Non-zero spontaneous magnetic moment along crystalline b-axis for rare earth orthoferrites

Rare earth orthoferrites demonstrate great application potentials in spintronics and optical devices due to their multiferroic and magnetooptical properties. In RFeO₃, magnetic R³⁺ undergo a spontaneous spin reorientation in a temperature range determined by R (rare earth), where the magnetic structure changes from Γ₂ to Γ₄. The b-axis component of their magnetic moment, M_b, is reported in numerous neutron diffraction studies to remain zero at all temperatures. More sensitive magnetometer measurements reveal a small non-zero M_b, which is minute above ∼200 K. M_b increases with cooling and reaches values of ∼10^–3 μ_B/f.u. at temperatures within or below the spin reorientation temperatures. Our results can be explained by assuming the Fe³⁺ spins as the origin of non-zero M_b, while R³⁺ spins suppress M_b. The representation analysis of point groups shows that non-zero M_b is associated with a small admixture of the Γ₃ phase to Γ₂ or Γ₄. Such a mixing of the three magnetic phases requires at least a fourth order of the spin Hamiltonian for RFeO₃ to describe the non-zero M_b.