Magnetic properties of syntheticP21/c(Mg-Fe)SiO3clinopyroxenes as observed from their low-temperature Mössbauer spectra and from SQUID magnetization measurements

Abstract

Magnetic properties of six Ca-free P2 1 /c clinopyroxenes along the clinoenstatite-clinoferrosilite join were studied by transmission 57 Fe Mössbauer spectroscopy (0.3-4.2 K) and SQUID magnetization measurements (2-300 K). The Fe x Mg 1-x SiO 3 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 temperatures TN were determined by Mössbauer thermoscanning with zero source velocity. The magnetic Mössbauer spectra were refined using a full hyperfineinteraction Hamiltonian approach assuming different Fe 2+ -Mg 2+ 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 5 D electronic level schemes as earlier determined from the paramagnetic Mössbauer spectra. The magnetic susceptibility measurements indicate a positive paramagnetic Curie temperature θ p for FeSiO 3 (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 further suggest ferromagnetic ordering of Fe 2+ 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 P2 1 /c clinopyroxenes are similar to that of orthopyroxenes with similar (Mg,Fe) compositions.