ZFC Curves Research Articles

Novel Magnetization Induced by Phase Coexistence in Multiferroic HoCrO3 Chromites

Single-phase HoCrO3 samples were prepared using the conventional solid-state reaction. The temperature dependence of dc magnetic properties show that the ZFC and FC magnetization curves are irreversible strongly and nearly symmetrical below 141.0 K under 100 Oe. Moreover, there appears a novel magnetization behavior with negative magnetization (diamagnetism-like) characteristics. This behavior exhibits the coexistence of canted antiferromagnetic and weak ferromagnetic phase and the existence of competition mechanism below Tc1 = 141.0 K. In addition, it is also proved that Ho ion ordered weak CAFM below 7.5 K. The current novel magnetization can be interpreted from the interaction between paramagnetic Ho3+ moments and canted Cr3+ moments.

Effect of Nickel Doping on the Magnetotransport Properties of Sm0.55Sr0.45MnO3 Manganites

We studied the effects of nickel (Ni) doping on the magneto-transport properties of Sm0.55Sr0.45MnO3 manganites near the metal-insulator transition. Various concentrations of Ni-doped Sm0.55Sr0.45MnO3 samples up to 10% were prepared (Ni was partially substituted at the Mn-site). The temperature dependence of resistivity and magnetoresistance were measured as a function of Ni concentrations at various applied magnetic fields. We observed a nonlinear reduction of the metal-insulator transition temperature (MIT) with increasing concentration of Ni, 5% of Ni was sufficient to completely suppress the insulator-metal transition. Moreover, we observed dramatic increases of the resistance of the doped material with an increasing Ni-doping (5% of Ni increases R by more than 1000 times). The resistivity peaks at various magnetic fields collapses on themselves at the high temperature ends above the MIT. We also performed magnetization versus temperature measurements on both Ni-free the Ni-doped samples for FC and ZFC states. The FC and ZFC curves rapidly decrease to paramagnetic state at 175 K and 130 K for ZFC and FC states, respectively. For other Ni-doped samples, we observed a reduction in the paramagnetic transition temperature with increasing Ni concentration.

Origin of Magnetic Anomalies below the Néel Temperature in Nanocrystalline LuMnO3

Rare earth manganites crystallize in distorted orthorhombic perovskite or hexagonal structures and exhibit quite interesting optical and magnetic properties dictated by the size of the rare earth ion. Many of these materials might exhibit both ferroelectric and magnetic ordering as well as magnetoelectric coupling. However, their physical properties at reduced particle sizes remain underexplored due to the challenges associated with their synthesis with a proper control over the crystalline phase. Here, we report the wet-chemical synthesis of the hexagonal phase of nanocrystalline LuMnO3 with an average crystallite size of ∼32 nm. The room-temperature Raman spectroscopy data are consistent with the calculated values of isomorphous hexagonal RMnO3 (R = rare earth atom) compounds with P63cm symmetry. The UV−vis-NIR spectra recorded in the diffused reflectance mode at room temperature show electronic transitions at 1.7 eV (729 nm), 2.3 eV (539 nm), and 5 eV (258 nm). The magnetization measurements show that the Néel temperature for the LuMnO3 is situated at around 89 K, which is in close proximity to the reported value of the bulk phase. We also observed two unique and field-dependent magnetic anomalies that were predicted earlier but never reported experimentally. The first anomaly is observed as a sharp bifurcation in the ZFC−FC curves below 44 K at a 100 Oe applied field, which is accompanied with a sudden rise in the coercivity and magnetization. A second transition is observed at 12 K as a sharp peak in the ZFC curves, which is accompanied with a dip in coercivity. We attribute the transition at 44 K to the reorientation of the Mn3+ ions due to the Dzyaloshinskii−Moriya interaction, and the transition at 12 K is explained by weak antiferromagnetic coupling between Mn−O−Mn in the ab plane, which becomes dominant at lower temperatures.

Flux creep relaxation in single crystal Hg-1201

Magnetization measurements on a single crystal of the high-Tc cuprate HgBa2CuO4 (Hg-1201) are performed for the two field orientations B||c and B||(a,b). Besides zero-field-cooling (zfc) – field-cooling (fc) and magnetization studies, the flux creep relaxation is determined. The zfc curves exhibit pronounced step-like anomalies in the mixed state above the lower critical field Bc1. They are interpreted as those points in the phase diagram where the penetrating flux fronts meet at the center of the sample. In order to obtain more information about the flux pinning mechanism flux creep relaxation experiments are performed at different temperatures in magnetic fields of 0.15 and 2 T. The resulting supercurrent dependence on the mean activation energy is analysed according to the collective pinning theory which predicts U ∝ ((j/jc)−μ −1). The μ-values for B||c are 0.5 at higher temperatures and 2 at 5 K, and they vary between 1.1 and 1.5 for B||(a,b). The observed behaviour is quite different from that found for powder samples.

Fabrication and magnetic property of α-Fe 2O 3 nanoparticles/TiO 2 nanowires hybrid structure

α-Fe 2O 3 nanoparticles/TiO 2 nanowires hybrid structure is fabricated by two-step hydrothermal treatment. TiO 2 nanowires are prepared by heating of titanate nanowires, which are obtained by hydrothermal treatment of TiO 2 powder and further repeated HCl treatment. α-Fe 2O 3 nanoparticles are deposited on the surface of TiO 2 nanowires by hydrothermal treatment in Fe(NO 3) 3 solution. The HRTEM images confirm the junctions between α-Fe 2O 3 nanoparticles and TiO 2 nanowires. The formation of hybrid structures has significant influence on the magnetic properties of α-Fe 2O 3. The Morin transition temperature of α-Fe 2O 3 nanoparticles/TiO 2 nanowires hybrid structure is 190 K, which is determined by the sharp peak in the differential ZFC curve. Whereas there is no observable Morin transition for the corresponding isolated α-Fe 2O 3 nanoparticles with similar average particles size of ca. 20 nm.

Irreversibility Line and Flux Relaxation in Bi:2223/Ag Tape Prepared by CTFF Technique: H//c axis

We have studied flux dynamics in Bi:2223/Ag tape prepared by the CTFF technique in the orientation of H//c axis. Irreversibility line (IL) of a Bi:2223/Ag tape has been determined from AC susceptibility and the DC magnetization technique based on FC and ZFC curves. We have observed that the irreversibility temperature determined from the AC susceptibility method locates at a higher temperature than that was determined from the DC magnetization method. This shift could be attributed to the viscous dissipation generated by the oscillation of flux lines. FC magnetization at 20 K and 30 K for the field range from 500 to 2500 Oe are the same and show a plateau implying the same amount of reversible magnetization due to the Meissner current. Remanent magnetization versus temperature curves, which give us information on the flux pinning mechanism of the vortices, are also presented.

Glassy magnetic behaviors in oxygen-deficient Sr2CoO4-δ films

We have fabricated c-axis-oriented Sr2CoO4-δ thin films under relatively low oxygen pressure by the pulsed laser deposition method (PLD) on LaAlO3 substrates. X-ray diffraction and high-pressure reflection high energy electron diffraction (RHEED) demonstrate that the films were well deposited. There exist two different characteristic temperatures, Tf and Ta, in the ZFC curve of temperature dependence of magnetization. Tf may correspond to the freezing temperature while Ta may be related with the anisotropic field of c-axis. Further investigations including magnetic relaxation, memory effect and hysteresis behaviour measurements suggest a main phase of spin glass, which starts the frozen-in process even above the freezing temperature. We ascribe the spin glass behaviour to the competition betwcen double-exchange and super-exchange interactions which could have originated from the coexistence of Co4+ and Co3+ caused by oxygen deficiency in the films.

Structural and magnetic properties of single-crystalline spinel systems ZnCr 2− x In xSe 4

The single crystals of In 3+-doped zinc chromium selenide spinel ZnCr 2Se 4 were prepared by a chemical transport method and characterized by X-ray diffraction, magnetic measurements and ESR spectroscopy in order to examine an effect of nonmagnetic indium on the structural and magnetic properties of the system. The X-ray diffraction analysis has shown that only small amount of In entered the octahedral sites in the cubic spinel structure with space group F d 3 ¯ m . As compared with parent ZnCr 2Se 4, the unit cell dimensions increased slightly with increasing In content, however at the In concentration of about 10%, the structures exhibited a certain amount of octahedral vacancies. For examined compounds the Neel temperature is very similar to that of the parent spinel ZnCr 2Se 4, whereas the paramagnetic Curie temperature is much lower (of about 59 K) than that for matrix. It means that the frustration introduced into systems by B-sublattice indium dilution reflects the change of ferromagnetic interactions in the magnetic exchange. The temperature dependence of the ESR linewidth over region ( T/ T N)>8 is interpreted by an occurrence of spin–phonon interaction. The interesting feature of the low-field magnetization is that the magnetic irreversibility observed in the separation of the FC and ZFC curves below T N is consistent with a new, weak resonance-like features.

Temperature dependence of magnetic properties in Fe/Fe–O nanoparticles dispersed in water

Commercial water dispersions of Fe/Fe–O nanoparticles (∼50–80 nm diameter) were prepared bare and treated with a biodegradable polymer to stabilize the suspension. Hysteresis loops and FC–ZFC curves of samples in liquid and dried form were measured by means of a vibrating sample magnetometer from 77 to 300 K. A marked dependence on the conditioning field is observed in all samples that display the Verwey transition at about 120 K. The role of magnetic interactions on dried samples was also investigated by means of magnetoresistance measurements as a function of temperature.

Implications of nanoparticle concentration and size distribution in the superparamagnetic behaviour of aging-improved maghemite xerogels

Evolution of static magnetic properties of a set of enhanced γ-Fe2O3/SiO2 nanocomposites with different iron concentration has been studied on the basis of their corresponding hysteresis loops, zero-field/field-cooled (ZFC/FC) magnetization curves and transmission electron microscopy images. The lack of coercivity in all compositions, as well as the fulfillment of the H/T scaling law by the magnetization above the blocking temperature of each system under study, evidence a superparamagnetic behaviour in the iron oxide nanoparticles. In order to study the influence of iron content in the unblocking processes of nanoparticles, ZFC curves under different applied magnetic fields have been fitted to a model considering the systems under study as a distribution of energy barriers. Depart from the superparamagnetic model is discussed considering interparticle interactions.

Magnetic behaviour of nanocrystalline Ni0.5Cu0.5Fe2O4 spinel ferrite

The magnetic behaviour of the nanocrystalline Ni0.5Cu0.5Fe2O4, synthesized by the co-precipitation method has been studied. The X-ray diffraction patterns confirm the formation of cubic spinel structure. The dc magnetization measurements show that the samples are superparamagnetic above the blocking temperatures and the blocking temperature increases with particle size. The reduction in saturation magnetization in the case of nanoparticles as compared to their bulk counterpart has been explained on the basis that the magnetic moments in the surface layers of a nanoparticle are in a state of frozen disorder. The hump in ZFC curve progressively shift to a lower temperature with increasing field and broader curves at higher field, suggests the spin-glass nature of the system.

Phase transition and magnetic properties of Mg-doped hexagonal close-packed Ni nanoparticles

Mg-doped Ni nanoparticles with the hexagonal close-packed (hcp) and face-centered cubic (fcc) structure have been synthesized by sol–gel method sintered at different temperatures in argon atmosphere. The sintering temperature played an important role in the control of the crystalline phase and the particle size. The pure hcp Mg-doped Ni nanoparticles with average particle size of 6.0 nm were obtained at 320 °C. The results indicated that the transition from the hcp to the fcc phase occurred in the temperature range between 320 °C and 450 °C. Moreover, the VSM results showed that the hcp Mg-doped Ni nanoparticles had unique ferromagnetic and superparamagnetic behavior. The unsaturation even at 5000 Oe is one of the superparamagnetic characteristics due to the small particle size. From the ZFC and FC curves, the blocking temperature T B of the hcp sample (6.0 nm) was estimated to be 10 K. The blocking temperature was related to the size of the magnetic particles and the magnetocrystalline anisotropy constant. By theoretical calculation, the deduced particle size was 6.59 nm for hcp Mg-doped Ni nanoparticles which was in agreement with the results of XRD and TEM.

Magnetic properties of a high energy ball-milled amorphous Gd 5Si 1.8Ge 1.8Sn 0.4 alloy

Amorphous Gd 5Si 1.8Ge 1.8Sn 0.4 has been obtained with the help of a high-energy ball milling technique. XRD and DSC measurements confirm the amorphous nature of the milled alloy. The separation of FC and ZFC curves below the ferromagnetic transition temperature indicates a spin glass-like state in the amorphous alloy. Thermal hysteresis has been significantly reduced in the high energy ball-milled alloy compared with that of a crystalline one. For the amorphous Gd 5Si 1.8Ge 1.8Sn 0.4, magnetic phase transition takes place in a wide temperature span and the RC value reaches around 495 J kg −1.

Coexistence of Superparmagnetism and Spin-Glass Like Magnetic Ordering Phenomena in a CoFe2O4−SiO2 Nanocomposite

The effect of interparticle interactions on the magnetic behavior of cobalt ferrite nanoparticles was investigated in a CoFe2O4/SiO2 nanocomposite with 50 wt % of magnetic phase. A sample of pure cobalt ferrite nanoparticles prepared by the same method was also studied, as reference. Both samples show irreversible magnetic behavior, when the DC magnetization, M(T), is observed by the zero field cooling−field cooling (ZFC−FC) measurements. For a field higher than 0.2 T, the ZFC curves of the two samples show a maximum and the corresponding temperature is proportional to the magnetic field raised to the 2/3 power (Tmax ∝ H2/3). This study shows that in both samples the MZFC(T) maxima are due to magnetic ordering phenomena rather than superparamagnetic blocking, pointing out that strong interactions among particles are present also in the nanocomposite. However, the presence of the silica matrix gives rise to the coexistence of two different magnetic behaviors. In fact, a secondary maximum at low temperature is observed in the MZFC(T) curves measured for the silica supported sample. The study of the Tmax vs H2/3 plot of such secondary MZFC(T) maximum shows that it is not due to collective magnetic ordering but rather to a superparamagnetic transition of a fraction of small nanoparticles. The coexistence of two populations of nanoparticles is in agreement with Mössbauer spectroscopy and TEM analysis.

Spin-glass-like ordering in ɛ-Fe 3− xNi xN (0.1 ≤ x ≤ 0.8) nanoparticles

ɛ-Fe 3− x Ni x N (0.1 ≤ x ≤ 0.8) nanoparticles are investigated for the magnetic interaction effects at low temperatures. The reduction of saturation magnetization ( σ s) values is attributed to spin pairing effects and surface spin canting. In ZFC and FC magnetization curves, the surface spin-glass layer ( T f1) and the reentrant spin-glass behavior ( T f2) are evidenced below 27 and 22 K respectively. The surface spin-glass phase is described by the random-field model of exchange anisotropy. Based on this theory, a surface spin-glass layer about 8 nm thick is determined. The reentrant spin-glass behavior occurs due to the disorder created by Ni substitution and is observed for 0.2 ≤ x ≤ 0.4. The spin-glass-like ordering in the representative ɛ-Fe 2.6Ni 0.4N system is verified using spin relaxation dynamics and ac susceptibility measurements. The dynamic scaling analysis, based on imaginary part of the ac susceptibility data, gives the parameters τ 0 = 10 −8 s, T c = 15.3 K, and zν = 4.5 and hence confirm the spin-glass-like ordering in ɛ-Fe 3− x Ni x N (0.1 ≤ x ≤ 0.8) nanoparticles.

Magnetic study of single domain ɛ-Fe 3N nanoparticles synthesized by precursor technique

Nanostructured single domain ɛ-Fe 3N particles have been prepared by chemical route. Magnetic dipolar interaction among particles has been observed. The FC and ZFC magnetization curves show the superparamagnetic blocking temperature around 125 K, at low applied field of 0.005 T. At higher applied field, these ZFC and FC magnetization curves merge because of increase in magnetization energy that overcomes the magnetic anisotropy and dipolar interaction energies. Mössbauer spectroscopy further corroborates the superparamagnetic nature at 300 K.

Temperature dependence of magnetic properties of Cu 80Co 19Ni 1 thin microwires

In the present work, we report the studies of temperature dependence of magnetic properties in thin microwires with composition Cu 80Co 19Ni 1. An extensive study of structural and magnetic characterization was realized. The structure was observed using X-ray diffraction with CuK α radiation. The magnetic measurements were carried out using a SQUID at temperatures between 5 and 300 K. The as-prepared Cu 80Co 19Ni 1 microwire presents a coercivity of about 80 Oe. The variation of the coercivity and remanent magnetization at 5–300 K were obtained from the hysteresis loops. From the difference of the ZFC and FC curves below T=100 K, we can assume the presence of small superparamagnetic grains embedded in the Cu matrix. Those superparamagnetic grains should be blocked at temperatures below the maximum of the magnetization observed below 50 K. The measurements show an unusual temperature dependence of the coercive field, consequence of a coexistence of blocked and unblocked particles, and the typical decreasing behaviour of the remanence increasing temperature.

Magnetic characterization of maghemite nanoparticles dispersed in surface-treated polymeric template

Magnetic characterization of maghemite nanoparticles dispersed in a polymeric template and treated under different chemical processes is reported in this work. Particle size estimated from magnetic measurements, DM ≈ 10 nm, for the free-surfactant sample, is consistent with values determined from XRD analysis and TEM images. The magnetic collapse of sextets towards a quadrupole doublet as the temperature is increased reveals the thermal relaxation of smaller ψ-Fe203 nanoparticles. Magnetic measurements show a strong irreversibility between ZFC and FC curves suggesting the occurrence of particle-particle interaction.

Magnetic properties of nanometric Fe-based particles obtained by laser-driven pyrolysis

Fe-based nanoparticles were prepared by laser pyrolysis method using a cross-flow reactor in which the CO 2 laser radiation orthogonally crosses with the stream of Fe(CO) 5, C 2H 2, and C 2H 4 gases. The as-synthesised powder was characterised by HRTEM, XRD, Mössbauer spectroscopy, and low- and high-temperature magnetic measurements. The as-synthesised powder consisted of α -Fe and Fe 3O 4/ γ -Fe 2O 3 nanoparticles embedded in a pyrolytic carbon matrix. The XRD pattern of the sample exhibited broad peaks. The crystallite size d XRD was estimated using the Scherrer formula and it was 1.8 nm for α -Fe and 1.6 nm for pyrolytic carbon. The as-synthesised nanopowder was superparamagnetic. The blocking temperature was determined from the maximum of the ZFC curve and it was 32 K. After the sample had been cooled down to 4 K, the Mössbauer–Lamb factor f strongly increased.

Reply to Comment on ‘HausmanniteMn3O4 nanorods: synthesis, characterization and magnetic properties’

We performed experiments to investigate the physical nature of the peak that appeared inthe ZFC curve of our former work in order to reply to the comment. The situation of theZFC curve under high applied field means that the viewpoint of the authors of thecomment is right.