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43637-AC10
Magnetization Reversal in Magnetic Nanostructures
We have investigated magnetization reversal in exchange biased thin films, nanodots, nanowires and other magnetic nanoparticles. The results so far have led to 9 publications in top physics and chemistry journals and 2 conference proceedings (1 invited). Our work have been recognized with 12 conference invited talks (including 2005 Fall MRS, 2006 Spring MRS, Intermag, AVS, and 2007 APS), 10 invited seminars at universities. In addition we have given over 30 contributed conference presentations.
Students supported by the grant have won individual recognitions for their work. Randy Dumas won a Leo M. Falicov Award (with a $1,000 prize) in November 2006 at the American Vacuum Society (AVS) 53rd International Symposium in San Francisco for best Graduate Research in the Magnetic Interfaces and Nanostructures Division. Joe Davies earned his Ph.D. in June 2007 and received a prestigious National Research Council Postdoctoral Fellowship. Jared Wong, an undergraduate specialist, won the 1st Prize Steven Chu Award (with a $500 prize) at the Fall 2006 American Physical Society - California Section Meeting for best undergraduate research.
Below are our main progresses over the past year:
1. Exchange biased ferromagnet/antiferromagnet (FM/AF) films:
1a. Irreversibility of Magnetization Rotation: Magnetization reversal via rotation is typical in FM/AF exchange biased systems. The reversibility of the rotation is a manifestation of the microscopic reversal process. We have investigated the magnetization reversal in Fe/epitaxial-FeF2 thin films using vector magnetometry and first order reversal curves. We find that the reversal is predominantly by rotation as the applied field makes an angle with the AF spin axis; mostly irreversible at small angles and reversible at larger angles. A modified Stoner-Wohlfarth model reproduces the overall trend of the irreversibility evolution. The remaining discrepancies between the modeled and measured irreversibility is attributed to local incomplete domain walls (Appl. Phys. Lett. 90, 032510).
1b. Rotational hysteresis of the exchange anisotropy direction: We have investigated the effects of rotating an applied field on the exchange anisotropy in Co/FeMn thin films. When the applied field is initially along the cooling field direction, the longitudinal hysteresis loop has a maximum coercivity and the transverse hysteresis loop is flat, indicating that the exchange field is along the cooling field direction. When the applied field angle is rotated away and then restored to the original field cooling direction, the exchange anisotropy direction has changed. The rotation of the exchange field direction trails the applied field and is hysteretic. The rotational hysteresis of the exchange field direction is due to the weak anisotropy in thin FeMn layers, and decreases with increasing FeMn thickness (J. Appl. Phys. 101, 09E508).
2. Magnetization reversal mechanisms of nanodots: Sub-100 nm nanomagnets not only are technologically important, but also exhibit complex magnetization reversal behaviors as their dimensions are comparable to typical magnetic domain wall widths. We have captured magnetic “fingerprints” of 109 Fe nanodots as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method. As the nanodot size increases from 52 nm to 67 nm, the FORC diagrams reveal striking differences, despite only subtle changes in their major hysteresis loops. The 52 nm nanodots exhibit single domain behavior and the coercivity distribution extracted from the FORC distribution agrees well with a calculation based on the measured nanodot size distribution. The 58 and 67 nm nanodots exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distribution and confirmed by micromagnetic simulations. Furthermore, the FORC method gives quantitative measures of the magnetic phase fractions, and vortex nucleation and annihilation fields (Phys. Rev. B 75, 134405).
3. Magnetic memory in Co/Pt films: we have investigated the microscopic magnetic memory effect (with Larry Sorensen's group at University of Washington) i.e., whether the microscopic magnetic configuration retains memory of its prior states during magnetic field cycling. Using a coherent x-ray speckle metrology to a series of Co/Pt thin films with varying degrees of disorder, the different magnetic domain configurations are manifested in speckle patterns. We find that both return-point and complementary-point memory are partial and imperfect in the disordered samples, and completely absent when the disorder is below a threshold level. Furthermore, we find that the return-point memory is always a little larger than the complementary-point memory (Phys. Rev. B 75, 144406).
