Collinear Spins Research Articles

Field-induced transformations of the spin ordering in Nd2CuO4

Using elastic neutron scattering, we show first that at zero field Nd 2 CuO 4 undergoes successive magnetic phase transitions associated with the Cu ++ spins. A phase I with La 2 NiO 4 magnetic type structure is stabilized between T N ≈ 250 K and T 1≈ 80 K and a phase II with a La 2 CuO 4 magnetic type structure is formed between T 1 and T 2≈ 30 K. Below T 2 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.

Molecular Field Theory for Randomly Substituted Ferrimagnetic Garnet Systems with Canted Local Spins

The theory given in a previous paper is extended and refined. The restriction of collinear spins is lifted and the idea of local random canting as suggested by Geller is introduced. It is assumed that when a magnetic ion has only one magnetic nearest neighbor, its spin need not be collinear with the total magnetization. The angle by which these spins are canted is treated as an adjustable parameter. This refinement of the theory makes it possible to reproduce in addition to the substitution dependence of the magnetic moments at 0°K, also the substitution dependence of the transition temperatures for a large variety of ferrimagnetic-substituted garnet systems. All of this agreement is obtained with a single adjustable parameter. When substitution is in the octahedral site, canting occurs in the tetrahedral site, and vice versa. It was found that spins in the ``d'' site having only one nearest magnetic neighbor are canted by about 60° when substitution is performed in the ``a'' site. Very small canting, if any at all, occurs in the ``a'' site when the substitution is in the ``d'' site. In double-substituted systems there is also no canting in the ``a'' site and the canting angles in the ``d'' site are ∼0° in Mg-substituted systems, ∼50° in Sc-substituted systems, and ∼75° in Zr-substituted systems.