Strong-disorder magnetic quantum phase transitions: Status and new developments

This article reviews the unconventional effects of random disorder on magnetic quantum phase transitions, focusing on a number of new experimental and theoretical developments during the last three years. On the theory side, we address smeared quantum phase transitions tuned by changing the chemical composition, for example in alloys of the type A$_{1-x}$B$_x$. We also discuss how the interplay of order parameter conservation and overdamped dynamics leads to enhanced quantum Griffiths singularities in disordered metallic ferromagnets. Finally, we discuss a semiclassical theory of transport properties in quantum Griffiths phases. Experimental examples include the ruthenates Sr$_{1-x}$Ca$_x$RuO$_3$ and (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ as well as Ba(Fe$_{1-x}$Mn$_x$)$_2$As$_2$.