Doping-induced quantum crossover in Er2Ti2−xSnxO7

Abstract

We present the results of the investigation of magnetic properties of the Er 2 Ti 2−x Sn x O 7 series. For small doping values, the ordering temperature decreases linearly with x, while the moment configuration remains the same as in the x = 0 parent compound. Around x = 1.7 doping level, we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time, we present evidence of a long-range order (LRO) in Er 2 Sn 2 O 7 (x = 2.0) below T N = 130 mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to x = 0. We discuss our results in the framework of a possible quantum phase transition occurring close to x = 1.7. Er 2 Ti 2 O 7 has the highest ordering temperature of all the rare-earth pyrochlores studied so far (T N = 1.23 K, Ref.[4]) with no apparent sample dependence, indicating a robust ground state. It orders into a noncoplanar k = 0 antiferromagnetic structure, described by the ψ 2 basis vector of the 5 irreducible representation [5]. Interestingly, another spin configuration (ψ 3 ) within 5 results with the same energy as ψ 2 (ψ 1 and ψ 2 in Ref. [6]). It has been long-proposed that ψ 2 is favored based on entropic grounds with both thermal [7] and quantum fluctuations [8]. This so-called "order-by-disorder" scenario has recently been verified both experimentally [9] and theoretically [10]. It is thus of great interest to investigate further the delicate balance between these degenerate states.One direction of investigation is a magnetic dilution of erbium sites by nonmagnetic ytrium ions, Er 2−x Y x Ti 2 O 7 . Niven et al. [11] have concluded from a heat capacity study that the dilution reduces the ordering temperature in a linear fashion, with a percolation threshold of ≈ 60 %. To the contrary, two independent theoretical studies [12,13] suggested an existence of a first order phase transition between ψ 2 and ψ 3 at around * ivica.zivkovic@epfl.ch 7% level. Indeed, recent neutron diffraction experiments [14] showed an instability of ψ 2 state and closing of an energy gap at somewhat higher level of doping, between 10% and 20%. They interpreted the results on the 20% sample as a frozen mosaic of ψ 2 and ψ 3 domains.The emergence of the ψ 3 state is explained through the "order-by-structural disorder" mechanism, which competes with the before-mentioned, entropy-based order-by-disorder [12]. When the system is found within such an intricate balance between the states, it is natural to consider effects of exchange energy tuning. A very general theoretical approach has been conducted by Wong et al. taking into account the anisotropic nearest-neighbor couplings [15]. Due to their very similar ionic radius, magnetic dilution with ytrium does not affect much the exchange interaction between erbium moments [11,14]. On the other hand, when doping is performed on the B site of the pyrochlore lattice, replacing Ti by either Sn or Ge, the ef...

Field-temperature phase diagram of the enigmatic Nd2(Zr1−xTix)2O7

“…Motivated by this very rich physics and by the competition between spin ice and the AIAO state [13,14], we investigate in this paper the field-temperature (H, T ) phase diagram of this material. Moreover, as documented in many pyrochlore compounds such as Tb 2 Ti 2 O 7 [15], Er 2 Ti 2 O 7 [16], or Yb 2 Ti 2 O 7 [17], weak substitution, often less than 5%, can change the physical properties leading to a quick collapse of the ground state. With this in view, the study was extended to the Nd 2 (Zr 1−x Ti x ) 2 O 7 family, using two nominal compositions x = 0.025 and x = 0.1.…”

Geometrically frustrated magnetism in the pyrochlore Er>sub<2>/sub<Ti>sub<2-x>/sub<Sn>sub<x>/sub<O>sub<7>/sub< and in the disordered fluorites R>sub<2>/sub<Zr>sub<2>/sub<O>sub<7>/sub< (>i<R >/i<= Dy, Ho, Tb)

“…Here, results obtained from specific-heat experiments for the Er 2 Ti 2−x Sn x O 7 series under zero magnetic field were reported. All compositions present clear transition peaks associated with long-range magnetic ordering at temperatures T N below 1 K. The long-range ordering and the magnetic ground states were confirmed by neutron studies [13]. Besides temperature, other factor as pressure, magnetic field, or doping level can constraint the order transition, and then a quantum phase transition can occur even at T = 0.…”

“…In XY pyrochlores it is also proposed that the introduction of large levels of random disorder induce spin-glass domains [33]. The nonmagnetic dilution of the Ti sublattice on Er 2 Ti 2 O 7 has been studied by our group in [13].…”

“…On the other hand, the spin-ice state appear to be very robust to the substitution of Ti by Sn. For Er 2 Sn 2 O 7 , long-range order is developed [13,14] and the magnetic ground state was reported to be Palmer-Chake type [15].…”

“…For Er 2 Ti 2 O 7 (x = 2) the magnetic moment of Er 3+ is proposed to exhibit XY anisotropy, in which the spins are constrained to lie in the plane perpendicular to their local [111] axis [20]. However, for Er 2 Sn 2 O 7 the magnetic state observed is reported as Palmer-Chake state [13,14]. We present an analysis of experimental thermal results at very low temperature including evidence that there is a change of spin anisotropy trough the series Er 2 Ti 2−x Sn x O 7 .…”