Magnetic Studies of COBALT(3) Benzenetricarboxylic ACID(2) Water(n): a Molecular-Based Disordered Magnet.

This dissertation is an examination of the magnetic properties of the molecular-based magnet rm Co_3BTCA_2(H_2O)_{n} . Interest in this compound arises from two sources: the apparent ferromagnetic interactions between the cobalt ions, and the effect of the disorder (and most likely frustration) present in the system on its critical behavior. The question of how disorder, or the absence of long-range structural order, will affect collective magnetic behavior has been of primary interest for many years. The compound is produced by a 98^ circC reaction of a cobalt perchlorate solution and a solution containing potassium hydroxide and 1,3,5 benzenetricarboxylic acid (BTCA). The precipitate that results has been found via X-ray studies to be amorphous. A variety of magnetic studies were performed from room temperature to 2 K, using techniques including vibrating sample magnetometry, SQUID magnetometry, alternating current susceptibility, and electron spin resonance. These studies indicate two transitions, one near 10 K and the other at 23 K. Below 10 K, behavior usually related to magnetic frustration is observed. The transition at 23 K resembles a ferromagnetic transition, while the transition at 10 K resembles a spin glass transition. These two transitions are common to disordered systems. Each of these transitions is analyzed in detail. The critical behavior of the 23 K transition is unique. In particular, the value of the critical exponent beta = 1.7, as determined by both a scaling analysis and the temperature dependence of the remanent magnetization, is not within the universality class of any ordered or disordered system. The absence of scaling and the lack of a divergence in the nonlinear susceptibility near 10 K indicate that the transition near 10 K is a result of nonequilibrium effects as opposed to a true thermodynamically driven phase transition. Preliminary work using other metal ions as the magnetic centers of the system, including manganese, nickel, and copper is also discussed.