Superparamagnetic Blocking Temperature Research Articles

Structural analysis of nickel doped cobalt ferrite nanoparticles prepared by coprecipitation route

Abstract Magnetic nanoparticles of nickel substituted cobalt ferrite (Ni x Co 1− x Fe 2 O 4 :0≤ x ≤1) have been synthesized by co-precipitation route. Particles size as estimated by the full width half maximum (FWHM) of the strongest X-ray diffraction (XRD) peak and transmission electron microscopy (TEM) techniques was found in the range 18–28±4 nm. Energy dispersive X-ray (EDX) analysis confirms the presence of Co, Ni, Fe and oxygen as well as the desired phases in the prepared nanoparticles. The selective area electron diffraction (SAED) analysis confirms the crystalline nature of the prepared nanoparticles. Data collected from the magnetization hysteresis loops of the samples show that the prepared nanoparticles are highly magnetic at room temperature. Both coercivity and saturation magnetization of the samples were found to decrease linearly with increasing Ni-concentration in cobalt ferrite. Superparamagnetic blocking temperature as determined from the zero field cooled (ZFC) magnetization curve shows a decreasing trend with increasing Ni-concentration in cobalt ferrite nanoparticles.

Evolution of size, morphology, and magnetic properties of CuO nanoparticles by thermal annealing

Cupric oxide (CuO) nanoparticles with different morphologies were synthesized by thermal annealing of the copper hydroxide at various temperatures. Significant changes in both the particle size and the morphology with the annealing temperature (TA) were observed. The average particle size (d) increases from 13 to 33 nm and the morphology varies from ellipsoidal to rodlike as TA increases from 150 to 550 °C. The formation of these morphologies is explained in terms of the variation in the interplanar H-bonds breaking rate with different temperatures. The magnetic measurements reveal the presence of weak ferromagnetic interaction and the blocking behavior in these nanoparticles. The magnetic field dependence of the superparamagnetic blocking temperature (TB) follows the Brown equation. In addition, the linear variation in zero field cooled susceptibility with particle size is consistent with the predictions of Néel model for the uncompensated spins. These surface spins are responsible for the observed anomalous magnetic properties of CuO nanoparticles.

Synthesis and Structural and Magnetic Characterization of Ni(Core)/NiO(Shell) Nanoparticles

A size series of ligand-stabilized Ni nanoparticles (NPs) with diameters between 8-24 nm was prepared by solution chemistry, followed by solution-phase oxidation with atmospheric oxygen at 200 degrees C to form Ni(core)/NiO(shell) NPs with shell thicknesses of 2-3 nm. In comparison with the oxidation of Fe and Co NPs, Ni NPs require higher temperatures for significant conversion to NiO. Transmission electron microscopy and electron diffraction show polycrystalline cores with predominantly amorphous shells. SQUID magnetometry measurements were performed to assess the effects of coupling between the ferromagnetic Ni cores and antiferromagnetic NiO shells. After intentional oxidation, the Ni(core)/NiO(shell) NPs have decreased superparamagnetic blocking temperatures (T(B)) and no exchange shift (H(EB)), but a small enhancement in the coercivity (H(C)) signifies weak exchange bias. These effects originate from the amorphous structure of the NiO shells and their thin layer thickness that renders the NiO moments incapable of pinning the core moment in moderate applied fields. The magnetocrystalline anisotropy constants before and after oxidation approach the value for bulk Ni and depend on the Ni core size and NiO shell thickness.

Synthesis and magnetic properties of gold coated iron oxide nanoparticles

We report on synthesis, structural, and magnetic properties of chemically synthesized iron oxide (Fe3O4) and Fe3O4@Au core-shell nanoparticles. Structural characterization was done using x-ray diffraction and transmission electron microscopy, and the magnetite phase of the core (∼6nm) and fcc Au shell (thickness of ∼1nm) were confirmed. Magnetization (M) versus temperature (T) data at H=200Oe for zero-field-cooled and field-cooled modes exhibited a superparamagnetic blocking temperature TB∼35K (40K) for parent (core-shell) system. Enhanced coercivity (Hc∼200Oe) at 5K along with nonsaturating M-H loops observed for Fe3O4@Au nanoparticles indicate the possible role of spin disorder at the Au–Fe3O4 interface and weak exchange coupling between surface and core spins. Analysis of ac susceptibility (χ′ and χ″) data shows that the interparticle interaction is reduced upon Au coating and the relaxation mechanism follows the Vogel–Fulcher law.

Synthesis and magnetic characterization of nickel ferrite nanoparticles prepared by co-precipitation route

Magnetic nanoparticles of nickel ferrite (NiFe 2O 4) have been synthesized by co-precipitation route using stable ferric and nickel salts with sodium hydroxide as the precipitating agent and oleic acid as the surfactant. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses confirmed the formation of single-phase nickel ferrite nanoparticles in the range 8–28 nm depending upon the annealing temperature of the samples during the synthesis. The size of the particles ( d) was observed to be increasing linearly with annealing temperature of the sample while the coercivity with particle size goes through a maximum, peaking at ∼11 nm and then decreases for larger particles. Typical blocking effects were observed below ∼225 K for all the prepared samples. The superparamagnetic blocking temperature ( T B ) was found to be increasing with increasing particle size that has been attributed to the increased effective anisotropy energy of the nanoparticles. The saturation moment of all the samples was found much below the bulk value of nickel ferrite that has been attributed to the disordered surface spins or dead/inert layer in these nanoparticles.

Magnetic properties of Fe–Ag granular alloys

A discontinuous Fe/Ag multilayer and a granular alloy, both with the overall nominal composition of Fe 10Ag 90, were studied by bulk magnetization and Mössbauer spectroscopy measurements. The superparamagnetic blocking temperatures, as measured by the low-field susceptibility, agree quite well but the external-field dependence of the magnetization shows large difference for the two samples at all temperatures and the grain sizes obtained from a Langevin fit to the bulk magnetization differ in almost a factor of two. The discrepancy is attributed to the different directions of anisotropy in the samples, as observed by Mössbauer measurements as well as by comparison of the magnetizations measured in parallel and perpendicular applied fields. Similar grain size is obtained for the two samples by both techniques when a simple large-field approximation ( M( H) ∼ M(∞)(1 − kT/ μH)) is applied in the calculations.

Exchange bias of singly invertedFeO/Fe3O4core-shell nanocrystals

Exchange bias (EB) is investigated in singly inverted $\text{FeO}/{\text{Fe}}_{3}{\text{O}}_{4}$ core-shell nanocrystals by vibrating sample magnetometry. A horizontal and vertical shift of the field-cooled (FC) hysteresis loop with respect to the zero-field-cooled (ZFC) hysteresis loop is observed below the blocking temperature of EB $({T}_{B}^{\text{EB}}\ensuremath{\approx}{T}_{N})$. An enhancement of the superparamagnetic blocking temperature $({T}_{B}^{\text{SP}}\ensuremath{\approx}190\text{ }\text{K})$ with respect to the expected value is observed from the ZFC/FC measurement of $m(T)$. Furthermore, the value of ${T}_{B}^{\text{SP}}$ is nearly constant for applied fields ranging from 25 to 200 Oe as a result of the exchange coupling at the antiferromagnetic/ferrimagnetic interface.

The correlation between superparamagnetic blocking temperatures and peak temperaturesobtained from ac magnetization measurements

We study the correlation between the superparamagnetic blocking temperatureTB and the peakpositions Tp observed in ac magnetization measurements for nanoparticles of different classes of magnetic materials. Ingeneral, Tp = α+βTB. Theparameters α andβ are different for thein-phase (χ′) and out-of-phase(χ′′) components anddepend on the width σV of the log-normal volume distribution and the class of magnetic material(ferromagnetic/antiferromagnetic). Consequently, knowledge of bothα and β is required if the anisotropy energy barrierKV and theattempt time τ0 are to be reliably obtained from an analysis based solely on the peak positions.

Magnetic and optical response of tuning the magnetocrystalline anisotropy in Fe 3O 4 nanoparticle ferrofluids by Co doping

Co x Fe 3− x O 4 (0⩽ x⩽0.10) nanoparticles coated with tetramethyl ammonium hydroxide as a surfactant were synthesized by a co-precipitation technique. The Fe:Co ratio was tuned up to x=0.10 by controlling the Co 2+ concentration during synthesis. The mean particle size, determined by transmission electron microscopy, ranged between 15±4 and 18±4 nm. The superparamagnetic blocking temperature and the magnetocrystalline anisotropy constant of the ferrofluids, determined using ac and dc magnetic measurements, scale approximately linearly with cobalt concentration. We also find distinct differences in the optical response of different samples under an applied magnetic field. We attribute changes in field-induced optical relaxation for the x=0 and 0.05 samples to differences in the anisotropic microstructure under an applied magnetic field.

Nanocrystalline spinel ferrites by solid state reaction route

Nanostructured NiFe2O4, MnFe2O4 and (NiZn)Fe2O4 were synthesized by aliovalent ion doping using conventional solid-state reaction route. With the doping of Nb2O5, the size of NiFe2O4 is reduced down to 33 nm. Similarly, nanostructured manganese ferrites (MnFe2O4) with diameters in the range of 45–30 nm were synthesized by Ti4+ ion doping. Particle diameters in all the specimens are found to decrease with increasing dopant content. The substitution of Nb5+ or Ti3+ ions essentially breaks up the ferrimagnetically active oxygen polyhedra. This created nanoscale regions of ferrites. Saturation magnetization and coercive field show a strong dependence on the size of the ferrite grains. Superparamagnetic behaviour is observed from the Mössbauer spectra of nanostructured NiFe2O4, if the particle size is reduced to 30 nm. Zero field cooled and field cooled curves from 30 nm sized MnFe2O4 particles showed a peak at T B (∼ 125 K), typical of superparamagnetic blocking temperature. These results are explained in terms of core/shell structure of the materials. The d.c. resistivity of the doped specimens decreases by atleast five orders of magnitude compared to pure sample. This is ascribed to the presence of an interfacial amorphous phase between the sites.

Crystallographically oriented Fe nanocrystals formed in Fe-implanted TiO2

A comprehensive characterization of the structural and magnetic properties of Fe-implanted rutile TiO2(110) is presented. Fe and FeTiO3 (ilmenite) nanocrystals (NCs) are identified by synchrotron-radiation x-ray diffraction. The majority of Fe NCs are crystallographically oriented with respect to the matrix following the relation Fe(001)[010]∥TiO2(110)[11̱0]. Postannealing induced the out-diffusion of Fe and the growth of FeTiO3 at the cost of Fe NCs. Mössbauer spectroscopy and superconducting quantum interference device (SQUID) magnetometry reveal the corresponding evolution of magnetic properties, i.e., magnetization, and superparamagnetic blocking temperature. We unambiguously identify Fe NCs as the origin of the ferromagnetism. These Fe NCs possess a uniaxial in-plane magnetic anisotropy, such that the two Fe[100] axes are inequivalent.

HIGH CURIE TEMPERATURE OF NANOSIZED NiZn FERRITE PARTICLES SYNTHESIZED BY A COMBUSTION METHOD

The magnetic properties of nanocrystalline powders of Ni 0.5 Zn 0.5 Fe 2 O 4, synthesized at low temperatures, by a combustion method, have been investigated. Different powders containing crystallites of comparable average size, obtained by varying the synthesis conditions, show a large difference in the superparamagnetic blocking temperature due to the wide difference in the size distribution. The ferrite nanoparticles show large Curie temperature enhancement up to ~ 100 K, when compared to that of the bulk.

Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.

Dominant factor of zero-field-cooled magnetization in discontinuous Fe films

The zero-field-cooled (ZFC) magnetization under the various field and temperature conditions has been investigated using the discontinuous Fe films. Among the various factors influencing the ZFC magnetization, it is found that either the thermal relaxation or the Langevin behavior dominates the ZFC magnetization, depending on the energy barrier distribution which is altered by the growth temperature. The peak temperature of ZFC magnetization follows the N\'eel-Brown model for the narrower energy barrier distribution. With broadening the energy barrier distribution for the higher growth temperature, the Langevin behavior of thermally fluctuated particles becomes dominant. The change of energy barrier distribution, namely, the dominant factor of ZFC magnetization, is explained by the broadening of size distribution and the degradation of crystallinity with increasing growth temperature. For both cases, we estimate the superparamagnetic blocking temperature ${T}_{B}$ and obtain the effective magnetic anisotropy and the effective volume from the field dependence of ${T}_{B}$. From the obtained values, we show the presence of interparticle interaction for the Fe grown at $323\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and discuss the effective magnetic anisotropy of randomly oriented particles grown above $573\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Magnetic Properties of Nickel–Zinc Ferrite Toroids Prepared from Nanoparticles

Toroids comprised of silica-coated 10 nm diameter nickel–zinc (Ni–Fe) ferrite nanoparticles (Ni0.5Zn0.5Fe2O4) have been fabricated by careful control of both the coating process and subsequent densification by viscous sintering. A narrow processing window is identified between a maximum temperature at which the nanoparticles coarsen, losing their super-paramagnetic properties, and a lower temperature required for viscous flow densification. Key to the successful fabrication was drying and cold isostatic pressing of the silica-coated nanoparticles; other routes invariably led to cracking during either drying or sintering. The super-paramagnetic blocking temperature, the coercive field, and remanent magnetization could all be controlled over a wide range by varying the thickness of the silica coating from 1 to 15 nm. The dipole–dipole coupling distance is estimated to be 4 nm. The high-frequency (1–500 MHz) properties were sensitive to the sintering temperature as well as the thickness of the silica coating. Toroids sintered at 1000°C or less exhibited no high-frequency magnetic losses and their permeability decreased with increasing temperature, suggesting that the permeability was controlled by thermally activated magnetization relaxation.

Epitaxial growth, magnetic properties, and lattice dynamics of Fe nanoclusters on GaAs(001)

Epitaxial bcc-Fe(001) ultrathin films have been grown at $\ensuremath{\sim}50\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ on reconstructed $\mathrm{Ga}\mathrm{As}(001)\text{\ensuremath{-}}(4\ifmmode\times\else\texttimes\fi{}6)$ surfaces and investigated in situ in ultrahigh vacuum (UHV) by reflection high-energy electron diffraction, scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and $^{57}\mathrm{Fe}$ conversion electron M\ossbauer spectroscopy (CEMS). For ${t}_{\mathrm{Fe}}=1$ ML (monolayer) Fe coverage, isolated Fe nanoclusters are arranged in rows along the [110] direction. With increasing ${t}_{\mathrm{Fe}}$ the Fe clusters first connect along the $[\ensuremath{-}110]$, but not along the [110] direction at 2.5 ML, then consist of percolated Fe clusters without a preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at ${t}_{\mathrm{Fe}}=4$ ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature-dependent in situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures ${T}_{B}$ of $62\ifmmode\pm\else\textpm\fi{}5$, $80\ifmmode\pm\else\textpm\fi{}10$, and $165\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for ${t}_{\mathrm{Fe}}=1.9$, 2.2, and 2.5 ML, respectively. At $Tl{T}_{B}$, freezing of superparamagnetic clusters is inferred from the observed quasilinear $T$ dependence of the mean hyperfine magnetic field $⟨{B}_{hf}⟩$. By combining the STM and CEMS results, we have determined a large magnetic anisotropy constant of $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ and $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{3}$ at ${t}_{\mathrm{Fe}}=1.9--2.2$ and 2.5 ML, respectively. For ${t}_{\mathrm{Fe}}\ensuremath{\leqslant}2.5$ ML, our uncoated ``free'' Fe clusters exhibit intrinsic magnetic ordering below ${T}_{B}$, contrary to literature reports on metal-coated Fe clusters on GaAs. Our present results demonstrate that the nature of the percolation transition for free Fe nanoclusters on GaAs(001) in UHV is from superparamagnetism to ferromagnetism. From the M\ossbauer spectral area, a very low Debye temperature ${\ensuremath{\Theta}}_{D}$ of $196\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is deduced for these uncoated Fe nanoclusters in UHV, indicating a strong phonon softening in the clusters.

A comparative Mössbauer study of the mineral cores of human H-chain ferritin employing dioxygen and hydrogen peroxide as iron oxidants

Ferritins are ubiquitous iron storage and detoxification proteins distributed throughout the plant and animal kingdoms. Mammalian ferritins oxidize and accumulate iron as a ferrihydrite mineral within a shell-like protein cavity. Iron deposition utilizes both O 2 and H 2O 2 as oxidants for Fe 2+ where oxidation can occur either at protein ferroxidase centers or directly on the surface of the growing mineral core. The present study was undertaken to determine whether the nature of the mineral core formed depends on the protein ferroxidase center versus mineral surface mechanism and on H 2O 2 versus O 2 as the oxidant. The data reveal that similar cores are produced in all instances, suggesting that the structure of the core is thermodynamically, not kinetically controlled. Cores averaging 500 Fe/protein shell and diameter ∼ 2.6 nm were prepared and exhibited superparamagnetic blocking temperatures of 19 and 22 K for the H 2O 2 and O 2 oxidized samples, respectively. The observed blocking temperatures are consistent with the unexpectedly large effective anisotropy constant K eff = 312 kJ/m 3 recently reported for ferrihydrite nanoparticles formed in reverse micelles [E.L. Duarte, R. Itri, E. Lima Jr., M.S. Batista, T.S. Berquó and G.F. Goya, Large Magnetic Anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles, Nanotechnology 17 (2006) 5549–5555.]. All ferritin samples exhibited two magnetic phases present in nearly equal amounts and ascribed to iron spins at the surface and in the interior of the nanoparticle. At 4.2 K, the surface spins exhibit hyperfine fields, H hf, of 436 and 445 kOe for the H 2O 2 and O 2 samples, respectively. As expected, the spins in the interior of the core exhibit larger H hf values, i.e. 478 and 486 kOe for the H 2O 2 and O 2 samples, respectively. The slightly smaller hyperfine field distribution DH hf for both surface (78 kOe vs. 92 kOe) and interior spins (45 kOe vs. 54 kOe) of the O 2 sample compared to the H 2O 2 samples implies that the former is somewhat more crystalline.

Magnetic irreversibility and the Verwey transition in nanocrystalline bacterial magnetite

The magnetic properties of biologically produced magnetite nanocrystals biomineralized by four different magnetotactic bacteria were compared to those of synthetic magnetite nanocrystals and large, high-quality single crystals. The magnetic feature at the Verwey temperature ${T}_{V}$ was clearly seen in all nanocrystals, although its sharpness depended on the shape of individual nanoparticles and whether or not the particles were arranged in magnetosome chains. The transition was broader in the individual superparamagnetic nanoparticles for which ${T}_{B}<{T}_{V}$, where ${T}_{B}$ is the superparamagnetic blocking temperature. For nanocrystals organized in chains, the effective blocking temperature ${T}_{B}>{T}_{V}$ and the Verwey transition is sharply defined. No correlation between particle size and ${T}_{V}$ was found. Furthermore, measurements of $M(H,T,\text{time})$ suggest that magnetosome chains behave as long magnetic dipoles where the local magnetic field is directed along the chain. This result confirms that time-logarithmic magnetic relaxation is due to the collective (dipolar) nature of the barrier for magnetic moment reorientation.

Magnetic study of single domain ɛ-Fe 3N nanoparticles synthesized by precursor technique

Nanostructured single domain ɛ-Fe 3N particles have been prepared by chemical route. Magnetic dipolar interaction among particles has been observed. The FC and ZFC magnetization curves show the superparamagnetic blocking temperature around 125 K, at low applied field of 0.005 T. At higher applied field, these ZFC and FC magnetization curves merge because of increase in magnetization energy that overcomes the magnetic anisotropy and dipolar interaction energies. Mössbauer spectroscopy further corroborates the superparamagnetic nature at 300 K.

Exchange bias in Co-Cr2O3 nanocomposites

The possibility of using exchange bias in a ferromagnetic-antiferromagnetic system to overcome the effect of superparamagnetism in small ferromagnetic nanoparticles is explored. We have prepared Co-Cr2O3 nanocomposite powders using a chemical method and shown that the effect of superparamagnetism in cobalt nanoparticles could be overcome using exchange bias between Co and Cr2O3. The superparamagnetic blocking temperature of 3 nm cobalt particles has been increased to above room temperature. The choice of Cr2O3 is vital as its TN is higher compared to other antiferromagnetic materials used for this purpose such as CoO. The field cooled and zero field cooled hysteresis measurements of the samples confirm the existence of exchange bias interaction in this system.