Magnetothermal study of a Dy-stuffed spin ice:Dy2(DyxTi2−x)

Abstract

We have studied the thermodynamics of the stuffed spin ice material, Dy 2 (Dy x Ti 2-x )O 7-x/2 , in which additional Dy 3+ replace Ti 4+ in the pyrochlore Dy 2 Ti 2 O 7 . Heat capacity measurements indicate that these materials lose the spin ice zero point entropy for x ≥ 0.3, sharply contrasting with results on the analogous Ho materials. A finite ac susceptibility is observed as T → 0 in both the Ho and Dy materials with x = 0.67, which suggests that spin fluctuations persist down to T ~ 0. We propose that both the entropy and susceptibility data may be explained as a result of domains of local pyrochlore type structural order in these materials.PACS numbers: 75.50. Lk, 75.30.Hx, 75.40.Cx, 75.30.Cr 2 Materials known as geometrically frustrated magnets have atomic moments which cannot simultaneously satisfy all spin-spin interactions due to their regular positions on a crystal lattice. The energy scales of the interactions in these magnets offer unique opportunities to study how frustrated thermodynamic systems settle into their lowest energy states [1,2,3]. Examples of novel ground states observed in geometrically frustrated magnets include spin-glass-like states despite the presence of minimal structural disorder [4,5,6,7], cooperative paramagnetic states, in which the spins are locally correlated yet continue to fluctuate as T ~ 0 [8,9,10,11], and spin ice states [12,13,14,15,16,17,18,19,20,21], in which the spins freeze into a state analogous to that of the protons in frozen water. In this paper we report experimental results for variants of two spin ice materials, formed by increasing the density of spins present in the materials.Pure spin ices have a pyrochlore lattice with the general formula A 2 B 2 O 7 , in which the A sites are occupied by either of the magnetic lanthanides Ho 3+ or Dy 3+ , forming a magnetic sublattice of corner sharing tetrahedra. The B sites, which are typically filled by either non-magnetic Sn 4+ or Ti 4+ , form a separate sublattice of corner sharing tetrahedra, which is offset from the A sites by the same distance as the side of a single tetrahedron. This results in 6 A nearest neighbors and 6 B nearest neighbors for each A or B site. The local crystal field in these materials produces a highly anisotropic, Ising like single ion ground state for the rare earth ions, constraining the spins to point either towards or away from the center of each magnetic tetrahedron [22,23]. Together with the strong dipolar interactions that are present [24], the minimum energy per tetrahedron results when two spins point into and two spins point out of each tetrahedron [12,16].3 This configuration has a high level of degeneracy, causing the spins to freeze into a disordered state with approximately the same zero point entropy as in water ice [17].Recent work by our groups has demonstrated that the pyrochlore lattice can be altered by adding additional rare-earth ions to the B site of the pyrochlore lattice, thus "stuffing" more moments into the system, a situation illustrated in Fig. 1 [...

Slow and static spin correlations in Dy2 +xTi2 −xO7 − δ

“…As the temperature is lowered the magnetic structure factor monotonically increases from S|Q| = 0 Å−1 to a maximum at ≈1.05 Å−1 . This is significantly different to the pure spin ice, where S|Q| peaks at 0.55 Å−1 , and is consistent with the more antiferromagnetic nature of the coupling first found by Ueland et al [33]. The scattering from Dy 2 Ti 2 O 7 is also considerably sharper than that in the stuffed compound, indicative of a longer correlation length, but neither sample shows magnetic Bragg peaks, indicating the absence of true long range magnetic order.…”

“…Ueland et al [33] studied the low temperature thermodynamic properties of stuffed dysprosium titanate, Dy 2+x Ti 2−x O 7−δ . They found that the additional Dy-Dy pathways released the zero-point entropy completely for x 0.3, which is significantly different to the results from Ho 2+x Ti 2−x O 7−δ [29].…”

New physics in frustrated magnets: Spin ices, monopoles, etc. (Review Article)

“…However, stuffed spin ices do not freeze down to the lowest temperatures, and have basically the same entropy as standard spin ices [248,249,250,251,252,253,254,255]. On the other hand, stuffed spin ice based on Dy does not manifest residual entropy, i.e., spin fluctuations persist there down to lowest temperatures [219,256]. Extra spins of Dy-based stuffed spin ice trap magnetic monopoles and obstruct flow of monopoles, introducing residual resistance.…”

Wasp – Waisted loop and spin frustration in Dy2−xEuxTi2O7 pyrochlore

“…[71] and Dy 2 (Ti 2-x Dy x )O 7-x/2 [72]. From inset (i) of figure 5a and 5b, it has been shown that T i peak is not observed for x = 1.8, 1.9 samples.…”