Investigators: A. J. Shapiro, R. D. Shull, V. I. Nikitenko (Guest Researcher), and V. S. Gornakov (Guest Researcher)
Objectives:
The primary goal of this project is one of the fundamental issues in magnetism - understanding of the microscopic mechanisms of magnetization reversal in quasi-two-dimensional nanocomposite magnetic materials. The advanced magneto-optical indicator film (MOIF) imaging technique developed by NIST and the Institute for Solid State Physics, Russian Academy of Sciences (ISSP RAS) is a simple, fast, illustrative, and sensitive tool for nondestructive and real-time visualization and characterization of materials magnetic microstructures at the micron scale. It opens perspectives for studies and solving practical and fundamental problems of materials science. In the last few years there has been a great deal of interest in studying the unique properties of nanocomposite materials. They may consist, for instance, of ultra thin layers. Thicknesses of such films are comparable to the domain wall width. In the films also a coexistence of ferromagnetic and antiferromagnetic exchange interactions is possible. This magnetic system is attractive for use as a memory element in future generation computers. For coupled magnetic systems, in particular, detailed knowledge of the magnetization reversal processes holds the key to unlocking the potential for practical applications. In addition, the advanced MOIF technique also can be used as a rapid non-destructive method for monitoring thin film quality during production.
Technical Description:
In the MOIF technique, a Bi-substituted yttrium iron garnet transparent film with in-plane anisotropy is placed on the top of a sample. Since the magneto-static field of the sample under study alters the magnetization of the film, it allows utilization of the film as an indicator of the magnetic field. When polarized light passes through the indicator film and is reflected back by an Al underlayer it undergoes the magneto-optic Faraday rotation which is proportional to a component of the magnetic stray field perpendicular to the indicator film plane. The interaction of the polarized light is imaged in a polarizing microscope. This allows visualizing, for example, the magnetization reversal processes, domain wall nucleation and motion, Bloch's lines, and crystal defects (e.g., dislocations, voids and sample edges) in magnetic materials.
The MOIF technique was applied to the investigation of the microscopic mechanisms in magnetization processes in three different types of materials: soft ferromagnet/hard ferro-magnetic, ferromagnet/antiferromagnetic (FM/AF) bilayers, and magnetic superlattices with nonmagnetic spacers.
In permanent magnets consisting of hard and soft magnetic phases coupled at the interfaces, the magnetization reversal limits the maximum achievable energy product. The hard magnetic phase provides the high anisotropy and coercive fields while the soft-magnetic phase enhances the magnetic moment. The soft-phase magnetization reversal occurs due to forming or unwinding of an exchange spring similar to the one which exists in a domain wall. Direct experimental studies of such phenomena were not available until now. The MOIF technique allows experimental investigation of the magnetization reversal process in exchange spring magnets such as thin film Sm-Co/Fe bilayer and Sm-Co/Co superlattice structures.
In the last few decades there has been a great deal of interest in studying the unique properties of exchange coupled ferromagnetic and antiferromagnetic ultra thin layers. Such a magnetic system is attractive for use as computer memory elements. For such an application, an understanding of the remagnetization process of these elements is mandatory. To date, the investigation of the microscopic magnetization reversal mechanisms in these systems has revealed drastic contradictions between theory and experiment in both the values of exchange bias field HE and coercive force HC. It is now well recognized that the understanding of the exchange-biased FM/AF thin film lies in the peculiarities of nucleation and motion of domain walls of both the constituent FM and AF layers. However, detailed experimental study of the domain wall dynamics of the FM/AF bilayers has been severely hampered by the complicated multi-domain structure which occurs during switching, occurring within a narrow field range, from one single-domain state to another with an opposite magnetization. Recently, macroscopic domain structures in an exchange-coupled bilayer of wedged-permalloy (Py=Ni81Fe19)/uniform-FeMn (Fe50Mn50), has been realized by exploiting the inverse dependence of the HE on the FM layer thickness. The magnetization switching process involves only two macroscopic domains. Taking advantage of this unusually simple domain pattern, we have used the magneto-optical indicator film (MOIF) technique to investigate the macroscopic mechanisms of the domain wall nucleation and motion.
Magnetic superlattices with nonmagnetic spacers can exhibit the giant magnetoresistance (GMR) effect due to inversion of the magnetization direction in adjacent layers in an external magnetic field. Utilization of this effect allows developing new devices for noninductive reading of magnetically recorded information. Spin reorientation phase transformations are responsible for the magnetization processes in magnetic superlattices with nonmagnetic spacers. We studied the correlation between the magnitude of the GMR and the micromechanism of the magnetization reversal of electrodeposited Co/Cu superlattices for a range of Cu spacer thicknesses.
Planned Outcome:
This project will contribute to the fundamental understanding of the micromechanism of the magnetization reversal processes in >quasi-two-dimensional nanocomposite magnetic materials. The MOIF imaging technique will be available for industrial applications: non-destructive quality magnetic materials control, research and development of new magnetic materials, domain wall behavior in ultra-thin layer structures, etc.
External Collaborations:
We conducted MOIF investigations as a part of collaborative studies of the following materials with the indicated companies and universities:
Accomplishments:
For the first time the remagnetization process was observed directly using the MOIF technique in epitaxial Sm-Co (35 nm)/Fe(50 nm) bilayer films, grown on Cr (20 nm) buffered MgO (100) substrate by sputtering. The macroscopic parameters of magnetization reversal were measured in a SQUID magnetometer. It revealed characteristic exchange-spring behavior where the reversal of the soft Fe layer is pinned at the interface by the SmCo hard layer. In order to investigate the magnetic spin rotation process inside the bilayer during remagnetization, a 0.3 mm hole was made in the sample, and the magnetostatic field (Hms) around the hole was visualized through the intensity changes of the double Faraday effect in a transparent indicator film with in-plane anisotropy. Black and white contrast on opposite sides of the microhole was observed, indicating the direction of magnetization in the sample around the hole. Analysis of the spin rotation processes in the soft ferromagnetic component during remagnetization have shown that spins rotate uniformly when the magnetic field was applied slightly off either side of the easy axis. When the field was aligned with the easy axis, no uniform spin rotation was observed. During rotational hysteresis, a unique spin behavior was observed: spin rotation was discovered to change sign without an accompanying change in the sign of the field rotation.
The features of the magnetization reversal processes in Permalloy (2 nm to 30 nm wedge)/ FeMn(30 nm)/Cu(30 nm)/Si bilayer system with the exchange anisotropy induced either perpendicular or parallel to the wedge direction were studied by an advanced MOIF technique and Vibrating Sample Magnetometry (VSM). When the exchange anisotropy is established perpendicular or parallel to the wedge direction, two macroscopic domains are observed. These domains are separated by a 180-degree wall in the perpendicular geometry and an intermediate band containing a large density of stripe-type microdomains in the parallel geometry. While the exchange field remains practically the same in both geometries, the coercivity and squareness of the loop are much less in the parallel geometry. Real-time magneto-optical indicator film images reveal distinct asymmetry in the motion of a single domain wall in a wedged-NiFe/uniform-FeMn bilayer due to the nucleation and behavior of an exchange spring in the antiferromagnetic layer. Magnetization reversal from the ground state begins at the thick end of the wedge where the exchange anisotropy field (HE) is minimal and the magnetostatic field (HMS) is maximal, whereas reversal into the ground state begins from the thin end where HE is maximal and HMS is minimal. We have directly observed macroscopic domain structures in a wedged- Permalloy / uniform-FeMn exchange-coupled bilayer with the anisotropy direction perpendicular to the wedge direction. Magnetization reversal from a fully magnetized state starts at the thick corners of the wedge where the exchange energy is minimal and the magnetostatic energy is maximal. The two edge domains then join and form a macroscopic reversal domain, separated from the original domain by a 180-degree macroscopic domain wall. The domain wall propagates toward and eventually vanishes at the thin end of the wedge where the exchange energy is maximal and the magnetostatic energy is minimal. The observed asymmetry in the domain wall motion, incompatible with a static AF spin structure, indicates the presence of a mobile domain wall (exchange spring) in the antiferromagnet.
We have investigated the elementary events of the magnetization reversal processes in anisotropic ferro- and antiferromagnetic superlattices [Co(1.6 nm)/Cu(dCu)]200 using MOIF and VSM techniques. Magnetization processes were analyzed through the intensity change due to the stray field at the edge of the multilayer, around domain walls and the deliberately created microhole. Specimens with different thicknesses of the non-magnetic spacer (dCu was varied in a range from 0.5 nm to 4 nm) were electrodeposited on the Si(001) substrates which were covered with a thin evaporated Cu seed layer. They were characterized by inplane fourfold symmetry of the magnetization and magnetoresistance. Peculiarities in the nucleation and motion of domain walls in superlattices with different types of interlayer exchange coupling between adjacent ferromagnetic layers were revealed. Nonuniform spin-reorientation transformations in the antiferromagnetic superlattice result in either a collinear or noncollinear magnetic phase, depending on the applied field direction. It was shown that the magnitude of the giant magnetoresistance effect (GMR) depends on the micromechanism of the magnetization reversal in electrodeposited Co/Cu superlattices.
Impact:
The advanced magneto-optical indicator film imaging technique developed by NIST and ISSP RAS is currently widely used in industrial and university laboratories in the USA and around the world for direct observation of the magnetization reversal processes in variety of magnetic materials. The results of our investigations have been transferred to our collaborators at the Argonne National Laboratory, Argonne, IL, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, and Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD.
Outputs:
Publications:
Nikitenko, V. I., Gornakov, V. S., Dedukh, L. M., Kabanov, Yu. P., Khapikov, A. F., Shapiro, A. J., Shull, R. D., and Chaiken, A. "Asymmetry of the Remagnetization Processes in Exchange-biased NiFe/NiO Bilayers," Journal of Magnetism and Magnetic Materials, 198-199, 500, 1999.
Nikitenko, V. I., Gornakov, V. S., Dedukh, L.M., Kabanov, Yu. P., Khapikov, A. F., Moffat, T. P., Shapiro, A. J., Shull, R.D., Shima, M., and Salamanca-Riba, L., "Direct Experimental Study of the Microscopic Remagnetization Mechanism in Co/Cu magnetic superlattices," Journal of Magnetism and Magnetic Materials, 198-199, 477, 1999.
Nikitenko, V. I., Gornakov, V. S., Dedukh, L. M., Shapiro, A. J., Shull, R. D., and Chaiken, A., "Influence of Crystal Lattice Defects on Domain Wall nucleation and Motion in Exchange-bias Films," Mat. Res. Soc. Symp. Proc. Vol. 517, 43, 1998.
Nikitenko, V. I., Gornakov, V. S., Shapiro, A. J., Shull, R. D., Kai Liu, Zhou, S. M., and Chien, C. L, "Asymmetry in Elementary Events of Magnetization Reversal Ferromagnetic/Antiferro-magnetic Bilayer," PRL, in press.
Jiang, J. S., Fullerton, E. E., Sowers, C. H., Inomata, A., Bader, S. D., Shapiro, A. J., Shull, R. D., Gornakov, V. S., and Nikitenko, V. I., "Spring Magnet Films," IEEE Trans. on Magn., in press.
Kai Liu, Zhou, S. M., Chien, C. L., Nikitenko, V. I., Gornakov, V. S., Shapiro, A. J., and Shull, R. D., "Anisotropy-Dependent Macroscopic Domain Structure in Wedged-Permalloy / Uniform-FeMn Bilayers," JMMM, in press.
Presentations:
Shull, R. D., Shapiro, A. J., Nikitenko, V. I., and Gornakov, V. S., "Domain Behavior in Magnetic Nanostructures as Revealed by MOIF Observations," TMS Annual Meeting, San Diego, CA, February 1999.
Hilt, Z., Shapiro, A. J., Shull, R. D., Nikitenko, V. I., Gornakov, V. S., Jiang, J. S., Sowers, C.H., Inomata, A., and Bader, S. D., "Magneto-Optical Indicator Film Investigation of the remagnetization Behavior of Exchange Spring Magnets," MRS Spring Meeting, San Francisco, CA, April 1999.
Jiang, J. S., Fullerton, E. E., Sowers, C. H., Inomata, A., Bader, S. D., Shapiro, A. J., Shull, R. D., Gornakov, V. S., and Nikitenko, V. I., "Spring Magnet Films," INTERMAG 99, Kyongju, Korea, May 1999.
Nikitenko, V. I., Shapiro, A. J., Gornakov, V. S., Shull, R. D., Kai Liu, Zhou, S. M., and Chien, C. L., "Domain Wall Motion and Nucleation in the Wedge-Shaped FeNi/FeMn Bilayers with Unidirectional Anisotropy," APS Meeting, Atlanta, GA. March 1999.
Gornakov, V. S., Nikitenko, V. I., Shapiro, A. J., and Shull, R. D., "Magnetic Domain Formation on Dislocations in the Antiferromagnetic layer of Exchange Biased Bilayer Films," APS Meeting, Atlanta, GA, March 1999.
Shull, R. D., Shapiro, A. J., Brown, H. R., Nikitenko, V. I., Gornakov, V. S., Jiang, J. S., Inomata, A., Sowers, C. H., and Bader, S. D., "Magnetization Reversal In Exchange Spring Magnets Observed Directly With A Magneto-Optical Indicator Film," Dresden, Germany, August 1999.
Kai Liu, Zhou, S. M., Chien, C. L., Nikitenko, V. I., Shapiro, A. J., Gornakov, V. S., and Shull, R. D., "Anisotropy-Dependent Macroscopic Domain Structure in Wedged-Permalloy / Uniform-FeMn Bilayers," 44th Annual Conference on Magnetism and Magnetic Materials, MMM'99, San Jose, CA, November 1999.
Gornakov, V. S., Nikitenko, V. I., Shapiro, A. J., Shull, R. D., Gökemeijer, N. J., and Chien, C. L., "Direct Observation of Antiferromagnetic Domains and Magnetization Processes in an Exchange-Biased Permalloy/FeMn Bilayer," 44th Annual Conference on Magnetism and Magnetic Materials, MMM'99, San Jose, CA, November 1999.