Magnetism + Nanoscience = Magnificence

Exploring nanomagnetism for the future

 

My research is mainly focused on magnetic nanostructures, including a variety of structures, such as nanorings, nanodots, nanotubes and nanonetworks. I use both top-down and bottom-up method to fabricate arrays of nano-entities.

Magnetic Nanorings

The magnetic configurations and reversal mechanisms of nanomagnets depend intricately on the geometrical shape and size of the entities and the competition between the magnetostatic and the exchange energies.  For example, an elongated nanomagnet can only acquire the single-domain state with magnetic poles at both ends and stray magnetic field in its vicinity. More interestingly, a circular magnetic disc can acquire the vortex state in which the magnetization forms a closure structure without magnetic poles nor stray field.

A more intriguing geometry is that of a magnetic nanoring which has no central area and therefore contains no vortex core in the vortex state. The diameter is between 50 nm and 1 um. It can be made of normal metal, magnetic metal, semiconductor, oxides and even polymers. I am working on the magnetic nanorings, typically made of cobalt or permalloy, that have novel properties. I have developed a lithographyless method for fabricating arrays of a large number of magnetic nanorings on a macroscopic area with ultrahigh density of 30 Giga rings/in2.

For symmetric nanorings (uniform cross section along the circumference), magnetometry measurements and micromagnetic simulation reveal that the nanorings can acquire two switching processes: one leads to the stable vortex states through domain wall annihilation; while the other involves only rotation of the onion states.

In 100 nm asymmetric nanorings, by tuning the asymmetry we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field.  The observed results have been accounted for by the dependence of the domain wall energy on the local cross section area of nanoring for which we have provided theoretical calculations.

Published work:

  1. Arrays of magnetic nanorings with ultrahigh areal densities,Advanced Materials 16, 2155 (2004).
  2. Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings, Phys. Rev. Lett. 96, 27205 (2006).
  3. Patterned Nanomagnets, Physics Today 60, 40 (2007).
  4. Current-induced multiple spin structures in 100-nm nanoring magnetic tunnel junctions, Phys. Rev. B(in press, 2008).

 
Symmetric nanoring

Domain pattern and Domain Wall Resistance

Magnetic anisotropy and domain patterns of Ni films, though been described in many textbooks, are still triggering interests due to the complicate domain structures and often counterintuitive magnetometry characterizations.  Perpendicular magnetic anisotropy (PMA) in Ni films, usually found in epitaxially grown samples and indicated by stripe domains patterns, is useful in studying the magnetoresistance of domain walls (DWs) because the anisotropic magnetoresistance effect (AMR) can be excluded when current is flowing perpendicular to the magnetization of all domains. We have grown Ni films, with thicknesses between 10 and 500 nm, by thermal evaporation on Si/SiO2 substrates. Stripe domains in Ni films thicker than 40 nm, as measured by magnetic force microscope, were maze-like after demagnezation in an out-of-plane field, but were almost perfectly aligned along the field direction after demagnezation in an in-plane field. The average size of stripe domains, varying between 100 and 260 nm and being larger if demagnetized perpendicularly, was found to be a power function of the film thickness with an exponent of 0.317 ± 0.005. A possible origin of the PMA is the residue stress resulting from lattice mismatch as big as 1%. Hysteresis loops revealed that the films had closed-domain structures with small remnant magnetizations (5%) along the out-of-plane direction. Magnetoresistance was measured by applying an out-of-plane field and an in-plane current perpendicular to the direction of stripe domains pre-aligned by in-plane demagnetization. The MR values of ~104 walls and domain walls were 0.1 – 0.5 %. A phenomenological model will be presented in this paper to evaluate the absolute resistivity of an individual domain wall.

Published work: 

  1. Determination of domain walls resistance in a cobalt thin film by means of thickness modulation, Appl. Phys. Lett. 88, 122503 ( 2006)
  2. Effect of Geometry on Magnetic Domain Structure in Thermally Evaporated Ni Strips, Phys. Rev. B 77, 132408 (2008).
  3. Perpendicular Magnetic Anisotropy and Domain Wall Resistance In Thermally Evaporated Nickel Films, Appl. Phys. Lett. (in prepartion 2008)

After in-plane demag


After out-of-plane demag

Magnetic Nanodots with Perpendicular Anisotropy

Nanodots consisting of Co/Pt multilayer have been fabricated using nanosphere lithography and ion beam etching. The nanodots retained the perpendicular anisotropy as that of the continuous films. However, the coercivity of the nanodots was greatly enhanced, more substantial for smaller nanodots. Concurrently, the remnant magnetization was reduced to 50~80%. Magnetic force microscopy revealed that the domain pattern of nanodots at the remnant state was largely single domain but with non-perpendicular stray fields on the edges, indicating non-collinear alignment of magnetizations in those regions.

Thin films of [Co/Pt]n multilayers were first deposited on Si(100) substrates. Monolayers of polystyrene spheres were then coated on top of the Co/Pt multilayers by direct immersion of the substrates in the nanosphere solutions. It is essential that the nanospheres are not in contact or pile up on the substrate. The areal coverage or the inter-sphere distances were controlled by the immersion time. The monolayer of PS spheres served as an etching mask in the following ion beam etching. A broad beam DC ion beam was used to mill the films. The milling time was controlled such that not only the film was completely milled away, but the Si substrate was slightly etched as well. The nanospheres were then removed leaving only the nanodots.

Published work: 

  1. Large Enhancement of Coercivity of Magnetic Co/Pt Nanodots with Perpendicular Anisotropy, J. Appl. Phys. 101, 09J101 (2007).
Co/Pt nanodots 

Other Magnetic Nanostructures

Direct electroplating Ni on laser modified Au surface yield beautiful 3D network with coral shapes.

By filling the empty spaces in ordered nanosphere crystals, macroporous Ni networks have been  fabricated with well controlled periodicity.

Ni nanotubes have been fabricated by phase-separation technique of co-plating Ni and Cu.

Published work:
  1. Magnetic and magneto-transport properties of electrodeposited magnetic nano-network on laser modified Au surface, J. Appl. Phys. 95, 6989 (2004)
  2. Fabrication and Magnetic Properties of Ordered Macroporous Nickel Structures, J. Electrochem. Soc. 154, D65 (2007).
  3. Exploiting finite size effects in a novel core/shell microstructure, J. Appl. Phys. 103, 064313 (2008)
electroplated Ni network


ordered porous Ni network