Superparamagnetic Blocking Temperature Research Articles

Cobalt nanoparticles formed in polysiloxane copolymer micelles: effect of production methods on magnetic properties

The effect of production method on the chemical and magnetic stability of a range of cobalt nanoparticles formed in polysiloxane copolymer micelles was studied. The nanoparticles were suspended in polydimethylsiloxane (PDMS) carrier fluids and are prototypes for use in biomedical applications. Three families of particles were investigated: cobalt particles in triblock copolymer micelles and silica-coated cobalt particles where the silica coating is formed either in toluene or PDMS carrier fluid. The silica coating effectively protects the cobalt particles from ongoing oxidation, with little change in the saturation magnetization of the particles observed over time and only a small shift in the centre of the field-cooled hysteresis loop observed. Temperature-dependent magnetization measurements show that the silica-coated particles have superparamagnetic blocking temperatures of 100–130 K compared with the non-silica-coated particles, which do not reach their characteristic blocking temperature below the melting point of the frozen fluids. This result is interpreted as a smaller magnetic core size in the silica-coated particles. The zero-field-cooled temperature-dependent magnetization of the silica-coated particles has two apparent superparamagnetic blocking temperatures, with a second, lower blocking temperature of approximately 15 K. This second apparent blocking temperature may be due to unreacted cobalt clusters in the suspensions or, alternatively, due to surface states on the nanoscale cobalt particles. The prototype silica-coated cobalt particles show promise for potential biomedical applications.

Size-Controlled Synthesis of Magnetite Nanoparticles in the Presence of Polyelectrolytes

Magnetite nanoparticles of nearly uniform size have been prepared by precipitating ferrous ions in the presence of two different polyelectrolytes, viz., poly(acrylic acid) and the sodium salt of carboxymethyl cellulose at high pH (∼13). The size of the magnetite nanoparticles can be controlled easily by varying the concentration of the polyelectrolyte in the medium. Transmission electron microscopy study indicates that the average particle size varies from 5 to 15 nm, depending on the concentration and the nature of the polyelectrolyte. X-ray diffraction study shows the presence of only magnetite phase. FTIR spectroscopy and thermogravimetric analysis confirmed the presence of polyelectrolyte on the magnetite surface. The magnetization and Mössbauer studies were performed on two samples with mean diameters 7.0 and 14.7 nm. Magnetization measurements suggest that both of these particles are of single magnetic domain. The measurements also estimate the superparamagnetic blocking temperature, TB = 145 K for smaller particles, while TB > 300 K for the larger particles. Mössbauer spectra at 300 K show only a quadrupole doublet for the smaller particles and mostly a magnetically separated sextet for the larger ones, indicating also the marked size dependence of moment dynamics.

Exchange bias and coercivity in oxygen-exposed cobalt clusters

Exchange bias and coercivity have been studied in nanometric-size cobalt clusters as a function of oxygen exposure under high vacuum. This technique allows looking at the early stages of oxidation, as well as studying exchange bias in small clusters before they are completely transformed into oxide. It is found that oxygen induces at first an increase in the coercive field and in the superparamagnetic blocking temperature, before any substantial loss in the magnetization, which indicates that this effect is due to adsorbed oxygen rather than to exchange coupling with an oxide overlayer.

A facile liquid foam based synthesis of nickel nanoparticles and their subsequent conversion to NicoreAgshell particles: structural characterization and investigation of magnetic properties

A facile route for the synthesis of nickel nanoparticles in stable aqueous foams is reported. The Ni nanoparticles were roughly 12–15 nm in size and were stable as aqueous suspensions or powders when oleic acid was used as a capping agent. These Ni nanoparticles were subsequently coated with a silver shell in view of the extra stability and the enhanced manipulative ability afforded by the silver nanocoating. This was accomplished by a simple transmetallation reaction wherein the nanoparticle surface nickel atoms act as localized reducing agents for the silver ions in solution. As the silver shell is formed through the surface reaction a reduction in the average size of the Nicore occurs. After the core–shell structure formation, the Nicore has an average diameter of 10–20 nm while the Agshell has a thickness of 2–4 nm. The pristine oleic acid coated Ni and NicoreAgshell nanoparticles were probed for their magnetic characteristics by a vibrating sample magnetometer. The nascent, oleic acid coated Ni nanoparticles display a low superparamagnetic blocking temperature, TB, of 20 K. The field dependent magnetic behaviour above and below TB displays the standard features corresponding to superparamagnetism, as expected for very small Ni crystallites suggesting again that each 12 nm particle is polycrystalline. The magnetic contribution in the NicoreAgshell system comes from only the Ni core and predictably, the blocking temperature of this system is below 12 K due to the smaller size of the Ni core.

Superparamagnetic Co 36C 64 granular film

Nanocomposite Co–C thin films of about 15 nm thick were prepared by pulsed filtered vacuum arc deposition. The films were characterized by X-ray photoelectron spectroscopy, non-Rutherford backscattering spectrometry, X-ray diffraction, and magnetic force microscopy. The as-deposited films were amorphous. After annealing at 350°C for 1 h in vacuum (<10 −3 Pa), the films were found to consist of nanocrystalline Co grains encapsulated in carbon. The superparamagnetism of the annealed Co 36C 64 film was demonstrated by the measurement of DC susceptibility and magnetic hysteresis using a SQUID magnetometer. The superparamagnetic relaxation blocking temperature was marked to be about 12 K by the peak of the zero-field-cooled magnetization under a field of 100 Oe. The magnetic properties of these annealed granular Co–C films transform from superparamagnetism to ferromagnetism when the Co concentration increases.

Relaxation times in exchange-biased nanostructures

We calculated the energy barrier, ΔE, for exchange-biased (EB) systems, using the ferromagnetic domain wall model. The temperature dependence of the EB is in good agreement with experimental results. For Fe–FeF2, Fe–MnF2, and Ni–NiO, ΔE is proportional to a power of the interfacial coupling constant and inversely to the ferromagnetic film thickness. The temperature and volume dependence of the relaxation time show that exchange coupling increases the superparamagnetic blocking temperature of nanostructured ferromagnets.

Quantum relaxation and quantum coherence in mesoscopic molecular magnets

The effect of a magnetic field ${B}_{\ensuremath{\perp}}$ on the resonant tunneling between different magnetic states of the molecular clusters ${\mathrm{Fe}}_{8}$ and ${\mathrm{Mn}}_{12}$ has been studied by measuring the magnetic specific heat of these compounds between $0.1 \mathrm{K}$ and $6 \mathrm{K}.$ At zero field, we have shown that tunneling proceeds via excited levels and found which states contribute most to quantum tunneling in ${\mathrm{Fe}}_{8}.$ As ${B}_{\ensuremath{\perp}}$ increases, the superparamagnetic blocking temperature decreases, thus indicating that tunneling proceeds through progressively lower lying excited levels. At high enough fields, incoherent tunneling is observed to proceed through the ground state, at rates faster than $1 {\mathrm{s}}^{\ensuremath{-}1}.$ For still higher fields, the tunnel splitting ${\ensuremath{\Delta}}_{t}$ of their $m=\ifmmode\pm\else\textpm\fi{}10$ magnetic ground states can be made large compared to perturbations such as hyperfine and dipolar interactions. Under those conditions, the equilibrium specific heat measured at the lowest temperature gives evidence for the presence of a ${\ensuremath{\Delta}}_{t},$ thus showing that coherent quantum mechanical tunneling can occur in these ``mesoscopic'' systems. For ${\mathrm{Mn}}_{12}$ the dissipation caused by the hyperfine interaction between the electronic and nuclear Mn spins has been studied in detail.

Physical and chemical characterization of therapeutic iron containing materials: A study of several superparamagnetic drug formulations with the β-FeOOH or ferrihydrite structure

The effectiveness of therapeutically used iron compounds is related to their physical and chemical properties. Four different iron compounds used in oral, intravenous, and intramuscular therapy have been examined by X-ray powder diffraction, iron-57 Mossbauer spectroscopy, transmission electron microscopy, BET surface area measurement, potentiometric titration and studied through dissolution kinetics determinations using acid, reducing and chelating agents. All compounds are nanosized with particle diameters, as determined by X-ray diffraction, ranging from 1 to 4.1 nm. The superparamagnetic blocking temperatures, as determined by Mossbauer spectroscopy, indicate that the relative diameters of the aggregates range from 2.5 to 4.1 nm. Three of the iron compounds have an akaganeite-like structure, whereas one has a ferrihydrite-like structure. As powders the particles form large and dense aggregates which have a very low surface area on the order of 1 m2 g−1. There is evidence, however, that in a colloidal solution the surface area is increased by two to three orders of magnitude, presumably as a result of the break up of the aggregates. Iron release kinetics by acid, chelating and reducing agents reflect the high surface area, the size and crystallinity of the particles, and the presence of the protective carbohydrate layer coating the iron compound. Within a physiologically relevant time period, the iron release produced by acid or large chelating ligands is small. In contrast, iron is rapidly mobilized by small organic chelating agents, such as oxalate, or by chelate-forming reductants, such as thioglycolate.

Structural and magnetic properties of nanoscale iron oxide particles synthesized in the presence of dextran or polyvinyl alcohol

Magnetite/maghemite nanoparticles were synthesized in the presence of either dextran or polyvinyl alcohol, yielding cluster- and necklace-like aggregates, respectively. Magnetization, Mössbauer spectroscopy, and microscopy measurements indicate that the arrangement of the particles within the aggregates affects the magnetic properties of the particles resulting in smaller particles in the clusters having higher superparamagnetic blocking temperatures than larger particles in the necklaces.

Magnetic dynamics of weakly and strongly interacting hematite nanoparticles

The magnetic dynamics of two differently treated samples of hematite nanoparticles from the same batch with a particle size of about 20 nm have been studied by M\"ossbauer spectroscopy. The dynamics of the first sample, in which the particles are coated and dispersed in water, is in accordance with the N\'eel expression for the superparamagnetic relaxation time of noninteracting particles. From a simultaneous analysis of a series of M\"ossbauer spectra, measured as a function of temperature, we obtain the median energy barrier ${K}_{\mathrm{Bu}}{V}_{\mathrm{m}}/k=570\ifmmode\pm\else\textpm\fi{}100$ K and the preexponential factor ${\ensuremath{\tau}}_{0}{=1.3}_{\ensuremath{-}0.8}^{+1.9}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ s for a rotation of the sublattice magnetization directions in the rhombohedral (111) plane. The corresponding median superparamagnetic blocking temperature is about 150 K. The dynamics of the second, dry sample, in which the particles are uncoated and thus allowed to aggregate, is slowed down by interparticle interactions and a magnetically split spectrum is retained at room temperature. The temperature variation of the magnetic hyperfine field, corresponding to different quantiles in the hyperfine field distribution, can be consistently described by a mean field model for ``super-ferromagnetism'' in which the magnetic anisotropy is included. The coupling between the particles is due to exchange interactions and the interaction strength can be accounted for by just a few exchange bridges between surface atoms in neighboring crystallites.

Superparamagnetic Behavior of Granular Co-C Films Consisting of Nanocrystalline Cobalt Encapsulated in Carbon

ABSTRACTNanocomposite Co-C thin films of about 15 nm thick were prepared by pulsed filtered vacuum arc deposition. The films were characterized by x-ray photoelectron spectroscopy, non-Rutherford backscattering spectrometry, x-ray diffraction, magnetic force microscopy and magnetic measurements. The as-deposited films were amorphous. After annealing at 350°C for one hour in vacuum (&lt; 10−3 Pa), the films were found to consist of nanocrystalline Co grains encapsulated in carbon. The superparamagnetism of the annealed Co36C64 film was demonstrated by the measurement of DC susceptibility and magnetic hysteresis using a SQUID magnetometer. The superparamagnetic relaxation blocking temperature was marked to be about 12K by the peak of the zero-field-cooled magnetization under a field of 100 Oe. The magnetic properties of these annealed granular Co-C films transform from superparamagnetism to ferromagnetism when the Co concentration increases.

Self-Organized Growth of Nanosized Vertical Magnetic Co Pillars on Au(111)

Starting from the conventional self-organized arrays of flat Co dots on Au(111), we extended the growth vertically to fabricate Co pillars. The principle is to deposit sequentially a fraction $x$ of an atomic layer (AL) of Co and $1\ensuremath{-}x$ AL of Au. At each step, the deposited Co atoms aggregate on top of the previous dots, increasing their height by 1 AL. Pillars with a 2:1 vertical aspect ratio were achieved. Because of the large number of atoms per pillar, the superparamagnetic blocking temperature is 300 K in the pillars, whereas it is only 20 K in conventional flat dots with the same lateral density.

Reversibility of the synthesis-decomposition reaction in the ball-milled Cu-Fe-O system

In this work, we present results from a ball-milling experiment carried out simultaneously on (a) spinel and (b) its precursor mixture , to investigate their evolution under identical conditions. Partial reversibility of the reaction was observed, resulting in nearly equal phase composition for both systems after 420 h of milling. X-ray data showed that the final admixture was composed mainly of nanosized and spinel phases. Almost coincident average grain sizes were found in both milled samples, although different size distributions were inferred from their magnetic behaviour. Superparamagnetic (SPM) relaxation effects were observed in the resulting particles at room temperature. Mössbauer and magnetization data taken below the SPM blocking temperature indicate partial formation of the (0 < x < 1) spinel solid solution, with cubic structure and high local disorder.

Deuteron spin-lattice relaxation in theMn12O12-acetate magnetic cluster

The dynamics of the fully deuterated polycrystalline molecular cluster nanomagnet ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}{[(\mathrm{C}\mathrm{D}}_{3}{\mathrm{C}\mathrm{O}\mathrm{O})}_{16}{(\mathrm{D}}_{2}{\mathrm{O})}_{4}{]\mathrm{\ensuremath{\cdot}}2\mathrm{C}\mathrm{D}}_{3}{\mathrm{COOD}\mathrm{\ensuremath{\cdot}}4\mathrm{D}}_{2}\mathrm{O}$ has been studied in a field of 6.3 T via deuteron spin-lattice relaxation time ${(T}_{1})$ measurements between room temperature and 4 K. The relaxation is due to the fluctuations in the hyperfine electron-nuclear coupling. The deuteron magnetization recovery was found to be of the stretched exponential type and the temperature dependence of the spin-lattice relaxation time parameter ${T}_{1}$ showed two pronounced minima, one around 70 K and another around 10 K. The two modes detected in this study are intermediate between the slow magnetization reorientation mode detected by susceptibility measurements and the fast spin modes detected by inelastic neutron scattering. Whereas the slow correlation time ${\ensuremath{\tau}}_{1}$ is of the order of days in the vicinity of the superparamagnetic blocking temperature 5 K, the fast correlation time ${\ensuremath{\tau}}_{2}$ is here still in the ${10}^{\ensuremath{-}6} \mathrm{s}$ range.

Effect of magnetic field on the superparamagnetic relaxation in granular Co-Ag samples

In order to study the effect of applied magnetic field on the superparamagnetic relaxation behavior of small Co particles, magnetization measurements were carried out on as-prepared and annealed granular samples of Co20Ag80 and Co25Ag75. Values of the superparamagnetic blocking temperature TB− were obtained from the characteristic peak in the zero-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease of TB− with applied magnetic field is quadratic. An estimate of the magnetic anisotropy “energy density” Ku yielded a value which is two orders of magnitude greater than the value for bulk cobalt. The results reported here underscore the importance of considering the effect of superparamagnetic relaxation on the performance of nanostructured magnetic materials.

Superparamagnetic behavior of structural domains in epitaxial ultrathin magnetite films

We provide evidence that thin films of magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ epitaxially grown on single-crystalline MgO(100), form domains with unique magnetic properties. Strong 180\ifmmode^\circ\else\textdegree\fi{} Fe-O-Fe superexchange paths at antiphase domain boundaries result in a frustration of the interdomain interactions. Probe-layer M\"ossbauer spectroscopy shows superparamagnetic behavior in the entire film, in spite of the nearly perfect layer-by-layer growth. Superconducting quantum interference device magnetometry is used to determine the temperature and thickness dependence of the superparamagnetic blocking temperatures.

Specific heat and magnetic relaxation of the quantum nanomagnetMn12Ac

We present specific-heat and ac-susceptibility measurements providing more evidence that magnetic quantum tunneling occurs in ${\mathrm{Mn}}_{12}$Ac spin clusters. The total anisotropy energy barrier determined by fitting an Arrhenius law to the magnetic relaxation time is underestimated when compared to the specific-heat data. The expected magnetic contribution using a quadratic spin Hamiltonian needs higher-order terms to improve the agreement with the experiments, as recently done for electron-paramagnetic-resonance spectra. In addition, another interesting effect was observed in the specific-heat measurements when a dc magnetic field is applied. The extra magnetic contribution due to Zeeman splitting is suppressed below the expected superparamagnetic blocking temperature in the time scale of the measurements.

Magnetic interactions in native horse spleen ferritin below the superparamagnetic blocking temperature

The magnetic interactions in a freeze-dried native horse spleen ferritin were quantified by measuring 5 K IRM and DCD remanence curves using a SQUID magnetometer. Irreversible and reversible components of magnetic moment were determined for a range of fields to ± 30 kOe and attributed to blocked and unblocked SPM particles, respectively. This separation allowed the proportion and mean magnetic properties of each population to be determined. By fitting to a modified Langevin, the mean magnetic moment per particle of the unblocked population was calculated to be 15 μ B, whilst constituting 67% of the magnetic moment at 5 K. Inter-particle interactions in ferritin were quantified using Henkel and Δ I plots, showing that ferritin particles weakly interact, with Δ I < 3%. This finding is consistent with a weak dipole-dipole interaction between otherwise magnetically independent particles.

Ferrite synthesis in microstructured media: Template effects and magnetic properties

Inverse micelles and organogels provide novel environments to synthesize ferrite particles. The fluid microstructure provides a template for the synthesis. Our experiments with ferrite synthesis in inverse micelles indicate the formation of superparamagnetic nanoparticles. Of interest is the encapsulation of these particles in polymer microspheres. The encapsulation is done using simple polymer precipitation in the micellar nonsolvent. The process results in a polymer-ferrite composite exhibiting supermagnetism. Low temperature spin glass properties of the composite are characterized through SQUID measurements. These composites have a superparamagnetic blocking temperature of 16 K and follow Curie–Weiss law at temperatures above 60 K with the fitted parameters: C=0.941 emu/g K, θ=−287 K, and TIP=0.0001 emu/g. Since the polymer used is polyphenol, a highly functionalizable material, the composite is well suited for applications in magnetic bioseparations and magnetic coatings.

Magnetic Properties of Multilayered Co-Cu Granular Composites

ABSTRACTThe magnetic properties of granular CO25Cu75/CU multilayer films prepared by sputtering onto heated glass substrates have been investigated in the range of ultrathin alloy films (5–250Á) and Cu spacer thicknesses (0–50Å). Both the coercive field at 10K and the maximum superparamagnetic blocking temperature TB are found to be sensitive to finite-size effects in single-layer films, and to interactions between layers in multilayer films. Our results indicate that the magnetic properties of granular alloy layers can be engineered to a large extent by selectively modifying the local environment of the Co particles.