Long-range magnetic ordering of S=1/2 linear trimers in A3Cu3(PO4)4 (A = Ca, Sr, and Pb)

Magnetic properties of S = 1/2 linear trimer cluster compounds A(3)Cu(3)(PO4)(4) (A = Ca, Sr, and Pb) were investigated. Magnetic susceptibility data for the three compounds showed that paramagnetic copper spins form trimers with the total spin of 1/2 below about 45K. Specific heat and magnetization measurements indicated that the trimer clusters undergo ferromagnetic long-range ordering at T-C = 0.91 K for A = Ca and antiferromagnetic long-range ordering at T-N = 0.91 K for A = Sr and T-N = 1.26 K for A = Pb. A(3)Cu(3)(PO4)(4) exhibited 1/3-magnetization plateau at least up to magnetic field of 55 T at 1.3 and 4.2 K. A(3)Cu(3)(PO4)(4) with A = Sr and Pb showed a spin-flop transition near 0.03 T in the antiferromagnetic state at 0.08 K. Specific heat data at magnetic fields clearly showed broad maxima at low temperatures due to the finite intra-chain interaction in one-dimensional arrays of the trimers. (c) 2005 Elsevier Inc. All rights reserved.