CdEr2Se4: A New Erbium Spin Ice System in a Spinel Structure

Abstract

Here we present a detailed study of the spinel CdEr 2 Se 4 and show it to be a new instance of spin ice, the first one in an erbium material and the first one in a spinel. Definitive experimental evidence comes from the temperature dependence of the magnetic entropy, which shows an excellent agreement with the predicted behavior for a spin ice state. Crystal field calculations demonstrate that the change in the local environment from that of the titanates completely alters the rare-earth anisotropy giving rise, in the case of Er 3þ , to the required Ising anisotropy, when The rare-earth (R) pyrochlores (R 2 M 2 O 7 ) have been found to display a rich variety of exotic magnetic behavior, a direct consequence of the impossibility, imposed by the geometry of the lattice, of finding a classical ground state by minimizing all pairwise exchange interactions. Theoretically, a system of antiferromagnetically coupled Heisenberg spins in a pyrochlore lattice should remain in a collective paramagnetic state as T ! 0 [1]. In practice, the macroscopic degeneracy can be lifted by lesser contributions to the spin Hamiltonian. In particular, the crystalline electric field (CEF) anisotropy term can dramatically alter the nature of the ground state, making it quasi specific to each rare-earth species. Thus, in the titanate series R 2 Ti 2 O 7 , the observed ground states in a zero applied field include nonconventional AFM states for the dipoleHeisenberg (R ¼ Gd [2]) and the XY (R ¼ Er [3,4]) systems, a spin liquid state (R ¼ Tb [5], half-way between Heisenberg and Ising), spin ice for the Ising Ho and Dy systems [6,7], and a dynamic disordered state (for Yb, with planar anisotropy [8][9][10]). The CEF anisotropy thus appears as the main cause for the rich phenomenology observed in the frustrated pyrochlores. But, at the same time, it constitutes a major limiting factor regarding the number of systems displaying a particular behavior as it effectively makes each R ion the only one of its kind.Normal spinels AR 2 X 4 constitute an alternative realm in which to look for exotic magnetism as the R ions occupying the octahedral sites in the structure form an identical pyrochlore sublattice to that in the titanates. In fact, work by Lau and co-workers on the series CdR 2 X 4 (X ¼ S, Se) [11] has shown clear indications of geometrical frustration in these materials. Interestingly, the local environment of the rare-earth in the spinels is different to that in the pyrochlores. This alters the CEF level scheme of the R ion and can potentially lead to completely different behavior for the same magnetic species in the two series despite the analogy in the overall geometry of the magnetic sublattice.In this Letter we show this to be the case. We present a detailed study of the cubic spinel CdEr 2 Se 4 (Fd 3m) and show that the change in the coordination environment of the Er 3þ ions from that of the pyrochlores Er 2 M 2 O 7 (M ¼ Ti, Sn) causes a change of the CEF anisotropy from planar to Ising, leading to spin ice behavior at low temperatures...

Nonmonotonic residual entropy in diluted spin ice: A comparison between Monte Carlo simulations of diluted dipolar spin ice models and experimental results

“…As is well known, a Pauling residual entropy is generically found in undiluted spin ice materials, 9,10,13,14,24,25 See, however, Ref. [26] where no residual Pauling entropy plateau is found in Dy 2 Ti 2 O 7 if extraordinary long relaxation time scales of several days is afforded below a temperature of order of 0.4 K. We return to this issue later in this Introduction.…”

“…11 More recently, Dy 2 Ge 2 O 7 and Ho 2 Ge 2 O 7 have been made through high pressure chemical synthesis, and shown to be spin ice systems through thermodynamic measurements, 12,13 as has CdEr 2 Se 4 , in which, unusually, Er 3+ is described by an Ising spin. 14 In all of these materials, the magnetic rare-earth Dy 3+ , Ho 3+ and Er 3+ ions sit on the vertices of a pyrochlore lattice of corner-sharing tetrahedra; the Ti 4+ , Sn 4+ and Ge 4+ ions are non-magnetic. Because of the very large single-ion anisotropy in these systems, the moments can be described below a temperature T ∼ 50 K as classical Ising spins pointing along their local [111] direction at their respective pyrochlore lattice sites.…”

Fluctuation-Driven Selection at Criticality in a Frustrated Magnetic System: The Case of Multiple-kPartial Order on the Pyrochlore Lattice

“…While this compound has been the subject of a number of investigations [20][21][22][27][28][29][30][31][32][33][34][35][36], the mechanism responsible for the selection of 4-k order has not been identified. Further, after fifteen years of research arXiv:1310.5146v1 [cond-mat.str-el] 18 Oct 2013 on the Tb 2 Ti 2 O 7 spin liquid candidate [37], evidence has recently begun accumulating that short-range magnetic correlations develop below T ∼ 0.4 K in the form of broad elastic neutron intensities at k = { 1 2 ) order is not unusual among the plethora of R 2 M 2 O 7 materials, or even AR 2 B 4 rare-earth spinels (A=Cd, Mg; B=S, Se and R is a rare-earth) [43][44][45].…”

Doping-induced quantum crossover in Er2Ti2−xSnxO7

“…The most notable example represents the case of a spin-ice manifold where excitations in the form of magnetic monopoles have been established [1]. A necessary prerequisite for the spin-ice ground state is the Ising-type of a single-ion anisotropy found in Ho 2 Ti 2 O 7 [1], Dy 2 Ti 2 O 7 [2], as well as in an erbium-based spinel compound CdEr 2 Se 4 [3]. To the contrary, the Er-based titanate, Er 2 Ti 2 O 7 , exhibits an almost ideal XY-anisotropy, with moments confined to the plane perpendicular to local 111 axes.…”