Richter-type magnetic after-effect in manganese ferrite

A study was made of magnetic after-effect of the Richter type in samples of non-stoichiometric manganese ferrites differing by their excess of manganese and having a different content of oxygen.     

Study of magnetic after-effect in magnesium manganese ferrites

The magnetic after-effect caused by the diffusion of electrons was studied in detail in MnMg ferrites of the series Mg _x Mn_1.15–x Fe_1.85O₄₊ ; the comparative measurements were also carried out on a sample of MgFe₂O₄₊₍<0). By combining the two methods, (a) investigation of the disaccommodation of initial permeability at different temperatures and (b) measurement of the displacement of the maximum of the temperature dependence tan with the frequency, it was possible to study the relaxation processes whose time constants lay between 0.5 sec and several hours, or between 10^–4 to 10^–7 sec. It was found that the processes taking place at low temperatures and studied by method (a) differ from those observed at high temperatures by method (b), particularly in the lower activation energies and greater dispersion of the time constants. In addition to these two main relaxation processes a weak relaxation superposed over the main disaccommodation was found in ferrites with non-zero content of manganese. An analysis of the intensity of different relaxation effects showed the participation of Mn ions in these relaxation effects and it was found that both main relaxations are probably equivalent as regards the final steady state, to the creation of which they lead; they differ however in the mechanism of electron diffusion by which this state is realized. The main features of the different diffusion mechanisms are discussed.

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, , MnMg Mg _x Mn_1.15–x Fe_1.85O₄₊ . MgFe₂O_{4+ (}<0). ()- , () tg , , 0,5 sec , 10^–4–10^–7 sec. , , () , , (), , , . , . Mn , , , , , ; , , , . .

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In conclusion the author thanks Dr. J. Bro and K. Závta for valuable discussion and F. Vilím for carefully carrying out the measurements.     

Magnetic aftereffects and the barkhausen effect in a magnesium manganese ferrite single crystal

The temperature dependence of the critical magnetic field [H₀(T)] of a magnesium manganese ferrite single crystal is analyzed, together with the changes taking place in the critical magnetic field with time, H₀(t), the temperature dependence of the domain-boundary stabilization field H_st(T), and the time required to establish the equilibrium state of the domain boundaries; the domain structure is examined, and the induced anisotropy constant is calculated; so is the activation energy of the process leading to the stabilization of the domain boundaries. The magnetic aftereffects and the Barkhausen jumps accompanying them are of a diffusion nature.     

Magnetic after-effect in Mn ferrite

The ballistic method was used to study the disaccommodation of magnetic inductionB in Mn_1.36Fe_1.64O₄ in a d.c. magnetic fieldH 0·040–5·5 Oe. The experimental results were interpreted from the point of view of the irreversible displacements of walls. It was shown that under certain assumptions it is possible to derive the statistical distribution of Bloch wall energies from the experimental curves for substances in which there exists magnetic diffusion aftereffect.     

Structural and magnetic studies of the Zn-substituted magnesium ferrite chromate

The formation of iron oxide nanoparticles in course of a sol-gel preparation process was traced by UV/Vis and ⁵⁷Fe Mössbauer absorption spectroscopy. Samples were extracted at different stages of the reaction. While spectra measured on samples extracted at low reactor temperatures showed the starting materials Fe(acac)₃ diluted in benzyl alcohol undergoing slow paramagnetic relaxation, a sample extracted at a reactor temperature of 180 °C gave clear evidence for emerging iron oxide nanoparticles. A prolonged stay at 200 °C results in a complete transformation from Fe(acac)₃ to maghemite nanoparticles.     

Effect of magnesium substitution on dielectric and magnetic properties of Ni-Zn ferrite

The dielectric and magnetic properties of Mg incorporated Ni-Zn spinel ferrites have been investigated. Ni_0.5−xZn_0.5Mg_xFe₂O₄ ferrites have been prepared by sol-gel auto-combustion technique. The as prepared ferrites were annealed at 673, 873 and 1073 K. The X-ray diffraction studies reveal the spinel structure of annealed ferrites. The TEM results are in agreement with XRD results. FTIR study has also been carried out to get insight into the structure of these ferrites. The dielectric measurements show that the dielectric constant (ε′), dielectric loss (tan δ) and conductivity (σ_ac) increase on incorporation of Mg in the Ni-Zn ferrite. ε′, tan δ and σ_ac also show dependence on temperature, frequency of external applied electric field and microstructure of the samples. The magnetic moment measurements reveal that the saturation magnetization (M_s) increases and coercivity (H_c) decreases with the increase in concentration of Mg²⁺ ions. M_s and H_c also show dependence on the annealing temperature.     

Effect of Cu-Cr co-substitution on magnetic properties of nanocrystalline magnesium ferrite

This study deals with the temperature and composition dependence of magnetization and magnetic anisotropy of Cu²⁺-Cr³⁺ co-substituted magnesium ferrite, Mg_1−xCu_xCr_xFe_2−xO₄ (x=0.0-0.5). The synthesized materials are characterized using thermo gravimetric analysis, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray fluorescence, Mössbauer spectrometer, superconducting quantum interference device magnetometer and vibrating sample magnetometer. The M-H loops measured up to 50 kOe at 300, 200 and 100 K, revealed narrow hysteresis curves with a coercive field and saturation magnetization varying for different compositions. The high field regimes of these loops are modeled using the Law of Approach to saturation to extract anisotropy information and saturation magnetization. Both the saturation magnetization and the anisotropy constant are observed to increase with the decrease in temperature while decrease with the Cu-Cr co-substituents for all the samples. Explanation of the observed behavior is proposed in terms of the preference of the co-substituent ions of Cu²⁺ and Cr³⁺ and their predominant choice to substitute into the octahedral sites of the cubic spinel lattice.     

Cationic behavior and the related magnetic and magnetotransport properties of manganese ferrite thin films

Manganese ferrite (Mn_xFe_3−xO₄) thin films have been prepared by a sol–gel method. The samples (x≤1.25) are polycrystalline containing well-defined grains and maintain cubic spinel structure with increasing lattice constant with x. The substituting Mn ions were found to have multi-valence, +2 and +3, by X-ray photoelectron spectroscopy. The saturation magnetization of the Mn-substituted films measured by vibrating sample magnetometry was found to increase from that of Fe₃O₄ at low x and then gradually decreases as x increases further. Such magnetic behavior can be explained in terms of the dominance of Mn²⁺ species over Mn³⁺ at low x and the gradual decrease in the population ratio Mn²⁺/Mn³⁺ as x increases. The observed decrease in the coercivity with increasing x implies the increase in octahedral Mn²⁺ population. The Mn-substituted samples exhibit magnetoresistance (MR) effect the maximum intensity of which is gradually reduced with x from that of Fe₃O₄. All samples show increasing MR with increasing external field (H), while their magnetization curves start to saturate near H=2 kOe. Such increasing MR with H can be explained in terms of the tunneling of spin-polarized carriers across grain boundaries. The reduction in the MR intensity with x can be partly explained in terms of the decrease in spin-polarized carrier density associated with octahedral occupation of Mn²⁺ ions.     

镍锰铁氧体纳米线阵列的制备与表征

采用真空负压灌注技术, 结合溶胶-凝胶法在多孔氧化铝模板的纳米孔洞中成功制备了平均直径为80 nm左右的Ni_{1- x}Mn_xFe₂O₄(x=0, 0.25, 0.5, 0.75) 纳米线阵列. XRD结果显示所制备的纳米线阵列为立方尖晶石结构, SEM和TEM的结果表明纳米线是由大量不同晶体取向的亚微晶粒联接组成. 磁测量结果显示, 随着Mn掺杂浓度的增加, 饱和磁化强度先增加而后减小, 这种变化与离子在尖晶石结构中的替代、占位变化有关. 相比于块体材料的NiFe₂O₄, 由于非线性磁结构比例的增加, 导致了线体NiFe₂O₄的饱和磁化强度降低.     

The infrared absorption spectrum and the inversion degree of manganese ferrite

Infrared spectra of MnFe₂O₄ with inversion degrees of 0.05 and 0.2 are reported. It can be concluded that the absorption observed at 335 cm^?1 has to be the third fundamental IR active mode.     

The photoluminescence of lead and manganese doped magnesium bromide

The excitation and emission spectra of MgBr₂6H₂O doped by Pb²⁺ and doubledoped by Pb²⁺ and Mn²⁺ are investigated. The 410 nm emission due to Pb²⁺ transitions was observed in both cases. In the emission spectrum of MgBr₂:Pb, Mn the 670, 630 and 540 nm bands can be observed as well; their appearance and intensity depend on the values and the ratio of concentrations of the Pb²⁺ and Mn²⁺ impurities. These bands correspond to the emission of Mn²⁺ which, in the low symmetry MgBr₂6H₂O base doped with Pb²⁺, can be located at various structural positions. The obtained results suggest that energy transfer in the MgBr₂:Pb, Mn occurs both from Pb²⁺ to Mn²⁺ and vice versa.     

The distribution of ions and their valencies in magnesium manganese ferrites

A model of the distribution of cations and valencies in non-stoichiometric magnesium manganese ferrites was proposed and its validity verified by measuring the saturation magnetic moments. The results of measuring the moment in stoichiometric ferrites to a certain extent confirms the correctness of the proposed model for this type of magnesium manganese ferrites, too. In a discussion of the distribution coefficient for manganese ions the close connection between this coefficient and the average valency of the manganese was pointed out. The question of the dependence of the distribution coefficient of magnesium ions on the composition is solved by introducing the maximum values of this coefficient and assuming that a simple proportionality holds for all compositions between the actual value of this coefficient and its maximum value.

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In conclusion the author thanks Dr. S. Krupika and K. Závta for valuable discussions.     

A novel proximity effect between superconductor and magnetic manganese oxide

A novel proximity effect between superconductor and magnetic manganese oxide has been found. To investigate interactions between superconductor and magnetic materials, we have fabricated both trilayered-type and coplanar-type junctions and investigated their current-voltage characteristics. The magnetic material was La_1-x Ca _x MnO _z (LCMO) or La_1-x Sr _x MnO _z (LSMO) manganese oxide. In YBCO/LCMO/YBCO junctions, supercurrents were observed through a magnetic barrier as thick as 500 nm. This phenomenon was also confirmed by coplanar junctions. The effect of magnetism of the barrier material was investigated. It was found that this proximity effect occurred only in a limited region where spin fluctuations are considered to exist.     

Micro Raman,Mossbauer and magnetic studies of manganese substituted zinc ferrite nanoparticles: Role of Mn

A series of Mn–Zn Ferrite nanoparticles (<15 nm) with formula Mn_xZn_1−xFe₂O₄ (where x=0.00, 0.35, 0.50, 0.65) were successfully prepared by citrate-gel method at low temperature (400 °C). X-ray diffraction analysis confirmed the formation of single cubic spinel phase in these nanoparticles. The FESEM and TEM micrographs revealed the nanoparticles to be nearly spherical in shape and of fairly uniform size. The fractions of Mn²⁺, Zn²⁺ and Fe³⁺ cations occupying tetrahedral sites along with Fe occupying octahedral sites within the unit cell of different ferrite samples are estimated by room temperature micro-Raman spectroscopy. Low temperature Mossbauer measurement on Mn_0.5Zn_0.5Fe₂O₄ has reconfirmed the mixed spinel phase of these nanoparticles. Room temperature magnetization studies (PPMS) of Mn substituted samples showed superparamagnetic behavior. Manganese substitution for Zn in the ferrite caused the magnetization to increase from 04 to18 emu/g and Lande's g factor (estimated from ferromagnetic resonance measurement) from 2.02 to 2.12 when x was increased up to 0.50. The FMR has shown that higher Mn cationic substitution leads to increase in dipolar interaction and decrease in super exchange interaction. Thermomagnetic (M–T) and magnetization (M–H) measurements have shown that the increase in Mn concentration (up to x=0.50) enhances the spin ordering temperature up to 150 K (blocking temperature). Magnetocrystalline anisotropy in the nanoparticles was established by Mossbauer, ferromagnetic resonance and thermomagnetic measurements. The optimized substitution of manganese for zinc improves the magnetic properties and makes these nanoparticles a potential candidate for their applications in microwave region and biomedical field.