Customizing Magnetic and Structural Properties of Nanoma-terials

Magnetic nanoparticles have attracted a lot of interest due to their importance and potential application in a variety of fields. The complex magnetic behavior exhibited by nanoparticles is governed by many factors, including their size, composition, shape, morphology, and shell-core structure. The increased surface to volume ratio and tailored structure in nanoparticles introduces many size dependent phenomena which may be used to change their chemical and physical properties. Nanostructured magnetic materials are suitable candidates for magnetic refrigeration due to a presence of a large magnetocaloric effect (MCE) in the superparamagnetic system and reduced hysteresis losses. Magnetocaloric nanoparticles have a broader entropy change over a wider temperature span which results in a higher relative cooling power (RCP). The RCP measures how much heat can be transferred between the cold and hot heat exchangers in an ideal refrigeration cycle. Increasing the RCP not only increases the amount of refrigeration obtainable from the particular refrigerant and field excursion, but also tends to increase the thermodynamic efficiency of the cycle. Improvement in RCP mainly relies on broadening the magnetic entropy change by either coupling two phases of magnetic materials with desirable properties or nanostructure synthesis with the main motivation rooted in their inherent tendency to have distributed exchange coupling, which will broaden the magnetic entropy curve. As a result, to have a comprehensive understanding of various factors that can affect the magnetic and structural properties of materials, in this research, different-sized yttrium-iron nanoparticles were synthesized through alkalide reduction chemical synthesis. Powder X-ray diffraction measurements at room temperature were carried out to study the crystal structures. Surface morphology and size of the synthesized powder alloys were characterized by scanning and transmission electron microscopy. The composition of the powders was determined from energy dispersive X-ray fluorescence. Magnetization measurements were performed using vector vibrating sample magnetometer (VVSM) with standard zero field cooling (ZFC), field cool cooling (FCC), and field cool warming (FCW) techniques. The XRD measurements of the annealed nanoparticle samples fit the yttrium-iron hexagonal closed pack structure and a phase prototype similar to Th 2 Ni 17 , with space group P6 3 /mmc(194). The average particle size of four samples were observed to be 22, 38, 62 and 76 nm, in good agreement with crystallite size estimated from XRD data using Scherrer relation. The magnetic and structural properties of these nanoparticles were measured at different fields and temperatures to study the effect of field, temperature, and particle size variations on the critical parameters, such as Curie temperature (T C ), blocking temperature (T B ), phase transitions, saturation magnetization (M s ), remnant magnetization (M r ), and coercivity (H c ). The temperature dependent magnetization, M(T), under different applied magnetic field up to 20 KOe indicated a change in samples’ T B , which is defined as the temperature corresponding to the maximum point on the ZFC curve after its divergence from the FCC curve. As shown in Fig. 1a, for the 38 nm sample, T B increases by increasing the applied field up to 2 KOe and then decreases with further increase in the applied field. The non-monotonic field dependence of the peak temperature in nanoparticles is attributed to the anisotropic energy barrier distribution of the particles, and to the slowly decreasing magnetization above T B . Moreover, T B increased as the size of nanoparticles increased (Fig. 1b). Results display that T C of nanoparticles depends on both size and shape conditions as it decreased by reducing the particles size. The shape effect is more prominent at sizes less than 40 nm. Additionally, the phase transition became more dominant in low applied filed whereas it is smoothened by increasing the field due to particles’ magnetic saturation. Moreover, magnetic couplings between particles appear stronger for larger particles. The saturation magnetization and coercivity of these nanoparticles were size and field dependent and they both decreased by reducing nanoparticles’ size. Furthermore, as shown in Fig. 2, $H_{C}$ and $M_{S}$ are temperature dependent and they both decrease by increasing temperature since more thermal energy is supplied and individual electron spins become more likely to be in higher energy states, pointing randomly, opposite to their neighbors and less aligned, leading to a reduction in the total magnetization. Therefore, a smaller field is required to reduce remnant magnetization to zero, leading to coercivity reduction. In summary, the comprehensive results of this research are promising and novel as they suggest that the physical and chemical properties of magnetic materials can be customized to suit them in a number of applications in variety of fields such as material science, medicine, engineering, nano-electronics, information technology, etc.