Successive spin reorientations and rare earth ordering in Nd0.5Dy0.5FeO3: Experimental and ab initio investigations

• Published in: Physical review. B 2020-10, Vol.102 (14)
• Main Authors: Singh, Ankita, Rajput, Sarita, Balasubramanian, Padmanabhan, Anas, M, Damay, Francoise, Kumar, C M N, Eguchi, Gaku, Jain, A, Yusuf, S M, Maitra, T, Malik, V K
• Format: Article
• Language: English
• Subjects: Anisotropy; Antiferromagnetism; Configurations; Diffraction; Dysprosium; Ferric ions; Heat exchange; Low temperature; Magnetic structure; Mathematical analysis; Metal ions; Phase transitions; Rare earth elements; Representations; Room temperature; Spin-orbit interactions
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.102.144432

In the present paper, the magnetic structure and spin reorientation of mixed rare-earth orthoferrite Nd0.5Dy0.5FeO3 have been investigated. At room temperature, our neutron-diffraction measurements reveal that the magnetic structure of Fe3+ spins in Nd0.5Dy0.5FeO3 belongs to Γ4 irreducible representation (Gx, Fz) as observed in both parent compounds ( NdFeO3 and DyFeO3). The neutron-diffraction study also confirms the presence of a spin-reorientation transition where the magnetic structure of Fe3+ spins changes from Γ4 to Γ2 (Fx, Gz) representation between 75 and 20 K while maintaining a G-type antiferromagnetic configuration. Such a gradual spin reorientation is unusual since the large single ion anisotropy of Dy 3 + ions is expected to cause an abrupt Γ4 → Γ1 (Gy) rotation of the Fe3+ spins. At 10 K, the Fe3+ magnetic structure is represented by Γ2 (Fx, Gz). Unexpectedly, the Γ4 structure of Fe3+ spins re-emerges below 10 K, which also coincides with the development of rare-earth (Nd3+/Dy3+) magnetic ordering having cRy configuration. Such re-emergence of a magnetic structure has been a rare phenomenon in orthoferrites. The absence of a second-order phase transition in rare-earth ordering, interpreted from heat capacity data, suggests the prominent role of Nd3+ − Fe3+ and Nd3+ − Dy3+ exchange interactions. These interactions suppress the independent rare-earth magnetic ordering observed in both parent compounds due to Nd3+/Dy3+ − Nd3+/Dy3+ exchange interactions. Our density-functional-theory calculations including Coulomb correlation and spin-orbit interaction effects (DFT + U + SO) reveal that the C-type arrangement of rare-earth ions (Nd3+/Dy3+), with Γ2 (Fx, Gz) configuration for Fe3+ moments, is energetically very close to a phase with the same rare-earth magnetic ordering but Γ4 (Gx, Fz) configuration of Fe3+ spins. Further, the Nd3+ − Fe3+ and Nd3+ − Dy3+ exchange interactions are observed to play significant roles in the complex Fe3+ spin reorientation with the re-emergence of Γ4 at low temperature. Consistent with the experimental observations, our calculations established the mixed phase (Γ2 and Γ4) to be the magnetic ground state of Fe3+ moments.