Magnetic thin-film diffraction from TAIPAN featured in Nature Communications

Bismuth Ferrite (BiFeO3) is a multiferroic compound that exhibits simultaneous long-range magnetic and charge order already at room temperature with fantastic potential in next generation spintronic technologies. One key feature of BiFeO3 is the presence of an unusually long-range cycloidal spin order whose propagation direction is coupled to the electric polarisation orientation. This link enables electric control of the dynamic magnetic ground state and forms the basis for the emerging spintronic research field of magnonics.
 
A mandatory requirement for applications such as spin-polarized tunneling devices is the availability of atomically precise thin films with thicknesses of just a few nanometers. Up to now, however, it is commonly accepted that strain in nanoscale thin films leads to a collapse of the cycloid, but the presence of the cycloid is a mandatory prerequisite for the multiferroic properties. This has thus far limited the potential of such thin films in practical devices.  The work reported in Nature Communications demonstrates that a large-scale uniform cycloidal spin order in 100 nm BiFeO3 thin films is possible through the careful choice of crystallographic orientation, and control of the electrostatic and strain-boundary conditions.
 

Neutron diffraction measurements were performed on the instrument TAIPAN at ANSTO, where the high flux, low background, and subsequent excellent signal-to-noise ratio, allowed investigation of the magnetism in thin films just 50-100 nm thick. Supplementary data were obtained in polarized-neutron mode using spin filters from our new 3He spin-filter polarisation station. In conjunction with X-ray diffraction mapping, an incommensurate spin cycloid with a unique [112] propagation direction was demonstrated. Remarkably, whilst this direction is different from bulk BiFeO3, the cycloid length and magnetic transition temperature retain values equivalent to the bulk.

 

 BiFeO3 Figure

Evidence for the magnetic spin cycloid along [112] in a 100 nm 1(10) BiFeO3 thin film: (a,b) Reciprocal space maps recorded on TAIPAN around the (½ ½ ½) and (½ ½ -½) reflection, (c,d) Schematic representations of the film orientation and spin cycloid.

Although this is only the first step, it is the most crucial in the long and complex task of making a fully integrated magnonic device. Given the unique crystallographic direction of the spin-wave propagation, it now becomes possible to couple the chirality of the spin cycloid to electric field (that is an applied electric field would change the polarization direction which in turn would have ramifications on the crystallographic orientation) in nanoscale BiFeO3 films, which opens up intriguing possibilities for magnetoelectrically coupled spin-wave devices.
 
The full reference is: Bertinshaw, J; Maran, R; Callori, SJ; Ramesh, V; Cheung, J; Danilkin, SA; Lee, WT; Hu, S; Seidel, J; Valanoor, N and Ulrich, C, Direct evidence for the spin cycloid in strained nanoscale bismuth ferrite thin films, Nat. Commun. 7, Art. No. 12664 (2016).  http://dx.doi.org/10.1038/ncomms12664

Published: 16/09/2016

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