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Rare earth orthoferrites demonstrate great application potentials in spintronics and optical devices due to their multiferroic and magnetooptical properties. In RFeO3, magnetic R3+ undergo a spontaneous spin reorientation in a temperature range determined by R (rare earth), where the magnetic structure changes from Γ2 to Γ4. The b-axis component of their magnetic moment, Mb, is reported in numerous neutron diffraction studies to remain zero at all temperatures. More sensitive magnetometer measurements reveal a small non-zero Mb, which is minute above ∼200 K. Mb increases with cooling and reaches values of ∼10–3 μB/f.u. at temperatures within or below the spin reorientation temperatures. Our results can be explained by assuming the Fe3+ spins as the origin of non-zero Mb, while R3+ spins suppress Mb. The representation analysis of point groups shows that non-zero Mb is associated with a small admixture of the Γ3 phase to Γ2 or Γ4. Such a mixing of the three magnetic phases requires at least a fourth order of the spin Hamiltonian for RFeO3 to describe the non-zero Mb.

Rare-earth orthoferrites are receiving ever-increasing attention for their potential applications in magneto-optical switching, multiferroics, and novel physics originating from complicated interactions between magnetic rare-earth and iron ions. In this work, a Dy0.5Pr0.5FeO3 single crystal was studied in comparison with DyFeO3 and PrFeO3 single crystals to ascertain the effects of interactions between rare-earth spins in Dy0.5Pr0.5FeO3 on its magnetic properties. Dy3+ and Pr3+ spins do not behave as separate entities in Dy0.5Pr0.5FeO3. The interaction between them was found to be the strongest below their antiferromagnetic ordering temperature. However, this interaction still persists to substantially higher temperatures. While the ordering temperature of Dy3+ spins is field-independent for DyFeO3, it becomes strongly field-dependent for Dy0.5Pr0.5FeO3. External field produces field polarization of nonordered rare-earth spins below ∼25 K for all three systems. High-field-induced spin transition of rare-earth spins was observed for Dy0.5Pr0.5FeO3 when a large field H ≥ 3.5 T is oriented along the crystalline a-axis at temperatures below and above the ordering temperature of the rare-earth spins, while the Fe3+ spin structure was not affected. This is different from the field-induced spin reorientation of the Dy3+ spin structure in DyFeO3, which occurs only when Dy3+ spins are ordered. The complicated behavior of rare earths uncovered in this work further deepens the understanding of such a complex material system.

Application of iron (Fe)- and silica (Si)-enhanced biochar compound fertilisers (BCF) stimulates rice yield by increasing plant uptake of mineral nutrients. With alterations of the nutrient status in roots, element homeostasis (e.g., Fe) in the biochar-treated rice root was related to the formation of biominerals on the plaque layer and in the cortex of roots. However, the in situ characteristics of formed biominerals at the micron and sub-micron scale remain unknown. In this study, rice seedlings (Oryza sativa L.) were grown in paddy soil treated with BCF and conventional fertilizer, respectively, for 30 days. The biochar-induced changes in nutrient accumulation in roots, and the elemental composition, distribution and speciation of the biomineral composites formed in the biochar-treated roots at the micron and sub-micron scale, were investigated by a range of techniques. Results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) showed that biochar treatment significantly increased concentrations of nutrients (e.g., Fe, Si, and P) inside the root. Raman mapping and vibrating sample magnetometry identified biochar particles and magnetic Fe nanoparticles associated with the roots. With Fe plaque formation, higher concentrations of FeOx− and FeOxH− anions on the root surface than the interior were detected by time-of-flight secondary ionization mass spectrometry (ToF-SIMS). Analysis of data from scanning electron microscopy energy-dispersive spectroscopy (SEM-EDS), and from scanning transmission electron microscopy (STEM) coupled with EDS or energy electron loss spectroscopy (EELS), determined that Fe(III) oxide nanoparticles were accumulated in the crystalline fraction of the plaque and were co-localized with Si and P on the root surface. Iron-rich nanoparticles (Fe–Si nanocomposites with mixed oxidation states of Fe and ferritin) in the root cortex were identified by using aberration-corrected STEM and in situ EELS analysis, confirming the biomineralization and storage of Fe in the rice root. The findings from this study highlight that the deposition of Fe-rich nanocomposites occurs with contrasting chemical speciation in the Fe plaque and cortex of the rice root. This provides an improved understanding of the element homeostasis in rice with biochar-mineral fertilization.