Antiferromagnetic order in the pyrochloresR2Ge2O7 (R=Er,

Zhiling Dun

¹

,

X. Li

²

,

R. S. Freitas

³

et al.

Abstract

Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er2Ge2O7 and Yb2Ge2O7 show that both systems are antiferromagnetically ordered in the Γ5 manifold. The ground state is a ψ3 phase for the Er sample and a ψ2 or ψ3 phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R2B2O7 (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction J± under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013) The pyrochlores R 2 B 2 O 7 (R: rare earth elements, B: transition metals) have been a hot topic due to their emergent physical properties based on the geometrically frustrated lattice [1,2]. Recent interest in pyrochlores is focused on systems with effective spin-1/2 R 3+ ions [3,4], in which the crystal electric field (CEF) normally introduces a well-isolated Kramers doublet ground state with easy XY planar anisotropy [5,6]. In these XY pyrochlores, the anisotropic nearest neighbor exchange interaction J ex = (J zz , J ± , J z± , J ±± ) between the R 3+ ions, plus the strong quantum spin fluctuations of the effective spin-1/2 moment, stabilize various exotic magnetic ground states [3].Er 2 Ti 2 O 7 and Yb 2 Ti 2 O 7 are two celebrated examples of the effective spin-1/2 XY pyrochlores. For Yb 2 Ti 2 O 7 , the local [111] Ising-like exchange interaction J zz is considerably larger than the XY planar interaction J ± [7]. An unconventional first order transition is observed [8], which has been proposed to be a splayed-ferromagnet (SF) state with Yb 3+ spins pointing along one of the global major axes with a canting angle [9]. Slight disorder between the Yb and Ti sites leads to a possible quantum spin liquid state [10,11]. For Er 2 Ti 2 O 7 , the Er 3+ spins are energetically favored to lie within the local XY plane due to the dominating J ± , in which a continuous U(1) symmetry is preserved in the Hamiltonian that allows the Er 3+ spins to rotate continuously in the XY plane [12][13][14]. Recently, both experimental and theoretical studies suggest that the quantum spin fluctuations lift the U(1) degeneracy with a small gap opening in the spin-wave spectrum and select an antiferromagnetic (AFM) ordering state (ψ 2 ) as the ground state for Er 2 Ti 2 O 7 . This is the so called "order by disorder" (ObD) mechanism [14][15][16][17][18], in which the ground state is selected through entropic effects. Meanwhile, an alternative CEF-induced energetic selection mechanism is proposed that will likewise result in the ψ 2 state with similar value of the gap [19,20]

Experimental evidence for field-induced emergent clock anisotropies in the XY pyrochlore Er2Ti2O7

“…We demonstrate that, depending on the field direction, a combination of two-, three-, and possibly six-fold Zeeman clock terms emerge and compete with the six-fold clock term present in zero field. We anticipate this work will help determine the ground state selection and field effects in other topical Γ 5 pyrochlores, such as NaCaCo 2 F 7 [29] [30,31].…”

Structural and magnetic properties of two branches of the tripod-kagome-lattice familyA2R3Sb3O14(ADun1, Trinh2, Lee3 et al. 2017Phys. Rev. BSelf Cite4513583View full textAdd to dashboardCite

“…In the pyrochlores, this chemical pressure effect has been proven effective for determining the magnetic ground states. For example, by replacing the Ti site with a smaller Ge ion or a larger Sn ion, the chemical pressure effect selects different magnetic ground states in the pyrochlore systems Yb 2 X 2 O 7 and Er 2 X 2 O 7 (X = Ge, Ti, Sn) 34,35 . In the TKL system, an obvious result of chemical pressure is the structural change.…”

“…In this TKL branch, the larger Zn 2+ ion is expected to modify the spin-spin interaction energies, akin to applying "chemical pressure" and thus allows rigorous theoretical tests of the low energy phases. Such chemical pressure has been proved to be crucial for selecting ground states in Er and Yb pyrochlores 34,35 . The questions raised above provide ample motivation for a systematic exploration of the large range of compounds based on the TKL structure.…”

Phase Competition in the Palmer-Chalker XY Pyrochlore Er2Pt2O<

“…Given that the lattice parameter of Er 2 Pt 2 O 7 differs by less then 0.5% from its titanate analog, it is surprising that the transition temperature is so substantially reduced. This dramatic reduction in T N occurs despite minimal structural modifications and a larger Curie-Weiss [12], Er2Ti2O7 [13], and Er2Pt2O7, filled in circles from [14] and open circles from our study. The latter two samples magnetically order with TN = 1.4 K and 1.2 K, respectively.…”

“…Of these states, all but Γ 7 have been experimentally observed in the XY pyrochlores. This XY family is made up of Yb 2 B 2 O 7 and Er 2 B 2 O 7 with B = Ge, Ti, and Sn, where: (i) The order-by-disorder candidate Er 2 Ti 2 O 7 orders into ψ 2 [17] (ii) Er 2 Ge 2 O 7 and Yb 2 Ge 2 O 7 have as-of-yet unidentified ordered states within Γ 5 (ψ 2 or ψ 3 ) [12,18] and (iii) Both Yb 2 Sn 2 O 7 [19,20] and some samples of Yb 2 Ti 2 O 7 [21][22][23][24][25] order into the Γ 9 splayed ferromagnetic state. This ensemble of magnetic ground states supports the picture of a rich phase space.…”