Neodymium magnets having Nd2Fe14B structure is one of the most famous materials among the permanent magnets. This type of magnet has wide applications in wind turbines, electric engines, computer hard drives and mobile phones etc. Nd2Fe14B magnets have the highest value of BHmax as compared with other magnetic materials such as Sm-Co, Sm-Fe-N, Alnico, etc. Therefore, Nd2Fe14B magnets are used when weight and size of the magnet have to be reduced, for example, production of cell phones, earpieces, personal laptops and so on. Production of magnetic Nd2Fe14B powders from Nd, Fe and B oxides by reduction-diffusion process with Ca or CaH2 is very popular and efficient way because of its simplicity and low price for initial materials. Micron- or nano-particles obtained by this method are very active due to small size and large surface area. Thus Nd2Fe14B phase obtained after reduction-diffusion process could be hydrided with the formation of Nd2Fe14BHx (x=1-5) during washing step since Nd2Fe14B interact with H2 produced as a result of reaction between water and residual calcium. Br and Hc for Nd2Fe14BHx phase are much less than those for Nd2Fe14B, hence total BHmax decreases dramatically. One of the ways to avoid having Nd2Fe14BHx in final powders is vacuum annealing of washed powder that lead to decomposition of Nd2Fe14BHx into Nd2Fe14B and hydrogen gas. We proposed the modification of washing process in order to exclude hydride phase formation and influence of wet ball milling in ethanol media was studied in this work. Parameters of milling process such as type of ball milling, rotation speed and milling time were varied and dependence of phase composition, particle size and magnetic properties on milling regimes was investigated. Finally, the single phase Nd2Fe14B powders were obtained.
Vitalii Galkin received his B.S. at 2014 and M.S. at 2016 from Peter the Great St. Petersburg Polytechnic University, Russia. He spent 3.5 months for his internship at Eindhoven Philips High Tech Campus (Netherlands) during his master course. Nowadays he is doing PhD in Peter the Great St. Petersburg Polytechnic University, Russia and works in Korea Institute of Geoscience and Mineral Resources (Rep. of Korea) as an exchange student.
Magnetic tunnel junctions (MTJs) have aroused intensive studies for applications in non-volatile magnetic random access memories (MRAMs). Unlike conventional insulator (I) based MTJs, where a ferromagnetic (FM) metal provides a spin-polarized, the spin-filter (SF) barrier plays an active role to produce high spin polarization and large TMR effect in NM/SF/I/SF/NM MTJs , where NM is non-magnetic metal. The fact of STT >> FLST renders the remarkable progress in STT writing technology in the so-called STT-MRAM, where spin-transfer (STT) and the fieldlike (FLST) are two components of the spin torque effect. Recently, Tang et al. employed the single-band tight-binding model to predict a dual control of giant FLST in spin-filter (SF) based MTJ provides a new avenue to achieve both ‘reading’ and ‘writing’ processes of nonvolatile FLST-MRAM [2,3], which may require lower critical current densities for magnetization switching and faster writing and reading speeds than the conventional STT-MRAM. In this study, we focus on the interfacial electronic properties of Cu/EuO/Cu MTJs. The semiconductor EuO exhibits spin-polarized conduction band edges due to the exchange splitting , which is paramagnet at room temperature and becomes Heisenberg ferromagnet at low temperature. The first-principles calculation is first applied to obtain the electronic properties of EuO and Cu bulks, including the orbital-resolved band structure and spin-polarized density of states. We further employ the first-principles calculation with nonequilibrium Green’s function method is to investigate the spin-polarized transport properties of Cu/EuO/Cu MTJs. (MOST 1062112-M-008-011- and 106-2633-M-008-002-)
Chia-Chia Chao (趙家加) is a graduate student in the department of Physics at National Central University, Taiwan. Her research interst is in the first-principles calculation for structural, electronic, and spin transport properties of insulator- and spin-filter-based magnetic tunnel junctions
Organic-based magnetic junctions have attracted intensive attentions due to their potential applications on data storage and magnetic sensor. The diversity and flexibility of molecular synthesis and the weak spin-orbital coupling property allow the electron spin state to be preserved with larger spin diffusion length. Extensive researches have demonstrated that the tunnelling spins across the so-called spinterface between ferromagnetic and organic molecules provides rich physics to closely correlate interfacial chemical bonding with spin transport property . Tang et al. has proposed the superior spin injection in amine-ended single-molecule magnetic junction (SMMJ) under stretching process , resulting from the strong hard-hard spinterfacial coupling between Co nanowire and amine linker. Furthermore, we predict anomalous magnetoresistance (MR) effect , including strain-induced sign reversal and bias-induced enhancement of MR value in amine-ended SMMJ with Co tip-like nanowire, which is in sharp contrast to normal MR effect in conventional magnetic tunnel junctions. Recently, we choose various kinds of Co-N contact geometries to investigate the hard-hard spinterfacial orbital coupling issue. Interestingly, the pronounced spin-↑ transmission peak near fermi energy (EF) can be well maintained no matter N ion contacts on top, bridge, or hollow site of Co tip atoms as long as there is an unpaired electron of N ion binding with Co. Therefore, the dissociated H ion turns out to be signiﬁcant by controlling number of unpaired electrons N ion have. These intriguing results may open up a new arena to engineer the spinterface between FM metal and organic molecule for desired magnetotransport properties and MR effect via the variety of choices in anchoring groups and contact geometries. (Contract No. MOST 105-2112-M-008010- and MOST 106-2112-M-008-011- )
Kuan-Rong Ching (江冠融) is a graduate student in the department of Physics at National Central University, Taiwan. His research interst is in the first-principles calculation for spin transport property of magnetic junction and the electronic property of transition metal dichalcogenides
Recently, magnetic tunnel junctions have attracted a lot of attentions owing to the potential applications in magnetic random access memories (MRAMs), magnetic field sensors, nonvolatile and other spintronic devices. Switching of magnetization state is one of the most important operating principle in magnetic tunnel junctions. The mechanism of spin transfer from conduction electrons to localized magnetic moments is caused by an torque applied to the layer’s magnetization. This phenomenon is called current-induced magnetization switching, which is obtained by using a current-induced spin transfer torque (STT). The reduction of the critical current density is a very important issue to realized the applications of STT magnetic random access memories (STT-MRAMs). In this study, huge current-induced STT effect in a magnetic tunnel junction with a superlattice tunneling barrier constituted by periodic layers of a nonmagnetic metal and an MgO insulator is discussed. The forbidden and allowed bands in the superlattice barrier are appropriately designed through modulating the thickness of the nonmagnetic metal layer in the superlattice barrier. The results demonstrate that the improvement of STT by band structure in superlattice magnetic tunnel junction can achieve four (three) orders greater than that of traditional single (double)-barrier structure for our previous work. In addition, the number of cells in the superlattice barrier are proportional to the STT and the charge current density that directly affects the energy consumption of device. The concept of superlattice can provide a novel physics mechanism and advantageous application in spintronics and nanoelectronic memory.
Peng Tseng is studing for a Ph.D. degree in National Taiwan University. During the postgraduate studies, he mainly researched in spintronics and graphene nanoelectronics. He has published 3 referred journal papers.