The behaviour of nanostructured magnetic materials produced by depositing gas-phase nanoparticles

Abstract

Depositing pre-formed gas-phase nanoparticles, whose properties can be widely varied, onto surfaces enables the production of films with designed properties. The films can be nanoporous or, if co-deposited with an atomic vapour, granular, allowing independent control over the size and volume fraction of the grains. This high degree of control over the nanostructure of the film enables the production of thin films with a wide variety of behaviour, and the technique is destined to make a significant contribution to the production of high-performance magnetic materials. Here we review the behaviour of magnetic nanoparticle assemblies on surfaces and in non-magnetic and magnetic matrices deposited from the gas phase at densities from the dilute limit to pure nanoparticle films with no matrix. At sufficiently low volume fractions (∼1%), and temperatures well above their blocking temperature, nanoparticle assemblies in non-magnetic matrices show ideal superparamagnetism. At temperatures below the blocking temperature, the magnetization behaviour of both Fe and Co particles is consistent with a uniaxial intra-particle magnetic anisotropy and an anisotropy constant several times higher than the bulk magnetocrystalline value. At relatively low volume fractions (≥5%) the effect of inter-particle interactions becomes evident, and the magnetization behaviour becomes characteristic of agglomerates of nanoparticles exchange coupled to form magnetic grains larger than a single particle that interact with each other via dipolar forces. The evolution of the magnetic behaviour with volume fraction is predicted by a Monte-Carlo model that includes exchange and dipolar couplings. Above the percolation threshold the films become magnetically softer, and films of pure clusters have a magnetic ground state that obeys the predicted magnetization behaviour of a correlated super-spin glass characteristic of random anisotropy materials. Magnetic nanoparticles in non-magnetic matrices show giant magnetoresistance behaviour, and the magnetotransport in deposited nanoparticle films is reviewed. Assembling Fe nanoparticles in Co matrices and vice versa is a promising technique for producing magnetic materials with a saturation magnetization that exceeds the Slater–Pauling limit. Structural studies reveal that the particles' atomic structure is dependent on the matrix material, and it is possible to prepare Fe nanoparticles with an fcc structure and, unusually, Co particles with a bcc structure. We also look to the future and discuss applications for materials made from more complex bi-metallic and core–shell nanoparticles.

Spin-dependent transport in cluster-assembled nanostructures: influence of cluster size and matrix material

“…No magnetic field effect, however, could be detected. The matrix, on the other hand, does intervene indirectly in the MTGV experiments since the magnitude of the cluster magnetic moment strongly depends on the surrounding medium [15,16]. We believe the different degree of quenching of the cluster magnetic moment [17] to be responsible for the difference in cluster size showing a comparable MTGV for Co-doped samples of Cu or Ag matrices.…”

Influence of size, shape and core–shell interface on surface plasmon resonance in Ag and Ag@MgO nanoparticle films deposited on Si/SiOx

“…Finally, the NPs can be covered with a transparent, dielectric shell, or embedded in a matrix in order to protect them from air exposure or to engineer the SPR energy position [9]. Realization of preformed, massselected, metal NPs by means of gas aggregation sources [19][20][21][22] allowed the systematic investigation of the NP structure and of the NP film morphology to be evaluated, relating them with "functional" properties, such as magnetic phase as a function of temperature [19]. Recently, it was also possible to co-deposit the preformed metal NPs and a flux of atoms obtained by evaporation, resulting in a core-shell structure with independently controlled core size and shell thickness [19,23,24].…”

“…Realization of preformed, massselected, metal NPs by means of gas aggregation sources [19][20][21][22] allowed the systematic investigation of the NP structure and of the NP film morphology to be evaluated, relating them with "functional" properties, such as magnetic phase as a function of temperature [19]. Recently, it was also possible to co-deposit the preformed metal NPs and a flux of atoms obtained by evaporation, resulting in a core-shell structure with independently controlled core size and shell thickness [19,23,24]. This method was also used to produce a non-native oxide shell and to study the evolution of the physical properties of the NP assemblies with increasing shell thickness, owing to a configuration where NPs are embedded in a metal [19] or oxide solid matrix [23,24].…”

Interaction effects in dilute cluster-assembled magnetic nanostructures

“…It has successfully been used to fit GMR curves of cluster-assembled samples. 15,16 Since scattering within the cluster can be neglected, 10,14,17 the model boils down to spin-disorder scattering 18 with a magnetoresistance ⌬R ϰ −M͑H , T͒ 2 , i.e.,…”