Indirect exchange coupling in magnetic multilayers, also known as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, is highly effective in controlling the interlayer alignment of the magnetization. This coupling is typically fixed at the stage of the multilayer fabrication and does not allow ex-situ control needed for device applications. In addition to the orientational control, it is highly desirable to also control the magnitude of the intralayer magnetization, ideally switch it on/off by switching the relevant RKKY coupling. Here we demonstrate a magnetic multilayer material, incorporating thermally-as well as field-controlled RKKY exchange, focused on to a dilute ferromagnetic alloy layer and driving it though its Curie transition. Such on/off magnetization switching of a thin ferromagnet, performed repeatably and fully reproducibly within a low-field sweep, results in a giant magnetocaloric effect, with the estimated isothermal entropy change of ∆S ≈ −10 mJ cm −3 K −1 under an external field of ∼10 mT, which greatly exceeds the performance of the best rare-earth based materials used in the adiabatic-demagnetization refrigeration systems.

We report on spin-vortex pair dynamics measured at temperatures low enough to suppress stochastic core motion, thereby uncovering the highly non-linear intrinsic dynamics of the system. Our analysis shows that the decoupling of the two vortex cores is resonant and can be enhanced by dynamic chaos. We detail the regions of the relevant parameter space, in which the various mechanisms of the resonant core-core dynamics are activated. We show that the presence of chaos can reduce the thermally-induced spread in the switching time by up to two orders of magnitude.Spin vortices carry a significant fundamental interest [1][2][3][4][5][6][7][8][9][10][11][12][13][14] due to their high variety in terms of physical layouts, spin configurations, and types of intraand inter-vortex interactions. Applications of relevance are memory [6,[15][16][17][18][19][20][21][22], rf signal sources [23][24][25] as well as biofunctional materials [26,27]. Vertically stacked vortices, with thin inter-vortex spacers, are rather unique as they can possess a very localized yet very strong core-core coupling potential [28][29][30]. Here we study such a tightly spaced vortex pair and focus on its most intriguing configuration, having parallel core polarizations and antiparallel vortex chiralities (referred to as the P-AP state; illustrated in Fig. 1a), as the collective dynamics it exhibits are entirely different from those of the individual vortices comprising the pair. We show that the system can be made bi-stable with the cores either magnetically and spatially coupled (diatomic-molecule type pair) or well separated (dissociated molecule), with highly nonlinear, chaos-enhanced switching dynamics.We develop an analytical model based on the Thiele equation [31] and show that the dynamics of the system can be reliably predicted with a set of only two time dependent first order equations -a bare minimum of dimensions required to have chaotic dynamics. Such simplicity sets our spin vortex pairs apart from other magnetic chaotic systems, studied with either continuous media models [32,33] or higher-dimensional models [34].The effects of thermal agitation on bi-stable, periodically driven systems are well understood. A dynamically meta-stable state appears as a result of the resonant excitation with its energy elevated, which enhances the stochastic escape rate [35,36]. Alternatively, when the external force oscillates slower than the systems characteristic response time, a stochastic resonance can be observed [37,38]. Thermal effects on chaotic systems were studied to a lesser degree, with one example being noise in Josephson junctions [39][40][41], important for building voltage standards. Only time-averaged characteristics are of interest in this case since the phase of the junction is a periodic variable, making individual switching events non-important. 1. (a) Schematic of the studied vortex pairs, having vertically tightly-spaced magnetic particles, each in a spin vortex state with parallel core polarizations (vertical arrows) and antiparalle...

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...

Vortex pairs in magnetic nanopillars with strongly coupled cores and pinning of one of the cores by a morphological defect, are used to perform resonant pinning spectroscopy, in which a microwave excitation applied to the nanopillar produces pinning or depinning of the cores only when the excitation is in resonance with the rotational or gyrational eigenmodes of the specific initial state of the core-core pair. The shift in the eigenmode frequencies between the pinned and depinned states is determined experimentally and explained theoretically, and illustrates the potential for multi-core spin-vortex memory with resonant writing of information on to various stable vortex pair states. Further, it is shown how the same resonant spectroscopy techniques applied to a vortex pair can be used as a sensitive nanoscale probe for characterizing morphological defects in magnetic films.

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