Magnetism and ε-τ Phase Transformation in MnAl-Based Nanocomposite Magnets

Abstract

Melt spun ribbons of Mn53Al45C2 and Mn52Al46C2 have been synthesized by rapid quenching of the melt with the purpose of monitoring the ε-τ phase transformation to show technologically feasible ways to increase magnetic parameters and to illustrate the viability of these alloys as the next generation of rare earth (RE)-free magnets. By differential scanning calorimetry (DSC), activation energies and temperatures of onset of the ε-τ phase transformation were obtained. Structural analysis was performed using X-ray diffraction (XRD) and the resulting XRD patterns were quantitatively assessed using full profile Rietveld-type analysis. Appropriate annealing was performed in order to enable the ε-τ phase transformation. While hcp ε-phase was found to be predominant in the as-cast samples, after appropriate annealing, the tetragonal τ-phase, the one that furnishes the relevant magnetic response, was found to be predominant with an abundance of about 90%. The data suggested a mechanism of hcp ε-phase decomposition controlled by the segregation towards the interfacial regions, having the rate of transformation governed by antiphase boundary diffusion processes. Magnetic measurements of annealed sample Mn53Al45C2, consisting of predominant tetragonal τ-phase, showed high values of magnetization and increased coercivity, consistent with an energy product of about 10 MGOe, similar with previously reported magnetization measurements, providing further insight into the realization of future class of RE-free low-cost permanent magnets.

Thermal Stability, Blocking Regime and Superparamagnetic Behavior in Mn-Al-C Melt Spun Ribbons

“…In our previous paper [48], we have demonstrated that partial replacement of Mn with small carbon addition was justified on the one hand by the need of obtaining a microstructure where carbon atoms act as nucleation centres for the formation of ordered L1 0 type phase in MnAl, and on the other hand to facilitate the occurrence of a magnetic interaction of the super-exchange type, where the exchange interaction between adjoining MnAl ordered grains/regions is intermediated and facilitated by carbon atoms, mostly situated in the interphase boundaries between neighboring ferromagnetic L1 0 regions/grains.…”

“…Structural studies by means of X-ray diffraction have shown [48] that for the two ribbons in the as-cast state, the ε hexagonal phase predominates. This result confirms the hints we have received from the scanning electron microscopy images, i.e., that the two samples are mostly single phased.…”

Ferromagnetic Mn–Al–C L10 Formation by Electric Current Assisted Annealing

“…This peak is related to the phase transition, in agreement with the values reported in literature for melt‐spun ribbons. [ 13,28 ]…”

“…τ phase transition, in agreement with the values reported in literature for melt-spun ribbons. [13,28] Comparatively to the DSC results, the relative electrical resistance variation (R=R 0 ) in function of the sample temperature using the in-house-built device is also shown in Figure 1. The R 0 value is defined as the initial electrical resistance while R is the resistance at the given sample temperature.…”

A Self-Assembly of Single Layer of Co Nanorods to Reveal the Magnetostatic Interaction Mechanism

“…Nanomagnets, with strong magnetic properties and small volume, have been considered to be the key materials to magnetic and electronic devices that exhibit important applications in artificial intelligence, intelligent robots, wind turbines, and electromobiles [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. Compared with rare-earth-based nanomagnets, the rare-earth-free nanomagnets earn more attention for their high chemical stability and low cost [ 13 , 14 , 15 , 16 ]. However, these rare-earth-free nanomagnets present small coercivity due to their relatively low magnetocrystalline anisotropy.…”