  We show that engineering of tunnel barriers forming at the interfaces of a
one-dimensional spin valve provides a viable path to a strong gate-voltage
tunability of the magnetoresistance effect. In particular, we investigate
theoretically a carbon nanotube (CNT) spin valve in terms of the influence of
the CNT-contact interface on the performance of the device. The focus is on the
strength and the spin selectivity of the tunnel barriers that are modelled as
Dirac-delta potentials. The scattering matrix approach is used to derive the
transmission coefficient that yields the tunneling magnetoresistance (TMR). We
find a strong non-trivial gate-voltage response of the TMR in the absence of
spin-orbit coupling when the energy of the incident electrons matches the
potential energy of the barrier. Analytic expressions for the TMR in various
limiting cases are derived. These are used to explain previous experimental
results, but also to predict parameters for device optimization with respect to
size and tunability of the TMR effect in the ballistic transport regime.
