required, vastly complicating the design. Spin caloritronics provides a complementary novel approach based on the spin Seebeck effect (SSE) . The SSE is based on the development of a spin current in a magnetic insulator (MI) as a result of a temperature gradient. This spin current is injected into an adjacent high-atomic-number metal (HM) and transformed into an electrical (charge) current via the inverse spin-Hall effect, transducing the thermal gradient to an electrical current. The physics of SSE devices is currently described by a model in which thermopower in MI/HM bilayers results from magnonic spin currents .
Optimisation of SSE devices in zero magnetic field requires knowledge of the MI magnetic microstructure and its correlation with the generated voltage. In the absence of an external magnetic field, magnetic inhomogeneity arising from the three-dimensional distribution of magnetic domains can strongly influence the magnon propagation and, therefore, the spin current at the MI/HM interface, consequently degrading the thermoelectric efficiency. The magnetic domain configuration is also affected by strain imposed by the substrate used to template the epitaxial growth of the MI, crystallographic and chemical defects, and the attached conducting HM layer.
A hard X-ray resonant nanobeam diffraction method was developed at beamline ID01, combining Fresnel Zone plate nanofocusing
optics with circularly polarised X-rays generated by a diamond phase plate retarder, to probe prototype SSE device structures consisting of 23-nm epitaxial thin films of Gd3Fe5O12 (GdIG) with 2.8 nm Pt surface layer. The experimental setup provided a new magnetic contrast imaging mechanism with structural and elemental specificity, allowing precise magnetic and structural information to be obtained simultaneously from nanoscale buried volumes. Magnetic information can be extracted from the diffracted X-ray intensity using flipping ratios. Two flipping ratios closely linked to the magnetism of the Gd3+ ions are employed: Fcir, measured with opposite circular polarised X-rays and Fπ, the normalised difference between intensities measured with π-polarised and the purely charge scattering component.
The structure and magnetism of the GdIG layer at low temperature are revealed in Figures 76 and 77 in X-ray nanobeam maps of a lithographically defined square, around which the GdIG thin film was removed. Maps of the flipping ratios Fcir and Fπ in Figures 76b and 76c show the distribution of magnetic domains within the GdIG layer. The small-scale distribution of domain directions is further apparent in Figure 77a, which shows maps of Fcir acquired at 7.938 keV at several scales from a region within Figure 76b. The domains exhibit a complex arrangement, in which some regions are clearly oriented with crystallographic axes.
Fig. 76: a) X-ray nanobeam magnetic diffraction microscopy with diagram of right (R) and left (L) circular polarisations and π linear polarisation. b) Optical micrograph of a patterned GdIG layer from which GdIG has been removed in the area outside the light
square. X-ray nanobeam diffraction maps of (c) circular-polarisation flipping ratio Fcir and (d) linear π-polarisation flipping ratio Fπ in the same region.