Antiferromagnetic Kitaev interaction in J_\rm{eff}=1/2 cobalt honeycomb materials Na3_3Co2_2SbO6_6 and Na2_2Co2_2TeO6_6

Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue to broaden and enrich the quantum magnetism research significantly. By carrying out inelastic neutron scattering experiments and subsequent analysis, we conclude that Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$ are new honeycomb cobalt-based AFM Kitaev systems. The spin-orbit excitons at 20-28~meV in both compounds strongly supports the idea that Co$^{2+}$ ions of both compounds have a spin-orbital entangled J_\rm{eff}=1/2 state. Furthermore, we found that a generalized Kitaev-Heisenberg Hamiltonian can well describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show large AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We should stress that our parameters' optimized magnetic structures are consistent with the magnetic structures reported from neutron diffraction studies. Moreover, there is also the magnon-damping effect at the higher energy part of the spin waves, as usually observed in other Kitaev magnets. We demonstrate that Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$ are the first experimental realization of AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.Comment: 28 pages, 9 figure

Static and dynamical properties of the spin-5/2 nearly ideal triangular lattice antiferromagnet Ba3MnSb2O9

We study the ground state and spin excitation in Ba3MnSb2O9, an easy-plane S = 5/2 triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, which could allow for isosceles triangular exchanges and biaxial anisotropy by symmetry, we observe no deviation from the behavior expected for spin waves in the in-plane 120o state. Even the easy-plane anisotropy is so small that it can only be detected by ESR in our study. In conjunction with the quasi-two-dimensionality, our study establishes that Ba3MnSb2O9 is a nearly ideal triangular lattice antiferromagnet with the quasi-classical spin S = 5/2, which suggests that it has the potential for an experimental study of Z- or Z2-vortex excitations

Ultralow-field magnetocaloric materials for compact magnetic refrigeration

Abstract Magnetic refrigeration around the liquid-helium temperature plays a critical role in many technological sectors. Even if gallium gadolinium garnet (GGG) has been regarded as the benchmark, its application is highly limited by the small magnetic entropy changes, the requirement of superconducting magnets, and the large device sizes. Here, we report that LiREF4 (RE = rare earth) single crystals exhibit significantly superior magnetocaloric performance levels to commercial GGG. Under a small magnetic field of 5 kOe, which can be easily achieved by a permanent magnet, the magnetic entropy change reaches a record-high value of 16.7 J kg−1 K−1 in LiHoF4 in contrast to the value of 1.0 J kg−1 K−1 in GGG. The combination of small driving fields, large entropy changes, and excellent thermal and/or magnetic reversibility enables this series to be employed as the ideal working material for compact magnetic refrigeration around the liquid-helium temperature

Anomalous Hall effect of the quasi-two-dimensional weak itinerant ferromagnet Cr

The anomalous Hall effect (AHE), a manifestation of the Hall effect driven by the Berry curvature, has numerous applications in spintronics and valleytronics. However, its realization in quasi-two-dimensional (quasi-2D) chromium tellurides remains puzzling. In this paper, we synthesize the Cr4.14Te8 single crystal and find that it shows weak itinerant ferromagnetic (FM) metallic behavior with a large magnetocrystalline anisotropy. At the same time, Cr4.14Te8 exhibits the AHE below the FM phase-transition temperature $T_{C}\sim 203\ \text{K}$ . By taking into account the scaling behavior between the anomalous Hall resistivity $\rho_{xy}^A$ and the longitudinal resistivity $\rho_{xx}$ , the origin of the AHE in this system is suggested to stem from the skew-scattering mechanism. Moreover, the possible magnetic ground state in Cr4.14Te8 has also been discussed to reveal the origin of AHE. Our results may be helpful for exploring the potential applications of these kinds of quasi-2D FM metals

Significant thermal Hall effect in the 3d cobalt Kitaev system Na2Co2TeO6

Kitaev physics has recently attracted attention in condensed matter for its anticipated quantum spin liquid (QSL) state. The thermal transport measurement is crucial for probing the features of charge-neutral quasiparticles. In this letter, we report a significant thermal Hall effect in Na2Co2TeO6 (NCTO), a Kitaev QSL candidate, when the magnetic field is applied along the out-of-plane direction of the honeycomb plane. The thermal conductivity (??xx ) and thermal Hall conductivity (??xy) in NCTO reveal distinct magnetic field dependences below and above the Neel temperature (TN ) of 27 K. For T > TN, ??xx has a monotonic decrease in the field dependence, while ??xy persists up to T ??? = 150 K. On the other hand, both ??xx and ??xy exhibit complex field dependence for T < TN

Controllable phase transition of two-dimensional ferromagnetic chromium telluride thin films grown by molecular beam epitaxy

Abstract Two-dimensional (2D) Cr(1+δ)Te2 materials exhibit strong magnetic ordering and high Curie temperatures, making them attractive for various applications. It is crucial to achieve controllable synthesis for their successful integration into device technologies. In this study, we present the synthesis of phase-controllable 2D Cr(1+δ)Te2 films on the Si (111) substrate via molecular beam epitaxy. The composition and phase transition of the as-grown Cr(1+δ)Te2 films are characterized by using in-situ reflection high-energy electron diffraction, scanning tunneling microscopy, ex-situ X-ray photoelectron spectroscopy, X-ray diffraction, and theoretical calculations. At low growth temperatures, by carefully adjusting the film thickness from 2 to more than 3 layers, we achieve precise control over the phase of Cr(1+δ)Te2, from CrTe2 to Cr intercalated Cr2Te3. At a relatively elevated growth temperature, it is demonstrated that the Cr(1+δ)Te2 phase is independent of the film thickness, only Cr2Te3 forms and its growth mode is thickness-dependent. These phase transitions at low growth temperatures and growth mode changes at elevated growth temperatures are attributed to interfacial effects and the phase stability of Cr(1+δ)Te2 compounds. Additionally, we utilize scanning tunneling spectroscopy and computations to gain insights into the electronic properties of Cr2Te3. The magnetic measurements reveal that the 30-nm Cr2Te3 film exhibits ferromagnetic behavior with a Curie temperature of about 180 K. Our work offers a robust method for the controllable growth of high-quality 2D Cr(1+δ)Te2 films on Si substrates, providing an ideal platform for investigating their intrinsic properties and advancing the development of 2D magnet-based spintronics devices

Field-induced quantum spin disordered state in spin-1/2 honeycomb magnet Na2Co2TeO6

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (⊥ b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets