Phase transitions of MnO3 compounds revealed by nonlinear magnetooptics

Magnetic second-harmonic generation (SHG) and three-photon difference-frequency generation (DFG) are used to investigate the magnetic phase diagrams of MnO₃ ^3- compounds with five- and sixfold coordination of the Mn³⁺ (3d⁴) ion. In the hexagonal manganites, the six basic antiferromagnetic structures following from a one-dimensional representational analysis of the hexagonal unit cell are clearly distinguished on the basis of their different selection rules for SHG. All structures break the geometric frustration of the unit cell by triangular antiferromagnetic ordering of the Mn spins. The stability and the mutual interactions of in-plane and inter-plane exchange and the in-plane anisotropy are investigated. The three quantities act as almost decoupled degrees of freedom, which leads to independent in-plane and inter-plane reorientations of the Mn spin lattice. DFG was observed in the orthorhombic manganites Pr_1-xCa_xMnO₃ and Nd_1-xSr_xMnO₃. The DFG reflects the fourfold anisotropy of the quasicubic crystal, and the independent tensor elements for DFG are determined quantitatively. A metal–insulator transition with a two-order-parameter coupling to antiferromagnetism and charge ordering leads to additional difference-frequency contributions, which allow us to observe the formation of ∼100 μm magnetic domains. The variety of results gained from the two vastly different groups of manganite compounds, with spectral and spatial resolution used as additional degrees of freedom, demonstrates the versatility of nonlinear magnetooptical experiments for the investigation of magnetic structures and symmetries. Received: 16 October 2001 / Published online: 2 May 2002