Eric E. Fullerton

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic–antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.

FeRh/FePt exchange spring films for thermally assisted magnetic recording media

The temperature-dependent magnetic response of exchange-coupled FePt/FeRh thin films is described. The FePt forms a high magnetocrystalline anisotropy, high-coercivity ferromagnetic layer. The FeRh layer is antiferromagnetic at room temperature but, upon heating above a transition temperature, becomes ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy, forming an exchange–spring system and significantly lowering the coercive field of the composite system. This feature opens intriguing possibilities for media applications for thermally assisted magnetic recording where the ferromagnetic phase of FeRh is exploited to help write the media while the antiferromagnetic phase supports the long-time stability.

Antiferromagnetically coupled magnetic media layers for thermally stable high-density recording

We describe a magnetic recording media composed of antiferromagnetically coupled (AFC) magnetic recording layers as an approach to extend areal densities of longitudinal media beyond the predicted superparamagnetic limit. The recording medium is made up of two ferromagnetic layers separated by a nonmagnetic layer whose thickness is tuned to couple the layers antiferromagnetically. For such a structure, the effective areal moment density (Mrt) of the composite structure is the difference between the ferromagnetic layers allowing the effective magnetic thickness to scale independently of the physical thickness of the media. Experimental realizations of AFC media demonstrate that thermally stable, low-Mrt media suitable for high-density recording can be achieved.

Hard/soft magnetic heterostructures: model exchange-spring magnets

An overview is provided of research on exchange-spring coupled magnetic films and multilayers, including fabrication methods, and the characterization and modeling of the magnetization reversal processes. For coupled hard/soft bilayers and multilayers the deposition process provides nanometer-scale control of thicknesses and magnetic anisotropy. Such magnetic heterostructures provide model systems for studying the exchange hardening mechanism. Recent work on epitaxial SmCo/Fe and SmCo/Co bilayers and superlattices that display many of the characteristic features of exchange-spring magnets is highlighted. Comparison of the experimental results with numerical simulations indicates that the exchange-spring behavior can be understood from the intrinsic parameters of the hard and soft layers. The simulations are extended to realistically estimate the ultimate gain in performance that can potentially be realized in permanent magnets based on the exchange-spring principle.

Compositionally controlled FePt nanoparticle materials

High temperature solution phase decomposition of Fe(CO)/sub 5/ and reduction of Pt(acac)/sub 2/ in the presence of stabilizers, oleic acid and oleyl amine, are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe(CO)/sub 5/ to Pt(acac)/sub 2/, and the compositions ranging from Fe/sub 30/Pt/sub 70/ to Fe/sub 80/Pt/sub 20/ are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess a disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces a change of internal particle structure and thus of the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around Fe/sub 55/Pt/sub 45/ will lead to the highly ordered fct phase. This Fe/sub 55/Pt/sub 45/ nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications.

Cargo-towing fuel-free magnetic nanoswimmers for targeted drug delivery.

Fuel-free nanomotors are essential for future in-vivo biomedical transport and drug-delivery applications. Herein, the first example of directed delivery of drug-loaded magnetic polymeric particles using magnetically driven flexible nanoswimmers is described. It is demonstrated that flexible magnetic nickel-silver nanoswimmers (5-6 μm in length and 200 nm in diameter) are able to transport micrometer particles at high speeds of more than 10 μm s(-1) (more than 0.2 body lengths per revolution in dimensionless speed). The fundamental mechanism of the cargo-towing ability of these magnetic (fuel-free) nanowire motors is modelled, and the hydrodynamic features of these cargo-loaded motors discussed. The effect of the cargo size on swimming performance is evaluated experimentally and compared to a theoretical model, emphasizing the interplay between hydrodynamic drag forces and boundary actuation. The latter leads to an unusual increase of the propulsion speed at an intermediate particle size. Potential applications of these cargo-towing nanoswimmers are demonstrated by using the directed delivery of drug-loaded microparticles to HeLa cancer cells in biological media. Transport of the drug carriers through a microchannel from the pick-up zone to the release microwell is further illustrated. It is expected that magnetically driven nanoswimmers will provide a new approach for the rapid delivery of target-specific drug carriers to predetermined destinations.

All-optical control of ferromagnetic thin films and nanostructures

All-optical magnetic state switching Magneto-optical memory storage media, such as hard drives, use magnetic fields to change the magnetization of memory bits, but the process is slow. Light can often reveal information about the magnetization state of a sample, such as its field direction. Lambert et al. show that under the right circumstances, light can also switch the magnetization state of a thin ferromagnetic film. Using light pulses instead of magnetic fields led to ultrafast data memory and data storage. Science, this issue p. 1337 The all-optical control of magnetization in thin ferromagnetic films is demonstrated. The interplay of light and magnetism allowed light to be used as a probe of magnetic materials. Now the focus has shifted to use polarized light to alter or manipulate magnetism. Here, we demonstrate optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits.

Nanoscale magnetic materials and applications

Spin Dynamics: Fast Switching of Macro-spins.- Core-Shell Magnetic Nanoclusters.- Designed Magnetic Nanostructures.- Superconductivity and Magnetism in Silicon and Germanium Clathrates.- Neutron Scattering of Magnetic Materials.- Tunable Exchange Bias Effects.- Dynamics of Domain Wall Motion in Wires with Perpendicular Anisotropy.- Magnetic Nanowires for Domain Wall Logic and Ultrahigh Density Data Storage.- Bit-Patterned Magnetic Recording: Nanoscale Magnetic Islands for Data Storage.- The Magnetic Microstructure of Nanostructured Materials.- Exchange-Coupled Nanocomposite Permanent Magnets.- High-Temperature Samarium Cobalt Permanent Magnets.- Nanostructured Soft Magnetic Materials.- Magnetic Shape Memory Phenomena.- Magnetocaloric Effect and Materials.- Spintronics and Novel Magnetic Materials for Advanced Spintronics.- Growth and Properties of Epitaxial Chromium Dioxide (CrO2) Thin Films and Heterostructures.- FePt and Related Nanoparticles.- Magnetic Manipulation of Colloidal Particles.- Applications of Magnetic Nanoparticles in Biomedicine.- Nano-Magnetophotonics.- Hard Magnetic Materials for MEMS Applications.- Solid-State Magnetic Sensors for Bioapplications.

Reducing the critical current for spin-transfer switching of perpendicularly magnetized nanomagnets

We describe nanopillar spin valves with perpendicular anisotropy designed to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability. By adjusting the perpendicular anisotropy and volume of the free element consisting of a [Co/Ni] multilayer, we observe that the critical current scales with the height of the anisotropy energy barrier and we achieve critical currents as low as 120 μA in quasistatic room-temperature measurements of a 45 nm diameter device. The field-current phase diagram of such a device is presented.

Separating dipolar broadening from the intrinsic switching field distribution in perpendicular patterned media

A critical requirement for bit patterned media applications is the control and minimization of the switching field distribution (SFD). Here, we use the ΔH(M,ΔM) method to separate dipolar interactions due to neighbor islands from the intrinsic SFD by measuring a series of partial reversal curves of perpendicular anisotropy Co∕Pd based multilayer films deposited onto prepatterned Si substrates. For a 100-nm-period island array the dipolar broadening contributes 22% to the observed SFD. For a 45-nm-period array this value increases to 31%. These results highlight the importance of quantifying long-range dipolar interactions for determining the intrinsic SFD of patterned media.