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43637-AC10
Magnetization Reversal in Magnetic Nanostructures

Kai Liu, University of California (Davis)

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We have investigated magnetization reversal in exchange biased thin films, nanodots and core-shell structured magnetic nanoparticles. The results so far have led to 13 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), 13 invited seminars at universities. In addition we have given over 40 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 won the 1^st Prize Steven Chu Award at the Fall 2006 American Physical Society - California Section Meeting for best undergraduate research.  

Below are our main progresses over the past year under time extension:

1. Arrays of magnetic nanodots:

a. Phase transition: We have investigated a temperature induced single domain - vortex state transition in 67 nm Fe nanodots using a first-order reversal curve (FORC) technique. Conventionally magnetization reversal in such nanomagnets via a vortex state is usually studied at the remanent state which has a characteristic flux closure state. However it is not always reliable to use the nanomagnet remanent magnetic configuration or the shape of the hysteresis loop to determine the presence of vortex state. In contrast, the irreversible nucleation/annihilation events are clear indications of a vortex state. These have been captured in our FORC analysis of the Fe nanodots. We find that both single domain and vortex state coexist in arrays of 67 nm Fe dots. At lower temperatures, it becomes harder to nucleate and annihilate vortices and the amount of single domain dots increases. [Appl. Phys. Lett. 91, 202501 (2007)].

b. Reversible vs irreversible magnetization reversal: Arrays of nanomagnets have important potential applications as future generation ultrahigh-density patterned magnetic recording media, in which each nanomagnet constitutes a single bit. We introduce a powerful technique to identify and quantify reversible and irreversible magnetization changes, a key challenge in characterizing these systems. The experimental protocol consists of measuring a few families of second-order reversal curves along selected profiles in the first-order-reversal-curve diagram, which then can be decomposed into truly irreversible switching events and reversible magnetization changes. The viability of the method is demonstrated for arrays of sub-100-nm Fe nanomagnets, which exhibit complex magnetization reversal processes. [J. Appl. Phys. 103, 07C518 (2008)].

2. Temperature Dependent Magnetization Reversal in (Co/Pt)/Ru Multilayers:

Antiferromagnetically coupled (Co/Pt)/Ru multilayers with perpendicular anisotropy have been shown to exhibit both vertically and laterally correlated magnetization reversal modes. We have investigated a multilayer film whose layer thicknesses have been tuned to be at the phase boundary between the two reversal modes. At high temperatures reversal is via vertically correlated stripe domains throughout the film thickness, similar to that in Co/Pt films without the Ru spacers.  At low temperatures antiferromagnetic (AF) interlayer exchange coupling dominates, and laterally correlated reversal is observed. Under magnetic field cycling, dipolar fields can transform the sample back into a vertically correlated domain state.  [Phys. Rev. B 77, 014421 (2008)].

3. Three-dimensionally Intercrossing Mn₃O₄ Nanowires:

We have investigated three-dimensional Mn₃O₄ nanostructures synthesized by a soft chemistry templating process using a block copolymer as the structure-directing agent. These materials have a unique structure of intertwining nanowires which are grown from faceted Mn₃O₄ particles and aligned spatially perpendicular to or along each facet. The magnetic measurements of such nanostructured films show clear signs of magnetic shape anisotropy, indicating that this templating synthesis approach could be a promising method to develop Mn₃O₄ nanostructures with desired anisotropic properties through dimensionality control. [Acta Materialia 56, 3516 (2008)].

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