In the rapidly developing field of spintronics, information is encoded and processed in the form of quantum spins. Although phenomena such as the giant magnetoresistance effect and spin-transfer torque have already found use in commercialized devices, two main challenges remaining in spintronic circuits based on electron transport are Joule heating and short decay lengths due to spin-flip scattering. The incorporation of superconducting elements into spintronic architectures has recently emerged as a potential solution to this problem. Recent advances in understanding the underlying physics of the interface between superconductors and ferromagnets has revealed a wealth of new features that can be enhanced and controlled to create improved spintronic devices. In this chapter, we present an introduction to the superconducting proximity effect in magnetic materials that opens the tantalizing prospect of combining the dissipationless transport offered by superconductors with the spin-polarized order existing in magnetic thin-film heterostructures. This includes an outline of theoretical frameworks and conventions in the field, as well as a discussion of some key experimental and theoretical advances that may indicate where the field is heading.
1.1 Introduction
In the rapidly developing field of spintronics, information is encoded and processed in the form of quantum spins – either in place of, or in conjunction with, the traditional charge-based processing schemes. This has high potential with respect to computational processing, both in terms of time and energy efficiency, as well as significantly increased stability and longer coherence times than conventional electronic components. Although several spintronic architectures have already been widely adopted, e.g. in hard drives and random access memory based on the giant magnetoresistive effect (GMR) [1, 2], two main challenges in spintronic circuits based on electron transport are Joule heating and short decay lengths due to spin-flip scattering. The incorporation of superconducting elements into existing spintronic architectures has recently emerged as a potential solution to this problem [3]. Recent advances in understanding the underlying physics of the interface between superconductors and ferromagnets has revealed a wealth of new features that can be enhanced and controlled to create improved spintronic devices. In this chapter, we present an introduction to the superconducting proximity effect that opens the tantalizing prospect of combining the dissipationless transport offered by superconductors with the spin-polarized order existing in magnetic thin-film heterostructures. This includes an outline of theoretical frameworks and conventions in the field, as well as a discussion of some key experimental and theoretical advances that may indicate where the field is heading.
Spin-polarized currents are typically generated by passing an electric current through a ferromagnet, such that the magnetization in the ferromagnet acts to align the electron spins. Spintronic nanostructures are designed as a series of thin-film layers of normal-metal and ferromagnetic elements, which can be incorporated into conventional semiconductor-based systems. Emergent features of such spintronic devices can then be used to harness and control aspects of the device, for which GMR provides an exemplary case. GMR manifests as a change in the electrical resistance according to the relative magnetization directions of adjacent ferromagnetic layers: the resistance is low for parallel alignment and high for antiparallel alignment. This effect has been implemented in a wide variety of experimental structures, an important example being spin valves [4, 5], which switch an electric current on or off based on a magnetic input signal. Experimentally, spin valves consist of two ferromagnetic layers and an interstitial normal metal. The coercivity of one ferromagnetic layer can be enhanced due to proximity with an antiferromagnetic base layer, meaning that the application of an external magnetic field can be used to control the magnetization of the other ferromagnet. Albert Fert and Peter Grünberg shared the 2007 Nobel Prize in Physics for the discovery of the GMR effect.
In order to utilize the GMR effect, one needs a way to alter the magnetization direction of a ferromagnet. Spin-polarized electric currents can induce magnetization dynamics via the so-called spin-transfer torque effect [6, 7], but this typically requires very large current densities of order 106 A/cm2. This causes excessive Joule heating and ultimately destroys the properties of the thin-film structure. Recent investigations of the proximity effect, where the properties of adjacent materials leak across the interfacial barrier, have indicated that it will be possible to harness the dissipationless currents offered by superconductors to overcome the problem of excessive heating by making these supercurrents spin-polarized [3].
Superconductivity was discovered as early as 1911, when Heike Kamerlingh Onnes observed that the electrical resistivtiy of certain materials vanished abruptly at cryogenic temperatures [8]. Kamerlingh Onnes received the Nobel Prize in Physics in 1913 for this discovery. The temperature at which the transition to zero resistivity occurs is called the critical temperature (Tc) of the material, and as a material transitions into this superconducting state it displays a second characteristic ...
