Temperature-dependent first-order reversal curve measurements on unusually hard magnetic low-temperature phase of MnBi

We have performed first-order reversal curve (FORC) measurements to investigate the irreversible magnetization processes in the low-temperature phase of MnBi. Using temperature-dependent FORC analysis, we are able to provide a clear insight into the effects of microstructural parameters such as grain diameter, shape, and surface composition on the coercivity of nucleation hardened permanent magnet MnBi. FORC diagrams of MnBi show a unique broadening and narrowing of the coercive field distribution with increasing temperature. We were able to microscopically identify the reason for this behavior, based on the shift in the single domain critical diameter from nearly 1 to 2 $\mu \text{m}$, thereby changing the dependence of coercivity with particle size. This is based on a strong increase in the uniaxial anisotropy constant with increasing temperature. Furthermore, the results also give an additional confirmation that the magnetic hardening in low-temperature phase MnBi occurs due to nucleation mechanisms. In our case, we show that temperature-dependent FORC measurements provide a powerful tool for the microscopic understanding of high-performance permanent magnet systems.

Figure 1

SEM images of the samples (a) HC 1, (b) HC 2, (c) SPS 2, and (d) SPS 1. The alignment direction (easy axis) is indicated on the top right corner of all images. The inset shows the particle size distribution of each of the samples. Hot compacted samples in particular have elongated particles in the direction of alignment (prolate ellipsoid) [25]. Spark-plasma sintered samples, on the other hand, have irregularly shaped particles.

Figure 2

Variation of lattice parameters for hexagonal LTP-MnBi with temperature for the four samples. The inset shows the unit-cell volume with temperature.

Figure 3

(a) Coercive field vs temperature for different samples. (b) Hysteresis loops for all the samples at 300 K. See the text for a detailed description.

Figure 4

FORC diagrams of the LTP-MnBi hard magnet with uniaxial anisotropy. (a) HC 1, hot compacted at 473 K. Mean diameter of the particles is 2.97 $\mu \text{m}$ (b) SPS 1, spark plasma sintered at 533 K. Mean diameter is 3.10 $\mu \text{m}$ (c) HC 2, hot compacted at 573 K. Mean diameter is 3.81 $\mu \text{m}$ (d) SPS 2, spark plasma sintered at 533 K with an atomic ratio of Mn:Bi as 45:55. Mean diameter of the particles is 2.09 $\mu \text{m}$. The temperature is given at the top right corner of each of the FORC diagrams.

Figure 5

Schematic representation of a coercive field dependent on particle diameter adopted from Ref. [40].

Figure 6

Schematic representation of the variation of the nucleation field dependence on the particle diameter with increasing temperature for LTP-MnBi.