Long-range magnetic order in the Heisenberg pyrochlore antiferromagnets Gd2Ge2O7 and

X. Li

¹

,

Yunqi Cai

²

,

Caixia Qi

³

et al.

Abstract

Gd 2 Sn 2 O 7 and Gd 2 Ti 2 O 7 have been regarded as good experimental realizations of the classical Heisenberg pyrochlore antiferromagnet with dipolar interaction. The former was found to adopt the Palmer-Chalker state via a single, first-order transition at T N ≈ 1 K, while the latter enters a distinct, partially ordered state through two successive transitions at T N1 ≈ 1 K and T N2 = 0.75 K, respectively. To shed more light on their distinct magnetic ground states, we have synthesized two more gadolinium-based pyrochlore oxides, Gd 2 Ge 2 O 7 and Gd 2 Pt 2 O 7 , under high-pressure conditions, and performed detailed characterizations via X-ray powder diffraction, dc and ac magnetic susceptibility, and specific heat measurements down to 100 mK. We found that both compounds enter a long-range antiferromagnetically ordered state through a single, first-order transition at T N = 1.4 K for Gd 2 Ge 2 O 7 and T N = 1.56 K for Gd 2 Pt 2 O 7 , respectively, with the specific-heat anomaly similar to that of Gd 2 Sn 2 O 7 rather than Gd 2 Ti 2 O 7 . Interestingly, the lowtemperature magnetic specific heat of both Gd 2 Ge 2 O 7 and Gd 2 Pt 2 O 7 was found to follow nicely the T 3 -dependence as expected for a three-dimensional antiferromagnet with gapless spin-wave excitations. We have rationalized the enhancement of T N in terms of the reduced Gd-Gd distances for the chemically pressurized Gd 2 Ge 2 O 7 and the addition of extra superexchange pathways through the empty Pt-e g orbitals for Gd 2 Pt 2 O 7 , respectively. Our current study has expanded the family of gadolinium-based pyrochlores, and permits us to achieve a better understanding on their distinct magnetic properties in a more comprehensive perspective.

High-Pressure Routes to New Pyrochlores and Novel Magnetism

“…Li et al [12] recently performed detailed studies on Gd 2 Ge 2 O 7 (GGO). As shown in Figure 2, at zero or small applied magnetic fields (H), the dc magnetic susceptibility (χ dc ), ac magnetic susceptibility χ dac , and specific heat of GGO consistently show an anomaly or peak at T N = 1.4 K, which represents a long-range magnetic ordering.…”

“…(color online) Temperature dependence of (a) dc magnetic susceptibility (χ dc ), (b) ac magnetic susceptibility χ ac , and (c) specific heat (C) under various fields for Gd 2 Ge 2 O 7 . From Ref [12]…”

Long-range dynamical magnetic order and spin tunneling in the cooperative paramagnetic states of the pyrochlore analogous spinel antiferromagnets CdYb2X4 ( X=S<

“…Following the rich phase diagrams discovered for the insulating pyrochlore compounds R 2 M 2 O 7 , where R is a trivalent rare-earth ion and M = Ti, Sn [1,2], there is a strong momentum to look at other compounds crystallizing within the same cubic crystal structure, but with different M tetravalent elements. For example, reports have been presented for M = Zr [3][4][5][6][7][8], M = Hf [9][10][11], M = Pb [12,13], M = Pt [14][15][16], and M = Ge [12,[16][17][18][19][20].…”

Structure and Dynamics of a Lattice of Tetragonal Germanates R2Ge2O7 (R = Tb–Lu, Y): Ab Initio Calculation

“…Test calculations were performed by an example of Gd 2 Ge 2 O 7 with a LATTICE DYNAMICS pyrochlore structure using two pseudopotentials: ECP53MWB-1 [19], which replaces inner shells up to the 4f shell inclusively, and ECP28MWB_SEG [21], which replaces inner shells only up to the 3d shell and leaves the 4f shell in the valence balance (i.e., it enters the valence basis set in this case and is described directly). The results are compared in Table 1, which also contains the experimental data from [22]. Parameter ρ = (Gd-O2/Gd-O1) characterizes the degree of distortion of the octahedron with a rare-earth ion at the center and oxygen ions at the vertices.…”