AVS2001 Session MI+NS-ThM: Magnetic Imaging and Spectroscopy

Thursday, November 1, 2001 8:20 AM in Room 110

Thursday Morning

Start	Invited?	Item
8:20 AM	Invited	MI+NS-ThM-1 Measurement of Spin Polarization using Andreev Reflection R.J. Soulen, M.S. Osofsky, G. Trotter (Naval Research Laboratory) Measurement of spin polarization using Andreev reflection A new class of electronics is emerging which relies on the ability of ferromagnetic materials to conduct spin polarized currents. The performance of devices based on this phenomenon is greatly enhanced as the spin polarization, P, of the ferromagnetic material approaches 100%. In the face of difficulties in measuring this important property, we have developed a very simple method to determine P in which a superconducting point is placed in contact with the candidate ferromagnetic material. The Andreev reflection process at the interface between the two metals is influenced by the spin polarization of the ferromagnet enabling the determination of P through measurement of the conductance of the contact. In a very short time we have been able to measure the spin polarization of several metals and conducting oxides: NixFe1-x; Ni. Co, Fe, NiMnSb, La0.7Sr0.3MnO3; CrO2, whose spin polarization ranges from 25% to over 90%. Our results compare well with other magnetic spectroscopy measurements of P where available. Our search continues for a material with 100% spin polarization.
9:00 AM	Invited	MI+NS-ThM-3 Point Contact Spectroscopy in Magnetic Fields M. Tsoi (IBM Almaden Research Center)
9:40 AM		MI+NS-ThM-5 Magnetocrystalline Anisotropy Probed using X-ray Magnetic Linear Dichroism S.S. Dhesi (ESRF, France); G. van der Laan (Daresbury Laboratory, UK); E. Dudzik (Hahn-Meitner-Institut, Germany); A.B. Shick (University of Davis, California) The anisotropy of the spin-orbit interaction,λ₂, in vicinal Co films has been measured using x-ray magnetic linear dichroism (XMLD). A linear increase in λ₂ with Co step density is found using a new sum rule and represents the first experimental confirmation that XMLD probes the magnetocrystalline anisotropy energy (MAE). X-ray magnetic circular dichroism (XMCD) is used to confirm that the XMLD arises from changes in the local step-edge electronic structure. The XMLD sum rule gives a larger MAE compared to macroscopic values and is discussed with respect to other local probes of the MAE.
10:00 AM	Invited	MI+NS-ThM-6 Soft X-ray Microscopy to Image Magnetic Domain Structures at High Resolution G. Schuetz (Universitat Wurzburg, Germany); P. Fischer (MPI-MF, Germany) X-ray magnetic circular dichroism (X-MCD) serves as huge element-specific magnetic contrast mechanism in combination with soft X-ray microscopy to image magnetic domains with a current lateral resolution down to 25nm. The sensitivity of X-MCD on the projection of the magnetization of the ferro(i)magnetic species along the photon propagation direction allows to study both in-plane and out-of-plane magnetized systems. The capability of this photon based microscopy to record the images in varying external magnetic fields and the high sensitivity down to thicknesses of a few nm is outlined and proofs this novel technique to be a promising tool for the study of the switching behaviour of individual layers in thin film magnetic media that are currently discussed (magnetic sensors, spintronic devices, etc.). Recent results obtained on nanostructured and multilayered systems will be presented together with micromagnetic simulations to get insight into the micromagnetic properties of these systems.
10:40 AM		MI+NS-ThM-8 Photoemission Electron Microscopy and X-Ray Magnetic Circular Dichroism of Ultrathin FeNi Alloy Films on Cu(111) Y. Sato, T.F. Johnson, S. Chiang (University of California, Davis); F. Nolting, A. Scholl (Lawrence Berkeley National Laboratory); X.D. Zhu, D.P. Land (University of California, Davis) We are studying the system of NiFe/Cu(111) to understand and control the surface/interface magnetism relevant to the application of the giant magnetoresistive effect to magnetic recording heads. We used the Photoemission Electron Microscope (PEEM2) at the Advanced Light Source to observe the domain structures of the alloy films. PEEM has the unique capability of imaging the film's magnetic structure with high spatial resolution and elemental specificity. Element specific magnetic contrast images and X-ray Magnetic Circular Dichroism (XMCD) spectra were obtained for eight different samples of varying Fe compositions at two different thicknesses. Samples with higher Fe content (x = 0.66, 0.74) were non-magnetic at room temperature. This trend of reduction in Curie temperature at higher Fe concentration agrees both with our XMLD data on the same system¹ and with previous work on FeNi/Cu(100).² We speculate this is a structure-driven effect related to the "Invar effect" in the bulk alloy. The PEEM images clearly show that Fe and Ni form a good alloy and have the same domain structures with their magnetization aligned. Further, we find a strong thickness and concentration dependence of the magnetic domain structures. For 5ML films, the domain structures appear to be strongly influenced by surface topography of the substrate. For 10ML films, however, the effect of the substrate features is already insignificant. At this thickness, the Fe concentration is also found to affect the size of the domains and the presence of an easy magnetization axis. ¹T.F.Johnson, S.Chiang, Y.Sato, et al., to be published ²F.O.Schumann, S.Z.Wu, G.J.Mankey and R.F.Willis Phys.Rev.B 56, 2668 (1997).
11:00 AM		MI+NS-ThM-9 Imaging Magnetization in MRAM Elements with Soft X-Ray Microscopy J.B. Kortright, G. Meigs, G.P. Denbeaux (Lawrence Berkeley National Laboratory); J.M. Slaughter, R. Whig, S.-I. Han (Motorola) The magnetic elements used to store information in MRAM devices will have dimensions of less than 1 micron laterally and roughly 5 nm in thickness. Such small dimensions make it difficult to directly observe field-dependent magnetization structure in individual elements, and possible interactions between elements, by conventional magnetic microscopy techniques. We are using scanning and imaging soft x-ray microscopes based on zone-plate lenses (with resolution approaching 30 nm) and resonant magnetic circular dichroism contrast to image magnetization structure during reversal in arrays of lithographically patterned bits on SiNx membrane substrates. Remnant magnetization structure and its evolution through reversal are clearly resolved, as is the dependence of this structure on element size, shape and cyclic reversal. Following a brief review of techniques, microscopy results relevant to MRAM applications and comparisons with micromagnetic theory will be presented. Work at LBNL was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Work at Motorola Labs was partially funded by DARPA.
11:20 AM	Invited	MI+NS-ThM-10 Imaging Magnetic Nanostructures by Spin-Polarized Scanning Tunneling Microscopy M. Bode, A. Kubetzka, O. Pietzsch, M. Kleiber, R Ravli@aa c@, R. Wiesendanger (University of Hamburg, Germany) Our recent progress in spin-polarized scanning tunneling microscopy (SP-STM) will be reviewed. By using magnetic thin film tips and spectroscopic techniques we could image the surface spin-structure of different surfaces and ultrathin films with a spatial resolution down to the atomic level. Namely, we will present results obtained on the topological antiferromagnet Cr(001),¹ on self-organized Fe-nanowires² and -islands,³ and on the antiferromagnetic monolayer of Mn/W(110).⁴ We will demonstrate that in-plane and out-of-plane spin-contrast can be achieved by choosing appropriate magnetic tip coatings and that the use of an antiferromagnetic tip material avoids any influence of the tip´s magnetic stray field on the sample´s domain structure. In contrast to most other electron-based microscopic techniques SP-STM as a near-field method can be applied even in large external magnetic fields up to several Tesla allowing the investigation of hysteresis effects in magnetically hard materials. ¹ ¹ M. Kleiber et al., Phys. Rev. Lett. 85, 4606 (2000). ² O. Pietzsch et al., Phys. Rev. Lett. 84, 5212 (2000). ³ A. Kubetzka et al., Phys. Rev. B 63, 140407 (2001). ⁴ S. Heinze et al., Science 288, 1805 (2000).