Magnetic ordering and phase transitions of RFe4Al8 (R = La, Ce, Y, Lu) compounds by neutron diffraction

We have studied the Fe magnetic ordering in the tetragonal RFe4Al8 (R = La, Ce, Lu, Y) antiferromagnetic compounds by neutron diffraction at temperatures between 1.5 and 240 K. The structure refinement in the paramagnetic state confirms the preference of the Fe atoms for the 8(f) positions. Below TN the four compounds were found to order with four distinct types of spiral structures requiring two sets of wave vectors for their description: (i) q(1) = 0, q(2) = (0, 0, q(z)) and (ii) q(1) = 0, q(3) = (q(x), q(x), 0). Most of the structures undergo magnetic phase transitions at lower temperatures. In all cases is the Fe moment value at 1.5 K close to 2.0 mu(B)/Fe atom. The wave vector length(s) is slightly temperature dependent in the high temperature region. The R = La compound orders with a double cone structure with the cone axis parallel to the wave vector q(2) = (0, 0, 0.265(5)). The cone axes of adjacent Fe chains extending along c have opposite signs. At 1.5 K the cone angle(s) of the Fe sublattices are Phi(c) = 60 degrees and Phi(c) + pi. The rotation angle is Phi = 95(1)degrees. For the RFe4Al8 compounds with R = Ce, Lu, Y the iron site splits into two independent sets (orbits) under the action of the in-plane wave vector q(3) = (q(x), q(x), 0), where q(x) has at 1.5 K the values 0.243(1), 0.145(3), 0.135(1) r.l.u., respectively. Between Fe moments of adjacent chains extending along c (different orbits) there is a non-zero phase while along the face diagonal(s) (the same orbit) the moments have the same phase. The moments of the Y and Lu compounds rotate within the plane of the wave vector (0 0 1) and form a cycloid spiral. The latter compound has in addition a small component along c which corresponds to a double cone cycloid spiral. The Ce compound has a simple spiral structure with the moments rotating in a plane perpendicular to the wave vector q(3) = (q(x), q(x), 0). The existence of small moment component along q, cannot be excluded from the present data. This would give rise to a double cone structure. (C) 1998 Elsevier Science B.V. All rights reserved.