Spin Disorder and Particle Size Effects in Cobalt Ferrite Nanoparticles with Unidirectional Anisotropy and Permanent Magnet-like Characteristics

Abstract

Spinel ferrites, especially cobalt ferrite, have the capability of preserving sufficient magnetization (M s) and coercive field (H c). However, precise nanostructuring is needed to sustain the steady ferrimagnetic (FiM) response with high H c and remnant magnetization (M r). By a two-step process involving co-precipitation and inert calcination, CoFe2O4 nanoparticles (NPs) with inherent lattice strain were achieved with appreciable magnetic anisotropy. The disordered spins on the NP surface are found to be the key for maintaining the anisotropy and squareness (M r/M s) of hysteresis loops, the extent of which is a function of the processing temperature and NP size. The freezing of surface spins results in 2.8 and 1.2 nm thick spin glass (SG) layers for the as-prepared and 350 °C-calcined CoFe2O4 NPs, respectively, analyzed at 5 K. The frozen surface spins pin the core FiM spins, resulting in the exchange bias (H eb) coupling. At room temperature, 20 nm NPs exhibit permanent magnet-like features with a maximum energy product, BHmax, of 2.4 MGOe, which is quite promising considering CoFe2O4 as an environmentally friendly alternative to the hazardous rare-earth-based permanent magnets. Our CoF2O4 NP ensemble delivers both intriguing low-temperature magnetic properties having academic and technological interests and worthwhile room-temperature features that have direct translational relevance.

Photodetectors with High Responsivity by Thickness Tunable Mixed Halide Perovskite Nanosheets

“…With increasing iodide fraction, the XRD peaks of NSs and NCs shift toward lower 2θ due to higher radius of I – (198 pm) than Br – (185 pm) (Figure b and Figure S1b). The NSs have a comparatively lesser compressive lattice strain than the NCs (Figure d), measured using the Williamson–Hall equation . The strain increases from Br 3 to I 3 NSs/NCs (Figures S2 and S3), because of which I 3 NSs/NCs tend to undergo a phase transition from the photoactive α- or γ-phase to the photoinactive δ-phase .…”

“…The NSs have a comparatively lesser compressive lattice strain than the NCs (Figure 1d), measured using the Williamson−Hall equation. 34 The strain increases from Br 3 to I 3 NSs/NCs (Figures S2 and S3), because of which I 3 NSs/NCs tend to undergo a phase transition from the photoactive αor γ-phase to the photoinactive δ-phase. 35 The strain values are 1.5 × 10 −3 , 6.7 × 10 −3 , and 9.8 × 10 −3 for Br 3 , Br 1.5 I 1.5 , and I 3 NSs and 5.4 The transmission electron microscope (TEM) images in Figure 2a−c show the average lateral xy dimensions of Br 3 , Br 1.5 I 1.5 , and I 3 NSs to be 176 ± 20, 210 ± 26, and 400 ± 29 nm, respectively.…”

“…The intrinsic strain of NSs and NCs were calculated by the Williamson–Hall equation, eq : where β hkl is the width of the XRD peak at full width half-maximum (fwhm), k is the Scherrer constant, λ is the wavelength of X-ray radiation, D is the crystallite size, and ε is the strain. The ε values were determined from the slope of β hkl cos θ versus 4­(sin θ) plots.…”

“…Calculation of Lattice Strain. The intrinsic strain of NSs and NCs were calculated by the Williamson−Hall equation, eq 3: 34 k D cos / 4 (sin )…”

A controllable one-pot hydrothermal synthesis of spherical cobalt ferrite nanoparticles: synthesis, characterization, and optical properties

“…2,[12][13][14][15][16] Cobalt ferrite nanoparticles have been synthesized using various synthesis methods such as mechanical milling, laser ablation, co-precipitation, plasma synthesis, hydrothermal, auto-combustion, microwave, sol-gel, and microemulsion. 14,17,18 Choice of the synthetic route and the applied reaction conditions have a great impact on the characteristics of the produced CoFe 2 O 4 nanoparticles such as morphology, crystallinity, surface area, as well as chemical and physical properties. 17,18 Among the synthesis routes, hydrothermal synthesis is a controllable, facile, eco-friendly, and energy-efficient route for the synthesis of pure CoFe 2 O 4 nanoparticles with tunable particle size.…”