Synthesis and Magnetic Properties of Ferrimagnetic CoFe₂O₄ Nanoparticles Embedded in an Antiferromagnetic NiO Matrix

Abstract

Ultrafine cobalt ferrite nanoparticles were prepared by forced hydrolysis in diethyleneglycol and, after centrifugation, dispersed in the same solvent to form a sol in which a layered hydroxyacetate nickel salt was precipitated. The as-obtained composite was moderately calcined in air to form nanoparticles constituted by a ferrimagnetic CoFe2O4 core embedded in well-crystallized antiferromagnetic NiO. Magnetic properties such as coercivity, hysteresis loop shift, and blocking temperature were analyzed regarding exchange bias features by comparing the magnetic data measured on these particles to those obtained on fresh CoFe2O4 particles mechanically dispersed in a diamagnetic Al2O3 matrix.

Exchange-biased NiFe2O4/NiO nanocomposites derived from NiFe-layered double hydroxides as a single precursor

“…For NF-2-500, the saturation magnetization (M s ) reached 8.10 emu/g, while for NF-3-500 and NF-4-500, the magnetization does not saturate and continues to increase quasi-linearly with the field. These differences can be explained as follows: firstly, with increasing content of the NiO phase, which is an antiferromagent below 525 K [37] …”

“…For instance, spinel ferrite/MnO core/shell nanoparticles can be fabricated by a chemical precipitation method [36]. CoFe 2 O 4 nanoparticles embedded in the NiO matrix have been prepared by the so-called "polyol method" [37], and NiFe 2 O 4 nanoparticles embedded in a NiO matrix can be synthesized by a chemical coprecipitation method [38]. All these studies showed that the exchange bias interaction between the heterogeneous spinel ferrite and oxide phases could improve the magnetic stability of the ferrite nanoparticles to a certain extent.…”

High Temperature Magnetic Stabilization of Cobalt Nanoparticles by an Antiferromagnetic Proximity Effect

“…In fact, it has been demonstrated [4] that ferromagnetic-antiferromagnetic (FM-AFM) interfacial exchangecoupling is an effective method, later patented by Seagate [12], to increase the effective K of FM nanoparticles. However, a T B enhancement beyond RT using this approach has been rarely reported [22][23][24][25][26] (where often broad particle size distribution can partly account for the "apparent" T B increase [22][23][24][25]). The reason for this scarcity is that high Néel temperature (T N ) AFMs tend to have a low anisotropy constant (e.g., NiO), and vice versa (e.g., CoO), while substantial values of both properties are required for high-temperature stabilization.…”

“…Despite the foreseeable change of recording paradigm from continuous to patterned media, where each bit is recorded in an individual nanostructure [7], the key for sustained storage density increase will remain the introduction of progressively more anisotropic (high K) materials [8], which allow for magnetic stability at very small volumes, V (i.e., blocking temperature, T B ∝ KV, above room temperature, RT). Two main strategies are largely investigated to achieve high K (both of them with implications in other active technologies beyond information storage, such as permanent magnets, magnetic hyperthermia or even sensors [5,[9][10][11]): (i) the use of compounds with intrinsically high magnetocrystalline anisotropy (such as FePt [3,8]) and (ii) the design of exchange-coupled nanocomposites [4,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].Unfortunately, most high-K materials require high-temperature annealing processes to obtain the desired phase, which could hamper their implementation in certain structures. Thus, FM-AFM exchange coupling alternatives may be an appealing option.…”

SrFe12O19/ZnO hybrid structures: Synthesis, characterization and properties

“…Especially, ferrite/metal oxide hybrid materials have attracted considerable attention because of their unique mechanical, electrical, magnetic and catalytic properties [2][3][4][5]. Recently, ZnO-based magnetic semiconductors have attracted increasing interest because of their unique properties with possible technological applications utilizing both the semiconductor physics and the ferromagnetism [6].…”