1/3 magnetization plateau and frustrated ferrimagnetism in a sodium iron phosphite

The sodium-iron phosphite NaFe₃(HPO₃)₂(H₂PO₃)₆ is studied by ac-magnetic susceptibility, pulsed-field magnetization, specific heat, and high-frequency electron spin resonance (HF-ESR) measurements combined with Mössbauer spectroscopy and density-functional calculations. We show that this compound develops ferrimagnetic order below T_C = 9.5 K and reveals a magnetization plateau at 1/3-saturation. The plateau extends to B_c ~ 8 T, whereas above B_c the magnetization increases linearly until reaching saturation at B_s ~ 27 T. The Mössbauer spectroscopy reveals two magnetically non-equivalent iron sites with the 2:1 ratio. The HF-ESR spectra are consistent with a two-sublattice ferrimagnet and additionally pinpoint weak magnetic anisotropy as well as short-range spin order that persists well above T_C. The ferrimagnetic order in the title compound is stabilized by a network of purely antiferromagnetic exchange interactions. The spin lattice comprises layers coinciding with the crystallographic (10-1) planes, with stronger couplings J_i ~ 1.5 K within the layers and weaker couplings J_i = 0.3−0.4 K between the layers. Both intralayer and interlayer couplings are frustrated. The ensuing ferrimagnetic order arises from a subtle interplay of the frustrated but nonequivalent exchange couplings.