Exchange-induced Tm magnetism in multiferroic h-TmMnO₃

Abstract

Analysis of (169)Tm Mössbauer spectra recorded for (hexagonal phase) h- TmMnO(3) confirms that the Mn sublattice orders magnetically below T(N)(Mn) = 82-83 K and reveals the growth of a local Tm moment at the 4b site that is induced by the Mn-Tm exchange interaction. The maximum hyperfine field recorded at the (169)Tm nucleus is 312 T, which is just under half of the free ion value and corresponds to a saturation moment of 3.29 µ(B). The temperature dependence of the fitted magnetic hyperfine interaction is closely represented by a simple two-singlet ground state model for the Tm(3+) crystal field scheme. The saturation molecular field is deduced to lie in the range B(Mn-Tm)(T = 0 K) = 1.2-2.3 T, dependent on the expectation value of the coupling α = ⟨0|J(z)|1⟩ between the two-singlet states. As observed elsewhere for other hexagonal manganites, there is no Mn-based exchange field at the second Tm site (the 2a site) which contributes a paramagnetic subspectrum down to the lowest experimental temperature of 4.2 K.