Ferromagnetic properties of glucose coated Cu2O nanoparticles

Magnetic properties of glucose coated cuprous oxide nanoparticles of different sizes have been studied. Unlike bulk Cu₂O, which shows diamagnetic behavior, the nanoparticles show superparamagnetic behavior. A superparamagnetic blocking temperature of 21 K is observed for 5 nm particles. A magnetic hysteresis loop with a coercivity of 406 Oe is observed for these particles at 5 K. The magnetization and the coercivity increase with decreasing particle size. The superparamagnetic behavior, along with the increase in magnetization and coercivity with decreasing particle size, is due to the enhanced surface contributions to the magnetism.     

Pulsed Field-Induced Magnetization Switching in Antiferromagnetic Ferrihydrite Nanoparticles

The dynamic magnetization switching of ferrihydrite nanoparticles has been investigated by a pulsed magnetometer technique in maximum fields H_max of up to 130 kOe with pulse lengths of 4, 8, and 16 ms. Ferrihydrite exhibits antiferromagnetic ordering and defects cause the uncompensated magnetic moment in nanoparticles; therefore, the behavior typical of magnetic nanoparticles is observed. The dynamic hysteresis loops measured under the above-mentioned conditions show that the use of pulsed fields significantly broadens the temperature region of existence of the magnetic hysteresis and the coercivity can be governed by varying the maximum field and pulse length. This behavior is resulted from the relaxation effects typical of conventional ferro- and ferrimagnetic nanoparticles and the features typical of antiferromagnetic nanoparticles.     

Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

Magnetic anomalies in single crystalline ErPd2Si2

Considering certain interesting features in the previously reported ¹⁶⁶Er Mössbauer effect, and neutron diffraction data on the polycrystalline form of ErPd₂Si₂ crystallizing in the ThCr₂Si₂-type tetragonal structure, we have carried out magnetic measurements (1.8–300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in features due to magnetic ordering in the plot of magnetic susceptibility χ versus temperature T at low temperatures. The χ(T) data reveal that there is a pseudo-low-dimensional magnetic order setting in at 4.8 K, with a three-dimensional antiferromagnetic order setting in at a lower temperature (3.8 K). A new finding in the χ(T) data is that, for H∥〈1 1 0〉 but not for H∥〈0 0 1〉, there is a broad shoulder in the range 8–20 K, indicative of the existence of magnetic correlations above 5 K as well, which could be related to the previously reported slow-relaxation-dominated Mössbauer spectra. Interestingly, the temperature coefficient of electrical resistivity is found to be isotropic; no feature due to magnetic ordering could be detected in the electrical resistivity data at low temperatures, which is attributed to magnetic Brillioun-zone boundary gap effects. The results reveal the complex nature of magnetism of this compound.     

Novel NdCo<Subscript>5</Subscript> nanoflakes and nanoparticles produced by surfactant-assisted high-energy ball milling

High-energy ball milling has been shown to be a promising method for the fabrication of rare earth—transition metal nanopowders. In this work, NdCo₅ nanoflakes and nanoparticles have been produced by a two-stage high-energy ball milling (HEBM), by first using wet HEBM to prepare precursor nanocrystalline powders followed by surfactant-assisted HEBM. NdCo₅ flakes have a thickness below 150 nm and an aspect ratio as high as 10²–10³; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes exhibited high coercivities at low temperatures, with values at 50 K of 3 and 3.7 kOe, respectively. The high values of coercivity can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior in contrast to the planar anisotropy of the bulk samples. Angle-dependent magnetization measurements at different temperatures were used to determine the spin reorientation transitions in the nanopowders and nanoparticles. The nanoparticles showed spin reorientation temperatures, T _SR1 = 276 and T _SR2 = 237 K which are lower when compared with the values of 290 and 245 K, respectively for bulk.     

Crystallization and magnetic properties of melt-spun neodymium-iron alloys

The magnetic and crystallization properties of melt-spun Nd_1?xFe_x alloys are reported. By using high purity constituents and an extremely fine orifice (100–150 μm), amorphous alloys were prepared over the interval 0.4 ? x ? 0.8. Their magnetic properties, taken between 20–850 K in fields up to 95 kOe, are interpreted on the basis of a sperimagnetic structure; at high field the alloys from collinear ferromagnetic structures. Room temperature coercivities of the amorphous alloys are relatively low (1.5–2.0 kOe) but increase substantially at reduced temperatures; at 20 K, a maximum coercivity of 52 kOe was found for a Nd_0.4Fe_0.6 alloy. X-ray diffraction indicates that the melt-spun alloys crystallize by the precipitation of Nd metal and an unidentified Nd-Fe phase. Changes in magnetization and coercivity during crystallization are reported.     

High coercivity nanocrystalline YCo5 powders produced by mechanical milling

High coercivity nanostructured YCo₅ powders were successfully prepared by mechanical milling of as-cast alloys and subsequent vacuum annealing. Almost single phase YCo₅ alloys, obtained by arc melting, were processed by high energy mechanical milling using a SPEX 8000 mill. After 4 h of milling, powders become nearly amorphous. DSC scans revealed the existence of an irreversible broad exothermic transition with a maximum at 516 °C associated with the crystallization process. Annealing in high vacuum at 800 °C during 2.5 min led to the formation of YCo₅ nanoparticles with an average particle size of 12 nm. A high intrinsic coercivity of 7.23 kOe together with a σ_r/σ_s ratio of 0.75 were obtained.     

Synthesis and characterization of L10 FePt nanoparticles from Pt–Fe3O4 core-shell nanoparticles

Pt/Fe₃O₄ core-shell nanoparticles have been prepared by a modified polyol method. Pt nanoparticles were first prepared via the reduction of Pt(acac)₂ by polyethylene glycol-200 (PEG-200), and layers of iron oxide were subsequently deposited on the surface of Pt nanoparticles by the thermal decomposition of Fe(acac)₃. The nanoparticles were characterized by XRD and HR-TEM. The as-prepared Pt/Fe₃O₄ nanoparticles have a chemically disordered FCC structure and transformed into chemically ordered fct structure after annealing in reducing atmosphere (4% H₂, 96% Ar) at 700 °C. The ordered fct FePt phase has high magnetic anisotropy with coercivity reaching 7.5 kOe at room temperature and 9.3 kOe at 10 K.     

Magnetic properties of marine tertiary tephra investigated over a wide temperature range

We report on a detailed investigation of the magnetic properties of tertiary tephra as a function of temperature and magnetic field, over a wide temperature. The tertiary tephra were chosen from two marine sites in the South Indian Ocean. The measurements were made using the electron spin resonance (ESR) technique in the temperature range 4–1100 K and a SQUID magnetometer at temperatures from 4 to 300 K.We show that the magnetic properties of the studied ashes are mainly due to two contributions, a ferrimagnetic one and a paramagnetic one. At moderate magnetic fields, B<1 T, in the large temperature interval, 20 K<T<1000 K, the magnetic response is dominated by the ferrimagnetic component present in the ashes. The experimental data concerning the hysteresis cycles and the Curie temperatures allow us to suggest that (titano-)magnetite and a haematite-like phase could be responsible for the observed ferrimagnetic behaviour. In addition, by electron microscopy investigations, two types of morphologies containing Fe ions have been detected: vitreous splinters, in which embedded crystallised nanoparticles (60–200 nm) are present, and smectite flakes which are crystallised and in which the Fe concentration is higher than in the splinters. We speculate that the ferrimagnetic contribution is due to the nanoparticles and the paramagnetic one results from lamellar flake-like particles.     

Particle size dependence of exchange-bias and coercivity in CuO nanoparticles

For CuO nanocrystals of size 6.6-37 nm, the exchange bias H_eb and coercivity H_c are measured at 5 K in zero-field-cooled (ZFC) and field-cooled (FC at 50 kOe) samples and their variations investigated as a function of particle size D. The similar 1/D variations observed for the difference coercivity ΔH_c=H_c(FC)−H_c(ZFC) and the interfacial exchange energy Δσ=H_ebM_fD are discussed in terms of the ferromagnetic magnetization M_f being produced by the uncompensated surface Cu²⁺ spins in the otherwise antiferromagnetically ordered CuO nanoparticles. This leads to the observation that the experimentally measured ΔH_c provides a good measure of Δσ in nanoparticle systems, with H_eb/ΔH_c varying as 1/M_fD.     

Synthesis of single-crystal Sm-Co nanoparticles by cluster beam deposition

Single-crystal Sm-Co nanoparticles have been successfully produced by a cluster beam deposition technique. Particles have been deposited by DC magnetron sputtering using high Ar pressures on both single-crystal Si substrates and Au grids for the magnetic and structural/microstructural properties, respectively. Oxidation of the particles is prevented by using carbon buffer and cover layers. Nanoparticles have a uniform size distribution with an average size of 4.2, 6 and 7 nm at 1, 1.5 and 2 Torr of Ar pressure, respectively. At 1 Torr, the particles have the disordered 1:7 structure and a high coercivity of 19 kOe at 10 K. These particles show a superparamagnetic behavior with a blocking temperature of T_B = 145 K. From this value of T_B and the particle volume, the value of anisotropy constant K is estimated to be around 2.2 × 10⁷ ergs/cc. Heat is introduced to the particles during their flight to the substrate to increase the particle size. Nanoparticles of SmCo₅ with an average size of 15 nm and high room temperature coercivity have been produced. No change in magnetic and structural properties of the samples has been observed even after 10 months. Cluster beam deposition could play a key role for the production of rare earth nanoparticles for many applications.     

Preparation of magnetic FeCo nanoparticles by coprecipitation route

Magnetic FeCo nanoparticles with high saturation magnetization (M_s = 148 emu/g) at 15 kOe were prepared by a coprecipitation route. The value of M_s for FeCo nanoparticles depends on the ratio of Fe to Co components. The size of the nanoparticles was confirmed by transmission electron microscopy (TEM) images, and morphology of the nanoparticles was obtained by field emission scanning electron microscopy (FE-SEM) images. The crystal structure of the nanoparticles dependent on annealing was characterized by X-ray diffraction data. The magnetic properties were characterized by saturation magnetization from a hysteresis loop by VSM.     

The influence of thermal annealing on the structural and magnetic properties of gold nanoparticles

In the past few years ferromagnetic-like behavior has been reported in metal gold nanoparticles coated with diverse organic surfactants. In this work we report on the effect of thermal annealing on the ferromagnetic-like behavior of oleic acid and oleylamine coated gold nanoparticles of about 7 nm size. The magnetic moment of the “as prepared” sample is about 3×10⁻² emu/g and the coercive field is 200 Oe at 10 kOe and 5 K, after the annealing the behavior changes from ferromagnetic-like to paramagnetic and the magnetization at 10 kOe decreases at a factor of 10. These results are compared with those obtained for oleylamine coated gold nanoparticles, which are diamagnetic at room temperature.     

Large vertical loop shifts in mechanically synthesized (Mn,Fe)2O3−t nanograins

Magnetic properties of zero field cooled (ZFC) and field cooled (FC) sample of (Mn,Fe)₂O_3−t nanograins have been investigated by magnetometry (up to 70 kOe) and Mössbauer spectroscopy (up to 60 kOe) in the temperature interval 4.2–300 K. Large horizontal (up to 0.8 kOe) and vertical (up to 80%) shifts of the magnetization hysteresis loops are observed in the FC regime. The obtained results are discussed in terms of exchange interaction between an antiferromagnetic core and a spin-glass-like state of the nanograins boundaries. It is shown that hysteresis loop shifts (horizontal and vertical) depend on the field cooling magnitude, an effect that can be understood by the change of the boundary magnetic structure induced by the external magnetic field. The vertical magnetization shift is described by a phenomenological model, which takes into account the magnetic interaction between the spin-glass like boundary spins and the applied field.     

Effect of Al doping on the martensitic transition and magnetic entropy change in Ni-Mn-Sn alloys

Effect of Al doping on the martensitic transition and magnetic entropy change in Mn₅₀Ni₄₀Sn_10−xAl_x was investigated. The experimental results show that the martensitic transition temperatures increase with the increase of Al content due to cell contraction, while the martensitic transition temperature range decreases rapidly. Mn₅₀Ni₄₀Sn₈Al₂ alloy has the largest value of  (3.14 J/kg K) for the magnetic field changing from 0 to 10 kOe, which is nearly twice as large as that of Mn₅₀Ni₄₀Sn₁₀ alloy. It is demonstrated that a larger can be obtained due to the sharper magnetization change around martensitic transition.     

Magnetic properties of nanoparticles of cobalt chromite

Magnetic properties of cobalt chromite nanoparticles of size 8-12 nm synthesized through conventional coprecipitation route are reported. Magnetization versus temperature measurement plot reveals a transition from paramagnetic to superparamagnetic (SPM) phase in contrast with the transition from paramagnetic to long-range ferrimagnetic phase at Curie temperature, T_c, reported in bulk. The blocking temperature, T_b, of SPM phase is found to be 50-60 K. On cooling in the presence of 10 kOe field these nanoparticles show an enhancement in coercivity and shifting of loop at 10 K, which is absent at 50 K. While the later observation supports the blocking temperature of the SPM phase, the former one is attributed to a disordered spin configuration at the surfaces and the distribution of nanoparticle sizes.     

Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films

The L1₀ ordered FePt films have been prepared at 300 °C with a basic structure of CrRu/MgO/FePt, followed by a post-annealing process at temperatures from 200 to 350 °C. The magnetic properties and the microstructure of the films were investigated. It is found that coercivity of FePt films increases greatly from 3.57 to 9.1 kOe with the increasing annealing temperature from 200 to 350 °C. The loop slope of the M–H curves decreases with the increasing annealing temperature, which is due to the grain isolation induced by MgO underlayer diffusion during the annealing process. The underlayer diffusion could be a useful approach to prepare the FePt-based composite films for high-density recording media.     

Size effect on the structure and magnetic properties of SmMn2O5 nanorods

Three sizes of SmMn₂O₅ nanorods that are labeled with (<L_C>) × axial lengths of 58(17) nm × 25(6) nm, 92(21) nm × 32(8) nm, and 126(25) nm × 52(13) nm were fabricated by the hydrothermal method. All the samples exhibited an antiferroicmagnetic (AFM) peak at approximately 6 K, which was associated with Sm magnetic ordering and no size independence. Another AFM magnetic ordering that belongs to the Mn ion was found with <L_C> = 58 nm, 92 nm, and 126 nm at 26 K, 28 K, and 30 K, respectively. The spin-orbit interaction increases with size in the magnetic susceptibility experiment. All the samples displayed a hysteresis loop at 2 K. The coercivity decreases as the size increases. The effects of the size on the crystal structure were elucidated from the Raman spectra of the <L_C> = 92 and 126 nm samples at various temperatures. The 126 nm sample displayed a red-shift for the Ag mode with warming, revealing that the Mn–O bonds are more sensitive to temperature in larger SmMn₂O₅ nanorods. These results demonstrate that the size effect importantly affects the structure and magnetic properties in SmMn₂O₅ multiferroic nanorods.     

Influence of solvothermal growth condition on morphological formation of hematite spheroid and pseudocubic micro structures and its magnetic coercivity

Influence of solvothermal growth condition on morphological formation and population of defects of hematite spheroid and pseudocubic micro structures and its magnetic properties were studied. Spheroid shaped crystals with different size were obtained from growth solution made of methanol, methanol-water, propanol and pseudocubic crystallites with dimension of 1.281 μm size were obtained with propanol-water solution combination at a growth temperature of 200 °C. UV absorption and magnetic properties of spheroid and pseudocubic micro structures were size and shape dependant. Spheroid shaped sample grown from precursor solution made of methanol gives intense UV absorption peaks at 360 nm and high coercivity (5.23 KOe) at room temperature. Reduction in magnetic coercivity and remanence of all samples at 5 K with respect to 300 K is attributed to antiferromagnetic nature of hematite with spheroid and pseudocubic morphology. High coercivity (6.2 KOe) at room temperature was observed from micro pseudocubic sample grown with propanol-water combination which is contributed to high aspect ratio, inter particle interaction and crystalline defects.     

One-pot synthesis and characterization of rhodamine derivative-loaded magnetic core–shell nanoparticles

A new method to produce elaborate nanostructure with magnetic and fluorescent properties in one entity is reported in this article. Magnetite (Fe₃O₄) coated with fluorescent silica (SiO₂) shell was produced through the one-pot reaction, in which one reactor was utilized to realize the synthesis of superparamagnetic core of Fe₃O₄, the formation of SiO₂ coating through the condensation and polymerization of tetraethylorthosilicate (TEOS), and the encapsulation of tetramethyl rhodamine isothiocyanate-dextran (TRITC-dextran) within silica shell. Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were carried out to investigate the core–shell structure. The magnetic core of the core–shell nanoparticles is 60 ± 10 nm in diameter. The thickness of the fluorescent SiO₂ shell is estimated at 15 ± 5 nm. In addition, the fluorescent signal of the SiO₂ shell has been detected by the laser confocal scanning microscopy (LCSM) with emission wavelength (λ_em) at 566 nm. In addition, the magnetic properties of TRITC-dextran loaded silica-coating iron oxide nanoparticles (Fe₃O₄@SiO₂ NPs) were studied. The hysteresis loop of the core–shell NPs measured at room temperature shows that the saturation magnetization (M _s) is not reached even at the field of 70 kOe (7T). Meanwhile, the very low coercivity (H _c) and remanent magnetization (M _r) are 0.375 kOe and 6.6 emu/g, respectively, at room temperature. It indicates that the core–shell particles have the superparamagnetic properties. The measured blocking temperature (T _B) of the TRITC-dextran loaded Fe₃O₄@SiO₂ NPs is about 122.5 K. It is expected that the multifunctional core–shell nanoparticles can be used in bio-imaging.