Exchange anisotropy of macrospins in superferrimagnetic structure

Context:

Magnetic nanoparticles (NPs) are used in many applications, including as potential building blocks for information storage. Single-domain magnetic nanoparticles present peculiar physical properties, which significantly differ from the same materials in the bulk state. In sufficiently small single-domain ferromagnetic particles (FMs), all spin moments rotate coherently and can thus be assimilated to macrospins [Stoner1948]. Moreover, the relatively large fraction of atoms occupying surface sites leads to a strong sensitivity of their magnetic properties (anisotropy, magnetic order, magnetization) to the surrounding environment, and many fundamental questions remain to be explored.

In this work, we will investigate the magnetic properties of NPs integrated in a superferrimagnetic (SFiM) heterostructure. A ferrimagnetic material (FiM) is characterized by the coexistence of two sublattices of spins, of different moments, coupled antiparallel by exchange. In the case of SFiM, these are two FM phases, of different magnetizations, coupled antiparallel by exchange at the interface. The concept of SFiM was proposed by Akdogan et al [Akdogan2014] to increase the resistance against demagnetization of permanent magnets. Its field of exploration has so far been restricted to bulk systems or micrometer-thick films [LeRoy2014], made of magnetically hard, multidomain FM grains, associated with a magnetically soft intergranular phase and supposedly exchange-coupled antiparallel.

Project:

The SFiM principle has not yet been investigated in single-domain nano-grain systems, whether embedded in a matrix or deposited on a surface. In such systems, the combination of an increased effective magnetic volume of the NP (including the exchange-coupled layer) and a reduced net magnetization resulting from the antiparallel interfacial coupling is expected to affect both the thermal stability (associated with the energy barrier separating two stable macrospin states) and the reversal field. The present project aims to experimentally quantify these effects.

The SFiM system considered in this work consists of assemblies of 3d transition metal nanoparticles (Co) deposited on (or embedded in) a rare-earth layer (Gd). In its pure solid state, Gd is ferromagnetic below 293 K and exhibits very low magnetocrystalline anisotropy due to its zero orbital angular momentum (L = 0), making it a suitable model system for investigating the SFiM mechanism.

Methodology:

The samples will be prepared using the facilities of PLYRA (Plateforme LYonnaise de Recherches sur les Agrégats) at the ILM-Tech platform. Co nanoparticles will be synthesized by the Low Energy Cluster Beam Deposition technique, based on laser vaporization and condensation of an atomic vapor, followed by low-energy deposition onto a substrate under ultra-high vacuum conditions (base pressure ~10⁻¹⁰ mbar). Prior to deposition, the nanoparticles will be mass-selected using an electrostatic deflector, enabling a relatively narrow size distribution (standard deviation inferior to 15%) centered on diameters between 1 and 8 nm. The Co/Gd heterostructures will be fabricated by sequential deposition of the nanoparticles and Gd atoms evaporated using an electron-beam source.  To investigate macrospin reversal and thermal stability, the PhD candidate will use a Superconducting Quantum Interference Device (SQUID) magnetometer available on the “Transport” characterization facility of the ILM-Tech platform.

References:

[Akdogan2014] O. Akdogan et al, J. Appl. Phys. 115 (17), 17A764 (2014) ; DOI:10.1063/1.4869067

[Le Roy2014] D Le Roy et al, J. Appl. Phys. 115 (17), 17A738 (2014) ;

DOI:10.1063/1.4867128

[Stoner1948] E.C. Stoner, E.P. Wohlfarth, Trans. Roy. Soc. (London) A240, 599 (1948) ; DOI:10.1098/rsta.1948.0007

The Institut Lumière Matière (iLM) is a joint research unit of the CNRS and Université Claude Bernard Lyon 1, located on the LyonTech-La Doua campus. With a staff of 310 members, including approximately one third PhD students and postdoctoral researchers, iLM is a center of excellence in physics and chemistry in the Auvergne-Rhône-Alpes region and enjoys international recognition for the quality of its research.

The institute structures its approach around a continuum linking fundamental research, responses to societal challenges, and innovation. This approach is accompanied by a collective commitment to scientific excellence, ethics, and responsibility in research.

The scientific activities of iLM are organized around six main thematic areas: 

- Advanced materials and optics

- Complex Matter and Out-of-Equilibrium Systems

- Nanosciences

- Optics, dilute media, and ultrafast processes

- Theory and modeling

- Living Systems, Health, and Environment

Strong background in Condensed Matter Physics or Materials Science. Some knowledge of programming. The application should include : CV, a statement of interest, and a Master's degree transcript (marks and ranking if available)