Complex Magnetism and Magnetic Structures of the Metastable HoMnO₃ Perovskite

Abstract

The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific heat, and neutron diffraction measurements. By cooling, HoMnO3 exhibits three singularities at T N = 41 K, T ≈ 26 K, and T ≈ 6.5 K, suggesting a rich magnetic phase diagram. The neutron diffraction data show that below T N = 41 K, the Mn3+ magnetic moments become ordered in an antiferromagnetic arrangement, adopting a modulated sinusoidal magnetic structure characterized by the wave vector k = (k x ,0,0) (k x = 0.40 at 41 K) and defined by the magnetic mode (C x ,0,0). When the temperature is decreased, the propagation vector varies and at T ≈ 29 K a transition to a commensurate magnetic structure defined by k = (1/2,0,0) takes place. Below T ≈ 22 K, a small ordered magnetic moment appears on the Ho3+ cations, strongly increasing below 9 K and reaching 7.3(1) μB at T = 1.8 K. The magnetic structure of the Ho3+ moments is defined by a (A x ,0,C z ) mode. The (H,T) phase diagram has been mapped out and the different magnetic structures interpreted on the basis of competing superexchange interactions.

Ga substitution as an effective variation of Mn-Tb coupling in multiferroicTbMnO3

“…29,34 Mncontaining DyMno 3 and HoMnO 3 show the same type of magnetic ordering but with doubling of unit cell along b-direction. 15,17 Expanding the same considerations on TMnO 3 , one can expect that strong J Mn−Tb prevents Tb of having regular τ Tb =1/2 b * structure presumably of G x A y -type (in one-dimensional case it would simply correspond to ↑↑↓↓).…”

“…5,14 In some cases, they even contribute to the magnitude of the polarization as has been shown for DyMnO 3 . 3,15 A necessary condition for this is the ordering of rare earths with the same propagation vector as Mn, behavior confirmed now for RMnO 3 with R = Gd, Tb, Dy and Ho above their own ordering temperatures, T R N < 10 K. 8,16,17 Here, Mn-magnetic sublattice polarizes R-spins and the induced R-moment is defined by the strength of exchange interaction between Mn-and R-ions, J Mn−R .…”

Distorted perovskite witheg1configuration as a frustrated spin system

“…The k s (∼0.295) at T N is incommensurate (IC) and decreases with decreasing T , and becomes nearly constant (k s =0.28) below ∼30 K. The anomalies in C/T and M at ∼27 K are in good agreement with the T where k s is locked at a constant value (T lock ). With further decreasing r R , Muñoz et al [7] reported that in polycrystalline HoMnO 3 (T N =41 K) the IC−to−commensurate (CM) magnetic phase transition takes place at ∼26 K, where the wave vector is (0, k s , 0) [0.4≤k s <0.5 (T -dependent) for the IC phase and k s = 1 2 for the CM one]. As displayed in Fig.…”

“…Recently, a similar spin structure, i.e. the up-up-down-down order in a MnO 2 plane (E-type AF order in the Wollan-Koehler notation [6]), is found in a manganite, HoMnO 3 [7] with a significantly distorted perovskite structure. This has to be a bridge between the well-understood A-type AF in manganites and the unique magnetic ground state in nickelates.…”

“…Let us first summarize in Fig. 1 the orbital (a) and spin (b) ordering temperatures (T OO and T N , respectively) on Mn sites of RMnO 3 as a function of φ, which is based on both the present and former studies [7][8][9][10]. Here, we adopt the φ at room temperature [11].…”