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 (Fe2‐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 Q4 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 long‐range 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.