Short-range magnetic order in two-dimensional cobalt-ferrite nanoparticle assemblies (original) (raw)
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Size and surface effects in the magnetic order of CoFe2O4 nanoparticles
Journal of Magnetism and Magnetic Materials, 2015
In this work, we have focused on the size dependence of the magnetic properties and the surface effects of CoFe 2 O 4 nanoparticles synthesized by high-temperature chemical method with diameter d$ 2, 4.5, and 7 nm, with narrow size distribution. transmission electron microscopy (TEM) images and X-ray diffraction (XRD) profiles indicates that samples with 7 and 4.5 nm present a high crystallinity while the 2 nm sample has a poor one. We have investigated by magnetization measurements and in-field Mössbauer spectroscopy the influence of the surface in the internal magnetic order of the particles. Particles with d¼7 nm have almost single domain behavior and the monodomain occupies approximately the whole particle. In the sample with d¼4.5 nm the surface anisotropy is large enough to alter the ferrimagnetic order in the particle shell. Then, a surface/volume ratio of $ 60% is the crossover between a single domain nanoparticle and a frustrated order in a magnetic core-shell structure, due to the competition between surface anisotropy and exchange interaction þcrystalline anisotropy in cobalt ferrite. In the d¼2 nm sample the poor crystallinity and the large surface/volume ratio avoid the ferrimagnetic order in the particle down to T ¼5 K.
Nature materials, 2004
The use of magnetic nanoparticles in the development of ultra-high-density recording media is the subject of intense research. Much of the attention of this research is devoted to the stability of magnetic moments, often neglecting the influence of dipolar interactions. Here, we explore the magnetic microstructure of different assemblies of monodisperse cobalt single-domain nanoparticles by magnetic force microscopy and magnetometric measurements. We observe that when the density of particles per unit area is higher than a determined threshold, the two-dimensional self-assemblies behave as a continuous ferromagnetic thin film. Correlated areas (similar to domains) of parallel magnetization roughly ten particles in diameter appear. As this magnetic percolation is mediated by dipolar interactions, the magnetic microstructure, its distribution and stability, is strongly dependent on the topological distribution of the dipoles. Thus, the magnetic structures of three-dimensional assembli...
Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles
Journal of Nanoparticle Research, 2011
Cobalt ferrite, CoFe 2 O 4 , nanoparticles in the size range 2-15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D TEM . Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M S , are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below *160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T 2 law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D TEM , and a thickness of 3.7 to 5.1 Å was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T B 160 K), observing a large hysteresis at magnetic fields H [ *1000 Oe compared to smaller ones (H B *1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T 0 , as deduced by assuming a Vogel-Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T b , observed in the FC magnetization curves.
Unravelling Finite Size Effects on Magnetic Properties of Cobalt Nanoparticles
The Journal of Physical Chemistry C, 2019
Using first principle calculations, the problem of scaling magnetic properties in nanoparticles is addressed. To this aim, the local electronic structure is characterized in cobalt quasi-spherical magnetic nanoparticles, in a large size range, from 0.5 to 2 nm of diameter. First, specific patterns of the magnitude of local spin magnetic moments are evidenced depending on the shape and the size of the nanoparticles. Then, effects of local structural environment (atomic coordination, structural deformations, finite size effects, shape changes) are unravelled. In small icosahedral nanoparticles, the local spin magnetic moment is found to decrease from the surface to the center. General rules driving charge transfers are observed whereby donor atomic sites are exclusively subsurface atoms and more unexpected vertex surface atomic sites. The variation of the magnetic moment is driven by the coupling between cluster microstructure and complex hybridization effects. In larger truncated octahedral clusters, whereas some properties are still found (quasi-zero inter-atomic site charge transfer, the reversal from anti-ferromagnetic to ferromagnetic coupling of electronic sp states with d ones at vertex atomic sites), other vary such as the behavior of the local spin magnetic moment which now presents weak oscillations.
Interparticle Interactions Effects on the Magnetic Order in Surface of Fe3O4 Nanoparticles
Journal of Nanoscience and Nanotechnology, 2008
We report interparticle interactions effects on the magnetic structure of the surface region in Fe 3 O 4 nanoparticles. For that, we have studied a desirable system composed by Fe 3 O 4 nanoparticles with d = 9 3 nm and a narrow size distribution. These particles present an interesting morphology constituted by a crystalline core and a broad (∼50% vol.) disordered superficial shell. Two samples were prepared with distinct concentrations of the particles: weakly-interacting particles dispersed in a polymer and strongly-dipolar-interacting particles in a powder sample. M H T measurements clearly show that strong dipolar interparticle interaction modifies the magnetic structure of the structurally disordered superficial shell. Consequently, we have observed drastically distinct thermal behaviours of magnetization and susceptibility comparing weakly-and strongly-interacting samples for the temperature range 2 K < T < 300 K. We have also observed a temperature-field dependence of the hysteresis loops of the dispersed sample that is not observed in the hysteresis loops of the powder one.
Investigation of the static and dynamic magnetic properties of CoFe2O4 nanoparticles
Journal of Magnetism and Magnetic Materials, 2008
We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300 K under zero external DC field for frequencies ranging from f ¼ 10 to 10,000 Hz. A prominent peak appears in both w 0 and w 00 as a function of T. The peak temperature T 2 of w 00 depends on f following the Vogel-Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at T B m $94 K in ZFC and 119 K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2 K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295 K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K ¼ 6 Â 10 6 erg/cm 3 . Mö ssbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mo¨ssbauer spectrum was fitted with two magnetic subspectra with internal fields H int of 490, 470 and 515 kOe, corresponding to Fe 3+ ions in A and B sites. r
Magnetic order in - nanoparticles: a XMCD study
Journal of Magnetism and Magnetic Materials, 2005
Spin-canting in spinel nanoparticles of maghemite has multiple origins, among which surface effects, finite-size effects or chemical disorder effects. XMCD at the Fe L 2;3 edges allows to separate the contributions of the magnetic moments of Fe 3þ ions in tetrahedral and octahedral sites of g-Fe 2 O 3 : We investigate three powders of g-Fe 2 O 3 synthetized via aqueous precipitation: particles of average diameter 2.7, 8 nm and particles of average diameter 8 nm coated with phosphoric acid. The relative contributions of the spins of the Fe 3þ Td and the Fe 3þ Oh ions are observed, varying the external magnetic field. Under high magnetic fields, a reduction of the magnetic contribution of the Fe 3þ Oh ions occurs for 8 nm phosphate-coated particles by comparison with the uncoated ones. A similar reduction appears at lower magnetic fields for the small 2.7 nm particles.
International Journal for Innovation Education and Research, 2021
In this study we report on the synthesis and characterization of cobalt ferrite (CoFe2O4) nanoparticles (NPs), synthesized by chemical co-precipitation in alkaline medium. Two samples were synthesized at two different temperatures, 35 and 90 oC. Both samples were characterized by Transmission Electron Microscopy (TEM), x-ray diffraction (XRD), and room-temperature (RT) magnetization. Two samples showed superparamagnetic behavior (SPM) at RT. TEM reveals morphological mean diameter increasing 5.8 nm to 10.4 nm, with the increase of the co-precipitation temperature. XRD confirm the inverse cubic spinel structure. The RT magnetization curves were analyzed by the first-order Langevin function averaged out by a lognormal distribution function of magnetic moments. This analysis showed saturation magnetization and magnetic moment increases from 60.2 to 74.8 emu/g and from 3.9 x 103 to 8.2 x 103 mB, respectively.
Chemistry of Materials, 2013
This paper focuses on the magnetic properties of CoFe 2 O 4 nanoparticles, discussing the influence of nanoparticles arrangements obtained by different synthesis methods. Using high thermal decomposition (HTD) and direct micellar (DM) routes, three samples of CoFe 2 O 4 nanoparticles with equal primary particle size (∼5 nm) were prepared. The HTD method allows one to obtain highly crystalline primary nanoparticles coated by oleic acid organized in a self-assembling arrangement (ACoFe HTD). The DM method results to be appropriate to prepare either irregular arrangements (IACoFe DM) or spherical iso-oriented nanoporous assemblies (SACoFe DM) of primary CoFe 2 O 4 nanocrystals. Despite the same particle size, magnetization measurements of the HTD sample show a tendency toward cubic anisotropy (M r / M s ≈ 0.7), while in DM samples, a uniaxial anisotropy (M r /M s ≈ 0.4) is observed. The comparison between IACoFe DM and SACoFe DM samples indicates that the ordering of nanocrystals at the mesoscopic scale induces an increase of the coercive field (μ 0 H c ≈ 1.17 T → μ 0 H c ≈ 1.45 T) and of the reduced remanent magnetization (M r /M s ≈ 0.4 → M r /M s ≈ 0.5). The reason for these differences is discussed. In particular, a detailed study on interparticle interactions is carried out, highlighting the influence of the molecular coating and the formation of spherical iso-oriented assemblies.
Dalton Transactions, 2014
The magnetic properties of copper ferrite (CuFe 2 O 4 ) nanoparticles prepared via sol-gel auto combustion and facile solvothermal method are studied focusing on the effect of nanoparticle arrangement. Randomly oriented CuFe 2 O 4 nanoparticles (NP) are obtained from the sol-gel auto combustion method, while the solvothermal method allows us to prepare iso-oriented uniform spherical ensembles of CuFe 2 O 4 nanoparticles (NS). X-ray diffractometry (XRD), atomic absorption spectroscopy (AAS), infra-red (IR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), 57 Fe Mössbauer spectroscopy and vibrating sample magnetometer (VSM) are used to investigate the composition, microstructure and magnetic properties of as-prepared ferrite nanoparticles. The field-dependent magnetization measurement for the NS sample at low temperature exhibits a steplike rectangular hysteresis loop (M R /M S ∼ 1), suggesting cubic anisotropy in the system, whereas for the NP sample, typical features of uniaxial anisotropy (M R /M S ∼ 0.5) are observed. The coercive field (H C ) for the NS sample shows anomalous temperature dependence, which is correlated with the variation of effective anisotropy (K E ) of the system. A high-temperature enhancement of H C and K E for the NS sample coincides with a strong spin-orbit coupling in the sample as evidenced by significant modification of Cu/Fe-O bond distances. The spherical arrangement of nanocrystals at mesoscopic scale provokes a high degree of alignment of the magnetic easy axis along the applied field leading to a step-like rectangular hysteresis loop. A detailed study on the temperature dependence of magnetic anisotropy of the system is carried out, emphasizing the influence of the formation of spherical iso-oriented assemblies. † Electronic supplementary information (ESI) available: Atomic absorption spectroscopy (AAS) analysis ), X-ray diffraction analysis, magnetic property analysis (orientation of magnetic easy axis). See