We demonstrate sharp thermally-induced switching between ferromagnetic and antiferromagnetic RKKY exchange in a spin-valve with the spacer incorporating a thin diluted ferromagnetic layer as the core. We illustrate the mechanism behind the effect as due to a change in the effective thickness of the spacer induced by the Curie transition into its paramagnetic state.The discovery of the indirect exchange coupling (IEC) of type Ruderman-Kittel-Kasuya-Yosida (RKKY) [1] and the giant magnetoresistance effect [2,3] in magnetic multilayers have broadened a number of fields of physics and technology [4]. The discovered IEC oscillates in magnitude and sign versus the spacing of the individual ferromagnetic layers in a metallic stack [5], yielding either parallel or antiparalel magnetic ground state of the multilayer, which is well explained theoretically as due to spindependent reflections and interference of conduction electrons within the nonmagnetic spacers [6][7][8][9][10]. This classical RKKY interaction is essentially independent of temperature [8,[11][12][13] and largely insensitive to any other external control parameter post-fabrication, which limits the use of the IEC in applications. The effect of alloying the spacer with nonmagnetic [14][15][16] and magnetic impurities [17][18][19] on RKKY was studied and explained in terms of an impurity-modified Fermi-surface topology and the corresponding significant changes in the RKKYoscillation periods. The magnetic state of the spacer and its dependence on temperature was, however, was not investigated. Skubic et al. [19] reported on the competition between antiferromagnetic (AFM) RKKY exchange and direct ferromagnetic exchange interactions in Fe/V/Fe multilayers, where the spacer (V) was uniformly alloyed with Fe, but did not discuss the effect of temperature on the competing interactions in the system. Recent attempts to enhance the thermal effect on RKKY and use it to control the IEC in Tb/Y/Gd [20] and Co/Pt [21] multilayers focused on thermally affecting the properties of the respective softer ferromagnetic layers (Gd and thin Co) and thereby the spin-dependent reflection at the respective ferromagnetic interfaces (Gd/Y and Co/Pt). Both studies report relatively weak RKKY, without direct FM-to-AFM thermal switching of the magnetization, with relatively broad thermal transitions (of the order of 100 K, to near full strength RKKY).Here, we focus on thermally altering the effective spacer thickness and demonstrate a magnetic phase transition in Fe/Cr-based multilayers with gradientdoped spacers from strongly ferromagnetic RKKY at low-temperature to strongly antiferromagnetic RKKY at high temperature, both of the order of 100 mT in strength. By optimizing the material system and tailoring the mechanism involved, which is principally different from the previous studies, we achieve direct and fully reversible thermal switching of the RKKY interaction, from strongly ferromagnetic to strongly antiferromagnetic, with very narrow transition widths, of the order of 10 K, es...
We propose a magnetic multilayer layout, in which the indirect exchange coupling (IEC also known as RKKY) can be switched on and off by a slight change in temperature. We demonstrate such on/off IEC switching in a Fe/Cr/FeCr-based system and obtain thermal switching widths as small as 10-20 K, essentially in any desired temperature range, including at or just above room temperature. These results add a new dimension of tunable thermal control to IEC in magnetic nanostructures, highly technological in terms of available materials and operating physical regimes. a) Electronic mail: dpol@kth.se.
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