We have used the AC susceptibility for MgAlB 2 samples to investigate the grain boundary effects and the Aldoping effects on the critical current density. The Cole-Cole plots were used to obtain the grain boundary effects on the critical current density. We found that the grain boundaries are highly transparent to current flow and have virtually no effects on the critical current density in MgB 2 pure and Al-doped samples. Moreover, the lower critical field H c1 is found to be enhanced with Al doping. The highest H c1 value is achieved with the 4.5 % concentration of Al.Keywords MgB 2 · Al doping · Grain boundaries
ExperimentalSeveral polycrystalline samples of Mg 1−x Al x B 2 have been synthesized by direct solid-state reaction of the elemental magnesium, aluminum, and boron (Eagle Picher 99 % purity). The starting materials powders (325 mesh) were mixed in the stoichiometric ratio and pressed into a pellet. The pellet was enclosed in a tantalum crucible and sealed by arc welding under argon atmosphere into a glove box. The arc welding was localized on the top of the crucible, and the sample was not exposed to high temperature during the welding process. The Ta crucibles were enclosed into a quartz tube then heated for 1 h at 800 • C and 2 h at 950 • C, and then furnace cooled to room temperature.Many high quality Mg 1−x Al x B 2 samples have been prepared using the method described above. In this study, we will focus on six samples in which x= 0, 0.025, 0.045, 0.065, 0.14, and 0.27.The X-ray diffraction patterns of all six samples are shown in Fig. 1. The peak 101 shows a broadening as the Al concentration increases. This is an indication of the reduction in the grain sizes. Another observation concerning the same peak 101 is its shift toward higher angle 2θ. This is also an indication of the reduction in the c-axis as predicted by Bragg's law. Figure 1 shows the typical peaks of MgB 2 , in addition to small quantities of MgO that increases gradually as the Al concentration increases. There are no peaks corresponding to Mg or B.
Results and DiscussionAC magnetic measurement, in which an AC field is applied to a sample and the resulting AC moment is measured, is an important tool for characterizing many materials. Because the induced sample moment is time-dependent, AC measurements yield information about magnetization dynamics which are not obtained in DC measurements, where the sample moment is constant during the measurement time.In AC magnetic measurements, a small AC drive magnetic field is superimposed on the DC field, causing a time-dependent moment in the sample. The field of the time-dependent moment induces a current in the pickup coils, allowing measurement without sample motion. The detection circuitry is configured to detect only in a narrow frequency band, normally at the fundamental frequency (that of the AC drive field).In order to understand what is measured in AC magnetometry, first consider very low frequencies, where the measurement is most similar to DC magnetometry. In this case, the magnet...