Tuning fragile moment transitions to quantum phase transitions.*

Lev Nikolayevich Tolstoy starts his novel Anna Karenina by stating that “All happy families are alike; every unhappy family is unhappy in its own way.”  A similar statement can be made about fragile magnetic phase transitions.  All local moment phase transition are alike; every fragile moment transition is fragile in its own way.  A fragile magnetic state [1] is one that has (i) reduced moment ordering and (ii) can be tuned by pressure, doping and/or applied field so that its transition temperature and associated fluctuations can be brought to low enough temperatures so as to allow a new phase or state to emerge.   Fragile magnetism appears to be a necessary, but certainly not sufficient condition for the type of high-Tc superconductivity found in cuprate, Fe-based, and even heavy-fermion superconductors.  Over the past years we have explored a number of fragile magnetic systems and continue to be amazed at the richness and complexity that emerges as ferromagnetic and antiferromagnetic phase transitions are tuned toward zero Kelvin.  In this talk I will motivate our interest in fragile magnets with data from the BaFe2As2 and CaFe2As2 systems and then discuss our recent results on a variety of rare earth based (Ce and Yb) and transition metal based (Co and Ni) intermetallic systems.  The avoided quantum criticality will play a prominent role in metallic systems that start with ferromagnetic ground states.

[1] Paul C. Canfield and Sergey Leokad’evich Bud'ko, “Preserved entropy and fragile magnetism” Reports on Progress in Physics, Volume 79, Number 8, 084506 (2016).