Thermoelectrical manipulation of nanomagnets

Department of Physics, University of Gothenburg, SE-412 96 Göteborg, Sweden
Journal of Applied Physics (Impact Factor: 2.18). 07/2010; 107(12):123706 - 123706-9. DOI: 10.1063/1.3437054
Source: IEEE Xplore


We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermoelectronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermoelectric manipulation of nanomagnets). Supporting experimental evidence of such thermally-controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel to antiparallel switching causes magnetoresistance oscillations whose frequency can be controlled by proper biasing from essentially dc to GHz. We discuss in detail an experimental realization of a device that can operate as a thermomagnetoresistive switch or oscillator.

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    • "temperature. One such system is strong/weak/strong ferromagnetic sandwich (F/f/F) where the weakly ferromagnetic spacer (f) has a lower Curie temperature (T C ) than that of the strong ferromagnetic outer layers (F) [14] [15]. Heating the structure through the T C of the spacer exchangedecouples the outer magnetic layers, so their parallel alignment below T C can be switched to antiparallel above T C . "
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    ABSTRACT: We investigate a novel type of interlayer exchange coupling based on driving a strong/weak/strong ferromagnetic tri-layer through the Curie point of the weakly ferromagnetic spacer, with the exchange coupling between the strongly ferromagnetic outer layers that can be switched, on and off, or varied continuously in magnitude by controlling the temperature of the material. We use Ni-Cu alloy of varied composition as the spacer material and model the effects of proximity-induced magnetism and the interlayer exchange coupling through the spacer from first principles, taking into account not only thermal spin-disorder but also the dependence of the atomic moment of Ni on the nearest-neighbor concentration of the non-magnetic Cu. We propose and demonstrate a gradient-composition spacer, with a lower Ni-concentration at the interfaces, for greatly improved effective-exchange uniformity and significantly improved thermo-magnetic switching in the structure. The reported magnetic multilayer materials can form the base for a variety of novel magnetic devices, such as sensors, oscillators, and memory elements based on thermo-magnetic Curie-switching in the device.
    Physical review. B, Condensed matter 08/2012; 86(21). DOI:10.1103/PhysRevB.86.214413 · 3.66 Impact Factor
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    ABSTRACT: We investigate the potential to use a magneto-thermo-electric instability that may be induced in a mesoscopic magnetic multi-layer (F/f/F) to create and control magnetic superstructures. In the studied multilayer two strongly ferromagnetic layers (F) are coupled through a weakly ferromagnetic spacer (f) by an "exchange spring" with a temperature dependent "spring constant" that can be varied by Joule heating caused by an electrical dc current. We show that in the current-in-plane (CIP) configuration a distribution of the magnetization, which is homogeneous in the direction of the current flow, is unstable in the presence of an external magnetic field if the length L of the sample in this direction exceeds some critical value Lc ~ 10 \mu m. This spatial instability results in the spontaneous formation of a moving domain of magnetization directions, the length of which can be controlled by the bias voltage in the limit L >> Lc. Furthermore, we show that in such a situation the current-voltage characteristics has a plateau with hysteresis loops at its ends and demonstrate that if biased in the plateau region the studied device functions as an exponentially precise current stabilizer.
    Physical review. B, Condensed matter 01/2012; 86(1). DOI:10.1103/PhysRevB.86.014436 · 3.66 Impact Factor
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