Different magnetic and magnetodielectric behavior of BaRFeO4 ferrites with R = Ho, Er, Tm, and Yb

New ferrites with the BaRFeO4 composition and R = Ho, Er, Tm, and Yb were prepared by a conventional solid-state method in air. Their crystal structures were refined by the Rietveld method using room-temperature synchrotron powder X-ray diffraction. They crystallize in space group Pnma and are isostructural with BaYFeO4. Their physical properties were investigated by magnetization and dielectric measurements and Mössbauer spectroscopy. They all show distinct magnetic behavior with three magnetic transitions for R = Ho (at TN3 = 6 K, TN2 = 37 K, and TN1 = 47 K) and R = Er (at TN3 = 4 K, TN2 = 37 K, and TN1 = 50 K), one magnetic transition for R = Tm (at TN1 = 46 K), and two magnetic transitions for R = Yb (at TN2 = 36 K and TN1 = 57 K). Mössbauer spectroscopy indicates long-range magnetic ordering below TN1. Dielectric measurements showed that all compounds exhibit different behavior of the dielectric constant as functions of temperature and magnetic field including evidence for spin-induced ferroelectric polarization. These features suggest different magnetic structures and complex 4f-3d exchange interactions providing ground for further experimental and theoretical studies.