Boosting the superconducting spin valve effect in a metallic superconductor/ferromagnet heterostructure

Abstract

Superconducting spin valves based on the superconductor/ferromagnet (S/F) proximity effect are considered to be a key element in the emerging field of superconducting spintronics. Here, we demonstrate the crucial role of the morphology of the superconducting layer in the operation of a multilayer S/F1/F2 spin valve. We study two types of superconducting spin valve heterostructures, with rough and with smooth superconducting layers, using transmission electron microscopy in combination with transport and magnetic characterization. We find that the quality of the S/F interface is not critical for the S/F proximity effect, as regards the suppression of the critical temperature of the S layer. However, it appears to be of paramount importance in the performance of the S/F1/F2 spin valve. As the morphology of the S layer changes from the form of overlapping islands to a smooth case, the magnitude of the conventional superconducting spin valve effect significantly increases. We attribute this dramatic effect to a homogenization of the Green function of the superconducting condensate over the S/F interface in the S/F1/F2 valve with a smooth surface of the S layer.It is well known that bringing together two antagonistic ordered states of matter, namely superconductivity (S) and ferromagnetism (F), in a thin S/F heterostructure gives rise to a variety of new physical phenomena, such as the S/F/S π-phase Josephson effect, the so-called cryptoferromagnetic state, conventional (singlet) and unconventional (triplet) superconducting spin valve effects (SSVE) (e.g., see a recent review [1] and references therein). The SSVE for a sequence of two metallic F layers and one S layer, S/F1/F2, was proposed theoretically in 1997 by Oh et al. [2]. This multilayer structure can be switched between the normal and superconducting states by changing the mutual orientations of the magnetizations of the F1 and F2 layers between parallel (P) and antiparallel (AP) configurations. The physical mechanism behind this effect involves manipulating the phase and amplitude of the superconducting wave function penetrating into the F1 layer and, hence, the superconducting critical temperature, Tc, by changing the magnetic state of the F1/F2 part of the heterostructure. A similar theory for a F1/S/F2 multilayer was proposed in 1999 by Tagirov [3] and Buzdin et al. [4]. Later, a triplet spin valve effect was described theoretically for S/F1/F2 structures by , who proposed another way to manipulate Tc, which is related to the formation of a long-range triplet component of the superconducting condensate at a non-collinear orientation of the F1 and F2 magnetizations.At present, there have been a number of experimental works confirming the SSVE (see, e.g.,

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