The magnetic structure of YMnO₃ perovskite revisited

The magnetic structure of the orthorhombic perovskite YMnO₃ has been investigated. A study on a polycrystalline sample based on neutron diffraction data and magnetization measurements has shown that YMnO₃ becomes magnetically ordered below T_N = 42 K. In the space group Pnma, the sinusoidal magnetic structure is defined by a (C_x,0,0) mode and characterized by the propagation vector k = (k_x,0,0). The k_x-component increases from 0.420(4), immediately below the ordering temperature, to 0.435(2) at T = 28.7 K. Below 28 K the k_x-component remains unchanged. The sinusoidal spin arrangement remains stable down to 1.7 K; at this temperature the amplitude of the sinusoid is A_k = 3.89(6) µ_B. YMnO₃ is the most distorted perovskite of the RMnO₃ series (R = rare earths); the observed sinusoidal magnetic structure is in contrast with those exhibited by the less-distorted members (i.e. LaMnO₃), which are commensurate-type antiferromagnetic structures.