David Dunlop

David Dunlop

Ph.D. | Professor Emeritus | Physics

Research

My research focuses on the magnetism of Earth materials and magnetic recording media, and on how magnetism can be used to decipher the early history of planets like the Earth and Mars.

1. Magnetism of Mars
The Mars Global Surveyor satellite revealed magnetic fields that date from Mars’ earliest history. They are ten times larger than any ancient magnetic field records on Earth. I am attempting to model and explain the sources of these fields, which must extend deep in Mars' lithosphere and involve much more iron-rich minerals than on Earth, and what they tell us about the thermal and tectonic evolution of Mars in the first few hundred million years of its history.

2. Early Earth's Magnetic Field
Paleomagnetism is the study of the ancient magnetic field of the Earth as recorded in rocks. The direction of the ancient field tells us how continents have rotated and changed latitude throughout geological history, while the intensity of the field holds the key to when the core dynamo field switched on and how it has evolved since. The oldest magnetic field record, from 3.5 billion years ago, was measured in our laboratory and we have pioneered multivectorial paleomagnetism which yields two magnetic field records of different ages from a single rock. The Canadian Shield has been the base for most of our field studies but we have worked as far afield as South Africa.

3. Novel Magnetic Recording Media
Needle-like particles have unique magnetic stability and are the best magnetic field recorders. They are binary because their magnetic vectors can only lie along the needle axis; this makes them ideal for digital recording. Examples are iron “nanowires�, chains of magnetic crystals used by birds and bacteria for navigation, and natural needle-shaped inclusions in otherwise non-magnetic silicate minerals. Recently we have measured individual milligram crystals of micas whose magnetic signals are just resolvable by today's most sensitive magnetometers. The experimental data are compared to models obtained by computer simulation and by microscopic observations of magnetic domains in the micas.

Publications

Dunlop, D. J., 2006. Inverse thermoremanent magnetization. J. Geophys. Res. 111, B12S02. doi:10.1029/ 2006JB004572 (12 pages).

Dunlop, D. J., and B. Carter-Stiglitz, 2006. Day plots of mixtures of superparamagnetic, single-domain, pseudo-single-domain and multidomain magnetites. J. Geophys. Res. 111, B12S09. doi:10.1029/ 2006JB004499 (10 pages).

Dunlop, D. J., O. Ozdemir, and D. G. Rancourt, 2006. Magnetism of biotite crystals. Earth & Planetary Science Letters 243, 805-819.

Carvallo, C., A. R. Muxworthy, and D. J. Dunlop, 2006. First-order reversal curves (FORC) diagrams of magnetic mixtures: Micromagnetic models and measurements. Physics of Earth & Planetary Interiors 154, 308-322.

Ozdemir, O., and D. J. Dunlop, 2006. Magnetic memory and coupling between spin-canted and defect magnetism in hematite. J. Geophys. Res. 111, B12S03. doi:10.1029/ 2006JB004555 (13 pages).

Milne, G. A., and D. J. Dunlop, 2006. Angular variation of the magnetic properties and reversal mode of aligned single-domain iron nanoparticles. J. Geophys. Res. 111, B12S08. doi:10.1029/ 2006JB004530 (10 pages).

Yu, Y. J., and D. J. Dunlop, 2006. Testing the independence of partial TRMs of single-domain and multidomain grains: Implications for paleointensity determination. J. Geophys. Res. 111, B12S31. doi:10.1029/ 2006JB004434 (9 pages).

Ozdemir, O., and D. J. Dunlop, 2006. Magnetic domain observations on magnetite crystals in biotite and hornblende crystals. J. Geophys. Res. 111, B06103. doi:10.1029/ 2005JB004090 (12 pages).

Fukuma, K., and D. J. Dunlop, 2006. Three-dimensional micromagnetic modeling of randomly oriented magnetite grains (0.03-0.3 μm). J. Geophys. Res. 111 (in press).

Dunlop, D. J., and O. Ozdemir, 2006. Magnetizations in rocks and minerals. Vol. 5 - Geomagnetism, Treatise on Geophysics, ed. G. Schubert, Elsevier Science, New York (in press)

Dunlop, D. J., and J. Arkani-Hamed, 2005. Magnetic minerals in the Martian crust. Journal of Geophysical Research 110, E12S04, doi:10.1029/2005JE002404 (11 pages).

Dunlop, D. J., 2005. Magnetic impact craters (News & Views). Nature 435, 156-157.

Dunlop, D. J., B. Zhang, and O. Ozdemir, 2005. Linear and nonlinear Thellier paleointensity behavior of natural minerals. Journal of Geophysical Research 110, B01103, doi:10.1029/ 2004JB003095 (15 pages).

Carvallo, C., D. J. Dunlop, and O. Ozdemir, 2005. Experimental comparison of FORC and remanent Preisach diagrams. Geophys. J. Int. 162, 747-754.

Ozdemir, O., and D. J. Dunlop, 2005. Thermoremanent magnetization of multidomain hematite. J. Geophys. Res. 110, B09104. doi:10.1029/2005JB003820 (8 pages).

Dunlop, D. J., and Y. J. Yu, 2004. Intensity and polarity of the geomagnetic field during Precambrian time. In Timescales of the Paleomagnetic Field, ed. J. E. T. Channell et al., American Geophysical Union Geophysical Monograph 145, 85-100.

Dunlop, D. J. and O. Ozdemir, 2001. Rock Magnetism: Fundamentals and Frontiers, 573 pp. Cambridge University Press, New York (paperback edition).