Muons and magnets

Abstract:

We report magnetic, transport, neutron diffraction, and muon spin rotation data showing that Pb2CoOsO6, a metallic double-perovskite with a centrosymmetric space group at room temperature exhibits a continuous second-order phase transition at 45 K to a magnetically ordered state with a noncentrosymmetric space group. The absence of inversion symmetry is very uncommon in metals, particularly metallic oxides. In contrast to the recently reported ferroelectriclike structural transition in LiOsO3, the phase transition in Pb2CoOsO6 is driven by a long-range collinear antiferromagnetic order, with propagation vector k=(12,0,12), which relieves the frustration associated with the symmetry of the magnetic exchanges. This magnetically driven loss of inversion symmetry represents another frontier in the search for novel metallic behavior.

Abstract:

© 2020 authors. We report on a comprehensive characterization of the newly synthesized Cu2+-based molecular magnet [Cu(pz)2(2-HOpy)2](PF6)2 (CuPOF), where pz=C4H4N2 and 2-HOpy=C5H4NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, μ+SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the two dimensional square-lattice S=12 antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of J/kB=6.80(5) K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to XY type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature XY anisotropy under the influence of the interlayer coupling, occurs at TN=1.38(2) K, as revealed by μ+SR. In applied magnetic fields, our H1-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at TN=2.8 K and 7 T.

Abstract:

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa₂Fe₄As₄F₂ above and below its superconducting transition temperature T _c ≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of Fe₂As₂ separated by insulating CaF₂ layers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energy E _R of the observed spin resonance is broadly consistent with the relationship E _R = 4.9k _B T _c, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value of E _R with that deduced from the total superconducting gap value integrated over the Fermi surface.