Thickness dependence of the triplet spin-valve effect in superconductor–ferromagnet–ferromagnet heterostructures

Abstract

Summary Background: In nanoscale layered S/F1/N/F2/AF heterostructures, the generation of a long-range, odd-in-frequency spin-projection one triplet component of superconductivity, arising at non-collinear alignment of the magnetizations of F1 and F2, exhausts the singlet state. This yields the possibility of a global minimum of the superconducting transition temperature T c, i.e., a superconducting triplet spin-valve effect, around mutually perpendicular alignment. Results: The superconducting triplet spin valve is realized with S = Nb a singlet superconductor, F1 = Cu41Ni59 and F2 = Co ferromagnetic metals, AF = CoOx an antiferromagnetic oxide, and N = nc-Nb a normal conducting (nc) non-magnetic metal, which serves to decouple F1 and F2. The non-collinear alignment of the magnetizations is obtained by applying an external magnetic field parallel to the layers of the heterostructure and exploiting the intrinsic perpendicular easy-axis of the magnetization of the Cu41Ni59 thin film in conjunction with the exchange bias between CoOx and Co. The magnetic configurations are confirmed by superconducting quantum interference device (SQUID) magnetic moment measurements. The triplet spin-valve effect has been investigated for different layer thicknesses, d F1, of F1 and was found to decay with increasing d F1. The data is described by an empirical model and, moreover, by calculations using the microscopic theory. Conclusion: The long-range triplet component of superconducting pairing is generated from the singlet component mainly at the N/F2 interface, where the amplitude of the singlet component is suppressed exponentially with increasing distance d F1. The decay length of the empirical model is found to be comparable to twice the electron mean free path of F1 and, thus, to the decay length of the singlet component in F1. Moreover, the obtained data is in qualitative agreement with the microscopic theory, which, however, predicts a (not investigated) breakdown of the triplet spin-valve effect for d F1 smaller than 0.3 to 0.4 times the magnetic coherence length, ξF1.

In situ transport characterization of magnetic states in Nb/Co superconductor/ferromagnet heterostructures

“…Here we observe the simplest MR curves, most consistent with the two-state theoretical prediction based on mono-domain simulations, as in Figure 1c . Namely, a P state at high field with large R xx (suppressed superconductivity) and an AP state at low fields with small R xx (enhanced superconductivity) [ 4 – 6 8 – 10 20 , 32 , 34 – 35 ]. However, this simple scenario does not explain the appearance of additional maxima between the P and AP states.…”

“…To facilitate this, we use dissimilar Co layers with thicknesses of 1.5 and 2.5 nm. Nb/Co MLs with similar Co thicknesses have been studied earlier and demonstrated good uniformity and perspectives for device applications [ 25 – 26 28 , 32 – 33 43 – 44 ]. Figure 1b shows a magnetization curve for a similar Nb(25 nm)/[Co(1.5 nm)/Nb(8 nm)/Co(2.5 nm)/Nb(8 nm)] 6 Co(1.5 nm)/Nb(25 nm) unpatterned ML film, deposited using the same setup (data from [ 43 ]).…”

Chirality of Bloch domain walls in exchange biased CoO/Co bilayer seen by waveguide-enhanced neutron spin-flip scattering

“…The exchange bias (EB) effect arises on the interface of antiferromagnetic (AF) and ferromagnetic (F) magnetic phases and leads to the shift of the hysteresis loop on value of H eb varying from several Oersteds to kOe (see review [1]). The effect was discovered 60 years ago, wellstudied to the date [1,2] and is utilized in creation of spin-valves including superconducting devices [3][4][5][6][7][8][9][10][11][12][13][14].…”

Reentrance phenomenon in superconductor/ferromagnet nanostructures and their application in superconducting spin valves for superconducting electronics

“…Indeed, recent calculations have shown [42] that not only the "direct switching" ( ) [42], which can be most spectacularly detected as a reentrant behavior of superconductivity as a function of the angle between magnetic moments of the F1 and F2 ferromagnetic layers. It has been shown that superconducting T c for the F1/N/F2/S structure, calculated with physical parameters derived from the experiments described above, keeps all features predicted in [42] for the simplified model, and the triplet component was detected in F1/N/F2/S multilayer structure [44][45][46][47][48] opening the perspectives for the triplet spin-valve design for superconducting electronics [49,50]. One can see that the functionality of the superconductor/ferromagnet layered nanostructures can be successfully utilized to build a novel base elements for superconducting electronics development.…”