Magnetic ordering in the ilmenite-hematite solid solution: A computational study of the low-temperature spin glass region

Abstract

Magnetic ordering in the ilmenite-hematite solid solution (Fe 2-xTixO3) has been investigated using Monte Carlo simulations, with particular emphasis on the low-temperature spin glass region of the phase diagram. Complex magnetic behavior is observed due to the presence of two competing magnetic order parameters: a "hematite-like" ordering with a two-layer repeat (Q2) and an "ilmenite-like" ordering with a four-layer repeat (Q4). The susceptibility and degree of magnetic order were calculated from the Fourier transform of the layer-averaged spin distribution, allowing long-range and short-range contributions from Q2 and Q4 to be analyzed separately. For x < 0.8, the ferrimagnetic (FM) phase remains stable down to 0 K. For x ≥ 0.8 a heterogeneous FM phase (HFM) followed by a modulated FM phase (MFM) develops. There is an increasing contribution from Q4 with increasing x, and a pronounced cusp in both Q2 and Q4 susceptibilities develops at 30 K. The superposition of Q2 and Q 4 leads to frustrated layers containing dynamically disordered spins. Freezing of this spin disorder below 30 K is responsible for the cusp in susceptibility, which can be classified as a reentrant spin glass (RSG) transition. A gradual loss of longrange FM order occurs as the percolation threshold is approached, resulting in a conventional spin glass (CSG) with no long-range order below 30 K for x ≥ 0.92. For x > 0.95, a transition to an antiferromagnetic (AF) phase occurs at 40-55 K, followed by an RSG transition at 20-30 K. Changes to the phase diagram caused by chemical clustering are determined using a preannealing algorithm. Clustering expands the AF field to x > 0.9 and the HFM field to x ≥ 0.55. The topology of the simulated phase diagram compares favorably with experiments but suggests that the nature of some phase boundaries should be reexamined from both experimental and computational perspectives. © 2009 by the American Geophysical Union.