Coupled oscillations of electron and nuclear spins and spontaneous polarization in multiferroics: Specific features of NMR

Coupled oscillations of electron and nuclear spins and spontaneous polarization in films of multiferroics have been analyzed. The analysis has been performed on the example of antiferromagnetic films of BiFeO₃ possessing weak ferromagnetism, magnetic anisotropy of the easy-plane type, and spontaneous polarization. The natural frequencies of the oscillations and the tensor of the high-frequency magnetoelectric susceptibility of the films (response to the ac magnetic and ac electric fields) have been calculated. Specific features of the susceptibility near the frequency of the nuclear magnetic resonance (NMR) have been investigated. The main dynamic characteristics of the system under consideration have been analyzed: the NMR frequencies, frequency shifts, enhancement coefficients. The possibility is discussed of the appearance of fundamentally new effects in these films in the region of NMR spectroscopy of magnets: (a) excitation of nuclear spins by an ac electric field with their subsequent detection with the use of the ac component of the magnetization (nuclear magnetoelectric resonance (NMER)); (b) excitation of the NMR signal by an ac magnetic field with its subsequent detection with the use of the ac component of the spontaneous polarization (nuclear electric-magnetic resonance (NEMR)).     

Specific features of magnetic order in a multiferroic compound CuCrO<Subscript>2</Subscript> determined using NMR and NQR data for <Superscript>63, 65</Superscript>Cu nuclei

Results of studying the paramagnetic and ordered phases of a CuCrO₂ single crystal using nuclear magnetic and nuclear quadrupole resonances on ^63,65Cu nuclei are presented. The measurements have been carried out in wide ranges of temperature (T = 4.2–300 K) and magnetic-field strength (Н = 0–94 kOe), with the magnetic fields being directed along a and c axes of the crystal. The components of the electric-field gradient tensor and the magnetic-shift tensor (K _a,c) have been determined. The temperature dependences K _a(H || a) and K _c(H || c) for the paramagnetic phase are described by the Curie–Weiss law and reproduce the behavior of the magnetic susceptibility (χ_a,c). The hyperfine field on a copper nucleus has been determined, which is equal to h _hf ^a,c = 33 kOe/μB. Below the temperature Т _N = 23.6 K, nuclear magnetic resonance and nuclear quadrupole resonance spectra for ^63,65Cu nuclei have been recorded typical of helical magnetic structures, which are incommensurable with the lattice period.     

Nuclear magnetic resonance in manganites

A brief survey of results concerning the study of manganites by the method of nuclear magnetic resonance (NMR) is presented. Experiments in which the nuclei of the majority of the elements that enter into the structural formulas of the manganites have been used as the local probes are considered. Primary attention is paid to the application of this local method for investigating the microscopic phase separation based on the data of the ⁵⁵Mn NMR, orbital ordering in LaMnO₃ based on the data of the ¹⁷O NMR, and low-frequency spin dynamics.     

Magnetic phase transitions in ferrimagnetic DyFe5Al7 near the compensation point

The magnetization curves of single-crystal DyFe₅Al₇ have been measured along the main crystal-lographic directions in the magnetically ordered state. The temperature dependences of the magnetic moments of the Dy and Fe sublattices and the parameter of the intersublattice exchange have been determined in the model of molecular field. Upon the measurements along the easy axis, jumps of magnetization, the values and critical fields of which strongly depend on temperature, have been revealed near the temperature of magnetic compensation. A model of the magnetization of a ferrimagnet with two magnetic sublattices coupled by weak intersublattice exchange has been suggested. Using this model, analytical expressions for the magnetization curve and critical field of the spin-reorientation transition have been obtained. A magnetic phase diagram has been constructed and magnetic structures that are realized in different fields at different temperatures have been determined. The generalization of the model to the case of a noncollinear Fe sublattice has been generalized.     

M?ssbauer studies of the magnetic phase transition in La(Fe0.88Si x Al0.12 ? x )13 compounds

Spectra of the nuclear gamma resonance for La(_Fe0.88Si _x Al_{0.12 ? x} )₁₃ compounds with different x values have been measured at 100 K. The compounds exhibit an antiferromagnetic ordering in the range of silicon concentrations x ?? 0.024 and a ferromagnetic ordering at x > 0.024. Analysis of the spectra has been carried out with the use of a two-core distribution of quadrupole shifts. It has been demonstrated that upon a transition from antiferromagnetic to ferromagnetic ordering, the average hyperfine field at the Fe nuclei increases by almost 40 kOe. It has been found that antiferromagnetic compounds are mainly characterized by a positive quadrupole shift, and ferromagnetic ones by a negative quadrupole shift. A model of the formation of a magnetic structure that explains the appearance of a layered antiferromagnetic ordering in La(Fe_0.88Si _x Al_{0.12 ? x} )₁₃ compounds has been proposed.     

Magnetic properties of <Emphasis Type="Italic">R</Emphasis>Co<Subscript>2</Subscript> compounds in the exchange-striction model of ferrimagnets

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo₂ ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (Т _С), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > Т _С. For an ErСо₂ sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.     

Ion beam analysis of corrosion films on a high magnesium alloy (Magnox Al 80)

Specific nuclear resonance reactions, ⁴He ion backscattering and ESCA have been used to investigate the depth distribution of oxygen, carbon, fluorine and magnesium in complex corrosion films formed on Magnox AL 80 after exposure to nuclear reactor coolant gas and to multi-component aqueous solutions.Results of experiments with water enriched in H₂¹⁸O have shown that magnesium hydroxide which is highly permeable to solution is formed at the metal/film interface. The oxide formed in hot CO₂ contains carbon, carbonate and traces of carbide-like material. This oxide hydrates uniformly in solutions of pH ? 11.5 but complete equilibration with solution is rather slow.In solutions containing fluoride ions which inhibit the corrosion reaction the product is a hydroxy-fluoride Mg(OH)_2?x and fluoride also exchanges to an appreciable extent with pre-formed magnesium hydroxide.     

Mössbauer investigation of Sn diffusion and segregation in grain boundaries of polycrystalline Nb

The parameters of nuclear γ resonance (Mössbauer) spectra on ^119mSn nuclei inserted into grain boundaries of polycrystalline Nb have been determined at the annealing temperatures of 680 to 994 K [0.25–0.36 melting temperature (T _m)]. Two components are observed in the nuclear emission γ resonance spectra at all of the annealing temperatures considered. One of them (component 1) is suggested to result from the ^119mSn atomic probes localized in grain boundary cores, while the other one (component 2) corresponds to that located in the near-boundary areas. Diffusion annealing temperature dependences of both components parameters have been analyzed. The coefficients and enthalpy of Sn grain-boundary segregation are determined based on this analysis, as well as the ratio of the diffusion pumping zone extension to the grain boundary width and the enthalpy of the atomic probes “pumping” from grain boundaries into the bulk.     

Nuclear magnetic resonance in magnets with a spiral magnetic structure

Specific features of nuclear magnetic resonance (NMR) in a ferromagnet with a crystallographic magnetic anisotropy of the easy-plane type placed in a constant magnetic field have been studied. The symmetry of the magnet admits the existence of the Dzyaloshinskii-Moriya interaction, which leads to the formation of a new ground state, namely, a soliton lattice (spiral structure). Within the spin-wave approximation, the following basic local characteristics of the NMR of this structure have been calculated: resonance frequency, enhancement factor, and line broadening and their field dependences have been investigated. The magnetic resonance susceptibility of the electron-nucleus spin system has been calculated; the shape of the NMR absorption line has been analyzed. The problem of the evolution of the NMR absorption line upon the change in the magnitude of an external magnetic field has been solved. The possibility of the experimental detection and investigation of the structural and dynamic features of the spiral magnetic structure by the NMR method is discussed.     

Controlling stress and diffusion for the low-temperature-ordering of L1<Subscript>0</Subscript> ordered FePt films

Low-temperature-ordering of L1₀ ordered FePt films have been extensively studied in recent years due to its potential for future application on magnetic perpendicular media. The predominant issue of the ordering process is the diffusion of iron and platinum atoms from a disordered to an ordered phase. The diffusion can be enhanced by adjusting diffusivity, providing extra energy, or reducing energy barriers. In addition to reducing the ordering temperature of FePt, (001)-oriented granular films require perpendicular media because the magnetic easy axis of the ordered phase is [001]. Atomic-scale multilayer deposition is proposed to achieve designed film structures.     

Improvement of the magnetic properties of nanocomposite permanent magnetic alloys with Dy and Ga substitutions

The Dy and Ga substituted NdFeB nanocomposite permanent magnetic alloys with high magnetic properties have been prepared by appropriate wheel speed of melt-spinning and post-annealing treatment. Under optimal conditions, compared with the best magnetic properties of ternary NdFeB alloy of J_r=1.18 T, H_ci=379.5 kA/m and (BH)_max=119.5 kJ/m³, the best magnetic properties of the alloy with Dy and Ga substitutions are J_r=1.16 T, H_ci=580.5 kA/m, and (BH)_max=162.7 kJ/m³. The XRD and TEM results showed that each of two alloys consists of hard magnetic 2:14:1 phase and soft magnetic α-Fe phase. The grain size of the 2:14:1 phase is about equal in the two alloys. The grain size and content of α-Fe phase in Dy and Ga substituted alloy are finer and lower, respectively.     

Electron spectroscopic studies on films of SnO2 and SnO2:Sb

Pure and antimony-doped SnO₂ films have been deposited by two different techniques: metal-organic chemical vapour deposition (MO-CVD) and spray pyrolysis of SnCl₄ in the presence of air with different concentrations of antimony. X-ray and UV photoelectron spectroscopic studies have been carried out on these films and the combined data from these studies provided information about the electronic properties of antimony-doped SnO₂ films and their compositions. That the binding energy of SnO is higher than that of SnO₂, contrary to expectations, is because of the fact that SnO₂ is more covalent than SnO. XPS has been used successfully to confirm the presence of Sb³⁺ and Sb⁵⁺ beyond a certain concentration of antimony (0.5 mol%) in the film, which is responsible for the decrease in conductivity and IR reflectivity. UV photoelectron spectroscopy of transparent, conducting SnO₂ films has been reported for the first time. The onset of photoemission in the valence band around 2.6 eV has been explained by invoking the idea of surface states near the valence band below the Fermi level.     

Etching of Laminates with Hydrogen Peroxide in Sulphuric Acid Solution

Three types of nuclear reaction have been applied to study chromium conversion coatings on zinc electroplate. The 1005keV resonance in the ⁵²Crlp, Y)⁵³Mn reaction has been used to determine the depth versus concentration profile of chromium in the coatings, while the ¹⁵O(d,α) ¹⁴N reaction has been used to profile oxygen content, or particle induced elastic forward scattering of protons (proton recoil) was used to determine the hydrogen profile in the samples. The results suggest that the newly introduced trivalent type treatments produce coatings containing comparable water/hydroxide contents to the establishedhexavalent type treatments.     

Magnetic properties of the Mn1.9 − x Co x Ge compounds with a hexagonal crystal structure

Concentrational dependences of the lattice parameters, spontaneous magnetic moment, and magnetic ordering temperature of nonstoichiometric Mn_{1.9 ? x} Co _x Ge (0.8 < x < 1.65) compounds with a hexagonal crystal structure of the Ni₂In type have been studied. As the Co content (x) increases from 1.1 to 1.2, the Curie temperature and magnetic moment were found to increase abruptly. The magnetization curves measured for oriented single crystals indicate the presence of a concentrational spin-reorientation transition in this system. The experimental data obtained are discussed taking into account the results of energy-band electronic-structure calculations.     

Induced magnetic anisotropy in permalloy films annealed with magnetic field

The effect of magnetic annealing on Fe₂₁Ni₇₉ films was investigated through the characterization of crystal structure and magnetic properties. Fe₂₁Ni₇₉ films were deposited on thermally oxidized Si substrates using a DC magnetron sputter. A highly a-axis oriented Fe₂₁Ni₇₉ film was grown on Si substrate by magnetic annealing. Columnar grain growth was dominant in the films annealed with magnetic field. Both the area surrounded by hysteresis curve, and coercivity, were increased by magnetic annealing, indicating that the magnetic annealing induced the anisotropy energy. Through EXAFS analysis, 1.19×10^?3 was obtained as the strain of a specimen annealed with magnetic field. This value confirmed that induced magnetic anisotropy was formed by atomic pair ordering.     

Electrochemical synthesis and characterization of tantalum alkoxides

Tantalum(V) propoxide (Ta(OPrⁿ)₅), isopropoxide (Ta(OPrⁱ)₅) and butoxide(Ta(OBuⁿ)₅) were synthesized by electrochemical reactions of corresponding alcohol at sacrificial tantalum anode in the presence of tetraethylammonium chloride as a conductive additive. The pure products were isolated by reduced pressure distillation under 5 kPa. The crystal of Ta(OPrⁱ)₅ was obtained by recrystallization from hexane at ?10 °C. These samples were characterized by Fourier transform infrared spectra (FT-IR), Raman spectra, nuclear magnetic resonance spectra (NMR), TG/DTA and ICP-MS. The results show that direct electrochemical synthesis of metal alkoxides has a high current efficiency and electrolysis yield. These alkoxides have a high purity of 99.97% and can be directly used as the precursor of Ta₂O₅ films.     

Carbazole-based aromatic amines having oxetanyl groups as materials for hole transporting layers

Several oxetane-functionalized aromatic amines have been synthesized by the multi-step synthetic rout. Full characterization of their structure by nuclear magnetic resonance and mass spectroscopy as well as IR spectrometry is presented. The synthesized materials were examined by various techniques including differential scanning calorimetry, thermogravimetry, UV spectrometry, electron photoemission and time of flight techniques. The electron photoemission spectra of the layers showed the ionisation potentials of 5.24–5.67 eV. Time-of-flight hole drift mobility in amorphous layers of bisphenol Z polycarbonate containing 33 wt. % of the electroactive materials ranges from 10⁻⁸ to 10⁻⁶ cm²/Vs at high electric fields.     

Microwave properties of powders produced by high-energy milling of iron with paraffin

The structural and phase composition and magnetostatic and microwave properties of powders produced by simultaneous milling of carbonyl iron and paraffin taken in volume ratios ranging from 10 : 90 to 80 : 20 have been studied. It has been shown that the shape of the powder particles depends substantially on the amount of Fe in the mixture. At an Fe content of 30 vol %, the particles acquire a platelet shape; at an Fe content of more than 40 vol %, the agglomeration of the particles is observed. At an Fe content above 30 vol %, the Fe₃C phase is formed in the particles, which leads to an increase in the coercive force and a decrease in the specific saturation magnetization of the powder. The frequency dependences of the microwave magnetic and dielectric permeabilities of the composite materials containing the prepared powders have been measured, and the frequency dependences of the intrinsic magnetic permeability of these powders have been determined. It has been shown that the static magnitude of the intrinsic magnetic permeability reaches a maximum in powders containing 30 vol % Fe in the initial mixture.     

Anomalous magnetism and electron paramagnetic resonance spectroscopy of the ZrNi1−xCrxSn solid solution

The static magnetic properties and electron paramagnetic resonance (EPR) spectra of ZrNi_1−xCr_xSn solid solution (0<x≤0.4) have been investigated in the temperature range 140–300 K. Materials were prepared by arc-melting of the pure metals. It was found that ferromagnetic ordering occurs in the samples with small chromium concentration. EPR spectra of the materials with x=0.05, 0.1, 0.2 consist of three absorption lines. The I and II lines are characterised by g=1.99±0.01, ΔH_pp=(120±5) G type and g=1.980±0.001, peak-to-peak width ΔH_pp=(10±1) G, respectively. They have been attributed to Cr³⁺ ions in Ni-sites of the lattice coupled by magnetic dipolar interaction (type I) and to exchange coupled Cr³⁺ pairs or clusters of more than two Cr³⁺ ions (type II). The third line detected in the samples with x=0.3, 0.4 characterised by g_eff=2.0003±0.0001 and ΔH_pp=(3.0±0.5) G has been interpreted as conduction electron spin resonance (CESR).     

Grain boundary defects-mediated room temperature ferromagnetism in Co-doped ZnO film

Zn_0.97Co_0.03O films with and without ZnO buffer layer have been fabricated by magnetron sputtering to investigate the role of grain boundary defects in ferromagnetic ordering in this system. The deposited wurtzite films with (0 0 2) preferred orientation all show intrinsic room temperature ferromagnetism based on the substitutional behavior of Co²⁺. We found that the ZnO/Co:ZnO film grows in smaller grain size, compared with Co:ZnO film, which leads to the increase in grain boundary defects. Meanwhile the increase in oxygen vacancies is confirmed by Co K-edge X-ray-absorption near-edge spectra and the enhancement of saturated magnetization is observed in ZnO/Co:ZnO film. Hence the most important factor for mediating ferromagnetism is proposed to be grain boundary defects, i.e., oxygen vacancies. Bound magnetic polaron mechanism is adopted to explain the intrinsic origin and the mediative effects of grain boundary defects on ferromagnetism in Co-doped ZnO films.