Evidence for Gapped Spin-Wave Excitations in the FrustratedGd2Sn2O7Pyrochlore Antiferromagnet from Low-Temperature Specific Heat Measurements

Abstract

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...

Long-range magnetic order in the Heisenberg pyrochlore antiferromagnets Gd2Ge2O7 and Li1, Cai2, Qi3 et al. 2016Phys. Rev. B

“…The different magnetic structures and the ordering processes between GTO and GSO are also echoed by their distinct low-energy spin excitations below T N . The magnetic specific heat C m (T) of GSO drops exponentially below 350 mK, 11 pointing to a conventional, gapped spin-wave excitations, which was confirmed by the inelastic neutron scattering. 15 Starting from the P-C state, Del Maestro and Gingras 27 actually predicted such conventional magnons gapped by the single-ion anisotropy and dipolar interactions.…”

“…40 Another noteworthy feature in Fig. 4(a) is that the drop of C(T) for GGO and GPO at the lowest temperature region is less profound as GSO 11 , implying the presence of different low-energy magnetic excitations.…”

“…4(a) in a double logarithmic scale in order to highlight the ultralow-temperature features. The C(T) data of their sister compounds, GSO 7,11 and GTO 22 , are also included for comparison. As can be seen, both GGO and GPO exhibit a single, sharp specific-heat peak, resembling that of GSO rather than GTO.…”

“…Because the Gd 3+ ion (S = 7/2, L = 0) has a very small intrinsic anisotropy, the insulating Gd 2 Ti 2 O 7 (GTO) and Gd 2 Sn 2 O 7 (GSO) oxides are expected to be good approximations of the classical Heisenberg antiferromagnet on the pyrochlore lattice with dipolar interaction. Since the pioneer study on GTO in this context by Raju et al, 6 the magnetic properties of these two compounds have been investigated with various experimental techniques, such as dc and ac magnetic susceptibility [6][7][8][9][10] , specific heat 6,7,11 , elastic and inelastic neutron scattering [12][13][14][15][16] , electron spin resonance [17][18][19][20] , Mössbauer 7,21 , and muon spin relaxation [22][23][24] . Besides, their interesting magnetic properties have also been the subject of several theoretical studies [25][26][27][28][29] .…”

Spin correlations in the dipolar pyrochlore antiferromagnet Gd2Sn2O7

“…In Gd 2 Sn 2 O 7 , a first-order transition to an ordered state occurs at T N = 1.0 K (≈ θ CW /10) [17], and neutrondiffraction measurements have shown that this is indeed the Palmer-Chalker state [18,19]. Recently, debate has focussed on the possible presence of spin fluctuations at temperatures much lower than T N [14,20,21,22]. Much less attention has been given to the paramagnetic phase; however, it is known to be strongly correlated, with approximately 60% of the total magnetic entropy associated with the development of short-range correlations above T N [17].…”