<p> Magnetic property of the symmetric Double Exchange Spring System, soft(S)/hard(H)/soft(S) ferromagnetic layers NiFe (Py)/SmFe/NiFe (Py), was investigated using Magneto Optical Kerr Effect (MOKE) measurement. Exchange spring magnet, H/S bilayer, shows a unique magnetic hysteresis curve due to the non-collinear magnetization developed by magnetic coupling of the two layers. In order to produce a symmetric non-collinearity in magnetization, the thicknesses of the two soft layers are controlled to be the same during the deposition. Due to the finite skin depth of MOKE measurement magnetic hysteresis loop for each soft layer could be measured separately by adjusting the right thickness of the layers. To measure the bottom soft layer transparent glass substrate was used for thin film deposition. We first observed an asymmetric hysteresis loop of the double exchange spring system by MOKE measurement. This is due to the existence of quadratic MOKE (QMOKE) signal caused by the in-plane magnetic anisotropy. Linear MOKE (LMOKE) and QMOKE signal were separated from the general MOKE signal using a symmetry operation. In general, the samples that have the induced easy axis have less significant QMOKE signal than those that do not have the easy axis. Second, from the LMOKE measurement, we found that the magnetic hysteresis loops for the bottom and the top S layers are not the same. The magnetic hysteresis loop from LMOKE measurements data was compared with the one measured by Alternating Gradient Magnetometer (AGM). AGM measures the total magnetization of the whole structure. Our data shows that the coercivity of the bottom S measured from LMOKE is closer to the coercivity of the first switching measured from AGM and is much smaller than that of the top S from MOKE. The coercivity of the top S from LMOKE is closer to that of the second switching from AGM. This indicates that the top S is, to some extent, strongly coupled with the hard layer causing them to switch together; while the bottom S is much less coupled so that its switching is spiral. In other words, the bottom S provides the non-collinearity as we expected from exchange spring magnets, while the top S does not. We found experimentally that within an acceptable uncertainty, the non-collinearity provided by the two soft layers is not symmetric despite the symmetry of the structure. A possible explanation of this general trend is that it is the difference in the surface roughness between the bottom Py/SmFe surface and the top Py/SmFe surface that causes the asymmetric non-collinearity, since we found that the interface between the top Py and the SmFe is rougher than the one between the bottom Py and the SmFe. Further investigation needs to be done to experimentally explain how the difference in the surface roughness affects the magnetic coupling between the soft and hard magnetic layers. </p>
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:1527351 |
| Date | 08 April 2014 |
| Creators | Yuan, Hanming |
| Publisher | California State University, Long Beach |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
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