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

We demonstrate a 20-fold enhancement in the strength of the RKKY interlayer exchange in dilute-ferromagnet/normal-metal multilayers by incorporating ultrathin Fe layers at the interfaces. Additionally, the resulting increase in the interface magnetic polarization profoundly affects the finite-size effects, sharpening the Curie transition of the multilayer, while allowing to separately tune its Curie temperature via intralayer magnetic dilution. These results should be useful for designing functional materials for applications in magneto-caloric micro-refrigeration and thermally-assisted spin-electronics.

Synthetic antiferromagnets (SAFs), fabricated either as continuous films or arrays of nanopillars, are magnetic multilayers composed of alternating ferromagnetic layers and nonmagnetic spacers, in which the layers' magnetic moments are arranged in antiparallel.
Since the discoveries of the oscillatory interlayer exchange coupling 1) and giant magnetoresistance, 2), 3) SAFs have enabled a range of spintronic devices 4), 5) such as spinvalve sensors 6), 7) and magnetic random access memory. 8)-10) SAFs favorably combine the advantages of both ferromagnetic and antiferromagnetic materials. For example, nanopatterned SAFs have negligible magnetic stray fields akin to antiferromagnets, which reduces crosstalk problems in magnetic memory arrays. At the same time, non-zero magnetization of the individual ferromagnetic layers comprising the SAF can be used to control its configuration by external magnetic fields as well as sense its individual stable magnetic states using spin currents -the tasks hardly achievable with ordinary antiferromagnets. Continuous SAF multilayers usually incorporate rather strong interlayer exchange known as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. 11),12) RKKYcoupled SAF materials demonstrate significant potential for such promising devices 13) as the race-track memory 14), 15) and the skyrmion-based logic. 16), 17) The useful functionality of SAF-systems extends to the realm of thermo-magnetic effects. 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. The addition of Co layers at the interfaces of Permalloy has been shown 24), 25) to enhance the interlayer exchange by up to several fold. The same effect of the interface polarization enhancement in lower-TC dilute-ferromagnet-based SAFs has not been studied in detail.
In this Letter, we demonstrate a 20-fold increase in the strength of the interlayer RKKY are explained in terms of a complex interplay between the thermally-enhanced finite-size effects and the Fe-enhanced magnetic polarization of the interfaces, well supported by atomistic spin simulations performed using the VAMPIRE software package. 26) Multilayers [Fe37Cr63(df)/Cr(dCr)]N (hereafter Fe-Cr/Cr) were grown on undoped Si (100) substrates at room temperature using an UHV dc magnetron sputtering system (AJA Inc.).
Layers of a dilute Fe37Cr63 binary alloy were deposited using co-sputtering from separate Fe and Cr targets. The Cr thickness of dCr = 1.2 nm was found to correspond to the strongest antiferromagnetic interlayer exchange coupling. At the same time, dCr ≥ 3.0 nm was determined to correspond to a completely decoupled state of the Fe-Cr layers. Since the interface roughness increases with the number of bilayer repetitions and can affect the RKKY interaction of the top layers, we have found the optimal number of the Fe-Cr/Cr bilayers to be N = 8, for which this effect is negligible.
Two characteristic sample series are studied in detail -the base multilayered structure   Figure 3 compares Ms and Jex obtained for the RKKY-coupled structures. As seen from Fig. 3

(b), the addition of interfacial Fe can increase
Jex by more than 20 times. This is a very strong enhancement considering that the maximum increase of the interlayer coupling using this approach was about 4 times, reported for Fe81Ni19/Co/Ag multilayers. 25) As we show in greater details below, it can be explained in terms of a considerable difference between the effective interatomic magnetic exchange  Figure 4(a) shows the local magnetization versus the monolayer's depth, m-vs-z, for a 17monolayer-thick Fe37Cr63-alloy film with and without a 2-ml-thick interfacial Fe layer; the simulation temperature chosen was T/TC0 = 0.55, where TC0 is the Curie temperature of the bulk Fe37Cr63 alloy. The layer without interfacial Fe, plotted in orange in Fig. 4(a), exhibits much lower local magnetization at the interfaces, which supports the interpretation of the thermally-enhanced finite-size effects (interface spin disorder) discussed above. In contrast, the m-vs-df profiles for the structures with interfacial Fe reveal significantly enhanced local magnetization at the interfaces, shown in blue in Fig. 4(a). The large difference in the interfacial magnetization for the two systems explains the observed large enhancement in the interlayer RKKY coupling, which should be proportional to the magnetic polarization of the interfaces.
The simulated total magnetization as a function of the Fe-Cr thickness, shown in Fig. 4(b), is in good agreement with the experimental data shown in Fig. 3