Unveiling the magnetic structure of BaFeO3-y: Shedding light on the elusive magnetic behavior

This work provides a comprehensive examination of the structural and magnetic properties of 6H-BaFeO 2.96 over a wide temperature range (10 K < T < 300 K) using neutron and synchrotron X-ray diffraction. Additionally, the local oxidation state of iron is examined using electron energy loss spectroscopy. No structural changes that could indicate a charge-order transition are observed in spite of a reported possible charge disproportionation of Fe 4+ into Fe (4+δ)+ and Fe (4-δ)+ phenomenon at low temperature. According to the magnetic characterization, BaFeO 2.96 orders antiferromagnetically at T N =156 K. The application of external magnetic fields strongly influences the magnetic behavior; the transition temperature shifts to lower values with the applied magnetic field and the long-range magnetic order melts with an applied field of 14 T. The magnetocaloric effect clearly shows at T N a change from negative to positive magnetic entropy. The Rietveld refinement of the neutron powder diffraction data collected at 10 K, gives a magnetic ordering with propagation vector [0, 0, ½] and three magnetically distinct sites for the Fe atoms. This magnetic structure consists of ferromagnetic Fe-sheets perpendicular to the c-axis with the magnetic moments along the [110] direction coupled both ferromagnetic and antiferromagnetically along the c-axis. The magnetic moment values of the three Fe ions are very different evidencing a delicate equilibrium of competing magnetic interactions. SYNOPSIS. The magnetic structure of BaFeO 2.96 is solved for the first time by Neutron Diffraction. Evidence of partial disproportionation is presented by Electron Energy Loss Spectroscopy. • The magnetic structure is commensurate with k = [0, 0, ½] and consists of FM Fe-sheets perpendicular to c-axis. • Fe-sheets with the magnetic moments along the [110] direction couple both FM and AFM along the c-axis. • Fe 4+ partial disproportionation is suggested by EELS, ocurring without structural distortion as proven by SXRD and ND.