Orientation Dependence of the Anomalous Hall Resistivity in Single Crystals of Yb14MnSb11

The Hall resistivity, electrical resistivity and magnetization of single crystals of the tetragonal ferromagnet Yb14MnSb11 are reported as a function of the direction of the current, I, and magnetic field, H with respect to the principal crystallographic axes. With I along the unique c direction and H in the a-b plane, the anomalous Hall resistivity in the limit of zero applied field is negative for all temperatures T less than Tc= 53 K. In this direction, the anomalous Hall effect behaves in a manner similar to that observed in other ferromagnets such as Fe, Co, Mn5Ge3, and EuFe4Sb12. However, with I in the a-b plane and H along the c direction, the anomalous Hall behavior is completely different. The anomalous Hall resistivity data are positive for all T less than Tc and a similar analysis of these data fails. In this direction, the anomalous response is not a simple linear function of the magnetization order parameter, and for a fixed temperature (T less than Tc) does not depend on the magnitude of the magnetization perpendicular to the current in the a-b plane. That is, when the magnetization and applied field are rotated away from the c direction, the anomalous Hall resistivity does not change. In all other soft ferromagnets that we have examined (including La doped crystals of Yb14MnSb11, i.e. Yb13.3La0.7MnSb11) rotation of the magnetization and magnetic field by an angle theta away from a direction perpendicular to I results in a decrease in both the anomalous and normal portions of the Hall resistivity that approximately scales as cos(theta). We suggest that the unique response exhibited by Yb14MnSb11 is a direct reflection of the delicate balance between ferromagnetism and Kondo screening.