Magnetic phase separation and strong enhancement of the Néel temperature at high pressures in a new multiferroic Ba3TaFe3Si2O14

Resumo

The high-pressure properties of a new multiferroic of the langasite family Ba₃TaFe₃Si₂O₁₄ were investigated in diamond-anvil cells (DAC) in the temperature range of 4.2–295 K by a new method of synchrotron Mössbauer spectroscopy. Strong enhancement of the Néel temperature T_N was observed at pressures above 20 GPa associated with the structural transformation. The highest value of T_N is about 130 K which is almost five times larger than the value at ambient pressure (about 27 K). It was suggested that the high value of T_N appears due to redistribution of Fe ions over 3f and 2d tetrahedral sites of the langasite structure. In this case, the short Fe-O distances and favorable Fe-O-Fe bond angles create conditions for strong superexchange interactions between iron ions, and effective two-dimensional (2D) magnetic ordering appears in the (ab) plane. The separation of the sample into two magnetic phases with different T_N values of about 50 and 130 K was revealed, which can be explained by the strong 2D magnetic ordering in the ab plane and 3D ordering involving inter-plane interaction.