Magnetic properties of synthetic P2₂/c (Mg-Fe)SiO₃ clinopyroxenes as observed from their low-temperature Mössbauer spectra and from SQUID magnetization measurements

Magnetic properties of six Ca-free P2₁/c clinopyroxenes along the clinoenstatite-clinoferrosilite join were studied by transmission ⁵⁷Fe Mössbauer spectroscopy (0.3-4.2 K) and SQUID magnetization measurements (2-300 K). The Fe_xMg_t-xSiO₃ members were synthesized in a multi-anvil press, and samples with x = 1.00, 0.91, 0.87, 0.78, 0.61, and 0.09 have been considered in this study. The magnetic order-disorder transition temperature T_N were determined by Mössbauer thermoscanning with zero source velocity. The magnetic Mössbauer spectra were refined using a full hyperfine-interaction Hamiltonian approach assuming different Fe²⁺-Mg²⁺ next-nearest-neighbor configurations of the probe nuclei to give a distinct spectral component. The different strengths of the M1 and M2 hyperfine fields can be related to the different ⁵D electronic level schemes as earlier determined from the paramagnetic Mössbauer spectra. The magnetic susceptibility measurements indicate a positive paramagnetic Curie temperature θ_p for FeSiO₃ (x = 1) above 50 K and a negative θ_p for x ≤ 0.91 above 200 K. The Néel temperatures as determined from the susceptibility curves are equal to those obtained from the thermoscanning, indicating that spin relaxation effects can be ruled out. The macroscopic magnetic results futher suggest ferromagnetic ordering of Fe²⁺ ions within ribbons consisting of two linear bands of M2 sites that enclose chains of M1 sites, and antiferromagnetic coupling between neighboring ribbons. Generally spoken the magnetic behavior of Ca-free P2₁/c clinopyroxenes are similar to that of orthopyroxenes with similar (Mg,Fe) compositions.