A method for measuring the Neel relaxation time in a frozen ferrofluid (original) (raw)
Related papers
Journal of Applied Physics, 2014
Rate of heat generated by magnetic nanoparticles in a ferrofluid is affected by their magnetic properties, temperature, and viscosity of the carrier liquid. We have investigated temperature dependent magnetic hyperthermia in ferrofluids, consisting of dextran coated superparamagnetic Fe 3 O 4 nanoparticles, subjected to external magnetic fields of various frequencies (188-375 kHz) and amplitudes (140-235 Oe). Transmission electron microscopy measurements show that the nanoparticles are polydispersed with a mean diameter of 13.8 6 3.1 nm. The fitting of experimental dc magnetization data to a standard Langevin function incorporating particle size distribution yields a mean diameter of 10.6 6 1.2 nm, and a reduced saturation magnetization ($65 emu/g) compared to the bulk value of Fe 3 O 4 ($95 emu/g). This is due to the presence of a finite surface layer ($1 nm thickness) of non-aligned spins surrounding the ferromagnetically aligned Fe 3 O 4 core. We found the specific absorption rate, measured as power absorbed per gram of iron oxide nanoparticles, decreases monotonically with increasing temperature for all values of magnetic field and frequency. Using the size distribution of magnetic nanoparticles estimated from the magnetization measurements, we have fitted the specific absorption rate versus temperature data using a linear response theory and relaxation dissipation mechanisms to determine the value of magnetic anisotropy constant (28 6 2 kJ/m 3) of Fe 3 O 4 nanoparticles. V
Nanoparticle Composition of a Ferrofluid and Its Effects on the Magnetic Properties
Langmuir, 2004
Experiments were carried out on a water-based ferrofluid (γ-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.
Magnetic behavior of zero-field-frozen ferrofluid
Solid State Communications, 2000
In contrast to the spin glass-like state described in the literature, an alternative approach to explain the temperature dependence of the magnetization in a zero-field-frozen ferrofluid (ZFFF) is proposed in this work. It is claimed that the presence of a well-defined peak in the magnetization (M) versus temperature (T ) curve results from the following combined effects: the temperature dependence of the reorientation of the magnetic moment associated to the nanomagnetic particle, saturation magnetization and magnetic anisotropy. The sample used in this work is a Nickel ferrite-based ferrofluid, which was investigated using magnetometry and transmission electron microscopy, the latter indicating the presence of nanomagnetics with a mean particle diameter of 11.1 nm and standard deviation of 0.37. Excellent agreement between theory and experiment is found for the M vs T curve using the ferrofluid sample containing 3 × 10 16 particle=cm 3 and submitted to magnetic fields of 1, 3 and 5 kG. ᭧
Enhancement of Neel Relaxation at Magnetic Heating Performance of Iron Oxide Nanoparticles
2021
The study is based on understand the titanium (Ti) doping effect to enhance the Neel relaxation at magnetic heating performance of magnetite (Fe3O4). Ti doped magnetite ((Fe1-x,Tix)3O4; x= 0.02, 0.03 and 0.05) superparamagnetic nanoparticles were synthesized via sol-gel technique. The analyses were performed for (Fe1-x,Tix)3O4 and core-shell (SiO2 coated (Fe1-x,Tix)3O4) nanoparticles in order to understand the influence of silica coating on the magnetic properties of nanoparticles. The target of study to enhance the Neel relaxation mechanism on magnetic heating. The interparticle spacing and Ti amount were two parameters that we focused on the study. The results provided that coating with SiO2 has no specific effect on heating performance of (Fe1-x,Tix)3O4 nanoparticles. While the increase in temperature (ΔT) under 150 kHz RF signal reached up to 22oC in 10 minutes for SiO2 coated (Fe0.97,Ti0.03)3O4 nanoparticles, which was very close value of uncoated Fe3O4 nanoparticles.
We report the systematic dc and ac susceptibility studies on the particle blocking and carrier fluid freezing effects on the magnetization and relaxation processes in two different ferrofluids composed of Fe 3 O 4 nanoparticles ͑mean size of ϳ14 nm͒ suspended in hexane and dodecane, which respectively have freezing temperatures below ͑178 K͒ and above ͑264 K͒ the blocking temperature of magnetic nanoparticles ͑ϳ200 K͒. Experimental results reveal that these effects play a key role in the formation of glasslike peaks and magnetic anomalies in ferrofluids. Quantitative fits of the frequency dependent ac susceptibility to the Vogel-Fulcher model = o exp͓E a / k͑T − T o ͔͒ clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics.
Journal of Magnetism and Magnetic Materials, 2001
The magnetic relaxation processes in two ferrofluids with Mn0.4Zn0.6Fe2O4 (sample F1) and Mn0.6Fe0.4Fe2O4 (sample F2) mixed ferrite particles, dispersed in n-decan and kerosene, respectively, are investigated through the determination of components /χ' and /χ'' of the complex magnetic susceptibility in the range of (2-30) MHz. The values of the saturation magnetization of the two ferrofluids are M∞=5.28kA/m for sample F1 and M∞=10.99kA/m for sample F2. A maximum of the imaginary component /χ'' was observed for both samples at frequencies of tens MHz. This maximum was assigned to relaxation processes of Néel type. The effective anisotropy constant /K of the particles from the studied samples was evaluated, using both static and dynamic measurements and the values were found to be K1=6.12×103Jm-3 for the ferrofluid F1, and K2=5.60×103Jm-3 for the ferrofluid F2. >From ferromagnetic resonance measurements, and based on the theoretical values computed for the Lande factor /(g), the effective anisotropy constants for the mixed ferrite particles in the studied ferrofluids and the anisotropy field values were determined using a new method. The values obtained in this way for the anisotropy constants K1 and K2 are compared to the ones determined from magnetic relaxation measurements.
Journal of Magnetism and Magnetic Materials, 2007
The magnetic properties of a ferrofluid are strongly influenced by its particle size distribution. We analyzed a ferrofluid with an unknown particle size distribution as well as fractionated samples of the original material. The ferrofluid in our investigations consists of a mixture of maghemite and magnetite. We investigated these different samples using temperature-dependent magnetorelaxometry method. The evaluation of the Ne´el relaxation signal allows us a direct determination of the energy barrier distribution, which is one of the most important parameters of such systems of magnetic nanoparticles. The calculated particle volumes were compared with particle sizes determined by transmission electron microscopy. r
A microscopic approach to heating rate of ferrofluid droplets by a magnetic field
Journal of Applied Physics, 2019
In this work, we study the heating process of colloidal ferrofluids by a magnetic field. The heating of the fluid occurs by the magnetic relaxation of the nanoparticles which provide thermal energy for the host liquid. In the limit of small volumes, the relaxation process occurs through the Néel mechanism since the magnetic nanoparticles present superparamagnetic behavior. Within this limit, we have used a microscopic model for the coupling to phonons and external magnetic field in order to model the relaxation mechanism and to obtain an expression for the heating rate of the fluid as a function of microscopic parameters. The analysis allows determining appropriate conditions for an optimal heating rate for ferrofluids based on superparamagnetic nanoparticles.