Ferromagnetic Order, Strong Magnetocrystalline Anisotropy, and Magnetocaloric Effect in the Layered Telluride Fe_3−δGeTe₂

The ternary transition-metal compound Fe_3−δGeTe₂ is formed for 0 < δ < 0.3. X-ray diffraction and Mössbauer spectroscopy reveal its layered crystal structure with occasional Fe vacancies in the Fe2 site, whereas no Fe atoms occupy the interlayer space, so that only van der Waals interactions exist between adjacent layers. We explore magnetic behavior and ensuing functional properties of Fe_2.9GeTe₂ via neutron diffraction, thermodynamic and transport measurements, Mössbauer spectroscopy, and electronic structure calculations. Below T_C = 225 K, Fe_2.9GeTe₂ is ferromagnetically ordered with the magnetic moments of 1.95(5) and 1.56(4) μ_B at T = 1.5 K, both directed along c, which is the magnetic easy axis. Electronic structure calculations confirm this magnetic structure and reveal a remarkably high easy-axis anisotropy of 4.2 meV/f.u. Mössbauer spectra reveal the magnetic ordering too, although a drastic influence of Fe vacancies on quadrupolar splittings and local magnetic fields has been observed. A moderate magnetocaloric effect with the magnetic entropy change upon the ferromagnetic ordering transition, −ΔS ∼ 1.1 J·kg^–1·K^–1 at 5 T, is found.