ترغب بنشر مسار تعليمي؟ اضغط هنا

Spin and orbital ordering in TlMnO3: Neutron diffraction study

132   0   0.0 ( 0 )
 نشر من قبل Dmitry Khalyavin
 تاريخ النشر 2019
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

Crystal and magnetic structures of the high-pressure stabilized perovskite phase of TlMnO3 have been studied by neutron powder diffraction. The crystal structure involves two types of primary structural distortions: a+b-b-octahedral tilting and antiferrodistortive type of orbital ordering, whose common action reduces the symmetry down to triclinic P -1. The orbital pattern and the way it is combined with the octahedral tilting are different from the family of LnMnO3 (Ln = lanthanide or Y) manganites who share with TlMnO3 the same tilting scheme. The experimentally determined magnetic structure with the k = (1/2,0,1/2) propagation vector and P_S-1 symmetry implies anisotropic exchange interactions with a ferromagnetic coupling within the (1,0,-1) planes and an antiferromagnetic one between them (A type). The spins in the primary magnetic mode were found to be confined close to the (1,0,-1) plane, which underlines the predominant role of the single ion anisotropy with the local easy axes of Mn3+ following the Jahn-Teller distortions of the octahedra. In spite of the same octahedral tilting scheme in the perovskite structures of both LnMnO3 and TlMnO3 manganites, a coupling of the secondary ferromagnetic component to the primary A-type spin configuration through antisymmetric exchange interaction is allowed in the former and forbidden in the latter cases.



قيم البحث

اقرأ أيضاً

We have analyzed the experimental evidence of charge and orbital ordering in La0.5Sr1.5MnO4 using first principles band structure calculations. Our results suggest the presence of two types of Mn sites in the system. One of the Mn sites behaves like an Mn(3+) ion, favoring a Jahn-Teller distortion of the surrounding oxygen atoms, while the distortion around the other is not a simple breathing mode kind. Band structure effects are found to dominate the experimental spectrum for orbital and charge ordering, providing an alternate explanation for the experimentally observed results.
Neutron diffraction measurements on a single crystal of CeGe1.76 reveal a complex series of magnetic transitions at low temperature. At T_N = 7 K, there is a transition from a paramagnetic state at higher temperature to an incommensurate magnetic str ucture characterized by a magnetic propagation vector (0 0 tau) with tau approx. 1/4 and the magnetic moment along the a axis of the orthorhombic unit cell. Below T_LI = 5 K, the magnetic structure locks in to a commensurate structure with tau = 1/4 and the magnetic moment remains along the a axis. Below T* = 4 K, we find additional half-integer and integer indexed magnetic Bragg peaks consistent with a second commensurately ordered antiferromagnetic state.
Orbital and spin ordering phenomena in strongly correlated systems are commonly studied using the local-density approximation + dynamical mean-field theory approach. Typically, however, such simulations are restricted to simplified models (density-de nsity Coulomb interactions, high symmetry couplings and few-band models). In this work we implement an efficient general hybridization-expansion continuous-time quantum Monte Carlo impurity solver (Krylov approach) which allows us to investigate orbital and spin ordering in a more realistic setting, including interactions that are often neglected (e.g., spin-flip and pair-hopping terms), enlarged basis sets (full d versus eg), low-symmetry distortions, and reaching the very low-temperature (experimental) regime. We use this solver to study ordering phenomena in a selection of exemplary low-symmetry transition-metal oxides: LaMnO3 and rare-earth manganites as well as the perovskites CaVO3 and YTiO3. We show that spin-flip and pair hopping terms do not affect the Kugel-Khomskii orbital-order melting transition in rare-earth manganites, or the suppression of orbital fluctuations driven by crystal field and Coulomb repulsion. For the Mott insulator YTiO3 we find a ferromagnetic transition temperature 50 K, in remarkably good agreement with experiments. For LaMnO3 we show that the classical t2g-spin approximation, commonly adopted for studying manganites, yields indeed an occupied eg orbital in very good agreement with that obtained for the full d 5-orbital Hubbard model, while the spin-spin e_g-t_{2g} correlation function calculated from the full d model is 0.74, very close to the value expected for aligned eg and t2g spins; the eg spectral function matrix is also well reproduced. Finally, we show that the t2g screening reduces the eg-eg Coulomb repulsion by about 10%
Sr2Cr3As2O2 is composed of alternating square-lattice CrO2 and Cr2As2 stacking layers, where CrO2 is isostructural to the CuO2 building-block of cuprate high-Tc superconductors and Cr2As2 to Fe2As2 of Fe-based superconductors. Current interest in thi s material is raised by theoretic prediction of possible superconductivity. In this neutron powder diffraction study, we discovered that magnetic moments of Cr(II) ions in the Cr2As2 sublattice develop a C-type antiferromagnetic structure below 590 K, and the moments of Cr(I) in the CrO2 sublattice form the La2CuO4 -like antiferromagnetic order below 291 K. The staggered magnetic moment 2.19(4){mu} B /Cr(II) in the more itinerant Cr2As2 layer is smaller than 3.10(6){mu}_B/Cr(I) in the more localized CrO2 layer. Different from previous expectation, a spin-flop transition of the Cr(II) magnetic order observed at 291 K indicates a strong coupling between the CrO2 and Cr2As2 magnetic subsystems.
We discuss the notions of spin-orbital polarization and ordering in paramagnetic materials, and address their consequences in transition metal oxides. Extending the combined density functional and dynamical mean field theory scheme to the case of mat erials with large spin-orbit interactions, we investigate the electronic excitations of the paramagnetic phases of Sr2IrO4 and Sr2RhO4. We show that the interplay of spin-orbit interactions, structural distortions and Coulomb interactions suppresses spin-orbital fluctuations. As a result, the room temperature phase of Sr2IrO4 is a paramagnetic spin-orbitally ordered Mott insulator. In Sr2RhO4, the effective spin-orbital degeneracy is reduced, but the material remains metallic, due to both, smaller spin-orbit and smaller Coulomb interactions. We find excellent agreement of our ab-initio calculations for Sr2RhO4 with angle-resolved photoemission, and make predictions for spectra of the paramagnetic phase of Sr2IrO4.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا