Influence of glycine addition on formation and magnetic properties of MnBi prepared by vacuum sintering technique

Abstract

An alternative way to increase a low-temperature phase MnBi (LTP-MnBi) content synthesized by low-temperature vacuum sintering is by using Mn powder with small particle sizes. To reduce the particle size by ball milling, glycine was added to prevent particle agglomeration and possible oxide formation. The ball-milled Mn and Bi powders were fixed at an atomic ratio of 1:1. The ball-milling time in glycine was between 1 and 3 hrs. The average particle size reduces from 400 µm (original) to 35-40 µm and 15-20 µm after grinding for 1 and 3 hr, respectively. Sintering of the mixtures was carried out at 275 °C for 12 hours at a vacuum pressure of less than 5×10−8 mbar. The LTP-MnBi samples sintered from 1 hr (MnG1Bi) and 3 hr (MnG3Bi) glycine-added Mn grinding times were investigated by XRD, SEM, XPS, and VSM. The coercivity values (H c) are 3.95 ± 0.04 and 2.35 ± 0.03 kOe for MnG1Bi and MnG3Bi, respectively. The saturation magnetization (M s) of MnG1Bi and MnG3Bi is relatively low (~0.18 emu/g) compared to the samples prepared by conventional ball-milling, i.e., without adding glycine in grinding. The XRD results show a few percentages of MnBi content in all samples, suggesting that glycine addition could prevent MnBi formation. The coverage of hydrocarbon (i.e., NH2-CH2-COOH) groups on Mn particles could prevent the diffusion mechanism during liquid-phase sintering. Moreover, it was found that the oxygen and carbon content in the MnGxBi were much higher than the conventional ball-milled MnBi.