Novel magnetic materials with chemically-tailorable interactions

Abstract

The investigation of compound KFe(C₂O₄)F was the impetus for this work as it was found to have interesting frustrated magnetic properties and became the first member of a family of transition metal oxalate fluorides which could be chemically tailored by cation substitution. Work to expand this family was undertaken to determine whether any other members exhibited novel magnetic properties. Depending on the cations and transition metals used, compounds with two distinct chemical formulae were synthesised; AM(C₂O₄)F and A₂M₂(C₂O₄)F₄ (A = K+, Rb+, Cs+, NH₄+; M = Mn²+, Fe²+, Co²+). Six compounds were isolated with the AM(C₂O₄)F formula; (NH₄)Mn(C₂O₄)F, KCo(C₂O₄)F, CsCo(C₂O₄)F, CsFe(C₂O₄)F, (NH₄)Fe(C₂O₄)F, and RbFe(C₂O₄)F while a further two were represented by the formula A2M2(C₂O₄)F₄; Cs₂Mn₂(C₂O₄)F₄ and (NH₄)₂Co₂(C₂O₄)F₄.

Through powder and single crystal X-ray diffraction, the AM(C₂O₄)F compounds could be divided into those with Pmmm space group symmetry [CsCo(C₂O₄)F and CsFe(C₂O₄)F], those with Pmc21 space group symmetry [(NH4)Mn(C₂O₄)F and RbFe(C₂O₄)F] and those with Cmc2₁ space group symmetry [KCo(C₂O₄)F and (NH4)Fe(C₂O₄)F]. A₂M₂(C₂O₄)F₄ compounds [Cs₂Mn₂(C₂O₄)F₄ and (NH₄)₂Co₂(C₂O₄)F₄] were found to crystalise with Cmmm space group symmetry.

The magnetic properties of all eight compounds were investigated initially by SQUID magnetometry, followed by elastic neutron diffraction to determine their magnetic ordering and structure. Additionally; CsCo(C₂O₄)F, CsFe(C₂O₄)F, Cs2Mn₂(C₂O₄)F₄ and (NH₄)₂Co₂(C₂O₄)F₄ were further investigated by µSR to probe spin behaviour over longer timescales. Neutron spectroscopy measurements were also taken for CsCo(C₂O₄)F and CsFe(C₂O₄)F to search for magnon excitations across temperature and energy ranges.