Epitaxial strain tunes spintronic behavior of multiferroic BiFeO₃

Researchers in pursuit of higher density memories have set their sights on a new generation of spintronic materials, in which both electron charge and spin are used to convey information. The magnetoelectric multiferroic BiFeO₃ (BFO), which exhibits direct coupling between ferroelectric and antiferromagnetic order, is particularly interesting for such an application. However, the complex interplay of strain and magnetic response in this system is only poorly understood. Now, Daniel Sando and colleagues at the Unité Mixte de Physique CNRS/Thales shed light on the fundamental mechanisms governing antiferromagnetism and demonstrate tunable control of this ordering for spintronics devices.

As described in the April 28 online edition of Nature Materials (DOI: 10.1038/nmat3629,) the researchers first deposited 70-nm thick BFO films onto different substrates using pulsed laser deposition, thereby imparting different in-plane strain states ranging from –2.6% compressive to 1.3% tensile strain. They next probed the hyperfine interactions of ⁵⁷Fe nuclei in BFO using Mössbauer spectroscopy, which allowed them to access the local magnetic environment around the Fe³⁺ ions. The results showed that at high strains, the typical helical antiferromagnetic spin cycloid vanishes.

Proposed magnetic phase diagram for BiFeO₃ as a function of strain state. For strains ranging from –1.6% to 0.5%, a Type-I (red) spin cycloid with wave vector along [10] or a Type-II (orange) spin cycloid with average wave vector along [110] is preferred. However, at higher strains a collinear antiferromagnetic ordering (blue) is preferred. Reproduced with permission from Nature Mater. (2013), DOI: 10.1038/nmat.3629. © 2013 Macmillan Publishers Ltd.

To better understand this, the researchers conducted Landau-Ginzburg and effective Hamiltonian theory calculations. These demonstrate that at low strain states one of two spin cycloid orderings is stable, while at higher strains a collinear antiferromagnetic ordering is preferred. The researchers confirmed these results using Raman spectroscopy and tested the effect of these strains on magnetic hysteresis. It is therefore possible to greatly change exchange bias and giant magnetoresistance (GMR) using strain. This understanding suggests that coupling BFO to a piezoelectric material such as PbZr_xTi_1–xO₃ (PZT) could enable antiferromagnetic ordering to be dynamically tuned, for use in a range of exciting device applications.