3D interconnected magnetic nanofiber networks with multifunctional properties

Summary form only given. Track-etched polymer membranes with crossed nanochannels have been revealed suitable as template for the electrodeposition synthesis of 3D interconnected magnetic nanofiber networks that present interesting magnetic and magneto-transport properties [1-3]. As shown in Fig. 1, such crossed nanowires systems are mechanically stable and self-supported after dissolution of the porous template. In addition, magneto-transport measurements can be easily performed on such interconnected nanofiber network films, with the flow of current restricted along the nanowire segments. The successive track-etch process allows the control of the template geometry such as the angles between the nanochannels and template normal, the volumetric porosity and the mean diameter that can be tuned between few tens of nm to few hundreds of nm. The template is partially filled from a previously sputtered metallic cathode by an electrodeposition process allowing the control of the material composition and nano-structuration, such as nanowires, nanotubes, core-shell nanocables or multilayered nanowires. The cathode is then locally removed to create a two-probe design suitable for easy electric measurements, as shown in Fig. 1c. Magneto-transport measurements are performed by sweeping the external magnetic field along the out-of-plane (OOP - z axis in Fig. 1c) and the in-plane (IP - y axis in Fig. 1c) directions. The magnetic and magneto-transport properties of crossed homogeneous nanowires (CNWs) and multilayered nanowires, synthesized by pulsed electrodeposition technique from a single electrolyte solution, have been investigated in details. A close relation between the CNWs magnetic and structural properties has been demonstrated in Co CNWs networks where the magnetic anisotropy can be accurately controlled by changing the electrolyte pH [2,4]. The crystalline structure can be switched from fcc to hcp with either the c-axis parallel or perpendicular to the NW axis such that the shape anisotropy and crystal anisotropy can either add or compete. A similar behavior was found in NiCo CNWs depending on the alloy composition in agreement with previous results obtained on bulk materials and films [3,5,6]. Neglecting the crossing zones that have a much smaller volume fraction than the NWs, the shape anisotropy of the fcc Ni-rich CNW network leads to the alignment of the magnetization of the mono-domain NWs with the current flow along the NWs axis leading to the highest resistive state, as shown in Fig. 2a. The application of a magnetic field in the OOP or IP direction decreases the resistance due to the AMR effect. The magnetocrystalline anisotropy present in hcp Co-rich alloy systems prevents the system to reach this highest resistance state and is also highlighted by the decrease of the squareness of the normalized hysteresis curves shown in Fig. 2b. An analytical model inherent to the topology of 3D nanowire networks has been validated, allowing to extract the AMR values from the measured magnetoresistance curves along the OOP and IP directions [2-3]. The AMR ratio variation with respect to the Ni content shown in Fig. 2c for the NiCo CNWs has been found coherent with previous results obtained on bulk materials and films [3,7]. The large AMR ratio obtained around 75% atomic of Ni makes this system very attractive to explore the presence of domain walls at the crossing zones using AMR measurements. By increasing the NW diameter, the crossing zones have larger volume which give rise to complex domain wall effect at low field. Finally, interconnected Co/Cu and NiCo/Cu multilayered 3D NWs networks display large CPP-GMR responses measured with the current flowing perpendicularly to the plane of the layers in the individual nanowire segments and thus overall in the plane of the 3D network film. Other 3D nanofiber network architectures based on nanotubes and core-shell nanocables are currently investigated. A wide range of controllable parameters that can be used to tune the magnetic and magnetotransport properties of mechanically stable and self-supported3D interconnected nanofiber networks. Numerical simulations are in progress to provide a better understanding of the studied systems. These mechanically stable 3D nanofiber superstructures have potential applications in many fields.