High-field magnetization and magnetoresistance of single crystal Yb1-xLuxB12 (x <= 1/4)

For single crystals of Yb1-xLuxB12 (x = 0, 1/8, 1/4), we have measured the magnetization and magnetoresistance up to 60 and 15 T, respectively. The fi rst-order metamagnetic transition for x = 0 becomes a second-order type for x = 1/8, keeping the critical field value for B parallel to[1 1 1] at 53 T . The longitudinal magnetoresistance for I = 0 is negative for B parallel t o[1 1 0] and [1 1 1] at all temperatures, whereas that for x = 1/4 at 1.5 K exhibits a positive maximum at around 5 T, followed by a change to negativ e values. These results suggest that Lu substitution results in the growth of a mid-gap state and weakens the anisotropy of the c-f mixing in YbB12. ( C) 2000 Elsevier Science B.V. All rights reserved.