Spin and lattice structures in materials with competing interactions investigated by neutron scattering techniques

Shen, Lingjia (2016). Spin and lattice structures in materials with competing interactions investigated by neutron scattering techniques. University of Birmingham. Ph.D.

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Abstract

Three magnetic materials, including γ-CoV $_2$ O $_6$ ,Pr $_0$ $_.$ $_5$ Ca $_0$ $_.$ $_5$ Mn $_0$ $_.$ $_9$ $_7$ Ga $_0$ $_.$ $_0$ $_3$ O $_3$ , and Y $_2$ (Cr $_1$ $_-$ $_χ$ Ga $_χ$ $_-$ $_0$ $_.$ $_5$ Sb $_0$ $_.$ $_5$ ) $_2$ O $_7$ (χ = 0.5, 0.6, 0.7, 0.8, 0.9), in which the magnetic or crystallographic structure shows strong correlations with the competing interactions (e.g. electron-lattice coupling, spin exchange interactions, single-ion anisotropy), have been explored by the neutron diffraction and small angle neutron scattering techniques.
For the quasi-one-dimensional magnet γ-CoV $_2$ O $_6$ , our neutron diffraction measurements have revealed a spatially inhomogeneous magnetic ground state, i.e. phase separation, below T $_N$ = 6.6 K.
For the strained manganese perovskite Pr $_0$ $_.$ $_5$ Ca $_0$ $_.$ $_5$ Mn $_0$ $_.$ $_9$ $_7$ Ga $_0$ $_.$ $_0$ $_3$ O $_3$ , we have observed the decoupling of magnetic field induced carrier delocalization and para-ferromagnetic transitions. Concomitantly, distinct responses of the crystallographic structure to these two transitions have also been established.
For the diluted pyrochlores Y $_2$ (Cr $_1$ $_-$ $_χ$ Ga $_χ$ $_-$ $_0$ $_.$ $_5$ Sb $_0$ $_.$ $_5$ ) $_2$ O $_7$ , we cannot detect any magnetic long range order in Y $_2$ CrSbO $_7$ (χ = 0.5), although it is well below the nonmagnetic site percolation limit (χ $_c$ = 0.61). We propose that the magnetism of Y $_2$ CrSbO $_7$ is governed by the bond disorder, caused by the weak ionic size mismatch between Cr $^3$ $^+$ and Sb $^5$ $^+$ and percolates at χ = 0.24, so that a spin-freezing transition is expected at very low temperatures according to the latest spin glass theories.