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Walther-Meißner-Institut (WMI), Bayerische Akademie der Wissenschaften
Chair for Technical Physics (E23), Technische Universität München

Spins Heat Up

BADW

March 07, 2012



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In a collaboration with partners from the University of Bielefeld, researchers from the Walther-Meißner-Institut (WMI), Bayerische Akademie der Wissenschaften and Technische Universität München, used focussed laser heating to generate local thermally driven spin currents and local thermo-galvanic voltages.


NIMNIMPure spincurrents are a fascinating topic in modern magnetism research. A spin current hereby "simply" is a directed flow of angularmomentum (spin) - in close analogy to the directed flow of electronic charge representing a conventional charge current. Intriguingly, however, pure spin currents can propagate both in electrical conductors (magnetic metals) and in electrical insulators exhibiting long-range magnetic order (magnetic insulators). Spin currents thus are not only a spin-addendum to charge transport, but a fundamentally different transport phenomenon.

One elegant way to generate pure spin currents relies on thermal non-equilibrium: a "hot" region in a magnetic material will emit more angular momentum than a cold one. In other words, the application of a thermal gradient allows to drive a spin current. A team of researchers at the Walther-Meißner-Institut (Bayerische Akademie der Wissenschaften and Technische Universität München) now has demonstrated in proof-of-principle experiments that this thermal gradient based spin current generation concept also works on local scales. In their experiments, Weiler et al. use a focussed laser beam to locally heat up a thin film made of the magnetic insulator yttrium iron garnet (YIG). The ensuing local spin current is detected in a thin Pt metal strip deposited ontop of the YIG. Weiler et al. hereby take advantage of the inverse spin Hall effect, which converts the spin current back into a charge current.

The key point behind the opto-thermal spin current generation approach established at the WMI now is that the spin current reflects the orientation of the magnetization in the YIG film. Scanning the laser beam across the sample thus allows to electrically image the local magnetization orientation in the magnetic insulator. Vice versa, controlling the orientation of the magnetization in the YIG film by means of an externally applied magnetic field makes it possible to control to polarity or even to switch off the local spin current.

In complimentary experiments, the WMI team furthermore could show that local laser heating also enables studies of the local magneto-thermo-galvanic properties of conductive ferromagnetic samples. This approach is analogous in many aspects to conventional, integral magneto-thermo-galvanic transport experiments -- however again with the added benefit of spatial resolution. In particular, it allows to generate local, magnetically controllable electro-motive forces, and to determine the local magnetic anisotropy.

Taken together, the interplay between local thermal gradients ("thermal landscapes"), magnetic properties and spin currents opens an avenue for the spatially-resolved investigation of spin-caloritronic effects, i.e., of the interaction of the local magnetic (spin), charge and heat degrees of freedom.


Journal reference

Local Charge and Spin Currents in Magnetothermal Landscapes
M. Weiler, M. Althammer, F. D. Czeschka, H. Huebl, M. S. Wagner, M. Opel, I-M. Imort, G. Reiss, A. Thomas, R. Gross, and S. T. B. Goennenwein
Phys. Rev. Lett. 108, 106602 (2012).

see also Viewpoint:

Spin-Heat Vision
Roberto Myers, Joseph Heremans
Physics 5, 29 (2012) | DOI: 10.1103/Physics.5.29

Acknowledgements:

This work has been supported by the Deutsche Forschungsgemeinschaft through SPP 1538 ''Spin Caloric Transport'' (project No. GO 944/4-1) and the German Excellence Initiative via the ''Nanosystems Initiative Munich'' (NIM).

Contact:

Dr. Sebastian T. B. Gönnenwein
Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften
und Physik-Department, TU München
Tel.: +49 (0)89 / 289 – 14226
E-Mail: Sebastian.Goennenwein@wmi.badw.de
Web: http://www.wmi.badw-muenchen.de/

Weiler PRL (2012)

Experimental setup: (a) The scannable laser beam generates a local temperature gradient normal to the ferromagnetic thin film plane. The dc voltage VANE which arises due to the anomalous Nernst effect depends on the local magnetization M at the position (x,y) of the laser beam. All investigated samples are patterned into 80µm wide and 900µm long Hall bars with contacts labeled as sketched. (b) VANE determined between contacts 2 and 4 as a function of the laser-spot position (x,y) and the external magnetic field magnitude µ0H in a 50 nm thick Co2FeAl film.