한국자기학회

(Department of Materoals Engineering, Chungnam National University), Toshihiro Shoyama(Department of Electronic Engineering, Tohoku University), Masakiyo Tsunoda(Department of Electronic Engineering, Tohoku University), Migaku Takahashil(Department of Electronic Engineering, Tohoku University), Tae-Hyo Lee(Department of Materials Engineering, Chungnam National University), Chong-Oh Kim(Department of Materials Engineering, Chungnam National University)

Abstract

Local M-H loops have been measured on the free layer of a tunneling magnetoresistance (TMR) junction using the magneto-optical Kerr effect (MOKE) system, with an optical beam size of about 2 µm diameter. Tunnel junctions were deposited using the DC magnetron sputtering method in a chamber with a base pressure of 3×10^-9 Torr. The relatively irregular variations of coercive force H_c(∼17.5 Oe) and unidirectional anisotropy field H_ua(∼7.5 Oe) in the as-deposited sample are revealed. After 200℃ annealing, H_c decreases to 15 Oe but H_ua increases to 20 Oe with smooth local variations. Two-dimensional plots of H_c and H_ua show the symmetric saddle shapes with their axes aligned with the pinned layer, irrespective of the annealing field angle. This is thought to be caused by geometric effects during deposition, together with a minor annealing effect. In addition, the variation of root mean square (RMS) surface roughness reveals it to be symmetric with respect to the center of the pinned-layer axis, with the roughness of 2.5Å near the edge and 5.8Å at the junction center. Comparison of surface roughness with the variation of H_ua suggests that the H_ua variation of the free layer is well described by dipole interactions related to surface roughness. As a whole, the reversal magnetization is not uniform over the entire junction area and the macroscopic properties are governed by the average sum of local distributions.

Keywords: Coercivity; Exchange coupling; MOKE; Surface roughness; TMR

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