X-ray magnetic circular dichroism study of the magnetic anisotropy on TbMnO3

The magnetic anisotropy of $\mathrm{TbMn}{\mathrm{O}}_{3}$ was explored by means of polarized x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) measurements at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ and $\mathrm{Tb}\phantom{\rule{0.16em}{0ex}}{M}_{4,5}$ edges as a function of temperature and magnetic-field intensity. The selective magnetometry measurements were compared with the macroscopic magnetic properties on single crystals. XMCD measurements at the $\mathrm{Tb}\phantom{\rule{0.16em}{0ex}}{M}_{4,5}$ edge as a function of the magnetic field reproduces quite well the macroscopic magnetic anisotropy at low temperatures with the Tb moments staying confined along their Ising axis within the $ab$ plane, whereas a weak XMCD signal is observed at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edge. These results point out that $\mathrm{T}{\mathrm{b}}^{3+}$ single-ion anisotropy is the only responsible for the magnetic anisotropy on this multiferroic compound at high magnetic fields. Moreover, we found $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ XMCD measurements show that the cycloidal antiferromagnetic order is almost unaffected by the applied magnetic field at low temperatures under an applied magnetic field. Therefore, we discuss that this strong Ising nature of $\mathrm{T}{\mathrm{b}}^{3+}$ ions, through a magnetocrystalline coupling mediated by the oxygen atoms, must play an important role in the field-induced electric polarization flop and therefore in the magnetoelectric coupling on $\mathrm{TbMn}{\mathrm{O}}_{3}$.