MAGNETISM IN HIGHLY ANISOTROPIC BORATES: EXPERIMENT AND THEORY M.A. Continentino, J.C. Fernandes, R.B. Guimar˜aes, B. Boechat and A. Saguia Instituto de F´ ısica, Universidade Federal Fluminense, Av. Litorˆanea, s/n, Campus da Praia Vermelha Niter´oi, 24.210-340, RJ, Brazil. 1.INTRODUCTION Highly anisotropic organic and inorganic materials have been the object of intensive stud- ies in the last decades. High T c superconductivity, quantum Hall effect, charge and spin density waves, disordered magnetic chains, plateau magnetization, solitons and many other phenomena are examples of the physical richness presented by this large class of compounds. It is not surprising that anisotropic materials became so attractive for solid state physicists. The 3d metal anhydrous borates, in which the boron ions have trigonal coordination forming the orthoborate (BO 3 ) 3− or pyroborate (B 2 O 5 ) 4− anion groups, present high in- ternal anisotropy revealed by the morphology of their crystals which appear as needles or plaquettes. The anhydrous borates possessing the orthoborate group are called or- thoborates and frequently oxy-borates since, in these compounds, there are one or two oxygen ions per chemical formula not directly bound to the boron ions. The oxy-borates present more than ten different crystallographic structures among which the warwickites and ludwigites are the easiest to be synthesized. The borates possessing the pyroborate