New attraction for heat-harvesting devices exploiting magnetism

16-12-2020

The combination of X-ray resonant scattering with X-ray nanobeam diffraction at beamline ID01 has revealed the role of magnetic domains on thermoelectric conversion in new spin caloritronic nanomaterials, which exploit magnons to convert waste heat into usable electricity. The observed spin texture is found to inhibit the thermoelectric conversion and is a first step towards highly efficient next-generation spintronics.

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Spintronic devices exploit both the charge and spin properties of the electron, with a wide scope of applications accessible by coupling spin, charge and heat currents in the same material. One promising extension is thermoelectric generation, broadly termed spin caloritronics [1]. If optimised, thermoelectric technologies can enable the direct conversion of thermal energy to electrical power. For efficient devices, good electric and poor thermal conductivity is required, vastly complicating the design. Spin caloritronics provides a complementary novel approach based on the spin Seebeck effect (SSE) [2]. 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 [3].

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.

 

evans_Fig1.jpg

Fig. 1: 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.

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 incident beam and Fπ, the normalised difference between intensities measured with π-polarised and the purely charged scattering component.

The structure and magnetism of the GdIG layer at low temperature are revealed in Figure 1 in X-ray nanobeam maps of a lithographically defined square around which the surrounding GdIG thin film was removed. Maps of the flipping ratios Fcir and Fπ in Figures 1b and 1c show the distribution of magnetic domains within the GdIG layer. The small-scale distribution of domain directions is further apparent in Figure 2a, which shows maps of Fcir acquired at 7.938 keV at several scales from a region within Figure 1b. The domains exhibit a complex arrangement in which some regions of domain walls are clearly oriented with crystallographic facets.

 

evans_Fig2.jpg (Figure 5)

Fig. 2: a) Nanobeam diffraction maps of Fcir at micrometre length-scales. b) Crystallographic tilt towards the [010] (vertical) direction. c) Integrated diffracted X-ray intensity at the 008 Bragg reflection. The magnetic response to the structural variation in (b) and (c) competes with the development of facets along directions of the lowest domain boundary energy.

Imaging combining nm-resolution structural and magnetic probes is an important step in addressing challenges in spintronics. The ability to reveal the coupling between magnetism and crystallographic structure is an important distinction between magnetic diffraction and magnetic imaging using X-ray spectroscopy. Ptychographic analysis will permit the simultaneous and rapid reconstruction of magnetic and structural information at the nanometre scale and in 3D, as is presently possible with studies of the crystallographic structure.  


Principal publication and authors
Resonant nanodiffraction X-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics, P.G. Evans (a), S.D. Marks (a), S. Geprägs (b), M. Dietlein (b,c), Y. Joly (d), M. Dai (a), J. Hu (a), L. Bouchenoire (e,f), P.B. Thompson (e,f), T.U. Schülli (e), M.-I. Richard (e,g), R. Gross (b,c,h), D. Carbone (i), D. Mannix (d,j), Sci. Adv. 6, 40 (2020); https://doi.org/10.1126/sciadv.aba9351.
(a) University of Wisconsin-Madison, Madison (USA
(b) Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, Garching (Germany)
(c) Physik-Department, Technische Universität München, Garching (Germany)
(d) Université Grenoble Alpes, CNRS, Institut Néel, Grenoble (France)
(e) ESRF
(f) University of Liverpool, Department of Physics, Liverpool (UK)
(g) Aix Marseille Université, CNRS, IM2NP UMR 7334, Université de Toulon, Marseille (France)
(h) Munich Center for Quantum Science and Technology (MCQST), Munich (Germany)
(i) MAX IV Laboratory, Lund (Sweden)
(j) European Spallation Source, Lund (Sweden)

References
[1] G.E. Bauer et al., Nat. Mater. 11, 391-399 (2012).
[2] K. Uchida et al., Nature 455, 778-781 (2008).
[3] S. Geprägs et al., Nat. Commun. 7, 10452 (2016).