Low Field Ac Susceptibility Measurements Research Articles

Entropic signatures of the skyrmion lattice phase in MnSi1–xAlx and Fe1–yCoySi

The entropic signatures of magnetic phase transitions in the skyrmion lattice host compounds MnSi0.962Al0.038 and Fe0.7Co0.3Si were investigated through low field magnetization and ac susceptibility measurements. These data indicate that the conical to skyrmion transition that occurs with the application of magnetic field in MnSi0.962Al0.038 is characterized by clear discontinuity in the magnetic entropy as expected for first order topological phase transition. These same magnetoentropic features are negligibly small in isostructural Fe0.7Co0.3Si due to the level of chemical substitution related disorder and differences in the spin dynamics (range and timescales). Despite the obvious similarities in the magnetic structures of these two compounds, the transitions between these phases is substantially different indicating a surprising non-universality to the magnetic phase transitions in this class of materials.

Magnetization reversal, magnetic anisotropy and ordering studies on quenched copper-ferri-aluminates

The magnetic properties of thermally quenched spinel ferrite system, CuAlxFe2-xO4 (x = 0.0, 0.2, 0.4 and 0.6), have been investigated by means of DC magnetization, hysteresis loop characteristics and thermal variation of low field ac susceptibility measurements. The system possesses a tetragonally distorted spinel structure and exhibits a non-collinear spin arrangement. The magnetic characteristics suggest that the ferrites are low anisotropy compounds, the negative magnetization observed for x = 0.6 composition at the compensation temperature T = 22 K is due to the copper ion moment with a net effect antiparallel to that of the Fe3+ ions with different temperature dependence giving rise to a compensation effect. The low-temperature anomalies on the zero field cooled curves are due to the coexistence and interplay between the ferrimagnetic long-range ordering and short-range ordering in the system.

Effect of process parameters on non-modulated Ni-Mn-Ga alloy manufactured using powder bed fusion

Increasingly, metal parts made by additive manufacturing are produced using powder bed fusion (PBF). In this paper we report upon the combined effects of PBF parameters, including power and scan speed, in layer-by-layer manufacturing of gas atomized non-modulated (NM) Ni-Mn-Ga alloy. The effects of process parameters upon PBF is studied by applying nine different parameter sets in the as-printed state and after homogenization and ordering. The chemical composition of the samples is analyzed using EDX attached to an SEM, and the crystal structures are determined by X-ray diffraction. The phase transformation temperatures are measured using a low-field ac susceptibility measurement system and the magnetic properties are measured with a vibrating sample magnetometer (VSM). Before the heat-treatment, all as-printed samples showed paramagnetic behavior with low magnetization and no phase transformations could be observed in the susceptibility measurements. After annealing, the samples recovered the ferromagnetic behavior with comparable magnetization to annealed gas atomized powder. The as-printed samples were composed of a mixture of different crystal structures. However, after annealing the original NM structure with a = b = 5.47 Å and c = 6.66 Å with a c/a -ratio of 1.22 was recovered and crystallographic twins could be observed in an SEM.

Magnetoelastic coupling at spin-glass-like transition in Sr3NiSb2O9

We report a characteristic spin-glass-like behavior in Sr3NiSb2O9, as confirmed from the low-field dc magnetometry and ac susceptibility measurements. The memory effects in the dc magnetometry exhibit similar characteristic features of the spin-glass systems. The ac susceptibility measurements provide that the frequency dependent peak-shift follows the conventional Vogel-Fulcher and dynamical scaling laws with the characteristic relaxation time to be ∼ 10−14 s. This relaxation time fits in the recommended range of ∼10−12−10−14 s for the classical spin-glasses. Low temperature synchrotron diffraction studies point to the strong magnetoelastic coupling close to the spin-glass-like transition. The transition is found to be associated with the step-like anomalous lattice contraction. Possible origin of magnetic frustration leading to the spin-glass ground state is discussed by correlating the microstructural results, as obtained from the analysis of synchrotron diffraction studies.

Glassy behavior of diluted Cu-Zn ferrites

The magnetic behavior of Zn substituted Cu-Zn spinel ferrites having chemical formula Cu1−xZnxFe2O4 (x = 0.7, 0.8, 0.9 and 1.0) has been studied by SQUID magnetometry, by means of magnetic hysteresis, field-cooled (FC) and zero-field-cooled (ZFC) magnetization, memory effect and low field ac susceptibility measurements. These measurements suggest that the ferrimagnetic phase of the x ≤ 0.8 samples is gradually turned into a spin glass (x ≥ 0.9). The compound with x = 0.9 exhibits the typical dynamical behavior of spin glasses, with indication of aging, rejuvenation and memory effects. The evolution of the magnetic properties of Cu-Zn spinel ferrites with substitution of Zn for Cu is discussed.

Synthesis and characterization of Fe0.6Zn0.4Fe2O4 ferrite magnetic nanoclusters using simple thermal decomposition method

This paper presents experimental results regarding the effect of the quantity of solvent on formation of the Fe–Zn ferrite nanoparticles during thermal decomposition. A ternary system of Fe0.6Zn0.4Fe2O4 has been synthesized by a thermal decomposition method using metal acetylacetonate in high temperature boiling point solvent and oleic acid. The X-ray diffraction study was used to determine phase purity, crystal structure, and average crystallite size of iron–zinc ferrite nanoparticles. The average crystallite size of nanoparticles was increased from 13nm to 37nm as a result of reducing the solvent from 30ml to 10ml in a synthesis batch. The diameter of particles and morphology of the particles were determined by transmission electron microscopy (TEM) and field emission scanning electron microscope (FESEM). Mid and far Fourier transform infrared (FT-IR) measurement confirmed monophasic spinel structure of ferrite. Furthermore, the DC magnetic properties of the samples were studied using the vibrating sample magnetometer (VSM). The largest Fe–Zn ferrite nanoparticles exhibited a relatively high saturation magnetization of 96emu/g. Moreover, Low-field AC susceptibility measurement indicated blocking temperature of nanoparticles around 170–200K.

Effect of Non-Ionic Surfactant Concentration on Microstructure, Magnetic and Dielectric Properties of Strontium-Copper Hexaferrite Powder

Strontium copper hexaferrite powder with composition Sr2Cu2Fe12O22 was synthesized in presence of a non-ionic surfactant Tween-80 using chemical co-precipitation route. The prepared samples were calcinated at 950 oC for 4 hrs in a furnace and then slowly cooled to room temperature to obtain Sr2Cu2Fe12O22 hexaferrite powder. The effect of surfactant concentration on phase formation, microstructure, magnetic and dielectric properties of Sr2Cu2Fe12O22 were investigated using XRD, SEM, TEM, VSM, dielectric and low field AC susceptibility measurement techniques. The XRD analysis reveals the formation of mixed phases of Y and M type hexaferrites. The synthesized samples exhibited magnetic properties typical for soft magnetic materials, with saturation magnetization typical for Y-type hexaferrites. The dielectric properties were studied within the frequency range 100 HZ to 2 MHz. SEM images show formation of non-uniform, spongy and porous structure. The low field AC susceptibility measurements indicate that formed Sr-Cu hexaferrite powder possesses ferrimagnetic to paramagnetic transition at Curie temperature.

Effect of mechanical milling induced strain and particle size reduction on some physical properties of polycrystalline yttrium iron garnet

The effect of mechanical milling induced strain and particle size reduction on structural, micro-structural and magnetic properties of un-milled and milled polycrystalline samples of yttrium iron garnet, Y3Fe5O12 (YIG), for different duration, 3, 6, 9 and 12 h, have been studied using X-ray powder diffractometry, scanning electron microscopy, infrared spectroscopy, Mossbauer spectroscopy, high field magnetization and low field (0.5 Oe) ac susceptibility measurements. YIG on milling is found to decompose into Y3Fe5O12 and yttrium ortho-ferrite, YFeO3, phases, as reflected from various characterizations. The infrared and Mossbauer spectroscopic studies give an indication for the loss of oxygen on milling. The shift in Neel temperature with particle size reduction has been described by finite size scaling. The observed changes in M–H loop are mainly due to strain induced increase in surface anisotropy on milling. The two transitions have been observed in the thermal variation of ac susceptibility curves, corresponding to ferrimagnetic YIG-phase and another due to weak ferromagnetic YFeO3-phase.

Magnetic and 57Fe Mössbauer study of magneto-electric GaFeO3 prepared by the sol–gel route

This work reports the preparation of magneto-electric GaFeO3 by the sol–gel route and its characterization by x-ray diffraction, dc-magnetization, ac-susceptibility, low temperature and high field 57Fe Mössbauer spectroscopy and dielectric constant measurements. The prepared samples are found to be single phase from x-ray diffraction studies. The crystallite sizes are found to be in the nano-regime for the samples sintered at low temperatures. From the temperature dependent dc-magnetization (M–T) measurements, bifurcation of the zero-field cooled (ZFC)–field cooled data and a cusp in the ZFC data are observed. With the help of low-field ac-susceptibility, 57Fe Mössbauer and detailed dc-magnetic measurements these features are explained in terms of the magnetic anisotropy of the sample ruling out phenomena like spin-glass and super-paramagnetism as quoted in the literature for this compound. Apart from this, very interesting and different M–H behavior mimicking composite two-phase magnets is observed for the samples sintered at different temperatures. A symmetric M–H loop is observed for samples sintered at low temperatures and a pinched M–H loop is observed for samples sintered at high temperatures. The observed magnetic properties are explained by estimating the Fe cation distribution using high field 57Fe Mössbauer spectroscopy measurements. An anomaly in the dielectric constant data at the Curie temperature indicates the ME coupling of the samples.

Negative magnetization, magnetic anisotropy and magnetic ordering studies on Al 3+-substituted copper ferrite

Detailed magnetic properties of Al 3+-modified CuFe 2O 4 spinel ferrite system: CuAl x Fe 2− x O 4; x = 0.0, 0.2, 0.4 and 0.6, have been studied by means of X-ray powder diffraction, field cooled (FC) and zero field cooled magnetization (ZFC) ( H = 10 mTesla, T = 4–325 K), magnetic hysteresis ( H ma x = 2 Tesla, T = 10 and 300 K) and low field (40 A/m) ac susceptibility ( T = 300–750 K) measurements. The system exhibits canted spin structure. It has been shown that the observed features of the FC–ZFC magnetization and ac susceptibility curves arise due to the low magneto crystalline anisotropy, not due to the cluster spin-glass-like magnetic ordering. The interesting features like low temperature cusp in the ZFC magnetization for all the compositions and negative magnetization for x = 0.6 composition have been observed. An attempt has been made to explain the negative magnetization within the framework of available models.

Structural and magnetic phase evolution study on needle-shaped nanoparticles of magnesium ferrite

Nanoparticles of magnesium ferrite (MgFe 2O 4) have been synthesized by chemical co-precipitation route. The microstructure, infrared spectral and magnetic properties have been studied by means of X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, infrared spectroscopy, high field magnetization, low field ac susceptibility and Mossbauer spectroscopic measurements. Transmission electron microscopy (TEM) observation showed that the nanocrystals present a needle-like shape with an aspect ratio of around 3.5 and long axis of 37 nm. The kinetics of needle-shaped nanocrystals formation has been discussed. The Mg 2+ seems to play key role in governing the rate of growth of growing planes of nanocrystals. The magnetic behaviour is explained by invoking the concept of super-paramagnetism assuming core–shell structure of the nanoparticles. Mossbauer spectral and susceptibility studies showed that the as-prepared nano-sized ferrite is super paramagnetic at room temperature and magnetic ordering evolves on baking and annealing the nanopowdered sample due to increase in the crystalline size. The high temperature annealing transforms the nanostructured ferrite to ordered magnetic structure of ceramic ferrite having long range ferrimagnetic ordering. Infrared (IR) spectral analysis reveals the weak high frequency shoulder due to disparity in the masses of cations present at tetrahedral sites, while splitting of low and high frequency absorption bands has been explained on the basis of Jahn–Teller effect in Fe 2+-ions which lead to a non-cubic component of the crystal field potential.

Study of magnetic ordering in MnAl xCr xFe 2−2xO 4

The spinel solid solution series MnAl x Cr x Fe 2−2 x O 4 with x=0.0–0.9 in steps of 0.1 was studied using powder X-ray diffraction profile refinement, magnetization measurement at 80 K using an applied magnetic field 3.18×10 5 A-turn/m, Mössbauer spectroscopy at 300 K and low field AC susceptibility measurements. We have seen that the increase of Cr 3+ ions and Al 3+ ions in B sites at the cost of Fe 3+ ions leads the present system towards the Néel type of collinear ordering of spins. The negative quadruple shifting seen in Mössbauer results and high value of oxygen positional ‘ u’ parameter indicate oblate type of deformation of 3d 5 shell. The results indirectly support the theory of “exchange disorder” of Fe 3+ ions in B sites as the cause of the reduction in magnetic moment for MnFe 2O 4 from Néel type of ordering. There is the emergence of ‘spin clusters’ in the long-range network of spins as indicated in the ac susceptibility and Mössbauer results at room temperature in x>0.7 compounds.

Structural and magnetic properties of zinc- and aluminum-substituted cobalt ferrite prepared by co-precipitation method

Spinal ferrites having the general formula Co1 − xZnxFe2 − xAlxO4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were prepared using the wet chemical co-operation technique. The samples were annealed at 800°C for 12 h and were studied by means of X-ray diffraction, magnetization and low field AC susceptibility measurements. The X-ray analysis showed that all the samples had single-phase cubic spinel structure. The variation of lattice constant with Zn and Al concentration deviates from Vegard’s law. The saturation magnetization σs and magneton number nB measured at 300 K using high field hysteresis loop technique decreases with increasing x, suggesting decrease in ferrimagnetic behaviour. Curie temperature TC deduced from AC susceptibility data decreases with x, suggesting a decrease in ferrimagnetic behaviour.

On the correlation between order parameter, superconducting volume fraction and critical current density in R: 123 superconductors

Resistivity and low field ac susceptibility measurements are made on R: 123 superconducting samples with different rare earth elements R. The order parameter dimensionality OPD is deduced from resistivity versus temperature plot using the Aslamazov and Larkin expression, while the analysis of the temperature dependence of ac susceptibility is done employing Beans’ critical state model and with the help of the Ravi expression. With increasing R, the critical temperatures T c are nearly kept constant (∼90 K), while the crossover temperatures T o are shifted to lower values. Moreover, the superconducting order parameter OPD is shifted toward 2D behavior. On the other hand, the values of superconducting volume fraction f g decrease with increasing ac field amplitude H m for all samples and it is higher in Er: 123 sample than in Nd: 123 sample. Although the values of critical current density J c at the peak temperature T m are nearly unchanged with increasing R, the values of J c ( T ) , at T < T m and T > T m , are found to be dependent on the chosen R. The correlation between the above calculated parameters against R is also mentioned.

Specific Heat of Dy.40 Cu.37 Y.23 Amorphous Alloy at Low Temperatures

The low temperature specific heat of the magnetic Dy0.40Cu0.37Y0.23 amorphous alloy have been investigated in the temperature range 2 to 50 K. The magnetic contributions to the specific heat, magnetic entropy changes, ordering temperatures and the effective magnetic spin have been estimated for this magnetic amorphous alloy. The magnetic specific heat show broad anomaly at a certain temperature, Tm of about 28 K. The value of Tm is higher than Tf (=23.5 K) determined from low field ac susceptibility measurements. The magnetic entropy changes between 0 K and Tm are estimated to be 84 % of the maximum theoretical value. The behaviors of this amorphous alloy agree quit well with those of condensed rare earth amorphous systems.

Spin-glass behavior in Pr 0.7Ca 0.3CoO 3 and Nd 0.7Ca 0.3CoO 3

With a view to investigate the effect of small size of A-site cations on the magnetic properties of the rare earth cobaltates, Ln 1− xA x CoO 3 ( Ln=rare earth, A=alkaline earth), we have investigated Pr 0.7Ca 0.3CoO 3 and Nd 0.7Ca 0.3CoO 3 in detail. For this purpose, we have carried out low-field DC magnetization and ac susceptibility measurements including a study of magnetic relaxation and memory effects. Both Pr 0.7Ca 0.3CoO 3 and Nd 0.7Ca 0.3CoO 3 show frequency-dependent transitions at 70 and 55 K respectively in the ac susceptibility data, due to the onset of spin-glass like behavior. Their relaxation behavior exhibits aging effects. In addition, memory effects are found in the magnetization behavior. These characteristics establish spin-glass behavior in both these cobaltates, a behavior that is distinctly different from that of La 0.7Ca 0.3CoO 3 and La 0.5Sr 0.5CoO 3 which show well-defined ferromagnetic transitions, albeit without long-range ordering.

Memory effects in a nanoparticle system: Low-field magnetization and ac susceptibility measurements

A dilute magnetic liquid with Co nanoparticles with an average diameter of 5 nm in hexane has been studied systematically following the experimental approach proposed recently for observing memory effects in magnetic nanoparticles. All phenomena reported previously have been observed, which were earlier ascribed to the memory effect. However, the standard experiments for observing memory effects (low frequency as susceptibility and low field dc magnetization measurements) do not show the memory effect. To understand those observations, very detailed physical pictures, based on the relaxation of the individual particle, are proposed here.

Localized canting of spins structure in the spinel oxide system: ZnzTizFe2−x−zCrx−zCoO4

The spinel oxide system ZnzTizFe2−x−zCrx−zCoO4; z=x2; x=0.60, 0.65, 0.70 and 0.80, was studied using neutron diffraction technique, low field DC magnetization measurements (ZFC–FC measurements), magnetic hysterisis, Mössbauer spectroscopy and low field AC susceptibility measurements. All the compositions show significantly less B-site magnetic moments at 10K temperature derived from neutron diffraction data than the free ions site moments deduced assuming collinear arrangement of spins. This combined with some other features seen in the low temperature neutron diffraction patterns suggest localized canting of spins (LCS) type of magnetic ordering in the present system where a long range order of longitudinal component of moments co-exists along with totally disordered transverse component of moments. The conclusion is also supported by the features seen in the other measurements. The magnetic moments derived from 10K neutron diffraction data are explained using the LCS approach for different exchange integrals ratios.

Critical state models for a granular ferromagnetic superconductor

Measurements of the magnetic moment M as a function of the magnetic field B can be used to extract information about the penetration behaviour of the magnetic field in a superconducting sample. In granular superconductors the response to an applied AC field is governed by both intra- and intergranular shielding currents. We have employed low-field AC susceptibility measurements to investigate the magnetic properties of a polycrystalline RuSr 2 GdCu 2 O 8 weak ferromagnet in its superconducting state. For fields below ≃1 mT the results are consistent with the Bean critical state model. For B>1 mT the Kim model is more suitable for the description of the sample's properties.

Collective dynamics of a highly dilute vortex lattice inYBa2Cu3O7−δthin films

The dynamics of highly dilute vortex ensembles in YBa 2 Cu 3 O 7 - thin films is investigated by low-field (5-90 Oe) ac susceptibility measurements of the dynamical relaxation rate Q. In a film with relatively weak pinning,flux bundling is found to be effective at all temperatures and down to the lowest fields. In a film with stronger pinning, single-vortex creep dominates only for T 40 K. The behavior of highly separated vortices is hence surprisingly collective in YBa 2 Cu 3 O 7 - Φ . Quantum creep is observed below 11 K, and at about 60 K the elastic vortex description breaks down as the importance of plastic creep gradually increases.