Magnetic structures of UNiAl in magnetic fields

UNiAl orders antiferromagnetically (AF) below T(N) = 19.3 K with a propagation vector q = (0.1, 0.1, 0.5). U moments oriented along the c-axis of the hexagonal structure are modulated sinusoidally within the basal plane with an amplitude of 1.24 mu(B)/U. The magnetic structure consists of three U moments (U(1), U(2), U(3)) coupled ferromagnetically within the basal plane. U(1) and U(2) are substantially larger than II, and invariant with held, whereas U(3) becomes enhanced in a held along c. The vector q remains intact in fields lower than the critical metamagnetic field (B(c) = 11.4 T at 1.5 K). Above B(c), UNiAl orders ferromagnetically with equal Umoments of 0.91 mu(B) in 11.8 T at 1.7 K. When decreasing field below B(c), the original propagation vector is lost and a new AF structure with q ' = (0.17, 0.03, 0.5) is established. The original q is recovered only by warming far above T(N) and subsequent zero-field cooling. (C) 1999 Elsevier Science B.V. All rights reserved.