  The change of a material's electrical resistance (R) in response to an
external magnetic field (B) provides subtle information for the
characterization of its electronic properties and has found applications in
sensor and storage related technologies. In good metals, Boltzmann's theory
predicts a quadratic growth in magnetoresistance (MR) at low B, and saturation
at high fields. On the other hand, a number of nonmagnetic materials with weak
electronic correlation and low carrier concentration for metallicity, such as
inhomogeneous conductors, semimetals, narrow gap semiconductors and topological
insulators, two-dimensional electron gas (2DEG) show positive, non-saturating
linear magnetoresistance (LMR). However, observation of LMR in single crystals
of a good metal is rare. Here we present low-temperature, angle dependent
magnetotransport in single crystals of the antiferromagnetic metal, TmB4. We
observe large, positive and anisotropic MR(B), which can be tuned from
quadratic to linear by changing the direction of the applied field. In view of
the fact that isotropic, single crystalline metals with large Fermi surface
(FS) are not expected to exhibit LMR, we attribute our observations to the
anisotropic FS topology of TmB4. Furthermore, the linear MR is found to be
temperature-independent, suggestive of quantum mechanical origin.
