ac susceptibility study on R(2)Fe(14)B single crystals (R=Y,Pr,Nd,Sm,Gd,Tb,Dy,Ho,Er,Tm)

The temperature dependence of the ac susceptibility chi=chi'+i chi('') has been measured on R(2)Fe(14)B single crystals with R=Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm as a function of temperature in the temperature interval from 4.2 to 300 K, as a function of the strength of the applied ac field (up to 800 A/m), as a function of frequency (ranging from 5 to 1000 Hz), as well as the dependence on the crystallographic orientation. A reversible rotation of magnetic moments, excited by applying the field perpendicular to the easy magnetization direction, is found to be the principal contribution to the susceptibility (chi'). The energy loss is fairly small in this case (chi('')approximate to 0) When the external field is applied parallel to the easy magnetization direction, domain-wall movement is the main contribution to the susceptibility (chi') and to the energy loss (chi('')) For Nd2Fe14B, a peak in chi('')(T) is detected at 135 K, the spin-reorientation temperature, in a 5 Hz held of 40 A/m applied along the [001] direction. This peak disappears upon increasing the frequency or upon changing the crystallographic orientation of the crystal. In addition, an anomaly in the temperature dependence of the susceptibility is detected around 220 K for R(2)Fe(14)B single crystals with R=Pr, Nd, Sm, Tb, and Dy. The presence of this anomaly depends on the crystallographic orientation. It can only be detected when the external field is applied parallel to the [001] direction. For all frequencies used, the temperature dependence of chi' is completely different from that of chi('') The chi' values are nearly independent of the frequency in our measuring range from 5 to 1000 Hz. However, the chi('') values increase with increasing frequency when the field is applied along the easy magnetization direction and decrease when the field is applied perpendicular to the easy magnetization direction.