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Kagome magnets with diverse topological quantum responses are crucial for next-generation topological engineering. The anisotropic magnetism and band evolution induced by ferromagnetic phase transition (FMPT) is reported in a newly discovered titanium-based kagome ferromagnet SmTi 3 Bi 4 , which features a distorted Ti kagome lattice and Sm atomic zig-zag chains. Temperature-dependent resistivity, heat capacity, and magnetic susceptibility reveal a ferromagnetic ordering temperature T c of 23.2 K. A large magnetic anisotropy, observed by applying the magnetic field along three crystallographic axes, identifies the b axis as the easy axis. Angle-resolved photoemission spectroscopy with first-principles calculations unveils the characteristic kagome motif, including the Dirac point at the Fermi level and multiple van Hove singularities. Notably, a band splitting and gap closing attributed to FMPT is observed, originating from the exchange coupling between Sm 4 f local moments and itinerant electrons of the kagome Ti atoms, as well as the time-reversal symmetry breaking induced by the long-range ferromagnetic order. Considering the large in-plane magnetization and the evolution of electronic structure under the influence of ferromagnetic ordering, such materials promise to be a new platform for exploring the intricate electronic properties and magnetic phases based on the kagome lattice.

Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh 6 Ge 4 and its distinction from other U-based heavy fermion metals such as UGe 2 , we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large- N Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three different ferromagnetic phases including a half-metallic spin selective Kondo insulator with a constant magnetization. The Fermi surface topologies are found to change abruptly between different phases, consistent with that observed in UGe 2 . At finite temperatures, we predict the development of Kondo hybridization well above the ferromagnetic long-range order and its relocalization near the phase transition, in good agreement with band measurements in CeRh 6 Ge 4 . Our results highlight the importance of magnetic anisotropy and provide a unified theory for understanding the ferromagnetic quantum phase transitions in heavy fermion metals.

This paper deals with the preparation of multiferroic Bi 1− x Ca x FeO 3 ( x  = 0–0.2) ceramics with the sol–gel method and a study on the influence of Ca 2+ doping on the structure, dielectric, and ferromagnetic properties of BiFeO 3 ceramics. The result shows that the XRD analysis reveals a phase transition in Ca-doped BiFeO 3 from rhombohedral to orthorhombic when x is greater than 0.1. The dielectric constant ( ε r ) of Bi 0.9 Ca 0.1 FeO 3 measured at 1 kHz is about seven times greater than that of BiFeO 3 , and Bi 0.8 Ca 0.2 FeO 3 is less than one-tenth of BiFeO 3 . It might be understood in terms of the dipole polarization, oxygen vacancy and lattice phase transition. Magnetic measurements show that the M-H of Bi 1 − x Ca x FeO 3 samples exhibit unsaturated and symmetric magnetic hysteresis loops with the increase of Ca 2+ , indicating the weakly ferromagnetic behavior. It indicates that there is coexistence of Fe 2+ and Fe 3+ in Bi 1 − x Ca x FeO 3 samples according to the XPS spectrum. The ratio of Fe 2+ /Fe 3+ increases with doping Ca 2+ content and the magnetic properties of BiFeO 3 are enhanced. It is evident that the ferromagnetic phase transition of BiFeO 3 samples occurs at 878 K by measuring the M–T and DSC curves. The T N of BiFeO 3 will be reduced from 644 to 638 K and the T M does not change slightly at 878 K with increasing Ca 2+ content. T N and T M of Bi 1 − x Ca x FeO 3 change depends mainly on the magnetic structure of relative stability and Fe–O–Fe super-exchange strength. Graphical abstract The grains of BiFeO 3 sample appear cube shaped, while more irregular grains of Bi 1 − x Ca x FeO 3 sample are formed with doping Ca 2+ from Fig. 2. M-H of BiFeO 3 exhibit saturated and symmetric magnetic hysteresis loops at room temperature with doping Ca 2+ , indicating that Ca 2+ doping can improve the ferromagnetic properties of BiFeO 3 . It is evident that the ferromagnetic phase transition of Bi 1- x Ca x FeO 3 samples occurs at 878K by measuring the M-T and DSC curves.

In condensed-matter systems, higher temperatures typically disfavour ordered phases, leading to an upper critical temperature for magnetism, superconductivity and other phenomena. An exception is the Pomeranchuk effect in 3 He, in which the liquid ground state freezes upon increasing the temperature 1 , owing to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite-temperature dynamics of spin and valley isospins in magic-angle twisted bilayer graphene 2 . Notably, a resistivity peak appears at high temperatures near a superlattice filling factor of −1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted-field magnetotransport and thermodynamic measurements of the in-plane magnetic moment show that the resistivity peak is connected to a finite-field magnetic phase transition 3 at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes the phase relative to an isospin-unpolarized Fermi liquid phase at higher temperatures. We find the entropy, in units of Boltzmann’s constant, to be of the order of unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered spins. In contrast to 3 He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness 4 , 5 , with implications for the nature of finite-temperature electron transport 6 – 8 , as well as for the mechanisms underlying isospin ordering and superconductivity 9 , 10 in twisted bilayer graphene and related systems. An electronic analogue of the Pomeranchuk effect is present in twisted bilayer graphene, shown by the stability of entropy in a ferromagnetic phase compared to an unpolarized Fermi liquid phase at certain high temperatures.

We report magnetometry data obtained on twin-free single crystals ofNa3Co2SbO6, which is considered a candidate material for realizing the Kitaev honeycomb model for quantum spin liquids. Contrary to a common belief that such materials can be modeled with the symmetries of an ideal honeycomb lattice, our data reveal a pronounced twofold symmetry and in-plane anisotropy of over 200%, despite the honeycomb layer’s tiny orthorhombic distortion of less than 0.2%. We further use magnetic neutron diffraction to elucidate a rich variety of field-induced phases observed in the magnetometry. These phases manifest themselves in the paramagnetic state as diffuse scattering signals associated with competing ferromagnetic and antiferromagnetic instabilities, consistent with a theory that also predicts a quantum spin liquid phase nearby. Our results call for theoretical understanding of the observed in-plane anisotropy and renderNa3Co2SbO6a promising ground for finding exotic quantum phases by targeted external tuning.

The physical properties of two-dimensional van der Waals crystals can be sensitive to interlayer coupling. For two-dimensional magnets1–3, theory suggests that interlayer exchange coupling is strongly dependent on layer separation while the stacking arrangement can even change the sign of the interlayer magnetic exchange, thus drastically modifying the ground state4–10. Here, we demonstrate pressure tuning of magnetic order in the two-dimensional magnet CrI3. We probe the magnetic states using tunnelling8,11–13 and scanning magnetic circular dichroism microscopy measurements2. We find that interlayer magnetic coupling can be more than doubled by hydrostatic pressure. In bilayer CrI3, pressure induces a transition from layered antiferromagnetic to ferromagnetic phase. In trilayer CrI3, pressure can create coexisting domains of three phases, one ferromagnetic and two antiferromagnetic. The observed changes in magnetic order can be explained by changes in the stacking arrangement. Such coupling between stacking order and magnetism provides ample opportunities for designer magnetic phases and functionalities.

Mechanical deformation of a crystal can have a profound effect on its physical properties. Notably, even small modifications of bond geometry can completely change the size and sign of magnetic exchange interactions and thus the magnetic ground state. Here we report the strain tuning of the magnetic properties of the A-type layered antiferromagnetic semiconductor CrSBr achieved by designing a strain device that can apply continuous, in situ uniaxial tensile strain to two-dimensional materials, reaching several percent at cryogenic temperatures. Using this apparatus, we realize a reversible strain-induced antiferromagnetic-to-ferromagnetic phase transition at zero magnetic field and strain control of the out-of-plane spin-canting process. First-principles calculations reveal that the tuning of the in-plane lattice constant strongly modifies the interlayer magnetic exchange interaction, which changes sign at the critical strain. Our work creates new opportunities for harnessing the strain control of magnetism and other electronic states in low-dimensional materials and heterostructures.A cryo-strain device capable of applying large, continuous strains to two-dimensional materials in situ enables the reversible tuning of magnetic order and spin-canting process of the layered magnetic semiconductor CrSBr.

At sufficiently low temperatures, a two-dimensional electron system placed in an external magnetic field can exhibit the so-called quantum Hall effect. In this regime, a variety of magnetic phases may occur, depending on the electron density and other factors. Wei et al. studied the properties of these exotic magnetic phases in graphene. They generated magnons—the excitations of an ordered magnetic system—that were then absorbed by the sample, leaving a mark on its electrical conductance. The magnons were able to propagate across long distances through various magnetic phases in the bulk graphene. Science , this issue p. 229 Transport measurements are used to monitor the propagation of magnons in a graphene sample. Spin waves are collective excitations of magnetic systems. An attractive setting for studying long-lived spin-wave physics is the quantum Hall (QH) ferromagnet, which forms spontaneously in clean two-dimensional electron systems at low temperature and in a perpendicular magnetic field. We used out-of-equilibrium occupation of QH edge channels in graphene to excite and detect spin waves in magnetically ordered QH states. Our experiments provide direct evidence for long-distance spin-wave propagation through different ferromagnetic phases in the N = 0 Landau level, as well as across the insulating canted antiferromagnetic phase. Our results will enable experimental investigation of the fundamental magnetic properties of these exotic two-dimensional electron systems.

The interplay between band topology and magnetism can give rise to exotic states of matter. For example, magnetically doped topological insulators can realize a Chern insulator that exhibits quantized Hall resistance at zero magnetic field. While prior works have focused on ferromagnetic systems, little is known about band topology and its manipulation in antiferromagnets. Here, we report that MnBi 2 Te 4 is a rare platform for realizing a canted-antiferromagnetic (cAFM) Chern insulator with electrical control. We show that the Chern insulator state with Chern number C  = 1 appears as the AFM to canted-AFM phase transition happens. The Chern insulator state is further confirmed by observing the unusual transition of the C  = 1 state in the cAFM phase to the C  = 2 orbital quantum Hall states in the magnetic field induced ferromagnetic phase. Near the cAFM-AFM phase boundary, we show that the dissipationless chiral edge transport can be toggled on and off by applying an electric field alone. We attribute this switching effect to the electrical field tuning of the exchange gap alignment between the top and bottom surfaces. Our work paves the way for future studies on topological cAFM spintronics and facilitates the development of proof-of-concept Chern insulator devices. Exotic states emerge from the interplay between band topology and ferromagnetism, but it remains less known in canted-antiferromagnetic phase. Here, the authors realize a canted-antiferromagnetic Chern insulator in atomically-thin MnBi 2 Te 4 with electrical control of chiral-edge state transport.

Understanding dissipation in 2D quantum many-body systems is an open challenge which has proven remarkably difficult. Here we show how numerical simulations for this problem are possible by means of a tensor network algorithm that approximates steady states of 2D quantum lattice dissipative systems in the thermodynamic limit. Our method is based on the intuition that strong dissipation kills quantum entanglement before it gets too large to handle. We test its validity by simulating a dissipative quantum Ising model, relevant for dissipative systems of interacting Rydberg atoms, and benchmark our simulations with a variational algorithm based on product and correlated states. Our results support the existence of a first order transition in this model, with no bistable region. We also simulate a dissipative spin 1/2 XYZ model, showing that there is no re-entrance of the ferromagnetic phase. Our method enables the computation of steady states in 2D quantum lattice systems. Our understanding of open quantum many-body systems is limited because it is difficult to perform a theoretical treatment of both quantum and dissipative effects in large systems. Here the authors present a tensor network method that can find the steady state of 2D driven-dissipative many-body models.