# Magnetic Nanowires
## 3 Results
### Spin Transport in Multilayered Nanowires
Equation (4) expresses that only a depth \(\ell_{\text{sf}}^{\text{FM}}\) on both sides of a magnetic layer is \"active\" in this limit. The variation as \(1/t_{\text{FM}}\) of Equation (4) is illustrated by the experimental results of Figure 9b for Co/Cu multilayers. Once \(p\) is determined by independent Magnetic Force Microscopy observations, the \(1/t_{\text{FM}}\) variation of Figure 9b can be used to derive the SDL length in Co, \(\ell_{\text{sf}}^{\text{Co}}=59\) nm at 77 K [5]. At room temperature, \(\ell_{\text{sf}}^{\text{Co}}=40\) nm. In Figure 9c, we show another example of variation of the GMR as \(1/t_{\text{FM}}\) for a collection of Py/Cu/Py trilayers [5]. The SDL in Py, \(3.3\) nm \(<\ell_{\text{sf}}^{\text{Py}}<5.3\) nm, has been derived from an analysis of these data with an expression similar to Equation (4). This value is in approximate agreement with that found for planar multilayers [86], \(\ell_{\text{sf}}^{\text{Py}}\approx 5.5\) nm. Despite their complex nano-architecture, interconnected NW networks made of Co/Cu, CoNi/Cu and Py/Cu multilayers were found to exhibit very large CPP-GMR responses reaching values of 40% at RT and 86% at \(T=15\) K for CoNi/Cu [64]. The same features were observed on the Co/Cu and Py/Cu CNW samples [65; 67], as shown in Figure 10a-c. The effects related to spin-transfer torque from a spin-polarized current in a nanosized ferromagnet have been studied in specially designed nanowires containing Py/Cu/Py and Co/Cu/Co pseudo spin-valves electrodeposited in anodic porous alumina supported on Si substrate (see Figure 11a). Single nanowire electrical connection techniques were developed for this purpose [30; 36; 41]. Switching phenomena driven by a spin-polarized current were clearly demonstrated in Py/Cu/Py trilayer nanowires [30]. Figure 9: (**a**) CPP-GMR versus applied field parallel to the layers at 77 K for Py/Cu (full line) and Co/Cu (dashed line) multilayered nanowires; (**b,c**) GMR versus \(t_{\text{FM}}\) at \(T=77\) K (**b**) for Co(\(t_{\text{Co}}\))/Cu(8 nm) multilayered nanowires and (**c**) for nanowires composed of [Py(\(t_{\text{Py}}\))/Cu(10 nm)/Py(\(t_{\text{Py}}\))] trilayers separated by 100 nm of Cu (open circles) or 500 nm of Cu (filled squares). The solid lines in (**b**) and (**c**) represent a variation as \(1/t_{\text{FM}}\) (adapted from [5]). figure adapted from ref. [5] with permission of the author. In Figure 11b, we show the variation of the differential resistance d\(V\)/d\(I\) versus injected direct current at small magnetic field. Starting from the P state at zero current, the injected current is first decreased toward negative values. We first observed a sharp increase of the resistance at \(-2.5\) mA due to the magnetization switching of the thin Py layer. The curve is hysteretic because the system remains in this high resistance state until the current is swept to a positive value. At a critical positive current of about +0.4 mA, the resistance flips back to the P state level. The system remains in this low resistance state at higher positive current. In addition, under specific magnetic field and injected current conditions, emission of microwave current was detected in Co/Cu/Co pseudo spin-valves with frequency in the GHz range and linewidth as low as 1.8 MHz [37; 41; 45]. Microwave signals have been obtained even at zero magnetic field and high frequency versus magnetic field tunability was demonstrated. From micromagnetic simulations, it appeared that the microwave emission is generated by the vortex gyrotropic motion which occurs in, at least, one of the two magnetic layers of our spin-valve structures. To stabilize the magnetic vortex state, a saturating perpendicular magnetic field is first applied to the sample before a decreasing ramp of magnetic field. The results reported in Figure 11c for the measured spin-transfer nano-oscillator containing a single spin-valve are related to the two-vortex state with vortices of opposite polarity present in both the magnetic layers of the spin-valve.