Special Seminar
Name: Dr. Ashish Chainani
Affiliation: National Synchrotron Radiation Research Center, Hsinchu, Taiwan
Title: “Element-specific Curie temperatures and Heisenberg criticality in ferrimagnetic Gd6(Mn1−xFex)23 via Kouvel-Fisher analysis”
Date & Time: Thursday, 17th October at 4:00 p.m.
Venue: Rajarshi Bhattacharya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
The origin of non-monotonic ferrimagnetic Curie temperatures (TC) coupled to monotonic composition-controlled magnetization in rare earth-transition metal alloys of the type Ra(M1−xM’x)b remains an important long-standing puzzle. In order to solve this puzzle, we have carried out T-dependent x-ray absorption and magnetic circular dichroism (XAS-XMCD), as well as low-T hard and soft x-ray photoemission spectroscopy of the series Gd6(Mn1−xFex)23, x=0.0-0.75. The Gd 3d core-level spectra indicate trivalent Gd3+ multiplets in the intermediate-coupling scheme with features due to L-S and j-J coupling [1]. XAS-XMCD spectra show that the net Mn-moment reduces and switches from parallel to antiparallel for x ≥0.2, while the Fe-moment is always antiparallel to the Gd-moment. Kouvel-Fisher analyses of XMCD data reveals distinct sublattice TC’s and 3D-Heisenberg criticality [2]. LDA+U band structure calculations (including Uff(Gd) ~6.5 eV and Udd(Mn,Fe) ~2 eV) show magnetic moments and density of states consistent with experiments[3].The magnetic phase diagram shows three regions characterized by (i) Mn-sublattice bulk-TC> Gd-sublattice TC, (ii) a reduced common-TC for all sublattices, and(iii) Fe-sublattice bulk-TC> Gd-sublattice TC. The Mn-moment switching and gradual increase of Fe-moment are shown to be the origin of non-monotonic TC’s with monotonic magnetization. The study indicates the importance of element-specific TC’s for tuning magnetic properties.
References:
[1] T. Ly Nguyen, Th. Mazet, D. Malterre, H. J. Lin, M. Yoshimura, Y. F. Liao, H. Ishii, N. Hiraoka, Y. C. Tseng, and A. Chainani, Hard x-ray photoemission spectroscopy of the ferrimagnetic series Gd6(Mn1-xFex)23, PRB 106, 045144 (2022).
[2] T. Ly Nguyen, Th. Mazet, E. Gaudry, D Malterre, F.-H. Chang,H.-J. Lin, C.-T. Chen, Y.-C. Tseng and A. Chainani, Element-specific Curie temperatures andHeisenberg criticality in ferrimagneticGd6(Mn1−xFex)23 via Kouvel-Fisheranalysis, Communications Materials, 5 :68 ( 2024).
[3] T. Ly Nguyen, C.-C. Yang, C.-H. Wang, Y.-W. Yang, Th. Mazet, E. Gaudry , D. Malterre,M. Yoshimura, Y. F. Liao, H. Ishii, N. Hiraoka, H. J. Lin, Y. C. Tseng, and A. Chainani, Crucial role of d-d Coulomb correlations in the magnetocaloric ferrimagnets Gd6(Mn1−xMx)23 (M = Fe, Co), PRB 109, 035102 (2024).