Ultrafast negative thermal expansion driven by spin-disorder

TL;DR

This study uses ultrafast X-ray diffraction to observe how laser excitation induces negative thermal expansion in a multilayer system, revealing rapid, spin-disorder-driven strain dynamics with an inverted bipolar strain pulse.

Contribution

It provides the first direct measurement of ultrafast negative thermal expansion driven by spin-disorder in a nanoscale multilayer system.

Findings

Negative thermal expansion occurs below Ho's Neel temperature.

Two distinct timescales of contractive stress are observed (instantaneous and 12 ps).

An inverted bipolar strain pulse propagates through the heterostructure.

Abstract

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.