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

Abstract

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 μm, 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.

Interpreting first-order reversal curves beyond the Preisach model: An experimental permalloy microarray investigation

“…FORC has been successfully utilized to study an extensive amount of different magnetic systems. Whether for magnetic particles in geologic compounds [4][5][6][7], permanent magnet research [8][9][10], magnetic temperature dependence [11][12][13], the study of nanowires [14][15][16][17][18], or nanostructured dot and antidot systems [19][20][21], first-order reversal curves have a broad range of applications. It has been shown that FORC can also be extended to even more sophisticated methods like second-order reversal curve (SORC) [22], or temperature FORC [23].…”

Consolidation of cobalt nanorods: A new route for rare-earth free nanostructured permanent magnets

“…Among the different candidates, Mn-and Fe-based alloys [7,8,9] are meeting the abundance criterion, while Co-based alloys [10,11,12] benefit from the high intrinsic anisotropy of cobalt.…”

MnBi Magnetic Material: A Critical Review

“…This limit equals the ideal nucleation field H N associated with two intrinsic parameters of material K u and M s , H N = 2K u /M s [12]. For MnBi, by using the approximation K u ≈ K 1 + 2K 2 [8], M s = 81.3 emu/g and ρ = 9.042 g/cm 3 , this limit, at room temperature is estimated approximately H N = 39 kOe Depending on the methods of estimation of K u , H N suffers a great fluctuation, up to 56 kOe as announcedin [13]. Particularly, this upper limit of coercivity should be for a dense MnBi bulk magnet owning an ideal microstructure with non-defected, non-interacted, single-domain-sized, and perfectly aligned grains assembly.…”

“…4. The scheme of variation of coercivity H c versus reduction of grain size D for ferromagnetic solids [13].…”