ac susceptibility of a sphericalNd2Fe14B single crystal

The external complex ac susceptibility ${\mathrm{\chi }}_{\mathrm{e}}$=${\mathrm{\chi }}_{\mathrm{e}}^{\prime }$-j${\mathrm{\chi }}_{\mathrm{e}}^{\mathrm{″}}$ of a spherical ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{14}$B single crystal has been measured as a function of temperature, field amplitude, and frequency in both directions, parallel and perpendicular to the c axis. ${\mathrm{\chi }}_{\mathrm{e}}^{\prime }$(⊥) for T>${\mathrm{T}}_{\mathrm{s}}$, the spin reorientation temperature, is due to domain-magnetization rotations (DMR’s). ${\mathrm{\chi }}_{\mathrm{e}}^{\prime }$(⊥) for T<${\mathrm{T}}_{\mathrm{s}}$ is due to comparable DMR’s and domain-wall displacements (DWD’s), ${\mathrm{\chi }}_{\mathrm{e}}^{\prime }$(∥) for the whole range is mainly due to DWD’s. DWD’s are usually enhanced by DMR’s. There is an anomalous sharp ${\mathrm{\chi }}_{\mathrm{e}}^{\prime }$(∥) peak at T=${\mathrm{T}}_{\mathrm{s}}$ for low fields and frequencies, which is ascribed to a technical magnetization mechanism when the first anisotropy constant equals zero. For data analyses and explanation, relations between the low-field susceptibility and the anisotropy constants, expressions of the domain-wall energy density, and a formula for ${\mathrm{\chi }}_{\mathrm{e}}^{\mathrm{″}}$ arising from eddy currents for a metallic magnetic sphere have been derived.