Ferromagnetic Susceptibility Research Articles

Magnetic and optical characterization of ferromagnetic [formula omitted] multilayers

We have investigated the magnetic and structural properties of Mn / SnO 2 multilayers deposited onto Si(1 0 0) substrates at room temperature using the magnetron sputtering method. The nominal Mn layer thickness ranged from 0.4 up to 10 nm. The multilayers structure was studied by X-ray and optical reflectivity. Magnetic measurements indicate the presence of two distinct contributions. One is ferromagnetic with an order temperature above 400 K for all samples and another paramagnetic. Both ferromagnetic and paramagnetic susceptibilities per Mn atom increase as the Mn layer thickness in the multilayers is reduced, corresponding to a more diluted situation. Oscillations in the optical reflectivity measurements are only observed for the sample with nominal Mn layer thickness of 0.4 nm (75 bilayers). In this case, the disappearance of the continuous Mn layers and therefore of the interfaces preserve the necessary transparency. In multilayers with thicker Mn layer, the effective absorption coefficient increases.

Onsager reaction-field approximation for a ferromagnet with a single-ion anisotropy

The Onsager reaction-field approximation is extended to the case of a Heisenberg ferromagnet with a uniaxial single-ion second-order anisotropy. For a paramagnetic phase of a three-dimensional ferromagnet with classical vector spins, the critical temperature of the ferromagnetic transition, susceptibility, pair longitudinal and transverse spin correlation functions, and the magnetic contribution to heat capacity have been calculated with allowance for Onsager corrections for the reaction field. It is shown that the critical exponents of the susceptibility, heat capacity, and correlation radius for an anisotropic ferromagnet in this approximation coincide with the critical exponents of the spherical model. The effect of Onsager corrections on the critical temperature of the transition of a ferromagnet with a d-dimensional hypercubic lattice has been analyzed to show that the corrections from the Onsager reaction field vanish in the limit of an infinite dimensionality of space.

ELECTRICAL RESISTIVITY AND AC SUSCEPTIBILITY STUDIES IN Sr1-xLaxMnO3

Electron-doped Sr 1-x La x MnO 3(0≤x≤0.50) and hole-doped (x=0.70 and 0.82) samples have been prepared by solid state route. X-ray diffraction patterns could be refined using P63 space group for x≤0.30 and [Formula: see text] space group for x≥0.40. Temperature variation of AC susceptibility measurements show that all the electron-doped materials exhibit paramagnetic to ferromagnetic transitions with the transition temperatures ranging from 375 to 393 K, followed by low temperature antiferromagnetic transitions. The ferromagnetic maximum susceptibility is found to increase systematically with La doping, i.e. with increase in Mn 3+ ions. The high temperature resistivity data could be analyzed using the Mott-variable range hopping model. The hole-doped materials exhibit ferromagnetic to paramagnetic transition.

A preconditioning strategy for microwave susceptibility in ferromagnets

3D numerical simulations of ferromagnetic materials can be compared with experimental results via microwave susceptibility. In this paper, an optimised computation of this microwave susceptibility for large meshes is proposed. The microwave susceptibility is obtained by linearisation of the Landau and Lifchitz equations near equilibrium states and the linear systems to be solved are very ill-conditioned. Solutions are computed using the conjugate gradient method for the normal equation (CGN Method). An efficient preconditioner is developed consisting of a projection and an approximation of an “exact” preconditioner in the set of circulant matrices. Control of the condition number due to the preconditioning and evolution of the singular value decomposition are shown in the results.

On the mixed susceptibility of ferromagnets

Based on the macroscopic approach developed by the authors, the frequency and orientational dependences of the mixed susceptibility and magnetic capacity of polydomain ferromagnets caused by shear of domain boundaries are described with allowance for the interaction of ferromagnet subsystems. The effect of constant displacing electric and magnetic fields is examined.

Effective-medium approach to the magnetic susceptibility of composites with ferromagnetic inclusions

A self-consistent effective-medium approximation is applied to the problem of ferromagnetic inclusions in the composite medium to determine the effective magnetic susceptibility. As the derivation utilizes the tensor form of ferromagnetic susceptibility, the obtained equations are valid in the vicinity of the ferromagnetic resonance, where the susceptibility of a ferromagnetic inclusion has significant off-diagonal components. In addition, the static magnetization of neighboring inclusions is taken into account through an effective magnetization of the effective medium. Three most interesting cases are examined, namely, a fully magnetized composite, a composite with single-domain particles, and a composite with demagnetized particles. The effective susceptibility of a magnetized composite is most interesting since the effective susceptibility has the tensor form, significantly varies with changing of the volume fraction of inclusions, which includes shifting of the resonance frequency, and differs from the noninteracting case even at very low volume fractions. In contrast, the effective susceptibility of a composite with isotropically oriented single-domain particles is scalar, without notable shift of resonance frequency, and varies only moderately with changing of the volume fraction. The case of composite with demagnetized inclusions is discussed only qualitatively since the derivation of effective susceptibility greatly depends on the particular physical state of the inclusions.

Microbial activities in forest soils exposed to chronic depositions from a lignite power plant

Atmospheric emissions of fly ash and SO 2 from lignite-fired power plants strongly affect large forest areas in Germany. The impact of different deposition loads on the microbial biomass and enzyme activities was studied at three forest sites ( Picea abies (L.) Karst.) along an emission gradient of 3, 6, and 15 km downwind of a coal-fired power plant (sites Ia, II, and III, respectively), representing high, moderate and low emission rates. An additional site (site Ib) at a distance of 3 km from the power plant was chosen to study the influence of forest type on microbial parameters in coniferous forest soils under fly ash and SO 2 emissions. Soil microbial biomass C and N, CO 2 evolved and activities of l-asparaginase, l-glutaminase, β -glucosidase, acid phosphatase and arylsulfatase (expressed on dry soil and organic C basis) were determined in the forest floor (L, Of and Oh horizon) and mineral top soil (0–10 cm). The emission-induced increases in ferromagnetic susceptibility, soil pH, concentrations of mobile (NH 4NO 3 extractable) Cd, Cr, and Ni, effective cation exchange capacity and base saturation in the humus layer along the 15 km long transect significantly ( P<0.05) reflected the effect of past depositions of alkaline fly ash. Soil microbial and biochemical parameters were significantly ( P<0.05) affected by chronic fly ash depositions. The effect of forest type (i.e. comparison of sites Ia and Ib) on the studied parameters was generally dominated by the deposition effect. Alkaline depositions significantly ( P<0.05) decreased the microbial biomass C and N, microbial biomass C-to-N ratios and microbial biomass C-to-organic C ratios. Microbial respiration, metabolic quotient (qCO 2) and the activities of l-asparaginase, l-glutaminase, β-glucosidase, acid phosphatase and arylsulfatase were increased by long-term depositions from the power plants. Acid phosphatase had the highest specific (enzyme activities expressed per unit organic C) activity values among the enzymes studied and arylsulfatase the lowest. The responses of the microbial biomass and soil respiration data to different atmospheric deposition loads were mainly controlled by the content of organic C and cation exchange capacity, while those of enzyme activities were governed by the soil pH and concentrations of mobile heavy metals. We concluded that chronic fly ash depositions decrease litter decomposition by influencing specific microbial and enzymatic processes in forest soils.

Harmonics of the ac susceptibility in ultrathin film ferromagnets

The initial ac susceptibility χ(ω) is used to monitor the odd harmonics in the response function. For an ultrathin film (Fe2/V5)50 superlattice we record for the first time harmonics χn(ω) up to 11ω in ultrathin films. A detailed Fourier analysis of χn(ω,T) yields the full temperature dependence of the hysteresis including the coercive field HC and the saturation magnetization MS in the vicinity of TC for 0.96<ε=T/TC<1.02.

Harmonics of the ac susceptibility in ultrathin film ferromagnets

Abstract The initial ac susceptibility χ(ω) is used to monitor the odd harmonics in the response function. For an ultrathin film (Fe2/V5)50 superlattice we record for the first time harmonics χn(ω) up to 11ω in ultrathin films. A detailed Fourier analysis of χn(ω,T) yields the full temperature dependence of the hysteresis including the coercive field HC and the saturation magnetization MS in the vicinity of TC for 0.96

Magnetic Salisbury screen and its high-frequency response

We have studied the possibility of constructing magnetic Salisbury screen, and the frequency response of reflectivity of the screen in microwave frequency range. We have found that magnetic Salisbury screen has a lower reflectivity when magnetic susceptibility satisfies χ″χ′. If the frequency response of magnetic susceptibility is in the relax type of domain wall resonance, we could make filmtype Salisbury screen, in which the thickness of the screen may be in the order of micrometer or even submicrometer. The reflectivity of the screen depends on the characteristic impedance zr of the magnetic film that is determined by ferromagnetic resonant frequency and susceptibility under a static field. The frequency response of reflectivity shows that the magnetic film Salisburyscreen has a broadband frequency range for absorbing microwaves well.

Temperature and field dependent susceptibilities of some Libyan desert glass samples

Magnetic susceptibilities of samples of Libyan desert glass have been measured by the Faraday technique. The samples were cut from specimens in which neither striae, nor spherulites, nor bubbles could be observed with the unaided eye. They were transparent with a pale yellow–green colour. The temperature dependent susceptibility can be explained as primarily due to a ‘Brillouin’ system of ‘non-interacting’ paramagnetic centres in a diamagnetic matrix. Assuming that Fe 3+ ions are the source of the para- and ferromagnetic susceptibilities, an analysis of the data shows that a fraction, about 0.4, of the Fe ions are single ions isolated in the matrix and the remainder are in pairs, with the exception of aggregates. The concentration of Fe atoms, determined by the susceptibility data, indicates that in the samples a fraction of the iron ions are anti-ferromagnetic coupled. Less than 2% of the Fe ions are in aggregates that are ferromagnetic at room temperature. Based on our analysis of the data we suggest that, in the samples, the distribution of iron ions is approximately random, with the ions with the smallest separations having ferromagnetic or anti-ferromagnetic interaction at temperatures <300 K.

Magnetic properties of two-dimensional random systems of Ising dipoles

Within the framework of the generalized mean-field theory, two-dimensional random systems of point Ising magnetic dipoles of both fluid and lattice type are considered. In the former case, the precise expression for the distribution function of random magnetic fields is found, the impossibility of ferromagnetic ordering established and the susceptibility of the system in a paramagnetic state is calculated. In the latter case, it has been shown that the ground state of the system of Ising dipoles placed in randomly filled sites of two-dimensional square lattice, depends on the fraction p of the filled sites: at p> p c=0.63, the system is ferromagnetic, otherwise—paramagnetic. The transition between those states is of percolation nature. Temperature dependencies of the magnetization in the ferromagnetic phase and susceptibility of the system in the paramagnetic phase have been found.

Glassy ferromagnetism and magnetic phase separation inLa1−xSrxCoO3

We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

Microbial biomass and enzyme activities in coniferous forest soils as affected by lignite-derived deposition

A forest ecosystem study was conducted along a deposition gradient of air pollutants in old Scots pine stands located near the industrial belt around the city of Bitterfeld in northeast Germany from 1999 to 2000. In order to estimate the impact of different atmospheric deposition loads on microbial biomass and enzyme activities, samples were taken from the forest floor (L, F, and H horizon) and the mineral topsoil (0–10 cm). The emission-induced increases in ferromagnetic susceptibility, soil pH, concentrations of mobile (NH4NO3 extractable) Cr and Ni, effective cation exchange capacity, and base saturation in the humus layer along the 25-km long transect reflected that great portions of the past depositions were characterized by alkaline fly ash. Alkaline depositions significantly (P <0.05) decreased the microbial biomass C and N contents, microbial biomass C-to-organic C ratios, and microbial respiration rates, but increased the metabolic quotient (qCO2) of the mineral topsoil and forest floor. Variations in microbial biomass and activity can mainly be predicted (r2 =0.60) by the concentrations of Ca, Zn and Cd in these forest soils. The specific activities (activity kg-1 organic C) of l-asparaginase, l-glutaminase, arylsulfatase, and in part, acid phosphatase were significantly (P <0.05) higher at forest sites receiving higher fly ash loads than those of the other sites, and thus followed the trend of the qCO2. In contrast, the specific activity of β-glucosidase was significantly (P <0.05) decreased at heavily affected sites compared to moderate and less affected sites, suggesting an inhibition of C mineralization in the forest floor of pine stands affected by predominantly alkaline emissions. A great portion (r2 =0.91) of the variation in the specific enzyme activity data in forest soils in emission areas can be predicted from a linear combination of the variables total organic C and NH4Cl-extractable Ca, pH and effective cation exchange capacity.

Stratigraphic distribution of lignite-derived atmospheric deposits in forest soils of the upper Lusatian Region, East Germany

Atmospheric fly ash emissions from lignite-fired power plantsin the Upper Lusatian and Turowan mining districts stronglyaffected large forest areas along the German-Polish border. Afield study was conducted in old spruce stands (Piceaabies (L.) Karst.) to assess the stratigraphic distributionof fly ash in the forest floor and mineral topsoil in the St. Marienthal forest area in the eastern part of Saxony,Germany. This forest area is subjected to long-termatmospheric depositions by two German and one Polish powerplants since the early 1900s. The three study sites arelocated along a fly ash deposition gradient of 3, 6 and 15 kmfrom the power plant in Turow (Sites Ia, II and III,respectively). An additional site (Site Ib) at a distance of 3 km from Turow was chosen to study the influence of vegetationtype on fly ash deposition intensity in forest soils. Samplesof the humic layer (Oi (L), Oe (F) and Oa (H) horizons) andmineral soil (0–10 cm) were taken in Spring and Fall 1999 andanalysed for their ferromagnetic susceptibility and total ashcontent. Particle size distribution, magnetic susceptibilityof individual size fractions, scanning electron microscopy(SEM), and energy dispersive X-ray microanalysis (EDX) wereperformed on selected samples to evaluate the origin ofmineral particles found in the forest floor. High magneticsusceptibility of the Oa and Oe horizons is a result of thelong-term accumulation of lignite-derived atmospheric depositsin the forest floors of the studied area. Pure conifer stands(year-round filtration of airborne pollutants) resulted inhigher inputs of ferromagnetic fly ash particles in forestsoils. Unusually high total ash contents for humic horizons(up to 77%) were determined in the Oa and Oe horizons atSites Ia and IIb, indicating the need for a new classificationsystems for the organic layer in forest soils near coal-firedpower plants. SEM revealed 4 typical phases of persistent flyash deposits formed by combustion of Lusatian lignite: (1) iron-containing `stable glasses’, (2) aluminium-iron-silicate-minerals,(3) slag fragments and (4) lignite-derived fossilcarbon. Particle size analysis, magnetic susceptibilitymeasurements and SEM-EDX techniques indicated that a greatportion of the mineral particles found in the humic horizonsof forests soils are from atmospheric sources. Fly ashaccumulation consisting of ferromagnetic minerals contributesmainly to the 125–63 and <63 μm fractions in soils. EDXanalysis revealed that atmospheric input of lignite-derivedfly ash increases the contents of the following ecologicalrelevant elements in soils: carbon, iron, aluminium, calcium,potassium, sulphur, titanium and sodium.

Field dependent susceptibility of the 2D ferromagnet (CH 3NH 3) 2CuCl 4 in the paramagnetic phase

The temperature dependence of the ac susceptibility ( χ) at constant applied magnetic field was investigated in the paramagnetic region of the quasi-2D ferromagnet (CH 3NH 3) 2CuCl 4. Above the Curie temperature ( T C=8.85 K) a maximum in the χ( T, H) curves was observed at T m( H). The temperature at the maximum increases with increasing applied field. This anomaly is related to short range fluctuations close the order transition. The behavior of T m( H) is governed by the gap exponent of the scaling function ( Δ= γ+ β). We found Δ=2.2±0.1 in very good agreement with the previously known values of γ and β.

A new method for the separation of paramagnetic and ferromagnetic susceptibility anisotropy using low field and high field methods

SUMMARY Most rocks contain both ferromagnetic and paramagnetic minerals that contribute to their bulk magnetic susceptibility and the anisotropy of magnetic susceptibility. Anisotropy of magnetic susceptibility techniques typically measure the net susceptibility and are not able to separate paramagnetic and ferromagnetic contributions. Since different minerals may form at various times and/or under different conditions, examination of their individual contributions provides unique information related to the rock’s formation and evolution. By subjecting a sample to high magnetic fields, the ferromagnetic minerals become saturated and the contribution of the paramagnetic minerals can be evaluated (the slope of the line at high field values, on a field vs magnetization plot). Using this approach, we developed a new technique that separates the ferromagnetic and paramagnetic components of standard 1 inch cylindrical samples using a Vibrating Sample Magnetometer. This separation is tested by artificially combining separate samples with known paramagnetic-only and ferromagnetic-only behaviour. By comparing the high-field results of a combined paramagnetic and ferromagnetic signal to the classic low field alternating current susceptibility of the paramagnetic-only signal, we demonstrate that the high field anisotropy is the result solely of the paramagnetic fabric even when the low field anisotropy of magnetic susceptibility is dominated by the ferromagnetic minerals. A ferromagnetic-only fabric is calculated for the combined paramagnetic and ferromagnetic rock, by tensor subtraction of the high field (paramagnetic-only) and low field (paramagnetic plus ferromagnetic) measurements on the same sample. Application of this technique to natural samples of combined paramagnetic and ferromagnetic behaviour is discussed.

The influence of adsorption of water molecules on magnetic susceptibility of amorphous ferromagnets

The reversible influence of weakly adsorbed water molecules on the magnetic susceptibility of amorphous ferromagnets of composition Fe 76.5Cu 1Nb 3Si 13.5B 6 is observed by a magneto-optical method at room temperature. The decrease of an amplitude of the domain wall (DW) oscillations is observed when the pressure of water vapour decreases from 400 to 1300 Pa. This effect is explained by appearance of surface perpendicular magnetic anisotropy, which is caused by the sample surface contraction. The surface contraction arises in consequence of the pressure on the walls of the micropores located on the surface of ferromagnets. The pressure is induced by H-bond adsorbed clusters of water molecules. The various magnetic charges emerge on the surfaces of domains through the presence of the surface perpendicular magnetic anisotropy. The field of magnetic charges causes an increase in the restoring force affecting the DW under its displacement from equilibrium position.

Damage correlations and fluctuations in 2D three-state Potts ferromagnet

We investigate, through the damage spreading method, the correlation length associated with the two-point correlation function between damage fluctuations, and the damage susceptibility of the square-lattice three-state Potts ferromagnet. We find that both quantities tend to diverge at the damage transition. These results can be interpreted as a first evidence of a possible fluctuation–dissipation-like relation between damage quantities in this model.

Differential approximants: An accurate interpolation from high-temperature series expansions to low-temperature behavior in two-dimensional ferromagnets

We introduce biased differential approximants to high-temperature series expansions of the susceptibility in two-dimensional Heisenberg ferromagnets, taking into account the exponential divergence at low temperature. They provide a remarkable continuous description of the susceptibility from high temperature down to the low-temperature regime.