Towards atomic-scale design: A theoretical investigation of magnetic nanoparticles and ultrathin films

Abstract

This paper reports recent theoretical work on nanostructured materials including magnetic alloyed CoRh nanoparticles that are good candidates to combine both a large magnetic moment and a high magnetic anisotropy, and nanoscale Mn films adsorbed on W substrates as an example of artificial magnetic material. It illustrates how modern atomistic modeling and simulation can fruitfully supplement the experiments performed in the laboratory by helping to resolve and understand the experimental information, by predicting new phenomena and providing useful hints to guide the development of innovative materials with original, specifically tailored properties.

Introduction

Triggered by the emerging opportunities in miniaturized nanoscale devices and the broad range of potential technological applications, an intense research field is nowadays concerned by the possibility of generating innovative nanostructured materials with controlled and adjusted properties. This is for instance the case in the information storage industry as the rapidly increasing amount of data requires the development of new recording media and devices with always bigger storage capacities, or in micro- and nanoelectronics with the aim to design smaller novel circuit elements.

In this context, magnetic nanostructured materials like magnetic nanoparticles and ultrathin films, are extensively studied as potential break-through technologies. Through the enhancement of surface or interface effects, the reduction of dimensionality gives indeed rise to unique properties that are often completely different from those observed in the bulk materials and that may lead to novel materials or devices. For example, recent experiments have demonstrated the interest of alloyed CoRh nanoparticles to combine both a large magnetic moment and a high anisotropy, while the reduction of size leads furthermore to a strong magnetic enhancement with respect to the bulk solids [1]. The epitaxial growth of nanoscale thin films on appropriate substrates also represents another way to modify considerably the properties of bulk solids through the stabilization of unusual crystalline phases, suggesting hereby the possibility of generating artificial materials. As a representative system, we consider here the case of ultrafine Mn films adsorbed on W substrates that exhibit at room temperature an unusual base-centered-cubic (bcc) structure and are magnetic, whereas this phase in the bulk state is only stable at very high temperature, beyond any magnetic ordering temperature [3], [4].

In such nanoscale systems, understanding the complex interplay between structure, chemical order and magnetism as well as the growth mechanisms in view of controlling and adjusting them, requires atomically resolved information which is often difficult to draw from laboratory experiments only. Many questions remain indeed open and need to be clarified. Another complication arises from the quantum behavior characteristic from the nanoscale which often escapes to the intuition one can try to infer from the macroscopic world. The lack might be compensated by the use of appropriate computational approaches using state-of-the-art quantum mechanical calculations, and possibly, less-demanding semi-empirical simulations with high-level interaction potentials, to get access to larger systems and hence to the mesoscopic or macroscopic properties.

In the following we illustrate this computational approach and its application to the two mentioned examples: small CoRh nanoparticles from atomic clusters to sizes of technological interest, and ultrathin Mn layers epitaxied on a W substrate.