Zero-field-cooled Measurements Research Articles

Characterization of YBa2Cu3O7−δ coplanar resonator for microwave kinetic inductance detectors

We demonstrate an improved YBa2Cu3O7−δ -based microwave kinetic inductance detector with a quality factor and noise equivalent power, at 10 K. Zero field cooled (ZFC) and field cooled (FC) measurements of the magnetic field dependence of the resonance characteristics, show substantially different behavior, indicating that both the screening currents and vortices play a role. The ZFC measurements exhibit a sharp decrease of the resonance frequency, , and at low fields, up to the full penetration field, revealing the dominant role of the screening currents. In contrast, the FC measurements exhibit a moderate decrease of and with field, revealing the role of vortices and reflecting the field dependence of the penetration depth in a d-wave superconductor.

Zero field cooled exchange bias effect in nano-crystalline Mg-ferrite thin film

The Zero Field Cooled (ZFC) Exchange Bias (EB) effect in a single phase nanocrystalline Mg-ferrite thin film, deposited on an amorphous quartz substrate using pulsed laser ablation technique, is reported. The film showed a high ZFC EB shift (HE ∼ 190 Oe) at 5 K. The ZFC EB shift decreased with increasing temperature and disappeared at higher temperatures (T > 70 K). This Mg-ferrite thin film also showed the Conventional Exchange Bias (CEB) effect, but unlike many CEB systems, the film showed a decrease in coercivity (HC) under the Field Cooled measurements. The film also showed the training effect in ZFC measurements, which followed the frozen spin relaxation behavior. The observed exchange bias could be attributed to the pinning effect of the surface spins of frozen glassy states at the interface of large ferrimagnetic grains.

Surface, structural and cryogenic magnetic studies of Cu, Co substituted NiFe2O4 thin films grown on Si substrate

Silicon (Si) is the most commonly used substrate material for fabricating integrated circuit; Si has very little fatigue effect. In the present work we developed pure NiFe2O4 and Cu, Co substituted NiFe2O4 thin films on Si substrate by metallo-organic decomposition (MOD) method using spin coating technique. The prepared thin films were characterized by XRD and Raman spectrometer for structural and phase identification. The topography and grain distribution of the prepared samples were analyzed by atomic force microscope (AFM). Magnetic hysteresis loops were measured at 10 K and 300 K using Superconducting quantum interference device (SQUID). The magnetization and coercive field increased with Cu, Co substitution. Temperature dependent magnetization (M-T) was carried out by zero-field cooled (ZFC) and field – cooled (FC) method. ZFC measurements revels that blocking temperature of the thin films is above 350 K.

Negative Zero-Field-Cooled Magnetization in YMn0.5Cr0.5O3 due to Giant Coercivity and Trapped Field.

We report an intensive study on negative magnetization under zero-field-cooled (ZFC) mode in YMn0.5Cr0.5O3 polycrystalline samples. It has been found that the magnetization reversal in ZFC measurements is strongly related to a giant coercivity of the oxide. The giant coercivity may result from the cooperative effect of magnetocrystalline anisotropy and the Dzyaloshinsky-Moriya interaction, especially at temperatures below 10 K. By fitting the high-temperature paramagnetic data under nominal zero field, the value of the trapped field in a superconducting magnet has been derived to be around several Oe, which further demonstrates that the negative ZFC magnetization is an artifact caused by negative trapped field in combination with the giant coercivity. Consequently, we suggest that one has to be cautious of trapped field in superconducting magnets in understanding negative ZFC magnetization, especially in "hard" magnetic samples.

Competing magnetic interactions and superparamagnetism like behaviour in xNiFe2O4 - (1-x)BaTiO3 (x = 0.2 and 0.3) nano composites

We report on structural, spin glass behaviour and competing magnetic interactions in xNiFe2O4 - (1-x)BaTiO3 (x=0.2 and 0.3) nanocomposites prepared by sol-gel method. The structure and surface morphology of the NiFe2O4-BaTiO3 composites were examined by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD measurements of composite materials show the presence of both NiFe2O4 and BaTiO3 phases. FESEM and energy dispersive X-ray (EDX) measurements indicate uniformly distributed nano size particles and presence of Ni, Fe, O, Ba and Ti elements respectively. Temperature variation Raman spectra indicates large structural disordering (Peak broadening and shift) around 395K due to the structural transformation of BaTiO3. Magnetization, zero field cooled (ZFC) and field cooled (FC) measurements were carried out from 2K to 320K for both composites. ZFC and FC measurements reveal the spin glass and Superparamagnetism like behaviour along with competing magnetic interaction in both composites. Saturation magnetization value is seen to increase from 6.7 to 10emu/g with increasing Ni ferrite composition from x=0.2 to x=0.3. Distribution function associated with magnetocrystalline anisotropy energy barrier exhibits broad peaks due to the random distribution of spins associated with different particle size in the x=0.3 composition.

Particle Interactions in Liquid Magnetic Colloids by Zero Field Cooled Measurements: Effects on Heating Efficiency

The influence of magnetic interactions in assemblies formed by either aggregated or disaggregated uniform γ-Fe2O3 particles are investigated as a function of particle size, concentration, and applied field. Hyperthermia and magnetization measurements are performed in the liquid phase of colloids consisting of 8 and 13 nm uniform γ-Fe2O3 particles dispersed in water and hexane. Although hexane allows obtaining the disagglomerated particle system, aggregation is observed in the case of water colloids. The zero field cooled (ZFC) curves show a discontinuity in the magnetization values associated with the melting points of water and hexane. Additionally, for 13 nm γ-Fe2O3 dispersed in hexane, a second magnetization jump is observed that depends on particle concentration and shifts toward lower temperature by increasing applied field. This second jump is related to the strength of the magnetic interactions as it is only present in disagglomerated particle systems with the largest size, i.e., is not observed fo...

The Verwey transition in nanostructured magnetite produced by a combination of chimie douce and spark plasma sintering

Magnetite nanoparticles about 10 nm sized were synthesized by the polyol method. Zero-field-cooled (ZFC)-FC measurements showed a blocking temperature ∼170 K and the absence of the Verwey transition. They were subsequently consolidated by spark plasma sintering at 750 °C for 15 min, leading to a high density (92% of the theoretical density), solid body, with grains in the 150 nm range. X-ray diffraction patterns exhibited a spinel single phase with cell parameters corresponding to the magnetite structure. Magnetic measurements showed a decrease of coercivity from 685 Oe (54.5 kA/m) at 118 K to 90 Oe (7.2 kA/m) at 139 K. ZFC measurements at 25 Oe presented a three-fold magnetization increase as temperature increased; a small transition between 116 and 117.5 K, followed by a larger one from 117.6 to 124 K. The first transition can be associated with a complex crystallographic transition and delocalization of Fe2+-Fe3+, while the second one can be attributed to spin reorientation due to the magnetocrystalline anisotropy constant (K1) change of sign as previously observed only in magnetite single crystals.

Structural and magnetic studies of Mg(1−x)ZnxFe2O4 nanoparticles prepared by a solution combustion method

Abstract Nanocrystalline Mg(1−x)ZnxFe2O4 (x = 0.0, 0.25, 0.5, 0.75 and 1) ferrites were prepared by a solution combustion method using a mixture of fuels. The structural and magnetic properties of the as-prepared samples were studied using powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis absorption spectra and vibrating sample magnetometer (VSM) measurements. The PXRD analysis of all the samples revealed the single phase cubic structure with the space group Fd 3 ¯ m . The average crystallite size determined from the PXRD data was found to increase from 12 nm to 25 nm with an increase in the Zn content from x = 0 to x = 1. The IR absorption spectra exhibited two prominent peaks, which are assigned to tetrahedral and octahedral vibrations. The saturation magnetization, magnetic moment and remenant magnetization increases with an increase in the zinc content up to x = 0.5 and decrease thereafter. The observed behavior is consistent with Yaffet–Kittel magnetic ordering. The ZFC (zero field cooled) and FC (field cooled) measurements of magnetization revealed that the blocking temperature is well below room temperature.

Correlation of crystal structure and magnetic properties of Co(1−x)NixFe2O4/SiO2 nanocomposites

The Co(1−x)NixFe2O4/SiO2 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanocomposites were prepared using the sol–gel method and characterized by Powder X-ray Diffraction (PXRD), Energy Dispersive X-ray Spectroscopy (EDX) and magnetic measurements. Moreover, Mössbauer spectroscopy (performed at room temperature and at 4.2K with applied field 6T) was performed to determine the cation distribution within the spinel network. The PXRD measurements confirmed the presence of the spinel phase in all samples. The particle diameters, obtained from the PXRD, were found out to vary between 15 and 24nm. The lattice parameters decrease linearly with increasing content of the Ni2+ ions. The ZFC (zero field cooled) and FC (field cooled) measurements of the magnetization showed that the blocking temperature is well above the room temperature for all samples. Both the coercivity and the saturation magnetization decrease linearly with increasing content of the Ni2+ ions. The room temperature Mössbauer spectra exhibit a sextet with asymmetric lines indicating a different crystal environment for the Fe3+ ions, accommodated in the tetrahedral and octahedral sites. By application of the external magnetic field at 4.2K, these overlapped sextets split up allowing determination of the ratio of the Fe3+(Oh)/Fe3+(Td) that exhibited slight decrease with increasing Ni content. The obtained results are discussed considering influence of the Ni doping.

Magnetic Properties of Magnetically Soft Nanocomposite Co–SiO2 Prepared via Mechanical Milling

Nanocomposite of Co-SiO2, a soft magnetic material, with Co weight fraction x = 0.3 and 0.7 was prepared via mechanical milling. The magnetic properties of these samples, both zero-field-cooled (ZFC) and field-cooled (FC), have been measured as a function of x, milling time, and temperature. The structural assessment of the composite indicates a presence of only ferromagnetic (FM) hcp-Co phase in the composite. However, reported magnetic properties of these composites appear to be dependent on the presence of antiferromagnetic (AFM) phases of cobalt oxide as well. The observed enhancement in ZFC coercivity and a reduction in saturation magnetization with the milling time are due to an increase in defect density upon milling. The ZFC coercivity for the x = 0.3 samples has been found to be much higher than the x = 0.7 samples for all milling times. The coercivity above 50 K depends on temperature according to the law corresponding to isotropic uniaxial superparamagnetic particles. Below 50 K the presence of an AFM phase Co3O4 (TN approximately 33 K) and increased interparticle interactions bring in a departure from that law. The saturation magnetization is found to be temperature dependent for the x = 0.3 samples and temperature independent for the x = 0.7 samples, which further provides evidence of the presence of higher AFM phase fraction in the composite with a low metal volume fraction. The FC magnetic measurements show a presence of an exchange bias field and an enhanced coercivity which are higher than the ZFC measurements. All magnetic measurements indicate that the overall magnetic properties of the composite are dictated by the presence of a trace amount of cobalt oxides.

Spin glass behavior in the CoSi1–xAlx alloys

In order to access the spin glass-like feature in the CoSi1−xAlx alloys, we measured the DC magnetization under zero-field-cooled (ZFC) and field-cooled (FC) procedures for CoSi0.85Al0.15 and CoSi0.70Al0.30 with various external magnetic fields. It has been found that the magnetic susceptibility and the irreversibility between ZFC and FC measurements in both compounds are very sensitive to the applied field. We associated the formation of the spin glass state with the Si/Al disorder in these materials.

On the low-temperature anomaly in the AC susceptibility of La 0.9Ca 0.1MnO 3

The substituted manganate La 0.9Ca 0.1MnO 3 undergoes a ferromagnetic transition at T c=157 K and a decrease in the AC susceptibility as well as zero-field-cooled (ZFC) magnetization measured using very low DC magnetic fields which is observed below ∼100 K. This low-temperature anomaly below ∼100 K is not observed in the field-cooled magnetization measured using the same small DC magnetic fields used for the ZFC measurements. Initial magnetization and magnetic hysteresis measurements show no difference in the M– H behavior above and below 100 K and indicate the multidomain ferromagnetic behavior of the compound. A small sharp increase in the coercivity of the material is observed below 100 K, corresponding to the drop in the AC susceptibility. The anomaly, at low temperatures, in the AC susceptibility and low-field ZFC magnetization of La 0.9Ca 0.1MnO 3 can be explained in terms of domain wall pinning effects.

Investigations of magnetic anisotropy in the complex low-temperature phase in Fe 2MnSi

Low-field (100 Oe) DC susceptibility zero-field-cooled (ZFC) and field-cooled (FC) measurements (using a SQUID magnetometer) have been made on a Fe 2MnSi single crystal. This compound exhibits a complex magnetic behaviour involving a transition from a paramagnetic state to a ferromagnetically ordered state at T c ∼ 220 K, followed by a transition to a low-temperature reordering phase at T R ∼ 60 K. Results confirm previously observed demagnetising effects from polycrystalline samples with similar compositions. Also, the ZFC measurements along separate crystallographic directions show that the magnetization increases most readily in the [1 0 0] direction compared with that of [1 1 0], indicating a form of anisotropy in the complex low-temperature phase in Fe 3 − x Mn x Si alloys. The FC measurements at this field are demagnetisation limited even in the re-ordered phase.

Size and Shape Effect on the Magnetic Properties of α-Fe<sub>2</sub>O<sub>3</sub> Nanoparticles

The magnetic properties of α-Fe 2 O 3 nanoparticles, prepared as powders by chemical route and as granular films by sputtering, have been investigated for different size (3 - 700 nm) and shape (rhombohedral, acicular and spherical). The particle size and shape effect on the Morin transition (T M = 263 K in the bulk system) has been investigated by analyzing the temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization. For rhombohedral (30 - 350 nm) and spherical (10 - 50 nm) particles, the Morin temperature was found to decrease with decreasing particle size and increasing magnetic field. On the other hand, acicular particles (350 - 700 nm) do not show the Morin transition, unless annealed. In samples characterized by a large distribution of particle size, the ZFC and FC measurements show the superimposed effects of the superparamagnetic behaviour of very small particles and the Morin transition for the larger ones. Only the superparamagnetic component is observed in granular films consisting of very small spherical particles (3 nm), whose average blocking temperature is about 140 K.

Correlation between peak and median blocking temperatures by magnetization measurement on isolated ferromagnetic and antiferromagnetic particle systems

The influence of the particle size distribution on the ratio of the peak temperature, T peak, to the blocking temperature, T Bm, in zero field cooled (ZFC) magnetization curves has studied for both ferromagnetic and antiferromagnetic particle systems. In both systems the ratio β=T peak/T Bm does not depend on the median particle volume. However, T Bm can be considerably different from T peak in both systems. These results show that the ZFC measurements can be used to determine T Bm values only if the particle size distribution of the system is known. Otherwise, the estimated T Bm values will have a large uncertainty, especially in systems with a broad particle size distribution.

Phase Diagram of a Reentrant Ising Spin Glass Fe0.6Mn0.4TiO3on the Magnetic Field-Temperature Plane

Behavior of the reentrant spin glass (RSG) material Fe 0.60 Mn 0.40 TiO 3 in magnetic fields H <7.1 kOe is studied by neutron scattering technique, and the H - T phase diagram was constructed. For H ≥0.93 kOe, the random field (RF) induced history dependent behavior was observed between zero-field-cooled (ZFC) measurements and field-cooled (FC) measurements below the Neel temperature T N ( H ). By examining the behavior of Bragg and diffuse scattering in ZFC measurements, the RSG transition temperature T RSG is determined. One finds that it appears within a gradual crossover region from weak irreversibility to SG-induced strong irreversibility observed in the previous magnetization measurements. The initial slope of the T RSG line is perpendicular to the temperature axis.

Muon-spin-relaxation studies of flux pinning in Bi2Sr2CaCu2O8 and Pb0.7Bi1.3Sr2CaCu2O8.

We have studied flux-depinning phenomena in the ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (Bi 2:2:1:2) and Pb-doped ${\mathrm{Pb}}_{0.7}$${\mathrm{Bi}}_{1.3}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (Pb-Bi 2:2:1:2) systems using the transverse-field muon-spin-relaxation (\ensuremath{\mu}SR) technique. Comparison of field-cooled (FC) and zero-field-cooled (ZFC) results with external fields applied along the c axis of single-crystal specimens defines an irreversibility temperature (depinning temperature) ${\mathit{T}}_{\mathrm{irr}}$: the FC and ZFC relaxation rates are essentially identical above ${\mathit{T}}_{\mathrm{irr}}$, while the relaxation rate in the ZFC measurements is larger than that in the FC measurements below ${\mathit{T}}_{\mathrm{irr}}$, reflecting the increased inhomogeneity of the local fields in the ZFC measurements due to flux pinning. The irreversibility line ${\mathit{T}}_{\mathrm{irr}}$(H) in the H-T phase diagram for Bi 2:2:1:2, obtained by \ensuremath{\mu}SR measurements for several fields, is compared with previous results from ac-susceptibility and mechanical-oscillator measurements. Using a superconducting-quantum-interference-device (SQUID) magnetometer, the time-dependent diamagnetic magnetization has been measured in the same Bi 2:2:1:2 crystals. We show that the results from \ensuremath{\mu}SR and these other techniques can be explained consistently within a framework of the flux-creep model. The irreversibility temperature in Pb-Bi 2:2:1:2, determined by \ensuremath{\mu}SR measurements, is significantly higher than that in the pure Bi 2:2:1:2 system. This result, together with the larger critical current and the higher activation energy ${\mathit{U}}_{0}$ in the Pb-Bi 2:2:1:2 crystals as found by the SQUID magnetization measurements, suggests an enhancement of flux pinning by the Pb doping. We also compare the \ensuremath{\mu}SR results in sintered ceramic, oriented film, and single-crystal specimens in the pure Bi 2:2:1:2 system, and discuss possible effects of sample morphology on \ensuremath{\mu}SR measurements.

Towards equilibrium in spin glasses (invited)

New nonequilibrium features of the magnetic susceptibility of spin glasses are reviewed. Experimental observations on the time variation of the zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities indicate that the same equilibrium value is obtained after the same characteristic time teq(T). ZFC and ac susceptibility measurements on metallic spin glasses unambiguously reveal the existence of aging of the zero-field spin-glass state. The possibility to probe the aging process from these measurements is outlined. The aging of the zero-field state ends at the characteristic time teq(T), where a spin-glass state at dynamic equilibrium is retained.