Spin dynamics and 1/3 magnetization plateau in the coupled distorted diamond chain compound K2Cu3(MoO4)4

We investigate magnetic properties of the s=1/2 compound K2Cu3(MoO4)4 by combining magnetic susceptibility, magnetization, specific heat, and electron spin resonance (ESR) with density functional calculations. Its monoclinic structure features alternating Cu2+ (s=1/2) monomers and edge-shared dimers linked by MoO4 units, forming a distorted diamond chain along the a axis. Antiferromagnetic order occurs at TN=2.3 K, as evident from a λ-type anomaly in specific heat and magnetic susceptibility derivatives. Inverse magnetic susceptibility reveals coexisting ferro- and antiferromagnetic interactions. Specific heat and ESR data show two characteristic temperatures: one at 20 K, associated with spin-singlet formation in Cu2O9 dimers, and another at 3.68 K, indicating short-range correlations between dimers and monomers. Magnetization measurements reveal a metamagnetic transition at 2.6 T and a critical magnetic field μ0Hc=3.4 T, where a 1/3 magnetization plateau emerges with saturation near 0.35 μB. Low-temperature specific heat and magnetization data reveal the suppression of long-range order at μ0Hc, enabling the construction of a temperature-magnetic field phase diagram showing multiple magnetic phases near the μ0Hc. Density functional theory confirms a distorted diamond chain with J1 dimers and competing J2, J4, J3, and J5 interactions with monomer spins as an effective low-temperature spin model.