Nearest-neighbor Exchange Interactions Research Articles

Large magnetocaloric refrigeration performance near room temperature in monolayer transition metal dihalides

Magnetocaloric effect (MCE) exhibits highly efficient and ecological cooling abilities for solid-state refrigeration in contrast to traditional vapor-compression refrigeration. Successive emerging two-dimensional (2D) magnetic materials provide a fertile platform for exploring low-dimensional MCE systems. Here, we focus on a series of 2D transition metal dihalides MX2 (M = Fe, Ru, Os; X = Cl, Br) to explore the maximum isothermal magnetic entropy change (−ΔSmagmax) and adiabatic temperature change (ΔTadmax) under external magnetic field. It is found that FeCl2, FeBr2, and RuCl2 have intrinsically sizable −ΔSmagmax, ΔTadmax, and high thermal conductivity near room temperature, demonstrating superior comprehensive refrigeration performance in comparison with other 2D magnets. It is revealed that strong nearest-neighbor ferromagnetic exchange interaction plays a decisive role in −ΔSmagmax, and the high lattice thermal conductivities of FeCl2 and RuCl2 are attributed to the longer phonon lifetime and larger group velocity of low-frequency acoustic branch. Moreover, moderate strain and carriers doping are able to effectively regulate Curie temperature and magnetocrystalline anisotropy energy and correspondingly enhance −ΔSmagmax. The present work provides important insights for the exploration of 2D magnets for magnetocaloric refrigeration near room temperature.

Two-dimensional ferromagnetism induced by Jahn-Teller distortion and orbital order

As the dimension of the system decreases, the quantum confinement effect and electronic correlation interaction inside the material will be correspondingly enhanced, often resulting in some novel physical properties. Recently, the freestanding perovskite oxide films down to the monolayer limit have been successfully prepared and can be transferred to any desired substrate, providing a great opportunity to explore the functional properties of two-dimensional perovskites. In perovskite materials, Jahn-Teller distortion and orbital order often cause a variety of correlated electronic behaviors. However, unlike van der Waals materials that retain their structural and chemical bonding characteristics when reduced to the monolayer limit, perovskite materials may undergo structural reconstruction when reduced to two dimensions. Therefore, whether Jahn-Teller distortion and related effects exist in the perovskite monolayer limit, and whether two-dimensional perovskite can exhibit some new properties different from its bulk phase, have become urgent issues to be solved. In this work, perovskite fluoride KCuF<sub>3</sub> and its monolayer have been comparatively studied by means of first-principles calculation, symmetry analysis, and Monte Carlo simulation methods, to reveal the change in lattice dynamics, structural, electronic, and magnetic properties caused by dimensionality reduction in perovskites. The results show that the cooperative Jahn-Teller distortion and the in-plane staggered orbital order occurring in the KCuF<sub>3</sub> bulk can be retained to the monolayer limit. However, unlike the bulk phase, the Jahn-Teller distortion mode appears as a soft mode of the prototype phase in the monolayer, and the insulating property of the monolayer does not rely on the emergence of the Jahn-Teller distortion, but is related to the enhancement of the electronic correlation effect. The staggered orbital order causes the nearest-neighbor exchange interaction to be ferromagnetic, resulting in the monolayer being a two-dimensional ferromagnetic insulator, different from the antiferromagnetic phase in the bulk. Monte Carlo simulations predict that the Curie temperature of the monolayer is about 5 K, which is much lower than the Néel temperature of the bulk phase, indicating that the disappearance of interlayer coupling leads to a significant reduction in the magnetic phase transition temperature. This work provides guidance and reference for the study of two-dimensional perovskite materials and the design of perovskite-based two-dimensional ferromagnets.

Weak trimerization in the frustrated two-dimensional triangular Heisenberg antiferromagnet LuyY1−yMnO3

To understand the 2D triangular Heisenberg antiferromagnetic system, we investigated the magnetic structures and the dynamics of ${\mathrm{Lu}}_{y}{\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{MnO}}_{3}$ in detail. The substitutions are adjusted to the Mn atomic position close to ${x}_{\text{Mn}}=\frac{1}{3}$. The neutron powder diffraction data claims that the magnetic structure of ${\mathrm{Lu}}_{y}{\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{MnO}}_{3}$ is described as a mixture of ${\mathrm{\ensuremath{\Gamma}}}_{3}$ ($P{6}_{3}^{\ensuremath{'}}c{m}^{\ensuremath{'}}$) and ${\mathrm{\ensuremath{\Gamma}}}_{4}$ ($P{6}_{3}^{\ensuremath{'}}{c}^{\ensuremath{'}}m$) at the ${x}_{\text{Mn}}$ position for $y=0.15$, 0.30, and 0.45. The ratio of ${\mathrm{\ensuremath{\Gamma}}}_{3}$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}$ depends on temperature and composition and the fraction of ${\mathrm{\ensuremath{\Gamma}}}_{3}$ increases upon cooling, while no clear trimerization was observed at the ${x}_{\text{Mn}}$ position. We estimated exchange parameters from the analysis of the low-energy part of the spin waves. The results showed a weak trimerization effect on cooling because the nearest-neighbor exchange interaction is slightly enhanced. The temperature dependence of the spin-wave dispersion around the $\mathrm{\ensuremath{\Gamma}}$ point shows that the spin gap closes with increasing temperature because the exchange interactions in the nearest Mn-Mn neighbor become smaller. Gapless diffusive magnetic excitation from a Mn triangular lattice has been observed in a wide range in $Q$ and $E$ space of ${\mathrm{Lu}}_{y}{\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{MnO}}_{3}$. We found that ${\mathrm{Lu}}_{0.7}{\mathrm{Y}}_{0.3}{\mathrm{MnO}}_{3}$ could be an ideal case to investigate the trimerization, frustrated magnetism, and magnetoelastic coupling often observed in two-dimensional triangular lattice Heisenberg antiferromagnet systems.

Force between magnetic nanoplates with dipolar interactions

This study considers the dependence of the force caused by the dipolar interaction between small low-dimensional magnets such as single-molecule magnets and two-dimensional magnets on the distance between them within the framework of the dipolar Ising model with nearest-neighbor exchange interactions and long-range dipolar interactions. In particular, we focus on the rapid change in the force between ferromagnetic and antiferromagnetic plates, which arise from the transition of the spin states and explain that this behavior originates from the spin frustrations between magnetic plates. Furthermore, the size and temperature dependence of the interaction energy are investigated using a Monte Carlo simulation.

Control of magnetic states and spin interactions in bilayer CrCl3 with strain and electric fields: an ab initio study

Using ab initio density functional theory, we demonstrated the possibility of controlling the magnetic ground-state properties of bilayer CrCl_{3} by means of mechanical strains and electric fields. In principle, we investigated the influence of these two fields on parameters describing the spin Hamiltonian of the system. The obtained results show that biaxial strains change the magnetic ground state between ferromagnetic and antiferromagnetic phases. The mechanical strain also affects the direction and amplitude of the magnetic anisotropy energy (MAE). Importantly, the direction and amplitude of the Dzyaloshinskii–Moriya vectors are also highly tunable under external strain and electric fields. The competition between nearest-neighbor exchange interactions, MAE, and Dzyaloshinskii–Moriya interactions can lead to the stabilization of various exotic spin textures and novel magnetic excitations. The high tunability of magnetic properties by external fields makes bilayer CrCl_{3} a promising candidate for application in the emerging field of two-dimensional quantum spintronics and magnonics.

Magnetic Behavior of Ising Nanowire with Mixed Integer Spins: A Monte Carlo Study

The magnetic and thermal properties of a ferrimagnetic mixed spin-1 and spin-2 cubic Ising nanowire are studied by using the Monte Carlo simulation. The influences of the nearest (JAB) and next-nearest neighbor (JA and JB) exchange interactions and the single-ion anisotropies (DA and DB) on the critical and compensation temperatures are illustrated. Moreover, the phase diagrams on the (temperature, anisotropy) plane are plotted for several values of JA/|JAB|. The system shows very rich and interesting behaviors, namely first and second order phase transitions, tricritical points and compensation phenomenon. Finally, the dependence of hysteresis loops on the anisotropies, the exchange interactions and the temperature is also investigated.

Quantum spin Hall insulator on the honeycomb lattice induced by ferromagnetic exchange interaction

We study the many-body instabilities of correlated electrons on the half-filled honeycomb lattice with enhanced exchange coupling. The system is described by an extended Hubbard model including the next-nearest-neighbor Coulomb repulsion (V2) and the nearest-neighbor exchange interaction (J). We use the truncated unity functional renormalization group approach to determine a schematic ground-state phase diagram with higher resolution in the parameter space of V2 and J. In the absence of the on-site repulsion and presence of sizable next-nearest-neighbor repulsion and enhanced nearest-neighbor exchange interaction, we encounter the quantum spin Hall phase, the spin-Kekulé phase, and the three-sublattice and the incommensurate charge-density-wave phases. We propose a scheme for combining consistently the truncated unity functional renormalization group and the mean-field approximation, which is distinct from the conventional one that directly uses the renormalization-group results as an input for the mean-field calculation. This scheme is used to study in detail the quantum spin Hall phase, presenting some characteristics like the bulk gap, the Chern number and the helical edge states.

Spin-wave dynamics in the KCeS2 delafossite: A theoretical description of powder inelastic neutron-scattering data

Layered rare-earth delafossites $ARX_2$ with $R$ = Yb(III) or Ce(III) have received a lot of interest as potential hosts for a quantum spin-liquid ground state. Some systems of this family that show no long-range order down to the lowest measured temperatures, such as NaYbO$_2$, NaYbS$_2$, and NaYbSe$_2$, are presumed to be in a quantum spin-liquid state. However, other isostructural compounds are known to order antiferromagnetically at subkelvin temperatures. Among them, KCeS$_2$ exhibits stripe-$yz$ magnetic order in the triangular-lattice planes that sets in below 400 mK. Here we investigate the spin-wave spectrum of this ordered phase with powder inelastic neutron scattering and describe it using a model Hamiltonian obtained from first-principles calculations based on the complete $|J,m_J\rangle$ multiplet description of the Ce sites with an anisotropic nearest-neighbor exchange interaction. Combing the current understanding of the exchange interaction in the systems with the effects of texturing in the powder, we have been able to model the inelastic neutron scattering spectrum with high fidelity.

Two-Dimensional Multiferroics with Intrinsic Magnetoelectric Coupling in A-Site Ordered Perovskite Monolayers

The magnetoelectric coupling effect in multiferroics provides a route to realize the control of magnetism by electric field. Here, we demonstrate the coexistence and coupling of ferroelectricity and ferromagnetism in designed A-site ordered perovskite oxide monolayers by combining symmetry analysis and first-principles calculation. These monolayers all exhibit a layered ordering and tilt distortion, and some of them exhibit rotation or Jahn-Teller distortion simultaneously, leading to the emergence of in-plane ferroelectricity. The Mn-based monolayers exhibit robust ferromagnetism, while some monolayers tend to form E-type spin order due to the splitting of the nearest-neighbor exchange interactions. Whether polarization reversal can lead to magnetization reversal depends on the mode of ferroelectric switching, that is, only the ferroelectric switching that reversing the tilt distortion can lead to magnetization reversal. This work demonstrates the feasibility of controlling the direction of magnetization by electric field in the monolayer limit of perovskites.

Instability of the magnetic state of MPX3(M=Mn,Ni;X=S,Se) monolayers induced by strain and doping

The instability of magnetic states in two-dimensional magnetic materials plays a crucial role in the application of spintronics. However, the instability mechanism and magnetic state diagram of $M\text{P}{X}_{3}$ ($M=\mathrm{Mn}$ and Ni and $X=\mathrm{S}$ and Se) monolayer with multiple magnetic states remains elusive. In our paper, we investigate the instability of magnetic states using ab initio calculation combined with an effective Heisenberg model. We obtain a series of magnetic state diagrams of $M\text{P}{X}_{3}$ ($M=\mathrm{Mn}$ and Ni and $X=\mathrm{S}$ and Se) monolayer under the approaches of strain and carrier doping. We clarify the instability mechanism that is mainly determined by the nearest-neighbor exchange interaction between magnetic ions, find a higher N\'eel temperature in ${\mathrm{NiPSe}}_{3}$ monolayer, and unveil a considerable increase in hole effective mass along the armchair direction in the ${\mathrm{NiPSe}}_{3}$ monolayer, which leads to giant anisotropy of carrier mobility. Our results provide a useful guidance to design high-performance spintronics devices based on the $M\text{P}{X}_{3}$ monolayer.

Frustrated Magnetic Interactions and Quenched Spin Fluctuations in CrAs

The discovery of pressure-induced superconductivity in helimagnets (CrAs, MnP) has attracted considerable interest in understanding the relationship between complex magnetism and unconventional superconductivity. However, the nature of the magnetism and magnetic interactions that drive the unusual double-helical magnetic order in these materials remains unclear. Here, we report neutron scattering measurements of magnetic excitations in CrAs single crystals at ambient pressure. Our experiments reveal well defined spin wave excitations up to about 150 meV with a pseudogap below 7 meV, which can be effectively described by the Heisenberg model with nearest neighbor exchange interactions. Most surprisingly, the spin excitations are largely quenched above the Néel temperature, in contrast to cuprates and iron pnictides where the spectral weight is mostly preserved in the paramagnetic state. Our results suggest that the helimagnetic order is driven by strongly frustrated exchange interactions, and that CrAs is at the verge of itinerant and correlation-induced localized states, which is therefore highly pressure-tunable and favorable for superconductivity.

Signatures of spin-liquid state in a 3D frustrated lattice compound KSrFe2(PO4)3 with S = 5/2

A quantum spin-liquid is a spin disordered state of matter in which spins are strongly correlated and highly entangled with low-energy excitations. It has been often found in two-dimensional S = ½, highly frustrated spin networks but rarely observed in three-dimensional (3D) frustrated quantum magnets. Here, KSrFe2(PO4)3, forming a complicated 3D frustrated lattice with a spin moment S = 5/2, is investigated by thermodynamic, neutron diffraction measurements and electronic structure calculations. Despite the relatively sizable Curie–Weiss temperature θCW = −70 K, a conventional magnetic long-range order is confirmed to be absent down to 0.19 K. The magnetic heat capacity data follow the power-law behavior at the lowest temperature region, supporting gapless excitations in a 3D spin-liquid state. Strong geometrical spin frustration responsible for the spin-liquid feature is understood as originating from the almost comparable five competing nearest-neighbor antiferromagnetic exchange interactions, which form the complicated 3D frustrated spin network. All these results suggest that the compound KSrFe2(PO4)3, representing a unique 3D spin frustrated network, could be a rare example of forming a gapless spin-liquid state even with a large spin moment of S = 5/2.

Hedgehog lattice and field-induced chirality in breathing-pyrochlore Heisenberg antiferromagnets

We theoretically investigate a ${J}_{1}\text{\ensuremath{-}}{J}_{3}$ classical Heisenberg model on the breathing pyrochlore lattice, where the nearest-neighbor (NN) exchange interactions for small and large tetrahedra, ${J}_{1}$ and ${J}_{1}^{\ensuremath{'}}$, take different values due to the breathing bond alternation and ${J}_{3}$ is the third NN antiferromagnetic interaction along the bond direction. It is found by means of Monte Carlo simulations that for large ${J}_{3}$, a hedgehog lattice, a three-dimensional periodic array of magnetic monopoles and antimonopoles, emerges in the form of a quadruple-$\mathbf{Q}$ state characterized by the ordering vector of $\mathbf{Q}=(\ifmmode\pm\else\textpm\fi{}\frac{1}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2})$, being irrespective of the signs of ${J}_{1}$ and/or ${J}_{1}^{\ensuremath{'}}$ as long as ${J}_{1}\ensuremath{\ne}{J}_{1}^{\ensuremath{'}}$. It is also found that in an applied magnetic field, there appear six quadruple-$\mathbf{Q}$ states depending on the values of ${J}_{1}$ and ${J}_{1}^{\ensuremath{'}}$, among which three phases including the in-field hedgehog-lattice state exhibit nonzero total chirality ${\mathbit{\ensuremath{\chi}}}^{\mathrm{T}}$ associated with the anomalous Hall effect of chirality origin. In the remaining two chiral phases, which are realized in the presence of ferromagnetic ${J}_{1}$ and/or ${J}_{1}^{\ensuremath{'}}$, the spin structure is not topologically nontrivial, in spite of the fact that ${\mathbit{\ensuremath{\chi}}}^{\mathrm{T}}\ensuremath{\ne}0$. The role of the topological objects of the monopoles in ${\mathbit{\ensuremath{\chi}}}^{\mathrm{T}}$ is also discussed.

Loss of classicality in alternating spin-/spin-1 chain, in the presence of next-neighbor couplings and Dzyaloshinskii–Moriya interactions

We have considered and alternating spin-/spin-1 chain with nearest-neighbor (NN) (J 1), next-nearest neighbor (NNN) (J 2) antiferromagnetic (AFM) Heisenberg interactions along with z-component of the Dzyaloshinskii–Moriya (DM) (Dz ) interaction. The Hamiltonian has been studied using (a) linear spin-wave theory and (b) density matrix renormalization group. The system had been reported earlier as a classical ferrimagnet only when NN exchange interactions are present. Both the AFM NNN interactions and DM interactions introduce strong quantum fluctuations and due to which all the signatures of ferrimagnetism vanishes. We find that the nonzero J 2 introduces strong quantum fluctuations in each of the spin sites due to which the z-components of both spin-1 and spin-1/2 sites average out to be zero. The ground state becomes a singlet. The presence of J 1 along with Dz introduces a short range order but develops long range order along the XY plane. J 1 along with J 2 induces competing phases with structure factor showing sharp and wide peaks, at two different angles reflecting the spin spiral structure locally as well as in the underlying lattice. Interestingly, we find that the D z term removes the local spin spiral structure in z-direction, while developing a spiral order in the XY plane.

Coexistence of spontaneous dimerization and magnetic order in a transverse-field Ising ladder with four-spin interactions

The spin-1/2 transverse field two-leg Ising ladder with nearest-neighbor exchange and plaquette four-spin interaction ${J}_{4}$ is studied analytically and numerically with the density matrix renormalization group approach. The quantum phase diagram in the transverse field $B$ versus ${J}_{4}$ plane has been obtained. There are three different phases: a paramagnetic (PM) phase for high values of the transverse field and, for low values of $B$, a ferromagnetic (FM) ordered phase for small ${J}_{4}$, and a dimerized-rung (DR) phase for large negative values of ${J}_{4}$. All phases are separated by quantum phase transition lines meeting at a multicritical point. The critical lines have been obtained by exploring the entanglement entropy. The results show that along the critical lines the central charge is $c=1/2$, while at the multicritical point one has $c=1$. The scaling dimension of the energy operator is ${X}_{\ensuremath{\epsilon}}=1$, in agreement with the universality class of the critical behavior of the quantum Ising chain. An effective field theory for the multicritical point is also discussed. The FM and the DR order parameters have also been computed and we found a region where the FM and the DR phases coexist.

Exchange interactions in the 1T-VSe2 monolayer and their modulation via electron doping using alkali metal adsorption and the electride substrate.

The modulation of exchange interactions in layered magnets has fundamental research value and potential applications in spintronics. Based on first-principles calculations, the exchange interactions in the experimentally controversial room-temperature ferromagnetic 1T-VSe2 monolayer are systematically studied. It is found that three shells of nearest-neighbor Heisenberg exchange interactions and higher-order interactions are crucial for an accurate description of the magnetism and its thermal stability in the 1T-VSe2 monolayer. Based on our understanding of tuning the magnetic interactions and the magnetic ground state in the 1T-VSe2 monolayer via external factors, two modulation methods, involving adsorption of the alkali metal lithium and the electride Ca2N substrate, are proposed. In both Li-VSe2 and VSe2/Ca2N systems, the strongly frustrated Heisenberg exchange interaction competes with the Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy, leading to complex magnetic ground states, such as antiferromagnetic spin spiral and periodic antiferromagnetic cycloidal states. Moreover, the higher-order exchange interactions play a crucial role in the stabilization of long-range double-row-wise antiferromagnetic states in Li-VSe2 and VSe2/Ca2N. These results highlight the effective manipulation of exchange interactions in two-dimensional magnets.

The role of epitaxial strain on the electronic and magnetic structure of La0.7Sr0.3MnO3/LaCoO3 bilayers

Realizing atomically flat interfaces between the ultrathin perovskite oxides is a challenging task, which usually possess different chemical environments, depending on the terminating lattice planes. Hence, tuning the interfaces across the heterostructures for desired electrical and magnetic properties is a powerful approach in oxide electronics. Focusing on these aspects, in the present work we employ a novel strategy of engineering the interfaces through the layer stacking sequence and degree of strain to probe the changes occurring in the local atomic environment at the interfaces, magnetic behaviour, and electronic properties of ferromagnetic bilayers La0.7Sr0.3MnO3 (LSMO)/LaCoO3 (LCO) grown by the pulsed laser deposition technique. The biaxial tensile strain experienced by these layers drives the ferromagnetic (FM) ordering temperatures to lower values as compared to their bulk counterparts. Interestingly, the bilayer sequence LCO (15 nm)/LSMO (5 nm) (BL2) exhibits large magnetocrystalline anisotropy (Ku ≈ 4.7 × 104 erg/cc) and weak anti-FM coupling across the interface of the two FM constituents, resulting in a partial compensation in the magnetic moment of the system within a specific temperature window (ΔT = 184 − 82 K). However, for T ≤ 82 K, the FM superexchange interaction between the trivalent Co high-spin and low-spin states dominates the overall magnetic ordering in BL2. The magnetodynamic features probed by the frequency dependent FM resonance (FMR) on this system yield the gyromagnetic ratio (γ/2π ∼ 29.22 GHz/T), demagnetization fields (4πMeff ∼ 3770 Oe), and effective damping constant (αeff ∼ 0.0143) for the BL2 configuration. Moreover, the strength of the nearest-neighbor exchange interaction Jeff in the BL2 configuration exhibits linear falloff with the increasing LCO layer thickness (2 nm ≤tLCO≤ 18 nm). This scenario is also consistent with the variation of the effective number of spins available per unit volume [10 cm−3 ≤ NV(×1022) ≤ 2 cm−3] with increasing tLCO. As tLCO approaches negligibly small values (<2 nm), the magnitude of Jeff/kB reaches its maximum ∼5.47 K (for LCO) and 21.93 K (for LSMO), which is in good agreement with Jeff/kB ∼ 5 ± 2 K (20 ± 2 K) for highly epitaxial LCO (LSMO) single layers. These results demonstrate that the layer sequence control of magnetic coupling across the interfaces opens a constructive approach for exploring the novel electronic devices.

Proposed Negative Thermal Expansion in Honeycomb-Lattice Antiferromagnets

We theoretically propose possible magnetism-induced negative thermal expansion in honeycomb-lattice antiferromagnets with edge-sharing networks of $MX_6$ octahedra where $M$ and $X$ are transition-metal and ligand ions, respectively. In this crystal structure, the nearest-neighbor exchange interaction is composed of two competing contributions, i.e., the antiferromagnetic contribution from a direct 180$^\circ$ $M$-$M$ bond and the ferromagnetic contribution from 90$^\circ$ $M$-$X$-$M$ bonds, amplitudes of which have different bond-length dependence. Numerical analysis of the spin-lattice model of the honeycomb-lattice antiferromagnets demonstrates that the negative thermal expansion can occur when the system enters the antiferromagnetic phase with lowering temperature so as to maximize the energy gain associated with the bond-length dependent antiferromagnetic exchange interaction. The present work provides a guiding principle for searching new materials and eventually contributes to diversify the family of materials that host the negative thermal expansion originating from the spin-lattice coupling on the honeycomb lattices or related crystal structures.

Environmental screening and ligand-field effects to magnetism in CrI3 monolayer

We study the microscopic origin of magnetism in suspended and dielectrically embedded CrI3 monolayer by down-folding minimal generalized Hubbard models from ab initio calculations using the constrained random phase approximation. These models are capable of describing the formation of localized magnetic moments in CrI3 and of reproducing electronic properties of direct ab initio calculations. Utilizing the magnet force theorem, we find a multi-orbital super-exchange mechanism as the origin of magnetism in CrI3 resulting from an interplay between ferro- and anti-ferromagnetic Cr-Cr d coupling channels, which is decisively affected by the ligand p orbitals. We show how environmental screening, such as resulting from encapsulation with hexagonal boron nitride, affects the Coulomb interaction in the film and how this controls its magnetic properties. Driven by a non-monotonic interplay between nearest and next-nearest neighbor exchange interactions we find the magnon dispersion and the Curie temperature to be non-trivially affected by the environmental screening.

Tuning exchange interactions in antiferromagnetic Fe/W(001) by 4d transition-metal overlayers

We use first-principles calculations based on density functional theory to study how the magnetic properties of an Fe monolayer on a W(001) surface---exhibiting a $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic ground state---can be modified by an additional $4d$ transition-metal overlayer. To obtain an overview of how the $4d$-band filling influences the exchange interactions in the Fe layer we have calculated the energy dispersion of spin spirals for $4d/\mathrm{Fe}/2\mathrm{W}$ unsupported quadlayers, in which the W(001) substrate is represented by only two atomic layers. Hybridization with the overlayer leads to a reduced ferromagnetic nearest-neighbor exchange interaction and the next-nearest neighbor exchange gains in strength. Surprisingly, we find that the $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic state is unfavorable for all systems with a $4d$ overlayer. For $4d$ overlayers from the beginning (Nb) or end (Pd) of the series we find a ferromagnetic ground state. As one moves to the center of the series there is a transition via a spin spiral (Mo, Rh) to a $p(2\ifmmode\times\else\texttimes\fi{}1)$ antiferromagnetic ground state (Tc, Ru). We have studied the Mo, Ru, and Pd overlayer on Fe/W(001) representing the surface by a sufficiently large number of W layers to obtain bulklike properties in its center. The energy dispersions of spin spirals show qualitatively the same results as those from the $4d/\mathrm{Fe}/2\mathrm{W}$ quadlayers. The Dzyaloshinskii-Moriya interaction calculated upon including spin-orbit coupling shows significant strength and considerable frustration effects. The calculated magnetocrystalline anisotropy energy is large as well. All $4d/\mathrm{Fe}/\mathrm{W}(001)$ films are potential candidates for complex noncollinear spin structures.