Electrically controllable spontaneous magnetism in nanoscale mixed phase multiferroics

Abstract

magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeo 3 . using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.

Mesoporous bismuth ferrite with amplified magnetoelectric coupling and electric field-induced ferrimagnetism

“…In specific, the Fe peak is centred at 710.8 eV, in good agreement with the expected value of 711 eV for Fe 3 þ , suggesting phase purity and a low-enough level of oxygen vacancies that they were not detected via XPS 40 . These data thus provide good evidence that the high M s values reported below do not arise from oxygen vacancies, as seen in some other sol-gel-derived materials, but more likely stem from strain effects, in closer analogy to epitaxial BFO systems 22,23,25 .…”

“…We also note that the low M s values in all unpoled samples preclude the possibility that iron oxide impurities or oxygen vacancies, as discussed above, from the sol-gel synthesis are the source of the high magnetization in poled samples. These M s values for electrically poled mesoporous BFO films demonstrate the highest values recorded for BFO films to date, greater even than values ranging from 0.2-0.5 m b per Fe reported in epitaxially strained BFO thin films 23,25 . Indeed, this magnetization change corresponds to a magnetoelectric coupling constant a ¼ m 0 DM/ E ¼ 1 Â 10 À 5 sm À 1 , which is on par with composite systems and much larger than that expected for single phase materials 42,43 .…”

“…For BFO in specific, thin films engineered with epitaxial strain show M S values up to 0.5 Bohr magnetons (m b ) per Fe (refs 22,25), which is substantially higher than the bulk value of 0.05 m b per Fe (refs 22,26). High values of 0.2-0.3 m b per Fe were also observed in highly strained mixed-phase (rhombohedral:super-tetragonal) epitaxial BFO 23 . While epitaxial strain in BFO has provided a fruitful method to tune materials properties, bulk, thin film, and nanostructured BFO have been made using a number of methods 12,14,19,[27][28][29][30][31][32][33][34][35][36][37] .…”

“…For example, epitaxy has been used to amplify magnetic polarizations in BFO and other materials by the introduction of lattice strains 5,[19][20][21][22][23] . Epitaxial strain has also been used to convert materials that are neither ferroelectric nor ferromagnetic to multiferroics at low temperatures 24 .…”

Electric field control of ferromagnetism using multi-ferroics: the bismuth ferrite story

“…6-8 emu ml −1 ) is not observable by the XMCD technique owing to the small magnitude of the moment. Our work has shown that this enhanced magnetic moment in the highly strained Rphase disappears around 150 • C; further, application of an electric field converts this mixed phase into the T-phase, and the enhanced magnetic moment disappears; reversal of the field brings the mixed phase back accompanied by the magnetic moment in the distorted R-phase [20]. These observations raise several questions: first, what is the magnetic state of the strained R-phase?…”