Abstract
Frustrated magnetic materials can show unconventional correlations such as quantum spin liquid states and monopole excitations in spin ices. These phenomena are observed on uniformly frustrated lattices such as triangular, kagome or pyrochlore types, where all nearest neighbour interactions are equivalent. Here we report incommensurate long-range spin amplitude waves in the spinels Fe2GeO4 and γ-Fe2SiO4 at low temperatures, which indicate that the degree of frustration may itself be a fluctuating quantity that can spontaneously order without a lattice distortion as a ‘frustration wave’. Fe2GeO4 with propagation vector (2/3 + δ 2/3 + δ 0) has ordered Fe2+ moments that vary between fully saturated 4 μB and 0 values, consistent with a frustration wave order. γ-Fe2SiO4 has a more complex (¾ + δ ¾ + δ 0) order that coexists with an ordered spin ice phase. Dynamic orbital fluctuations are proposed to give rise to locally correlated patterns of ferromagnetic and antiferromagnetic interactions consistent with the observed orders.
Highlights
Frustrated magnetic materials can show unconventional correlations such as quantum spin liquid states and monopole excitations in spin ices
For a collinear spin order on a simple lattice in which exchange couplings are equivalent and are either fully frustrated or unfrustrated, the degree of frustration simplifies to F = Nf/N, where Nf is the number of frustrated interactions and N is the total number of interactions around each spin
Alternating Current (AC) measurements show no frequency dependence in the low-temperature features, indicating an absence of the spin–glass behaviour (Fig. 2b)
Summary
Frustrated magnetic materials can show unconventional correlations such as quantum spin liquid states and monopole excitations in spin ices. These phenomena are observed on uniformly frustrated lattices such as triangular, kagome or pyrochlore types, where all nearest neighbour interactions are equivalent. Previous studies have established that both materials have magnetic transitions near 10 K10–14, but the lowtemperature spin orders are not reported and preliminary abstracts have differing results[15,16] Our investigation of their magnetic structures has led to the discovery of frustration wave order as a class of ground states, where spin–spin interactions become spatially non-uniform within a structurally uniform lattice
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