Spin caloritronics in magnetic/non-magnetic nanostructures and graphene field effect devices Dejene, Fasil

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1 University of Groningen Spin caloritronics in magnetic/non-magnetic nanostructures and graphene field effect devices Dejene, Fasil DOI: /nphys2743 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2015 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Dejene, F. (2015). Spin caloritronics in magnetic/non-magnetic nanostructures and graphene field effect devices [Groningen]: University of Groningen DOI: /nphys2743 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Summary Electrons in a (magnetic) conductor transport charge, heat and angular momentum (spin) when a voltage or temperature gradient is applied. The past century has seen great progress in the understanding of the coupled flow of charge and heat (thermoelectricity) as well as charge and spin (spintronics) both in bulk and thin metallic films. While most electronic devices we use today are based on the charge property of the electron, it is not uncommon to find applications that make use of the interaction of the charge with other transport properties. For instance, accurate measurement of temperatures and solid-state refrigeration applications are based on two of the most common thermoelectric effects the Seebeck effect, the conversion of heat into an electrical current, and its inverse the Peltier effect, respectively. The significant improvement in the data-storage and information processing technologies we witnessed over the past decade hinges on the giant (tunneling) magnetoresistance effect used in the read-heads of the magnetic hard disk drive. Very recently, the combination of spintronics and thermoelectricity has led to the birth of spin caloritronics a field envisioned to provide multifunctional spintronic concepts that may, in the future, provide alternative ways for managing heat flow at the nanoscale and controlling spin information by using heat. The research described in this thesis was aimed at gaining a deeper understanding of the origin and working principles of various spin-dependent thermoelectric effects in ferromagnetic/normal metal structures. Spin caloritronics in metals (chapters 3 5) In a ferromagnetic metal, due to the strong exchange interaction, the Fermi energy density of states for spin up and spin down electrons is shifted with respect to each other. The transport of charge and heat can thus be described by a two-spin

3 118 Summary channel model, one for spin up (majority spins) and another for spin down (minority spins) with each spin-channel having its own electrical and thermal conductivities as well as Seebeck and Peltier coefficients. A charge current flowing through a ferromagnet is thus accompanied by a spin-polarized current that, when injected into another nonmangetic metal (N), results in a non-equilibrium magnetization (spin accumulation). A spin valve device, comprising of two F layers separated by an N layer, is prototypically utilized to study the process of electrical spin injection, transport and detection in various systems. In addition to charge current, heat current driven spin injection into non magnetic materials has recently been demonstrated by Slachter et al., in nonlocal spin valve devices, where a Joule-heated ferromagnet is used to inject spins into an adjacent normal metal. The size of the spin accumulation is proportional to the spin-dependent Seebeck coefficient, the difference in the Seebeck coefficients of spin up and spin down electrons, of the ferromagnetic metal used. In chapter 3, we verified this earlier work in specifically designed nanopillar spin valve devices and determined the spin-dependent Seebeck coefficient for permalloy and cobalt. The Thomson-Onsager relation, that relates the Seebeck coefficient with the Peltier coefficient, also predicts a spin current driven heating/cooling effect (spin-dependent Peltier effect). Flipse et al. demonstrated this process earlier from which a spin-dependent Peltier coefficient of 1 mv was obtained that was in agreement with the Thomson Onsager reciprocity relation. This symmetry relation was rigorously tested in chapter 5 by measuring both spin-dependent quantities in a single device. The reciprocity relation holds both in the linear as well as in the nonlinear regime. In the latter, contributions from nonlinear thermoelectric effects cause deviation in the current-voltage relationships. In chapter 4 the first experimental observation of the magnetic heat valve was presented. Due to the spin-dependence of the thermal conductivity, a heat current through a ferromagnetic metal is also spin polarized, that when injected into a non-magnetic metal, causes a spin heat accumulation (SHA) or a difference in the effective temperature of spin up and spin down electrons. In a pillar spin valve, it is possible to modulate the heat conductance of the spin valve by changing the relative magnetization direction of the ferromagnets. When the two ferromagnetic layers are aligned parallel (antiparallel) to each other, the total heat conductance of the nanopillar is larger (smaller) corresponding to the absence (presence) of SHA in the normal metal spacer. This non-equilibrium SHA thermalizes by inelastic scattering mediated by either electron-electron or electron-phonon interactions and/or spin-flip scattering processes. The length scale over which the spin temperature thermalizes is directly linked to the inelastic scattering length in the metal. This technique therefore offers a unique possibility to estimate the inelastic scattering length at low energies and elevated temperatures, not accessible by other spectroscopic methods.

4 Summary 119 Manipulation of spin currents by a magnetic insulator (chapter 6) The long-term goal of spintornics is to achieve efficient manipulation of a spin current using an external gate fabricated atop a spin-transport channel as in the Datta-Das spin field effect transistor. In chapter 7 we demonstrated an alternative spin current manipulation technique using a nonlocal spin valve fabricated on a magnetic Yttrium Iron Garnett (YIG) substrate. Although exchange of electrons across the metal/yig is not allowed, spins can be exchanged due to the spin-mixing interface conductance. When the spin magnetic moment in the metal is aligned (antialigned) with the magnetization of the YIG, most of the spins are back-reflected. However, when the spin magnetic moment is perpendicular to the YIG magnetization, majority of the spins are absorbed by the YIG thereby resulting in a reduction in the nonlocal spin valve signal. We quantified the results by using a three dimensional spin transport model as well as comparing the results with devices fabricated on standard SiO 2 substrate, for which the parameters governing spin transport are well known. We also quantified the size of the spin-mixing conductance and highlighted the role of thermal magnons and other interfacial spin-orbit induced spin-relaxation mechanisms for the observed small modulation in the signal. Thermoelectric effects in graphene (chapter 7) In recent years, extensive research has been devoted to two dimensional systems such as metal chalcogenides, topological insulators and graphene. Graphene is a two dimensional one atom thick honeycomb lattice of carbon atoms that has prominent electronic, spintronic and thermoelectric properties. It was discovered in 2004 by Andre Geim and Konstantin Noveselov for which they shared the 2010 Nobel prize in physics. It has well documented properties, among others, long spin relaxation times, very large thermal conductivity and Seebeck coefficient. Graphene s unique electronic band structure presents the possibility of tunning the Seebeck and Peltier coefficient from large negative values (in the electron regime) to large positive values (in the hole regime) opening up possibilities for tunable thermoelectric conversion or refrigeration applications. In chapter 7, we studied electronic and thermoelectric properties of single and bilayer graphene in a device architecture that allowed us to detect both the Seebeck and Peltier effect in a single device. The devices studied for this purpose have, in addition to conventional electrical contacts, a micropatterned electrical heater as well as a thermocouple that is used to measure local temperature changes at a graphene/metal interface. Two separate measurements aimed at the understanding of the Peltier heating/cooling and Seebeck effects were performed. In the first experiment, by sending an electrical current through a metal/graphene interface

5 120 Summary and tunning the charge carriers in the graphene, it was possible to reversibly heat or cool the interface. In another measurement, on the same device, we performed thermopower measurements where the graphene was subjected to an in-plane heat current and the thermovoltage that develops over the graphene was obtained as a function of the carrier density. Using a three dimensional thermoelectric model, we further verified the validity of the Thomson-Onsager reciprocity relation between the Peltier and Seebeck coefficients. Conclusion Spin caloritronics is a broad research field that encompasses the study of the coupling between heat and spin transport in metallic nanostructures, magnetic insulators, magnetic tunnel junctions and other technologically important devices. Although it is yet at its infancy, it has already provided us with additional insight on the thermoelectrical transport properties of nanoscale spintronic devices. While heating effects remain detrimental to the performance of current microprocessors, future spin caloritronic devices might be useful for nanoscale heat scavenging and waste heat management applications as well as adding more functionality to current spintronic devices.