Thermally induced antiferromagnetic exchange in magnetic multilayers

We demonstrate sharp thermally induced switching between ferromagnetic and antiferromagnetic RKKY (Ruderman-Kittel-Kasuya-Yosida) 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 being due to a change in the effective thickness of the spacer induced by the Curie transition into its paramagnetic state. The ability to switch between ferromagnetic and antiferromagnetic states in a magnetic multilayer by a slight change in temperature may lead to new types of spin-thermoelectronic devices for use in such applications as memory or oscillators.

Figure 1

Illustration of magnetization states in F/S/F trilayers with (a) uniform nonmagnetic (N) or paramagnetic (P) spacers, S = N(P); (b) uniform ferromagnetic spacer, S = F*; (c) composite N/P/N; and (d) composite N/F*/N spacers. Curie transition in F* within the composite spacer (transition from case (c) to case (d) on increasing temperature) changes the sign of RKKY as a result of a change in the effective thickness of the composite spacer.

Figure 2

Room-temperature VSM M-B loops for Fe/S/Fe trilayers with (a) uniform spacer S = ${\mathrm{Fe}}_x{\mathrm{Cr}}_{100-x}$(1.5) and (b) composite spacer S = Cr(0.4)/${\mathrm{Fe}}_x{\mathrm{Cr}}_{100-x}$(0.7)/Cr(0.4), for different Fe concentrations, $x$. The inset in panel (a) shows data for the exchange pinned structure ${\mathrm{Fe}/\mathrm{Fe}\text{−}\mathrm{Cr}/\mathrm{Fe}/\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}$ ($x=15\%$). $B_{\text{pin}}$ is the effective field of exchange pinning by antiferromagnetic IrMn.

Figure 3

Temperature dependence of (a) normalized remanent magnetic moment ($m_{\text{r}}$), (b) exchange field ($B_{\text{ex}}$) and the corresponding RKKY coupling strength $J_{\text{RKKY}}$ (right scale), and (c) minor-loop coercivity ($B_{\text{c}}$) for F/S/F trilayers with S = Cr(0.4)/${\mathrm{Fe}}_x{\mathrm{Cr}}_{100-x}$(0.7)/Cr(0.4), $x=15$%, 17.5%, and 20%. (d) M-B loops for $x=17.5\%$ for representative temperatures above and below the effective Curie point of the gradient spacer (dashed lines indicate the center of the loops). Data were obtained using MOKE magnetometry.

Figure 4

Temperature dependence of (a) normalized remanent magnetization and (b) exchange field for F/S/F trilayers with S = $\mathrm{Cr}(d_{\text{Cr}}$)/${\mathrm{Fe}}_{15}{\mathrm{Cr}}_{85}$($d$)/$\mathrm{Cr}(d_{\text{Cr}}$), $d\left(d_{\text{Cr}}\right)=3\left(6\right)$, 7(4), 9(3), 11(2), and 15(0) Å. (c) Initial slope of the $B_{\text{ex}}\left(T\right)$ dependence and critical temperature $T_{\text{C}}^{*}$ vs thickness $d$ for the spacer with constant concentration $x=15\%$.