Enhanced exchange and reduced magnetization of Gd in an Fe/Gd/Fe trilayer

The exchange interaction of Gd adjacent to Fe has been characterized by transport measurements on a double spin valve with a Fe/Gd/Fe trilayer as the middle layer. Our measurements show that the ferromagnetism of the Gd is enhanced by the presence of the Fe, and it remains ferromagnetic over its Curie temperature up to a thickness no smaller than 1 nm adjacent to the Fe. This thickness is more than double what has been reported before. Additionally, the saturation magnetization of the thin Gd layer sandwiched in Fe was found to be half of its bulk value. This reduced magnetization does not seem to be related to the proximity of Fe but rather to the incomplete saturation of Gd even for very high fields.

Figure 1

(a) STEM-EELS map of the Fe $L_{2,3}$ edge of the DSV with 4 nm of Gd. (b) Hysteresis loop for this sample indicating the switching of the different layers. (c) MR curve for this sample, with an indication of the configuration of each layer at different points of the loop.

Figure 2

Room-temperature MR curve (a) for a DSV with 0 nm of Gd (i.e., 6 nm of a continuous Fe layer in the middle of the DSV) and (b) for a DSV with 2 nm of Gd. The inset in (b) is a STEM-EELS map at the Fe $L_{2,3}$ edge. The scans of this sample show a Gd thickness between 2.5 and 2.8 nm.

Figure 3

MR loop for the sample with 4 nm of Gd at 25 K. The shape of this loop is very similar to the one shown in Fig. 2a for no Gd, which shows that the two Fe layers are exchange coupled through the Gd. The inset shows a sequence for several temperatures for a low-field zoom of the loop. The vertical scale of the inset is shown on its right.

Figure 4

Schematic MR loops for a DSV (a) when it is Fe aligned and (b) when it is Gd aligned. Each schematic has an arrow diagram representing the direction of the layers at the saturating field (the field that saturates the pinned layer) in each case. For higher fields, the sample goes into a twisted state represented by the dotted line.

Figure 5

MR loops measured at 10 K for a DSV with the structure described in Fig. 1 and varying the thickness of the Gd layer, indicated in each plot. The transition from (a) Fe aligned to (d) Gd aligned happens between (b) $t_{\mathrm{Gd}}$ = 8 nm and (c) $t_{\mathrm{Gd}}$ = 10 nm.

Figure 6

Hysteresis loop with the field applied perpendicular to the sample plane for a thin film of Ta(3 nm)/Fe(6 nm)/Ta(3 nm) (circles) and a thin film of Ta(3 nm)/Gd(4 nm)/Ta(3 nm) (triangles), measured at 10 K.