Magnetic ordering of the Nd6Fe13-xGa1+x (x=0,1) and Pr6Fe13-xGa1+x (x=0,1) compounds:: a neutron diffraction study

Neutron powder diffraction performed on R6Fe13-xGa1+x (R=Pr, Nd; x=0,1) compounds has shown a selective distribution of the Ga atoms among the five non-rare earth positions filling up successively the 4a Si and the 16l(2) Fe-4 sites of the prototype Nd6Fe13Si structure with increasing x. The magnetic ordering is associated in all cases with the wave vector q = (0,0,1) (I-p magnetic lattice with anticentering translation). For the Nd6Fe12Ga2 and Pr6Fe12Ga2 compounds the ordered moment values observed at 1.5 K of 2.3-2.9 mu(B)/R atom, are below the free ion R3+ value and the ordered Fe moment values fall into the range 2.1-2.4 mu(B)/Fe atom, in good agreement with Mossbauer measurements. The Nd compounds display almost easy axis behavior below T-N and spin reorientation transitions at lower temperatures to an easy cone. In contrast the Pr compounds have an easy plane moment arrangement over the entire ordered regime. These differences are discussed using a crystal-field model with several competing anisotropies. (C) 2003 Elsevier B.V. All rights reserved.