FIELD-INDUCED TRANSFORMATIONS OF THE SPIN ORDERING IN ND2CUO4

Using elastic neutron scattering, we show first that at zero field Nd2CuO4 undergoes successive magnetic phase transitions associated with the Cu++ spins. A phase I with La2NiO4 magnetic type structure is stabilized betweenTN ≈ 250K and T1≈ 80K and a phase II with a La2CuO4 magnetic type structure is formed between T1 and T2≈ 30K. Below T2 a phase III with a magnetic structure similar to that of phase I, is stabilized down to the lowest temperatures. When a small magnetic field, up to about 2 Tesla, is continuously applied perpendicular to the tetragonal axis along (1,-1,0), the magnetic reflections ( 1 2, 1 2,l) with odd integers l continuously vanish in the stability region of phases I and III, while the intensity of those with even integers l increases. However in the region of phase II the opposite occurs: even l reflections continuously vanish and odd l reflections are enhanced. The results indicate that at relatively small field of about 0.7 Tesla, a single magnetic domain with collinear spins is stabilized. Further, detailed studies of the reversibility of these transformations upon the applied field show no significant hysteresis, suggestive of a continuous transformation from non collinear spins structures at zero field into their collinear analogues at high field. © 1990.