Energy and momentum relaxation through the Curie temperature in an itinerant ferromagnet

TL;DR

This study uses advanced THz spectroscopy techniques to separately measure energy and momentum relaxation in a ferromagnetic transition of Ca₂RuO₄, revealing that momentum relaxation decreases while energy relaxation remains unchanged across the transition.

Contribution

It demonstrates the independent measurement of energy and momentum relaxation rates in a ferromagnetic transition using combined linear and non-linear THz spectroscopy.

Findings

Momentum relaxation rate decreases at the transition

Energy relaxation rate remains unaffected by magnetic order

Energy relaxation primarily via phonons, not magnetic excitations

Abstract

In this work, we combine conventional linear response time-domain THz spectroscopy with non-linear THz-pump THz-probe techniques to study metallic strained thin films of $\mathrm{Ca}_2\mathrm{RuO}_4$, which undergo a transition into a ferromagnetic state at 10 K. Such measurements allowing us to independently measure momentum and energy relaxation rates. We find that while the momentum relaxation rate decreases significantly at the ferromagnetic transition, the energy relaxation rate remains unaffected by the emergence of magnetic order. This shows that the dominant changes to scattering across the transition correspond to scatterings that relax momentum without relaxing energy. It is consistent with a scenario where energy is not carried off by coupling to collective magnetic degrees of freedom. Instead, the principal channel for energy relaxation remains the conventional one e.g. coupling to acoustic phonons. This observation validates the approximation used in the conventional understanding of resistive anomalies of ferromagnets across the Curie temperature, which due to critical slowing down, spin fluctuations can be treated as effectively static and scattering off of them elastic. This scenario can likely be extended to resistive anomalies at other phase transitions to charge- and spin-density wave states in kagome metals or pnictide system