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

Electron paramagnetic resonance (EPR) of the systems of exchange-coupled ions differing strongly by spin-lattice relaxation times are discussed. The physical description of a rapid process which modulates and averages out the other, more slow one, is extended to the situations when paramagnetic ions participate in several exchange processes. The layered copper-neodymium squarate, [CuNd2(C4O4)4H2O16] · 2H2O (I), and the one-dimensional copper-lanthanide Cl3-acetates, CuLn2× (Cl3CCOO)8 · 6H2O (Ln = Nd3+ (II), Pr3+ (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 temperatures the short spin-lattice relaxation time of Nd3+ averages out the Cu-Nd exchange interaction. The manifestation of this interaction should be temperature dependent when governed by the spin-lattice relaxation of one of the partners. The collective Cu spin-system exists in I atT > 250 K (JCu-Cu≈ 0.1 cm−1) and gradually transforms into Cu-Nd spin-system which dominates atT < 20–30 K (JCu-Nd ≈ 0.4 cm−1) withJCu-Nd >JCu-Cu >JNd-Nd⋅ Polymer chains in II may be considered as weakly interacting asymmetrical triads −Nd1−Cu-Nd2− (with the largest components ofJij tensor ≈ 0.3 cm−1). For III the lowest state of Pr3+ was proved to be a nonmagnetic singlet withJCu-Pr equal to zero. Very weak, temperature-dependent exchange interaction between Cu2+ ions via two Pr3+ ions (JCu-Cu ≈ 0.015 cm−1 atT = 4.2 K) was observed.