In order to understand the magnetic anisotropy in depth, we evaluate the g-factors using the density functional theory calculations. Figure shows the molecular unit of the coordination polymer. The ellipsoid centered at the copper atom visualizes the g-factors in three orthogonal directions, g x = 2.0361435, g y = 2.0426129, and g z = 2.1254711. Panel c shows the spin structure in which four sequential spins within two adjacent unit cells realize zero net magnetism. The first (second) and third (fourth) spins point in opposite directions. When a weak magnetic field is applied, some of the four spins flip the direction, which results in the fast increase of the magnetization in low fields. As the field is raised further, the spins start to revolve towards the field orientation, which explains the linear dependence at higher fields in the magnetization isotherm at 2 K. When the field is switched off, the spin orientations in panel (b) are recovered so that no hysteresis is observed.