6 March 2000
Investigators: R. D. McMichael, A. J. Shapiro, H. J. Brown, D. E. Mathews, Gino Serpa, Chan-Gyu Lee, and Jason Eicke
Objectives:
This project seeks to provide measurement methods, computational methods, and data on the thermal stability, exchange biasing and micromagnetics of thin magnetic films to the magnetic recording, magnetic sensor, and other magneto-electronic industries.
Technical Description:
The technical area addressed by this project includes the processing and micromagnetics of thin magnetic films. Specifically, this project is concerned with the thermal stability of "spin valve" multilayer films, the micromagnetics of thin films, including the ferromagnet/antiferro-magnet interface in exchange biased layers, and dynamic measurements of thin magnetic films. In FY1999, the emphasis has been on exchange biasing and micromagnetics.
The exchange biasing effect is technologically important for pinning the magnetization of thin films, and it depends on the micromagnetic spin configuration at and near the interface between the ferromagnetic film and an antiferromagnet. Measurement methods and meaningful characterization of the exchange biasing and associated effects are important for device design using currently available materials and materials with stronger exchange bias that will be needed in the future.
Micromagnetic modeling techniques are also developed and evaluated for predicting hysteretic behavior and magnetic domain configurations in small elements patterned from thin films and multilayers. Control of domain configuration is important to the design of linear, low-noise read heads and other sensors, and to the control of coercivity in memory elements.
Planned Outcome:
At the conclusion of this project, measurement methods, models and data will be available for industry to evaluate different schemes for magnetization control in magnetic thin films used in devices. Micromagnetic models and measurement methods will be available for exchange biasing materials selection, and micromagnetic computational methods and domain control methods will be available for device design.
External Collaboration:
On exchange biased materials, Mark Stiles, PL, NIST, collaborated on theoretical modeling of exchange F/AF interfaces and exchange bias effects. Seagate provided specially deposited samples for exchange bias and rotatable anisotropy measurements. On micromagnetics, the Center for Theoretical and Computational Materials Science (MSEL) supported a micromagnetics workshop and graduate student help. Mike Donahue and Don Porter, NIST, ITL, are the main authors of the OOMMF public micromagnetics software, and have contributed solutions to micromagnetic standard problem #2. The Institute for Magnetics Research, George Washington University, collaborated on micromagnetic modeling of domain wall trap structures and contributed to OOMMF public code. IFW of Dresden, Germany, INESC, of Lisbon, Portugal, and Max-Planck-Institut fur Metallforschung of Stuttgart, Germany contributed solutions to micromagnetic standard problems #2 and #3.
Accomplishments:
A method for determining the hysteretic, "unused" fraction of the ferromagnet/antiferro-magnet interaction in exchange biasing applications was refined and tested on a wider range of materials. In exchange biasing applications, stable antiferromagnetic spin configurations give rise to an effective field, the exchange bias field. Unstable antiferromagnetic spin configurations give rise to less-desirable hysteretic effects. Following up on the initial technique development and measurements on NiO-biased thin films in FY1988, we have applied our ferromagnetic resonance technique to NiMn-biased films provided by Seagate and to CoO-biased films prepared at NIST. We find that the presence of an unstable fraction is common to all these films. The NiO work was published in Phys. Rev. B, the CoO work has been submitted to J. Appl. Phys. and the NiMn work is in progress.
In collaboration with Mark Stiles (NIST, PL) the effects of stable and unstable ferromagnet-antiferromagnet interfacial interactions were modeled. These results were presented at the 42nd annual Conference on Magnetism and Magnetic Materials and at the Centennial Meeting of the American Physical Society. A paper has been published in Physical Review B, and a second paper on the temperature dependence of the effect is scheduled for publication November 1, 1999.
To control the magnetization in thin films, a possible alternative to exchange biasing is uniaxial anisotropy. We have discovered a technique for producing strong uniaxial anisotropy in thin films by oblique-incidence deposition of tantalum underlayers, and we have produced anisotropy fields on the order of 0.1 T (1 kOe) in thin films of NiFe and Co. These fields are an order of magnitude larger than the magnetocrystalline anisotropy fields in similar films. This work is in progress.
Our work in standard problems for micromagnetic modeling has continued to gain international attention. Three papers were published (one from NIST) in the J. Appl. Phys on standard problem #2 and one paper on standard problem #1. Another paper outlining an analytical solution and a more accurate computation has been submitted for publication. Three invited presentations have been given on micromagnetic standard problems in FY1999.
Impact:
Our work on exchange bias phenomena is part of a large community effort to understand and improve exchange bias materials to satisfy industrial demand for strong exchange bias fields in materials with high blocking temperatures and high thermal stability. Our development and publication of the ferromagnetic resonance method and model provide a more complete method of measuring exchange bias phenomena.
Published results on our micromagnetic standard problem #2 have been used by researchers and Lawrence Livermore National Labs as a benchmark for their computational methods. Our recent collaborations with ITL have resulted in an analytical solution for the small-particle limit and enhanced accuracy computations that agree to within 0.01 %. The standard problems have also been receiving international attention as evidenced by our invitation to speak on standard problems to a European Community supported workshop.
Outputs:
Publications:
Stiles, M. D. and McMichael, R. D., "Temperature Dependence of Exchange Bias in Ferromagnet/antiferromagnet Bilayers," accepted for publication in Phys. Rev. B.
McMichael, R. D., Donahue, M. J., Porter, D. J., and Eicke, J. "Comparison of Magnetostatic Field Calculation Methods on 2-D Square Grids as Applied to Micromagnetic Standard Problem #2," J. Appl. Phys., 85 5816, 1999.
Stiles, M. D. and McMichael, R. D., "Model for Exchange Bias in Polycrystalline Ferromagnet-antiferromagnet Bilayers," Phys. Rev. B., 59, 3722, 1999.
McMichael, R. D., Stiles, M. D., Chen, P. J., and Egelhoff, W. F., Jr., "Ferromagnetic Resonance Studies of NiO-coupled Thin Films of Ni80Fe20," Phys. Rev. B., 58, 8605, 1998.
Presentations:
McMichael, R. D., "Experiences with Micromagnetic Standard Problems," Invited, NIST Workshop on Preisach Modeling, January 1999.
Eicke, J., "Element Shaping for Reduced Switching Field," Invited, NIST Workshop on Preisach Modeling, January 1999.
McMichael, R. D., "Micromagnetics -- Magnetism on the Mesoscale," Invited, poster component, APS Topical Group on Magnetism Display in the APS centennial meeting exhibits, Atlanta, GA, March 1999.
Stiles, M. D. (in collaboration with R. D. McMichael) "Hysteretic Effects in Exchange Biased Multilayers," Invited, APS Centennial Meeting, Atlanta, GA, March 1999.
McMichael, R. D., "Relaxation in Ferromagnetic Thin Films," Invited, Surface Science Lunch Bunch, NIST, May 3, 1999.
McMichael, R. D., "Standard Problems in Micromagnetics," Invited, DOE Computational Materials Sciences Network - Magnetism Workshop, Washington, DC, July 15, 1999.
McMichael, R. D., "Standard Problems in Micromagnetism," Invited, COST P3 Workshop - Simulation of Physical Phenomena in Technological Applications, Helsinki, Finland, September 1999.
McMichael, R. D., "Separation of Reversible and Irreversible Effects in Exchange-biased Films by Ferromagnetic Resonance," 43rd Annual Conference on Magnetism and Magnetic Materials, Miami, FL, November 1999.
Stiles, M. D. and McMichael, R. D., "Irreversible Effects in Exchange-biased Magnetic Layers," 43rd Annual Conference on Magnetism and Magnetic Materials, Miami, FL, November 1999.
McMichael, R. D. "Comparison of Magnetostatic Field Calculation Methods on 2-D Square Grids as Applied to Micromagnetic Standard Problem #2," 43rd Annual Conference on Magnetism and Magnetic Materials, Miami, FL, November 1999.
McMichael, R. D., "Contrasting the Temperature Dependence of Thin Films Biased by CoO," APS Centennial Meeting, Atlanta, GA, March 1999.
McMichael, R. D., "Element Shaping for Reduced Switching Field," MRS Spring Meeting, San Francisco, CA, April 1999.
Patents Pending:
Domain Wall Traps for Low Field Switching of Magnetic Elements
Robert D. McMichael
Disclosure Filed
6 March 2000