Experimental Magnetization Curves Research Articles

The Langevin formula for describing the magnetization curve of a magnetic liquid

It has been shown that, at the initial stage of the magnetization curve, the magnetic susceptibility of magnetic liquid determined as χ = Mμ0/B (M is the magnetization, B is the magnetic induction in a sample) obeys the Curie law, and the magnetic susceptibility determined as χ = M/H (H is the magnetic field intensity in a sample) obeys the Curie–Weiss law. Since the Curie law is a particular case of the Langevin dependence, it is assumed that an experimental magnetization curve is described by the Langevin formula with a Langevin parameter ξ = PB/kT, where P is the magnetic moment of a particle and T is the temperature. Experimental verification has shown that, at parameter ξ, the mean relative deviation between the values of M measured and calculated by the Langevin formula is 5%. This deviation can be accounted for by the influence of dispersion of the magnetic moments of nanoparticles.

Mechanics of magnetic fluid column in strong magnetic fields

Elastic-and magnetic properties of magnetic fluid confined by ponderomotive force in a tube fixed in horizontal position are considered. The system is placed in a strong magnetic field under the influence of external static and dynamic perturbations. An experimental setup has been developed. A theoretical basis of the processes of magnetic colloid elastic deformation has been proposed. The values of the static ponderomotive elasticity coefficient and the elasticity coefficient under dynamic action are experimentally determined. The calculations of the saturation magnetization for two magnetic fluid samples, carried out according to the equation containing the dynamic elasticity coefficient, are in good agreement with the experimental magnetization curve. The described method is of interest when studying magnetophoresis and aggregation of nanoparticles in magnetic colloids.

Magnetically coupled clusters in aggregated maghemite ferrofluid: Mössbauer and magnetization study

Mossbauer spectroscopy in a weak static magnetic field and measurements of isothermal magnetization loops were used to study the effect of polymer coating of the γ-Fe 2O3 nanoparticles on the magnetic properties of concentrated ensembles of such nanoparticles. It was found that the individual coating of the nanoparticles by a ∼ 1 nm layer of the polymer leads to the observable changes in the shapes of the Mossbauer spectra and the magnetization curves of the ensembles. Modeling of the experimental magnetization curves in the classical Langevin model and analysis of the Mossbauer spectra in the generalized multi-level relaxation model revealed that the establishment of interparticle magnetic dipole interactions leads to both a ∼ 30 % increase in the magnetic anisotropy constant and a ∼ 35 % increase in the width of the hysteresis loop.

B11-P-01Magnetocrystalline anisotropy of hexagonal Co by relative intensities of electron magnetic circular dichroic signals

Energy integrated signal intensities of Electron Magnetic Circular Dichroism (EMCD) in TEM-EELS (I EMCD ) are proportional to the spin moment of the probed atoms (Mz) projected along the external magnetic field [ 1 ], i.e., I EMCD = KMZ, where the coefficient K depends significantly on the measurement conditions of the EELS detector position on the diffraction plane, sample thickness, convergent/collection angles and I EMCD has not been used to quantitatively compare Mz between different orientations or different materials. We try the quantitative comparison by setting similar measurement conditions for the Co L2,3 EMCD with z//c and zic: The results show clear differences in the EMcD signal intensities, which are fitted with the theoretical curves calculated by the Dyndiff code [ 2 ], relative projected spin moment magnitude (M z┴c / M z//c ) is estimated to be 0.66 ± 0.07, reasonably consistent with 0.75 ± 0.05 from the experimental magnetization curves of a planer shape Co.

High-field magnetic behavior and forced-ferromagnetic state in anErFe11TiHsingle crystal

The crystal-field and exchange parameters are determined for the single-crystalline hydride $\mathrm{ErF}{\mathrm{e}}_{11}\mathrm{TiH}$ compound by analyzing the experimental magnetization curves obtained in magnetic fields of up to 60 T. By using the calculated parameters we succeeded in modeling theoretical magnetization curves for $\mathrm{ErF}{\mathrm{e}}_{11}\mathrm{TiH}$ up to 200 \cyrchar\CYRT{} and to study in detail the transition from ferrimagnetic to a ferromagnetic state in the applied magnetic field.

Strongly enhanced exchange bias of top-pinned Co/IrMn films with Py spacers

This work investigates the effects of inserting an ultrathin ferromagnetic Py or Co spacer layer (SL) at the interface of ferromagnet/antiferromagnet films, namely Co/SL(Py)/IrMn and Py/SL(Co)/IrMn. The exchange bias field, HEB, of the two series of films changes with the spacer thickness in very different manners. Whereas HEB of the Py/SL(Co)/IrMn series decreases monotonously with the Co layer thickness, a fourfold initial bias enhancement is obtained for Py spacers with thicknesses up to 1 nm. Moreover, the Co/SL(Py)/IrMn series shows a maximum HEB more than 50% higher than that of the Py/IrMn film which, in turn, is almost three times higher than the HEB of the control Co/IrMn film. Simulations of experimental magnetization curves through a polycrystalline model for exchange bias showed that these variations, which could yield improvement in the performance of modern spintronic devices, are predominantly governed by modifications of the interfacial exchange interactions. Several mechanisms that could be responsible for the observed behavior are discussed.

Features of Magnetic, Magnetoelectric and Magnetoelastic Properties of HoAl<sub>3</sub>(BO<sub>3</sub>)<sub>4</sub>

The main features of the magnetic, magnetoelastic and record magnetoelectric properties of a HoAl3(BO3)4 alumoborate single crystal have been studied experimentally and theoretically. ForB||awe have observed a giant polarization ΔPba(Ba) ≈ –5240 µС/m2 induced by field of 9 T at T = 5 K. We have calculated the magnetic characteristics of HoAl3(BO3)4 in the framework of a crystal-field model. The parameters of the crystal-field in HoAl3(BO3)4 have been determined. Good agreement of experimental and calculational magnetization curves Mc,^с(В) and the temperature dependences of magnetization Mс,^с(T) was achieved. For interpretation of magnetostriction in HoAl3(BO3)4 the field and temperature dependences of the multipole moments have been calculated.

On the extended elliptic critical-state model for hard superconductors

The magnetic behavior of an irreversible type-II superconducting slab under the action of in-plane crossed fields is investigated within both the original elliptic critical-state model and the extended one, which was recently proposed by Clem. In particular, we study the suppression of the remanent magnetization of a PbBi specimen by a sweeping external transverse magnetic field. It is found that both elliptic critical-state models reproduce the main features of available experimental magnetization curves. We also show that the average magnetizations, corresponding to diamagnetic and paramagnetic initial states at a static bias field Hz, are asymmetrically reduced by the action of an oscillating transverse field Hy. If the amplitude of the oscillations of Hy is as large as the first penetration field HP, the resulting state becomes paramagnetic after various cycles of Hy. Such a kind of paramagnetism is attributed to the anisotropy, induced by flux-line cutting effects, in the critical current density. In PbBi samples, paramagnetism is expected to be manifest in a wide range of values of the static bias field Hz.

A density functional theory approach to the magnetic properties of a coupled single-molecule magnet (Mn7)2 complex — An entangled qubit pair candidate

The exchange coupling constants of a Mn14 complex constituted by two weakly coupled Mn7 moieties were calculated using two different density functional theory (DFT) approaches: the Perdew–Burke–Ernzerhof (PBE) functional with a numerical basis set and the hybrid Becke, three-parameter Lee–Yang–Parr (B3LYP) functional employed with a Gaussian basis set. The sign and relative strength of the exchange coupling constants calculated with both methods were consistent; as expected, the values calculated with the PBE functional were slightly overestimated, as corroborated by comparison with the experimental magnetic susceptibility curve. Both methods gave a ground spin configuration of S = 3/2 for the Mn7 moiety, which was weakly antiferromagnetically coupled with the other Mn7 fragment, leading to an S = 0 ground spin configuration for the entire Mn14 complex.

Calculation of magnetization curves for orthorhombic magnets based on the néel theory of magnetic phases

A theoretical consideration of the processes of magnetization of orthorhombic crystals has been performed based on the Neel theory of magnetic phases. It has been established that orthorhombic magnets are characterized by linear magnetization curves. The main mechanisms of achieving the state of magnetic saturation of orthorhombic magnets along various directions of magnetization have been considered. A comparison of the theoretical and experimental magnetization curves for single crystals of the orthorhombic compound FeB has been performed. A good coincidence of the theoretical and experimental curves made it possible to determine the saturation field for FeB single crystals in different crystallographic directions and the magnitudes of the constants of magnetocrystalline anisotropy of the FeB compound.

Coupled Magneto-Mechanical Analysis Considering Permeability Variation by Stress Due to Both Magnetostriction and Electromagnetism

A general model for the coupled analysis of magneto-mechanical systems is developed by minimizing the continuum energy functional of the system using the calculus of variation. This approach, which is in contrast with the traditional approach of minimizing after discretization, allows the use of strain and stress tensors, vector identities and the divergence theorem, and results in coupled governing equations of the system with three coupling terms; the magnetic stress tensor, the magnetostriction stress tensor, and the magnetostriction reluctivity. The model uses the information contained in the set of experimental magnetostriction curves dependent on stress to calculate the permeability variation due to stress. The governing equations are then discretized using the Galerkin method resulting in methods for the calculation of nodal magnetic and magnetostriction forces including the coupling effects. Finally the model is applied to a simple 2D problem and the flux density distributions using the proposed method and the traditional method of using experimental magnetization curves are compared.

Magnetic properties study of iron-oxide nanoparticles/PVA ferrogels with potential biomedical applications

A study of the magnetic behavior of maghemite nanoparticles (NPs) in polyvinyl alcohol (PVA) polymer matrices prepared by physical cross-linking is reported. The magnetic nanocomposites (ferrogels) were obtained by the in situ co-precipitation of iron salts in the presence of PVA polymer, and subsequently subjected to freezing–thawing cycles. The magnetic behavior of these ferrogels was compared with that of similar systems synthesized using the glutaraldehyde. This type of chemical cross-linking agents presents several disadvantages due to the presence of residual toxic molecules in the gel, which are undesirable for biological applications. Characteristic particle size determined by several techniques are in the range 7.9–9.3 nm. The iron oxidation state in the NPs was studied by X-ray absorption spectroscopy. Mössbauer measurements showed that the NP magnetic moments present collective magnetic excitations and superparamagnetic relaxations. The blocking and irreversibility temperatures of the NPs in the ferrogels, and the magnetic anisotropy constant, were obtained from magnetic measurements. An empirical model including two magnetic contributions (large NPs slightly departed from thermodynamic equilibrium below 200 K, and small NPs at thermodynamic equilibrium) was used to fit the experimental magnetization curves. A deviation from the superparamagnetic regime was observed. This deviation was explained on the basis of an interacting superparamagnetic model. From this model, relevant magnetic and structural properties were obtained, such as the magnitude order of the dipolar interaction energy, the NPs magnetic moment, and the number of NPs per ferrogel mass unit. This study contributes to the understanding of the basic physics of a new class of materials that could emerge from the PVA-based magnetic ferrogels.

A theoretical analysis of the magnetic properties of the low dimensional bis(2-chloropyrazine)dichlorocopper(II) molecule-based magnet

The First-principles Bottom-up (FPBU) theoretical procedure is applied to rationalize the different macroscopic magnetic properties of the bis(2-chloropyrazine)dichlorocopper(II), Cu(2-Clpz)2Cl2, crystal. It is found that this crystal presents three non-negligible antiferromagnetic through-space interactions: J(d1)=−7.53cm−1, J(d3)=−0.15cm−1, and J(d4)=−1.47cm−1. The two strongest magnetic couplings generate a magnetic topology of alternating 2D layers, which are then weakly interconnected by J(d3): a quasi-2D magnetic topology. The computed magnetic susceptibility curves using a model space for the real quasi-2D, and both enforced 2D and 1D magnetic topologies (obtained by deleting J(d3) and J(d3) and J(d4), respectively, from the computed magnetic topology) are close and similar to the experimental curve. This similarity explains why the experimental magnetic susceptibility curve of the Cu(2-Clpz)2Cl2 crystal could be fitted by a 1D model. Heat capacity and magnetization simulations provide unequivocal signatures of magnetic dimensionality. Finally, comparison among five bis(substituted − pyrazine or pyridine) dichlorocopper(II) coordination complexes will enable us to draw qualitative conclusions about the relationship between substituent positions and magnetic dimensionality.

Thermophysical and magnetic studies of two paramagnetic liquid salts: [C4mim][FeCl4] and [P6 6 6 14][FeCl4

The density, heat capacity and viscosity of two magnetic ionic liquids sharing the same Fe (III)-containing anion, specifically, [C4mim][FeCl4] and [P66614][FeCl4], have been determined, along with their volumetric expansivity, and thermal decomposition temperatures. The magnetic properties of the two compounds have been studied using SQUID magnetometry. Both [C4mim][FeCl4] and [P66614][FeCl4] are paramagnetic, but while in the first case the magnetic moment value is 5.8μB/Fe, therefore close to that of Fe3+, for the latter it is only 4.8μB/Fe. The effective concentration of magnetic sites is almost three times greater for [C4mim][FeCl4] as compared to [P66614][FeCl4]. In this latter case, the experimental isothermal magnetization curves do not follow a Brillouin-type of behavior; alternatively, the results were discussed using the Spin Hamiltonian formalism in terms of both the distortion of the Iron site and covalence effects.

Magnetization-based assessment of correlation energy in canted single-chain magnets

We demonstrate numerically that for the strongly anisotropic homometallic S=2 canted single-chain magnet described by the quantum antiferromagnetic Heisenberg model the correlation energy and exchange coupling constant can be directly estimated from the in-field-magnetization profile found along the properly selected crystallographic direction. In the parameter space defined by the spherical angles (\phi, \theta) determining the axes orientation, four regions are identified with different sequences of the characteristic field-dependent magnetization profiles representing the antiferromagnetic, metamagnetic and weak ferromagnetic type behavior. These sequences provide a criterion for the applicability of the anisotropic quantum Heisenberg model to a given experimental system. Our analysis shows that the correlation energy decreases linearly with field and vanishes for a given value H_{cr} which defines a special coordinates in the metamagnetic profile relevant for the zero-field correlation energy and magnetic coupling. For the single-chain magnet formed by the strongly anisotropic manganese(III) acetate meso-tetraphenylporphyrin complexes coupled to the phenylphosphinate ligands, the experimental metamagnetic-type magnetization curve in the c direction yields an accurate estimate of the values of correlation energy \Delta_{\xi}/k_B = 7.93 K and exchange coupling J/k_B=1.20 K.

Investigation of structural and magnetic properties of Ni0.5Zn0.5Fe2O4 nano powders prepared by self combustion method

Nano powders of Ni0.5Zn0.5Fe2O4 have been synthesized by the self-combustion method at a relatively low temperature of 473K under conditions of non-uniform and uniform heating. Rietveld fitting of X-ray diffractograms confirm the formation of the pure spinel phase in both samples. Transmission electron microscopy indicates that the sample prepared under non-uniform heating has a bimodal particle size distribution (average values 16nm and 6nm) while the one prepared under uniform heating has a very narrow particle size distribution (average size 4nm). Low temperature, in-field Mössbauer spectroscopic studies clearly show surface spin contributions. The hyperfine fields and Curie temperature of the non-uniformly heated sample are in good agreement with those reported for the corresponding bulk samples. Despite having a very narrow particle size distribution, the uniformly heated sample has a large anisotropy distribution which is evident in the broad transition visible in the temperature dependent magnetization curve. It is also corroborated by the fact that the experimental magnetization curve at room temperature requires two Langevin functions for satisfactory reproduction.

Magnetic Phases of ZnCr2O4Revealed by Magneto-Optical Studies under Ultra-High Magnetic Fields of up to 600 T

Faraday rotation and magneto-optical absorption spectral measurements were conducted on a spinel oxide, ZnCr 2 O 4 , a prototype of three-dimensional geometrically frustrated magnet. The measurements were carried out at temperatures down to 4.6 K under ultra-high magnetic fields of up to 600 T. The ultra-high magnetic fields were generated by the electro-magnetic flux compression method. We obtained a precise magnetization curve up to a fully polarized phase, where the phase transition takes place above 400 T. The experimental magnetization curves were compared with those obtained by Monte Carlo calculations with an effective spin model including spin–lattice coupling up to fully saturated magnetization. The absorption spectral peaks of the intra- d -band transitions in Cr 3+ ions as well as the exciton–magnon–phonon transition were used for monitoring the crystal and magnetic structures subjected to a strong external magnetic field. A novel magnetic phase was found prior to the fully polarized phase, whi...

Magnetic anisotropy in epitaxial Mn5Ge3films

High crystalline quality Mn 5 Ge 3 films with thicknesses ranging 4–200 nm have been grown on Ge(111) substrates by solid phase epitaxy. The basal hexagonal plane of Mn 5 Ge 3 is in epitaxy with the Ge(111) plane. Magnetic properties of the films have been investigated as a function of the film thickness and the magnetization curves have been analyzed using a theory that includes a description of magnetic domains in uniaxial thin films. The results clearly indicate the existence of a critical thickness below which the magnetic stripe phase disappears. We have determined the value of this thickness to lie between 10 and 25 nm from the analysis of experimental magnetization curves and the theoretical fit of the in-plane remanent magnetization. Although analogies can be drawn between the behavior observed in our system and that of hcp Co, we have shown that the critical thickness is considerably smaller in Mn 5 Ge 3 ; this has the potential to open new fields of applications for Mn 5 Ge 3 thin films in magnetic recording and spintronics.

Homopolar Magnetic Bearing Saturation Effects on Rotating Machinery Vibration

An objective in the design of high performance machinery is to minimize weight so magnetic bearings are often designed to operate slightly lower than their magnetic material saturation. Further weight reduction in the bearings requires operation in the nonlinear portion of the - curve. This necessitates a more sophisticated analysis at the bearing and rotordynamic system levels during the design stage. This paper addresses this problem in a unique manner by developing a fully nonlinear homopolar magnetic bearing model. The nonlinear dynamics of a permanent magnet-biased homopolar magnetic bearing (PMB HoMB) system with a flexible rotor is analyzed. Nonlinear effects due to power amplifier voltage and current saturation and position dependent reluctances are also included in the model. A new curve fit model of the - curve is shown to have significantly better agreement with the measured counterpart than conventional piecewise linear. The modified Langmuir method, with a novel correction terms for the weak flux region, is used to form an analytical model of the experimental magnetization curve of Hiperco 50. High static and dynamic loads applied to the rotor force the magnetic bearing to operate in a flux saturated state. The response of the heavily loaded 4-DOF rotor-bearing system shows that limit cycle stability can be achieved due to the magnetic flux saturation or current saturation in the amplifier. The stable limit cycle prevents the linear model instability, creating what is experimentally observed as a “virtual catcher bearing.” To the authors' knowledge this is the first explanation of this commonly observed phenomenon.

Size‐Dependent Nonlinear Weak‐Field Magnetic Behavior of Maghemite Nanoparticles

The magnetic behavior at room temperature of maghemite nanoparticles of variable sizes (from 7 to 20 nm) is compared using a conventional super quantum interference device (SQUID) and a recently patented technology, called MIAplex. The SQUID usually measures the magnetic response versus an applied magnetic field in a quasi-static mode until high field values (from -4000 to 4000 kA m(-1)) to determine the field-dependence and saturation magnetization of the sample. The MIAplex is a handheld portable device that measures a signal corresponding to the second derivative of the magnetization around zero field (between -15 and 15 kA m(-1)). In this paper, the magnetic response of the size series is correlated, both in diluted and powder form, between the SQUID and MIAplex. The SQUID curves are measured at room temperature in two magnetic field ranges from -4000 to 4000 kA m(-1) (-5T to 5T) and from -15 to 15 kA m(-1). Nonlinear behavior at weak fields is highlighted and the magnetic curves for diluted solutions evolve from quasi-paramagnetic to superparamagnetic behavior when the size of the nanoparticles increases. For the 7-nm sample, the fit of the magnetization with the Langevin model weighted with log-normal distribution corresponds closely to the magnetic size. This confirms the accuracy of the model of non-interacting superparamagnetic particles with a magnetically frustrated surface layer of about 0.5 nm thickness. For the other samples (10-nm to 21-nm), the experimental weak-field magnetization curves are modeled by more than one population of magnetically responding species. This behavior is consistent with a chemically uniform but magnetically distinct structure composed of a core and a magnetically active nanoparticle canted shell. Accordingly the weak-field signature corresponds to the total assembly of the nanoparticles. The impact of size polydispersity is also discussed.