Takaya Mitsui

We investigate the microscopic origins of magnetic anisotropy in the ferromagnetic binary alloy Fe3Si film while applying reversible strain on a ferroelectric Pb(Mg,Nb)O3–PbTiO3 substrate. X-ray magnetic circular dichroism with the introduction of reversible strain clearly demonstrates the change in the magnetic easy axis direction, even with an anisotropy energy of 1.6 × 103 J/m3. The strain-dependent Mössbauer spectroscopy detects changes in the quadrupole shift, which are related to variations in the electric field gradient from the valence electron distribution. The density-functional-theory calculation also supports the electron quadrupole contribution to magnetic anisotropy. These results indicate that the strain-dependent changes in the magnetic easy axis in Fe3Si arise from the formation of the charge quadrupole at the strained Fe sites. This characteristic is notably different from that of the ferromagnetic Heusler alloy Co2FeSi, where one of the Fe sites in Fe3Si are replaced by Co. This study provides a significant approach to understanding and designing interfacial multiferroic materials.

Electronic nematic states, characterized by broken rotational symmetry, are prevalent in correlated materials. In most iron-based superconductors, the nematic anisotropy aligns with the Fe–Fe direction of the iron square lattice. However, recent investigations propose a unique form of nematicity oriented along the diagonal Fe–As direction in heavily hole-doped AFe2As2 (A=Rb or Cs). Yet, the transport studies focusing on the fluctuations of such nematicity yield conflicting outcomes regarding the presence and orientation of the nematic fluctuations. Here, we report high-resolution heat capacity measurements conducted under in-plane field rotation in RbFe2As2. While no discernible anomaly associated with the nematic transition is found in the temperature dependence of specific heat, the field-angle dependence near the superconducting transition (at ∼2.8 K) reveals clear 2-fold oscillations within the plane, providing thermodynamic evidence for the presence of diagonal nematicity. Moreover, we find that Mössbauer spectroscopy sensitively probes the nematic transition at ∼50 K with no evidence of static magnetism. These findings imply that the diagonal nematicity in RbFe2As2 has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.

A novel three-dimensional surface analysis system for iron-based solid materials has been developed. This system is composed of a synchrotron Mössbauer source, an X-ray focusing device, a precision stage, a gas flow proportional counter for conversion electron measurements, and multiple multichannel scalers. The performance was evidenced by determining the spatial distribution of the iron oxide abundance on a laser-ablated iron foil surface. This system can be widely utilized as a powerful analytical tool in steel science for corrosion, welding, and laser surface modification.