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 tempe rature interval from 4.2 to 300 K, as a function of the strength of th e applied ac field (up to 800 A/m), as a function of frequency (rangin g from 5 to 1000 Hz), as well as the dependence on the crystallographi c orientation. A reversible rotation of magnetic moments, excited by a pplying the field perpendicular to the easy magnetization direction, i s 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 magnetizatio n direction, domain-wall movement is the main contribution to the susc eptibility (chi') and to the energy loss (chi('')) For Nd2Fe14B, a pea k in chi('')(T) is detected at 135 K, the spin-reorientation temperatu re, in a 5 Hz held of 40 A/m applied along the [001] direction. This p eak disappears upon increasing the frequency or upon changing the crys tallographic orientation of the crystal. In addition, an anomaly in th e 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 p resence of this anomaly depends on the crystallographic orientation. I t can only be detected when the external field is applied parallel to the [001] direction. For all frequencies used, the temperature depende nce of chi' is completely different from that of chi('') The chi' valu es are nearly independent of the frequency in our measuring range from 5 to 1000 Hz. However, the chi('') values increase with increasing fr equency when the field is applied along the easy magnetization directi on and decrease when the field is applied perpendicular to the easy ma gnetization direction.