R-Type Ferrites (RTF) constitute a large isostructural class of transition element oxides featuring quasi-two-dimensional Kagome sublattices. We review a decade of experiments covering a large range of compositional variations, ATM5O11 (A= alkali earth, T= 3d element, M= Ru and 3d elements). The physical properties of RTF are remarkably sensitive to atomic disorder, and crucial roles for spin-orbit interactions and magnetic frustration are indicated. The availability of single-crystal samples has revealed a startling panorama of RTF ground states and exotic physical properties:
Disordered iron-bearing RTF are narrow-gap semiconductors with colinear ferrimagnetic order well above 300 K with T_C proportional to the semiconducting gap, and properties suitable for spin injection applications. In contrast, disordered Co- and Mn-bearing RTF exhibit canted, “all-in/all-out” ordering substantially below 300 K in the frustrated Kagome sublattice. Atomically ordered RTF (T= Ni, Zn, M= Ru) do not order to below 4 K, and display spin fluctuations and unusual non-Fermi-liquid behavior. Many RTF compositions display large anomalous topological Hall effects that involve Berry phase effects in the magnetization textures.
In spite of the wide variations of ground states and physical properties, the lattice parameters of RTF vary by as little as 0.1%, which suggests fabrication of epitaxial RTF heterostructures with unique physical properties may be feasible for a range of applications.
*Research supported by U.S. DoE Grant #DE-FG02-97ER45653 and the Kentucky Science and Engineering Foundation.