  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.
