  In weakly ferromagnetic materials, already small changes in the atomic
configuration triggered by temperature or chemistry can alter the magnetic
interactions responsible for the non-random atomic-spin orientation. Different
magnetic states, in turn, can give rise to substantially different macroscopic
properties. A classical example is iron, which exhibits a great variety of
properties as one gradually removes the magnetic long-range order by raising
the temperature towards and beyond its Curie point of
$T_{\text{C}}^{0}=1043$\,K. Using first-principles theory, here we demonstrate
that uniaxial tensile strain can also destabilize the magnetic order in iron
and eventually lead to a ferromagnetic to paramagnetic transition at
temperatures far below $T_{\text{C}}^{0}$. In consequence, the intrinsic
strength of the ideal single-crystal body-centered cubic iron dramatically
weakens above a critical temperature of $\sim 500$\,K. The discovered
strain-induced magneto-mechanical softening provides a plausible atomic-level
mechanism behind the observed drop of the measured strength of Fe whiskers
around $300-500$\,K. Alloying additions which have the capability to partially
restore the magnetic order in the strained Fe lattice, push the critical
temperature for the strength-softening scenario towards the magnetic transition
temperature of the undeformed lattice. This can result in a surprisingly large
alloying-driven strengthening effect at high temperature as illustrated here in
the case of Fe-Co alloy.
