Experimental and theoretical analysis of the upper critical field in ferromagnet–superconductor–ferromagnet trilayers

Abstract

Abstract. The upper critical magnetic field H c2 in thin-film FSF trilayer spinvalve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses d F1 of the bottom and d F2 of the top Cu 41 Ni 59 F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H c2 is measured in the temperature range 0.4 − 8 K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu 41 Ni 59 /Nb/Cu 41 Ni 59 spin-valve core trilayers.Experimental and theoretical analysis of the upper critical field in FSF trilayers 2

A Peculiar Solitary Re-entrant Superconductivity Induced by an External Magnetic Field in Ferromagnet-Superconductor Heterostructures

“…It is well known that the magnetic field monotonically decreases the critical temperature, which agrees with our results. Note that in recent work [11], the re-entrant superconductivity was observed with increasing magnetic field in the symmetrical Cu 41 Ni 59 /Nb/Cu 41 Ni 59 trilayers. This agrees well with our theoretical results (see also our recent work [21]).…”

“…We use a numerical approach proposed in our recent work [21], where good agreement with the experimental data for the symmetrical CuNi/Nb/CuNi trilayer in an external magnetic field [11] was obtained. All lengths related to the S and F 1,2 layers are normalized on the coherence lengths ξ s0 = υ s /2π T cs and spin stiffness length a f = υ f /2I , respectively.…”

“…Our theoretical approach taking into account the asymmetry and triplet superconducting correlations is valid in the external magnetic field presence as in the real experimental setup for the FS spin valve explorations [11,19,20,30]. We predict an unusual behavior of the difference t versus external magnetic field in the FSF system and discuss its application to the spin valve problem.…”

“…Firstly, the appearance of the re-entrant superconductivity was observed in bilayers V/Fe [8] and Nb/CuNi [9]. Recently, the re-entrant superconductivity with increasing magnetic field predicted in work [10] was observed in the symmetrical CuNi/Nb/CuNi trilayer [11]. Lastly, the solitary superconductivity was also recently theoretically proposed for clean FS systems [12,13].…”

“…Moreover, the superconducting switch based on the four-layered FSFS system can have up to seven different states [17]. An implementation of the spin valve can be found in the reviews [2][3][4] and recent works [11,[18][19][20][21] (see also references therein).…”