NMR of Colossal Magnetoresistive Manganites and Expanded Lattice Intermetallics at High Pressure

A survey is given of NMR studies of colossal magnetoresistive manganese perovskites (RE,Ca,Sr,Ba)MnO₃ (RE = rare earth) and the interstitially modified permanent magnet materials, RE₂Fe₁₇A_x (A = N,C,H) at ambient pressure and at applied pressures of up to 10 kbar. The different pressure behavior of the Mn hyperfine field found in the metallic and in the insulating manganite compounds is discussed and related to the micro- and macroscopic properties of these magnetically inhomogeneous materials. In the RE₂Fe₁₇A_x compounds a different pressure behavior of the hyperfine field at the RE sites with different number and type of interstitial atom neighbors is discussed in terms of local compressibility. The influence of the interstitial modification and the applied pressure on the RE hyperfine field and quadrupole splitting is analyzed and the impact of the chemical pressure and covalent effects on these quantities is compared. A comparison of the behavior of the magnetic state of the 4f electron shell with pressure and with interstitial modification is made and the relation to the magnetocrystalline anisotropy is discussed.