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

Abstract

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 (J _Cu-Cu≈ 0.1 cm⁻¹) and gradually transforms into Cu-Nd spin-system which dominates atT < 20–30 K (J _Cu-Nd ≈ 0.4 cm⁻¹) withJ _Cu-Nd >J _Cu-Cu >J _Nd-Nd ^⋅ Polymer chains in II may be considered as weakly interacting asymmetrical triads −Nd₁−Cu-Nd₂ ⁻ (with the largest components ofJ _ij tensor ≈ 0.3 cm⁻¹). For III the lowest state of Pr³⁺ was proved to be a nonmagnetic singlet withJ _Cu-Pr equal to zero. Very weak, temperature-dependent exchange interaction between Cu²⁺ ions via two Pr³⁺ ions (J _Cu-Cu ≈ 0.015 cm⁻¹ atT = 4.2 K) was observed.