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

Abstract

We investigate magnetic properties of the $s$ = 1/2 compound K${}_2$Cu${}_3$(MoO${}_4$)${}_4$ by combining magnetic susceptibility, magnetization, specific heat, and electron spin resonance (ESR) with density functional calculations. Its monoclinic structure features alternating Cu${}^{2+}$ ($s$ = 1/2) monomers and edge-shared dimers linked by MoO${}_4$ units, forming a distorted diamond chain along the $a$-axis. Antiferromagnetic order occurs at $T_{\mathrm{N}}$ = 2.3 K, as evident from a $\hat{I}»$-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 Cu${}_2$O${}_9$ 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 $\hat{I}{¼}_0{H}_c$ = 3.4 T, where a 1/3 magnetization plateau emerges with saturation near 0.35 $\hat{I}{¼}_{\mathrm{B}}$. Low-temperature specific heat and magnetization data reveal the suppression of long-range order at $\hat{I}{¼}_0{H}_c$, enabling the construction of a temperature-magnetic field phase diagram showing multiple magnetic phases near the $\hat{I}{¼}_0{H}_c$. Density functional theory confirms a distorted diamond chain with $J_1$ dimers and competing $J_2$, $J_4$, $J_3$, and $J_5$ interactions with monomer spins as an effective low-temperature spin model.