Jeongjae Lee

Tuning the interlayer exchange coupling between single perpendicularly magnetized CoFeB layers

We experimentally study the tunability of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interlayer exchange coupling (IEC) in Pt/CoFeB/Pt/Ru/Pt/CoFeB/Pt stacks with perpendicular magnetic anisotropy (PMA). The perpendicular magnetization of a single Pt/Co60Fe20B20/Pt (at. %) shows full remanence and square hysteresis loops for a CoFeB thickness range of 0.60–1.0 nm. By inserting a Pt layer between the Ru and CoFeB, the PMA of the ultrathin CoFeB layers is stabilized and the IEC can be tuned. In particular, we show that the IEC versus Pt thickness exhibits a simple exponential decay with a decay length of 0.16 nm.

Metal–Organic Nanosheets Formed via Defect-Mediated Transformation of a Hafnium Metal–Organic Framework

We report a hafnium-containing MOF, hcp UiO-67(Hf), which is a ligand-deficient layered analogue of the face-centered cubic fcu UiO-67(Hf). hcp UiO-67 accommodates its lower ligand:metal ratio compared to fcu UiO-67 through a new structural mechanism: the formation of a condensed “double cluster” (Hf12O8(OH)14), analogous to the condensation of coordination polyhedra in oxide frameworks. In oxide frameworks, variable stoichiometry can lead to more complex defect structures, e.g., crystallographic shear planes or modules with differing compositions, which can be the source of further chemical reactivity; likewise, the layered hcp UiO-67 can react further to reversibly form a two-dimensional metal–organic framework, hxl UiO-67. Both three-dimensional hcp UiO-67 and two-dimensional hxl UiO-67 can be delaminated to form metal–organic nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate bonds and is effected under mild conditions, suggesting that defect-ordered MOFs could be a productive route to porous two-dimensional materials.

DYNAMIC SCALING OF MAGNETIC HYSTERESIS IN MICRON-SIZED NI80FE20 DISKS

The scaling of the magnetic hysteresis loop area of permalloy disks (20–400 μm diam) has been studied as a function of applied field amplitude H0 and frequency Ω using scanning Kerr microscopy. An increase in the dynamic coercivity with reduced size is observed for d<100 μm in the frequency range studied (0.1–800 Hz). However, the loop area A follows the scaling relation A∝H0αΩβ, with α≈0.14 and β≈0.50 throughout the entire size range studied. Our results demonstrate that the dynamic scaling behavior is universal even though the lateral size influences the domain structure and magnetic reversal behavior.

DOMAIN IMAGING DURING SOLITON PROPAGATION IN A 3D MAGNETIC RATCHET

The propagation of a kink soliton through a perpendicularly magnetized antiferromagnetically coupled multilayer stack has been imaged by scanning laser Kerr microscopy. The soliton behavior allows layer-by-layer reversal leading to clear evidence of changes of switching behavior of different layers in the stack. We find that the density of domain nucleation sites is dependent on the configuration of the neighboring layers as well as height up the stack. By growing a series of single layer and coupled trilayer samples, we are able to explain the trends in nucleation seen in the soliton stack in terms of pinhole and orange peel coupling, in agreement with STEM (Scanning transmission electron microscope) imaging.

Controlling nucleation in perpendicularly magnetized nanowires through in-plane shape

The nucleation field of perpendicularly magnetized nanowires can be controlled by changing their width, so that below a critical width the nucleation field decreases as the width decreases. Placing pads at the ends of the nanowires prevents any reduction in coercivity with width, demonstrating that at small widths domain walls nucleate from the ends of the wires. Using this technique, we are able to create asymmetric nanowires with controlled nucleation at a defined point. We also show how dipole fields from a neighboring wire in close proximity can be used to shift the hysteresis loop of the asymmetric nanowire, creating a simple NOT gate. These results show how control of the in-plane shape of perpendicularly magnetized nanoscale elements can directly lead to device functionality.

Dynamic selective switching in antiferromagnetically-coupled bilayers close to the spin reorientation transition

We have designed a bilayer synthetic antiferromagnet where the order of layer reversal can be selected by varying the sweep rate of the applied magnetic field. The system is formed by two ultra-thin ferromagnetic layers with different proximities to the spin reorientation transition, coupled antiferromagnetically using Ruderman-Kittel-Kasuya-Yosida interactions. The different dynamic magnetic reversal behavior of both layers produces a crossover in their switching fields for field rates in the kOe/s range. This effect is due to the different effective anisotropy of both layers, added to an appropriate asymmetric antiferromagnetic coupling between them. Field-rate controlled selective switching of perpendicular magnetic anisotropy layers as shown here can be exploited in sensing and memory applications.

Soliton propagation in micron-sized magnetic ratchet elements

We demonstrate ratchet soliton propagation in individual patterned antiferromagnetically coupled superlattice elements down to 3 μm diameter using magneto-optical Kerr effect measurements. The bulk switching and soliton propagation fields are investigated as a function of the element size. It is found that on the length scale investigated here we do not see significant variation in ratchet behavior depending on the element size. The margin for soliton propagation and additional features related to downscaling are discussed.

Temperature dependence of spin waves in Co/CoO bilayers

Brillouin light scattering measurements of spin-wave frequencies in an exchange coupled ferromagnet/antiferromagnet epitaxial Co/CoO bilayer are reported. A striking temperature dependence of the measured spin-wave frequencies in the cobalt layer in the range 77–300 K was observed which has been demonstrated to be due to exchange coupling to the ultrathin (7 A) CoO layer as antiferromagnetic order develops. The temperature dependence of the spin-wave frequency demonstrates that interface exchange coupling occurs in the absence of the unidirectional anisotropy. A study of the mode line width shows a broadening with reducing temperature which indicates that locally ordered antiferromagnet regions persist above the Neel temperature and play a central role in determining the magnetic behavior of the bilayer system.

A robust soliton ratchet using combined antiferromagnetic and ferromagnetic interlayer couplings

A sharp magnetic soliton can be created and propagated in a vertical ratchet structure based on magnetic layers with out-of-plane anisotropy using a combination of antiferromagnetic and ferromagnetic interlayer couplings. This allows the use of identical magnetic layers in the stack, which simplifies the implementation of the ratchet compared to schemes which use alternating layer thicknesses. The ratchet behavior is analyzed using an Ising-macrospin approximation and conditions are derived for the propagation of a soliton, which is demonstrated experimentally. Values extracted from the experimental data for the coercivities and interlayer couplings show significant variation, which demonstrates the robustness of the soliton propagation.

Magnetic properties and interlayer coupling of epitaxial Co/Cu films on Si

Thin films of Co and Co/Cu/Co trilayers with wedged Cu interlayers were grown epitaxially on Cu buffer layers on hydrogen passivated Si(001) wafers. We find that single Co layers have a well-defined four-fold anisotropy but with smaller in-plane anisotropies than observed in Co grown on Cu crystals. Ruderman–Kittel–Kasuya–Yosida (RKKY) interlayer coupling is observed in one Co/Cu/Co sample which is the smoothest of the films as measured by atomic force microscopy. Some of the films also form a dot-like structure on the surface. Intermixing at elevated temperatures between the Cu buffer and Si limits the ability to form flat surfaces to promote RKKY coupling.