Magnetic domain and local atomic structures of the Fe<Subscript>0.94</Subscript>Si<Subscript>0.06</Subscript> alloy before and after thermomagnetic treatment in an alternating-current magnetic field

The changes observed in the local atomic structure of an iron alloy with 6 at % Si as a result of thermomagnetic treatment in an alternating-current magnetic field and their correlation with magnetic characteristics, such as the domain structure and magnetic properties, have been investigated using Mössbauer spectroscopy. It has been shown that the destabilization of the domain structure is caused by the destruction of uniaxial magnetic anisotropy in domains due to a disordered distribution of pairs of silicon atoms along the 〈100〉 easy magnetization axes. Annealing and cooling without an external magnetic field and in an alternating-current magnetic field lead to a change in the magnetic texture of single-crystal samples of the siliconiron alloy. When the samples are annealed in an alternating-current magnetic field, the magnitude of this effect depends on the orientation of the magnetic field with respect to the crystallographic axes of the sample.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Magnetic resonance of the RbMnBr<Subscript>3</Subscript> antiferromagnet under pressure

The antiferromagnetic resonance spectrum was experimentally studied for a noncollinear RbMnBr₃ antiferromagnet under a mechanical pressure applied to the sample in various directions. It is shown that the incommensurate magnetic phase existing in the initial system in the presence of regular crystallographic distortions is sensitive to the pressure applied along one of the sample axes. The critical transition field to the commensurate phase decreased under pressure. It was also found that the pressure influences the uniaxial anisotropy appearing in the crystal basal plane in the presence of orthorhombic distortions. These effects were analyzed with allowance for the domain structure of the sample.     

Quantum oscillations in dual-layered quasi-two-dimensional organic metal (ET)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>4</Subscript>Cl<Subscript>2</Subscript>)

We study the effect of interionic anisotropy on the phase states of a non-Heisenberg ferromagnet with magnetic ion spin S = 1. It is shown that depending on the relation between the interionic anisotropy constants, uniaxial and angular ferromagnetic and nonmagnetic phases exist in the system. We analyze the dynamic properties of the system in the vicinity of orientational phase transitions, as well as a phase transition in the magnetic moment magnitude. It is shown that orientational phase transitions in ferromagnetic and nematic phases can be first- as well as second-order.     

Synthesis and investigation of precipitated ensembles of magnetic nanoparticles with controlled easy axis orientation

Two samples with immobilized magnetic nanoparticles are synthesized and investigated. The first sample has randomly oriented easy axes of magnetic anisotropy; the second sample has preferentially aligned easy axes, produced via the precipitation of a colloid of ferromagnetic particles in the presence of a magnetic field. It is shown that the precipitation of an aqueous suspension of nanoparticles in the presence of a magnetic field greatly changes the anisotropy of a sample, compared to one prepared by precipitation without a magnetic field. The second sample exhibits a preferential direction of the easy axes of magnetic anisotropy that coincides with the direction of the external magnetic field applied in the drying process of sample preparation.     

Controlled-synthesis,characterization, and magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanostructures

Fe₃O₄ nanostructures with different morphologies, including uniform nanoparticles, nanorods and nanowire bundles, have been successfully synthesized via a facile hydrothermal route. Based on the observation of TEM images, the growth mechanism of one-dimensional Fe₃O₄ nanostructures is in accordance with Ostwald ripening process. From the hysteresis loops of as-prepared Fe₃O₄ products, we found that the morphology has great influence on the magnetic properties. The uniform Fe₃O₄ nanoparticles have higher saturation magnetization and lower coercivity than that of Fe₃O₄ nanorods and nanowires bundles. These phenomena attribute to the high shape anisotropy of nanorods and nanowire bundles, which prevent them from magnetizing in directions other than along their easy magnetic axes. PACS 81.07.-b; 75.50.Bb; 75.30.Gw; 81.10.Dn; 81.16.Be     

Anisotropy of the heat capacity of a mixed-state superconducting Nd<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> single crystal for various magnetic field orientations with respect to the crystallographic axes

The anisotropy in the superconducting properties of single-crystal Nd_1.85Ce_0.15CuO₄ was studied from measurements of the heat capacity within the temperature interval 2–40 K in zero magnetic field and in a magnetic field of 8 T. We report on the first observation of heat capacity jumps occurring at the superconducting transition for various magnetic field orientations with respect to the crystallographic axes and on a strong anisotropy of the magnetic contribution to heat capacity in magnetic fields oriented in the a-b plane and perpendicular to it. These measurements yielded the anisotropy in the electronic heat capacity coefficient γ_n(H) and in the superconducting transition temperature T_c(H). The angular dependence of the Sommerfeld coefficient γ_n in the a-b plane observed in a magnetic field of 8 T exhibits four-lobe symmetry and zero gap direction of the order parameter. A comparison of the results obtained on the Nd_1.85Ce_0.15CuO₄ single crystal with the data available for La_1.85Sr_0.15CuO₄ permits one to conclude that the mechanisms of superconductivity in the electron-and hole-doped superconductors are similar.     

Theoretical prediction of a surface-induced spin-reorientation phase transition in BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanocrystals

The equation of the magnetization of a hexagonal crystal is derived for the first time for an arbitrary orientation of the external magnetic field relative to the crystallographic c axis. In order to clarify the magnetization mechanism for a real ensemble of small particles in the framework of the given problem, surface anisotropy (which is significant for nanosize objects) was taken into account along with crystalline magnetic anisotropy and anisotropy in the particle shape. Model computer experiments prove that the magnetization curves for nanocrystals oriented in a polar angle range of 65–90° exhibit an anomaly in the form of a jump, indicating a first-order spin-reorientation phase transition. This explains a larger steepness of the experimental curve reconstructed taking into account the interaction between particles as compared to the theoretical dependence obtained by Stoner and Wohlfarth [IEEE Trans. Magn. MAG 27 (4), 3469 (1991)]. An analysis of variation of the characteristic anisotropy surface and its cross section with increasing ratio |K₂|/K₁ of the crystalline magnetic anisotropy constants upon a transition from a macroscopic to a nanoscopic crystal shows that surface anisotropy leads to a change in the magnetic structure. As a result, an additional easy magnetization direction emerges in the basal plane apart from the easiest magnetization direction (along the c axis). The direction of hard magnetization emerges from the basal plane, the angle of its orientation relative to the c axis being a function of the ratio | K₂|/K₁.     

Influence of the inhomogeneity of local magnetic parameters on the curves of magnetization in an ensemble of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

Methods have been proposed and tested for analyzing local magnetic parameters in a system of single-domain ferromagnetic nanoparticles using their magnetization curves. The magnetic inhomogeneity in ensembles of Fe₃C nanoparticles encapsulated in carbon nanotubes has been investigated. It has been established that the Fe₃C nanoparticles encapsulated in carbon nanotubes are characterized by two-modal distribution functions of the local magnetic anisotropy energy. The particle distribution over the blocking temperature is reconstructed from the experimental temperature dependence of the coercive force. The allowance made for the inhomogeneity of the local magnetic parameters of the Fe₃C nanoparticles, which were studied by the proposed methods, explains the discrepancy between the magnetic anisotropy energy determined by the method of the magnetization approaching saturation and the magnetic anisotropy energy estimated from the coercive force of single-domain nanoparticles.     

Structural and magnetic properties of size-controlled Mn<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles and magnetic fluids

Mn_0.5Zn_0.5Fe₂O₄ ferrite nanoparticles with tunable Curie temperature and saturation magnetization are synthesized using hydrothermal co-precipitation method. Particle size is controlled in the range of 54 to 135 Å by pH and incubation time of the reaction. All the particles exhibit super-paramagnetic behaviour at room temperature. Langevin’s theory incorporating the interparticle interaction was used to fit the virgin curve of particle magnetization. The low-temperature magnetization follows Bloch spin wave theory. Curie temperature derived from magnetic thermogravimetric analysis shows that Curie temperature increases with increasing particle size. Using these particles magnetic fluid is synthesized and magnetic characterization is reported. The monolayer coating of surfactant on particle surface is confirmed using thermogravimetric measurement. The same technique can be extended to study the magnetic phase transition. The Curie temperature derived using this measurement complies with the low-temperature magnetic measurement. The room-temperature and high-temperature magnetization measurements are also studied for magnetic fluid systems. The magnetic parameters derived for fluid are in good agreement with those obtained for the particle system.     

Novel NdCo<Subscript>5</Subscript> nanoflakes and nanoparticles produced by surfactant-assisted high-energy ball milling

High-energy ball milling has been shown to be a promising method for the fabrication of rare earth—transition metal nanopowders. In this work, NdCo₅ nanoflakes and nanoparticles have been produced by a two-stage high-energy ball milling (HEBM), by first using wet HEBM to prepare precursor nanocrystalline powders followed by surfactant-assisted HEBM. NdCo₅ flakes have a thickness below 150 nm and an aspect ratio as high as 10²–10³; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes exhibited high coercivities at low temperatures, with values at 50 K of 3 and 3.7 kOe, respectively. The high values of coercivity can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior in contrast to the planar anisotropy of the bulk samples. Angle-dependent magnetization measurements at different temperatures were used to determine the spin reorientation transitions in the nanopowders and nanoparticles. The nanoparticles showed spin reorientation temperatures, T _SR1 = 276 and T _SR2 = 237 K which are lower when compared with the values of 290 and 245 K, respectively for bulk.     

Phase separation effects in charge-ordered Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> thin film

Compressed Pr_0.5Ca_0.5MnO₃ films (250 nm) deposited on LaAlO₃ have been studied by Electron Spin Resonance technique under high frequency and high magnetic field. We show evidences for the presence of a ferromagnetic phase (FM) embedded in the charge-order phase (CO), in form of thin layers which size depends on the strength and orientation of the magnetic field (parallel or perpendicular to the substrate plane). This FM phase presents an easy plane magnetic anisotropy with an anisotropy constant 100 times bigger than typical bulk values. When the magnetic field is applied perpendicular to the substrate plane, the FM phase is strongly coupled to the CO phase whereas for the parallel orientation it keeps an independent ferromagnetic resonance even when the CO phase becomes antiferromagnetic.     

α-Fe_2O_3纳米粒子Morin相变的Raman光谱研究

本文首次用Raman光谱法研究了粒子尺寸和Co2 +包附对α-Fe2 O3纳米粒子Morin相变温度TM的影响机制。测量结果显示纳米粒子α -Fe2 O3的振动模相对大块样品发生了红移和宽化 ,粒子愈小红移和宽化愈显著。表明表面Fe3+离子与配位氧离子的键距增大 ,键长呈从体相Fe-O键长过渡到表面Fe -O键长的某种分布。这种小尺寸引起的键长改变导致α -Fe2 O3粒子的单离子各向异性能减小 ,因此降低了样品的TM。分析显示Co2 +包附导致α -Fe2 O3粒子TM 的大幅下降可能是由单离子各向异性的减小和Co2 +与Fe3+之间的磁偶极子相互作用各向异性共同作用的结果     

Superspin-glass like behavior of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> obtained by mechanochemical milling

Single-phase perovskite compound La_0.7Ca_0.3MnO₃ was synthesised by a high-energy ball milling in a single step processing. Structure and morphology characterizations revealed nanoparticle nature of this mixed valent manganite with the average particle diameter of 9 nm. Comprehensive set of magnetic measurements showed that the system can be described as an ensemble of interacting magnetic nanoparticles where each particle possesses high magnetic moment, i.e., superspin. Furthermore, magnetic behavior showed contributions from both superspin-glass (SSG) and superparamagnetic (SP) states, and the prevailing properties depended on the experimental conditions. It was established that SSG state dominated in low magnetic fields up to 500 Oe while in higher applied fields suppression of collective behavior occurred and individual characteristics of nanoparticles prevailed. It was also concluded that the applied method of synthesis produced system with high magnetic anisotropy as well as with the large nanoparticle shell whose thickness amounts 30% of a particle diameter.     

Dipolar anisotropy in quasi-2D honeycomb antiferromagnet MnPS<Subscript>3</Subscript>

The nature of the anisotropy in magnetic systems which show isotropic Heisenberg exchange is crucial in determining their magnetic properties. This is particularly true in low-dimensional systems in which the very existence of long-range order depends on the anisotropy. The honeycomb lattice MnPS₃ system has been studied as an example of a magnetically quasi-two-dimensional system of unusual symmetry. In this paper the effect of the dipole-dipole interaction in MnPS₃ on the magnetic ordering is explored through modelling. It is found that the dipolar anisotropy can explain the spin directions both in zero field and above the spin flop phase transition, but it is important that real rather than idealised atomic coordinates are used; this latter consideration is significant because in performing theoretical calculations, it may sometimes be assumed that small deviations away from the ideal can be ignored, but in truth they determine key aspects of the behaviour.     

A sonochemical-assisted synthesis and annealing temperature effect of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> nanoparticles

This paper reports on the sonochemical-assisted synthesis of La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles (NPs) which have a single-crystalline perovskite structure. The average particle size of LSMO NPs was controlled from about 40 to 120 nm by changing the annealing temperatures from 750 to 1050°C. The particle size, electrical resistivity, and ferromagnetic transition temperature of LSMO NPs were strongly dependent on the annealing temperature. A substantial decrease in resistivity and an enhancement in the insulator–metal transition temperature were found on increasing the annealing temperature. Furthermore, the enhancement in magnetization and paramagnetic–ferromagnetic (PM–FM) transition temperatures was observed as the annealing temperature increases.     

The magnetic and hyperthermia studies of bare and silica-coated La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> nanoparticles

The magnetic nanoparticles of La_0.75Sr_0.25MnO₃ perovskite manganite with a controlled size were prepared via sol–gel procedure, followed by thermal treatment and subsequent mechanical processing of the resulting raw product. The prepared materials were structurally studied by the XRD and TEM methods and probed by DC magnetic measurements. The nanoparticles of the mean crystallite sizes 11–40 nm exhibit T _C in the range of ≈310–347 K and the sample possessing 20-nm crystallites was identified as the most suitable for hyperthermia experiments. In order to obtain a colloidally stable suspension and prevent toxic effects, the selected magnetic cores were further encapsulated into silica shell using tetraethoxysilane. The detailed magnetic studies were focused on the comparison of the raw product, the bare nanoparticles after mechanical processing and the silica-coated nanoparticles, dealing also with effects of size distribution and magnetic interactions. The heating experiments were carried out in an AC field of frequencies 100 kHz–1 MHz and amplitude 3.0–8.9 kA m⁻¹ on water dispersions of the samples, and the generated heat was deduced from their warming rate taking into account experimentally determined thermal losses into surroundings. The experiments demonstrate that the heating efficiency of the coated nanoparticles is generally higher than that of the bare magnetic cores. It is also shown that the aggregation of the bare nanoparticles increases heating efficiency at least in a certain concentration range.     

Synthesis of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles with tunable magnetism by the modified hydrothermal method

Cobalt ferrite (CoFe₂O₄) nanoparticles were synthesized by using the hydrothermal route with the addition of trisodium citrate dihydrate (Na₃CA·2H₂O). The formation of CoFe₂O₄ nanoparticles with size ranging from 13 to 19 nm was confirmed by X-ray diffraction, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy; the clear-cut sharp of the nanoparticles was observed by transmission electron microscopy. By these characterization methods, the evolution of lattice constant and morphologies of the nanoparticles with the addition of Na₃CA·2H₂O is observed. Furthermore, the magnetic hysteresis loops measured at room temperature indicate that the magnetic properties of the products also show clear relationship with the masses of Na₃CA·2H₂O. For example, coercivity and high-field paramagnetic susceptibility increase with the increasing masses of Na₃CA·2H₂O, whereas the saturation magnetization and the effective magnetic anisotropy constant have the maximum values as the mass of Na₃CA·2H₂O is 1 g. This change of magnetic properties is related with the expanded lattice and the varied size and shape because of the addition of Na₃CA·2H₂O.     

La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin films on SrTiO<Subscript>3</Subscript> and CaTiO<Subscript>3</Subscript> buffered Si substrates: structural,static, and dynamic magnetic properties

Nearly 50-nm thick La_0.7Sr_0.3MnO₃ (LSMO) films were grown on Si substrates using molecular beam epitaxy on (001) Si substrates over-layered by a 20 nm thick SrTiO₃ (STO) or by a 20 nm thick CaTiO₃ (CTO) film. In addition, a reference LSMO film was directly deposited on a (001) STO substrate by pulsed laser deposition. For all the samples, X-ray diffraction revealed an excellent epitaxy of the LSMO film and small mosaicity around (001), with in-plane [100] and [010] cubic axes. The LSMO/CTO films are in-plane compressed while the LSMO/STO ones are in-plane extended. The temperature dependence of their static magnetic properties was studied using a SQUID, showing a Curie temperature overpassing 315 K for all the samples. Hysteresis loops performed at room temperature (294 K) with the help of a vibrating sample magnetometer (VSM) are also discussed. At 294 K Micro-strip ferromagnetic resonance (MS-FMR) was used to investigate the dynamic magnetic properties. It allows concluding to a strong anisotropy perpendicular to the films and to a weak fourfold in-plane anisotropy with easy axes along the [110] and [1[`1]0 1\bar{1}0 ] directions. Their values strongly depend on the studied sample and are presumably related to the strains suffered by the films.