Disorder promotes ferromagnetism: Rounding of the quantum phase transition in Sr_{1-x}Ca_xRuO_3

TL;DR

Disorder introduced by ionic size differences in Sr_{1-x}Ca_xRuO_3 extends ferromagnetism and smears the quantum phase transition, highlighting the significant role of correlated disorder in itinerant magnets.

Contribution

This study demonstrates how disorder affects the quantum phase transition in Sr_{1-x}Ca_xRuO_3, showing disorder extends ferromagnetism and smears the transition, supported by experimental and theoretical analysis.

Findings

Disorder extends the ferromagnetic phase in Sr_{1-x}Ca_xRuO_3.

The quantum phase transition is completely destroyed by disorder.

Correlated disorder enhances ferromagnetism more than random disorder.

Abstract

The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr1-xCaxRuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.