Magneto-elastic properties and electronic structure analysis of the (Fe1-xNix)2P system

For the (Fe1-xNix)(2)P compounds with x = 0.015, 0.02, 0.025 and 0.25, both X-ray diffraction and chi(ac) susceptibility experiments under pressure up to 1.5 GPa were performed from 80 to 420 K. For all the studied compounds, a clear cut metal atom distribution was found on the basis of the Fe2P structure-type (SG: P62m), the 3g sites being fully occupied by Fe, while both Fe and Ni are randomly distributed on the 3f sites. The variation of the ordering temperature under pressure partial derivativeT(C)/partial derivativep increases from negative values for all studied compounds up to zero for the Ni-rich one (Fe0.75M0.25)2P. Additionally for the latter, neutron diffraction experiments were carried out in the 10-290 K range to characterise the transformation between the ferromagnetic (F) and paramagnetic (P) states. An electronic structure analysis of the (Fe1-xNix)(2)P compounds performed using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) shows that the total energy calculation fully supports the Ni preference for the 3f occupancy. Accounting to this trend, the calculated total magnetic moment was found to decrease almost linearly with x, in very fair agreement with the experimental data when measured for x < 0.25. However, the theoretical values of local magnetic moments computed in (Fe0.75Ni0.25)2P remain in satisfying agreement with the results of neutron diffraction. Hence, it is suggested that the strong electron polarisation at E-F (P similar to 90%) as established theoretically for the (Fe1-xNix)(2)P compounds having the smallest Ni content, can be responsible for the marked magneto-elastic phase transition as observed, e.g. in (Fe0.975Ni0.025)2P. (C) 2004 Elsevier B.V All rights reserved.