University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Xiaoshan Xu Papers Research Papers in Physics and Astronomy 2018 A brief review of ferroelectric control of magnetoresistance in organic spin valves Xiaoshan Xu University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/physicsxu Part of the Atomic, Molecular and Optical Physics Commons, Condensed Matter Physics Commons, and the Engineering Physics Commons Xu, Xiaoshan, "A brief review of ferroelectric control of magnetoresistance in organic spin valves" (2018). Xiaoshan Xu Papers. 11. https://digitalcommons.unl.edu/physicsxu/11 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Xiaoshan Xu Papers by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. J Materiomics 4 (2018) 1e12 Contents lists available at ScienceDirect J Materiomics journal homepage: www.journals.elsevier.com/journal-of-materiomics/ A brief review of ferroelectric control of magnetoresistance in organic spin valves Xiaoshan Xu Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA article info abstract Article history: Magnetoelectric coupling has been a trending research topic in both organic and inorganic materials and Received 16 September 2017 hybrids. The concept of controlling magnetism using an electric field is particularly appealing in energy Received in revised form efficient applications. In this spirit, ferroelectricity has been introduced to organic spin valves to 8 November 2017 manipulate the magneto transport, where the spin transport through the ferromagnet/organic spacer Accepted 11 November 2017 interfaces (spinterface) are under intensive study. The ferroelectric materials in the organic spin valves Available online 13 November 2017 provide a knob to vary the interfacial energy alignment and the interfacial crystal structures, both are critical for the spin transport. In this review, we introduce the recent efforts of controlling magnetore- Keywords: Organic spin valve sistance of organic spin valves using ferroelectricity, where the ferroelectric material is either inserted as Ferroelectricity an interfacial layer or used as a spacer material. The realization of the ferroelectric control of magneto Spinterface transport in organic spin valve, advances our understanding in the spin transport through the ferro- Magnetoresistance magnet/organic interface, and suggests more functionality of organic spintronic devices. © 2018 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Introduction . .................................................. 1 2. Diffusive spin valves . .................................................. 3 3. Tunneling spin valves . .................................................. 4 4. Organic spin valves . .................................................. 4 5. Ferroelectric interfaces . .................................................. 5 6. Control of spin transport using ferroelectric interfaces . ..................................... 6 6.1. Diffusive organic spin valves and interfacial energy alignment . .........................7 6.2. Tunneling organic spin valves and the interfacial crystal structure . .........................9 7. Conclusion and outlook . ................................................. 10 Acknowledgements . ........................10 References................................................................. ................................ ........................10 1. Introduction read heads of the hard disks for much larger information density, has been realized; the fundamental research was awarded Nobel Giant magnetoresistance has been a successful example of prizes in 2007. The concept of manipulating the spin degree of nanotechnology in which transport of spin-polarized current freedom of electrons to control the electrical transport, has now through interfaces is manipulated in nanoscale to vary the resis- evolved into a large active field of research and technology, i.e. tance of the devices. Wide application of this effect, such as in the spintronics, with emphasis more and more on spin transport and application potentials in information storage and processing, sen- sors, energy generation, etc. [1e5]. The effect of giant magnetoresistance can manifest in a trilayer E-mail address: [email protected]. Peer review under responsibility of The Chinese Ceramic Society. junction shown in Fig. 1(a). The junction contains a non-magnetic https://doi.org/10.1016/j.jmat.2017.11.003 2352-8478/© 2018 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). 2 X. Xu / J Materiomics 4 (2018) 1e12 Fig. 1. (a) Schematics of an FM/NM/FM trilayer spin valve. (b) and (c) are the magnetic-field dependence of resistance of the spin valve for positive (normal) and negative (inverse) MR respectively. (d) Schematics of the electronic structure of the FM and NM materials, where n[ and nY are the number of states of the up spin and down spin respectively. (NM) spacer layer sandwiched by two ferromagnetic (FM) elec- Organic spin valves are trilayer structures including organic trodes. Depending on the alignment of the magnetization of the semiconductors (OSC) or insulators as the spacer materials. The two FM electrodes, the resistance of the junction changes between long spin life time of the organic materials [18,19] (due to the weak high and low values. The trilayer junction can then be regarded as a spin-orbit coupling in the light elements such as carbon and spin valve, in the sense that the electrical current can be turned on hydrogen), is appealing for spin transport. In addition, the flexi- and off using the external magnetic field which controls the bility, environment friendliness, and the vast chemical diversity of alignment of the magnetization of the two FM electrodes. organic materials suggest great application potentials of organic Depending on the spacer materials and the thickness, three cate- spintronic devices. Organic spin valves generally belong to the gories of spin valves are mostly studied. 1) If the spacer material is a latter two categories introduced above, where the two FM elec- metal, the magnetoresistance (MR) is expected to be small trodes are decoupled in terms of exchange interactions [6]. The compared with the volume resistance [6]. Superlattice-fashioned alignment of the magnetization of the two FM electrodes, can be structures were adopted to enhance MR by increasing the num- tuned by an external magnetic field, based on their difference in ber of interfaces while keeping the thickness of the junction and magnetic coercivity. The MR has a butterfly-like shape, as illus- the volume resistance constant [7e11]. In this case, the thickness of trated in Fig. 1(b) and (c). If the resistance of the spin valve is high the spacer is often less than a few nanometers; the MR has to do (low) when the magnetization of the two FM layers are antiparallel, with the indirect exchange coupling between the magnetic layers it is called normal (inverse) or positive (negative) MR. [12,13]. 2) If the spacer is a thick semiconductor, the two FM layers Encouraged by early promising results on organic spin valves are magnetically decoupled and the transport through the spacer [20,21], efforts on understanding the fundamental mechanism becomes diffusive. In this case, the MR hinges on spin injection, and on realizing organic spintronic devices, has been growing which is actually difficult for a metal/semiconductor ohmic contact. rapidly. However, several key issues, such as spin injection and Tunneling through a barrier between the FM and the semi- spin polarization at the FM/organic interfaces, are still not fully conductor provides a more efficient route for the spin injection understood in organic spin valves. To tackle these key issues, be- [14e17]. 3) If the spacer is a thin insulator, the electrical transport is ing able to tune the crystal structure and electronic structure at based on spin-conserved tunneling between the two electrodes. the FM/organic interfaces (spinterface [22]) appears to be critical, Therefore, the MR is related to the alignment of spin polarization of because the sign and magnitude of the MR is determined by the the initial and final states of the tunneling. spin polarization at these interfaces, which are sensitive to the X. Xu / J Materiomics 4 (2018) 1e12 3 electronic structure. The electronic structure at the FM/spacer a ð À a Þ a ð À a Þ interface is a result of coupling between that of the two materials, 1 ¼ 1 F 1 F þ N 1 N 2 l =s l =s which is based on the crystal structure and the energy level RB ðaF À aNÞ F F N N alignment of the two materials. Ferroelectric (FE) materials, either as an interfacial material or as the spacer itself, offer tunabilities to where lF and lN are the spin diffusion length [39,40] of the FM and the electronic structures at the interface, by changing the energy NM layers respectively, sF and sN are the conductivity of the FM level alignment or the interfacial crystal structure. Besides the and NM layers