Superparamagnetic properties of La1 − x Sr x Mn0.925Zn0.075O3 (x = 0.075, 0.095, and 0.115) lanthanum manganites

Lanthanum-strontium manganites doped with zinc are studied by the method of electron magnetic resonance. Nano-objects with ferromagnetically correlated spins, which behave themselves like superparamagnetic particles in the magnetic resonance spectrum, have been found in the paramagnetic phase. The temperature dependences of the resonance magnetic field and magnetic resonance linewidth for La_{1 ? x} Sr _x Mn_0.925Zn_0.075O₃ ceramic samples at temperatures ranging from 100 to 340 K have been analyzed on the basis of the Raikher-Stepanov theory of superparamagnetic particles. The magnetic moment, anisotropy field, and characteristic size of the regions of the ferromagnetically correlated spins have been determined.     

Very high field magnetization and AC susceptibility of native horse spleen ferritin

The magnetization of native horse spleen ferritin protein is measured in pulsed magnetic fields to 55 T at T=1.52 K. The magnetization rises smoothly with negative curvature due to uncompensated Fe³⁺ spins and with a large high field slope due to the underlying antiferromagnetic ferritin core. Even at highest fields the magnetic moment is only ∼4% of the saturation moment of the full complement of Fe³⁺ in the ferritin molecule. The AC magnetic susceptibility, χ_AC(T,f), responding to the uncompensated spins, reaches a maximum near the superparamagnetic blocking temperature with the temperature of the maximum, T_M, varying with excitation frequency, T_M⁻¹ α log f for 10?f?10⁴ Hz.     

Frequency dependent magnetic susceptibility and spin glass freezing inPtMn alloys

We report measurements of the low-field complex magnetic susceptibility on Pt_1?x Mn _x forx=0.01, 0.025 and 0.05 and for frequencies ν between 10 and 4,000 Hz. A strong frequency dependence of the freezing temperatureT _f is observed: ΔT _f /T _f Δ lnv=0.025 (decade ν)^?1 for all three alloys. These results as well as previous other measurements are interpreted in terms of a phenomenological model.     

Finite size effect on Gd doped CoGdxFe2−xO4 (0.0≤x≤0.5) particles

Nanoparticles of CoGd_xFe_2−xO₄ (where x=0.0, 0.1, 0.3, 0.5) series have been prepared by chemical co-precipitation. The effect of Gd³⁺ ion concentration on crystalline phase, crystallinity, crystallite size, molecular vibrations and magnetic resonance has been investigated in detail. The crystallinity decreases with an increase in Gd³⁺ ion concentration and changes the structural parameters. The spin lattice relaxation has been correlated with the doping ion concentration. Similarly, the superparamagnetic behavior of these particles has been observed with EPR spectroscopy.     

Mössbauer study of the re-entrant spin-glass behaviour in the mixed spinel ferrites Mg(0.9+x)Fe2(1−x)Ni0.1TixO4 (x=0.5 and 0.6)

Samples of the mixed spinel ferrite series Mg_(0.9+x)Fe_2(1−x)Ni_0.1Ti_xO₄ with x=0.5 and 0.6, prepared by solid state reaction of the appropriate oxides, have been investigated with ⁵⁷Fe Mössbauer spectroscopy. The as-prepared samples are found to be mainly superparamagnetic due to magnetic cluster formation. Samples after at least three times reheated exhibit spectra, which can be rather interpreted by a transversal relaxation of the spin above and spin-glass behaviour below the respective freezing temperatures T_f. External-field spectra reveal the canting to occur only on the octahedral sites. From the derived transition temperatures and thresholds together with data from earlier investigated sample with x=0.7 a compositional magnetic phase diagram for this spinel series is obtained.     

Distinction of spin-glass freezing from superparamagnetic blocking

Experimental results on Eu_xSr_1?xS provide clear evidence for a cooperative phenomenon at the spin-glass transition, as distinguished from ordinary thermal blocking of superparamagnetic clusters. Only below the percolation threshold x_p = 0.13 can single-clusters aspects be separated clearly (superparamagnetic regime). In the spin-glass regime for x >x_p, susceptibility and remanent magnetization are studied near the freezing temperature in dependence on temperature, magnetic field and observation time. The anomalous slow relaxations of the remanent magnetization, which follow a power-law, exhibit a strong variation just near T_f0, the transition temperature deduced from static magnetization measurements. In addition, T_f values derived from ac-_χ measurements are distinctly frequency dependent; the frequency variation decreases towards low frequencies and seems to saturate near the T_f0 value. The strong sensitivity of _χ(T_f) to even small applied fields can be represented by a universal function independent of concentration. All these results emphasize the importance of the interactions among the spin clusters of spin glasses which are partially frustrated.     

High-frequency properties of discontinuous FeCoSi/native-oxide multilayer films

Discontinuous [FeCoSi (d)/native-oxide]₅₀ multilayer films were fabricated by DC magnetron sputtering without any post-deposition treatment. The films exhibit good soft magnetic properties with initial permeability μ_i larger than 100, the saturation magnetization 4πM_s and the in-plane uniaxial anisotropy field H_k increase as the magnetic FeCoSi layer thickness d is increased from 5.5 to 20.5 Å. As a consequence, the ferromagnetic resonance frequencies f_r of the films increase from 2.0 to 3.9 GHz. The combination of high f_r and large μ_i makes these films potential candidates for magnetic devices applied in the high-frequency range. The origin of the excellent high-frequency properties in discontinuous FeCoSi/native-oxide multilayer films is discussed.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

Mössbauer study of nanocrystalline Ni–Zn ferrite

Ni_1−xZn_xFe₂O₄ (0.0⩽x⩽1.0) nanoparticles have been prepared by the polyvinyl alcohol (PVA) sol–gel method. The lattice parameter of Ni–Zn nanoparticle is larger than that of the bulk material. The Mössbauer spectra of the samples showed the presence of ultrafine particles exhibiting superparamagnetic relaxation at room temperature and an ordered magnetic structure at 77 K.     

Structure and magnetoresistance of a Ni79.7Fe14.0Co2.8Zr2.0Cu1.5 thin film

The structure and magnetoresistance R of thin films based on Ni₈₀Fe₂₀ permalloy doped with Co, Zr, and Cu have been examined by X-ray diffraction analysis and resistance measurement. The films have been obtained by ion plasma sputtering on oxidized silicon, fused quartz, and glass ceramic cold substrates. It has been shown that the structure of a film in the initial state is a mixture of solid solutions based on two phases: Ni(fcc) particles with a size of L ≈ 8 nm and (Zr_0.67Ni_0.22O_0.11)γ particles with a size of L ≈ 12 nm. The R(H) dependences on the strength and direction of the magnetic field H have been obtained at room temperature for film samples in the initial state and after isothermal annealing at 653 K for 1 h. According to R(H) dependences and X-ray diffraction analysis, films in the initial state are assumingly in a superparamagnetic state, whereas they exhibit ferromagnetic properties after isothermal annealing.     

Medium range ordering and some magnetic properties of amorphous Fe90Zr7B3 alloy

High-resolution electron microscopy (HREM) reveals in the as-quenched Fe₉₀Zr₇B₃ alloy the existence of medium range ordered (MRO) regions 1-2 nm in size. Transmission Mössbauer spectroscopy confirms that these regions are α-Fe MRO ones. Above the Curie point of the amorphous phase (T_C=(257±2)K) they behave like non-interacting superparamagnetic particles with the magnetization decreasing linearly with the temperature. For these particles the average magnetic moment of 390μ_B and the average size of 1.7 nm, in excellent agreement with HREM observations, were estimated. The maximum of the isothermal magnetic entropy change at the maximum magnetizing field induction of 2 T occurs at the Curie temperature of the amorphous phase and equals to 1.05 Jkg⁻¹ K⁻¹. The magnetic entropy changes exhibit the linear dependence on the maximum magnetizing field induction in the range 0.5-2 T below, near and above T_C. Such correlations are attributed to superparamagnetic behavior of α-Fe MRO regions.     

Radiative lifetimes and tensor polarizabilities of 1s4f 1 F and 1s5f 1 F level of He I

He atoms have been excited by Ne⁺ ion impact and the depolarization of the fluorescence lines at 668 nm and 492 nm by magnetic and electric fields has been studied. The Ne⁺ ion energy could be chosen such that pure cascade level crossing signals were observed. From the widths of magnetic depolarization signals the radiative lifetimes τ(1s4f ¹ F)=74(2) ns and τ(1s5f ¹ F)=133(5) ns have been determined. By investigating the electric field splitting of the magnetic depolarization signals the tensor polarizabilities ¦α _ten(1s4f ¹ F)¦=0.58(1) kHz/(V/cm)² and ¦α _ten(1s5f ¹ F)¦=4.2(1) kHz/(V/cm)² have been deduced. From the latter value a mean frequencyv(1s5g?1s5f)=14.4 GHz of the transitions between the levels of the 1s5f configuration and those of the 1s5g configuration has been derived.α _ten(1s4f ¹ F) depends sensitively on the singlet-triplet mixing in the 1s4f configuration and thus a mixing coefficient could be deduced for this configuration.     

Logistic equation of arbitrary order

The paper is concerned with the new logistic equation of arbitrary order which describes the performance of complex executive systems X vs. number of tasks N, operating at limited resources K, at non-extensive, heterogeneous self-organization processes characterized by parameter f. In contrast to the classical logistic equation which exclusively relates to the special case of sub-extensive homogeneous self-organization processes at f=1, the proposed model concerns both homogeneous and heterogeneous processes in sub-extensive and super-extensive areas. The parameter of arbitrary order f, where −∞<f<+∞, depends on both the coefficient of external resource utilization u=N/K, where 0<u<1, and the internal microscopic character of realized processes related to the depth of feedback β. The coefficient β directly influences self-organization of processes by the change of microscopic parameters V_i, S_i, i and Z, where V_i is the number of references (visit) to the ith component of the system during the service of each task, S_i is the time of serving the task by the ith component, and Z is the think time of a given process. In the general case of complex system, parameters V_i, S_i, i and Z can have values in the range from 0 to +∞. In this way the new equation includes all possible cases of a complex executive system’s operation. Furthermore, it allows us to define the optimal matching point between X and N with f as the parameter. It also helps to balance the load in complex systems and to equip artificial systems with self-optimization mechanisms similar to those observed in natural systems.     

Low-frequency alternative-current magnetic susceptibility of amorphous and nanocrystalline Co60Fe20B20 films

This investigation experimentally studies the low-frequency alternating-current magnetic susceptibility (χ_ac) of amorphous and nanocrystalline CoFeB films by measuring the magnetic field established by passing currents of various frequencies through such films of various thicknesses (t_f). A CoFeB film is sputtered onto a glass substrate with t_f from 100 Å to 500 Å under the following conditions: (a) As-deposited films were maintained at room temperature (RT) and (b) films were post-annealed at T_A=150 °C for 1 h. The samples thus obtained are analyzed in a magnetic field that was generated by an alternating current (AC) at various frequencies from 10 Hz to 25,000 Hz. The experimental results demonstrate that the χ_ac declines as the thickness of the as-deposited sample and the post-annealed sample (T_A=150 °C) increases because the lower coercivity (H_c) of thinner CoFeB films is similar to a soft magnetic characteristic and is associated with a higher χ_ac value. The best χ_ac value is obtained at a thickness of 100 Å under both conditions. The χ_ac value of the post-annealed sample exceeds that of the RT sample at thicknesses from 100 Å to 500 Å because the magneto crystalline anisotropy of the post-annealed sample yields the highest χ_ac value at the optimal resonance frequency (f_res), at which the spin sensitivity is maximal. The X-ray diffraction patterns (XRD) of as-deposited CoFeB films reveal their amorphous structure. The XRD results for the post-annealed films include a main peak at 2θ=44.7° from the body-centered cubic (BCC) nanocrystalline CoFe that indicated a (110) textured structure. Post-annealing treatment caused that the amorphous structure to become more crystalline by a thermally driven process, such that the χ_ac value of the post-annealed sample exceeded that of the RT sample. This experimental result demonstrates that the χ_ac value decreased as the thickness of the thin film increased. Finally, the CoFeB thin films had the best χ_ac at low frequency (<50 Hz) following post-annealing treatment. The results obtained under the two conditions indicate that the maximum χ_ac value and the optimal f_res of a 100 Å-thick CoFeB thin film were 1.6 and 30 Hz, respectively, following post-annealing at T_A=150 °C for 1 h, suggesting that a 100 Å-thick CoFeB thin film that has been post-annealed at T_A=150 °C can be utilized as a gage sensor and in transformer applications at low frequencies.     

Synthesis and characterization of Ni-Zn ferrite nanoparticles

Nickel zinc ferrite nanoparticles Ni_xZn_1−xFe₂O₄ (x=0.1, 0.3, 0.5) have been synthesized by a chemical co-precipitation method. The samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, electron paramagnetic resonance, dc magnetization and ac susceptibility measurements. The X-ray diffraction patterns confirm the synthesis of single crystalline Ni_xZn_1−xFe₂O₄ nanoparticles. The lattice parameter decreases with increase in Ni content resulting in a reduction in lattice strain. Similarly crystallite size increases with the concentration of Ni. The magnetic measurements show the superparamagnetic nature of the samples for x=0.1 and 0.3 whereas for x=0.5 the material is ferromagnetic. The saturation magnetization is 23.95 emu/g and increases with increase in Ni content. The superparamagnetic nature of the samples is supported by the EPR and ac susceptibility measurement studies. The blocking temperature increases with Ni concentration. The increase in blocking temperature is explained by the redistribution of the cations on tetrahedral (A) and octahedral (B) sites.     

Magnetic,optical and relaxometric properties of organically coated gold–magnetite (Au–Fe3O4) hybrid nanoparticles for potential use in biomedical applications

We present the magnetic, optical and relaxometric properties of multifunctional Au–Fe₃O₄ hybrid nanoparticles (HNPs), as possible novel contrast agents (CAs) for magnetic resonance imaging (MRI). The HNPs have been synthesized by wet chemical methods in heterodimer and core–shell geometries and capped with oleylamine. Structural characterization of the samples have been made by X-ray diffraction and transmission electron microscopy, while magnetic properties have been investigated by means of Superconducting Quantum Interference Device-SQUID magnetometry experiments. As required for MRI applications using negative CAs, the samples resulted superparamagnetic at room temperature and well above their blocking temperatures. Optical properties have been investigated by analyzing the optical absorbtion spectra collected in UV–visible region. Relaxometric measurements have been performed on organic suspensions of HNPs and Nuclear Magnetic Resonance (NMR) dispersion curves have been obtained by measuring the longitudinal 1/T₁ and transverse 1/T₂ relaxation rates of solvent protons in the range 10 kHz/300 MHz at room temperature. NMR relaxivities r₁ and r₂ have been compared with ENDOREM^®, one of the commercial superparamagnetic iron oxide based MRI contrast agents. MRI contrast enhancement efficiencies have been investigated also by examining T₂-weighted MR images of suspensions. The experimental results suggest that the nanoparticles' suspensions are good candidates as negative CAs.     

The effect ofs-d exchange interaction on the nuclear spin relaxation in dilute alloys

The effect ofs-d exchange interaction on the relaxation rate of host nuclear spins has been investigated in dilute magnetic alloys. The leading logarithmic corrections to the Korringa relaxation rate have been calculated together with some lower order logarithmic terms to avoid a divergence at the Kondo temperature (T _k ). The obtained formulae are valid at and aboveT _k and in weak or strong magnetic fields (ω _i ?T or ω _i ?T, respectively, where ω _i is the Larmor frequency of the impurity spin). In second order of the perturbation theory our result reproduces that obtained by Giovannini and Heeger apart from a numerical factor of the order of unity and from some contribution of order (J/? _f )².     

Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH₂O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe³⁺ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT ².     

Use of ferromagnetic resonance to determine the size distribution of γ-Fe2O3 nanoparticles

Ferromagnetic resonance (FMR) experiments were performed as a function of temperature (10-300 K) on γ-Fe₂O₃ nanoparticles prepared by a sol-gel method. By measuring at several temperatures the relative intensity of the spectrum due to superparamagnetic particles and the anisotropy field of the spectrum due to ferrimagnetic particles, we determined the size distribution of γ-Fe₂O₃ nanoparticles. It was found to be a log-normal distribution with a most probable diameter D_m=8.1 nm and a standard deviation σ=0.25. Transmission electron microscopy measurements performed on the same samples yielded a log-normal distribution with D_m=11.2 nm and σ=0.23. The difference is attributed to the existence of a disordered surface layer in the particles.