Spin caloritronics

Journal name:
Nature Materials
Volume:
11,
Pages:
391–399
Year published:
DOI:
doi:10.1038/nmat3301
Published online

Abstract

Spintronics is about the coupled electron spin and charge transport in condensed-matter structures and devices. The recently invigorated field of spin caloritronics focuses on the interaction of spins with heat currents, motivated by newly discovered physical effects and strategies to improve existing thermoelectric devices. Here we give an overview of our understanding and the experimental state-of-the-art concerning the coupling of spin, charge and heat currents in magnetic thin films and nanostructures. Known phenomena are classified either as independent electron (such as spin-dependent Seebeck) effects in metals that can be understood by a model of two parallel spin-transport channels with different thermoelectric properties, or as collective (such as spin Seebeck) effects, caused by spin waves, that also exist in insulating ferromagnets. The search to find applications — for example heat sensors and waste heat recyclers — is on.

At a glance

Figures

  1. Non-local detection of thermally injected spin accumulation (spin-dependent Seebeck effect).
    Figure 1: Non-local detection of thermally injected spin accumulation (spin-dependent Seebeck effect).

    a, Sketch of the measuring device. b, Schematics of thermal spin injection by the spin-dependent Seebeck effect across an FM|NM interface (symbols are explained in the text). For the purpose of illustration, the charge Seebeck coefficients have been chosen to be small in order not to mask the spin accumulation generated by the spin-dependent Seebeck effect. Figure reprinted from ref. 22, © 2010 NPG.

  2. Device geometry of the spin-dependent Peltier effect.
    Figure 2: Device geometry of the spin-dependent Peltier effect.

    a, Scanning electron microscopy image of the measuring device. The colours represent the different materials used. Yellow: gold top contact; grey: platinum bottom contacts; blue: cross-linked PMMA; red: constantan (Ni45Cu55). b, Schematic representation of the device. Current is sent from contact 1 to 2 while the voltage is recorded between contacts 3 and 4. Contacts 1, 2, 5 and 6 are used for four-probe spin-valve measurements. The thermocouple is electrically isolated from the bottom contact by an Al2O3 (green) layer. The perpendicular giant magnetoresistance stack for the spin-accumulation injection consists of 15 nm Ni80Fe20 (permalloy)|15 nm Cu|15 nm Ni80Fe20 (ref. 23). The observed spin-dependent Peltier coefficients are consistent with the results from ref. 22 and the spin-dependent Kelvin–Onsager relation Π(s) = S(s)T. Figure reprinted from ref. 23, © 2012 NPG.

  3. Collective spin dynamics and spin Seebeck effect in magnetic insulators.
    Figure 3: Collective spin dynamics and spin Seebeck effect in magnetic insulators.

    Spin currents can be carried by a, free electrons and b, spin waves (adapted from ref. 45, © 2010 NPG). c, Spin currents injected into a paramagnetic metal (Js) can be detected by the inverse spin Hall effect in terms of the electromotive force EISHE generated by the spin–orbit interaction. The observable is the voltage V that builds up normal to the spin current. d,e, The spin Seebeck effect can be observed in two different configurations, the longitudinal and the transverse, as illustrated schematically in d and e respectively. f,g, Experimental data are shown for the longitudinal (f)86 and transverse (g) spin Seebeck effect in yttrium iron garnets69, 85.

  4. Thermal fluctuations and spin currents.
    Figure 4: Thermal fluctuations and spin currents.

    a, A bilayer of a ferromagnet (FM) and a paramagnetic metal (NM). b, An NM|FM|NM sandwich with temperature bias TRTL, where TR and TL are the temperatures of the left and right reservoirs, and TF the temperature of the magnetic order. The fluctuations in the magnetization direction vector m(t) pump spin currents Jspump and Jstorque. They are cancelled on average by fluctuating Johnson–Nyquist spin currents at thermal equilibrium75, but in the presence of a temperature bias, net spin (and heat) currents flow with magnitudes indicated by the thickness of the arrows.

  5. Multifunctional magnetic nanomachine, consisting of a magnetic nanowire of length l, containing a domain wall centred at position rw.
    Figure 5: Multifunctional magnetic nanomachine, consisting of a magnetic nanowire of length l, containing a domain wall centred at position rw.

    The wire is in electrical and thermal contact with reservoirs that allow application of a temperature or voltage bias. The wire is mounted such that it can rotate around the x axis. A magnetic field and mechanical torque can be applied along x. Figure reprinted with permission from ref. 54, © 2010 APS.

  6. Hall effects in ferromagnets.
    Figure 6: Hall effects in ferromagnets.

    A sketch of the configuration for a, anomalous Hall effects and b, planar Hall effects. Source, drain, left and right Hall contacts connect to reservoirs at controlled temperature and electrochemical potentials. The arrow denotes whether the magnetization direction is normal to or in the film plane. The signal modulation as a function of in-plane angle is also referred to as anisotropic. A typical thermoelectric experiment consists of measuring the transverse Hall voltage difference induced by a source–drain temperature bias, the anomalous Nernst effect in configuration a or planar or anisotropic Nernst effect in configuration b. Note that the effects in the absence of magnetization receive the label 'spin', such as the inverse spin Hall effect in Fig. 4c, referring to the Hall voltage induced by a source–drain spin current.

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Affiliations

  1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

    • Gerrit E. W. Bauer &
    • Eiji Saitoh
  2. Kavli Institute of NanoScience, Delft University of Technology, 2628 CJ Delft, The Netherlands

    • Gerrit E. W. Bauer
  3. CREST, Japan Science and Technology Agency, Sanbancho, Tokyo 102-0075, Japan

    • Eiji Saitoh
  4. Zernike Institute for Advanced Materials, University of Groningen, The Netherlands

    • Bart J. van Wees

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