Disorder Promotes Ferromagnetism: Rounding of the Quantum Phase Transition inSr1−xCaxRuO3

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 Sr(1-x)Ca(x)RuO3 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.

Muon Spin Relaxation and Neutron Diffraction Studies of Cluster-Glass States in Sr1−xLaxRuO3

“…This theory applies to the Sr 1−x Ca x RuO 3 system and successfully explains the dependence of the Ca concentration on the low-temperature dc magnetization data. 33) In Sr 1−x La x RuO 3 , however, the result (3) clearly contradicts this assumption, suggesting that the properties of cluster-glass states in Sr 1−x La x RuO 3 cannot be explained simply in terms of the distribution of La atoms in real space.…”

Ferromagnetic Cluster-Glass State in Itinerant Electron System Sr1−xLaxRuO3

“…22,23) Suppression of the ferromagnetism has also been found in the Sr site-substituted system Sr 1−x Ca x RuO 3 . [24][25][26][27][28][29] Substituting Ca for Sr increases the Ru-O-Ru bond angle and thereby enhances the RuO 6 octahedra rotation in the distorted perovskite structure. This variation is expected to strongly affect the electronic structure.…”

“…A recent magnetization study has revealed that the ferromagnetic-to-paramagnetic quantum phase transition driven by the substitution is totally destroyed by the disorder, and the FM phase is extended to over a wide x range. 29) It is thus considered that both the electronic correlation and disorder play an important role in this system. Sr 1−x La x RuO 3 also shows a suppression of ferromagnetism generated by substituting La for Sr. 31,32) The La substitution also enhances the RuO 6 octahedra rotation.…”

Anomalous quantum criticality in an itinerant ferromagnet

“…Alternatively, T C is suppressed all the way to absolute zero on substitution of Sr 2 þ by the smaller Ca 2 þ ions at a critical concentration x c E0.7, where Sr 1 À x Ca x RuO 3 (SCRO) turns from a ferromagnetic (FM) to a paramagnetic ground state. Whether a quantum critical point (QCP) may be present at x c is a matter of current debate 2,4,7,8 . While specific heat 3 and NMR 4 measurements both concluded that the self-consistent renormalization theory 9 , equivalent to the Hertz-Millis model 10,11 , could describe the underlying physics of an FM QCP in SCRO, recent Kerr effect measurements on a composition-spread epitaxial film showed that a possible quantum phase transition (QPT) around a (reduced) critical concentration x c were smeared by disorder originating from the difference of ionic radii between Sr and Ca ions 8 .…”

“…Whether a quantum critical point (QCP) may be present at x c is a matter of current debate 2,4,7,8 . While specific heat 3 and NMR 4 measurements both concluded that the self-consistent renormalization theory 9 , equivalent to the Hertz-Millis model 10,11 , could describe the underlying physics of an FM QCP in SCRO, recent Kerr effect measurements on a composition-spread epitaxial film showed that a possible quantum phase transition (QPT) around a (reduced) critical concentration x c were smeared by disorder originating from the difference of ionic radii between Sr and Ca ions 8 . In addition, on the basis of muon-spin-rotation (mSR) experiments, it was argued that a spontaneous phase separation may be a common feature in FM systems near their QPTs, leading to a suppression of dynamic critical behaviour 7,12 .…”