Anisotropy and relaxation processes of uniaxially oriented CoFe2O4 nanoparticles dispersed in PDMS

When a uniaxial magnetic field is applied to a non-magnetic dispersive medium filled with magnetic nanoparticles, they auto-assemble into thin needles parallel to the field direction, due to the strong dipolar interaction among them. We have prepared in this way magnetically oriented nanocomposites of nanometer-size CoFe₂O₄ particles in a polydimethylsiloxane polymer matrix, with 10% w/w of magnetic particles. We present the characteristic magnetic relaxation curves measured after the application of a magnetic field forming an angle α with respect to the needle direction. We show that the magnetic viscosity (calculated from the logarithmic relaxation curves) as a function of α presents a minimum at α=0, indicating slower relaxation processes associated with this configuration of fields. The results seems to point out that the local magnetic anisotropy of the nanoparticles is oriented along the needles, resulting in the macroscopic magnetic anisotropy observed in our measurements.     

Structural and magnetic properties of size-controlled Mn<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles and magnetic fluids

Mn_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles with tunable Curie temperature and saturation magnetization are synthesized using hydrothermal co-precipitation method. Particle size is controlled in the range of 54 to 135 Å by pH and incubation time of the reaction. All the particles exhibit super-paramagnetic behaviour at room temperature. Langevin’s theory incorporating the interparticle interaction was used to fit the virgin curve of particle magnetization. The low-temperature magnetization follows Bloch spin wave theory. Curie temperature derived from magnetic thermogravimetric analysis shows that Curie temperature increases with increasing particle size. Using these particles magnetic fluid is synthesized and magnetic characterization is reported. The monolayer coating of surfactant on particle surface is confirmed using thermogravimetric measurement. The same technique can be extended to study the magnetic phase transition. The Curie temperature derived using this measurement complies with the low-temperature magnetic measurement. The room-temperature and high-temperature magnetization measurements are also studied for magnetic fluid systems. The magnetic parameters derived for fluid are in good agreement with those obtained for the particle system.     

The structural instabilities in ferronematic based on liquid crystal with negative diamagnetic susceptibility anisotropy

The studied ferronematic is a nematic liquid crystal (ZLI1695) of low negative anisotropy of the diamagnetic susceptibility (χ_a<0) doped with the magnetic particles Fe₃O₄. Structural instabilities are interpreted within Burylov and Raikher's theory. The high magnetic fields were oriented perpendicular (Freedericksz transition) or parallel to the initial director. Using capacitance measurements the Freedericksz threshold magnetic field of the ferronematic B_FN, and the critical magnetic field B_max, at which the initial parallel orientation between the director and the magnetic moment of magnetic particles breaks down, have been determined. The values of these quantities have been used to estimate the surface density of the anchoring energy W of liquid crystal molecules on the surface of the magnetic particles. The obtained values indicate a soft anchoring of the liquid crystal on the magnetic particles with a preferred parallel orientation of the magnetic moment of magnetic particles and the director.     

Magnetic stray fields of patterned permalloy structures investigated by photoemission electron microscopy

Using a photoemission electron microscope we determined magnetic stray fields at the edges of permalloy (Ni₈₀Fe₂₀) particles. X-ray magnetic dichroism was used for visualization of magnetic domains. The values of the stray fields were deduced from the deflection of electrons in the image due to the Lorentz force. The stray fields are responsible for the magnetic interaction of adjacent particles with distances much larger than the thickness. The measured magnetic stray field is about 0.023 T for rectangular particles with a thickness of 30 nm and lateral sizes of tens of microns. PACS 68.37.Xy; 75.40.Cx; 75.75.+a     

Size determination of metallic iron particles by temperature dependent Mössbauer and ferromagnetic resonance studies

The thermal decomposition products of NaX·Fe(CO)₅ after various steps of heat treatment, in vacuum as well as under gas atmosphere (Ar, H₂), have been investigated by temperature-dependent Mössbauer and ferromagnetic resonance measurements. The measured spectra of both techniques consistently are analyzed using superparamagnetic relaxation behavior of magnetic particles. From this analysis information concerning composition and size of magnetic iron particles is derived.     

Morphology and magnetic properties of ultrafine ZnFe2O4 particles

Well-crystallized ultrafine ZnFe₂O₄ particles of several nanometers in size have been prepared by the coprecipitation method, and their particle morphology and magnetic properties, especially at low temperatures, examined. Room-temperature X-ray diffraction, transmission electron microscopy, magnetization measurements at various temperatures from 300 K to 4.2 K, and Mössbauer spectroscopy at various temperatures from 300 K to 4.2 K, and at 4.2 K with a longitudinal magnetic field of 16.4 kOe applied have been employed. The formation of short-range and long-range magnetic order in small ZnFe₂O₄ particles above and below approximately 30 K is discussed. Below 30 K, the appearance of spontaneous magnetization and its hysteretic property is confirmed for small ZnFe₂O₄ particles.     

Magnetische Systeme mit Inhomogenitäten

Within the molecular-field approximation the Gibbs free energyG is calculated for a system with inhomogeneous distribution of magnetic particles. Some properties of such a system derivable fromG are compared with magnetic and calorimetric measurements on Ce_1-xGd_xRu₂ alloys.     

Magnetic properties of Co2.4Al0.6O4/SiO2 nanocomposite obtained via sol-gel method

Nanocomposite made of 10 wt% of Co_2.4Al_0.6O₄ particles dispersed in an amorphous SiO₂ matrix has been synthesized by a sol-gel method. X-ray diffraction, transmission electron microscopy and magnetic measurements have been used to characterize the properties of nanocomposite. Most of the particles are well crystallized and have an average diameter below 100 nm. Smaller particles with size below 10 nm have also been observed. A large value of the effective magnetic moment per Co²⁺ ion of 5.08 μ_B and negative and the low Curie-Weiss paramagnetic temperature Θ∼−6 K, obtained from the high-temperature susceptibility data, indicate a possible mixing of Co²⁺ and Co³⁺ ions between tetrahedral and octahedral sites of the spinel crystal lattice. The measurements of static and dynamic magnetic susceptibilities have shown that Co_2.4Al_0.6O₄ particles in SiO₂ matrix display a spin glass behavior at low temperatures.     

Magnetic phase transitions and phase diagrams of DyMn2Ge2

The experimental studies of magnetic phase transitions in the layered tetragonal intermetallic compound DyMn₂Ge₂ are continued. The existence of spontaneous phase transitions is confirmed by the results of measurements of the temperature dependences of lattice parameters and the initial magnetic susceptibility. The measurements in strong (up to 50 T) and ultrastrong (up to 150 T) fields revealed two new field-induced magnetic transitions. The inclusion of the exchange interaction between next-to-nearest layers of manganese and the crystal field effects for the rare-earth subsystem along with the antiferromagnetic exchange interaction of nearest Mn layers has made it possible to describe the magnetic properties of DyMn₂Ge₂ in a wide range of magnetic fields. The parameters of these interactions are determined from a comparison of the experimental and theoretical magnetization curves and H-T phase diagrams.     

Synthesis of copper-nickel nanoparticles prepared by mechanical milling for use in magnetic hyperthermia

Mechanical alloying of a mixture of copper and nickel powders has been applied for the preparation of copper-nickel alloy particles in the nanometer range. The particles were designed to be used for controlled magnetic hyperthermia applications. The milling conditions were optimized using the desired alloy composition. Utilizing a ball-to-powder mass ratio of 20, we could obtain a nanocrystalline Cu_27.5Ni_72.5 (at%) alloy with a crystallite size of around 10 nm and a Curie temperature of 45 °C.Thermal demagnetization in the vicinity of the Curie temperature of the nanoparticles was determined by thermomagnetic measurements using an adapted TGA-SDTA apparatus. The size and morphology of the particles were determined by XRD measurements and TEM analyses. The magnetic properties were also examined with a VSM. The magnetic heating effects were measured for the powdered material.     

Growth of CuPd nanoalloys encapsulated in carbon-shell

Phase characteristics of the partially oxidized cobalt particles were investigated. It is shown that the studied system consists of Co, CoO, and Co₃O₄ phases. The paper presents results for the magnetic characteristics of Co particles covered by a CoO shell. Magnetic measurements were performed at 77 K. It is shown that the shapes of the hysteresis loop of the investigated system are different for different cases of cooling, with and without applications of external magnetic field. Structural investigations were performed on single particles.     

Suitability of water based magnetic fluid with CoFe2O4 particles in hyperthermia

Magnetic susceptibility of the magnetic fluid with CoFe₂O₄ particles was measured in the low-frequency range in order to determine mean values of the magnetic grain size and the saturation magnetisation. The volume concentration of the solid phase (CoFe₂O₄) was also found. Results of the calorimetric measurements, in the range from 70 kHz to 1.7 MHz, confirmed the suitability of use of the medium studied in magnetic fluid hyperthermia, especially in the frequency region 600–800 kHz. The H²–law-type dependence of the specific absorption rate on the square amplitude of the magnetic field testified to the presence of superparamagnetic particles in the magnetic fluid. The minimum magnetic field intensity needed for successful hyperthermal treatment was experimentally determined.     

Alignment of Anisotropic Particles by Magnetic Field as Seen by NMR

We applied ¹³C and ²⁰⁵Tl NMR for studying alignment of particles of graphene and high temperature superconductor (Tl_0.5Pb_0.5)(Ba_0.2Sr_0.8)₂Ca₂Cu₃O_y caused by magnetic field. These compounds have layered structure and reveal anisotropic magnetic susceptibility. We found that the field of 8 T causes minor alignment of powder graphene and somewhat better alignment of fluffy graphene particles. Herewith the effect of alignment is well pronounced in ²⁰⁵Tl spectra of the superconducting particles fixed in epoxy in the field of 8 T. This effect is reflected in the ²⁰⁵Tl line shape measured in a magnetic field of 1.17 T and becomes much more pronounced in measurements made in high magnetic field of 8 T. Spectra simulations allow determining the degree of the particles’ alignment.     

Magnetic relaxation in nanocrystalline iron-oxides

Nanocrystalline Fe oxides are compacted solids of fine particles. By compacting interfaces are formed between adjacent particles. Since collective magnetic relaxation and superparamagnetism depend on the anisotropic energy, it is expected that the existence of interfaces and related interface anisotropy will change the relaxation behavior of nanocrystalline Fe oxides. Measurements on nanocrystalline α-Fe₂O₃ and γ-Fe₂O₃ have shown that the exchange interaction in the interfaces is more effective in suppressing superparamagnetic relaxation upon compacting than the dipole interaction.     

Synthesis protocol influence on aqueous magnetic fluid properties

Magnetic colloids containing superparamagnetic Fe₃O₄ nanoparticles have been prepared by co-precipitation method. Three samples of citric acid coated magnetic colloids containing magnetic nanoparticles (ultra-fine particles of Fe₃O₄) have been obtained following three different preparation protocols. Physical tests have been performed on these samples of the magnetic colloids prepared by us (consisting mainly of Fe₃O₄ ultra-fine particles stabilized with citric acid (C₆H₈O₇) and immersed in water), in order to reveal their microstructural and rheological features. Transmission electron microscopy (TEM) and magnetic measurements were the investigation methods used for the assessing of the magnetic nanoparticles size. The dimensional distribution of the ferrophase physical diameter was comparatively presented using the box-plot statistical method. Infrared absorption spectra have been recorded aiming to get some information on the magnetic fluid composition.     

Mössbauer,X-ray diffraction and AC susceptibility studies on nanoparticles of zinc substituted magnesium ferrite

Nanoparticles of zinc substituted Mg-ferrite with compositions Mg_(1-x)Zn _x Fe₂O₄ (x = 0.15, 0.30 and 0.50) having particle sizes in the range 6.4 nm to 21.4 nm prepared by the co-precipitation method were characterized by ⁵⁷Fe Mössbauer spectroscopy, X-ray diffratometry and AC magnetic susceptibility measurements. Mössbauer measurements at room temperature and down to 20 K clearly indicate presence of superparamagnetic particles in all the samples. AC magnetic susceptibility data show lowering of blocking temperature with decrease of particle size. Superparamagnetic relaxation was observed for larger particle size in samples with higher Zn content, which is attributed to the weakening of A-B exchange interaction in ferrite lattice due to replacement of Fe^{3 +} in tetrahedral site by Zn^{2 +} ions.Received: 16 April 2004, Published online: 23 July 2004PACS: 75.50.Tt Fine-particle systems; nanocrystalline materials - 76.80. + y Mössbauer effect; other gamma-ray spectroscopy - 75.30.Cr Saturation moments and magnetic susceptibilitiesS. Das: Present address: Department of Physics, Jadavpur University, Kolkata - 700032, India     

Magnetic shielding and spin-rotation interaction in ground state alkali molecules

Precision measurements of nuclear magnetic dipole moments in alkali molecules are performed using atom-molecule exchange optical pumping. A comparison with measurements on alkali atoms gives the magnetic shielding differences between atoms and molecules and an approximate value for the spin-rotation interaction constant of the alkali molecules. It is reported on experiments on³⁹K₂ and⁸⁷Rb₂.     

Cems study of corrosion on iron foil at low temperatures

Conversion electron Mössbauer measurements with a proportional counter at 6.3, 78 and 300 K have been done to investigate the corrosion of the surface of iron foils, which consists of small particles of γ-Fe₂O₃. When the average particle sizes of the corroded layer are approximately 4 and 5 nm, magnetic hyperfine splitting was observed in the spectra at 6.3 and 78 K. However, for a corroded layer composed of smaller particles, the splitting was noticed only at 6.3 K. The anisotropy constantK of the small particles was estimated using the magnetic hyperfine fields obtained from the spectra at 6.3 and 78 K.     

An investigation of the magnetic hyperfine interactions in LiFeS2 by Mössbauer spectroscopy and magnetic susceptibility measurements

Mössbauer spectroscopy of LiFeS₂ in a wide temperature range indicates that iron is in the trivalent state with high spin configuration having a magnetic hyperfine interaction at 4.4 K. The magnetic susceptibility measurements indicates antiferromagnetic ordering in LiFeS₂.