School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, ADFA, Canberra, AUSTRALIA* Technische Universitaet Muenchen, FRM II, Garching, Germany**
â—‹Stewart J Campbell* Michael Hofmann**
Rare earth (R) magnetic materials are used extensively in society and have numerous technological applications. When combined with transition metal (T) and metalloid (X) elements, rare earth intermetallic compounds, R-T-X, exhibit a wide range of interesting and useful properties. The prime examples are magnets based on the Nd2Fe14B phase which exhibits the largest energy product (BH) discovered for a magnetic material.
A brief overview of the basic properties of rare earth intermetallic compounds and their applications in modern society will be presented. Interest will then focus on RT2X2, the 1-2-2 series. RMn2X2 compounds are of particular interest as, unlike most transition metals, the Mn atoms carry a magnetic moment. For example, as LaMn2Si2 is predominantly ferromagnetic and YMn2Si2 antiferromagnetic; compounds of intermediate composition in La1-xYxMn2Si2 exhibit mixed magnetic interactions leading to a complex magnetic phase diagram. RMn2(Si,Ge)2 with R=Yb and Eu exhibit a wide range of unusual properties as well as intermediate valences associated with the transition from a divalent to a trivalent state. An overview of our neutron diffraction investigations of YbMn2(Si,Ge)2 (~1.8-723 K; p ~ 0-2.7 GPa) and EuMn2(Si,Ge)2 will also be presented. This has led to a magnetic phase diagram for YbMn2Si2-xGex of features consistent with a valence change between the trivalent behaviour of YbMn2Si2 and the divalent-like behaviour of YbMn2Ge2 around xc ~ 1.6. The volume changes in EuMn2Si2 have been analysed for the first time in terms of the interconfigurational fluctuation model, leading to a valence transition temperature of Tv ~ 527 K, occupation probabilities for the Eu2+ state and average Euval values.