Study of iron, ferri and sperimagnetismo in bilayers and amorphous films of R-Co (R = Y, Gd, Tb).

Author(s)

Taeko Yonamine Fukuhara

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Física (IF/SBI)
• Defense date: 2000-09-22

Examining board members

Antonio Domingues dos Santos; Marcia Carvalho de Abreu Fantini; Fernando Jose Gomes Landgraf; Miguel Alexandre Novak; Luis Gustavo Pereira

• Advisor: Antonio Domingues dos Santos

Abstract

In this work we present the magnetic and magneto-optical study of thin films of R-Co and Y-Co/R-Co bilayer, R=Gd, Tb. Three different kínds of magnetism are found in these films: ferromagnetism (Y-Co), ferrimagnetism (Gd-Co) and sperímagnetísm (Tb-Co). The samples were deposíted by DC magnetron sputteríng on glass substrate, and were protected from oxídatíon by 30nm thick Sí3N4, layers. Magnetic characterization of the samples was done using SQUlD, VSM and transverse magneto-optícal Kerr effect (TMOKE) magnetometers. A large doma in wall (DW), localízed at the YCo2/ Gd Co2, amorphous bilayer interface, is formed duríng a macrosropic ferri-ferromagnetíc transitíon. A model to analyze the DW ís based on Zeeman, anísotropy and exchange energies. As a result, we obtained three interdependent equations, relatíng the DW thickness , the relative position of the wall , and the exchange constant A. In this PhD thesis we present the temperature dependence of , and A. The model was ímproved to account for the unusual DW present at the bílayer interface. About 60% of the DW was inside the Y-CO layer, and the DW was found to be quite large, occupying ~64% of the volume of the bilayer. The exchange constant A presents a monotonically decreasing value, ranging from ~3.1 xl0-7 erg/cm at 25K to ~2.0 Xl0-7 erg/cm at room temperature. Gd was replaced by Tb and a study of the DW in amorphous YCo2/TbCo2, bilayer is presented. The main difference between Gd-Co and Tb-Co films is the kind of magnetism they present, the first is ferrimagnetic and the second is sperimagnetíc. As a consequence, the Tb based film shows a much higher anisotropy field than the Gd-Co film, at room temperature. The high Tb-Co anisotropy effect on the bílayer is a DW almost totally localized in the Y-Co layer. This behavior was used to determine the dependence on the magnetic field and the exchange constant A for different temperatures. A ranged from 1.8xl0-7 erg/cm at 150K to 1.4xl0-7 erg/cm at 300K and, for all temperatures, decreases from ~l00nm, at the transition field (Ht), to ~20nm at lkOe. In order to increase the understanding of the Gd and Tb based bilayers, the temperature dependence of the saturation magnetization of R-Co (R=Gd,Tb) films was simulated by the mean field theory. The values of exchange integrais JR-Co between R and Co atoms are in agreement to the literature. We also studied some ternary tilms, but in less detail. The magnetic and magneto-optical characterization of R-FeCo (R=Gd, Tb) films and GdFeCo/Tb-FeCo bilayer revealed an in-plane and perpendicular anisotropy in Gd and in Tb based fiims, respectively. (AU)