SPIN-SINGLET GROUND-STATE WITH ENERGY GAPS IN CU2PO4 - NEUTRON-SCATTERING, MAGNETIC-SUSCEPTIBILITY, AND ESR MEASUREMENTS

Magnetic properties of Cu2PO4 powder were studied by neutron-scatterin g, magnetic-susceptibility, and electron-spin-resonance (ESR) measurem ents. It has been proved that this cuprate has a spin-singlet ground s tate separated from the excited states by finite-energy gaps. The spin susceptibility and the ESR integrated intensity show a broad maximum around 100 K followed by a rapid decrease to zero with decreasing temp erature. The neutron-scattering measurements clearly exhibit magnetic excitations around 14.4 and 17.9 meV. The excitation energies are near ly temperature and Q independent, but the intensity of the excitations does decrease with increasing temperature. The experimental results a re consistent with a model of a four-spin (S=1/2) chain with alternati ng exchange interactions. The two exchange interactions are J(1)=-55 K (ferromagnetic) and J(2)=95 K (antiferromagnetic).