MAGNETIC BEHAVIOUR OF IMPERFECT QUASI ONE DIMENSIONAL INSULATORS FeMgBO4 AND FeMg2BO5 : SPIN GLASS SYSTEMS ? A. Wiedenmann and P. Burlet J. Phys. Colloques, 39 C6 (1978) C6-720-C6-722 DOI: 10.1051/jphyscol:19786320