forms of ReS$_2$ is of particular interest, due to its comparatively weak inter-planar interaction. However, the degree of inter-layer interaction and the question of whether a transition from indirect to direct gap is observed on reducing thickness (as in other TMDs) are controversial. We present a direct determination of the valence band structure of bulk ReS$_2$ using high resolution angle resolved photoemission spectroscopy (ARPES). We find a clear in-plane anisotropy due to the presence of chains of Re atoms, with a strongly directional effective mass which is larger in the direction orthogonal to the Re chains (2.2 $m_e$) than along them (1.6 $m_e$), in good agreement with density functional theory calculations. An appreciable inter-plane interaction results in an experimentally-measured difference of ~100-200 meV between the valence band maxima at the Z point (0,0,1/2) and the $\Gamma$ point (0,0,0) of the three-dimensional Brillouin zone. This leads to a direct gap at Z and a close-lying but larger gap at $\Gamma$, implying that bulk ReS2 is marginally indirect. This may account for recent conflicting transport and photoluminescence measurements and the resulting uncertainty about the direct or indirect gap nature of this material.

Microscopic Analysis of the Different Perchlorate Anions Intercalation Stages of Graphite

Driven by the perspective of large-scale, high-quality graphene production via chemical routes, the investigation of electrochemical anion intercalation between the basal graphite planes has seen a renewed interest among the scientific community. At relatively high electrochemical potentials, when oxidation occurs, graphite electrodes undergo significant anion intercalation processes. The latter swell the uppermost graphite layers (i.e., graphene sheets), reduce the interplane interaction and favor the graphite delamination in liquid. Different intercalation stages are observed in a perchloric acid electrolyte, which are usually interpreted in terms of different perchlorate penetration depths. Nonetheless, the understanding of the morphological changes occurring at the electrode surface during the different intercalation stages is still not completely clear. We combine different microscopy techniques including optical, scanning electron and electrochemical atomic force microscopies to analyze the morpholo...