Current Research

    Our group's research focuses on understanding the role of randomness and disorder in nanostructured magnetic material. Systems with grain sizes from 5 nm to 50 nm are fabricated using cluster-beam assembly. The clusters are collected on a cold finger and compacted, and the resulting structures are composed of crystalline grains (with sizes from 5 to 50 nm) that are separated by an interphase region. The atoms in the crystallites are shown as open circles and the interphase atoms are shown as solid circles. This ultra-high-vacuum-based fabrication technique is extremely versatile. For example, nanostructures can be made from a single type of atom (i.e. nickel) so that the clusters and the interphase are chemically alike but structurally different. Co-deposition can be used to make systems consisting of clusters of more than one type. Careful selection of materials allows the production of systems in which the crystallites and the interphase atoms are chemically distinct. Our cluster-beam deposition technique provides high monodispersity, good control over the average crystallite size and the versatility to deposit any material that can be sputtered. Our current research projects include:
'Ordering' in systems with competing interactions: Some early measurements of elemental magnetic nanostructures suggest that the interplay between exchange coupling and magnetocrystalline anisotropy produce a state similar to a spin glass or random anisotropy magnet; however, many of the early studies were troubled by contamination and widely varying sample morphologies. We are fabricating nanostructures with high chemical purity and well-controlled morphologies to determine whether structural disorder alone can produce spin-glass-like behavior and how this behavior is related to canonical systems such as Cu_1-xMn_x.
The effects of disorder in magnetic alloys: Many alloys derive their magnetic properties from the order imposed by a regular crystalline structure. The magnetic properties can be profoundly affected when that order is interrupted, for example, by making small grains or introducing defects. We are interested in alloys that are ferromagnetic in their bulk form and exhibit spin glass behavior in their nanostructured forms. These materials include GdAl₂, SmCo₅ and Co₂Ge, which have been studied using ball milling to produce chemical disorder. Our studies of mechanically milled SmCo₅ show that the magnetic coercivity depends critically on the presence of vacancies and antisite defects, and that these milled materials exhibit spin-glass-type behavior. Our current projects focus on identifying the origin of glassy behavior in mechanically milled and cluster-beam deposited systems and understanding how disorder can be used to produce high-energy-product permanent magnets.
Exchange-coupled magnetic systems: An 'exchange spring' mechanism couples the magnetizations of nanometer-sized hard magnetic grains interspersed with soft magnetic grains. The resulting structures can have very high energy products. We are using our cluster-beam deposition technique to fabricate systems such as SmCo:Co with the goal of optimizing structural parameters such as grain size and volume fraction to produce materials with desirable magnetic properties and understanding the fundamental origins of the exchange-spring behavior.
Chemically Synthesized Magnetic Systems: A joint project with Reuben Rieke in the Chemistry Department investigates the production of novel magnetic materials using chemical synthesis. The very high chemical reactivity of the metals produced using the Rieke technique allows for the creation of metastable alloys such as Ni₃C. We are studying the carbonization process in nickel and cobalt nanoparticles and are developing new techniques for producing magnetic alloys such as NiCo and MgCo.

Recent Key Publications

P. M. Shand, T. M. Pekarek, R. Skomski, V. Petkov and D. L. Leslie-Pelecky, “Magnetic Transitions in Disordered GdAl₂”, Phys. Rev. B 68, 214404 (2003).

P. M. Shand, C. Stark, T. Pekarek, R. Brown, Lanping Yue, D. L. Leslie-Pelecky, “Curie-Weiss Analysis of Ferromagnetic and Glassy Transitions in Nanostructured GdAl₂”, J. Appl. Phys. 93, 6525-6527 (2003).

R. Skomski, D. Leslie-Pelecky, R.D. Kirby, A. Kashyap, D.J. Sellmyer, “Coercivity of Disordered Nanostructures”, Scripta Mater. 48, 857-862 (2003).