Coercivity mechanism of rare-earth free MnBi hard magnetic alloys

Abstract

In this work, we present and discuss results concerning the hard magnetic behavior of rare earth-free MnBi alloys obtained by suction casting technique. The physics of coercivity for these type of alloys is based on the nucleation process of reverse domains, which in turn is determined by the alloy microstructure features such as phase distribution, morphology, grain size and in particular, defects, which are characteristic of real materials. The microstructure of the as-cast alloy presented here comprises the formation of the Low Temperature Intermetallic Phase (LTIP)-MnBi, interspersed within Bi-and Mn-rich areas. A considerable intrinsic coercivity field of 238 kA/m together with a saturation magnetization of 0.04 T were observed. The nucleation controlled mechanism of this alloy was described in terms of the Kronmüller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces and local demagnetizing fields. A notorious switching of coercivity mechanism associated with domain wall pinning was found to be produced upon annealing of the alloy at 583 K for 24 hrs, yielding a drastic reduction of coercivity (down to 16 kA/m). The key microstructural feature determining the switching of coercivity mechanism is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of LTIP.

Structural and Fluorescence Studies of Polycrystalline α‐Al2O3 Obtained From Sulfuric Acid Anodic Alumina

“…[26] The mass loss of 9.5% at around 967-972 C corresponds to the release of SO 2 and O 2 . [27][28][29][30][31][32] And then the gradual mass loss, corresponding to the SO 2 release, [29] of about 4.3% total is observed in the temperature range of 980-1250 C (Figure 4). DSC curve shows a very sharp exothermic peak at around 967 C and another broader peak onset at around 1194 C (see Figure 4).…”

“…The as-anodized and 800 C-treated anodic alumina specimens are essentially Xray amorphous ( Figure 5 obtained and it agrees with the formation of equilibriumshaped crystals. [27][28][29][30][31][32][33][34]…”

Highly Ordered Anodic Porous Alumina Formation by Self‐Organized Anodizing

“…The thermal characteristics of highly ordered porous alumina structures formed on aluminum have been studied widely [299][300][301][302][303][304][305][306], with thermal analyses being performed in order to collect data on chemical resistance [299,300], structural and optical properties [292,[300][301][302][303], thermal conductivity and diffusivity along the channel axis [304], self-repair rearrangement of ordered nanopore arrays induced by long-term heat treatment [305], and mechanical properties [306]. The fracture mechanism, Youngs modulus, hardness, fracture toughness [306] and fracture behavior in cylindrical ordered porous alumina [307] were also investigated.…”

“…The separation of aluminum oxide from the remaining aluminum substrate might also be performed by using electrochemical etching in 20% HCl, with an operating potential between 1 and 5 V [302]. However, the most widespread method is based on a wet chemical removal of aluminum.…”