Search for 2D Ferromagnets: Molecular Beam Epitaxy is a Critical Tool

正Van der Waals materials with ferromagnetic properties have attracted recent interest because of their promise to enable combination of materials with atomically sharp interfaces, facilitating spin transport and spin injection across these interfaces. The discovery of ferromagnetic ordering to persist to the twodimensional( D) limit for Cr_2Ge_2Te_6~([1]) and CrI_3~([2]) has sparked interest of 2D ferromagnetism and their potential for fundamentally new magnetic van der Waals heterostructures and spintronics applications.     

On Cluster Properties of Classical Ferromagnets in an External Magnetic Field

Correlation functions of ferromagnetic spin systems satisfying a Lee-Yang property are studied. It is shown that, for classical systems in a non-vanishing uniform external magnetic field h, the connected correlation functions decay exponentially in the distances between the spins, i.e., the inverse correlation length (“mass gap”), m(h), is strictly positive. Our proof is very short and transparent and is valid for complex values of the external magnetic field h, provided that \(\mathrm {Re}\, h \not = 0\). It implies a mean-field lower bound on m(h), as \(h \searrow 0\), first established by Lebowitz and Penrose for the Ising model. Our arguments also apply to some quantum spin systems.     

On the spin wave problem in the Heisenberg model of Ferromagnetism

A simple theoretical model has been developed which accounts for the characteristics — voltage, current, resistance, inductance, and energy versus time — of a flash X-ray discharge. The model adopted is valid for single as well as multi-pinched discharges provided that no instabilities are occurring. The calculated curves presented are found to be in good accordance with experimental recordings earlier obtained.     

Ferromagnetism in the Mott insulator Ba2NaOsO6

Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d1 ferromagnetic Mott insulator with an ordered moment of approximately 0.2microB per formula unit and TC=6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet ground state anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.     

缺陷TiO2中铁磁性的第一性原理计算

计算钛空位（V_Ti）和氧空位（V_O）共存情况下未掺杂金红石TiO₂的铁磁性.发现V_O可以产生局域磁矩,由它引起的自旋极化比V_Ti引起的更加局域,导致V_O之间的铁磁耦合作用弱于V_Ti之间的铁磁耦合作用.V_Ti之间的铁磁耦合在引入V_O之后进一步加强.V_O引入的电子调制两个分离的V_Ti之间的长程铁磁耦合.加入V_Ti之后,两个V_O的磁矩猝灭,当V_O的数量多于V_Ti的数量二倍时,V_O会对磁矩有贡献.结果与实验发现的V_O可以提高铁磁有序,并且总的磁矩会随着V_O数量的增多而增加的结果符合很好.     

Ferromagnetism of LaCrOS2

Magnetic measurements and neutron diffraction show that LaCrOS₂ is a ferromagnet. Tc = 35 K. The magnetic space group is P b’ n m’.     

Topological Aspects of Solitons in Ferromagnets

Two kinds of topological soliton （skyrmion and magnetic vortex ring） in ferromagnets are studied. They have the common topological origin, a tensor Hαβ = n→·（δαn→×βδn→）, which describes the non-trivial distribution of local orientation of magnetization n→ at large distances in space. The topological stability of skyrmion is protected by the winding number. Knot-like topological defect as magnetic vortex rings is also studied. On the assumption that magnetic vortex rings are geometric lines, we present their δ-function distribution in ferromagnetic materials. Furthermore, it is briefly shown that Hopf invariant is a proper topological invariant to describe the topology of magnetic vortex rings.     

Ferromagnetism in the Hubbard model

Whether spin-independent Coulomb interaction can be the origin of a realistic ferromagnetism in an itinerant electron system has been an open problem for a long time. Here we study a class of Hubbard models on decorated lattices, which have a special property that the corresponding single-electron Schrödinger equation hasN _d-fold degenerate ground states. The degeneracyN _d is proportional to the total number of sites ||. We prove that the ground states of the models exhibit ferromagnetism when the electron filling factor is not more than and sufficiently close to=N _d/(2||), and paramagnetism when the filling factor is sufficiently small. An important feature of the present work is that it provides examples of three dimensional itinerant electron systems which are proved to exhibit ferromagnetism in a finite range of the electron filling factor.     

Topological Classification of the Magnetization States in the Hole-Including Ferromagnets

In this paper the problem of topological classification of the magnetization states in the hole-including ferromagnets is comprehensively studied. The main result is that the set of homotopy classes of magnetization states, in a ferromagnet including a family of noninteracting holes of cylinder, sphere and annulus types with numbers m, k and l, respectively, can be constructed into a group isomorphic to z^k+l, the (k + l)-dimensional discrete vector group.     

Ferromagnetism in carbon-doped In2O3 thin films

Carbon-doped In₂O₃ thin films exhibiting ferromagnetism at room temperature were prepared on Si (100) substrates by the rf-magnetron co-sputtering technique. The effects of carbon concentration as well as oxygen atmosphere on the ferromagnetic property of the thin films were investigated. The saturated magnetizations of thin films varied from 1.23 to 4.86 emu/cm³ with different carbon concentrations. The ferromagnetic signal was found stronger in samples with higher oxygen vacancy concentrations. In addition, deposition temperature and different types of substrates also affect the ferromagnetic properties of carbon-doped In₂O₃ thin films. This may be related to the oxygen vacancies in the thin film system. The experiment suggests that oxygen vacancies play an important role in introducing ferromagnetism in thin films.     

Spin diffusion in Heisenberg Ferromagnets in the high temperature limit

A study is made of the time dependence of the memory function given by the mode-mode theory for the Heisenberg Ferromagnet in the high temperature limit. For very long times this function is found to go as $\text{t}{}^{\mathrm{<mtext>?5</mtext><mtext>2</mtext>}}$ exp (-Dq²t/2). This leads to the conclusion that spin diffusion is not predicted in the asymptotic time region, in contrast to what has been stated before.     

Features of Spin Waves Focusing in Ferromagnets

Physics of the Solid State - Abstract—Features of focusing spin waves in ferromagnets with magnetic moment exchange interaction with the closest neighbors and second neighbors are explored....     

Ferromagnetism in the degenerate Hubbard model

We study the ground state of the doubly degenerate Hubbard model in the strong coupling limit. For this limit, we obtain new exact results: when there are N ? 1 electrons, the ground state is ferromagnetic and there is a ferromagnetic orbital ordering; when the number of electrons is between N and 2N, the ground state is also ferromagnetic due to the intra-atomic Hund's coupling. For this second case, we give an estimate of the Curie temperature.     

Ferromagnetism of the electron gas

The ground-state energy of the ferromagnetic electron gas is calculated for the relative polarizationζ=0−1 and the interelectron separationr _s =5−12. The method consists in describing the electron gas approximately by a quadratic boson Hamiltonian, and contains the random-phase approximation as a special case. Numerical studies show that in both the random-phase and the present approximations the paramagnetic state has the lowest energy: the energy increases withζ for all values ofr _s considered. In the present approximation instabilities are found to occur forr _s above a critical value, due to exchange processes of finite momentum transfers. Forζ=0 this critical value ofr _s is 9.4; it decreases with increasingζ. However, the fully-polarized state (ζ=1), which lies above the rest, is always stable. The conclusions are as follows: (1) Forr _s <9.4 the electron gas is paramagnetic. (2) Atr _s =9.4 it goes over to the fully-polarized ferromagnetic state. (3) This phase transition requires an energy absorption of 0.03 rydberg per electron. (4) The fully-polarized state is not obtainable as the limitζ→1.     

Studies of the Ferromagnetic State in Quasi-One-Dimensional Organic Polymer Ferromagnets

The possible ferromagnetic ground state of π-conjugated alternant hydrocarbon chains with unequal number of atoms on two sublattices is studied by the peierls-Hubbard model. Allowing for full lattice relaxation, a set of self-consistent iterative equations are established to study this system. The stabilities of the possible ferromagnetic ground states with respect to two kinds of unsymmetric site energies are also studied. We find that the ground state of the system is a stable ferromagnetic state as a result of the topological structure of the network and the electron-electron interaction. And the unsymmetric site energies, induced by substitutions, will severely weaken the stability of the ferromagnetic ground state.