Magnetic state modification induced by superconducting response in ferromagnet/superconductor Nb/Co superlattices

C. Monton, F. de la Cruz, and J. Guimpel

Phys. Rev. B 77, 104521 – Published 19 March 2008

Abstract

Magnetization measurements in superconductor/ferromagnet $Nb ∕ Co$ superlattices show a complex behavior as a function of temperature, applied field, and sample history. Based on a simple model, it is shown that this behavior is due to an interplay between the superconductor magnetization temperature dependence, the ferromagnet magnetization time dependence, and the stray fields of both materials. It is also shown that the magnetic state of the Co layers is modified by the Nb superconducting response, implying that the problem of a superconductor/ferromagnetic heterogeneous sample has to be solved in a self-consistent manner.

Received 14 August 2007

DOI:https://doi.org/10.1103/PhysRevB.77.104521

Authors & Affiliations

C. Monton, F. de la Cruz, and J. Guimpel

Centro Atómico Bariloche, Instituto Balseiro, Comisión Nacional de Energía Atómica, Universidad Nacional de Cuyo, (8400) San Carlos de Bariloche, Argentina

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Vol. 77, Iss. 10 — 1 March 2008

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Images

Figure 1
Main panel: magnetization $M$ as a function of applied field $H_{a}$ for the $[Nb (44 nm) ∕ Co (10 nm)] \times 19$ superlattice at $T = 7 K > T_{C S} = 6.2 K$ . Panels (a)–(d) show the temperature $T$ dependence of the superconducting magnetic flux response $Δ ϕ$ in units of the superconducting flux quantum $ϕ_{0}$ at different applied fields and ferromagnetic layer initial states, as indicated by gray dots and connecting arrows in the main panel. Arrows in panels indicate the direction of the $T$ sweeps. Panel (a): $H_{a} = - 22 Oe$ , $+ FC$ . Panel (b): $H_{a} = 75 Oe$ , $- FC$ . Panel (c): $H_{a} = 38 Oe$ , $- FC$ . Panel (d): $H_{a} = 22 Oe$ , $- FC$ .Reuse & Permissions
Figure 2
Hysteresis loop for the superlattices $[Nb (44 nm) ∕ Co (10 nm)] \times 19$ at $T = 7 K$ , normal state (upper panel), and $[Nb (44 nm) ∕ Co (0.7 nm)] \times 7$ at $T = 3 K$ , superconducting state (lower panel). Arrows show the corresponding scale for the longitudinal (black circles) and transverse (open triangles) signals. The $t_{Co} = 0.7 nm$ sample is not ferromagnetic.Reuse & Permissions
Figure 3
Schematics of the toy model behavior. Panel (a): at $T > T_{C S}$ , the normal Nb ellipsoid experiences an effective field due to the applied field (straight lines) and the ferromagnetic Co ellipsoid stray field (dipolelike lines). Panel (b): as $T$ is reduced below $T_{C S}$ , the magnetic flux expulsion from the Nb ellipsoid modifies the effective field over the Co ellipsoid.Reuse & Permissions
Figure 4
Time $t$ dependence of the sample magnetization change $Δ ϕ$ at $T > T_{C S}$ (triangles) and $T < T_{C S}$ (circles) for $22 Oe - FC$ magnetic initial state. The inset shows the temperature dependence of the sample magnetization change $Δ ϕ$ . Dashed lines in the inset show the temperatures at which drifts were measured.Reuse & Permissions
Figure 5
Model prediction for the temperature $T$ dependence of the sample magnetization change, $Δ M_{T} = M_{T} (T) − M_{0}$ , where $M_{0}$ is the magnetization for the initial simulated data point at $T > T_{C S}$ . Panels (a) and (b) show the results for two sets of parameters qualitatively equivalent to the experimental data in panels (c) and (a) of Fig. 1. Panel (a): $H_{a} = 22.85 Oe$ , $M_{0} = 5.12 emu {cm}^{- 3}$ . Panel (b): $H_{a} = - 17.15 Oe$ , $M_{0} = 5.12 emu {cm}^{- 3}$ .Reuse & Permissions
Figure 6
Magnetic flux expulsion, $Δ ϕ$ , for a $[Nb (44 nm) ∕ Co (7.5 nm)] \times 19$ superlattice with $T_{C S} = 5.9 K$ , and model prediction, $Δ M_{T}$ , for $H_{a} = 15 Oe$ and $M_{0} = 9 emu {cm}^{- 3}$ . The inset shows the dependence on simulated data point number, i.e., time, for the superconducting ellipsoid magnetization $M_{SC}$ , the ferromagnetic ellipsoid magnetization $M_{FM}$ , and the sample’s total magnetization, $M_{T}$ .Reuse & Permissions

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