Zero-field-cooled Curves Research Articles

Canted spin structure in clusters of the (La 0.7Ca 0.3) 1− xMn 1+ xO 3 perovskites

In this paper, we report on the effect of excess manganese on the magnetic properties of the (La 0.7Ca 0.3) 1− x Mn 1+ x O 3 perovskites. All samples display a colossal magnetoresistance effect which becomes considerably enhanced with increasing manganese content. The magnetic behaviour of manganites studied was shown to depend strongly on the values of manganese doping and applied magnetic field. For the samples with x=0.1 and 0.2, in addition to an anomaly connected with paramagnet–ferromagnet transition, there are anomalies in the field-cooled (FC) and zero-field-cooled (ZFC) magnetization at low temperatures. In low magnetic fields ( H<150 Oe), the M FC( T) curves have a kink near T C, and the FC and ZFC magnetization drop appears below 45 K. In applied fields of H>200 Oe, a jump upwards takes place in the M FC( T) and M ZFC( T) curves at the same temperature. A low-temperature anomaly in AC susceptibility also indicates a change in magnetic ordered state. From M( T, H) and χ( T) dependencies, one can assume the existence of regions with different magnetic order at T<45 K: a main part of the sample being a ferromagnetically ordered one and a minor one being a noncollinear state. The latter is most probably to present mesoscopic inhomogeneity containing clusters with canted spin structure. The large irreversibility in the FC and ZFC magnetization arises due to the blocking of cluster magnetic moment below T C.

Abnormal temperature dependence of low-field magnetization of FeCr2−xAlxS4

In this letter, the abnormal temperature dependence of low-field magnetization of FeCr2−xAlxS4 (0⩽x⩽0.7) is reported. In addition to a large irreversibility between zero-field-cooling (ZFC) and field-cooling (FC) magnetization for all samples, a cusp is also observed in both FC and ZFC curves for x⩽0.2. The cusp and the irreversibility are suppressed under an applied field of 0.5 T. The study of magnetic hysteresis loop reveals that this abnormal low-field magnetic behavior is closely related to coercitivity of the system. Accordingly, the abnormal magnetic behavior is explained qualitatively by taking account of large magnetic anisotropy and domain wall pinning of nonmagnetic ions.

Magnetic Properties of LnTi0.5V0.5O3 (Ln=Ce and Pr)

Magnetic properties of LnTi0.5V0.5O3 (Ln=Ce and Pr), which are solid solutions between the perovskites LnTiO3 and LnVO3, have been investigated. Their crystal structures are an orthorhombic perovskite type (Pnma), where the Ti and V ions are settled with random occupation at the same crystallographic site. DC susceptibility–temperature (χ–T) curves exhibit magnetic transitions at TN∼50 K. These temperatures are considerably lower than those of the end compounds ∼116–145 K. The deviation between FC (field-cooled) and ZFC (zero-field-cooled) curves, and the susceptibility cusps only in the ZFC curves are also observed below TN2∼30 K. Magnetization at 6 K changes almost linearly with logarithm of time. AC susceptibility–temperature (χ(ν)–T) curves show the maximums of both real and imaginary parts of χ(ν). The susceptibility maximum temperatures exhibit temperature dependence. These magnetic properties suggest the formation of spin-glasses, arising from the random occupation of the Ti and V ions.

Magnetic Anisotropy of Co-substituted Bi-DyAlIG Nanoparticles.

The magnetic properties of the superparamagnetic Co-substituted Bi-DyAJIG (BiDy2CoxAlyFe5-x-yO12: 0≤x≤0.2, 0≤y≤1.1) nanoparticles were investigated. On the basis of the magnetic data, it is inferred that the magnetic moments of Dy ions cant from those of the iron ions. The canting angle was estimated to be 53°, which is larger than that previously observed in the sputtered films. The reason of the canting is possibly the surface and/or size effect of the nano-size garnet particles. The Co substitution was found to be effective to increase the coercive-force of the nanoparticles for magnetic recording storage media. The bifurcation temperature of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations is also increased by the Co substitution. The ZFC and FC magnetization curves for the BiDy2CoxAly, Fe5-x-yO12 nanoparticles suggest that Co substitution enhances magnetic anisotropy of the nanoparticles, which is in accordance with increase in coercive-force Hc.

Effect of sample shape on the zero-field-cooled magnetization behavior: comparative studies on NiFe 2O 4, CoFe 2O 4 and SrFe 12O 19

The zero-field-cooled (ZFC) magnetization behavior of powders and sintered solid pieces having arbitrary shapes and sizes of three different ferrimagnetic oxides, NiFe 2O 4, CoFe 2O 4 and SrFe 12O 19, has been compared to distinguish the contribution from magnetocrystalline and shape anisotropies on the general characteristics of the ZFC magnetization curves. It is observed that the general shape of the ZFC magnetization curve is mainly determined by the magnetocrystalline anisotropy. The minor effect on the shape of M ZFC curve due to sample geometry, apart from the changes in the magnitude of magnetization due to demagnetization effects, decreases with increasing anisotropy of the material.

Magnetic Properties of Gadolinium Intercalated Graphite; Gd-GIC

Magnetic properties of the gadolinium intercalated graphite compound, Gd-GIC, were investigated. This is the first report on the magnetic properties of a trivalent rare-earth GIC. Differences between zero field cooled (ZFC) and field cooled (FC) susceptibility curves were observed. The ZFC curve shows a sharp decrease below 4 K indicating the existence of a disordered magnetic state. Charge transfer from the Gd atoms to the graphite host was concluded from the temperature dependence and hysteresis of the susceptibility. The orbital susceptibility does not reveal any significant anisotropy, which also indicates substantial charge transfer, supporting results of previous photoemission studies.

Comparison of the zero-field-cooled magnetization behavior of some ferromagnetic and ferrimagnetic systems

The zero-field-cooled (ZFC) magnetization behavior of two ferromagnetic oxides, SrRuO 3 and La 2/3Ca 1/3MnO 3, and two ferrimagnetic oxides, MnCo 2O 4 and NiFe 2O 4, has been compared. It has been observed that a peak or maximum in M ZFC( T) is obtained (at T p⩽ T c) in a given temperature range only when measured with magnetic fields less than a critical value, H cr, which is temperature dependent. H cr corresponds to the field below which the contribution to initial magnetization is mainly from domain wall displacements. M ZFC increases with increasing temperature as long as H cr is greater than the applied field and the applied magnetic field overcomes this critical field above T p. The results indicate that there is a direct correlation between the shapes of M ZFC( T) curves and the domain structure of a material. A drastic decrease of M ZFC below T p is observed for high coercivity (hard) materials when measured with small DC magnetic fields, compared to a broad maximum obtained for the soft materials, because of domain wall pinning effects.

Magnetic properties of U2V3Ge4

We have measured the temperature and magnetic field dependence of the magnetization for the U2V3Ge4 system. A relatively high ferromagnetic TC of 63 K is observed for this system even though the U–U distance is less than the Hill limit of 3.5 Å. The field dependence of the magnetization at low temperatures exhibits hysteresis with a large coercive field indicating a high anisotropy for this compound. Below TC both the zero field cooled (ZFC) and field cooled (FC) magnetization increase as the temperature is decreased, but the ZFC magnetization deviates from the FC data, reaches a broad maximum, and then drops significantly when the temperature is lowered further. This irreversible behavior in the ZFC and FC curves can still be observed in an applied field of H=50 kOe. The anomalous magnetization of this system can be interpreted in terms of intrinsic domain wall pinning which usually occurs in systems with high anisotropy.

Nonstoichiometric Zinc Ferrite Nanocrystals: Syntheses and Unusual Magnetic Properties

This paper demonstrates that, by using oil-in-water micelles, nonstoichiometric zinc ferrite nanoparticles are fabricated. Control in size and composition is obtained. The nanocrystals are characterized by a spinel structure with two sublattices in tetrahedral and octahedral sites. Repartition of Fe3+ in the two sublattices is disordered. The particles are ferrimagnetic with a noncollinear structure (the magnetic moments are not strictly parallel). This is more pronounced with the smallest particles. Zinc ferrite nanocrystals exhibit unusual magnetic properties. The anisotropy constant, K, of nanocrystals differing by their sizes is determined by Mossbauer spectroscopy and from zero field cooled (ZFC) curves. The field cooled (FC) curves show a peculiar behavior with a decrease in the magnetization at very low temperature. The ZFC curves are highly sensitive to the applied field, indicating that Langevin law cannot be applied for zinc ferrite nanocrystals. These unusual magnetic properties are attributed ...

The particle interaction effects in the field-cooled and zero-field-cooled magnetization processes

The present theories explaining the mechanism of particle interaction within a fine particle system driven by the thermal agitation assign the increase of the interaction strength either to an increase of the particle anisotropy due to the environment reaction to its dipole moment, or to the occurrence of a collective state. The particle interaction effects on the field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves are the anisotropy effect, referring to the increase of the temperature T MAX, corresponding to the ZFC curve maximum, with increasing sample volume concentration, and the mean-field effect, referring to the flattening of both, FC and ZFC, magnetization curves with increasing sample demagnetizing factor, without altering T MAX in the low applied field limit. We demonstrate that the Onsager mean-field model is able to recover an increase of the particle anisotropy with increasing sample volume concentration using a cavity having the shape of an oblate ellipsoid, the eccentricity increasing with increasing sample volume concentration. The proposed explanation is the formation of particle clusters having a uniaxial symmetry in the particle arrangement (chain-of-particles). We show that the anisotropy effect of interactions is due to not only an increase of the particle anisotropy with increasing sample volume concentration, but also to a temperature-dependent interaction field distribution due to the local non-homogeneity of the particle dispersion. The proposed model is able to recover the experimental FC and ZFC initial susceptibility curves for various concentrations of γ-Fe 2O 3 nanoparticle systems.

Possible interpretation on the existence of an anomalous inversion of some ZFC and FC transport characteristics in YBCO and BSCCO ceramic superconductors

We performed a comparative and systematic study of voltage versus current and critical current versus applied magnetic field characteristics measured in zero field cooling (ZFC) and field cooling (FC) conditions, in YBCO and BSCCO 2223 ceramic superconductors. We found an anomalous inversion of ZFC and FC curves at low applied magnetic fields and temperatures in several BSCCO samples; i.e., ZFC curves showed stronger superconductivity than the FC ones. This is in clear contrast to YBCO ceramics. Besides, the studied curves for BSCCO ceramics presented a much weaker dependence on cooling conditions than the ones corresponding to YBCO. We also made a semi-quantitative interpretation of the anomalous inversion through the investigation of the internal magnetic field.

Anomalous inversion of voltage versus current curves in ZFC and FC conditions in BSCCO ceramic superconductors

An anomalous inversion of voltage versus current curves measured in zero-field cooling (ZFC) and field-cooling (FC) conditions at low applied magnetic field was found in several BSCCO (2223) ceramic superconductors. In other words, at low magnetic-field values of ZFC curves are unexpectedly below the corresponding FC ones. This is in contrast with YBCO ceramic where the order is reversed. Results are discussed in terms of intergranular critical current and the effect on the weak-link array of the internal magnetic field. Pinning energies dependences with current and magnetic field and temperature for both cooling conditions were also performed.

The magnetic domain effect in the local canted spin ferrite Zn0.5Co0.5Fe2-xCrxO4: A macroscopic and mesoscopic study

AC susceptibility, zero-field-cooled (ZFC) magnetization and field-cooled (FC) magnetization have been reported for the Zn0.5Co0.5Fe1.1Cr0.9O4 disordered ferrite system. A polarization analysis of the transmitted neutron beam in both ZFC and FC conditions has also been reported for Zn0.5Co0.5Fe2-xCrxO4(x=0.9 and 1.0) systems. The low-field AC susceptibility shows a broad peak at a temperature (well below the Neel temperature) where a strong irreversibility between the ZFC and FC curves starts. The presence of a ferrimagnetic-like domain structure over the entire temperature range (below the Neel temperature) is confirmed from the presence of significant depolarization up to 12 K, the lowest measured temperature. The observed features of chi AC, magnetization and neutron depolarization measurements have been analysed using a model of thermally activated domain wall movement of ferrimagnetic arrangement of local canted spins.

NOVEL BINARY SPIN GLASS ALLOYS SYNTHESIZED BY MECHANICAL MILLING

A few novel binary spin glass alloys have been successfully synthesized by mechanical milling of ordered intermetallic compounds in a high energy ball mill. These alloys are amorphous Co 2 Ge , atomically disordered crystalline GdAl 2, and ball-milled crystalline and amorphous CoZr. The characteristic features of spin glasses are observed; these are (a) the sharp cusps at the freezing temperature T f in both ac and dc magnetic susceptibility versus temperature curves and their peculiar sensitivity to the external magnetic field; (b) the irreversibility, i.e. the difference in value and shape between the magnetisation versus temperature curves after zero field cooling (ZFC) and field cooling (FC) at temperatures below T f and (c) the displacement of the FC magnetisation curve relative to ZFC curve and the corresponding remanence in the FC curve at low temperatures. The freezing temperatures are 41, 65, 35, and 11 K, respectively, which are defined by the sharp cusps in the lowest field ac susceptibility versus temperature curves. The freezing temperatures are lowered with increasing external field. The common feature of these materials is that all are binary alloys with a rather high concentration of the magnetic component. The discovery of these novel spin glasses is of significance because they not only represent new classes of spin glass materials but also demonstrate the use of mechanical milling as a novel technique to synthesize various new materials such as spin glasses. The results are reviewed briefly.

Observations of magnetization reversal and magnetic clusters in copper ferrite films

The observation of a magnetization reversal and a superparamagnetic behavior are reported for films of copper ferrite sputtered in an oxygen atmosphere. X-ray diffraction studies indicate the presence of 14-nm nanoscale crystals of CuFe2O4 in the films. Data on the temperature dependence of the film magnetization M for zero-field-cooled (ZFC) and field-cooled (FC) show that ZFC and FC curves separate at a temperature Tf and that Tf decreases with increasing static field strength. For T&amp;lt;Tf, M vs T data for low fields indicate the presence of a compensation point in virgin samples. For T≳Tf, a superparamagnetic character is evident form the high-field magnetization data. An average cluster size of 16 nm obtained from such data is in excellent agreement with the results of x-ray diffraction studies.

Irreversibility line of Y 1−xCa xSrBaCu 2.9Co 0.1O 6+z (0

We present detailed measurements on the complex susceptibility, χ′ (1500 Hz) and field cooled (FC) and zero field cooled (ZFC) DC magnetization, M, of Y 1- x Ca x SrBaCu 2.9Co 0.1O 6+ z (0⩽ x⩽0.25) as a function of temperature and DC field, H (0 to 800 Oe). We confirm earlier published data (on YBaCuO) that T c increases as a function of x(Ca). Further, Ca enhances the irreversibility line derived both from the χ data (ILAC) and the M- H (ILDC) data. The data are analyzed using the relation H= K(1- T irr/ T c) n . Here T irr represents the peak T p of the χ data in the case of AC susceptibility measurements, whereas T ∗ represents the temperature where the FC and the ZFC curves meet. The value of K is found to increase as a function of x(Ca) which is taken to indicate enhanced flux pinning due to Ca. However, the ILAC does not coincide with the ILDC. Some possible causes are discussed.

Magnetic study of M-type doped barium ferrite nanocrystalline powders

We have studied the static magnetic properties of three different M-type doped barium ferrite compounds prepared by the glass crystallization method. The zero-field-cooled (ZFC) and field-cooled (FC) processes have been recorded at low field and they all show the typical features of a small particle system. The ZFC curves display a broad peak at a temperature TM, which depends on the distribution of particle volumes in the sample. Isothermal magnetization curves M(H) at several temperatures and saturation magnetization Ms as a function of temperature have been measured for the Co-Ti sample (BaFe10.4Co0.8Ti0.8O19). The dependence on temperature of the macroscopic magnetic parameters has been analyzed. The distribution of blocking temperatures is studied from the derivative of the remanent-to-saturation magnetization ratio with respect to temperature and it is fitted to a lognormal distribution, leading to a mean blocking temperature 〈TB〉=(81±40) K. The distribution of volumes of the magnetic unit is also obtained from this fitting. The dependence on temperature of the coercive field follows a Tk-law below 35 K. The value of the k exponent is discussed within the scope of two models: (i) the aligned case (k=0.5) and (ii) the random case (k=0.77).

Quantitative analysis of zero field cooling and field cooling curves to determine the critical current density and lower critical field

Field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves were measured for the single crystals of YBa 2Cu 3O 7 and (La 1−xSr x) 2CuO 4 (x = 0.065) under several magnetic fields. Using the extended Bean's critical state model, J c and lower critical field H c1(0) were estimated as fitting parameters to explain FC and ZFC curves consistently. The J c(T) values thus obtained well agreed with the reported ones obtained by the other method.

Temperature dependence of the magnetization in high fields in the diluted antiferromagnet Fe46%Zn54%F2

Magnetization measurements on the diluted antiferromagnet Fe46%Zn54%F2 were performed in fields up to H≂8 T. The difference between the Néel temperature TN = Tc (H=0) and the temperature where (δM/δT)H is maximum scales as H2/φ, with φ=1.46(2) and φ=1.50(2) for zero-field-cooled (ZFC) and field-cooled (FC) procedures, respectively. The temperature where the ZFC and FC curves split from one another, Teq(H), yields a larger φ=1.56(4). The variation of the magnetization in the FC case, for T &amp;lt; Teq(H), is interpreted as being internal to the domains, while the larger value of (δM/δT)H in the ZFC case compared to the FC is attributed to the critical behavior in the random-field Ising model (RFIM) regime around the transition temperature Tc(H).

Glass behavior and the H-T phase diagram of the high-Tc ceramic superconductors YBa2Cu3O7, EuBa2Cu3O7, and GdBa2Cu3O7.

We report systematic field-cooled (FC) and zero-field-cooled (ZFC) diamagnetic susceptibility measurements in polycrystalline samples of the high-${T}_{c}$ oxide superconductors R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, where R=Y, Eu, or Gd. We focus mainly on the temperature range near ${T}_{c}$, where the FC and ZFC curves split up below a characteristic temperature, similar to the behavior of spin glasses. This temperature depends on the applied magnetic field and defines a crossover line in the H-T plane separating ergodic and nonergodic behavior, in close resemblance to what is found in conventional spin glasses. We suggest that this is a general characteristic of the ceramic oxide superconductors. Moreover, our data allow us to obtain a more detailed high-temperature H-T phase diagram, where a lower temperature bound for the superconducting glasslike state is proposed.