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Magnetism and Structure of Thin 3d Transition Metal Films : XMCD and EXAFS using Polarized Soft X-Rays

by Hahlin, Anders

Abstract (Summary)
In this Thesis the magnetic and structural properties of thin epitaxial Fe, Co, and Ni films are discussed. Some of the in-situ prepared samples were used to characterize the degree of circular polarization of the newly installed beamline D1011 at MAX-lab. By means of x-ray magnetic circular dichroism (XMCD) and utilizing the associated magneto optic sum rules, the orbital (ml) and spin (ms) moments are determined directly in mB/atom with elemental specificity. The extended x-ray absorption fine structure (EXAFS) measurements yield site specific information on the local crystallographic structure.These measurements were performed using the circular x-rays of several beamlines. The influence of the degree of spatial source coherence lspat of the x-rays was characterized by means of Fresnel diffractometry. A correlation between enhanced XAS white line intensities and higher values of lspat was established for 20 ML Fe, Co, and Ni films on Cu(100).The degree of circularly polarized x-rays (Pc) at beamline D1011 at MAX-lab was characterized by studying Fe films on Cu(100) by means of XMCD. The maximum value of Pc is experimentally determined to Pc =0.85.The Au/Co/Au trilayer system was studied as a function of Co thickness, temperature, and Au cap thickness. A 10 mono-layer (ML) Co film, with an Au cap of 20 Å, shows a spin reorientation transition (SRT) from an in-plane to an out-of-plane easy direction as the temperature is lowered from 300 K to 200 K. The magnetic properities of these Co films are very different to what is found for bulk samples due to, in particular, the broken symmetry at the interfaces.The thickness dependent spin reorientation transition in the Fe/Ag(100) system was characterized by means of XMCD and EXAFS measurements. 3 ML Fe films show an out-of-plane easy direction with an 125% enhanced orbital moment as compared to the 25 ML Fe in-plane film. Simulations of the Fe L-edge EXAFS indicate the bulk Fe bcc structure for film thicknesses of 6-25 ML Fe. For 3 ML Fe strong deviations from this bcc phase is observed.Ultrathin Co films deposited on flat and vicinal Cu(111) in the thickness region 1-25 ML were studied by means of XMCD and scanning tunneling microscopy (STM). The vicinal Cu(111) Co deposition leads to the formation of elongated islands preferentially oriented along the step edges. In connection to this particular Co growth mode we observe an increase of both the orbital and the spin moment on the vicinal Cu(111) of about 25% relative to what was observed for Co on flat Cu(111).
Bibliographical Information:

Advisor:

School:Uppsala universitet

School Location:Sweden

Source Type:Doctoral Dissertation

Keywords:NATURAL SCIENCES; Physics; Physics; thin film; xmcd; exafs; anisotropy; coherence; circular polarization; magnetism; Fe; Co; Ag; Fysik; Physics; fysik

ISBN:91-554-5632-4

Date of Publication:01/01/2003

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