Coexistence of singlet and ordered S=12 moments in the ground state of the triclinic quantum magnet CuMoO₄

CuMoO₄ is a triclinic quantum magnet based on S=1/2 moments at the Cu2⁺ site. It has recently attracted interest due to the remarkable changes in its chromic and volumetric properties at high temperatures and in its magnetic properties at low temperatures. This material exhibits a first-order structural phase transition at T_C ∼ 190 K as well as a magnetic phase transition at T_N ∼ 1.75 K. We report low-temperature heat capacity measurements as well as extensive elastic and inelastic neutron scattering measurements on powder samples taken above and below T_N. We observe neutron diffraction consistent with a simple (1/2, 0, 0) antiferromagnetic structure indicating a doubling of the a-axis periodicity below T_N. In addition, inelastic neutron scattering above a spin gap of ∼2.3 meV is consistent with triplet excitations out of paired S=1/2 moments which form singlet dimers. Low-lying spin wave excitations are also observed and these originate from ordered S=1/2 moments below T _N. Taken together these measurements show the ground state of CuMoO₄ to display both nonmagnetic singlets and ferromagnetically coupled spins coexisting within an antiferromagnetic structure below T _N ∼ 1.75 K.