Yujia Yang

τ-MnAl with high coercivity and saturation magnetization

In this paper, high purity τ-Mn54Al46 and Mn54−xAl46Cxalloys were successfully prepared using conventional arc-melting, melt-spinning, and heat treatment process. The magnetic and the structural properties were examined using x-ray diffraction (XRD), powder neutron diffraction and magnetic measurements. A room temperature saturation magnetization of 650.5 kAm-1, coercivity of 0.5 T, and a maximum energy product of (BH)max = 24.7 kJm-3 were achieved for the pure Mn54Al46 powders without carbon doping. The carbon substituted Mn54−xAl46Cx, however, reveals a lower Curie temperature but similar saturation magnetization as compared to the carbon-free sample. The electronic structure of MnAl shows that the Mn atom possesses a magnetic moment of 2.454 μB which results from strong hybridization between Mn-Al and Mn-Mn. We also investigated the volume and c/a ratio dependence of the magnetic moments of Mn and Al. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume ex...

Theoretical analysis and simulation of the influence of self-bunching effects and longitudinal space charge effects on the propagation of keV electron bunch produced by a novel S-band Micro-Pulse electron Gun

As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. The self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagat...

Study on the maximum stable output of a novel s-band micro-pulse electron gun

A novel S-Band Micro-Pulse electron Gun (MPG) which works at the left crossover point energy EcI on the Secondary Electron Yield (SEY) curve was proposed. The working principles of the MPG were presented. The maximum output beam current limited by space charge effects and beam loading effects was investigated by theoretical analysis. The result shows that the maximum beam current is decided by the parameters of the MPG such as resonant frequency, cavity length, shunt impedance and the secondary emission property of the cathode and grid, having nothing to do with the power flowed into the cavity. The low shunt impedance and the low slope of the SEY curve can help increasing the maximum beam current. According to the principles, a MPG with the frequency of 2.856 GHz has been designed and constructed. The steady working state was achieved by using oxygen free copper and molybdenum grid with different transmission coefficient. It was found a good agreement between the analysis and the experiments.A novel S-Band Micro-Pulse electron Gun (MPG) which works at the left crossover point energy EcI on the Secondary Electron Yield (SEY) curve was proposed. The working principles of the MPG were presented. The maximum output beam current limited by space charge effects and beam loading effects was investigated by theoretical analysis. The result shows that the maximum beam current is decided by the parameters of the MPG such as resonant frequency, cavity length, shunt impedance and the secondary emission property of the cathode and grid, having nothing to do with the power flowed into the cavity. The low shunt impedance and the low slope of the SEY curve can help increasing the maximum beam current. According to the principles, a MPG with the frequency of 2.856 GHz has been designed and constructed. The steady working state was achieved by using oxygen free copper and molybdenum grid with different transmission coefficient. It was found a good agreement between the analysis and the experiments.

Studies of Mn-based permanent magnetic materials MnX(X=Al, Ga)

Since Mn atoms in the system of Mn-based materials can have large magnetic anisotropy and high magnetic moments (almost twice as high as Fe atoms in pure Fe metal), ternary or quaternary Mn-based compounds show potential as hard magnetic materials with high performance and medium price[1-3]. However, the knowledge of the magnetic moment formation and the mechanism of the magnetic coupling between Mn moments in Mn-based materials are very limited, preventing the exploitation of the large reservoir of ternary and quaternary Mn-based compounds in the search for novel magnetic materials. In the current study, the magnetic and structural properties of Mn-based intermetallic compounds MnX(X=Al, Ga) have been investigated, for the goal of searching novel Mn-based materials suited for permanent magnets. We successfully produced single-phase materials of MnAl and MnGa using different methods. MnAl and Mn-Ga show promising hard magnetic properties, and exhibit various magnetic behaviors including spin reorientation and exchange bias. The origin and modification of the magnetocrystalline anisotropy and ferromagnetism in these compounds have been explored.

Study of metamagnetism in Sm(Ni 0.5 Fe 0.4 Cu 0.1 ) 7

Metamagnetism, a sudden increase in the magnetization of a material with a small change of external magnetic field, are calling more attentions because of their rich magnetic phenomena and scientific significance . There are quite different physical causes for different types of metamagnets . In this work, metamagnetism has been found in arc-melting Sm(Ni0.5Fe0.4Cu0.1)7 . The physical property measurement system (PPMS) is used to measure the magnetic properties . The hysteresis loops at different temperatures (T>5K) are shown in Fig .1 . Both sides of the hysteresis loops exhibit obvious metamagnetic behaviors . The hysteresis loops show wasp-waisted character . At the beginning of the curves the magnetization increases rapidly and then gets saturation at about 1T . When field increases continuously to the critical magnetic fields (Hcm) metamagnetic behavior appears . The lower the temperature is, the higher the Hcm is . The magnetization appears to saturate when field increases to a higher value . The magnetization stays at high magnetization state until the field decreases to about 1T . The magnetizatic behaviors at another side of the hysteresis loops are the same . The critical magnetic fields (Hcm) corresponding to the metamagnetic points increases with the temperature decreasing by an exponential dependence. The spin reverse model with thermal activation (TA) is used to explain the relation of the critical field to temperature . The expression can be written as Hcm(T)=Hcm(0)exp(-kT/U), where Hcm(T) is the critical field at T, Hcm(0) is the calculated value at 0K, k is the Boltzmann constant, U is the energy needed to turnover one spin. The fitting results show U≈6 .6×10-15erg . At temperatures, below 5K, the smooth jumps turn into step-like jumps. The number of the steps and the values of critical fields vary with different samples . The inset in Fig .2 is the enlargement of one step with a field interval of 0.05T . When repeating the magnetization process, the hysteresis loops can coincide compactly, which is different from the usual Barkhausen jumps. The XRD results show that the main phase is hexagonal P6/mmm structure and the easy magnetization direction at room temperature is along c axis, which may show high anisotropy constants, where macroscopic quantum tunneling (MQT) may happen. As temperature gets extremely low, the thermal activation can be ignored and the quantum behavior becomes obvious . One possible explanation is as follows: the MQT happens first, which leads to a release of thermal energy and increase of sample temperature, followed by a huge magnetization reversal due to the external magnetic field . A step-like magnetic jump appears . The differences of step numbers and Hcm values between different polycrystalline samples are thought to be related to the relative orientation of the crystalline grain and the field direction . It seems that the MQT and TA models have solved the problem well . Another possible explanation is the narrow domain-wall pinning, but such a mechanism would have difficulty accounting for the presence of multistep jumps and the transition from one smooth jump to several sharp jumps just by changing few kelvins .

Research and development of rare earth-iron interstitial compounds

In 1990, the discovery of interstitial nitrogen atom effect on rare earth-iron intermetallics attracted much interest in the field of magnetism and magnetic materials. The nitrogenation effect is typically demonstrated in 2:17 and 1:12 rare earth - iron compounds. After nitrogenation, the intrinsic magnetic properties of the 2:17 and 1:12 nitrides are marvelously improved. So their intrinsic magnetic properties are comparable to those of Nd2Fe14B, even better in terms of Cuire temperature and corrosion resistance[1-2]. At the same time, neutron diffraction studies indicated that after nitrogenation, the 2:17 and 1:12 nitrides remain their original crystal structure. The nitrogen atoms occupy definite interstitial sites. On the basis of neutron diffraction data, theoretical calculations showed that the nitrogen atoms located at the interstitial sites effectively modify the crystal field interaction of 4f electrons of rare earth atoms, as well as the 3d electronic structures of iron atoms. All the modifications by interstitial nitrogen atoms result in a fundamental change in the magneto-crystalline anisotropy of rare earth atoms, and an increase in magnetic moment of iron atoms as well as a significant enhancement for Fe-Fe exchange interactions. As a result, in the rare earth materials family, a new series of rare earth-iron intermetallics with outstanding intrinsic magnetic properties were found based on the interstitial nitrogen atom effect, which was named as interstitial compounds[3-4]. Since then a quarter of a century has passed, tireless effort has been made in the world wide to bring the interstitial compounds into commercial use. Up to date, a variety of materials has been developed not only for the applications in the field of permanent magnets but also for the microwave absorbers based on the interstitial compounds.

The shell thickness dependence of the exchange bias of Fe/Fe 3 O 4 core-shell nanoparticles

Exchange bias (EB) effect has been intensively studied for more than half century due to its applications in spintronics devices, such as giant magneto resistance (GMR) read heads and magnetic random access memory (MRAM).[1] Except for conventional ferromagnetic (FM)-antiferromag-netic (AFM) system, EB has also been reported in other magnetic systems, such as ferromagnetic (FM)-ferromagnetic (FIM) system, spin glasses (SG) and nanoparticles with surface spin disorder. The most studied FM-FIM system is the Fe/Fe3O4 core shell structure. It is believed that the inter-facial SG state plays a major role in the EB of this system.[2,3] However, all the studies have aimed to the stably oxidized Fe nanoparticles. The effect of the FIM thickness on the EB in this system has been neglected. In this study, a controlled oxidizing procedure was adopted to obtain Fe/Fe3O4 nanoparticles with different Fe3O4 thickness. The Fe3O4 thickness dependence of the EB was studied.