Structure, morphology, and magnetic properties of Fe nanoparticles deposited onto single-crystalline surfaces

Abstract

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...

Ensemble magnetic behavior of interacting CoFe nanoparticles

“…The black dashed line in Figure 3B shows a fit using Equation (1) to the data, r bulk ls is fixed at 0.044 [31]. Only the outermost layer of atoms are expected to have a reduced coordination so d is fixed at an the results of our study are combined with data from previous studies [25,[28][29][30]. The black dashed line is a fit using the model described in the text.…”

“…We have studied the magnetic properties of densely packed Co 30 Fe 70 nanoparticles. XMCD spectroscopy combined with AFM size determination reveal the Fe orbital to spin moment ratio m l /m s decreases rapidly with increasing particle size.…”

“…Co 30 Fe 70 nanoparticles were pre-formed in vacuum using an Oxford Applied Research gas-aggregation cluster source. This instrument uses a specially modified magnetron sputtering source to super-saturate metallic vapor in an inert gas atmosphere, pre-forming particles in a 5-15nm diameter size range [15].…”

“…There remain many fascinating questions to be answered, particularly regarding particle ensembles with interaction forces present. Here we present an X-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometry (VSM) study of densely packed pure Co 30 Fe 70 nanoparticles grown by in situ gas-aggregation technique.…”

Morphological and Interfacial Studies of Fe Clusters on SrTiO₃(001) Surfaces

“…Nano-sized clusters and particles are attracting much attention for both fundamentally and technologically due to their unique properties [1][2][3][4][5]. Metal clusters on oxide substrates are of particular interest due to their applicability in nanomagnetism, catalysis, and so on.…”