Relief of frustration through spin disorder in multiferroic Ho1−xYxMnO3

The magnetic phase diagram of single crystalline Ho1−xYxMnO3 below TN is determined by measuring the magnetic susceptibility (χ), specific heat (Cp), dielectric constant (ε), and thermal conductivity (κ). Y doping enhances the P6̱3cm̱ magnetic phase below TSR and therefore increases TSR; only the P6̱3cm̱ phase exists below TN for x⩾0.9 samples. Y doping also reduces the P63cm phase below T2. The anomaly around TSR appears in the dielectric constant but is absent in χ and Cp for Ho0.3Y0.7MnO3 and Ho0.2Y0.8MnO3 samples, which indicates that the multiferroicity at TSR for HoMnO3 is not necessarily related to the Ho3+ spin ordering. Specific heat measurements show that the large electronic specific heat (γ) of HoMnO3 is related to the disorder of Ho3+ spins, and is correlated with the relief of frustration in the Mn sublattice. The thermal conductivity data support an enhanced spin-lattice interaction due to strong spin fluctuations in the geometrically frustrated Mn-spin system.