Orientation of Magnetized MnBi in a Strong Static Magnetic Field
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e MnBi (low-temperature phase (LTP)) undergoes a first-order phase transition from the ferromagnetic state to the paramagnetic state at a peritectic temperature of 630 K as temperature increases. This transition is accompanied by a structural transformation from a NiAs-type hexagonal structure to a distorted Ni2In-type cubic structure (MnBi1.08, high-temperature phase (HTP)). The growth direction of the LTP is the same as the easy c-axis of its crystal structure.[1–3] Thus, MnBi compounds feature unusual magnetic[4–6] and magneto-optical[7] properties and have been investigated extensively. MnBi has a strong uniaxial anisotropy along the c-axis at room temperature.[8,9] Owing to many interesting effects (such as orientation, refinement,[10,11] the convection suppression effect,[12,13] and the thermal electromagnetic effect[14]), strong static magnetic fields (SSMFs) have been widely applied during the solidification and processing of alloys with high properties. Recently, SSMFs have been used to
TIANXIANG ZHENG, YUNBO ZHONG, LICHENG DONG, BANGFEI ZHOU, and ZHONGMING REN are with the State Key Laboratory of Advanced Special Steels, Shanghai University, Shanghai 200072, China. Contact email: yunboz@staff.shu.edu.cn FRANCOIS DEBRAY and ERIC BEAUGNON are with the LNCMI, CNRS/UJF/INSA/UPS, 38042 Grenoble, France. Manuscript submitted July 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
obtain highly oriented LTP microstructure with magnetic anisotropic properties.[9,15,16] The distribution of LTPs could be adjusted by controlling the solidification process strategy and the magnetic parameters, such as magnetic flux density and its gradient BdB/dz.[15] The application of a radial gradient magnetic field produces a ringlike LTP-rich layer at a cooling rate (R) of 0.1 K/ min and highly oriented LTPs at a cooling rate of 0.18 K/min under 10 T.[16] In this study, Bi-4.5 wt pct Mn alloys were bulk solidified with and without an SSMF at two different cooling rates. This alloy was chosen for its low melting point (approximately 640 K) and wide temperature gap at the semisolid state (about 100 K). Another important reason is that the LTP is the main primary phase during the solidification process. Thus, an SSMF can easily affect the precipitation process of the LTP. On the basis of previous studies,[2,17] the size of the LTP reaches approximately 50 to 200 lm when R is 10 K/min. Thus, to study the effects of an SSMF on the morphology of LTPs with different grain sizes, two different cooling rates (namely, 5 and 60 K/ min) were applied during the bulk solidification process of Bi-4.5 wt pct Mn alloys. The purpose of this article is to report the effects of magnetic forces on the distribution and morphology of LTPs in an SSMF. A new multilayered microstructure of LTPs is reported and the physical mechanism is discussed. Samples used in this experiment were prepared from high-purity substances (4 N) in a quartz tube and melted in a high-frequency induction furnace under the protection of pure argon (5 N) gas. A solidified ingot, 10 mm in length a
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