Спиральное магнитное упорядочение и переход металл--диэлектрик в модели Хаббарда на треугольной решeтке

The magnetic phase diagrams of the two-dimensional Hubbard model for isotropic and anisotropic triangular lattices are constructed within the Hartree-Fock and slave boson approximations. The triangular lattice specific non-collinear and spiral magnetic states, as well as phase separation between them, are shown to be realized in a wide range of model parameters along with collinear magnetic states (stripe antiferromagnetic and ferromagnetic). Phase transitions of the first and second order are found, and the boundaries of the phase separation regions are determined. A comparison of the two approximations, Hartree-Fock and slave boson, shows that electronic correlations suppress magnetic states, the region of paramagnetism being expand, for values U/t>5. At the same time, when the Fermi level is near the van Hove singularity, electron correlations do not change the diagrams qualitatively, which is consistent with the previously obtained result for square and cubic lattices. The results are compared with the data available in the literature for other methods and approaches.

Thermodynamics of free and bound magnons in graphene

Symmetry-broken electronic phases support neutral collective excitations. For example, monolayer graphene in the quantum Hall regime hosts a nearly ideal ferromagnetic phase at specific filling factors that spontaneously breaks the spin-rotation symmetry1–3. This ferromagnet has been shown to support spin-wave excitations known as magnons that can be electrically generated and detected4,5. Although long-distance magnon propagation has been demonstrated via transport measurements, important thermodynamic properties of such magnon populations—including the magnon chemical potential and density—have not been measured. Here we present local measurements of electron compressibility under the influence of magnons, which reveal a reduction in the gap associated with the ν = 1 quantum Hall state by up to 20%. Combining these measurements with the estimates of temperature, our analysis reveals that the injected magnons bind to electrons and holes to form skyrmions, and it enables the extraction of free magnon density, magnon chemical potential and average skyrmion spin. Our methods provide a means of probing the thermodynamic properties of charge-neutral excitations that are applicable to other symmetry-broken electronic phases.

Zeroth-Order Nucleation Transition under Nanoscale Phase Separation

Materials with nanoscale phase separation are considered. A system representing a heterophase mixture of ferromagnetic and paramagnetic phases is studied. After averaging over phase configurations, a renormalized Hamiltonian is derived describing the coexisting phases. The system is characterized by direct and exchange interactions and an external magnetic field. The properties of the system are studied numerically. The stability conditions define the stable state of the system. At a temperature of zero, the system is in a pure ferromagnetic state. However, at finite temperature, for some interaction parameters, the system can exhibit a zeroth-order nucleation transition between the pure ferromagnetic phase and the mixed state with coexisting ferromagnetic and paramagnetic phases. At the nucleation transition, the finite concentration of the paramagnetic phase appears via a jump.

A diagrammatic approach to variational quantum ansatz construction

Variational quantum eigensolvers (VQEs) are a promising class of quantum algorithms for preparing approximate ground states in near-term quantum devices. Minimizing the error in such an approximation requires designing ansatzes using physical considerations that target the studied system. One such consideration is size-extensivity, meaning that the ground state quantum correlations are to be compactly represented in the ansatz. On digital quantum computers, however, the size-extensive ansatzes usually require expansion via Trotter-Suzuki methods. These introduce additional costs and errors to the approximation. In this work, we present a diagrammatic scheme for the digital VQE ansatzes, which is size-extensive but does not rely on Trotterization. We start by designing a family of digital ansatzes that explore the entire Hilbert space with the minimum number of free parameters. We then demonstrate how one may compress an arbitrary digital ansatz, by enforcing symmetry constraints of the target system, or by using them as parent ansatzes for a hierarchy of increasingly long but increasingly accurate sub-ansatzes. We apply a perturbative analysis and develop a diagrammatic formalism that ensures the size-extensivity of generated hierarchies. We test our methods on a short spin chain, finding good convergence to the ground state in the paramagnetic and the ferromagnetic phase of the transverse-field Ising model.

Low energy electrodynamics of CrI3 layered ferromagnet

We report on the optical properties from terahertz (THz) to Near-Infrared (NIR) of the layered magnetic compound CrI3 at various temperatures, both in the paramagnetic and ferromagnetic phase. In the NIR spectral range, we observe an insulating electronic gap around 1.1 eV which strongly hardens with decreasing temperature. The blue shift observed represents a record in insulating materials and it is a fingerprint of a strong electron-phonon interaction. Moreover, a further gap hardening is observed below the Curie temperature, indicating the establishment of an effective interaction between electrons and magnetic degrees of freedom in the ferromagnetic phase. Similar interactions are confirmed by the disappearance of some phonon modes in the same phase, as expected from a spin-lattice interaction theory. Therefore, the optical properties of CrI3 reveal a complex interaction among electronic, phononic and magnetic degrees of freedom, opening many possibilities for its use in 2-Dimensional heterostructures.

Anomalous Nernst effect dependence on composition in Fe100-X Rh X alloys

We studied the anomalous Nernst effect in CsCl-type Fe100-X Rh X (X = 45, 48, 50, 52, 54, 60) with a thickness of 50 nm deposited on a thermally oxidized Si substrate. Samples with X < 48 certainly have a ferromagnetic phase, exhibiting the anomalous Nernst effect. The composition dependence of the anomalous Nernst coefficient S yx agreed with the transverse thermoelectric conductivity α yx. |S yx | and |α yx | were maximized at X = 48, which has a ferromagnetic state close to the phase transition state. The maximization of |α yx | at X = 48 can be explained using band structure-based calculations , where |α yx | rapidly increases near the phase transition.

Ferrimagnetic–ferromagnetic phase transition in Mn4N films favored by non-magnetic In doping

The ferrimagnet Mn4N forms a family of compounds useful in spintronics. In a compound comprising non-magnetic and magnetic elements, one basically expects the compound to become ferromagnetic when the proportion of the magnetic element increases. Conversely, one does not expect ferromagnetism when the proportion of the non-magnetic element increases. Surprisingly, Mn4N becomes ferromagnetic at room temperature when the Mn content is decreased by the addition of In atoms, a non-magnetic element. X-ray magnetic circular dichroism measurement reveals that the magnetic moment of Mn atoms at face-centered sites, Mn(II), reverses between x = 0.15 and 0.27 and aligns parallel to that of Mn atoms at corner sites, Mn(I), at x = 0.27 and 0.41. The sign of the anomalous Hall resistivity also changes between x = 0.15 and 0.27 in accordance with the reversal of the magnetic moment of the Mn(II) atoms. These results are interpreted from first-principles calculation that the magnetic moment of Mn(II) sites which are the nearest neighbors to the In atom align to that of Mn(I) sites.

Systematic study of the synthesis of nanostructured multiferroic films and their structural, electronic and magnetic properties

<p>Multiferroics are unique materials that display multiple ferroic properties (ferroelectricity, ferromagnetism and ferroelasticity) simultaneously. A number of materials containing bismuth have intrinsic multiferroic properties, including BiFeO₃ and BiCrO₃. Among them, BiFeO₃ has attracted widespread attention because BiFeO₃ was the first material to display multiferroic behaviour at ambient temperature. A weak ferromagnetism occurs only at low temperatures depending on synthesis conditions. This thesis reports the structural, magnetic and optical properties of nanostructured BiFeO₃ thin films prepared by two novel approaches of ion beam sputtering and ion implantation techniques. Nanocrystalline BiFeO₃ films were prepared at ambient temperature by sputtering and thermal annealing at 500 °C in an oxygen atmosphere. The annealing resulted in the formation of multiferroic BiFeO₃ phase with a reduction of iron oxide and bismuth phases. Superparamagnetism was observed and could be attributed to magnetite and maghemite nanoparticles. The magnetic properties were mainly due to magnetite and maghemite nanoparticles. The saturation magnetic moment was 60% lower after annealing, which was due to Fe in phases of iron oxide being incorporated into BiFeO₃ nanoparticles. An exchange bias was observed before and after annealing. The exchange bias cannot be attributed to BiFeO₃ structure. Instead, the exchange has likely arisen from magnetite and maghemite cores with spin-disordered shells. Piezoelectric responses measured by piezoelectric force microscopy confirmed the presence of BiFeO₃ ferroelectric material. The Magneto-optical Kerr effect (MOKE) and optical studies were used to calculate an anomalously high Verdet constant. The MOKE and magnetic circular dichroism (MCD) displayed a significant modification in function of the wavelength. Further increasing the annealing temperature lead to an increase in iron oxide phases, while increasing the annealing duration reduced the iron oxide phases, however this increases the fraction of Bi₂Fe₄O₉ and Bi₂O₃. Another approach to synthesise BiFeO₃ thin film was investigated by bismuth ion implantation into iron oxide thin film. An as-made iron oxide film subsequently implanted with bismuth and annealed showed a 6.5% reduction of the ferromagnetic phase fraction. An annealed iron oxide film subsequently implanted with bismuth and annealed show that the ferromagnetic phase was present at less than 4% while Fe₃O₄ and γ-Fe₂O₃ increased to 7%. The coercive field is affected by annealing. However, this field is not affected by the bismuth implantation. For the first-time, a preliminary investigation reporting the implantation of Bi then Fe then O into SiO₂:Si was made with the aim to synthesise BiFeO₃ films and magnetic nanoparticles. The implantation of Fe then O then Bi into SiO₂:Si contained a mix of iron oxides: α-Fe₂O₃ and Fe₃O₄, as confirmed by Raman spectroscopy and X-ray diffraction, while γ-Fe₂O₃ was most likely also present in the film. The as-implanted sample displayed a sign of a superparamagnetic phase that was lost with annealing the sample. Preliminary investigations of another multiferroic material, BiCrO₃, were carried out. Thin films of BiCrO₃ were prepared by ion beam sputtering and annealing the sample in an oxygen atmosphere which lead to BiCrxOy with chromium oxides and bismuth oxide phases. Magnetic enhancement was observed when annealing above 700 °C. Annealing in an oxygen atmosphere followed by an argon atmosphere created a superparamagnetic phase that was not visible under other annealing conditions.</p>

Systematic study of the synthesis of nanostructured multiferroic films and their structural, electronic and magnetic properties

<p>Multiferroics are unique materials that display multiple ferroic properties (ferroelectricity, ferromagnetism and ferroelasticity) simultaneously. A number of materials containing bismuth have intrinsic multiferroic properties, including BiFeO₃ and BiCrO₃. Among them, BiFeO₃ has attracted widespread attention because BiFeO₃ was the first material to display multiferroic behaviour at ambient temperature. A weak ferromagnetism occurs only at low temperatures depending on synthesis conditions. This thesis reports the structural, magnetic and optical properties of nanostructured BiFeO₃ thin films prepared by two novel approaches of ion beam sputtering and ion implantation techniques. Nanocrystalline BiFeO₃ films were prepared at ambient temperature by sputtering and thermal annealing at 500 °C in an oxygen atmosphere. The annealing resulted in the formation of multiferroic BiFeO₃ phase with a reduction of iron oxide and bismuth phases. Superparamagnetism was observed and could be attributed to magnetite and maghemite nanoparticles. The magnetic properties were mainly due to magnetite and maghemite nanoparticles. The saturation magnetic moment was 60% lower after annealing, which was due to Fe in phases of iron oxide being incorporated into BiFeO₃ nanoparticles. An exchange bias was observed before and after annealing. The exchange bias cannot be attributed to BiFeO₃ structure. Instead, the exchange has likely arisen from magnetite and maghemite cores with spin-disordered shells. Piezoelectric responses measured by piezoelectric force microscopy confirmed the presence of BiFeO₃ ferroelectric material. The Magneto-optical Kerr effect (MOKE) and optical studies were used to calculate an anomalously high Verdet constant. The MOKE and magnetic circular dichroism (MCD) displayed a significant modification in function of the wavelength. Further increasing the annealing temperature lead to an increase in iron oxide phases, while increasing the annealing duration reduced the iron oxide phases, however this increases the fraction of Bi₂Fe₄O₉ and Bi₂O₃. Another approach to synthesise BiFeO₃ thin film was investigated by bismuth ion implantation into iron oxide thin film. An as-made iron oxide film subsequently implanted with bismuth and annealed showed a 6.5% reduction of the ferromagnetic phase fraction. An annealed iron oxide film subsequently implanted with bismuth and annealed show that the ferromagnetic phase was present at less than 4% while Fe₃O₄ and γ-Fe₂O₃ increased to 7%. The coercive field is affected by annealing. However, this field is not affected by the bismuth implantation. For the first-time, a preliminary investigation reporting the implantation of Bi then Fe then O into SiO₂:Si was made with the aim to synthesise BiFeO₃ films and magnetic nanoparticles. The implantation of Fe then O then Bi into SiO₂:Si contained a mix of iron oxides: α-Fe₂O₃ and Fe₃O₄, as confirmed by Raman spectroscopy and X-ray diffraction, while γ-Fe₂O₃ was most likely also present in the film. The as-implanted sample displayed a sign of a superparamagnetic phase that was lost with annealing the sample. Preliminary investigations of another multiferroic material, BiCrO₃, were carried out. Thin films of BiCrO₃ were prepared by ion beam sputtering and annealing the sample in an oxygen atmosphere which lead to BiCrxOy with chromium oxides and bismuth oxide phases. Magnetic enhancement was observed when annealing above 700 °C. Annealing in an oxygen atmosphere followed by an argon atmosphere created a superparamagnetic phase that was not visible under other annealing conditions.</p>

Fеatures of exchange interactions of the B-sublattice ions in the La0,5Sr0,5Co1–x Nix O3–d system

A comprehensive study of the crystal structure, magnetic and magnetotransport properties of the La0.5Sr0.5Co1–x Nix O3–d cobaltite system (x = 0.1–0.16) was carried out. The X-ray measurement results indicate that the unit cell of all solid solutions of the system is cubic and is described by the space group Pm3m. It is found that with an increase in the 540 Doklady of the National Academy of Sciences of Belarus, 2021, vol. 65, no. 5, рр. 539–545 Ni content, the Curie temperature (TC) decreases from 230 to 180 K, as well as magnetization values. The magnetic transition is blurred across the field. The iodometric studies show that the concentration of Co4+ ions in all samples does not exceed 35 %. The chemical substitution of Co ions by Ni ones does not result in significant modification of the unit cell parameters, which may indicate a spin crossover of Co ions. The temperature dependence of resistivity is metallic in character, which indicates the stability of the main conducting ferromagnetic phase. The nature of exchange interactions of different signs between B-sublattice ions completely determines the behavior of the system. An increase in the content of Ni ions leads both to decrease the component of ferromagnetic exchange interactions between Co3+ ions in the intermediate spin state and to increase the fraction of antiferromagnetic and weaker ferromagnetic interactions. In addition, presumably the Co4+ ion can stabilize the high spin state of the closestCo3+ ion and in the next two coordination spheres it can stabilize the Co3+ ion in the low spin state, i. e. the ferromagnetic complexes Co4+–Co3+ (HS) are shielded by the diamagnetic shell of low spin Co3+ ions, which results in decreasing the magnetization values.