X-ray absorption and magnetic circular dichroism studies of the ferrimagnetic series Gd₆(Mn_1-xFe_x)₂₃

Truc Ly Nguyen^1,2*, F. H. Chang², H. J. Lin², C. T. Chen², Th. Mazet³, D. Malterre³, Y. C. Tseng^1,4, A. Chainani²

¹International College of Semiconductor Technology, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
²National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
³Universitè de Lorraine, CNRS, Institut Jean Lamour, F-54000 Nancy, France
⁴Dept. of Materials Science Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan

* Presenter:Truc Ly Nguyen, email:trucleenguyen@gmail.com

We study the electronic structure of the ferrimagnetic series Gd₆(Mn_1-xFe_x)₂₃, x = 0.0 – 0.75, which show a large magnetocaloric effect and have potential as magnetic refrigerator materials near room temperature. The ferrimagnetic transition temperature Tc decreases from Tc = 489 K for x = 0.0, down to Tc = 125 K for x = 0.3 and then increases up to Tc = 467 K for x = 1.0. On the other hand, the net bulk magnetization reduces systematically upon Fe substitution. We have carried out temperature (T)-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies at the Dragon Beamline (BL 11A) of the Taiwan Light Source. The aim is to understand the origin of the variation in Tc and magnetization across the series. The Mn and Fe L-edge XAS show typical metal-like spectra while the Gd M-edge XAS spectra confirm trivalent Gd³⁺ features for the series. The Gd M-edge XMCD spectra indicates nearly full Gd³⁺ spin magnetic moment of the Gd 4f⁷ configuration with S = 7/2 state for all x. Surprisingly, while the net Mn magnetic moment is coupled parallel to the Gd magnetic moment in Gd₆Mn₂₃, upon Fe substitution, the net Mn magnetic moment switches and becomes antiparallel to the Gd moment. The Fe magnetic moment is also antiparallel to Gd moments and increases systematically with x. Using the sum-rule analysis, the estimated spin and orbital moments of Gd, Mn, and Fe are found to be consistent with the bulk net magnetization reported in earlier work. We also report the Gd XMCD spectra for all x, and for Fe at x = 0.5 and 0.75 as a function of T. The T-dependent XMCD intensity shows a systematic increase on decreasing T. It follows a power law dependence with exponent β = 0.38 ± 0.02 below Tc, indicating validity of a 3-D Heisenberg model for the magnetic behavior. The element specific XMCD shows that : (i) the bulk Tc is determined by the Mn sublattice for Gd₆Mn₂₃, and Gd moments exhibit a lower Tc compared to Mn moments (ii) For intermediate compositions (x = 0.2 to 0.5), the Gd and Fe sublattices show the same Tc (iii) For x = 0.75, the Fe moments determine the bulk Tc which is again larger than the Gd sublattice Tc. The overall results indicate that the ferrimagnetic bulk Tc’s are derived from the transition metal sublattices, and the systematic reduction in net magnetization is caused by the antiparallel coupling of the Mn and Fe sublattices obtained by Fe substitution.

Keywords: Electronic structure, ferrimagnetic order, x-ray absorption, x-ray magnetic circular dichroism