Phonon evidence for a partial, re-entrant spin reorientation near room temperature in LaFe$_{1-x}$Ni$_x$O$_3$

Overview & Synopsis

Spin reorientation near room temperature offers a route to controlling antiferromagnetic order, yet LaFeO$3$, which lacks a magnetic rare-earth ion, is generally expected to retain its spin orientation along the $a$ axis below $T{\mathrm N}\approx740$~K. Here, we investigate LaFe$_{1-x}$Ni$_x$O$_3$ ($x=0$ and 0.1) using temperature-dependent magnetization and Raman spectroscopy over 200--400~K. Both compositions exhibit magnetic anomalies at 285.6 and 323~K, respectively, accompanied by correlated deviations of 12 Raman modes from their conventional anharmonic behavior. Principal component analysis and a global fitting approach reveal a common temperature-dependent phonon response, with mode-dependent amplitudes and signs. To interpret these anomalies, we develop a Landau-based spin-phonon coupling model describing complete, partial re-entrant, and incipient spin reorientation. Comparison with the experimental Raman profiles favors a partial, re-entrant rotation of the N'eel vector in both compositions, centered near 290~K. Ni substitution narrows the characteristic reorientation interval from approximately 116 to 84~K and enhances the magnetization anomaly by a factor of four. An additional feature near 262~K in LaFeO$_3$ is consistent with the Morin transition of trace $α$-Fe$_2$O$_3$. These findings suggest that partial spin reorientation can emerge without magnetic rare-earth ions and establish collective Raman phonon anomalies as a sensitive indirect probe of magnetic anisotropy changes in orthoferrites.