  We measure the transient strain profile in a nanoscale multilayer system
composed of Yttrium, Holmium and Niobium after laser excitation using ultrafast
X-ray diffraction. The strain propagation through each layer is determined by
transient changes of the material-specific Bragg angles. We experimentally
derive the exponentially decreasing stress profile driving the strain wave and
show that it closely matches the optical penetration depth. Below the Neel
temperature of Ho, the optical excitation triggers negative thermal expansion,
which is induced by a quasi-instantaneous contractive stress, and a second
contractive stress contribution rising on a 12 ps timescale. These two
timescales have recently been measured for the spin-disordering in Ho [Rettig
et al, PRL 116, 257202 (2016)]. As a consequence we observe an unconventional
bipolar strain pulse with an inverted sign travelling through the
heterostructure.
