  Thermal expansion, magnetostriction and magnetization measurements under
magnetic field and hydrostatic pressure were performed on a
$\mathrm{UAu_{2}Si_{2}}$ single crystal. They revealed a large anisotropy of
magnetoelastic properties manifested by prominent length changes leading to a
collapse of the unit-cell volume accompanied by breaking the fourfold symmetry
(similar to that in $\mathrm{URu_{2}Si_{2}}$ in the hidden-order state) in the
antiferromagnetic state as consequences of strong magnetoelastic coupling. The
magnetostriction curves measured at higher temperatures confirm a bulk
character of the 50 K weak ferromagnetic phase. The large positive pressure
change of the ordering temperature predicted from Ehrenfest relation
contradicts the more than an order of magnitude smaller pressure dependence
observed by the magnetization and specific heat measured under hydrostatic
pressure. A comprehensive magnetic phase diagram of $\mathrm{UAu_{2}Si_{2}}$ in
magnetic field applied along the $c$ axis is presented. The ground-state
antiferromagnetic phase is suppressed by a field-induced metamagnetic
transition that changes its character from the second to the first order at the
tricritical point.

Strongly Correlated Electrons