  We report on the use of helium ion implantation to independently control the
out-of-plane lattice constant in epitaxial La0.7Sr0.3MnO3 thin films without
changing the in-plane lattice constants. The process is reversible by a vacuum
anneal. Resistance and magnetization measurements show that even a small
increase in the out-of-plane lattice constant of less than 1% can shift the
metal-insulator transition and Curie temperatures by more than 100 {\deg}C.
Unlike conventional epitaxy-based strain tuning methods which are constrained
not only by the Poisson effect but by the limited set of available substrates,
the present study shows that strain can be independently and continuously
controlled along a single axis. This permits novel control over orbital
populations through Jahn-Teller effects, as shown by Monte Carlo simulations on
a double-exchange model. The ability to reversibly control a single lattice
parameter substantially broadens the phase space for experimental exploration
of predictive models and leads to new possibilities for control over materials'
functional properties.
