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The defence will be in French.
Abstract: This thesis focuses on the study of the structural, magnetic, and electronic properties of NiFe₂O₄, with the primary goal of testing the hypothesis of a ferroelectric transition below 98 K and exploring its behavior under extreme pressure and temperature conditions. High-quality single-crystal samples were used to ensure reliable characterization.
Magnetization, pyroelectric current, and conductivity measurements performed between 10 K and 300 K revealed no evidence of ferroelectricity, thus excluding the existence of intrinsic multiferroic order. In parallel, structural investigations (single-crystal X-ray diffraction, Raman spectroscopy, and azimuthal scans of forbidden reflections) confirmed that the correct space group is Fd-3m, ruling out alternative scenarios proposed in the literature (polar phase P4₁22 or F-43m).
XANES and XMCD spectroscopies were then employed to probe the electronic and magnetic states of the compound. At ambient pressure, no significant change was detected as a function of temperature, confirming the robustness of the cubic structure and magnetic order. Under hydrostatic pressure, however, up to 55 GPa, distinct transitions were observed. At 300 K, an abrupt structural transition occurs at 32 GPa, without phase coexistence, from Fd-3m to an orthorhombic phase of type Pbcm or Bbmm. At 20 K, the transition is gradual, extending between 34 and 38 GPa, and involves phase coexistence. The structure of the high-pressure, low-temperature phase could not be determined and requires further investigation.
In addition, XANES spectra reveal electronic modifications at the Fe edge (from 30 GPa at 300 K and 32 GPa at 20 K) and the Ni edge (from 30 GPa at 300 K), while XMCD data show a progressive reduction of magnetic order at lower pressures (from 20 GPa at the Ni edge at 300 K, 30 GPa at the Fe edge at 300 K, and 28 GPa at the Fe edge at 20 K). This decoupling between structural and magnetic transitions points to complex mechanisms driven by reduced interatomic distances and enhanced 3d–2p hybridization, promoting electronic delocalization and weakening local magnetic moments.
In conclusion, this work establishes that NiFe₂O₄ is not ferroelectric and confirms its assignment to the Fd-3m space group. Nevertheless, under pressure, it exhibits distinct structural and magnetic transitions, whose nature depends on temperature, highlighting the importance of extreme conditions in revealing electronic and magnetic states not accessible at ambient conditions.
