Magnetic Behavior of the FeSiO3–MgSiO3 Orthopyroxene System From NGR in 57Fe

The magnetic behavior of (Fe_xMg_1−x)SiO₃ with 0.269≤x≤1.0 was investigated between 300° and 1.7°K using NGR of ⁵⁷Fe. For $\text{x≳0.76}$ magnetic ordering was observed at the following temperatures: x=1.0, T_C=38°K; x=0.866, T_C=18°K; x=0.758, T_C=11°K. Ferrous ions occupy two different lattice sites (M₁ and M₂) of low symmetry. The resonance spectra consist of two superimposed hyperfine patterns, which reflect different electronic ground states for Fe²⁺ at the two sites. The M₂ site is characterized by a smaller magnetic hyperfine field and a more negative isomer shift compared to the M₁ site. Typical values for FeSiO₃ at 4.2°K are 113 kOe, 1.29 mm/sec, 45 MHz at M₂ and 290 kOe, 1.34 mm/sec, 69 MHz at M₁. Above T_C two quadrupolar doublets are observed. The quadrupole coupling of the M₁ site shows a strong temperature dependence, while that of the M₂ site remains fairly constant. The magnetically more dilute samples $\text{(x≲0.4)}$ did not show ordering in the temperature region above 1.7°K, but produced paramagnetic relaxation spectra with only the smaller of the two hyperfine fields present, since only the M₂ site is occupied sufficiently with Fe²⁺ for the resonance to be observed.