ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
Physics
Notes:
The rare-earth trifluoromethanesulfonates R(CF3SO3)3⋅9H2O (RTFMS), form hexagonal crystals structurally similar to the ethylsulfates R(C2H5SO4)3⋅9H2O (RES). While the magnetic properties of RTFMS and RES analogs for many different rare earths are quite similar, PrTFMS has been found to exhibit long-range spin order (TN=0.31 K) while PrES shows only short-range order. In an attempt to understand this difference, electron paramagnetic resonance has been studied at and below 4.2 K at X band (∼9.2 GHz) and K band (∼23.5 GHz) for 1% and 3% concentrations of Pr3+ substituted in LaTFMS crystals. For Pr3+, the crystal field of C3h symmetry leaves an accidentally degenerate ground doublet and a singlet first excited state. There is a magnetic dipole moment along the c axis while an electric dipole moment develops in the perpendicular (xy) plane. Random distortions split the doublet by an amount Δ. Its Hamiltonian, in an effective spin S=1/2 formalism, is H= g(parallel)βHzSz +AIzSz+(ΔxSx+ΔySy) +γ(ExSx+EySy), where Δ2=Δx2+Δy2. Single-ion resonances are well described by this Hamiltonian with g(parallel)≈1.74, g⊥≈0, A≈0.086 cm−1, γ≈2.3×10−31 C m, and Δ/k=0.224 K. At K band, additional lines due to interacting pairs of Pr3+ ions have been identified and analyzed using an interaction Hamiltonian of the form Si⋅K⋅Sj.Coupling constants have been determined for both nearest-neighbor (nn) pairs along the c axis and next-nearest-neighbor (nnn) pairs whose axis makes an angle of ∼65° with the c direction. nn and nnn interactions are of opposite sign, comparable magnitudes, and much larger than those of magnetic dipoles. They explain the Weiss constant observed in concentrated PrTFMS. The nnn coupling, which appears to be more important than in PrES, probably accounts for long-range ordering seen in PrTFMS although the detailed mechanism is unclear.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.353475
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