Surface induced suppression of magnetization in nanoparticles

A model based on competing exchange interactions is presented for the investigation of nanoparticle magnetization. The ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions contribute differently at the nanoparticle surface and interior, leading to reduced ferromagnetic order at the surface. This model predicts an unconventional temperature dependence of magnetization and a surface magnetically ‘dead layer’. This is confirmed by temperature dependent magnetization and Mössbauer measurements of FePt nanoparticles. The effects are sensitive to particle size and surface terminations. (Some figures in this article are in colour only in the electronic version) Nanoparticle magnetism is a subject of active research because of its potential applications in data storage, spintronics and biomedicine [1]. The size and temperature dependence of magnetization as well as magnetization reversal were studied both experimentally and theoretically in a variety of systems [2–7]. The results are system dependent suggesting strong effects of the surface and the nature of magnetic interactions in nanoparticles. The surface effects were studied theoretically in various aspects. First, the surface may introduce large magnetocrystalline anisotropy because of its lowered symmetry. Second, the exchange interaction near the surface can be modified because some neighbouring atoms are missing and the electronic structure of surface atoms is different from its bulk counterparts. While the enhancement of saturation magnetization (Ms) is observed in small elemental metallic clusters [7], many thin film and nanoparticle systems (commonly oxides) have reduced Ms [8]. Earlier, this effect was explained by a magnetically ‘dead layer’ [9], or random canting of the surface spins caused, for example, by competing AFM exchange interactions between sublattices [10]. Other studies suggest that the spin canting persists throughout the volume of the particle, and therefore a finite size rather than surface effect [11]. To further complicate the issue, both theoretical and experimental work suggest that surface atoms may have either enhanced or quenched moment, depending on their chemical environment [12, 13]. In this paper we show that the competition between FM and AFM interactions in nanoparticles can lead to surface induced suppression of the magnetization. The symmetry of surface termination of the nanoparticle may lead to a magnetic structure that is paramagnetic, non-collinear or FM, depending on the relative strength of the competing exchange interactions. We consider model particles with the shape of truncated octahedron (TO) terminated by (1 1 1) and (0 0 1) facets, mimicking experimentally synthesized nanoparticles [14]. The surface ordering temperature can be lower than that of the interior of the particle, resulting in a magnetically ‘dead layer’ at finite temperatures. This model is consistent with several of our experimental observations in FePt nanoparticles: (1) a significant paramagnetic volume fraction at finite temperatures, which is enhanced with decreasing particle size and diminished by cooling; (2) a large reduction of the zero temperature magnetization from bulk values; (3) an unconventional temperature dependence of the magnetization which becomes more prominent in smaller sized