Long-range Exchange Interaction Research Articles

Critical behavior and tunable magnetocaloric effect in (Gd1-xErx)3Al2 alloys

The magnetic properties, magnetocaloric effect, and the critical behavior near Curie temperature (TC) were systematically studied in a series of (Gd1-xErx)3Al2 (x = 0, 0.1, 0.2, 0.3) alloys. As the Er content increases, both the TC and lattice constant gradually decrease. The maximum magnetic entropy change and refrigerant capacity remain relatively high, ranging from −5.8 to −8.5J/kg K and 209.0 to 290.8 J/kg respectively, for a field change of 5 T. The critical exponents, derived from the field dependence of magnetic entropy change, conform to scaling theory and are consistent with those obtained using the Kouvel–Fisher method. These findings suggest that the reliability of the obtained critical exponents and imply the presence of long-range exchange interactions in (Gd1-xErx)3Al2 alloys.

Experimental and computational Insights Into the magnetic anisotropy and magnetic behaviour of layered room-temperature ferromagnet Cr1.38Te2

We investigate the structural, magnetocrystalline anisotropy, critical behaviour, and magnetocaloric effect in the layered room-temperature monoclinic ferromagnet Cr1.38Te2. The critical behavior is studied by employing various techniques such as the modified Arrott plot (MAP), the Kouvel-Fisher method (KF), and the critical isothermal analysis (CI) around the Curie temperature (T C ) of 316 K. The derived critical exponents are self-consistent and obey the rescaling analysis. The Monte-Carlo simulations reproduce the experimentally obtained critical exponents. However, the derived critical exponents do not suggest any single universality class of the magnetic interactions. On the other hand, the renormalization group (RG) theory suggests 3D-Ising type long-range exchange interactions [J(r)], decaying with distance (r) as J(r) = r −(d+σ) = r −4.73. Further, magnetocrystalline anisotropy energy density (K u ) is found to be temperature dependent. The ground state magnetic easy-axis (b-axis) is identified by analyzing the magnetocrystalline anisotropy energy (MAE) using the density functional theory calculations. Maximum entropy change - ΔSmmax ≈2.51 J/kg-K is found near the T C .

Critical behaviors of van der Waals itinerant ferromagnet Fe3.8GaTe2

The critical behavior of the van der Waals ferromagnet Fe3.8GaTe2 was systematically studied through measurements of isothermal magnetization, with the magnetic field applied along the c-axis. Fe3.8GaTe2 undergoes a non-continuous paramagnetic to ferromagnetic phase transition at the Curie temperature T c ∼ 355 K. A comprehensive analysis of isotherms around T c utilizing the modified Arrott diagram, the Kouvel–Fisher method, the Widom scaling law, and the critical isotherm analysis yielded the critical exponent of β= 0.411, γ = 1.246, and δ = 3.99. These critical exponents are found to be self-consistent and align well with the scaling equation at high magnetic fields, underscoring the reliability and intrinsic nature of these parameters. However, the low-field data deviates from the scaling relation, exhibiting a vertical trend when T < T c. This discrepancy suggests the occurrence of a first-order phase transition in the crystal under a low magnetic field when T< T c. Mössbauer spectra were employed to provide insights into the critical behaviors of magnetic moments at different sites, including (Fe1)A, (Fe1)B, and Fe2. The results are consistent with the isothermal magnetization analysis. Additionally, the renormalization group theory analysis reveals that the spin coupling within the Fe3.8GaTe2 crystal follows the three-dimensional Heisenberg ({d:n} = {3:3}) type with long-range magnetic and exchange interaction decays with a distance as J(r) ≈ r −4.80.

Longitudinal-transverse splitting and fine structure of Fermi polarons in two-dimensional semiconductors

Interaction of excitons with resident charge carriers in semiconductors gives rise to bound three-particle complexes, trions, whose optical response is conveniently described in the framework of many-body correlated Fermi polaron states. These states are formed as a result of correlation of photocreated trion with the Fermi sea hole and possess the angular momentum component of ±1 depending on the helicity of the photon. We study theoretically the energy spectrum fine structure of Fermi polarons in two-dimensional semiconductors based on transition metal dichalcogenides. We demonstrate both by the symmetry analysis and microscopic calculation that the Fermi polarons with nonzero in-plane wavevector k are split, similarly to the neutral exciton states, into the linearly polarized longitudinal and transverse, with respect to the k, states. The origin of this longitudinal-transverse splitting is the long-range electron-hole exchange interaction that can be also described as the interaction of Fermi polarons with their induced electromagnetic field. The effective Hamiltonian describing the Fermi polaron fine structure is derived, and its parameters are determined from the microscopic model.

Biaxial strain tuning of exciton energy and polarization in monolayer WS2

We perform micro-photoluminescence and Raman experiments to examine the impact of biaxial tensile strain on the optical properties of WS2 monolayers. A strong shift on the order of −130 meV per % of strain is observed in the neutral exciton emission at room temperature. Under near-resonant excitation, we measure a monotonic decrease in the circular polarization degree under the applied strain. We experimentally separate the effect of the strain-induced energy detuning and evaluate the pure effect coming from the biaxial strain. The analysis shows that the suppression of the circular polarization degree under the biaxial strain is related to an interplay of energy and polarization relaxation channels as well as to variations in the exciton oscillator strength affecting the long-range exchange interaction.

Universal magnetic proximity effect in ferromagnet-semiconductor quantum well hybrid structures.

Hybrid ferromagnet-semiconductor systems possess new outstanding properties, which emerge when bringing magnetic and semiconductor materials into contact. In such structures, the long-range magnetic proximity effect couples the spin systems of the ferromagnet and semiconductor on distances exceeding the carrier wave function overlap. The effect is due to the effective p-d exchange interaction of acceptor-bound holes in the quantum well with d-electrons of the ferromagnet. This indirect interaction is established via the phononic Stark effect mediated by the chiral phonons. Here, we demonstrate that the long-range magnetic proximity effect is universal and observed in hybrid structures with diverse magnetic components and potential barriers of various thicknesses and compositions. We study hybrid structures consisting of a semimetal (magnetite Fe3O4) or dielectric (spinel NiFe2O4) ferromagnet and a CdTe quantum well separated by a nonmagnetic (Cd,Mg)Te barrier. The proximity effect is manifested in the circular polarization of the photoluminescence corresponding to the recombination of photoexcited electrons with holes bound to shallow acceptors in the quantum well induced by magnetite or spinel itself, in contrast to interface ferromagnet in case of metal-based hybrid systems. A nontrivial dynamics of the proximity effect is observed in the studied structures due to recombination-induced dynamic polarization of electrons in the quantum well. It enables the determination of the exchange constant Δexch ≈ 70μeV in a magnetite-based structure. The universal origin of the long-range exchange interaction along with the possibility of its electrical control offers prospects for the development of low-voltage spintronic devices compatible with existing solid-state electronics.

How Accurately Can DFT Describe Non-valence Anions?

Weakly bound non-valence anions are molecular systems where the excess electron stabilizes in a very diffuse orbital whose size, shape, and binding energy (∼1-100 meV) are governed by the long-range electrostatic potential of the molecule. Its binding energy comes mainly from charge-dipole or charge-multipole interactions or dispersion forces. While highly correlated methods, like coupled cluster methods, are considered to be the state of the art for describing anionic systems, especially when the electron lies in a very diffuse orbital, we consider here the possibility to use DFT-based calculations. In such molecular anions, the outer electron experiences long-range exchange and correlation interactions. We show that DFT can describe long-range bound states provided that a correct asymptotic exchange and correlation potential is used, namely, that from a range-separated hybrid functional. This opens an alternative to the computationally demanding highly correlated method calculations. It is also suggested that the study of weakly bound anions could help in the construction of new DFT potentials to study systems where nonlocal effects are significant.

Excitation transfer in disordered spin chains with long-range exchange interactions

We examine spin excitation or polarization transfer via spin chains with long-range exchange interactions in the presence of diagonal and off-diagonal disorder. To this end, we determine the mean localization length of the single-excitation eigenstates of the chain for various strengths of the disorder. We then identify the energy eigenstates of the system with large localization length and sufficient support at the chain boundaries that are suitable to transfer an excitation between the sender and receiver spins connected to the opposite ends of the chain. We quantify the performance of two transfer schemes involving weak static couplings of the sender and receiver spins to the chain, and time-dependent couplings realizing stimulated adiabatic passage of the excitation via the intermediate eigenstates of the chain which exhibits improved performance.

Theoretical study on the magnetic properties of cathode materials in the lithium-ion battery.

The layered LiMO2 (M = Co, Ni, and Mn) materials are commonly used as the cathode materials in the lithium-ion battery due to the distinctive layer structure for lithium extraction and insertion. Although their electrochemical properties have been extensively studied, the structural and magnetic properties of LiNiO2 are still under considerable debate, and the magnetic properties of monoclinic LiMnO2 are seldom reported. In this work, a detailed study of LiNiO2, LiMnO2, and a half-doped material LiNi0.5Mn0.5O2 is performed via both first-principles calculations and Monte Carlo simulations based on the effective spin Hamiltonian model. Through considering different structures, it is verified that a structure with a zigzag-type pattern is the most stable one of LiNiO2. Moreover, in order to figure out the magnetic properties, the spin exchange interactions are calculated, and then magnetic ground states are predicted in these three systems. The results show that LiNiO2 forms a spiral order that is caused by the competition from both the short-range and long-range spin exchange interactions, whereas the magnetic ground state of LiMnO2 is collinearly antiferromagnetic due to its nearest and next-nearest neighbor antiferromagnetic spin exchange interactions. However, LiNi0.5Mn0.5O2 is collinearly ferrimagnetic because of the ferromagnetic nearest neighbor Ni-Ni and Mn-Mn exchange interactions. Our work demonstrates the competition between the different exchange interactions in these cathode materials, which may be relevant to the performance of the lithium-ion battery.

RKKY Interaction in Spin Valve Structure with LaAlO&lt;sub&gt;3&lt;/sub&gt; Spacer

RKKY interaction is an indirect interaction between localized magnetic moments mediated by conduction electrons. RKKY exchange interaction is mostly found in multilayer involving conductors as the non-magnetic spacer. However, recent experiment shows that LaAlO3 spacer can intermediate long-range exchange interaction even though it is an insulator. Here we study the mechanism of RKKY interaction in LaAlO3. To study the effect of charge transfer, we perform a density functional theory (DFT) approach to obtain the band structure of LaAlO3 with charge doping. Undoped LaAlO3 has an indirect band gap around 3 eV with peak valence at R symmetry point and lowest conduction point at Γ point. We show that the conducting characteristic arises from Fermi energy shifts via charge doping. By adding charge doping, we show that LaAlO3 can have conductor characteristics, in agreement with the charge transfer experiment on LaAlO3. The spin-polarized conduction electron is measurable as additional magnetization of the system. Furthermore, the conducting LaAlO3 is able to mediate RKKY interaction in spin valve structure.

Intrinsic chiral field as vector potential of the magnetic current in the zig-zag lattice of magnetic dipoles

Chiral magnetic insulators manifest novel phases of matter where the sense of rotation of the magnetization is associated with exotic transport phenomena. Effective control of such phases and their dynamical evolution points to the search and study of chiral fields like the Dzyaloshinskii–Moriya interaction. Here we combine experiments, numerics, and theory to study a zig-zag dipolar lattice as a model of an interface between magnetic in-plane layers with a perpendicular magnetization. The zig-zag lattice comprises two parallel sublattices of dipoles with perpendicular easy plane of rotation. The dipolar energy of the system is exactly separable into a sum of symmetric and antisymmetric long-range exchange interactions between dipoles, where the antisymmetric coupling generates a nonlocal Dzyaloshinskii–Moriya field which stabilizes winding textures with the form of chiral solitons. The Dzyaloshinskii–Moriya interaction acts as a vector potential or gauge field of the magnetic current and gives rise to emergent magnetic and electric fields that allow the manifestation of the magnetoelectric effect in the system.

Coherent spin dynamics of excitons in strained monolayer semiconductors

Back in 1958 E.I. Rashba predicted the so-called resonant (long-range exchange) interaction between an electron and a hole in Wannier-Mott excitons. This interaction underlies exciton fine structure and its spin dynamics. Here, the author develops a model of the coherent exciton spin-valley dynamics in two-dimensional transition metal dichalcogenides caused by the interplay of the wave-vector dependent exchange interaction ($L\phantom{\rule{0}{0ex}}T$ splitting) and wave-vector independent anisotropic splitting induced by elastic strain. The author uncoversdifferent regimes of coherent spin dynamics of two-dimensional excitons and outlines the possibilities to disentangle the strain-induced from the $L\phantom{\rule{0}{0ex}}T$ splitting.

Magnetocaloric effect in multilayers studied by membrane-based calorimetry

We study magnetic multilayers, incorporating dilute ferromagnetic spacers between strongly-ferromagnetic layers exhibiting a proximity-enhanced magnetocaloric effect (MCE). Using magnetometry and direct measurements of the adiabatic temperature change based on a nanomembrane-calorimetry, we find that the MCE in the studied multilayer is indeed enhanced compared to that in the bulk spacer material. We develop a phenomenological numerical model of the studied trilayer and find that a long-range exchange interaction through the weakly-ferromagnetic spacer is required to adequately describe the magnetic and magnetocaloric properties of the system.

Fractional dynamics and modulational instability in long-range Heisenberg chains

We study the effective dynamics of ferromagnetic spin chains in presence of long-range interactions. We consider the Heisenberg Hamiltonian in one dimension for which the spins are coupled through power-law long-range exchange interactions with exponent α. We add to the Hamiltonian an anisotropy in the z-direction. In the framework of a semiclassical approach, we use the Holstein–Primakoff transformation to derive an effective long-range discrete nonlinear Schrödinger equation. We then perform the continuum limit and we obtain a fractional nonlinear Schrödinger-like equation. Finally, we study the modulational instability of plane-waves in the continuum limit and we prove that, at variance with the short-range case, plane waves are modulationally unstable for α<3. We also study the dependence of the modulation instability growth rate and critical wave-number on the parameters of the Hamiltonian and on the exponent α.

Spin-ice behavior of mixed pyrochlore Dy2GaSbO7 exhibiting enhanced Pauling zero-point entropy

Herein, the low-temperature magnetothermal properties and spin-ice physics of a mixed pyrochlore, Dy2GaSbO7 (DGS), are investigated through dc magnetization, ac susceptibility, and heat capacity experiments, and crystal-field computation. A Curie-Weiss (CW) fit to the dc susceptibility data yields effective magnetic moment, μeff = 10.0 μB/Dy3+ and CW temperature, θCW = +1.62 K. Magnetization attains a saturation value of Msat = 5.06μB/Dy above H ≈ 8 T and at T = 2 K, which is half of the μeff due to the effective Ising-type anisotropy along local 〈111〉 axes of the elementary Dy4O′ tetrahedron, illustrating DGS as an effective spinS∼ = 1/2 system. The dc susceptibility data are interpreted satisfactorily within the framework of a crystal-field theory and a mean-field approximation taking account of long-range dipolar interactions and anisotropic exchange interactions between the nearest-neighbor Dy3+-ions. AC susceptibility exhibits anomaly at a temperature Tice ≈ 3.13 K at frequency of f = 111 Hz, which is a signature of dynamical freezing of spins into a disordered state in compliance with the spin-ice rules. Heat capacity at zero field also shows a peak at 1.36 K and a low kink at 3.25 K which smears out on application of the magnetic field. The calculated magnetic entropy of DGS at zero field is 2.82 J K−1(mol-Dy)−1, which is much less than the Pauling spin-ice value of 4.1 J K−1mol−1. The spin-ice entropy is restored with a value of 4.50 J K−1(mol-Dy)−1 at a field of 1 T. The zero-point entropy of DGS becomes 2.94 J K−1(mol-Dy)−1 which is larger than Pauling zero-point entropy of 1.686 J K−1mol−1 for cannonical dipolar spin-ice materials, e.g., Dy2Ti2O7 and Ho2Ti2O7. The enhanced entropy of DGS may be induced due to random chemical disorder and varied chemical pressure of B-site (GaSb tetrahedra) ions. Thus, the present study creates a new pathway to extend the horizon of spin-ice physics.

Valley-magnetophonon resonance for interlayer excitons

Heterobilayers consisting of MoSe2 and WSe2 monolayers can host optically bright interlayer excitons with intriguing properties such as ultralong lifetimes and pronounced circular polarization of their photoluminescence due to valley polarization, which can be induced by circularly polarized excitation or applied magnetic fields. Here, we report on the observation of an intrinsic valley-magnetophonon resonance for localized interlayer excitons promoted by invervalley hole scattering. It leads to a resonant increase of the photoluminescence polarization degree at the same field of 24.2 Tesla for H-type and R-type stacking configurations despite their vastly different excitonic energy splittings. As a microscopic mechanism of the hole intervalley scattering we identify the scattering with chiral TA phonons of MoSe2 between excitonic states mixed by the long-range electron hole exchange interaction.

MN4 embedded graphene layers: Tunable decay rate of the RKKY interaction

One of the most important tasks in the development of high-performance spintronic devices is the preparation of two dimensional (2D) magnetic layers with long-range exchange interactions. MN4 embedded graphene (MN4-G) layers, with M being transition metal elements, are experimentally accessible 2D layers, which exhibit interesting magnetic properties. In this paper, by employing the spin-polarized density functional theory (SP-DFT), we study MN4-G layers with a special focus on the behavior of the indirect M-M exchange interactions, and demonstrate that the MN4-Gs with M = Fe, Mn and Co, are 2D anisotropic magnetic layers with Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By examining the electronic configurations of the M atoms for various M-M spacers, we demonstrate that the RKKY interaction in such layers are tunable and exhibiting an unusual prolonged decay (r to the power of minus n, n between 0.5 and 2). In addition, we investigate the influence of the CuN4 moiety in the graphene host, and show that, in contrary to the other MN4-Gs, the 2D CuN4-G layer behaves as decoupled one-dimensional spin chains, regardless of the spacer lengths.

Optical Signatures of Periodic Magnetization: The Moiré Zeeman Effect.

Detecting magnetic order at the nanoscale is of central interest for the study of quantum magnetism in general, and the emerging field of moiré magnets in particular. Here, we analyze the exciton band structure that arises from a periodic modulation of the valley Zeeman effect. Despite long-range electron-hole exchange interactions, we find a sizable splitting in the energy of the bright circularly polarized exciton Umklapp resonances, which serves as a direct optical probe of magnetic order. We first analyze quantum moiré magnets realized by periodic ordering of electron spins in Mott-Wigner states of transition metal dichalcogenide monolayers or twisted bilayers: we show that spin valley-dependent exciton-electron interactions allow for probing the spin-valley order of electrons and demonstrate that it is possible to observe unique signatures of ferromagnetic order in a triangular lattice and both ferromagnetic and Néel order in a honeycomb lattice. We then focus on semiclassical moiré magnets realized in twisted bilayers of ferromagnetic materials: we propose a detection scheme for moiré magnetism that is based on interlayer exchange coupling between spins in a moiré magnet and excitons in a transition metal dichalcogenide monolayer.

Quantum dynamics of disordered arrays of interacting superconducting qubits: Signatures of quantum collective states

We study theoretically the collective quantum dynamics occurring in various interacting superconducting qubits arrays (SQAs) in the presence of a spread of individual qubit frequencies. The interaction is provided by mutual inductive coupling between adjacent qubits (short-range Ising interaction) or inductive coupling to a low-dissipative resonator (long-range exchange interaction). In the absence of interaction the Fourier transform of temporal correlation function of the total polarization ($z$-projection of the total spin), i.e. the dynamic susceptibility $C(\omega)$, demonstrates a set of sharp small magnitude resonances corresponding to the transitions of individual superconducting qubits. We show that even a weak interaction between qubits can overcome the disorder with a simultaneous formation of the collective excited states. This collective behavior manifests itself by a single large resonance in $C(\omega)$. In the presence of a weak non-resonant microwave photon field in the low-dissipative resonator, the positions of dominant resonances depend on the number of photons, i.e. the collective ac Stark effect. Coupling of an SQA to the transmission line allows a straightforward experimental access of the collective states in microwave transmission experiments and, at the same time, to employ SQAs as sensitive single-photon detectors.

Long-range order and quantum criticality in antiferromagnetic chains with long-range staggered interactions.

We study quantum phase transitions in Heisenberg antiferromagnetic chains with a staggered power-law decaying long-range interactions. Employing the density-matrix renormalization group (DMRG) algorithm and the fidelity susceptibility as the criticality measure, we establish more accurate values of quantum critical points than the results obtained from the spin-wave approximation, quantum Monte Carlo, and DMRG in literatures. The deviation is especially evident for strong long-range interactions. We extend isotropic long-range interactions to the anisotropic cases and find that kaleidoscope of quantum phases emerge from the interplay of anisotropy of the long-range exchange interaction and symmetry breaking. We demonstrate nonfrustrating long-range interactions induce the true long-range order in Heisenberg antiferromagnetic chains with a continuous symmetry breaking, lifting the restrictions imposed by the Mermin-Wagner theorem.