Spin-glass order induced by dynamic frustration

Abstract

Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order 1 . It is generally accepted that both frustration and disorder are essential ingredients in all spin glasses, so it was surprising that PrAu 2 Si 2 , a stoichiometric compound with a well-ordered crystal structure, was reported 2 to show spin-glass freezing. Here, we report on inelastic neutron scattering measurements of crystal-field excitations, which show that PrAu 2 Si 2 has a singlet ground state and that the exchange coupling is very close to the critical value to induce magnetic order. We propose that spin-glass freezing results from dynamic fluctuations of the crystal-field levels that destabilize the induced moments and frustrate the development of long-range magnetic correlations. This novel mechanism for producing a frustrated ground state could provide a method of testing the concept of 'avoided criticality' in glassy systems.Frustration arises from competing interactions that favour incompatible ground states 1,3 . For example, in rare-earth intermetallic compounds, the magnetic moments of the f electrons on each rare-earth site interact with neighbouring moments through Ruderman-Kittel-Kasuya-Yosida exchange interactions that oscillate in sign with increasing separation. Except for specific classes of geometrically frustrated lattices, well-ordered crystal structures produce either ferromagnetic or antiferromagnetic order, depending on the energy minimization of the interactions between all the neighbouring moments. However, when there is disorder, in either the site occupations or the exchange interactions, the additional randomness can prevent a unique ordered ground state. Instead, these systems may form spin glasses, which possess a multitude of possible disordered ground states, into one of which the system freezes below the glass transition temperature, T g (ref. 1). In the thermodynamic limit, spin glasses show broken ergodicity, preventing significant fluctuations to any of the other degenerate spin configurations.In recent years, two stoichiometric intermetallic compounds have shown evidence of spin-glass order, URh 2 Ge 2 (ref. 4) and PrAu 2 Si 2 (ref. 2), both nominally with the same tetragonal (ThCr 2 Si 2 -type) crystal structure. In both samples, classic spin-glass behaviour was observed (T g = 11 K and 3 K, respectively), with a frequency-dependent peak in the Figure 1 Crystal-field transition in PrAu 2 Si 2 . Inelastic neutron scattering from PrAu 2 Si 2 measured at 1.5 K (red symbols) with error bars derived from the square root of the raw data counts. The solid line is a fit to the singlet-doublet crystal-field transition at ∆ = 0.7 meV (dashed line) and an elastic peak from nuclear incoherent scattering (dotted line). The shaded area represents double scattering from the transition at 2∆. The inset illustrates the mechanism for induced-moment formation, in which interionic exchange coupling, J ex , admixes the excited magnetic doublet into the singlet grou...

Ground State of the Easy-Axis Rare-Earth Kagome LangasitePr3Ga5SiO14

“…The latter admixes excited CF states into the ground CF singlet, which otherwise has a quenched total angular momentum in isolated ions. This is not in contradiction with the Van-Vleck paramagnet picture, because in PGS J e /∆ CF is far below the critical value which would allow ground-state moments to form spontaneously [32].…”

Structural and magnetic properties of the5d2double perovskitesSr2BReO6(Aczel1, Zhao2, Calder3 et al. 2016Phys. Rev. BSelf Cite223244View full textAdd to dashboardCite

“…On the other hand, if the intramultiplet splitting δ for the non-cubic systems becomes significant and creates a singlet ground state, there are three main possibilities based on the strength of the exchange interactions J ex between the magnetic ions. It is well-known that there is a critical value x c = (J ex /δ) c above which magnetic moments are induced in singlet systems [49] and typical long-range magnetic order can still arise. On the other hand, for x c > J ex /δ, spontaneous magnetism becomes impossible and therefore non-magnetic singlet ground states are realized instead.…”

“…On the other hand, for x c > J ex /δ, spontaneous magnetism becomes impossible and therefore non-magnetic singlet ground states are realized instead. The third case with x c ≈ J ex /δ proposed for the spin glass PrAu 2 Si 2 is perhaps the most interesting, as it has been suggested that the spin freezing results from dynamic fluctuations of the crystal-field levels that destabilize the induced moments and frustrate the development of long-range magnetic order [49]. This mechanism can serve as the source of both the frustration and disorder that are generally accepted to be essential ingredients in all spin glasses, and it can provide a natural explanation for why no obvious source of disorder has ever been found in 5d 2 spin glass DPs.…”

Magnetic field and temperature dependence of the amplitude-modulated magnetic structure ofPrNi2Si2determined by single-crystal neutron diffraction

“…[1][2][3] This may arise from the topology of the lattice ͑geometric frustration͒ or from competing magnetic interactions. Magnetic frustration leads often to complex magnetic structures-e.g., incommensurate, noncollinear, or amplitude-modulated-or ground states that are not magnetically ordered, such as spin glasses, spin liquids, and nematic order.…”