Engineered Self-assembled Magnetic Nanochains INVITED

Abstract

Bottom-up fabrication of magnetic nanostructures composed of particles with desired sizes and composition can result in significant magnetic properties desired for practical applications in high performing magnetics, spintronics, and magnetic sensors [1, 2]. Magnetic and magneto-transport properties are strongly size-dependent, and the ability to adjust nanoparticle size leads to a high control on nanostructures' behavior [3, 4]. Generation of nanoparticles by aerosol methods provides great control of particle formation, size and deposition [5, 6].Here, we present a novel bottom-up approach for generating self-assembled magnetic nanostructures [7]. The method uses an aerosol technique based on spark ablation to produce nanoparticles (NPs) with tunable compositions guided onto any substrate of choice using combined electric and magnetic fields. Using external magnetic fields, magnetic NPs can be assembled one-by-one along any direction to form 1, 2, or 3-D structures, see Fig. 1. In these systems, both shape and magneto-crystalline contributions play a key role in the magnetic anisotropy resulting in enhanced magnetic properties such as higher coercivity [8].The presented technique provides a new approach for self-assembling of 1, 2, and 3-D magnetic nanostructures with building blocks that can be tailored for a specific application. Fig. 1 (a-c) shows the magnetic-field-induced self-assembly of vertical and in-plane structures. The magnetic field used to control the self-assembly also aligns the easy axis of the NPs, resulting in a higher anisotropy compared to a chain with randomly oriented particles. Fig. 1 (d) shows the possibility of particle size modulation, which is similar to diameter modulation in magnetic nanowires [9]. As illustrated in Fig. 1 (e), these structures can be directly integrated on the suitable substrates for, e.g., transport measurements or practical applications, which suggests a great advantage over the common solution-based methods [10]. Furthermore, the particle generation technique can be used to generate a vast number of material systems that can be used as building blocks. We have produced nanochains (NCs) composed of Ni, Fe, Co [7, 11], and as shown in Fig. 1 (f, g), CoPd (alloy) and CoAu (segmented/1D granular) structures. By tuning the segmented structures or chemical composition of alloys, a variety of systems can be achieved with possible applications in magnetism and spintronics [5].Fig. 2 shows the NCs transferred to a carbon-supported copper grid where they were imaged using SEM (a & d) and by Scanning Transmission X-ray Microscopy (STXM) using circular polarized X-rays. The measurements were performed at the PolLux beamline at the Swiss light source (SLS) [12]. X-ray magnetic circular dichroism (XMCD) provides magnetic contrast (b-c & e-f) with a resolution approaching that of a single particle when combined with SEM images from the same NC. Applying a strong ex-situ magnetic field produces a mostly homogeneous remanent magnetization along the NCs with only minor indications of domain formation. Applying an in-situ field of -140 mT resulted in that the XMCD contrast of several more regions along the NC turned bright. These results demonstrate that an ex-situ field can produce a strong remanent magnetization and that the weak in-situ fields can be used to switch the magnetization in some regions. It is argued that these observations are due to larger multi-domain particles that behave as nucleation centers to initiate domain formation. As shown in Fig. 2 (g), micromagnetic simulations on NCs show that the magnetization is energetically favorable along the long axis of the chains and in the same direction (single domain). However, when there is a particle with a diameter larger than the single domain size, the NC breaks up into two domains.Magneto-transport in these structures is of interest since the magnetized and demagnetized states of the chains can result in a magnetoresistance response, and also, the segmented chains composed of different materials (Such as Co/Au in Fig. 1) can act as 1-D magnetic multilayers. To investigate the magneto-transport in these systems, NCs are self-assembled directly onto the surface of a Si-chip insulated with SiO2 and pre-patterned with aligned markers. Contacts to the NCs are defined via electron beam lithography and thermal evaporation, and magneto-transport properties along with micromagnetic simulations will be discussed. **