Micromagnetic Modeling and its Implications for Paleointensity Studies Koji FUKUMA

Transcription

1 Journal of Geography Micromagnetic Modeling and its Implications for Paleointensity Studies Koji FUKUMA Abstract Since three-dimensional micromagnetic modeling was introduced to rock magnetism in the late 1980s, we have gained clearer perspectives beyond the classical Neel's theory. at present, we can predict the magnetic properties of even non-uniformly magnetized grains, and directly compare simulated and experimental results, which makes interpretations of measurement data much easier and more appropriate. The longstanding issue of the pseudo-single-domain can be regarded as arising from the vortex structure of magnetization based on micromagnetic modeling results. Recently, micromagnetic modelings have been applied to simulate hysteresis measurements, low-temperature magnetometry, and First-Order Reversal Curves FORC. Calculations of demagnetizing energy are still demanding in terms of computation load, but we can obtain magnetization structures of micron-size magnetites whose Bitter patterns are observable under an optical microscope. In conjunction with micromagnetic modelings, FORC methods should be useful for illustrating how seriously magnetic interaction affects paleointensity data from sediments. Key words rock magnetism, paleomagnetism, micromagnetic modeling, paleointensity I Glen, 1982 Department of Environmental Systems Science, Doshisha University

2 Tauxe, 1993, 2005 Dunlop and Ozdemir, 1997 Pseudo-Single- Domain Stacey, 1958, Williams and Dunlop, 1995 Neel, 1949 Newell and Merrill, 1999 Hubert and Schafer, 1998; Kronmuller and Fahnle, , II Williams and Dunlop 1989 Schabes and Bertram Williams and Dunlop 1989

3 Newell et al., 1993a Fabian and Heider III 3 Butler and Banerjee, Williams and Dunlop Newell et al., 1993b; Fukuma and Dunlop, Williams and Dunlop, 1995 / Williams and Dunlop, Fukuma and Dunlop, submitted Williams and Dunlop Kittel

4 Kittel, 1949 IV 4 wasp waisted Tauxe et al., 1996 Williams and Dunlop 1995 Tauxe et al wasp waisted Schabes and Bertram, K Verwey 5 10 Verwey Muxworthy and Williams Verwey FORC First Order Reversal Curves 6 Pike et al., 1999 FORC 3 FORC Carvallo et al., 2003, 2004; Muxworthy and Dunlop, 2003 FORC V 0.07 Williams and Dunlop, 1995 <111> 2

5 7 Ozdemir et al., 1995 Halgedahl and Fuller, U Geiss et al., Fukuma and Dunlop, Rave et al Williams and Wright Wright et al., U 3 2 8, 1984 VI Tauxe, Brunhes Guyodo and Valet, 1999, Kawai et al., Verosub, 1977 Dunlop, 1981 Amerigian 1981 in press

6 King et al., 1983 Levi and Banerjee, 1976 Sagnotti et al., 2003 Sugiura, 1979 Yamazaki and Ioka, 1997 Dunlop, 1981 Yoshida and Katsura, 1985; Fukuma, 1992 Chang and Kirschvink, 1989 FORC Pike et al., 1999 FORC Pike et al., 2001 FORC Muxworthy and Dunlop, 2003 FORC FORC

7 5 Verwey stoichiometric 120 K stoichiometric 6 FORC First Order Reversal Curves Reversal curve Butler, R.F. and Banerjee, S.K : Theoretical single-domain garain size range in magnetite and titanomagnetite. J. Geophys. Res., 80, Carvallo, C., Muxworthy, A.R., Dunlop, D.J. and Williams, W : Micromagnetic modeling of firstorder reversal curve FORC diagrams for singledomain and pseudo-single-domain magnetite. Earth Planet. Sci. Lett., 213, Carvallo, C., Ozdemir, O. and Dunlop, D.J : First-order reversal curve FORC diagrams of elongated single-domain grains at high and low temperatures. J. Geophys. Res., 109, doi: / 2003JB Chang, S.-B.R. and Kirschvink, J.L : Magnetofossils, the magnetization of sediments, and the evolution of magnetite biomineralization. Ann. Rev. Earth Planet. Sci., 17, : Dunlop, D.J : The rock magnetism of fine particles. Phys. Earth Planet. Int., 26, Dunlop, D.J. and Ozdemir, O : Rock Magnetism : Fundamentals and Frontiers. Cambridge University Press. Fabian, K. and Heider, F : How to include magnetostriction in micromagnetic models of titanomagnetite grains. Geophys. Res. Lett., 23, Fukuma, K : A numerical simulation of magnetostatic coagulation in a fluid. Geophys. J. Int., 111, : Fukuma, K. and Dunlop, D.J : Grain size dependence of two-dimensional micromagnetic structures for pseudo-single-domain magnetite m. Geophys. J. Int., 134, Fukuma, K. and Dunlop, D.J. submitted : Threedimensional micromagnetic modeling of randomly oriented magnetite grains m. J. Geophys. Res. Geiss, C.E., Heider, F. and Soffel, H.C : Magnetic domain observations on magnetite and titanomaghemite grains m. Geophys. J. Int., 124, Glen, W : The Road to Jaramillo : Critical Years of the Revolution in Earth Science. Stanford University Press. Guyodo, Y. and Valet, J.-P : Global changes in intensity of the Earth's magnetic field during the past 800 kyr. Nature, 399, Halgedahl, S. and Fuller, M : The dependence of magnetic domain structure upon magnetization state with emphasis upon nucleation as a mechanism for pseudo-single-domain behavior. J. Geophys. Res., 88, Hubert, A. and Schafer, R : Magnetic Domains. Springer-Verlag. Kawai, N., Sato, T., Sueishi, T. and Kobayashi, K : Palaeomagnetic study of deep-sea sediments from the Melanesian basin. J. Geomag. Geoelectr., 29, King, J.W., Banerjee, S.K. and Martin, J : A new rock-magnetic approach to selecting sediments for geomagnetic paleointensity studies : Application to paleointensity for the last 4000 years. J. Geophys. Res., 88, Kittel, C : Physical theory of ferromagnetic domains. Rev. Mod. Phys., 21, Kronmuller, H. and Fahnle, M : Micromagnetism and the Microstructure of Ferromagnetic Solids. Cambridge University Press. Levi, S. and Banerjee, S.K : On the possibility of obtaining relative paleointensities from lake sediments. Earth Planet. Sci. Lett., 29, Muxworthy, A.R. and Dunlop, D.J : First-order reversal curve FORC diagrams for pseudo-singledomain magnetites at high temperature. Earth Planet. Sci. Lett., 203, Muxworthy, A.R. and Williams, W : Micromagnetic models of pseudo-single domain grains of magnetite near the Verwey transition. J. Geophys. Res., 104,

8 Neel, L : Theorie du trainage magnetique des ferromagnetiques en grains fins avec applications aux terres cuites. Ann. Geophys., 5, Newell, A.J. and Merrill, R.T : Single-domain critical sizes for coercivity and remanence. J. Geophys. Res., 104, Newell, A.J., Williams, W. and Dunlop, D.J. 1993a : A generalization of the demagnetizing tensor for nonuniform magnetization. J. Geophys. Res., 98, Newell, A.J., Dunlop, D.J. and Williams, W. 1993b : A two-dimensional micromagnetic model of magnetizations and fields in magnetite. J. Geophys. Res., 98, Ozdemir, O., Xu, S. and Dunlop, D.J : Closure domains in magnetite. J. Geophys. Res., 100, Pike, C.R., Roberts, A.P. and Verosub, K.L : Characterizing interactions in fine magnetic particle systems using first order reversal curves. J. Appl. Phys., 85, Pike, C.R., Roberts, A.P. and Verosub, K.L : First-order reversal curve diagrams and thermal relaxation effects in magnetic particles. Geophys. J. Int., 145, Rave, W., Fabian, K. and Hubert, A : Magnetic states of small cubic particles with uniaxial anisotropy. J. Magn. Magn. Mat., 190, Sagnotti, L., Rochette, P., Jackson, M., Vadeboin, F., Dinares-Turell, J., Winkler, A. and Team, M.-N. S : Inter-laboratory calibration of low-field magnetic and anhysteretic susceptibility measurements. Phys. Earth Planet. Int., 138, Schabes, M.E. and Bertram, H.N : Magnetization processes in ferromagnetic cubes. J. Appl. Phys., 64, Stacey, F.D : Thermoremanent magnetization TRM of multidomain grains in igneous rocks. Phil. Mag., 3, Sugiura, N : ARM, TRM and magnetic interactions : Concentration dependence. Earth and Planet. Sci. Lett., 46, Tauxe, L : Sedimentary records of relative paleointensity of the geomagnetic field : Theory and practice. Rev. Geophys., 31, Tauxe, L., Mullender, T.A.T. and Pick, T : Potbellies, wasp waists, and superparamagnetism in magnetic hysteresis. J. Geophys. Res., 101, Tauxe, L., Bertram, H.N. and Seberino, C : Physical interpretation of hysteresis loops: Micromagnetic modeling of fine particle magnetite. Geochem. Geophys. Geosyst., 3 10, 1055, doi: / 2001GC : Verosub, K.L : Depositional and post depositional processes in the magnetization of sediments. Rev. Geophys. Space Phys., 15, Williams, W. and Dunlop, D.J : Three-dimensional micromagnetic modeling of ferromagnetic domain structure. Nature, 337, Williams, W. and Dunlop, D.J : Simulation of magnetic hysteresis in pseudo-single-domain grains of magnetite. J. Geophys. Res., 100, Williams, W. and Wright, T.M : High-resolution micromagnetic models of fine grains of magnetite. J. Geophys. Res., 103, Wright, T.M., Williams, W. and Dunlop, D.J : An improved algorithm for micromagnetics. J. Geophys. Res., 102, Yamazaki, T. and Ioka, N : Cautionary note on magnetic grain-size estimation using the ratio of ARM to magnetic susceptibility. Geophys. Res. Lett., 24, Yoshida, S. and Katsura, I : Characterization of fine magnetic grains in sediments by the suspension method. Geophys. J. R. astr. Soc., 82,

9 2 Pictorial 2 Micromagnetic Structures of Magnetite Grains Magnetization structures of 0.09-, 0.6-, and 0.9-micron magnetite grains. Microscopic magnetization directions are shown by arrows on surfaces, and the color variations represent vertical components of magnetizations upward : red, downward : blue. Koji FUKUMA 3 Pictorial 3 Magnetic Lines of Force outside the Earth's Core 3485 km 6371 km Yellow lines denote outward magnetic lines of force that come out of the core, while light blue lines denote inward ones that enter the core. The red sphere corresponds to the core of radius 3485 km. Parallels of latitude, lines of longitude, and the coastline are drawn on the Earth's surface of radius 6371 km. The radial component of the magnetic field, Br, at the Earth's surface is positive on the southern hemisphere and negative on the northern hemisphere upon the whole; that is, magnetic lines of force come out of the south and enter into the north of the Earth. However, the distribution of Br at the core surface is complicated; some magnetic lines of force come out of and again enter the core on the southern hemisphere, as seen beneath the south of Africa. Masaki MATSUSHIMA