Energy barrier analysis on hot-deformed Nd-Fe-B magnets

One approach to solve the so-called coercivity problem on Nd-Fe-B permanent magnet is to investigate the magnetization reversal process. The magnetization reversal is a transition event between two energy local minima separated by an energy barrier. The energy barrier E_b is a function of magnetic field H and is generally given as the form of E_b = E₀(1 - H/H₀)ⁿ, where E₀ is the barrier height at zero magnetic field, H₀ the intrinsic coercivity without thermal fluctuation, n the constant. These parameters strongly depend on the magnetization reversal process, but the experimental determination of n is somewhat difficult. Recently, we have proposed the novel experimental method to determine the value of n based on the magnetic viscosity measurements [1]. The parameters of E₀, H₀ and n also depend on the field direction with respect to the magnetic easy axis. Thus using a highly oriented sample is indispensable for this analysis. A newly developed hot-deformed Nd-Fe-B magnet with Nd-Cu eutectic diffusion under expansion constraint is a good sample for this analysis, because it exhibits very high coercivity of about 20 kOe and excellent c-axis orientation [2]. In this study, energy barrier of the hot-deformed Nd-Fe-B magnets with and without Nd-Cu diffusion are analyzed and the magnetization reversal process and the temperature dependent coercivity are discussed.