Abstract
We report detailed thermal expansion and magnetostriction experiments on GdCoIn$_5$ and GdRhIn$_5$ single crystal samples that show a sudden change in the dilation at a field B$^\ast$ for temperatures below the N\'eel transition temperature TN. We present a first-principles model including crystal-field effects, dipolar and exchange interactions, and the dependence of the exchange couplings with lattice distortions in order to fully account for the magnetostriction and magnetic susceptibility data. The mean-field solution of the model shows that a transition between metastable states occurs at the field B$^\ast$. It also indicates that two degenerate phases coexist in the sample at temperatures below TN. This allows to explain the lack of observation, in high resolution x-ray experiments, of an orthorhombic distortion at the N\'eeel transition even though the magnetic structure breaks the tetragonal symmetry and the magnetoelastic coupling is significant. These conclusions could be extended to other tetragonal Gd-based compounds that present the same phenomenology.
Highlights
Rare-earth magnetic compounds are among the strongest permanent magnets and present the highest magnetostrictive responses ever recorded. These remarkable properties stem from the large magnetic moments of the rare-earth ions with partially filled f-shells and the magnetic anisotropy associated with crystal-field effects and spin-orbit couplings
The magnetic structure of these compounds is mainly determined by the RudermanKittel-Kasuya-Yosida (RKKY) exchange interactions between the magnetic moments of the rare earth ions and by the crystal field, which dominate over the dipolar interaction
We present below the main experimental and theoretical results on the magnetostriction and thermal expansion data for the GdCoIn5 and GdRhIn5 compounds
Summary
Rare-earth magnetic compounds are among the strongest permanent magnets and present the highest magnetostrictive responses ever recorded. A second-neighbor AFM coupling can lead to a C-AFM order where chains of parallel-aligned moments order antiparallel (antiferromagnetically) between them (see Fig. 1) [5, 6] If these chains are aligned along the basal plane, it is expected that below the AFM ordering temperature TN , the lattice lowers its symmetry to orthorhombic [7]. Symmetryconserving distortions at the AFM transition have been detected [4], high-precision x-rays experiments (upto |a − b|/a ∼ 2 × 10−4) do not show any difference between a and b lattice parameters [7, 8] Such an intriguing absence of lattice symmetry breaking at the Neel transition in Gd-based AFM systems has been referred to as the magnetoelastic paradox [7].
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