Diverse magnetic-field-versus–temperature phase diagrams in breathing pyrochlore magnets
The pyrochlore lattice, a three-dimensional network of corner-sharing regular tetrahedra, is a prototypical system that hosts strong geometrical frustration. By alternately expanding and contracting adjacent tetrahedra so that the nearest-neighbor exchange interactions within the small and large tetrahedra become inequivalent, one can introduce an additional “breathing” degree of freedom. A-site–ordered chromium spinels AA'Cr4X8 serve as model materials for breathing pyrochlore lattice magnets. Among them, CuInCr4S8 realizes a specific combination in which the exchange interaction J within the small tetrahedra is antiferromagnetic, whereas the interaction J' within the large tetrahedra is ferromagnetic. In this compound, the four spins on each large tetrahedron behave as a ferromagnetic cluster at sufficiently low temperatures, allowing the system to be viewed effectively as an S = 6 antiferromagnet on a face-centered-cubic (fcc) lattice. Because geometrical frustration persists on the fcc lattice, the interplay with spin–lattice coupling and external magnetic fields is expected to yield nontrivial magnetic states.
We performed magnetization measurements on CuInCr4S8 up to 150 T and confirmed that, as in conventional chromium spinel oxides, the compound exhibits a wide 1/2-magnetization plateau over 60–110 T [A]. By further conducting detailed measurements of magnetostriction, dielectric permittivity, and the magnetocaloric effect, we uncovered a variety of magnetic phases immediately below the 1/2-plateau [B]. In particular, a pocket-like phase (“A phase”) that appears only at finite temperatures shows anomalous magnetostrictive and dielectric responses, suggesting the emergence of multi-Q states such as a magnetic skyrmion lattice. Following this discovery, we also established that in CuGaCr4S8, the Cu and Ga ions are crystallographically ordered, and J is antiferromagnetic whereas J' is ferromagnetic, as in CuInCr4S8 [C]. The field-induced properties of CuGaCr4S8 likewise share features with those of CuInCr4S8, including a robust 1/2-magnetization plateau. Taken together, these results strongly suggest that breathing pyrochlore magnets with antiferromagnetic J and ferromagnetic J' provide an excellent platform for realizing novel magnetic phenomena.
[B] M. Gen et al., Phys. Rev. Research 4, 033148 (2022). (Original paper [15])
[C] M. Gen et al., Phys. Rev. Mater. 7, 104404 (2023). (Original paper [24])
We performed magnetization measurements on CuInCr4S8 up to 150 T and confirmed that, as in conventional chromium spinel oxides, the compound exhibits a wide 1/2-magnetization plateau over 60–110 T [A]. By further conducting detailed measurements of magnetostriction, dielectric permittivity, and the magnetocaloric effect, we uncovered a variety of magnetic phases immediately below the 1/2-plateau [B]. In particular, a pocket-like phase (“A phase”) that appears only at finite temperatures shows anomalous magnetostrictive and dielectric responses, suggesting the emergence of multi-Q states such as a magnetic skyrmion lattice. Following this discovery, we also established that in CuGaCr4S8, the Cu and Ga ions are crystallographically ordered, and J is antiferromagnetic whereas J' is ferromagnetic, as in CuInCr4S8 [C]. The field-induced properties of CuGaCr4S8 likewise share features with those of CuInCr4S8, including a robust 1/2-magnetization plateau. Taken together, these results strongly suggest that breathing pyrochlore magnets with antiferromagnetic J and ferromagnetic J' provide an excellent platform for realizing novel magnetic phenomena.
References
[A] M. Gen et al., Phys. Rev. B 101, 054434 (2020). (Original paper [3])[B] M. Gen et al., Phys. Rev. Research 4, 033148 (2022). (Original paper [15])
[C] M. Gen et al., Phys. Rev. Mater. 7, 104404 (2023). (Original paper [24])
