Template-Free Self-Assembly of Multi-Component Magnetic Nanostructures with Tunable Composition, Size and Magnetic Properties

The ability to tune nanostructures’ composition will open up new approaches to produce hard-soft bimagnetic nanomaterials, which provide promising strategies for developing new sustainable permanent magnets for green technologies on a macroscopic scale¹. Here we present a gas-phase template-free technique for generating multi-component magnetic nanostructures with tunable properties by directed self-assembly of nanoparticles (NPs). We demonstrate that, depending on the carrier gas used during synthesis, the structure and composition, i.e., metallic and oxide phases, of Co-Ni bimetallic NPs can be tuned along with their magnetic properties. The resulting NPs are self-assembled into 1D nanochains (NCs), which are characterized using XRD, electron microscopy, SQUID magnetometry, and micromagnetic simulations.<br/><br/>A custom-built spark discharge generator (SDG) is used to study the effect of the carrier gas on the structure and composition of Co-Ni NPs produced with Co-Ni alloyed electrodes (Co:Ni = 66.5:33.5 at.%). Three different carrier gases are used: 95% N₂ + 5% H₂ (N₂/H₂), N₂, and air. The structure and composition of the particles is analyzed with powder XRD and TEM. Different crystallinity is observed depending on the carrier gas used. The TEM data confirmed these results. Homogeneous single-phase CoNi (2.05 Å, corresponding to CoNi {111}) and CoO (2.45 Å, corresponding to CoO {111}) are detected for particles produced with N₂/H₂ and air, respectively. Two-phase particles, composed of CoNi and CoO, are found to be generated with N₂. The oxidative/reductive environment, i.e., the oxygen content, within the SDG explain the different particle compositions obtained². Considering that Co and Ni are ferromagnetic and their oxides are antiferromagnetic, these NPs are expected to exhibit different magnetic properties. Metallic particles produced with N₂/H₂ show larger coercivity than mixed metallic/oxide particles generated with N₂, for which exchange-bias (EB) is observed. The EB phenomenon is extensively studied in this work. As expected, the sample generated with air do not display any magnetic response as the particles were fully oxidized.<br/><br/>The NPs are then self-assembled by means of an external magnetic field during deposition, which attracts them to the already deposited NPs via dipole-dipole interactions, forming NCs along the external magnetic field direction³. Two particle sizes are employed for this study, 25 and 50 nm, by simple size-selection in the gas-phase using differential mobility analyzers before deposition. When N₂/H₂is used, the NCs present a strong magnetization direction along the chain in agreement with the SQUID measurements. Interestingly, NCs formed by 25 nm particles show a two-fold increase in the remanence magnetization and coercivity compared to NCs formed by 50 nm particles. When N₂ is employed, the remanent magnetization and coercivity are decrease, but in turn there is an enormous increase in the EB, especially for NCs formed by 25 nm particles. Additionally, we show that the EB can be increased even further by increasing the length of the NCs, i.e., depositing NPs for longer times, making this system appealing for magnetoresistive read heads and sensors. These results present a promising method for tuning magnetic NPs by simply changing the carrier gas to generate nanostructured materials with designed sizes, compositions, and morphologies tailored for specific applications.<br/><br/>1. Skomski, R., and Coey, J., <i>Physical Review B </i>(1993) <b>48</b> (21), 15812<br/>2. Ternero, P.<i>, et al.</i>, <i>Journal of Aerosol Science </i>(2023) <b>170</b>, 106146<br/>3. Preger, C.<i>, et al.</i>, <i>Nanotechnology </i>(2021) <b>32</b> (19), 195603