2007/02/28 by Adrian Del Maestro, Michel J. P. Gingras · 3 citations
Materials Science · Physics and Astronomy · #Advanced Condensed Matter Physics #Multiferroics and related materials #Physics of Superconductivity and Magnetism #cond-mat.mtrl-sci #cond-mat.stat-mech
paper · pdf · doi:10.1103/physrevb.76.064418
published as Phys. Rev. B 76, 064418 (2007) · 12 pages, 13 figures; shortened introduction and added 1 figure
arxiv created 2007/06/25 · openalex publication_date 2007/08/15 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
The Gd2Sn2O7 pyrochlore Heisenberg antiferromagnet displays a phase transition to a four sublattice N'eel ordered state at a critical temperature Tc\ensuremath∼1\phantom\rule0.3em0exK. The low-temperature state found via neutron scattering corresponds to that predicted by a classical model that considers nearest-neighbor antiferromagnetic exchange and long-range dipolar interactions. Despite the seemingly conventional nature of the ordered state, the specific heat Cv has been found to be described in the temperature range 350\phantom\rule0.3em0exmK\ensuremath\leqslantT\ensuremath\leqslant800\phantom\rule0.3em0exmK by an anomalous power law Cv\ensuremath∼T2. A similar temperature dependence of Cv has also been reported for Gd2Ti2O7, another pyrochlore Heisenberg material. Such behavior is to be contrasted with the typical T3 behavior expected for a three-dimensional antiferromagnet with conventional long-range order which is then generally accompanied by an exp(\ensuremath-\ensuremathΔ∕T) behavior at lower temperature where anisotropy effects induce a gap \ensuremathΔ to collective spin excitations. Such anomalous T2 behavior in Cv has been argued to be correlated to an unusual energy dependence of the density of states which also seemingly manifests itself in low-temperature spin fluctuations found in muon spin relaxation experiments. In this paper, we report calculations of Cv that consider spin-wave-like excitations out of the N'eel order observed in Gd2Sn2O7 via neutron scattering. We argue that the parametric Cv\ensuremath∝T2 does not reflect the true low-energy excitations of Gd2Sn2O7. Rather, we find that the low-energy excitations of this material are antiferromagnetic magnons gapped by single-ion and dipolar anisotropy effects, and that the lowest temperature of 350\phantom\rule0.3em0exmK considered in previous specific heat measurements accidentally happens to coincide with a crossover temperature below which magnons become thermally activated and Cv takes an exponential form. We argue that further specific heat measurements that extend down to at least 100\phantom\rule0.3em0exmK are required in order to ascribe an unconventional description of magnetic excitations out of the ground state of Gd2Sn2O7 or to invalidate the standard picture of gapped excitations proposed herein.