Mass-filtered Fe and FeCo nanoparticles are deposited under soft-landing and ultra-high
vacuum conditions on the bare W(110) surface. The structure and the stoichiometry of
FeCo alloy nanoparticles are determined by ex situ high resolution transmission electron
microscopy (HRTEM) and energy-dispersive x-ray spectroscopy (EDX) on single
nanoparticles, respectively, proving a crystalline structure. In situ scanning tunneling
microscopy (STM) shows that the clusters are irregularly distributed on the W(110)
surface, that no fragmentation occurs, that steps do not act as preferential adsorption sites,
and yields strong evidence for a partial flattening of the particles upon deposition.
The magnetic anisotropy energy (MAE) of mass-filtered pure Fe nanoparticles is
investigated by means of x-ray magnetic circular dichroism (XMCD) at the iron
L3
edge. The magnetization loops reveal an uniaxial magnetic anisotropy with the magnetic
hard axis being perpendicular to the surface. The experimentally determined MAE is
compared to calculated shape and interface anisotropy contributions of partially
flattened nanoparticles according to the STM observations on the FeCo clusters.
Summary
Background: Magnetic nanostructures and nanoparticles often show novel magnetic phenomena not known from the respective bulk materials. In the past, several methods to prepare such structures have been developed – ranging from wet chemistry-based to physical-based methods such as self-organization or cluster growth. The preparation method has a significant influence on the resulting properties of the generated nanostructures. Taking chemical approaches, this influence may arise from the chemical environment, reaction kinetics and the preparation route. Taking physical approaches, the thermodynamics and the kinetics of the growth mode or – when depositing preformed clusters/nanoparticles on a surface – the landing kinetics and subsequent relaxation processes have a strong impact and thus need to be considered when attempting to control magnetic and structural properties of supported clusters or nanoparticles.
Results: In this contribution we focus on mass-filtered Fe nanoparticles in a size range from 4 nm to 10 nm that are generated in a cluster source and subsequently deposited onto two single crystalline substrates: fcc Ni(111)/W(110) and bcc W(110). We use a combined approach of X-ray magnetic circular dichroism (XMCD), reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) to shed light on the complex and size-dependent relation between magnetic properties, crystallographic structure, orientation and morphology. In particular XMCD reveals that Fe particles on Ni(111)/W(110) have a significantly lower (higher) magnetic spin (orbital) moment compared to bulk iron. The reduced spin moments are attributed to the random particle orientation being confirmed by RHEED together with a competition of magnetic exchange energy at the interface and magnetic anisotropy energy in the particles. The RHEED data also show that the Fe particles on W(110) – despite of the large lattice mismatch between iron and tungsten – are not strained. Thus, strain is most likely not the origin of the enhanced orbital moments as supposed before. Moreover, RHEED uncovers the existence of a spontaneous process for epitaxial alignment of particles below a critical size of about 4 nm. STM basically confirms the shape conservation of the larger particles but shows first indications for an unexpected reshaping occurring at the onset of self-alignment.
Conclusion: The magnetic and structural properties of nanoparticles are strongly affected by the deposition kinetics even when soft landing conditions are provided. The orientation of the deposited particles and thus their interface with the substrate strongly depend on the particle size with consequences regarding particularly the magnetic behavior. Spontaneous and epitaxial self-alignment can occur below a certain critical size. This may enable the obtainment of samples with controlled, uniform interfaces and crystallographic orientations even in a random deposition process. However, such a reorientation process might be accompanied by a complex reshapin...
A continuously working arc cluster ion source was used to prepare mass-filtered Fe nanoparticles with a mean size of 6 to 10 nm. Their structure was determined by HRTEM. The nanoparticles were deposited into an Al matrix and additionally on a W(110) substrate for comparison. Within the matrix they maintain their spherical shape and do not show any magnetic anisotropic effect. For uncapped Fe nanoparticles on a tungsten surface, a flattening is observed by STM and the nanoparticles exhibit a distinct magnetic anisotropy with the easy magnetization axis being in the surface plane. Correlating both investigations we conclude that this observation is due to shape and interface-induced contributions to the magnetic anisotropy.
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