Giant magnetocaloric effect driven by indirect exchange in magnetic multilayers

Abstract

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.

Influence of Nanosize Effect and Non-Magnetic Dilution on Interlayer Exchange Coupling in Fe–Cr/Cr Nanostructures

“…In particular, the antiferromagnetic coupling between the layers forms a basis for the production of synthetic antiferromagnets, which are used in magnetic sensors [16,17] and as elements of magnetic random access memory (MRAM) [18][19][20], being also promising systems for antiferromagnetic spintronics [21], which is being actively developed now [22,23]. The indirect interlayer exchange coupling can lead to interesting interesting thermomagnetic phenomena in multilayered FM/NM systems near the Curie temperature for the FM layers [24][25][26][27][28][29]. For instance, such phenomena as the temperature-induced switching of the interlayer coupling from the ferromagnetic to antiferromagnetic type [17,28] and the giant magnetocaloric effect [29] have been demonstrated recently for systems based on multilayered Fe-Cr/Cr nanostructures ( Fig.…”

“…The indirect interlayer exchange coupling can lead to interesting interesting thermomagnetic phenomena in multilayered FM/NM systems near the Curie temperature for the FM layers [24][25][26][27][28][29]. For instance, such phenomena as the temperature-induced switching of the interlayer coupling from the ferromagnetic to antiferromagnetic type [17,28] and the giant magnetocaloric effect [29] have been demonstrated recently for systems based on multilayered Fe-Cr/Cr nanostructures ( Fig. 1, ), where Fe-Cr are layers of dilute ferromagnetic alloys Fe Cr 100− with the Curie temperature close to room temperature (Fig.…”

Inverse magnetocaloric effect in synthetic antiferromagnets

“…A large MCE was predicted for a system of macrospins (∼100 µ B ) embedded into a weakly magnetic matrix, amplified by the interparticle exchange [19,21]. Subsequent experiments on magnetic nano-composites [22,23] and RKKY-coupled superlattices [20] have indeed shown large isothermal entropy changes of up to ∆S max = -0.4 J/kg K in a field of 7 T. A conceptually different multilayer system, based on thermally controlled RKKY [30][31][32], showed a giant MCE of ∆S max = -1.4 J/kg K in fields as low as 10 mT [24]. The magneto-calorically active layer was a dilute ferromagnet undergoing an RKKY-induced magnetic phase transition on antiparallel-to-parallel magnetization switching in the system.…”

“…Nanostructuring was recently shown to yield greatly enhanced MCE in materials containing no rare-earth elements [2,[19][20][21][22][23][24] via controlling the relevant anisotropy and exchange parameters [25]. The approach exploits the finite-size effects in nanostructures, where the interface and bulk contributions are energetically comparable.…”

Tuning thermo-magnetic properties of dilute-ferromagnet multilayers using RKKY interaction

“…18)-23) Incorporating dilute-ferromagnets (e.g., FexCr1-x) with relatively low Curie point (TC near room temperature) into RKKY-coupled Fe/Cr-based multilayers was experimentally shown to enable thermally-controlled antiferromagnetic exchange 21), 22) as well as a giant magnetocaloric effect. 23) These were explained in terms of a thermally-driven competition between the intra-and inter-layer exchange interactions, when the two are tuned to be comparable in magnitude. Since the interlayer RKKY-exchange is an interfacial effect and is usually much weaker that the intra-layer exchange, an addition of an ultra-thin strongly-ferromagnetic layer (e.g., Fe) at the FexCr1-x/Cr interfaces allows to substantially enhance the interlayer coupling.…”