Microplasma-Based Growth of Biphasic NiFe₂O₄/NiO Nanogranular Films for Exchange Bias Applications

Abstract

Biphasic, nanostructured NiFe 2 O 4 /NiO films exhibiting exchange bias were synthesized using high-pressure microplasma-based deposition. Organometallic Ni and Fe precursors were dissociated via electron impact in the hollow cathode region of a supersonic plasma jet with oxygen to form a directed flux of Ni and Fe oxide species that were subsequently spray-deposited on SiO 2 /Si substrates to form nanostructured thin films. Exchange bias (H E ) and coercivity (H C ) enhancement were observed for all films containing NiO after field cooling, indicating an exchange interaction between NiFe 2 O 4 and NiO. H E and H C increased with NiO content, which may be explained by a concomitant increase in NiFe 2 O 4 /NiO interfacial density. Large H E (∼250 Oe) and H C (∼700 Oe) were measured at 20 K, and exchange bias persisted at high temperatures (305 K) where an H E and H C of 80 and 400 Oe were observed, respectively. ■ INTRODUCTIONIntimate contact between a ferromagnet (FM) and an antiferromagnet (AFM) can result in exchange bias (EB), a phenomenon where the strong magnetic anisotropy of the AFM causes a shift (H E ) in the magnetic behavior of the softer, FM material after the system is field-cooled through the Neél temperature (T N ) of the AFM. These systems are characterized by pinning of magnetic spins where exchange coupling at the FM/AFM interface imparts supplementary magnetic anisotropy opposing the moment reversal of the FM in moderate to large reverse fields. 1 In addition, increased coercivity (H C ) and opening of hysteresis loops (M R /M S ; remanence/saturation magnetization) are well-known effects of exchange coupling in FM/AFM systems, which are of particular interest for permanent magnet design. 2−4 Exchange bias has also been studied in other inhomogeneous systems, such as those involving ferrimagnets (FiM) and spin glass (SG) phases as well as for applications including data storage (M-RAM), microelectronics (spin and tunneling valves), magnetic sensors (read/write heads), and medicine (drug delivery). 5,6 More recently, there has been renewed interest in exchange biasing of magnetic nanoparticles (d < 20 nm) for applications in ultrahigh density recording 6−9 to surpass the superparamagnetic limit, i.e., the temperature above which ferromagnetism becomes thermally unstable due to a decrease in magnetic anisotropy energy with size.A wealth of fabrication techniques have been explored to synthesize particle-based exchange bias systems, albeit with mixed results. The first and most widely studied method consists of chemically modifying transition metal-based FM nanoparticles using partial oxidation, nitridation, or sulfidization (e.g., Ni−NiO, 10 FeCo−FeCoO, 11 Fe−FeS, 12 Co−CoN 13 ), which can result in exchange bias effects. 14 Issues with these approaches include a limited suite of material combinations and difficulty in independently controlling the (oxide) shell thickness 15 and crystallinity. 16 The resulting "disordered" structures often lead to weak exchange bias and the onset of superparamagn...

Evolution of Microstructure and Exchange Bias Characteristics of NiFe2O4/NiO nanocomposite

“…With increasing sintering temperature, the size of particles become larger and the NiFe 2 O 4 particles may form ferromagnetic type clusters in NiO matrix as result of which effective coupling among the NiFe 2 O 4 and NiO decreases. Similar types of observations have also been reported [19,33,[60][61][62]. Hence it is clear from the study that relatively larger size NiFe 2 O 4 particles or more ferromagnetic NiFe 2 O 4 phase in the system is not the good choice for the occurrence of EB in this system.…”

“…Pebley et al [33] have synthesized nanostructured NiFe 2 O 4 / NiO films using high-pressure microplasma-based deposition by varying Ni molar compositions (x Ni ). These authors reported that the presence of both NiFe 2 O 4 and NiO in the samples prepared with x Ni = 0.49−0.74.…”

“…In another work, Pebley et al, [33] synthesized NiFe 2 O 4 / NiO nanocomposites by varying Ni molar fraction and studied the EB characteristics of the samples with Ni molar fraction. These authors reported that both C H and H EB under FC condition increase with increasing Ni molar fraction in the sample.…”

Exchange bias and spin glass behavior in biphasic NiFe2O4/NiO thin films

“…1 and detailed elsewhere [35]. The MP source consists of a stainless steel capillary tube (i. d.¼500 μm) operating as the cathode and a grounded ring as the anode with Macor insulator.…”

Microplasmas for direct, substrate-independent deposition of nanostructured metal oxides

“…As such, we continually seek to develop general and tunable methods that can easily and rapidly create nanostructured functional materials. For example, atmospheric pressure plasmas [5,6], plasma sprays [7][8][9], and microplasmas [10][11][12][13][14][15][16][17][18][19][20] have shown much promise toward this goal. Extending and adapting such methods in a generic way to different material systems and deposition situations, as well as understanding how plasma operating conditions affect growth processes, is critical for their implementation.…”