Phase diagram of Ba 2 NaOsO 6, a Mott insulator with strong spin orbit interactions

Abstract

We report 23 Na nuclear magnetic resonance (NMR) measurements of the Mott insulator with strong spin-orbit interaction Ba 2 NaOsO 6 as a function of temperature in different magnetic fields ranging from 7 T to 29 T. The measurements, intended to concurrently probe spin and orbital/lattice degrees of freedom, are an extension of our work at lower fields reported in [1]. We have identified clear quantitative NMR signatures that display the appearance of a canted ferromagnetic phase, which is preceded by local point symmetry breaking. We have compiled the field temperature phase diagram extending up to 29 T. We find that the broken local point symmetry phase extends over a wider temperature range as magnetic field increases.

Phase transition preceding magnetic long-range order in the double perovskite Ba2NaOsO6

“…netization data obtained on the same Ba 2 NaOsO 6 single crystal. Using general thermodynamic considerations and the Fisher formula [22] we separate magnetic and non-magnetic contributions to the specific heat, and present evidence for a largely field-independent phase transition occurring at T s ≈ 9.5K, slightly above the magnetic ordering temperature of T c ≈ 7.5 K. This transition appears as a broadened step in the temperature dependence of the specific heat, and its evolution with applied magnetic field suggests that this phase corresponds to the broken local point symmetry phase seen in NMR experiments in high fields [19][20][21]. Our specific heat results, in conjunction with the NMR findings, indicate that this new phase between T c and T s is nematic in origin as predicted by theoretical models [8,11].…”

“…This state is unstable against a Jahn-Teller distortion [10,11,15] that induces a tetragonal paramagnetic phase with a two fold degenerate ground state as also indicated by measurements of the entropy balance across the magnetic transition yielding a value close to R ln 2 rather than R ln 4 [17]. Furthermore, recent high-field NMR experiments [19][20][21] reveal that the magnetically ordered state of Ba 2 NaOsO 6 is composed of an exotic non-collinear magnetic structure in which successive ferromagnetic Os-Na layers that subtend a canting angle of ≈ 134 • resulting in a small net magnetic moment along (110). This long-range magnetic order is driven by orbital ordering and has an over-all orthorhombic symmetry.…”

“…This long-range magnetic order is driven by orbital ordering and has an over-all orthorhombic symmetry. In addition, at high magnetic fields and several kelvin above the magnetic transition temperature, signatures of orbital ordering into a state that preserves time reversal symmetry but breaks the four-fold lattice symmetry of the high-temperature paramagnetic phase have been observed in NMR experiments [19][20][21]. However, the evolution of this phase at low fields has not been reported so far.…”

“…Above the magnetic transition and in the field range of |µ 0 H| < 1T the specific heat can be Shown are the fields of the peak in the specific heat marking the transition to long-range magnetic order (magenta) and the high-temperature shoulder (light blue) marking the beginning of the nematic phase. Also included are the high-field data from NMR experiments [21] for the magnetic transition (purple) and nematic phase (phase of broken local point symmetry) (dark blue). NMR data were taken in fields applied along a (001) direction while the specific heat was measured in (110)-fields.…”

“…Shown are the fields of the peak in the specific heat (respectively the zero crossing in ∂C/∂H) marking the transition to long-range magnetic order and the high-temperature shoulder. Also included are the high-field data from NMR experiments for the magnetic transition and the transition to a phase of broken local point symmetry [21]. The evolution of the latter boundary with applied magnetic field suggests that the high-temperature shoulder represents the continuation of the broken local point symmetry phase to low fields.…”