6 March 2000
Investigators: R. D. Shull, A. J. Shapiro, R. D. McMichael, L. J. Swartzendruber, H. J. Brown, R. V. Drew, and D. E. Mathews
Objectives:
This program focuses on developing an understanding of the magnetic behavior of low dimensional systems, as in systems wherein one or more characteristic dimensions have been reduced to nanometer sizes. For these new materials, it is not known whether their exciting novel behavior is due to new physics or to a logical extension of large-size behavior to small dimensions. Consequently, implementation of this new type of material into marketable products is significantly delayed. NIST is providing the measurement science to answer this critical unknown and to identify where standards may be required as the field becomes more mature.
Technical Description:
Since the magnetic behavior of nanomaterials is largely unknown, much of the focus in this effort is directed toward measuring the magnetic characteristics of this new class of materials and checking if they are consistent with present theories explaining the behavior of conventional materials. For instance, a characteristic feature of conventional ferromagnets is the presence and morphology of magnetic domains in the material. Magnetic anisotropy is required for such a domain structure to exist, and conventional wisdom would argue that the normal sources of magnetic anisotropy would average to zero in nanocrystalline and nanocomposite materials. Consequently, efforts are ongoing to image the domain structure in these new materials and their dynamics if they exist. Imaging by means of a ferrofluid decoration technique at domain walls as well as by means of a magneto-optic indicator film (the MOIF technique developed in our laboratory in collaboration with a group from Chernogolovka, Russia) and on electrodeposited nanocrystalline Ni are being pursued. In addition, domain kinetics are being investigated in nanometer-thick bilayer combinations of materials with dissimilar magnetic characters, like antiferromagnetism and ferromagnetism with various degrees of magnetic softness.
In conventional materials, the material will magnetize along the easy axis of magnetization, so that in a polycrystalline material the magnetization will fluctuate on a scale of the material's grain size. Small angle neutron scattering (SANS) is a useful method for determining such magnetic fluctuations, and this technique was applied for the first time to a single phase nanocrystalline material, electrodeposited nanocrystalline Ni, in order to observe anticipated nanometer-scale magnetic fluctuations. This material is uniquely suited for this examination because it possesses few pores, and therefore most scattering at small angles was predicted to be magnetic in origin.
In the Ba-Fe-Ti-O system, it has been found that in many of the compounds in the system the Fe is physically located into nanometer-sized regions of the crystal structure. However the magnetic character of these various structurally-related compounds has been found to change by great amounts, possibly due to size and interaction effects. In order to understand this extreme variability, in collaboration with T. Vanderah (Ceramics Division), several Ba-Fe-Ti-O compounds are being investigated using Mössbauer spectroscopy and magnetic susceptibility. This technique is particularly useful for detecting size effects in the magnetic character of the material, and to detect variations in magnetic moment for different Fe atoms in the structure.
In conventional ferromagnets, when a non-magnetic species is added to that material, the magnetic coercivity will normally increase due to retardation of magnetic domain wall motion. In order to assess whether that large-scale phenomenon still persists at small dimensions, the magnetic behavior of thin films of Fe and Co containing varying amounts of nanometer-sized C60 carbon was investigated using magnetometry, MOIF imaging, and electron microscopy. This was a collaborative effort with Rice University, experts at working with C60 complexes. Surprisingly, this year it was found that C60 inclusion in thin films of Fe and Co affects their coercivities differently. The relationship between coercivity and magnetocrystalline anisotropy in mechanically milled Fe-Al and Fe-Al-Ge nanocrystalline materials was also examined in a collaboration with Caltech.
Magnetic nanocomposites exhibiting superparamagnetism were discovered at NIST to possess enhanced magnetocaloric effects, a finding which has opened up the possibility for magnetic refrigeration devices operating at much higher temperatures and at much lower magnetic fields than were previously possible. This year Dy-Al-Fe garnet nanocomposites were found to have enhanced magnetocaloric effects similar to those previously discovered in the Gd-Ga-Fe garnet nanocomposites.
Planned Outcome:
It is anticipated that as a result of this program, one will possess an improved prediction capability of magnetic properties of magnetic nanomaterials in different morphologies. Success in this area will provide an improved capability to engineer magnetic properties by design. In addition, it is anticipated that improved characterization techniques for magnetic nanomaterials will be developed, thereby leading to improved quality control by manufacturers. Furthermore, it is anticipated that one will be better able to exercise control over the flux dynamics in small magnetic devices. By exercising leadership roles in the scientific community, these improved capabilities would be transferred to industry, e.g., as by the organization of and participation in workshops and symposia in the area, and by publications and presentations at national and international meetings.
External Collaborations:
In collaboration with the University of Toronto (U. Erb) and the University of Saarlandes (J. Weissmueller), SANS measurements were performed on electrodeposited nanocrystalline Ni. In this collaboration U. Erb provided the samples and J. Weissmueller analyzed the SANS measurements. In addition, two students from the University of Toronto (J. McCrea and G. Hibbard) worked in our laboratory this summer examining the energy losses in these materials. In a collaboration with the Russian Academy of Sciences at Chernogolovka, Russia (V. Nikitenko), a special magnetic domain imaging technique called MOIF has been developed. This technique has been used jointly to image several nanocrystalline and nanocomposite materials. In collaboration with Rice University (R. Barrera and A. Chang), the magnetic character of thin films of Fe and Co containing carbon in the C60 form prepared at Rice University were investigated. In a collaboration with Caltech (B. Fultz and H. Frase) the magnetization behavior of a series of nanocrystalline materials was correlated with the magnetocrystalline anisotropy of the material, and the magnetic behavior of Co/Pt multilayers was examined in a collaboration with George Washington University (L. Bennett and R. Fry). A collaboration with Argonne National Laboratory (S. Bader and S. Jiang) was also successful this year in examining the remagnetization behavior of SmCo/Fe spring magnets; while a collaboration with the Johns Hopkins University (C.L. Chien and Kai Liu) was successful at examining the remagnetization behavior of wedge-shaped FM/AF materials. R. Shull was elected as the Chairman of the International Committee on Nanostructured Materials. A interagency group with R. Shull as one of the organizing members comprised of NIST, NSF, ONR, DOC/TA, AFOSR, NIH, and NASA organized a national workshop on the status and trends in research on nanomaterials and nanodevices.
Accomplishments:
The remagnetization behavior of an epitaxial antiferromagnet/ferromagnet bilayer (possessing magnetic exchange anisotropy) was shown to be accomplished by two completely different processes for remagnetization in the two different directions. This is the first time such a phenomenon has been observed in these materials, and may help answer the 40 year old question of why the coercivity of such materials is so low. Examination of materials prepared by depositing the magnetic species in the opposite order unequivocally showed that the asymmetry in remagnetization behavior is due to defects, like dislocations, in the antiferromagnet. Also, examination of a sample with a wedge-shaped ferromagnetic layer showed that the remagnetization behavior is dependent on the relative strengths of magnetostatic and exchange interactions on the ferromagnet.
The magnetic coercivity of thin films of Fe containing nanometer-sized C60 complexes of carbon was found to increase with the C60 content as in conventional ferromagnets. However, the magnetic coercivity of thin films of Co containing the C60 complexes decreased with increased C60 content! It is felt this difference between Fe and Co films was due to differences in morphology caused by the addition of the C60.
SANS observations of nanocrystalline Ni resulted in a finding that the magnetic correlation length was not the same as the material's grain size. Since one of the big unanswered questions in nanocrystalline magnetic materials is the effect on the magnetic moment of the atoms near grain boundaries where the local atomic environment is significantly changed, these results have attracted great interest. Specifically, these results provide some of the strongest evidence to date that the grain boundary atoms' magnetic strength is not diminished.
Examination of the magneto-optic Kerr effect in layered Co/Pt films, envisioned for magneto-optical data storage, showed that the remagnetization behavior is accompanied by more than one magnetic phase reversal. In addition, significant time dependence effects were observed and modeled. In nanocrystalline Fe-Al and Fe-Al-Ge alloys, it was found that the major contribution to magnetic coercivity was residual stress in the material, not the magnetocrystalline anisotropy.
NIST was joined by NSF, ONR, DOC/TA, AFOSR, NIH, and NASA in organizing a national workshop to review of the status and trends in nanotechnology. A report of this workshop is in final review and will be used to justify a National Initiative in this area. Several agencies are now using this report to reassess where to put their available research dollars.
Impacts and Technical Highlights:
NIST is now considered a pre-eminent organization on magnetic nanocomposite properties. As a result, NIST is consulted by industry and other national research organizations in assisting them to take advantage of properties discovered in the area and to help establish a national policy toward research in the area.
As a result of NIST research on magnetic nanocomposite refrigerants, many groups around the world have initiated research activities in the area, including in China, Germany, France, Great Britain, Japan, and the United States.
Following the national workshop on nanomaterials organized by people (including R. D. Shull) from NIST, NSF, ONR, DOC/TA, AFOSR, NIH, and NASA, a document summarizing the activity and needs in this field was submitted to the White House and a National Initiative on Nanotechnology was developed for funding in FY2001.
Outputs:
Publications:
Weismuller, J., McMichael, R. D., Michels, A., and Shull, R. D., "Small Angle Neutron Scattering by the Magnetic Microstructure of Nanocrystalline Ferromagnets Near Saturation," J. Res. Natl. Inst. Stand. Technol. 104, 261, 1999.
Nikitenko, V. I., Gornakov, V. S., Shapiro, A. J., Shull, R. D., Kai Liu, Zhou, S. M., and Chien, C. L., "Effect of Exchange Anisotropy on Elementary Events of the Magnetization Reversal Processes in Ferromagnetic/Antiferromagnetic Bilayers," J. Appl. Phys., in press.
Fry, R. A., Bennett, L. H., Della Torre, E., Shull, R. D., Egelhoff, W. F., Farrow, R. F. C., and Lee, C. H., "Magneto-optical Measurements of Co-Pt (111) Multilayers," J. of Mag. Magn. Materials 193 (1-3), 162, 1999.
Vanderah, T. A., Wong-Ng, W., Toby, B. H., Browning, V. M., Shull, R. D., Geyer, R. G., and Roth, R. S., "Characterization of Ternary Compounds in the BaO: Fe2O3: TiO2 System: Ba6Fe45Ti17O106 and BaFe11Ti3O23," J. Solid State Chemistry 143, 182, 1999.
Presentations:
Shull, R. D., "Magnetization Reversal in Exchange Spring Magnets Observed Directly with a Magneto-optical Indicator Film," International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials (ISMANAM-99), Invited Talk, Dresden, Germany, August 1999. Shull, R. D., "Magnetic Domains in Nanocrystalline and Nanocomposite Materials," Materials Science Department Seminar Speaker, Darmstadt University, Invited Talk, Darmstadt, Germany, August 1999.
Shull, R. D., "Magnetization Reversal in Thin Film Bilayers With Exchange Anisotropy Observed Directly by the MOIF Technique," FIM-99 (Frontiers in Magnetism) Conference, Royal Institute of Technology, Invited Talk, Stockholm, Sweden, August 1999.
Shull, R. D., "Low Field, High Temperature Magnetic Refrigerants for the 21st Century," FIM-99 Frontiers in Magnetism) Conference, Royal Institute of Technology, Invited Talk, Stockholm, Sweden, August 1999.
Shull, R. D., "Magnetic Properties of Nanostructured Films," NATO Advanced Research Workshop on Nanostructured Films and Coatings, Invited Talk, Santorini, Greece, June 1999.
Shull, R. D., "AC Susceptometry," Royal Institute of Technology, Invited Talk, Stockholm, Sweden, April 1999.
Shull, R. D., "Properties of Magnetic Nanocomposites," ICMAT-ICR, Invited Talk, Rome, Italy, April 1999.
Shull, R. D., "Magnetic Materials Group Programs," University of Bologna Seminar, Invited Talk, Bologna, Italy, April 1999.
Shull, R. D., "Magnetic Properties of Nanocomposites," Fourth International Conference on Metastable Phases (IWOMP-IV), Invited Talk, Bologna, Italy, April 1999.
Shull, R. D., "Domain Behavior in Magnetic Nanostructures as Revealed by MOIF Observations," Nanostructured Hybrid Materials Symposium, TMS Annual Meeting, Invited Talk, San Diego, CA, March 1999.
Shull, R. D., "Magnetic Nanocomposites," Seminar Speaker, Johnson Space Center, Invited Talk, Houston, TX, December 1998.
Shull, R. D., "New Directions in Magnetism: Magnetic Nanocomposites," Seminar Speaker, Department of Mechanical Engineering and Materials Science, Rice University, Invited Talk, Houston, TX, December 1998.
Shull, R. D., "Magnetic Activities in the NIST Magnetic Materials Group," Seminar Speaker, Patrick Air Force Base, Invited Talk, Melbourne, FL, November 1998.
Shull, R. D., "Direct Experimental Study of the Magnetization Reversal Process in Epitaxial and Polycrystalline Films with Unidirectional Anisotropy," 43rd Annual Conference on Magnetism and Magnetic Materials, Miami, FL, November 1998.
Shull, R. D., "Domain Behavior in Magnetic Nanostructures as Revealed by MOIF Observations," American Vacuum Society Annual Meeting, Baltimore, MD, November 1998.
Shull, R. D., "Magnetic Exchange Bias in a NiO/FeNi Antiferromagnetic/Ferromagnetic Bilayer," TMS Fall Meeting, Rosemont, IL, October 1998.
Shull, R. D., "Magnetic Dendrimers and Other Nanocomposites," American Chemical Society Annual Meeting, Invited Talk, Boston, MA, August 1998.
6 March 2000