Magnetic properties of Fe ₂GeMo ₃N; An experimental and computational study

A polycrystalline sample of Fe ₂GeMo ₃N has been synthesized by the reductive nitridation of a mixture of binary oxides in a flow of 10% dihydrogen in dinitrogen. The reaction product has been studied by magnetometry, neutron diffraction and Mössbauer spectroscopy over the temperature range 1.8 ≤ T/K ≤ 700. The electronic structure and magnetic coupling have been modelled by Density Functional Theory (DFT) and Monte Carlo methods. Fe ₂GeMo ₃N adopts the cubic η-carbide structure with a = 11.1630(1) Å at 300 K. The electrical resistivity was found to be ∼0.9 mΩ cm over the temperature range 80 ≤ T/K ≤ 300. On cooling below 455 K the compound undergoes a transition from a paramagnetic to an antiferromagnetic state. The magnetic unit cell contains an antiferromagnetic arrangement of eight ferromagnetic Fe ₄ tetrahedra; the ordered atomic magnetic moments, 1.90(4) μ _B per Fe atom at 1.8 K, align along a <111> direction. DFT predicts an ordered moment of 1.831 μ _B per Fe. A random phase approximation to the DFT parameterised Heisenberg model yields a Néel temperature of 549 K, whereas the value of 431 K is obtained in the classical limit for spin. Monte Carlo calculations confirm that the experimentally determined magnetic structure is the lowest-energy antiferromagnetic structure, but with a lower Néel temperature of 412 K. These results emphasise the potential of these computational methods in the search for new magnetic materials.