Strongly Exchange Coupled Core|Shell Nanoparticles with High Magnetic Anisotropy: A Strategy toward Rare-Earth-Free Permanent Magnets

Abstract

Antiferromagnetic­(AFM)|ferrimagnetic­(FiM) core|shell (CS) nanoparticles (NPs) of formula Co0.3Fe0.7O|Co0.6Fe2.4O4 with mean diameter from 6 to 18 nm have been synthesized through a one-pot thermal decomposition process. The CS structure has been generated by topotaxial oxidation of the core region, leading to the formation of a highly monodisperse single inverted AFM|FiM CS system with variable AFM-core diameter and constant FiM-shell thickness (∼2 nm). The sharp interface, the high structural matching between both phases, and the good crystallinity of the AFM material have been structurally demonstrated and are corroborated by the robust exchange-coupling between AFM and FiM phases, which gives rise to one among the largest exchange bias (H E) values ever reported for CS NPs (8.6 kOe) and to a strongly enhanced coercive field (H C). In addition, the investigation of the magnetic properties as a function of the AFM-core size (d AFM), revealed a nonmonotonous trend of both H C and H E, which display a maximum value for d AFM = 5 nm (19.3 and 8.6 kOe, respectively). These properties induce a huge improvement of the capability of storing energy of the material, a result which suggests that the combination of highly anisotropic AFM|FiM materials can be an efficient strategy toward the realization of novel rare-earth-free permanent magnets.

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

“…In addition to its potential spintronics applications [107,108], EB is an important tool in understanding fundamental nanoscale spin ordering, knowledge of which is essential to tailor the anisotropic magnetic properties of nanoparticle systems for potential rare-earth-free permanent magnet applications [109] and for biomedical applications such as magnetic hyperthermia [42,110,111,112] via a controllable exchange coupling mechanism. In particular, Lee et al have shown that the effective anisotropy can be tuned by using exchange-coupled core/shell nanoparticles (exchange anisotropy), which, in effect, increase the heating efficiency of MNPs [110].…”

Tunnel Magnetoresistance in Self-Assemblies of Exchange-Coupled Core/Shell Nanoparticles

“…[27] In this regard, an exciting possibility is the fabrication of devices based on self-assemblies of exchange coupled core/shell MNPs with tailored magnetic properties. [28] The coercive field in these systems can be finely modified through the interface magnetic coupling [29][30][31][32][33][34], the core size and shell thickness, [35][36][37] or the magnetic anisotropy of the components. [23,[38][39][40] Devices of this type should provide a way to manipulate at will the characteristic switching field of TMR by controlling the magnetic coupling across the core/shell interface.…”

Novel Ba-hexaferrite structural variations stabilized on the nanoscale as building blocks for epitaxial bi-magnetic hard/soft sandwiched maghemite/hexaferrite/maghemite nanoplatelets with out-of-plane easy axis and enhanced magnetization

“…An alternative approach to enhance MS in hard-magnetic materials is the exchange coupling low-MS materials (e.g., BHF) with high-MS softmagnetic materials. [23][24][25][26][27][28][29][30][31][32][33][34][35][36] Actually, in recent years, there has been an increased interest in hard-soft-magnetic core/shell nanoparticles due to the large number of potential applications. 25 However, most of the studied systems have polycrystalline shells, which inevitably results in sub-optimal properties.…”