Noncollinear ferrimagnetic structures and spin dynamics in Al-doped lithium ferrites

The new non-collinear magnetic order, in an Al-doped lithium ferrite (Al-LFO), ( Li 0.5 Al 0.5 ) ( Fe 0.75 Li 0.25 ) 2 Fe 3 O 8 (denoted as Li50Al50), could be determined in the magnetic space group P 4 3 2 ′ 1 2 ′ using high-resolution neutron powder diffraction (HRNPD) at 3 and 50 K. The largest spin-canting angle of the non-collinear octahedral B-sublattice determined from the structural analysis using 3K-HRNPD approaches the Yafet-Kittel angle 21° at 0 K, extrapolated from saturation magnetization. The spin-canting textures shown at 3 and 50 K are thermally averaged at elevated temperatures to reveal collinear spin alignments along [001] at 300 K under zero external fields. The non-collinearity of magnetic spins in Al-LFOs arises from multiple spin-reorientation (SR) processes without magnetic phase transitions below T c (e.g., 788 K in Li50Al50). The SRs are activated above 100 K, showing Arrhenius behavior, but blocked at 5 K without altering the spontaneous magnetic polarization, as indicated by ac magnetic susceptibility data. The dilute effect of L i + replacing F e 3 + ( 20 % in Li60Al40 or 25 % in Li50Al50) because of the respective A l 3 + -substitution for 40 % and 50 % L i + in the geometrically frustrated B-sublattice (the hyperkagome lattice) enhances not only the lattice-distortion-induced electric polarization, but also the intrinsic spin dynamics. All SR-related features in both Al-LFOs are effectively quenched above 0.5 kOe, where all magnetic spins of F e 3 + are collinearly aligned parallel to the external magnetic fields, independent of frequency and temperature. These findings could be applied to further develop LFO-type multiferroics, e.g., for photovoltaic applications.