Zero-field-cooled Curves Research Articles

Unusual magnetic properties of NiO nanoparticles embedded in a silica matrix

We have observed unusual magnetic properties of NiO (nickel oxide) nanoparticles embedded in a silica matrix. The sample was synthesized by a method based on the contribution of sol–gel and combustion processes. X-ray powder diffraction (XRPD) of the sample shows the formation of the nanocrystalline NiO phase whereas transmission electron microscope (TEM) reveals spherical-shaped nanoparticles of about 4 nm diameter. Moreover, HRTEM images show lattice fringes of the nanoparticles and defects in the crystal structure. The temperature and field dependence of the magnetization are also measured. The zero-field-cooled (ZFC) measurements show two maximums, one sharp and narrow at low temperatures ∼6.5 K and an other broad one at higher temperature ∼64 K. The FC magnetization shows a continuous increase upon lowering the temperature. The M(H) measurements reveal that NiO nanoparticles display anomalous hysteretic behaviors at low temperatures (below the low temperature maximum in the ZFC curve, 2 K and 5 K) showing that the magnetization initial curve lies below the hysteresis loop for a certain field range. Moreover, jump of the magnetization at low temperatures (2 K and 5 K) are also observed. These features represent novel magnetic properties for nanosized NiO which may be attributed to the surface spins. Moreover, these results indicate that the NiO nanoparticle consists of magnetically disorder shell and antiferromagnetically order core with an uncompensated magnetic moment.

ZnFe2O4 Nanoparticles: Different Magnetic Behavior When They Are Hosted in Porous Structures

ZnFe2O4 nanoparticles have been prepared encased into sepiolite and faujasite matrixes. Transmission electron microscopy (TEM) images show hosted particles from 1 to 3 nm. From the zero field cooling (ZFC) and field cooling (FC) magnetic susceptibility data, the blocking temperatures distribution has been obtained, which reflects the particle size distribution. In both samples, the particles seem to present two well-defined sizes. Taking into account the adsorption results, the smallest particles must be located in the micropores of the sepiolite and faujasite structures, while the largest ones are probably hosted in structural defects. On the other hand, when the thermal energy overcomes the anisotropy barrier, at temperatures higher than TB, the abrupt decreasing of magnetization observed in the ZFC and FC curves would indicate that the hosted nanoparticles are free to rotate. This leads one to believe that they do not present any mechanical impediment that could avoid their free rotation. This fact can be attributed to the good dispersibility of the nanoparticles in the matrixes, which implies noninteracting particles, and to the negligible interaction with the surface of the cavities.

Spin-glass-like behavior in hollandite Ba1+δMn8O16 nanoribbons synthesized by molten-salt method

Abstract Single-crystal hollandite-type manganese oxides Ba 1+ δ Mn 8 O 16 nanoribbons (width of 30–500 nm and up to several hundred micrometers long) are synthesized by a simple molten-salt method. At 43 K a rapid increase of magnetization is observed; the zero field-cooled (ZFC) and field-cooled (FC) magnetization curves diverge below this temperature with a sharp cusp around 26 K in the ZFC curve revealing spin-glass-like behavior in the Ba 1+ δ Mn 8 O 16 nanoribbons. Measurements of ac-susceptibility indicate the coexistence of a weak ferromagnetic phase with a spin-glass component in the nanoribbons, which is distinct from the canted antiferromagnetic state found in the counterpart bulk sample.

Magnetic properties of YFe 12− xMo x compounds and magnetocaloric effect of YFe 9.5Mo 2.5

The characterization and magnetic properties of YFe 12− x Mo x ( x=2.0, 2.5 and 3.0) with the ThMn 12-type structure, and the magnetocaloric effect of YFe 9.5Mo 2.5 were investigated. A directional growth was observed in YFe 10Mo 2 alloy. A broad peak in the zero-field-cooling (ZFC) magnetization curve of the YFe 12− x Mo x compounds is ascribed to the existence of ferromagnetic clusters with different site moments and scattered orientations of the moments. The broad range of the peak is reduced with increasing Mo content. A weak peak is observed near 190 K in the ZFC curve of YFe 9Mo 3, which is associated with the 8i sites being mostly occupied by Mo atoms. YFe 9.5Mo 2.5 has a magnetic entropy change of −1.09 J/kg K for a field change of 5 T at 277 K.

Magnetic properties of Fe3O4 nanoparticles coated with oleic and dodecanoic acids

Magnetic nanoparticles (NP) of magnetite (Fe3O4) coated with oleic acid (OA) and dodecanoic acid (DA) were synthesized and investigated through transmission electron microscopy (TEM), magnetization M, and ac magnetic susceptibility measurements. The OA coated samples were produced with different magnetic concentrations (78%, 76%, and 65%) and the DA sample with 63% of Fe3O4. Images from TEM indicate that the NP have a nearly spherical geometry and mean diameter ∼5.5 nm. Magnetization measurements, performed in zero-field cooled (ZFC) and field cooled processes under different external magnetic fields H, exhibited a maximum at a given temperature TB in the ZFC curves, which depends on the NP coating (OA or DA), magnetite concentration, and H. The temperature TB decreases monotonically with increasing H and, for a given H, the increase in the magnetite concentration results in an increase in TB. The observed behavior is related to the dipolar interaction between NP, which seems to be an important mechanism in all samples studied. This is supported by the results of the ac magnetic susceptibility χac measurements, where the temperature in which χ′ peaks for different frequencies follows the Vogel–Fulcher model, a feature commonly found in systems with dipolar interactions. Curves of H versus TB/TB(H=0) for samples with different coatings and magnetite concentrations collapse into a universal curve, indicating that the qualitative magnetic behavior of the samples may be described by the NP themselves, instead of the coating or the strength of the dipolar interaction. Below TB, M versus H curves show a coercive field (HC) that increases monotonically with decreasing temperature. The saturation magnetization (MS) follows the Bloch’s law and values of MS at room temperature as high as 78 emu/g were estimated, a result corresponding to ∼80% of the bulk value. The overlap of M/MS versus H/T curves for a given sample and the low HC at high temperatures suggest superparamagnetic behavior in all samples studied. The overlap of M/MS versus H curves at constant temperature for different samples indicates that the NP magnetization behavior is preserved, independently of the coating and magnetite concentration.

Effect of Bivalent Cation Ca-Doping on Magnetic Properties in Multiferroic YMnO3 Manganites

Effect of bivalent cation Ca-doping on magnetic properties in multiferroic YMnO3 manganites was systemically studied by DC and AC magnetic measurements. Series of samples were prepared by solid-state reaction method with composition Y1−x Ca x MnO3 (x=i/8,i=2,3,…,7). The results show that each of the zero-field-cooling (ZFC) curves has a clear cusp and it shifts to higher temperature with increasing Ca-doping x. From the χ −1–T curve, the temperature of charge-ordering (CO) state T CO is given at about 244.0 K and this indicates that the CO state is dominated by the cooperative Jahn–Teller effect in Y1/2Ca1/2MnO3 with small A-site cation, for which there is a different behavior compared to the other manganites of larger 〈r A 〉. When x=2/8 and 7/8 for Y1−x Ca x MnO3, the magnetization curve shows strong irreversibility between ZFC and FC data. The variation of AC susceptibility presents a maximum cusp at the beginning temperature of the bifurcation between ZFC and ZF curve and shifts to higher temperature with increasing measurement frequency for AC susceptibility. These results can be characterized by spin-glass, indicating that much more itinerant eg electrons are promoting the magnetic transition.

Dimensionality effects on the magnetisation processes in arrays of superparamagnetic nanoparticles

We analyse the effect of inter-particle coupling and clustering on the zero-field cooled (ZFC) magnetisation processes of superparamagnetic nanoparticles. The static interaction field in dense assemblies of nanoparticles of different dimensionalities (1D, 2D and 3D) is investigated by numerical micromagnetic simulations. An analytical model is then used in order to evaluate the influence of the dimensionality of these systems on the topology of zero-field cooled magnetisation processes. Mutual static magnetic interactions are almost negligible in 3D arrays but become very important in systems of reduced dimensionality such as chains or layers of nanoparticles and they can account for up to 20% shift in the peak temperature of ZFC curves. The present results can be useful in evaluate the influence of interactions on the topology of ZFC magnetisation curves.

Magnetic properties and spin-glass-like behavior in stoichiometric Mn3In compound

Magnetic and transport properties of stoichiometric Mn3In compound are extensively studied by means of dc magnetization, ac susceptibility, magnetic relaxation, and electrical resistivity measurements. It is found that Mn3In is a ferrimagnet with Curie temperature of 80 K and that a metallic spin-glass-like (SGL) state exists below 45 K, which is indicated by the difference between zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves, invariable FC moments, large remanence effect, magnetic relaxation, and electrical resistivity abnormality. Moreover, the SGL state in Mn3In exhibits some anomalous behaviors, such as no cusp in the ZFC curve at the frozen temperature Tf, a remarkable high-field irreversibility, and a high-field relaxation process. The presence of the SGL state in the Mn3In compound without doping or substitution is discussed in terms of magnetic cluster mode.

The effect of randomly oriented anisotropy on the zero-field-cooled magnetization of a non-interacting magnetic nanoparticle assembly

A model based on localized partition function and master equation was set up to calculate the zero-field-cooled (ZFC) and field-cooled (FC) curves of a non-interacting magnetic nanoparticle assembly with randomly oriented anisotropy. The peak temperature of the ZFC curve corresponds to the highest energy barrier that acts against the unblocking process, and could be described well by an equation covering the heating rate effect. The predicted H 2/3 field dependence of the peak temperature is in good agreement with published results.

Effects of Ca substitution on the magnetic properties in La1-xCaxMn0.90Cu0.10O3 (x=0.05–0.30) manganites

Systematic AC and DC magnetic measurements were carried out to investigate the magnetic properties of La1-xCaxMn0.90Cu0.10O3 (x=0.05–0.30) system. A striking paramagnetism (PM) to ferromagnetism (FM) transition is observed at the FM ordering temperature TC. Below TC, the samples with x≤.0.20 show a visible unexpected drop in zero-field cooled (ZFC) DC magnetization curves near 100 K, which is believed to be associated with the domain wall pinning effects. As for the sample with x=0.30, there is no anomalous behavior near 100 K, while another different magnetic phase transition is shown up below 50 K in both FC and ZFC curves. Detailed experimental results show that the phenomenon is resulted from the phase competition rather than solely magnetic interactions.

On the synthesis and magnetic properties of multiwall carbon nanotube–superparamagneticiron oxide nanoparticle nanocomposites

Multiwall carbon nanotubes (MWCNTs) possessing an average inner diameter of 150 nmwere synthesized by template assisted chemical vapor deposition over an aluminatemplate. Aqueous ferrofluid based on superparamagnetic iron oxide nanoparticles(SPIONs) was prepared by a controlled co-precipitation technique, and this ferrofluidwas used to fill the MWCNTs by nanocapillarity. The filling of nanotubes withiron oxide nanoparticles was confirmed by electron microscopy. Selected areaelectron diffraction indicated the presence of iron oxide and graphitic carbon fromMWCNTs. The magnetic phase transition during cooling of the MWCNT–SPIONcomposite was investigated by low temperature magnetization studies and zero fieldcooled (ZFC) and field cooled experiments. The ZFC curve exhibited a blocking at∼110 K. A peculiar ferromagnetic ordering exhibited by the MWCNT–SPION composite aboveroom temperature is because of the ferromagnetic interaction emanating from theclustering of superparamagnetic particles in the constrained volume of an MWCNT. Thiskind of MWCNT–SPION composite can be envisaged as a good agent for variousbiomedical applications.

Magnetic Properties of $\gamma$-Fe$_{2}$O$_{3}$ Nanoparticles Precipitated in Alginate Hydrogels

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Influence of the nanoparticle size on the blocking temperature of interacting systems: Monte Carlo simulations

Abstract Zero field cooling curves (ZFC) under a relatively big magnetic field have been simulated in order to study the influence of the nanoparticle concentration on the rate of increase of the blocking temperature and its dependence with the nanoparticle size. Results show that for all nanoparticle concentrations the blocking temperature increases linearly with the nanoparticle size. The rate of increase of the blocking temperature is larger for larger nanoparticle concentrations, although it tends to a constant value for very large interactions between particles.

Fabrication and magnetic properties of Fe 3O 4 nanowire arrays in different diameters

Fe 3O 4 nanowire arrays with different diameters of D=50, 100, 150 and 200 nm were prepared in anodic aluminum oxide (AAO) templates by an electrodeposition method followed by heat-treating processes. A vibrating sample magnetometer (VSM) and a Quantum Design SQUID MPMS magnetometer were used to investigate the magnetic properties. At room temperature the nanowire arrays change from superparamagnetism to ferromagnetism as the diameter increases from 50 to 200 nm. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the blocking temperature T B increases with the diameter of nanowire. The ZFC curves of D=50 nm nanowire arrays under different applied fields ( H) were measured and a power relationship between T B and H were found. The temperature dependence of coercivity below T B was also investigated. Mössbauer spectra and micromagnetic simulation were used to study the micro-magnetic structure of nanowire arrays and the static distribution of magnetic moments of D=200 nm nanowire arrays was investigated. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in nanowires.

Nanometre Ni and core/shell Ni/Au nanoparticles with controllable dimensions synthesized in reverse microemulsion

Ni nanoparticles with different diameters were chemically synthesized in reverse microemulsion. Furthermore, core/shell Ni/Au nanoparticles with different core diameters and shell thickness were chemically synthesized from the above Ni nanoparticles through an in situ redox–transmetalation method in reverse microemulsion. The X-ray diffraction patterns revealed the presence of crystalline nickel and gold of the core/shell Ni/Au nanoparticles. The spherical Ni nanoparticles and the core/shell structured Ni/Au nanoparticles could be clearly observed by the transmission electron microscope. The diameter of the Ni nanoparticles with narrow size distribution could be controlled to range from about 8 to 30 nm. The diameter of the core/shell Ni/Au nanoparticles could be controlled to range from about 14 to 30 nm, with about 7–21 nm core diameter and about 3–7 nm shell thickness. The ZFC (zero-field-cooled) and FC (field-cooled) curves of the core/shell Ni/Au nanoparticles indicated that the blocking temperature increased from 16 to 53 K as the diameter of the Ni cores increased from about 7 to 15 nm.

Magnetic field‐dependence study of the magnetocaloric properties of a superparamagnetic nanoparticle system: a Monte Carlo simulation

AbstractThe influence of the applied magnetic field on the magnetocaloric properties of a fine magnetic particle system has been studied using a Monte Carlo technique. By simulating zero field cooling (ZFC) curves under different strengths of the applied magnetic field, we have analyzed the variation of the entropy for temperatures above the maximum of the ZFC curves, where the process is reversible. The entropy curves have been observed to behave in a different fashion at low values of the magnetic field, where the peak only slightly shifts to higher temperatures with increasing fields. For larger fields, the peak rapidly shifts to higher temperatures, while the overall shape of the curve broadens over a wide temperature range. It is also observed that the blocking temperature as a function of the magnetic field shows the feature of a change from a bell‐like shape to a monotonically decreasing function, resembling what is found experimentally for intermediate values of the sample concentration. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spin-glass transition in single-crystalBiFeO3

Magnetic properties of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ (BFO) single crystals with rhombohedral symmetry $(R3c)$ were measured in the temperature range of $4.2--350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Zero field cooled (ZFC) and field cooled magnetization curves split below $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a sharp cusp around $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the ZFC curves revealing spin-glass behavior in BFO single crystals. The observed coercive field increases with decreasing temperature, suggesting week ferromagnetism below $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and spin glass behavior below $\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The cusps around the freezing temperature $[{T}_{f}(\ensuremath{\omega})]$ which are frequency dependent confirm the spin-glass behavior in BFO single crystals with spin freezing temperature $({T}_{\mathit{SG}}=29.4\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The critical exponent $z\ensuremath{\nu}=1.38$ agrees rather well with experimental values in $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{O}}_{3}$ $(z\ensuremath{\nu}=1.0)$ [K. De et al., J. Appl. Phys. 99, 13908 (2006)] and the mean field theory $(z\ensuremath{\nu}=2.0)$ of Kirkpatrick and Sherrington [Phys. Rev. B 17, 4384 (1970)].

Surface spin glass behavior in sol–gel derived La0.7Ca0.3MnO3 nanotubes

We report the fabrication of La(0.7)Ca(0.3)MnO(3) nanotubes (LCMONTs) with a diameter of about 200 nm, by a modified sol-gel method utilizing nanochannel alumina templates. High resolution transmission electron microscopy confirmed that the obtained LCMONTs are made up of nanoparticles (8-12 nm), which are randomly aligned in the wall of the nanotubes. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves as well as a cusplike peak in the ZFC curve gives strong support for surface spin glass behavior.

Magnetic and Transport Properties of Ni Doped Pr0.5Ca0.5Mn1-xNixO3-δ

ABSTRACTStructural, magnetic and transport properties of Pr0.5Ca0.5Mn1-xNixO3 (x =0, 0.04, 0.07, 0.1) series have been investigated. The substitution of Mn ions by Ni induces drastic changes in the magneto-transport properties of Pr0.5Ca0.5MnO3. The physical behavior depends drastically on O stoichiometry. For the low Ni doping x=0.04, 0.07, spin glass configuration is rather favored than ferromagnetism; while for x=0.10, the ferromagnetic phase is significantly suppressed. Pronounced irreversibility between zero-field cooled (ZFC) and field cooled (FC) magnetization and a kink in the ZFC curve observed for x=0.04 and 0.07 are indicative of spin-glass-like state. Applied hydrostatic pressure of about 10 kbar reduces the temperature of charge ordering in an x=0 sample by about 10 K indicating a pressure induced suppression of the Jahn-Teller distortions and of the electron-phonon coupling. Electron magnetic resonance (EMR) unambiguously evidences appearance of FM phase in Ni doped manganites. Temperature dependence of EMR spectra parameters allow us to speculate on the effect of magnetic inhomogeneities.

Silica Encapsulated Ni Nanoparticles: Variation of Optical and Magnetic Properties with Particle Size

Ni nanoparticles embedded in SiO2 matrix were prepared by sol-gel process. The molar percentages of Ni were varied from 2 to 20% of total SiO2 present in the matrix. Transmission electron microscope (TEM) images revealed that particle sizes varied from 8.0-15.7 nm at an annealing temperature of 773 K with variation of concentration. The optical absorption spectra revealed that the surface plasmon resonance (SPR) peak in the UV region of the spectrum shifted with the particle diameter (D) from that at 247.3 nm for D = 8.0 nm to 250.7 nm for D = 15.7 nm. In hysteresis loop measurements the magnetizations (M) of the nanocomposites also increased with higher Ni content in the matrix and did not saturate in the measuring limit of the magnetic filed (H) of 4 KOe. The anhysteric curves for different samples were analyzed with the law of approach to saturation (LAS). The zero field cooled (ZFC) and field cooled (FC) magnetization measurements at 50 Oe showed increasing broadening of the ZFC curve with the higher Ni content. To calculate the average blocking temperature ((T(B a distribution of the blocking temperatures (T(B)) was assumed to initiate theoretical fittings and it was found to be increasing with the Ni concentration in the matrix.