Strain, domain walls, and cycloidal spin textures in multiferroic BiFeO3 thin films

The popular multiferroic BiFeO3 (BFO) is both a ferroelectric and an antiferromagnet (AFM) at room temperature. A promising future direction for this material is in antiferromagnetic spintronics, since in AFMs the spins can respond more quickly than in ferromagnets. Moreover, BFO possesses magnon (spin wave) excitations in the GHz-THz range which could be harnessed in future low energy magnonics technologies. These spin-based computing approaches would not require the movement of charge and could thus offer significant energy savings. An important prerequisite for the magnonic excitations in BFO is to control the magnetic ordering, as this dictates the properties of the magnon modes. The magnetic structure of BFO is characterized by a long-range spin cycloid which arises through the influence of the ferroelectric polarization on the magnetic order. In epitaxial thin films of BFO, stimuli such as doping, strain, and substrate orientation can influence the cycloid.[1] More recently, it has become apparent that the nanoscale domain walls can also disrupt this cycloidal modulation.[2,3] In this presentation, I will give an insight into my work of the past ~10 years in understanding cycloidal spin textures in epitaxial BFO films. I will start with an introduction into the cycloid and the various experimental probes (neutron diffraction, Mössbauer spectroscopy, Raman spectroscopy) used to characterise it, and then discuss the effect of biaxial epitaxial strain.[4] I will then describe the influence of substrate crystallographic orientation and film thickness on the spin cycloid,[5–7] including how novel types of spin cycloids – not present in bulk BFO – can be formed. In the second part of the talk, I will discuss how new real-space imaging probes are providing unprecedented insight into the influence of strain and domain walls on the spin cycloid.[2] I will conclude with perspectives on how finer details of the cycloid (such as its anharmonicity and period) are affected by domain walls and applied magnetic field and provide a sneak preview into new neutron diffraction datasets on new low-symmetry triclinic phases of BFO.[8] The detailed knowledge we have gained into the spin textures in BFO will help us to engineer thin film systems useful for future magnonic architectures.