Understanding the interaction of soft and hard magnetic components in NdFeB with first-order reversal curves

First-order reversal curve (FORC) measurements are a powerful tool to study magnetization reversal processes and interactions in heterogeneous systems with broad coercivity distributions. In NdFeB hard magnets an additional soft magnetic component is often observed possibly originating from damaged surface grains. Here we use FORC to study the reversal processes and interactions in these permanent magnets at different temperatures between 50 and 350 K. The measured reversal curves reveal a strongly coupled switching of the soft and hard magnetic components above 250 K. Below this temperature the two components are decoupled and switch almost independently. This decrease in effective interaction at lower temperatures is also observed in the FORC diagrams by a relative reduction in intensity of the so called interaction peak. This result proves that FORC is a powerful method, contributing to a better understanding of magnetization reversal processes and interactions in permanent magnets.

Figure 1

Applied field over time for one single first-order reversal curve measurement. The segments where the field is driven from saturation $H_{\mathrm{sat}}$ to reversal field $H_{\mathrm{r}}$ and from $H_{\max }$ to $H_{\mathrm{sat}}$ are displayed in green, the actual measurement of a FORC in red, and short waiting times in blue.

Figure 2

Hysteresis loop at 300 K of sintered, commercial NdFeB magnet with (red) and without (blue) demagnetizing correction. The soft and hard magnetic switching regions are marked by ellipses. The sample geometry is shown in the inset.

Figure 3

NdFeB FORC densities at 300 K without (top) and with (bottom) demagnetization correction. Positive FORC peaks are shown in red, negative peaks in blue. Three characteristic features are visible: (a) and (b) switching of soft and hard magnetic components, respectively, and (c) interaction peak.

Figure 4

Demagnetizing field corrected FORC density at room temperature projected onto the minor loops. The origin of the three FORC peaks visible in Fig. 3 is again labeled with (a), (b), and (c).

Figure 5

NdFeB FORC densities between 50 and 350 K. Top left: The 50 K FORC density is projected onto the minor loops. Note the axis scaling in the left and right column is not the same for a better visibility of the FORC densities.

Figure 6

Relative intensities of the three FORC peaks over temperature.