Collective Phenomena in the LiHoxY1−xF4 Quantum Ising Magnet: Recent Progress and Open Questions

Abstract

In LiHoxY1−xF4, the magnetic Holmium Ho3+ ions behave as effective Ising spins that can point parallel or antiparallel to the crystalline c-axis. The predominant inter-Ho3+ interaction is dipolar, while the Y3+ ions are non-magnetic. The application of a magnetic field Bx transverse to the c-axis Ising direction leads to quantum spin-flip fluctuations, making this material a rare physical realization of the celebrated transverse field Ising model. The problems of classical and transverse-field-induced quantum phase transitions in LiHoxY1−xF4 in the dipolar ferromagnetic (x = 1), diluted ferromagnetic (0.25 ≲ x < 1) and highly diluted x ≲ 0.25 dipolar spin glass regimes have attracted much experimental and theoretical interest over the past twenty-five years. Two questions have received particular attention: (i) is there an antiglass (quantum disordered) phase at low Ho3+ concentration and (ii) what is the mechanism responsible for the fast Bx-induced destruction of the ferromagnetic (0.25 ≲ x < 1) and spin glass (x ≲ 0.25) phases? This paper reviews some of the recent theoretical and experimental progress in our understanding of the collective phenomena at play in LiHoxY1−xF4, in both zero and nonzero Bx.