Formation of correlated spin systems in one- and two-dimensional copper and lanthanide compounds

Electron paramagnetic resonance (EPR) of the systems of exchange-coupled io ns differing strongly by spin-lattice relaxation times are discussed. The p hysical description of a rapid process which modulates and averages out the other, more slow one, is extended to the situations when para magnetic ion s participate in several exchange processes. The layered copper-neodymium s quarate, [CuNd2(C4O4)(4)H2O16]. 2H(2)O (I), and the one-dimensional copper- lanthanide Cl-3-acetates, CuLn(2)X (Cl3CCOO)(8). 6H(2)O (Ln = Nd3+ (II), Pr -(3+) (III)), were investigated by X-band EPR in the temperature range 4.2- 300 K. These compounds were shown to consist of subunits admitting exchange interaction between Cu ions, Cu-Nd ions and Nd-Nd ions. At high temperatur es the short spin-lattice relaxation time of Nd3+ averages out the Cu-Nd ex change interaction. The manifestation of this interaction should be tempera ture dependent when governed by the spin-lattice relaxation of one of the p artners. The collective Cu spin-system exists in I at T > 250 K (J(Cu-Cu) a pproximate to 0.1 cm(-1)) and gradually transforms into Cu-Nd spin-system w hich dominates at T < 20-30 K (J(Cu-Nd) approximate to 0.4 cm(-1)) with J(C u-Nd) > J(Cu-Cu) > J(Nd-Nd). Polymer chains in II may be considered as weak ly interacting asymmetrical triads -Nd-1-Cu-Nd-2- (with the largest compone nts of J(if) tensor 30.3 cm(-1)). For III the lowest state of Pr3+ was prov ed to be a nonmagnetic singlet with J(Cu-Pr) equal to zero. Very weak, temp erature-dependent exchange interaction between Cu2+ ions via two Pr3+ ions (J(Cu-Cu) approximate to 0.015 cm(-1) at T=4.2 K) was observed.