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Specific heat measurements in zero magnetic field are presented on a homologous series of geometrically frustrated, antiferromagnetic, Heisenberg garnet systems. Measurements of Gd3Ga5O12, grown with isotopically pure Gd, agree well with previous results on samples with naturally abundant Gd, showing no ordering features. In contrast, samples of Gd3Te2Li3O12 and Gd3Al5O12 are found to exhibit clear ordering transitions at 243 mK and 175 mK respectively. The effects of low level disorder are studied through dilution of Gd 3+ with non-magnetic Y 3+ in Gd3Te2Li3O12. A thorough structural characterization, using X-ray diffraction, is performed on all of the samples studied. We discuss possible explanations for such diverse behavior in very similar systems.

We have measured the low-temperature specific heat of the geometrically frustrated pyrochlore Heisenberg antiferromagnet Gd2Sn2O7 in zero magnetic field. The specific heat is found to drop exponentially below approximately 350 mK. This provides evidence for a gapped spin-wave spectrum due to an anisotropy resulting from single ion effects and long-range dipolar interactions. The data are well fitted by linear spin-wave theory, ruling out unconventional low-energy magnetic excitations in this system, and allowing a determination of the pertinent exchange interactions in this material.In the insulating R 2 M 2 O 7 magnetic pyrochlores, the rare-earth ions (R 3+ ) sit on a lattice of cornersharing tetrahedra, resulting in geometrical frustration for antiferromagnetic nearest-neighbor exchange between Heisenberg spins. As such, these materials display a wide variety of interesting and unusual behaviors. Examples of observed phenomenology include spin ice [1], spin glass [2,3], and spin liquid [4] behaviors as well as long-range magnetic order [5,6,7,8,9,10,11] and perhaps some novel type of hidden order [12]. A common trend observed in all of these systems is an apparent persistence of spin dynamics down to the lowest temperatures as revealed by muon spin relaxation (µSR) [3,4,7,8,9,11,12] Perhaps what is most perplexing, is that PSDs have been found in pyrochlores, such as Gd 2 Ti 2 O 7 (GTO) [9,11] and Gd 2 Sn 2 O 7 (GSO) [7,8,13], which, according to neutron scattering experiments [6,10], display long-range magnetic order below a critical temperature T c = 0.74 K for GTO [6] and T c = 1.0 K for GSO [10]. This is highly unusual since conventional wisdom suggests that collective magnon-like excitations, hence spin dynamics, should freeze out in the limit of zero temperature. Similarly, the Gd 3 Ga 5 O 12 antiferromagnetic garnet (GGG), which displays rather extended magnetic correlations below 140 mK [14] that are now theoretically rationalized [15], also exhibits PSDs below 100 mK [16]. It is tempting to speculate that the unusual low-energy excitations giving rise to PSDs may result from a remnant of the high frustration that these systems possess and the lack of long-range order that would occur were it not for perturbatively small exchange interactions beyond nearest-neighbor [17], dipolar interactions [18,19] or single-ion anisotropy [20].We see Gd 2 Sn 2 O 7 (GSO) as a key system to investigate in seeking to convincingly demonstrate, via measurements of the low temperature magnetic specific heat, C m (T ), the existence of unconventional low-energy excitations, as suggested by the observation of PSDs. Previous C m measurements on this material between 350 mK and 800 mK (see Fig. 1) were found to be parametrized by a C m (T ) ∼ T 2 law [5]. Such a temperature dependence of C m is unusual since conventional antiferromagnetic magnon excitations without a gap lead to C m (T ) ∼ T 3 or, with an anisotropy energy gap ∆, to C m (T ) ∼ exp(−∆/T ). Such an unconventional C m (T ) behavior further argues for the existe...

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at 0°, we obtain for this sample S= 1.129M? at 4.21°K, while the 2D hard-disk gas calculation yields S = 0.969Nk. The b for which the formula agrees with the experimental entropy is 8.8 A 2 , which lies well within its range of uncertainty. The entropy comparison indicates that the assumptions of a 2D gas state at 4° and of essentially complete order at low temperatures are correct. The transition at x g = i thus appears to involve a change from a 2D gaslike state at high temperatures to an ordered array at low temperatures; and since He 4 at "critical coverage" follows the logarithmic dependence of latticegas theory, the extinction of thermal mobility must in some sense be proportional to the degree of spatial order. If mobility is so intimately related to the spatial order, it is a collective property of the system and cannot be treated in terms of noninteracting single-particle Bloch states. The transitions therefore bear some resemblance to the metal-insulator transition of certain three-dimensional electronic systems. 11 We wish to thank C. E. Campbell, M. Schick, and P. R. Zilsel for continuing stimulating conversations and the free exchange of ideas. Their