X-rays magnetic diffraction study of Eu8Ga16Ge30 intermetallic clathrate compound

Author(s)

Larissa Sayuri Ishibe Veiga

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin
• Defense date: 2011-04-27

Examining board members

Carlos Manuel Giles Antunez de Mayolo; Marcos de Abreu Ávila; Flávio César Guimarães Gandra

• Advisor: Carlos Manuel Giles Antunez de Mayolo

Abstract

In this work we studied the magnetic behavior of Eu8Ga16Ge30 clathrate compound, the only clathrate known so far where the guest positions are fully occupied by a rare-earth element. This compound crystallizes in a cubic unit cell (space group Pm_3n) and presents two types of cages formed by Ga-Ge atoms (a smaller cage formed by 20 atoms and a larger cage, formed by 24 atoms) inside which Eu+2 guest ions reside. The magnetic moments of these ions order ferromagnetically below TC _ 36 K through RKKY interaction. The aim of this thesis was the investigation of the local magnetic properties of the crystallographic sites occupied by Eu ions, which inside of the larger cages, are located in four off-center positions contained in planes perpendicular to the faces of the cubic unit cell. In this work we describe the synthesis process of Eu8Ga16Ge30 compound. Eu8Ga16Ge30 single crystal was grown from metallic flux method with excess of Ga. We also describe the structural characterization of the samples by high resolution x-ray diffraction and x-ray powder diffraction. Electrical resistivity measurements were performed and showed anomalies not only at TC but also at T* _ 24 K. The same behavior was observed in the literature which indicates that the anomaly found in T* has a magnetic nature. The magnetic properties were studied by macroscopic measurements such as magnetic susceptibility and magnetization versus applied magnetic field. The local magnetic behavior of the compound was probed by dichroic resonant diffraction of circularly polarized x rays. This technique was used to measure site-specific magnetism of the Eu8Ga16Ge30 compound. It was able to provide information about the magnetism of the crystallographic sites 2a and 24k occupied by Eu ions through the study of magnetic hysteresis obtained in the diffraction condition at different temperatures. It was observed that the magnetic behavior of the 2a and 24k crystallographic sites are different for the temperature of 8 K. However, no difference was found for the hysteresis of the two sites on T = 30 K. The results suggest that the magnetic hysteresis of the 24k site at 8 K is formed by several contributions indicating the existence of a magnetic structure where the magnetic moments located on this site are arranged along three mutually perpendicular directions. We also found less structured magnetic hysteresis at T = 30 K which may be related to the presence of a magnetic structure in which all magnetic moments are oriented along a single direction (easy axis of magnetization along [001]). However, the mechanism that describes the magnetic behavior of this compound (spin reorientation or two temperatures of magnetic ordering) has not been fully elucidated (AU)

• FAPESP's process: 08/08900-2 - X-ray diffraction study of intermetallic clathrates compounds
• Grantee: Larissa Sayuri Ishibe Veiga
• Support Opportunities: Scholarships in Brazil - Master