Dominant role of the epitaxial strain in the magnetism of core-shell Co/Au self-organized nanodots

Increasing Magnetic Anisotropy in Bimetallic Nanoislands Grown on fcc(111) Metal Surfaces

The magnetic properties and the atomic scale morphology of bimetallic two-dimensional nanoislands, epitaxially grown on fcc(111) metal surfaces, have been studied by means of Magneto-Optical Kerr Effect and Scanning Tunneling Microscopy. We investigate the effect on blocking temperature of one-dimensional interlines appearing in core-shell structures, of two-dimensional interfaces created by capping, and of random alloying. The islands are grown on Pt(111) and contain a Co-core, surrounded by Ag, Rh, and Pd shells, or capped by Pd. The largest effect is obtained by Pd capping, increasing the blocking temperature by a factor of three compared to pure Co islands. In addition, for Co-core Fe-shell and Co-core Fe_xCo_1−x-shell islands, self-assembled into well ordered superlattices on Au(11,12,12) vicinal surfaces, we find a strong enhancement of the blocking temperature compared to pure Co islands of the same size. These ultra-high-density (15 Tdots/in²) superlattices of CoFe nanodots, only 500 atoms in size, have blocking temperature exceeding 100 K. Our findings open new possibilities to tailor the magnetic properties of nanoislands.

Dynamic magnetic characterization and magnetic particle imaging enhancement of magnetic-gold core-shell nanoparticles

Multifunctional nanoparticles with a magnetic core and gold shell structures are emerging multi-modal imaging probes for disease diagnosis, image-guided therapy, and theranostic applications. Owing to their multi-functional magnetic and plasmonic properties, these nanoparticles can be used as contrast agents in multiple complementary imaging modalities. Magnetic particle imaging (MPI) is a new pre-clinical imaging system that enables real-time imaging with high sensitivity and spatial resolution by detecting the dynamic responses of nanoparticle tracers. In this study, we evaluated the dynamic magnetic properties and MPI imaging performances of core-shell nanoparticles with a magnetic core coated with a gold shell. A change in AC hysteresis loops was detected before and after the formation of the gold shell on magnetic core nanoparticles, suggesting the influence of the core-shell interfacial effect on their dynamic magnetic properties. This alteration in the dynamic responses resulted in an enhancement of the MPI imaging capacity of magnetic nanoparticles. The gold shell coating also enabled a simple and effective functionalization of the nanoparticles with a brain glioma targeting ligand. The enhanced MPI imaging capacity and effective functionality suggest the potential application of the magnetic-gold core-shell nanoparticles for MPI disease diagnostics.

Growth of Co Nanomagnet Arrays with Enhanced Magnetic Anisotropy

A trigon structure formed by submonolayer gadolinium deposition onto Au(111) is revealed as a robust growth template for Co nanodot arrays. Scanning Tunneling Microscopy and X-Ray Magnetic Circular Dichroism measurements evidence that the Co nanoislands behave as independent magnetic entities with an out-of-plane easy axis of anisotropy and enhanced magnetic anisotropy values, as compared to other self-organized Co nanodot superlattices. The large strain induced by the lattice mismatch at the interface between Co and trigons is discussed as the main reason for the increased magnetic anisotropy of the nanoislands.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin–orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L1₀ structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

The design and assembly of nanostructures exhibiting ferromagnetic hysteresis at room temperature are recognized goals for high-density data storage. Here, the authors engineer nanostructures with atomically sharp bimetallic interfaces and interlines, which exhibit large magnetic anisotropy and high temperature hysteresis.

Directed growth of mixed self-assembled monolayers on a nanostructured template: a step toward the patterning of functional molecular domains

We report on the elaboration of networks of SAM domains. More precisely, we show the feasibility in making arrays of functionalized alkylthiol nanodomains bordered with an alkylthiol matrix. The several step process relies on the replication of a self-organized cobalt array grown on Au(111). The SAM process takes place in solution. The chemical affinity of thiol for gold leads to the selective grafting of molecules on the surface. After having removed the inorganic array, alkylthiol functionalized with a terthiophene unit is grafted in free gold areas. The efficiency of the replication of the initial template depends on the stability of the first SAM. We also investigate electronic tunnel transport through oligothiophene islands with the STM. The variation of the molecular contrast with bias voltage between the two molecular species indicates a potential resonant tunneling mechanism through the orbitals of the aromatic compound.

Spin-wave-assisted thermal reversal of epitaxial perpendicular magnetic nanodots

The magnetic susceptibility of self-organized two-dimensional Co nanodots on Au(111) has been measured as a function of their size in the 2-7 nm diameter range. We show that the activation energy for the thermal reversal displays a power law behavior with the dot volume. Atomic scale simulations based on the Heisenberg Hamiltonian show that this behavior is due to a deviation from the macrospin model for dot size as small as 3 nm in diameter. This discrepancy is attributed to finite temperature effects through the thermal excitation of spin-wave modes inside the particles.