ZFC Curves Research Articles

Effects of disordered surface structure on the magnetic properties of nanocrystalline MnFe2O4

In this study, a detailed investigation of the magnetic properties of nanocrystalline manganese ferrite (MnFe2O4) (average crystallite size is 8.4nm) synthesized by ball milling technique is presented. DC and AC magnetization measurements reveal that disordered surface spin structure plays an important role on the magnetic behavior of nanocrystalline MnFe2O4. Temperature dependent DC field biased susceptibility measurements, performed by field cooled and zero field-cooled procedures, have shown a clear field and temperature dependent irreversibility. Although, the real part of the ZFC susceptibility possesses a reduction up to 25K, FC susceptibility exhibits the conventional increasing trend. The temperature at which the difference between the FC and ZFC lines becomes zero is inversely proportional to the DC bias field. While the difference between the two lines reduces by the increase of the field, the temperature where ZFC curves make a dip is determined to be independent of the DC bias field.

Size distribution of FeNiB nanoparticles

Two samples of amorphous nanoparticles FeNiB, one of them with SiO2 sheath around the core and one without, were investigated by transmission electron microscopy and magnetic measurements. The coating gives mean particle diameters of 4.3 nm compared to 7.2 nm for the uncoated particles. Magnetic measurements prove superparamagnetic behaviour above 160 K (350 K) for the coated (uncoated) sample. With use of effective anisotropy constant Keff – determined from hysteresis loops – size distributions are determined both from ZFC curves, as well as from relaxation measurements. Both are in good agreement and are very similar for both samples. Comparison with the size distribution determined from TEM pictures shows that magnetic clusters consist of only few physical particles.

Preparation and size-dependent magnetism of highly dispersed iron silicide nanoparticles on silica

Fe3Si nanoparticles with a size of about 6–9 nm well dispersed on silica have been prepared by pyrolysis of ferrocene–polydimethylsilane composites at 600 °C. Powder XRD patterns, TEM and HRTEM images, and XPS spectra revealed that the average particle size increased with increasing pyrolysis temperature and a new phase, Fe5Si3, appeared when the pyrolysis temperature was above 800 °C. 57Fe Mössbauer spectra, M − H curves and FC and ZFC curves demonstrated that the as-prepared nanoparticles presented superparamagnetic behavior at 27 °C and ferromagnetic behavior at −268 °C, and their particle sizes had a great impact on their magnetic properties. The Fe3Si nanoparticles on silica may find applications in magnetically recording materials, magnetically separable catalysts and electrode materials for spintronic devices.

Iron oxide nanotubes synthesized via template-based electrodeposition

Considerable effort has been invested in the development of synthetic methods for the preparation iron oxide nanostructures for applications in nanotechnology. While a variety of structures have been reported, only a few studies have focused on iron oxide nanotubes. Here, we present details on the synthesis and characterization of iron oxide nanotubes along with a proposed mechanism for FeOOH tube formation. The FeOOH nanotubes, fabricated via a template-based electrodeposition method, are found to exhibit a unique inner-surface. Heat treatment of these tubes under oxidizing or reducing atmospheres can produce either hematite (α-Fe2O3) or magnetite (Fe3O4) structures, respectively. Hematite nanotubes are composed of small nanoparticles less than 20 nm in diameter and the magnetization curves and FC-ZFC curves show superparamagnetic properties without the Morin transition. In the case of magnetite nanotubes, which consist of slightly larger nanoparticles, magnetization curves show ferromagnetism with weak coercivity at room temperature, while FC-ZFC curves exhibit the Verwey transition at 125 K.

STUDY OF BIFERROIC PROPERTIES IN THE La0.37Ca0.17Ba0.43Mn0.52Ti0.44Zr0.04O3 COMPLEX PEROVSKITE

In this paper, details of synthesis and structural, morphological, electrical, and magnetic characterization of the new La 0.37 Ca 0.17 Ba 0.43 Mn 0.52 Ti 0.44 Zr 0.04 O 3 multiferroic complex perovskite are reported. Mixtures with 50% mass of ferromagnetic lanthanum calcium manganite La 0.67 Ca 0.33 MnO 3 and ferroelectric barium-lanthanum zirconate titanate Ba 0.9 La 0.067 Ti 0.91 Zr 0.09 O 3 were prepared by the solid state reaction technique. Patterns of X-ray diffraction showed that the materials have reacted resulting in a new perovskite-like structure with tetragonal symmetry, space group P4mm(#99). The structure of the material was refined using the Rietveld method through the GSAS code. ZFC and FC magnetization curves show the occurrence of two phase transitions at 42.25 K and 203.9 K which have been associated with two different magnetic regimes. Hysteresis curves measured confirm that the relationship between the applied field and the magnetization does not evidence a linear behavior. These curves also show that in the low temperature regime the magnetic memory of the material is greater than in the high temperature region. AC impedance as a function of temperature measurements show the same two regions observed in the magnetization curves. The ferroelectric behavior with relative permittivity of 153.12 is observed by polarization curves performed at room temperature in the synthesized materials.

Morphology and magnetism of multifunctional nanostructured [formula omitted]-[formula omitted] films: Simulation and experiments

This paper introduces a new approach for simulating magnetic properties of nanocomposites comprising magnetic particles embedded in a non-magnetic matrix, taking into account the 3D structure of the system in which particles’ positions correctly mimic real samples. The proposed approach develops a multistage simulation procedure in which the size and distribution of particles within the host matrix is firstly attained by means of the Cell Dynamic System (CDS) model. The 3D structure provided by the CDS step is further employed in a Monte Carlo (MC) simulation of zero-field-cooled/field-cooled (ZFC/FC) and magnetic hysteresis loops (M×H curves) for the system. Simulations are aimed to draw a realistic picture of the as-produced ultra-thin films comprising maghemite nanoparticles dispersed in polyaniline. Comparison (ZFC/FC and M×H curves) between experiments and simulations regarding the maximum of the ZFC curve (TMAX), remanence (MR/Ms) and coercivity (HC) revealed the accuracy of the multistage approach proposed here while providing information about the system's morphology and magnetic properties. For a typical sample the value we found experimentally for TMAX (54K) was very close to the value provided by the simulation (53K). For the parameters depending on the nanoparticle clustering the experimental values were consistently lower (MR/Ms=0.32 and HC=210Oe) than the values we found in the simulation (MR/Ms=0.53 and HC=274Oe). Indeed, the approach introduced here is very promising for the design of real magnetic nanocomposite samples with optimized features.

Enhancement of magnetic and ferroelectric behaviour in (Ca, Co) co-doped HoMnO3 multiferroics

The effect of sintering temperature on structural, electrical and magnetic behaviours of polycrystalline samples of Ho0.9Ca0.1Mn0.9Co0.1O3 prepared by the solid state reaction route sintered at three different temperature 1250°C, 1350°C, 1450°C for 10h are investigated. XRD, SEM, magnetization, dielectric and ferroelectric measurements were carried out. Experimental results showed the nucleation of orthorhombic phase as the sintering temperature increases from 1250°C to 1450°C. Ferroelectric (Tc) and antiferromagnetic transition temperature (TN) increases with increase in sintering temperature. Strong bifurcation of FC and ZFC curve in sample sintered at 1450°C showed a clear onset of ferromagnetic state around 165°K, which is confirmed from M to H graph at 165°K. All the sample showed ferroelectric behaviour at room temperature which are leaky in nature. Sintering temperature along with Ca and Co doping in HoMnO3 ceramics plays an important role in phase transformation along with enhancement in multiferroic properties.

Spin-Glass-Like Behavior and Concentration Phase Transitions in Model of Monolayer Two-Sublattice Magnetics

Two-sublattice magnets with competing direct exchange interactions can exhibit spin-glass properties. It is shown that the Ising model on a 2D lattice with z = 8 nearest neighbors takes place divergence between the ZFC and FC magnetization curves. It was shown, that a weak antiferromagnetic interaction between ferromagnetically ordered sublattices with approximately similar values of the magnetic moments in the sites can lead to very complex magnetic properties. Theoretical magnetic phase diagram for the structures with random dilution was built.

Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle

Mn doped TiO2 nanoparticles are synthesized by sol–gel method. Incorporation of Mn shifts the diffraction peak of TiO2 to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO2 show peaks at g = 1.99 and 4.39, characteristic of Mn2+ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO2, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn3O4. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn2+ ions via oxygen vacancy (F+ center). Increase in Mn concentration increases the fraction of Mn3O4 phase and thereby increases the paramagnetic ordering.

Controllable organic-phase synthesis of cuboidal CoO mesocrystals and their magnetic properties

A one-pot process is proposed for the synthesis of cuboidal CoO mesocrystals via the decomposition of cobalt(II) oleate complexes in the presence of oleic acid, water, sodium oleate, and high boiling point (b.p.) organic solvents. The formation of faceted mesocrystals is attributed to the preferential attachment of sodium oleate on the {100} planes of the CoO structure. Meanwhile, the effective aggregation and fusion of the primary nanoparticles is attributed to the presence of free oleic acid and water. It is shown that the size of the mesocrystal cuboids can be tuned between approximately 60 nm and 200 nm by adjusting the ratio of sodium oleate to cobalt oleate and choosing an appropriate high b.p. organic solvent. The as-prepared and aged (45 days) CoO mesocrystals exhibit spontaneous magnetization at low temperatures, as a result of residual spins. The additional peak found in the ZFC curve of the aged mesocrystals at 54 K suggests that surface oxidation leads to the formation of a Co3O4 shell in which symmetry-breaking Co3+ cations enhance the TN. Finally, the S-shaped magnetization regions and vertical shifts of the field-cooled loops in the hysteresis curves of the as-prepared and aged mesocrystals indicate that the residual spins are either rotatable or tightly pinned by the antiferromagnetic lattices.

Spin-glass-like behavior of surfactant capped Co50Ni50 nanoparticles

We investigated static and dynamic magnetic properties of surfactant capped 25nm Co50Ni50 particles. These nanoparticles exhibit the superparamagnetic behavior at 300K. Interestingly, a freezing temperature (Tf∼16K) is observed in the ZFC curve, which corresponds to surface spin-glass-like transition. The significant high coericivity (Hc∼910Oe) is observed at 5K as compared to Hc∼180Oe at 77K and is attributed to the random freezing of surface spins at T⩽Tf. The spin-glass-like behavior of nanoparticles is further verified by ac-susceptibility measurements as a function of temperature at different frequencies. The fits of Arrhenius law, Vogel–Fulcher law and dynamic scaling equation to data confirm the spin-glass-like behavior of Co50Ni50 nanoparticles.

Metal Ions Induce Growth and Magnetism Alternation of α‐Fe2O3 Crystals Bound by High‐Index Facets

In this study, quasi-cubic and hexagonal bipyramid α-Fe(2)O(3) polyhedrons with high-index facets exposed were controllably synthesized by applying metal ions Zn(2+) or Cu(2+) as structure-directing agents. The growth of the α-Fe(2)O(3) nanostructures with high-index facets were induced by metal ions without the addition of any other surfactants. The quasi-cubic form controlled by Zn(2+) looks like a cube but has an angle of approximately 86° bound by (012), (10-2), and (1-12) facets, whereas the hexagonal bipyramid form controlled by Cu(2+) has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape- and surface-dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240 K and might be spin-canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi-cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal-ion-directing technique could provide a universal method for shape- and surface-controlled synthesis of nanocrystals with high-index facets exposed.

Synthesis, structure and magnetic properties of La3Co2SbO9: A double perovskite with competing antiferromagnetic and ferromagnetic interactions

The synthesis, structural characterization, and magnetic properties of La3Co2SbO9 double perovskite are reported. The crystal structure has been refined by X-ray and neutron powder diffraction data in the monoclinic space group P21/n. Co2+ and Sb5+ have the maximum order allowed for the La3Co2SbO9 stoichiometry. Rietveld refinements of powder neutron diffraction data show that at room temperature the cell parameters are a=5.6274(2)Å, b=5.6842(2) Å, c=7.9748(2) Å and β=89.999(3)°. Magnetization measurements indicate the presence of ferromagnetic correlations with TC=55K attributed to the exchange interactions for non-linear Co2+–O–Sb5+–O–Co2+ paths. The effective magnetic moment obtained experimentally is μexp=4.38μB (per mol Co2+), between the theoretical one for spin only (3.87μB) and spin-orbit value (6.63μB), indicating partially unquenched contribution. The low magnetization value at high magnetic field and low temperature (1μB/f.u., 5T and 5K) and the difference between ZFC and FC magnetization curves (at 5kOe) indicate that the ferromagnetism do not reach a long range order and that the material has an important magnetic frustration.

Physical properties ofAxFe2−yS2(A=K, Rb, and Cs) single crystals

We successfully synthesized two new compounds Rb$_x$Fe$_{2-y}$S$_2$ and Cs$_x$Fe$_{2-y}$S$_2$ which were isostructural with K$_x$Fe$_{2-y}$Se$_2$ superconductor. We systematically investigated the resistivity, magnetism and thermoelectric power of $A_x$Fe$_{2-y}$S$_2$ ($A$=K, Rb and Cs) single crystals. High temperature resistivity and magnetic measurements show anomalies above 500 K depending on $A$ which are similar to $A_x$Fe$_{2-y}$Se$_2$. Discrepancy between ZFC and FC curves was observed in K$_x$Fe$_{2-y}$S$_2$ and Rb$_x$Fe$_{2-y}$S$_2$, while it disappears in Cs$_x$Fe$_{2-y}$S$_2$. Our results indicate the similar magnetism between $A_x$Fe$_{2-y}$S$_2$ and $A_x$Fe$_{2-y}$Se$_2$ at high temperature.

Production of superparagnetic nanospheres for hyperthermic therapy of surface (skin) cancer diseases

The idea of invention is that superparamagnetic nanoceramic particles are received in rotating cathode equipment. Microstructure of powder is studied by electronic microscopy and X-ray analysis. Also, powder homogeneity, particle size distribution and, respectively, stability and magnetic susceptibility are studied by magnetometric method. Average sizes of particles are 30-100 nm. The powder is homogeneous and in the process of analysis it was revealed its good stability. Research object is superparamagnetic powder. Relative value of its magnetic susceptibility is 1.00037. Sample magnetization measurements are done at cooling in zero magnetic field (ZFC) and in nonzero field (FC) modes, also magnetic hysteresis curves are measured at low and high temperatures compared to the mean blocking temperature (TB) for measured samples. From the state of maximum on the ZFC curve it was deduced that temperature of blocking in the given superparamagnetic subsystem of particles is TB ~ 60 K.

Coalescence-free L1 ordering of embedded CoPt nanoparticles

We have synthesized diluted samples of CoPt clusters and investigated by various techniques the effect of a 2 h anneal at 750 K. Transmission electron microscopy observations have put into evidence L10 chemical ordering without any detectable coalescence upon annealing. Magnetic measurements on CoPt clusters embedded in amorphous carbon have been used to provide additional and stronger evidence of the absence of coalescence. We show how the analysis of normalized ZFC curves can be used as a convenient and powerful technique to detect subtle variations in the cluster size distribution.

Low temperature properties of multiferroic [formula omitted] compound

Single-phase BiFe 0.9Cr 0.1O 3 compound with a grain size of ∼ 63 nm was prepared by the sol–gel technique. A well-defined nonlinearity in the field dependences of magnetization as a result of the collapse of the space-modulated spin structure and the strong coupling between Fe 3+ and Cr 3+ superexchange interaction indicated a weak ferromagnetic character, and the observed significant deviation between the ZFC and FC curves suggested a spin-glass-like behavior at low temperatures. The low temperature dielectric constant showed two subtle anomalies at 50 and 230 K, which are related to the spin-glass behavior and magnetoelectric coupling. Moreover, the magnetoelectric coupling was estimated by determining the variation of the dielectric constant with applied magnetic field.

Magnetic‐field influence on magnetization dependence of temperature in Cu56Ga27Mn17 annealed microwires

AbstractGlass‐coated Cu56Ga27Mn17 magnetic microwires, resulting from thermal annealing at 523 K for 1 h, have been structural and magnetically characterized. The crystalline structure shows the coexistence of four crystalline phases: a hexagonal phase with a = b = 0.4536 nm and c = 0.8386 nm lattice parameters, a cubic B2 one having a lattice parameter a = 0.29208 nm, a second fcc Cu‐rich minority phase with a = 0.3592 nm, and a monoclinic CuO appearing at the sample surface. ZFC–FC–FH thermomagnetic curves performed at different applied magnetic fields in the CuGaMn microwires annealed at 523 K, show a clear coexistence of two ferromagnetic phases with a Curie temperature around 150 K for the major phase while that corresponding to the second one is above room temperature. The field effect on ZFC–FC–FH magnetization behavior displays that the increase in the applied field modifies and shifts two peaks observed in the ZFC curve, also reducing the thermal hysteresis between FC and FH curves existent in the applied field range of 50–500 Oe. The magnetization behavior exhibited by the CuGaMn annealed microwires depending on the temperature range and magnetic field value will be discussed on the basis of the coexistence of antiferromagnetic and ferromagnetic interactions.

NEW MAGNETIC BEHAVIOR OF LaCoO3 SINGLE CRYSTAL AT LOW TEMPERATURE

The LaCoO 3 single crystal, 4 mm in diameter and 30 mm in length, has been grown by optical floating zone method. The magnetic measurements show that the coercivity and the remanence are 5 Oe and 1.5 × 10-3 emu/g at 5 K, respectively. The ZFC and FC magnetic susceptibility curves overlap and the 1/χ(T) curve shows a linear variation over the interval 5 K < T < 35 K in 1000 Oe. As the temperature increases from 35 K to 90 K, the ZFC curve in 1000 Oe firstly reduces nonlinearly, and then appears as a wave crest. The χ under ZFC is higher than that under FC in the temperature range of 55~90 K. In 50 000 Oe, a slope change of 1/χ(T) at about 12 K is observed.

Magnetic susceptibility curves of a nanoparticle assembly, I: Theoretical model and analytical expressions for a single magnetic anisotropy energy

We study a model system made of non-interacting monodomain ferromagnetic nanoparticles, considered as macrospins, with a randomly oriented uniaxial magnetic anisotropy. We derive a simple differential equation governing the magnetic moment evolution in an experimental magnetic susceptibility measurement, at low field and as a function of temperature, following the well-known Zero-Field Cooled/Field Cooled (ZFC/FC) protocol. Exact and approximate analytical solutions are obtained, together for the ZFC curve and the FC curve. The notion of blocking temperature is discussed and the influence of various parameters on the curves is investigated. A crossover temperature is defined and a comparison is made between our progressive crossover model (PCM) and the crude “two states” or abrupt transition model (ATM), where the particles are assumed to be either fully blocked or purely superparamagnetic. We consider here the case of a single magnetic anisotropy energy (MAE), which is a prerequisite before considering the more realistic and experimentally relevant case of an assembly of particles with a MAE distribution (cf. part II that follows).