Tuning the Crystal Structure and Magnetic Properties of FePt Nanomagnets

Abstract

Ferromagnetic nanoparticles are important materials for nanotechnology both in practical applications and in fundamental research. The different purposes for which they are used have posed significant challenges in their preparation. For example, the development of highly sensitive magnetoresistive (MR) sensors makes it possible to detect the binding interactions between DNA or protein molecules, which are attached by magnetic beads.[1] To achieve high selectivity and sufficient signal-to-noise ratio, oxidation-resistive ferromagnetic nanomagnets are more favorable than the superparamagnetic microbeads usually used. Nanocomposite exchangespring magnets were expected to have unusually high energy products (BH) max (magnetic flux density times magnetic field strength), which requires the fabrication of crystallographically coherent hard and soft magnetic phases with a proper exchange coupling. [2,3] This requires nanometer-scale structure and property control of both the hard and soft magnetic components. Future extremely high density magnetic storage media demand high anisotropic magnetic materials, such as L1 0 -phase FePt, to overcome the superparamagnetic limit.[4]Chemical ordering and orientation control are essential prerequisites for the popular wet-chemical approach, [5] and these are difficult to achieve by post-deposition annealing. [6] We have recently reported a vacuum technique based on the gasphase condensation principle to prepare directly ordered FePt nanoparticles. [7] Here we present a general methodology of tuning the crystal structure and magnetic properties of FePt binary alloy nanomagnets.The final crystal structure of nanoparticles is a result of their thermal history, which involves both thermodynamic and kinetic factors. The binary phase diagram of equiatomic FePt alloy shows that it has two solid-state phases: fcc disordered (face-centered cubic or A1 phase) and fct ordered (face-centered tetragonal or L1 0 phase), with fcc as the high-temperature stable phase.[8] Many of the preparation techniques are kinetically controlled, so the fcc disordered phase dominates their final products. To get the fct ordered phase post-deposition annealing is generally used. This annealing process is a first-order phase transformation process, which requires nucleation sites for the ordering to originate. In FePt thin films, grain boundaries and crystal defects can supply such starting points for phase transformation. FePt nanoparticles are more difficult to transform because they do not have conventional grain boundaries and do not have many defects either.[9] Accordingly, any post-annealing approach usually results in particle agglomeration and twin formation, which will limit their applications. An alternative, and also optimal, way to prepare nanoparticles with the desired crystal structure is to control the thermal environment for their nucleation and growth, which is the main focus of this work. There are several different ways to control the thermal environment for particle nucleation and growth by ...

Synthesis and Characteristics of a Phase‐Change Magnetic Material

“…It is not easy to alter their properties by phase change or other physical means once they have been synthesized. [7][8][9] Therefore, it is highly desirable to synthesize a new material exhibiting both magnetic and phase-change features. Here we show a new material synthesized by introducing a new degree of freedom, associated with the spin of carriers, into a chalcogenide-based phase-change material.…”

Magnonics: Spin Waves on the Nanoscale

“…[13] Top-down fabrication processes, as discussed here, might then be overtaken by bottom-up approaches. [39][40][41] Intentionally introduced nanodefects should then be explored to provide storage devices for SW information. Advanced device concepts will address periodically patterned magnets as metamaterials, where the field-controlled and programmable dynamic response offers multifunctionality up to optical frequencies by the design of the nanostructures.…”

Direct Gas‐Phase Synthesis of Heterostructured Nanoparticles through Phase Separation and Surface Segregation

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Heterostructured nanoparticles (NPs) offer great opportunities for applications in areas such as biomedicine, renewable energy, information processing, and data storage by virtue of their embedded multiple functionalities and enhanced magnetic, [1,2] optical, [3] and catalytic performance [4] arising from the interactions of the individual components within the NPs.To understand the fundamental nature of these interactions, as well as to utilize them more efficiently, it is imperative to develop synthesis methods to prepare heterostructured NPs with high-quality interfaces and surfaces. Several heterostructures have been synthesized by the seed-growth approach using wet chemical methods, [5][6][7][8][9] sometimes followed by a chemical vapor deposition (CVD) process.

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“…In the experimental setup used in this study (see Supporting Information), [2] the materials (Co and Au in this case) have been vaporized by the bombardment of Ar + ions from a solid-state composite target. The energetic atoms leave the target surface and lose their energy through collisions with Ar molecules.…”