Elastic softness of low-symmetry frustrated $A$Ti$_2$O$_5$ ($A$ = Co, Fe)

Tadataka Watanabe; Kazuya Takayanagi; Ray Nishimura; Yoshiaki Hara; Dharmalingam Prabhakaran; Roger D. Johnson; Stephen J. Blundell

doi:10.1103/PhysRevB.111.024426

paper

Elastic softness of low-symmetry frustrated $A$ Ti $_{2}$ O $_{5}$ ( $A$ = Co, Fe)

arXiv:2408.03783 · doi:10.1103/PhysRevB.111.024426

Abstract

Orthorhombic pseudobrookites CoTi $_{2}$ O $_{5}$ and FeTi $_{2}$ O $_{5}$ have a low-symmetry crystal structure comprising magnetic Co $^{2 +}$ /Fe $^{2 +}$ ions and nonmagnetic Ti $^{4 +}$ ions, where the orbital-nondegenerate Co $^{2 +}$ /Fe $^{2 +}$ ions form one-dimensional chains running along the orthorhombic $a$ axis. These compounds undergo an antiferromagnetic phase transition at $T_{N} \sim$ 26 K for CoTi $_{2}$ O $_{5}$ and $T_{N} \sim$ 40 K for FeTi $_{2}$ O $_{5}$ . Ultrasound velocity measurements on single crystals of CoTi $_{2}$ O $_{5}$ and FeTi $_{2}$ O $_{5}$ reveal that CoTi $_{2}$ O $_{5}$ exhibits unusual elastic softness above $T_{N}$ in the symmetry-lowering elastic mode of $a c$ -plane shear elastic modulus, inconsistent with the structural symmetry breaking caused by antiferromagnetic ordering at $T_{N}$ . This suggests the presence of two distinct types of magnetostructural fluctuations above $T_{N}$ that should be a precursor to the symmetry-lowering lattice distortion at $T_{N}$ . In contrast, FeTi $_{2}$ O $_{5}$ exhibits either negligible or smaller elastic softness, indicating weaker spin-lattice coupling. These findings highlight CoTi $_{2}$ O $_{5}$ and FeTi $_{2}$ O $_{5}$ as unique spin-latticed-coupled frustrated systems with low crystal symmetry, where, while the exchange interactions are quasi-one-dimensional in nature, the frustration is released by further lowering the crystal symmetry through three-dimensional spin-lattice coupling, which is stronger in CoTi $_{2}$ O $_{5}$ than in FeTi $_{2}$ O $_{5}$ .

9 pages, 8 figures, accepted for publication in Phys. Rev. B

Elastic softness of low-symmetry frustrated ATi2​O5​ (A = Co, Fe)

Abstract

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Elastic softness of low-symmetry frustrated $A$ Ti $_{2}$ O $_{5}$ ( $A$ = Co, Fe)