AVS1997 Session MI-WeM: Micromagnetics: Modeling and Imaging

Wednesday, October 22, 1997 8:20 AM in Room J3

Wednesday Morning

Start	Invited?	Item
8:20 AM	Invited	MI-WeM-1 Development of the Second-Harmonic Magneto-Optic Kerr Effect as an Interfacial Magnetometry Tool T.J. Silva (National Institute of Standards & Technology); T.M. Crawford, C.T. Rogers (University of Colorado, Boulder) We will review the current state of the second-harmonic magneto-optic Kerr effect (SH-MOKE). We will first present a brief history, from the initial theoretical predictions of SH-MOKE to the application of the effect to perform magnetometry on technologically-relevant materials and devicesfoot1-6. Special attention will be paid to the surprising enhancement (~1000x) to contrast found with SH-MOKE over its linear analog. The enhancement was not predicted by inital theoretical considerations. There is a rapidly growing database of materials which exhibit enhanced SH-MOKE contrast (Fe, Co, Ni, NiFe, PtMnSb, CoZrMo, FeN), suggesting the need for a universal explanation for the observed magnitude of the effect foot7-9. Studies of Co/Au multilayers and our own investigation of SH-MOKE's dependence on film thickness and surface oxide growth for Permalloy (Ni₈₁Fe₁₉) provide strong evidence for the interfacial sensitivity of SH-MOKEfoot10. As such, SH-MOKE presents itself as a very sensitive tool for studies of interfacial magnetism which may be applied to a wide variety of magnetic materials, including many which are of technological interest to the data storage community. We have studied lithographically-defined spin-valve structures and have found that SH-MOKE has the unique ability to permit magnetometry on a single magnetic interface in a multilayer structurefoot11. Recently, we have expanded the practical use of SH-MOKE to include dynamical studies of magnetization reversal in Permalloy filmsfoot12. We will discuss the practical considerations for the measurement of second-harmonic generation from metallic films, including the technical requirements for ultrafast laser pulses , the sensitive detection of the weak optical second harmonic signal, and the role played by the angle of incidence. We will also present the results of our attempts to increase the spatial resolution of the technique and how the optical damage threshold of any given material sets the fundamental limits for SH-MOKE. ¹R. J. Romagnoli, P. E. Ferguson, Optica Acta, vol. 18, pg. 191 (1971). ²R.-P. Pan, H. D. Wei, Y. R. Shen, Phys. Rev. B, vol. 39, pg. 1229 (1989). ³W. Heubner, K.-H. Bennemann, Phys. Rev. B, vol. 40, pg. 5973 (1989). ⁴T. M. Crawford, C. T. Rogers, T. J. Silva, Y. K. Kim, Appl. Phys. Lett., vol. 68, pg. 1573 (1996). ⁵T. M. Crawford, C. T. Rogers, T. J. Silva, Y. K. Kim, IEEE Trans. Magn., vol. 32, pg. 4087 (1996). ⁶T. M. Crawford, C. T. Rogers, T. J. Silva, Y. K. Kim, J. Appl. Phys., vol. 81, pg. 4354 (1997). ⁷J. Reif, J. C. Zink, C.-M. Schneider, J. Kirschner, Phys. Rev. Lett., vol. 67, pg. 2878 (1991). ⁸G. Spierings, V. Koutsos, H. A. Wierenga, M. W. J. Prins, D. Abraham, Th. Rasing, J. Magn. Magn. Mat., vol. 121, pg. 109 (1993). ⁹J. Reif, C. Rau, E. Matthias, Phys. Rev. Lett., vol 71, pg. 1931 (1993). ¹⁰T. J. Silva, T. M. Crawford, C. T. Rogers, Y. K. Kim, OSA Technical Digest, vol. 11, pg. 299 (1996). ¹¹T. M. Crawford, C. T. Rogers, T. J. Silva, Y. K. Kim, IEEE Intermag '97 Digest, pg. EA-09 (1997). ¹²T. M. Crawford, C. T. Rogers, T. J. Silva, Y. K. Kim, This AVS meeting (1997).
9:00 AM		MI-WeM-3 Surface Magnetization Dynamics in NiFe Thin Films T.M. Crawford, C.T. Rogers (University of Colorado, Boulder); T.J. Silva (National Institute of Standards & Technology) As disk-drive areal densities rise, driving frequencies for heads and media must also increase. The fundamental magnetization switching time in the constituent materials will eventually limit the attainable frequency. Previous measurements of switching speeds in NiFe, an important head material, were performed in the 1950’s and 1960’s using an inductive technique, and demonstrated switching speeds of ~1 ns, with a time resolution of 350 ps¹. Recently, ultrafast pump-probe experiments have been used to study bulk response times in thin-film heads, with 50 ps time-resolution, by stroboscopically probing the magnetization directly using the polar magneto-optic Kerr effect². As multilayered devices, such as the spin valve, are developed as read sensors, there is a necessity to measure and characterize the magnetic response of surfaces and buried interfaces. Experiments on NiFe and spin valve component interfaces, using the second-harmonic magneto-optic Kerr effect (SH MOKE), have demonstrated its sensitivity to surface and interface magnetism ³^,⁴. We have now measured the magnetization dynamics at the surface of a longitudinally-magnetized NiFe thin film in a pump-probe experiment, using SH-MOKE as the probe. We have observed the free-induction decay of the surface magnetization with ~80 ps resolution, using both microstrip lines and photolithograpically patterned coplanar waveguides. The different waveguide dimensions allow variable field-pulse amplitudes, ranging from 8 A/m (0.1 Oe) to >8 kA/m (100 Oe). We will discuss the effects of field amplitude and film thickness on the NiFe time-response and damping constant. ¹W. Dietrich, W. E. Proebster, and P. Wolf, I.B.M. J. Res. Dev. 4, 189, 1960. ²M. R. Freeman, A. Y. Elezzabi, and J. A. H. Stotz, J. Appl. Phys. 81, 4516, 1997. ³T. M. Crawford, C. T. Rogers, T. J. Silva, and Y. K. Kim, Appl. Phys. Lett. 68, 1573, 1996. ⁴T. M. Crawford, C. T. Rogers, T. J. Silva, and Y. K. Kim, J. Appl. Phys. 81, 4354, 1997.
9:20 AM		MI-WeM-4 Wide-Field Kerr Characterization of Magnetic Tunnel Junctions P.L. Trouilloud (IBM T.J. Watson Research Center); Yu Lu (IBM T.J. Watson Research Center and Brown University); D.W. Abraham, R.A. Altman, W.J. Gallagher (IBM T.J. Watson Research Center); A.C. Marley, S.S.P. Parkin (IBM Almaden Research Center) The switching behavior of small magnetic tunnel junctions as measured by their magnetoresistance displays evidence of complex domain/micromagnetic activity. This behavior is consistent over a range of dimensions from 50µm to 1µm devices. In order to clarify the micromagnetic mechanisms active in these magnetoresistive (R vs H) loops, we have measured Kerr (M vs H) loops for devices with widths down to 2.5µm. Kerr loops and vectorial magnetization maps were reconstructed from wide-field longitudinal Kerr images. The vectorial reconstruction was necessary to visualize the more complex vortex-like structures observed in these devices. Single or averaged loops were measured at up to video-frame rates with applied hard-axis bias fields. As in the R vs H data, the Kerr images show more reproducible magnetic behavior on one edge of the hysteresis loops. As found in micromagnetic computations, 360 degree walls are often present and require large fields to be eliminated. If the field excursion is not large enough, magnetic reversal is observed to nucleate from the 360 degree walls rather than from the edges of the device.
9:40 AM		MI-WeM-5 X-ray Magnetic Circular Dichroism Imaging of Domains Caused by Anomalous Uniaxial Anisotropy in Co/Cu(001) T. Droubay (University of Wisconsin, Milwaukee); G. De Stasio (CNR, Italy); B.P. Tonner (University of Wisconsin, Milwaukee) X-ray magnetic circular dichroism imaging has been used to study the domain structure formed in ultra-thin films of Co grown on Cu(100). These films exhibit a uniaxial in-plane anisotropy, which breaks the four-fold symmetry of the substrate. A possible origin of this uniaxial anisotropy is the presence of steps and step-bunches, which could result from small mis-alignments of the sample surface with the [001] direction. We have characterized the magnetic anisotropy of a particular crystal using element-specific XMCD magnetometry, and subsequently investigated the magnetic domain structure using an x-ray photoelectron emission microscope (X-PEEM) with circularly polarized radiation at the Co 2p edge. The images show 'stripe' domains with an extreme aspect ratio; widths of about 5 microns but hundreds of microns long. Images taken after applying fields along and perpendicular to the uniaxial asymmetry axis support the conclusion that these domains are associated with step bunches. We will show the relationship between the magnetic and principle crystal axes and explain the prominent features in the hysteresis curves by correlation with the magnetic domain micrographs.
10:00 AM		MI-WeM-6 Magnetic Force and Dissipation Microscopy P.H. Grutter, Y. Liu (McGill University, Canada) An implicit assumption often made in magnetic force microscopy (MFM) is to neglect any distortions of the sample micromagnetic domain structure as a result of the tip stray field. We have developed a method allowing the quantification of this influence by directly monitoring the magnetic energy transferred between the tip and the sample in a standard vacuum AC-MFM by measuring changes in the damping of the oscillating tip. We measure this magnetic dissipation with a sensitivity of better than 10^-18 W and a resolution at least as high as the simultaneously acquired standard constant force gradient MFM images. Damping contrast is strongly correlated with micromagnetic structure. On some samples (such as NiFe, 4 nm thin Co films, terfenol-D) the damping losses are in quantitative agreement with magnetoelastic losses in the sample. On magnetic recording tracks, very large damping signals are observed. This has direct consequences in the interpretation of traditional MFM images acquired with detectors that convolute frequency and damping information.
10:20 AM		MI-WeM-7 Mapping Local Susceptibility using a Scanning Co-axial Write/Read Head. V. Ström, K.V. Rao (Royal Institute of Technology, Sweden) Susceptometry has always been concerned about bulk properties, and since this means that the measured quantity is an average over the sample volume, there are comparatively few investigations of the directional dependency of susceptibility. In this work we present a new approach to study local magnetic susceptibility of planar samples. This technique is very suitable for mapping directions and values of the "elastic", and "lossy" (dissipative) suspectibility and their dependencies on frequency and field strength. A somewhat related and new technique which also yields local information is the 'magnetic extension', MFM, of the conventional STM and AFM microscope wherein the probing tip is magnetic and interacts magnetically with the sample. Unfortunately, this means that the measured quantity is related to the "tip-sample" system, which in general could be rather complicated, since the tip's magnetization can be effected by the sample and vice versa. Our local susceptibility measurement approach essentially breaks this interdependence between the sample and the sensing probe which is a great asset. The probe in our technique consists of two inductive heads with their gap directions perpendicular to each other and parallel to the sample surface. Such a probe is sensitive to the difference in the susceptibility for two orthogonal directions. With two mappings using the Head in two positions at 45° between the scans, we demonstrate a method to map "the direction and the value of the local uniaxial susceptibility" on a surface. Examples of our studies on samples with considerable uniaxial susceptibility like a magnetic strip on a credit card, and isotropic floppy disk with disoriented g-Fe₂O₃ particles will be presented. To further illustrate the capabilities of our approach we monitor and explain the changes in the susceptibility scans on a metglas 2605SC ribbon subjected to a static field with different strengths and directions. At present, the Head gap-width limits the spatial resolution of the extracted magnetic information. A theoretical scheme to extract the strength and direction of the local susceptibility for structures with uniaxial anisotropy will also be presented.
10:40 AM		MI-WeM-8 Quantitative Analysis of Data Tracks in Magnetic Recording Media by Magnetic Force Microscopy. A. Moser, M.E. Best, D.K. Weller (IBM Almaden Research Center); T. Minvielle, P. Baumgart (IBM Storage Systems Division) We present a quantitative analysis method of magnetic force microscope (MFM) images of written data tracks in longitudinal magnetic recording disks. Since an MFM tip provides much better spatial resolution than a magneto-resistive element in the read gap of a recording head, the analysis of MFM images provides complementary information about media noise sources. Of particular importance are quantities like cross track correlation width (W_C), transition width (W) and transition jitter (sigma) which we derive in an automated fashion using the image processing software PV-Wave. All data are extracted by taking various averages over many bit transitions and comparing the averages with the original data. The MFM measurements were carried out with a DI 5000 instrument. Results will be presented for data patterns written at different densities and in different media. Statically derived signal to noise ratios will be compared with respective dynamic data. Finally, we will compare our method with that suggested by Glijer and Suzuki ¹. ¹P. Glijer and T. Suzuki, J. Mag. Soc. Jap, Vol. 20, No. 5 (1996), 905 - 911
11:00 AM	Invited	MI-WeM-9 Micromagnetic Behavior in GMR Multilayers Devices J.-G. Zhu, Y. Zheng (Carnegie Mellon University) Magnetoresistance in GMR multilayers depends on relative magnetization orientation between adjacent layers. In GMR multilayer devices, including spin valve device, domain configurations can be very complex due to intra-layer and inter-layer magnetic interactions. Complicated domain configurations and magnetization switching processes can cause performance degradation and instability. This paper concerns micromagnetic behavior in spin valve, pseudo spin valve and multilayer devices with various geometries. Magnetization processes and switching properties are characterized via dynamic micromagnetic modeling. The model combines the classic micromagnetic theory with the Landau-Lifshitz-Gilbert equations. This dynamic approach allow us to study transient states of magnetization processes, including formation of domain walls and nucleation of new domains. In addition to the effect of various geometric structures, correlations with device microstructure and film intrinsic magnetic properties have been systematically investigated.
11:40 AM		MI-WeM-11 Anisotropy Design in Magnetic Media: A Micromagnetics Study J.H. Kaufman, T. Koehler, A. Moser, D.K. Weller, B.A. Jones (IBM Almaden Research Center) We have investigated the characteristics of model magnetic media using different strategies for the "anisotropy design". We modeled media with intra-grain anisotropy oriented random in the plane of the media, random in 3-d, and aligned along the track direction, and for a large range of anisotropy field Hk and intergranular exchange J. We report a possible scaling function which collapses data for all three geometries onto a single universal curve. The rescaled function was then analyzed along with the signal and noise functions from the various model materials. For any geometry we find a relation between exchange and anisotropy which maximizes the signal to (media) noise function.