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• The magnetic decoupling between the Cu1 ferromagnetic chain and the Cu2 antiferromagnetic chain has been revealed. • The high-field magnetization presents a metamagnetic transition at 1.7 T and a half-saturated ferromagnetic (FM)-like magnetization process. • The antiferromagnetic resonances of the antiferromagnetically coupled Cu1 FM chains were observed, showing the presence of significant magnetic anisotropy. The high-field magnetization and electron spin resonance (ESR) are reported for the two-dimensional triangular-lattice antiferromagnet Cu 2 (OH) 3 HCO 2. The compound has an antiferromagnetic (AFM) order at T N = 4.9 K with the Curie-Weiss temperature θ CW = − 3.8 K. The high-field magnetization up to 50 T exhibits a metamagnetic transition at 1.7 T followed by a half-saturated ferromagnetic (FM)-like magnetization process, showing the magnetic decoupling between the Cu1 FM chain and the Cu2 AFM chain. The unsaturated magnetization at 50 T serves as an evidence of much larger AFM exchange in the Cu2 chain. The temperature-dependent ESR spectra are in line with the development of magnetic correlations and AFM ordering. The observed ESR modes below T N originate from the AFM resonances of the antiferromagnetically coupled Cu1 FM chains. The zero-field gap of 111 GHz and the zero-frequency field of 2.4 T suggest the presence of significant magnetic anisotropy. [ABSTRACT FROM AUTHOR]