Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

Strongly interacting superspins in Fe3O4 nanoparticles

In this paper we report structural and magnetic properties of Fe₃O₄ nanoparticles synthesized by thermal decomposition of ball milled iron nitrate and citric acid in N₂ and air ambient. The XRD pattern of samples which are prepared in air shows some impurity phases, while the samples synthesized in the N₂ atmosphere are almost pure Fe₃O₄ phase. The result shows that by increasing the particle size, the magnetization of the samples increases. The increase of magnetization by increasing the particle size could be attributed to the lower surface spin canting and surface spin disorder of the larger magnetic nanoparticles. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel -Brown model for superparamagnetic relaxation, but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass like phase in these samples.     

Non-equilibrium effects in the magnetic behavior of Co3O4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co₃O₄ nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co₃O₄ nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.     

Crystal and magnetic structure of Ho2NiGe6

The crystal and magnetic structure of Ho₂NiGe₆ was studied by powder neutron diffraction. The paramagnetic neutron diffraction data confirmed the Ce₂CuGe₆-type crystal structure reported earlier for this compound. Below the Néel temperature equal to 11 K the Ho magnetic moments form a uniaxial antiferromagnetic ordering. The Ho magnetic moments equal to 8.16(7)μ_B at 1.5 K are parallel to the b-axis. The data are compared with those published for HoNi_0.46(6)Ge₂.     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

High heat generation ability in AC magnetic field for nano-sized magnetic Y3Fe5O12 powder prepared by bead milling

Nano-sized magnetic Y₃Fe₅O₁₂ ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9 μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y₃Fe₅O₁₂ ferrites. The highest heat ability in the AC magnetic field was for the fine Y₃Fe₅O₁₂ powder with a 15-nm crystallite size (the samples were milled for 4 h using 0.1 mm? beads). The heat generation ability of the excessively milled Y₃Fe₅O₁₂ samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4 h using 0.1 mm? beads, the heat generation ability (W g⁻¹) was estimated using a 3.58×10⁻⁴ fH² frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which is the highest reported value of superparamagnetic materials.     

Neutron diffraction studies of Tb2Rh3Si5 compound

In this work neutron diffraction studies of Tb₂Rh₃Si₅ compound are reported. The compound crystallizes in the monoclinic crystal structure of Lu₂Co₃Si₅-type. At 1.5 K an antiferromagnetic ordering with a propagation vector k=(1/2;1/2;1/2) was observed. The Tb magnetic moments of 9.8(2) μ_B form a non-collinear magnetic structure. In the vicinity of Néel temperature of 8 K a change of the magnetic ordering is evidenced. The change seems to be connected with phase transition from commensurate to incommensurate sine-wave modulation of the Tb magnetic moments.     

Study on dielectric and magnetic properties of Ho3Fe5O12 ceramics

Ho₃Fe₅O₁₂ ceramics with garnet structure were prepared by the solid-state reaction method. The results revealed the existence of Fe²⁺ ions have intensive influence on dielectric and magnetic properties of Ho₃Fe₅O₁₂ ceramics, which could be further confirmed by oxygen treatment. With a magnetic field lower than 10 kOe, the ME coefficient reaches 33 ps m⁻¹ at room temperature. And the ME coupling was further verified by dielectric anomaly near Néel temperature.     

Influence of organic precursor on the structural and magnetic properties of Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles were synthesized by the coprecipitation method. With the addition of the sucrose as chelating agent (sucrose) the size of the particles was reduced from 54 nm to 19 nm. The Co₃O₄ nanoparticles exhibit a cubic spinel structure identified for X-ray diffraction (XRD) and confirmed by Rietveld refinement. Scanning Electron Microscopy (SEM) images exhibit a spherical-like morphology and confirm the decrease of the particle size observed by XRD. The magnetic measurements as a function of temperature using a superconducting quantum interference device (SQUID) show a large surface anisotropy for samples obtained with the addition of sucrose accompanied by an exchange Bias effect indicating also the existence of a weak ferromagnetism. A decrease of the Néel temperature from the bulk (and other nanostructures-type) was observed, which can be associated with finite-size effect in the nanoparticles' shape.     

Biosynthesis and magnetic properties of mesoporous Fe3O4 composites

Magnetic Fe₃O₄ materials with mesoporous structure are synthesized by co-precipitation method using yeast cells as a template. The X-ray diffraction (XRD) pattern indicates that the as-synthesized mesoporous hybrid Fe₃O₄ is well crystallized. The Barrett-Joyner-Halenda (BJH) models reveal the existence of mesostructure in the dried sample which has a specific surface area of 96.31 m²/g and a pore size distribution of 8-14 nm. Transmission electron microscopy (TEM) measurements confirm the wormhole-like structure of the resulting samples. The composition and chemical bonds of the Fe₃O₄/cells composites are studied by Fourier transform infrared (FT-IR) spectroscopy. Preliminary magnetic properties of the mesoporous hybrid Fe₃O₄ are characterized by a vibrating sample magnetometer (VSM). The magnetic Fe₃O₄/cells composites with mesoporous structure have potential applications in biomedical areas, such as drug delivery.     

Neutron diffraction study on composite compound Nd2Co7

The crystallographic and the magnetic structures of the composite compound Nd₂Co₇ at 300 K are investigated by a combined refinement of X-ray diffraction data and high-resolution neutron diffraction data. The compound crystallizes into a hexagonal Ce₂Ni₇-type structure and consists of alternately stacking MgZn₂-type NdCo₂ and CaCu₅-type NdCo₅ structural blocks along the c axis. A magnetic structure model with the moments of all atoms aligning along the c axis provides a satisfactory fitting to the neutron diffraction data and coincides with the easy magnetization direction revealed by the X-ray diffraction experiments on magnetically pre-aligned fine particles. The refinement results show that the derived atomic moments of the Co atoms vary in a range of 0.7 μ_B-1.1 μ_B and the atomic moment of Nd in the NdCo₅ slab is close to the theoretical moment of a free trivalent Nd³⁺ ion, whereas the atomic moment of Nd in the NdCo₂ slab is much smaller than the theoretical value for a free Nd³⁺ ion. The remarkable difference in the atomic moment of Nd atoms between different structural slabs at room temperature is explained in terms of the magnetic characteristics of the NdCo₂ and NdCo₅ compounds and the local chemical environments of the Nd atoms in different structural slabs of the Nd₂Co₇ compound.     

Magnetoelastic properties of Nd6Fe13Cu intermetallic compound

Magnetoelastic properties of Nd₆Fe₁₃Cu intermetallic compound are reported. To study the magnetoelastic behaviour of this compound, the thermal expansion as well as the longitudinal (λ_l) and transverse (λ_t) magnetostriction were measured by using the strain gauge method in the selected temperature range of 80-500 K under applied magnetic fields up to 1.5 T. An anomaly and invar-type effects are observed in the linear thermal expansion and α(T) curves at the Néel temperature. The linear spontaneous magnetostriction decreases sharply by approaching the Néel temperature and also shows the short-range magnetic ordering effects when antiferromagnetic-paramagnetic transition occurs. In the low field region, the absolute values of the anisotropic magnetostriction are small and then start to increase with applied magnetic field. Each isofield curve of the anisotropic magnetostriction passes through a minimum and then approaches to zero with increasing temperature. This magnetostriction compensation arises from the difference in the magnetoelastic coupling constants of the sublattices in this compound.     

焙烧温度对SiO2负载的Co3O4纳米粒子表面氧缺陷浓度和CO氧化催化性能的影响

以传统的浸渍法,在不同焙烧温度下制备了用于CO氧化反应的Co₃O₄/SiO₂催化剂．通过激光拉曼光谱(Raman)、X射线光电子能谱(XPS)、X射线衍射(XRD)、程序升温还原(TPR)和X射线吸收精细结构谱(XAFS)表征了该系列催化剂的结构．在所有的催化剂中,XRD和Raman光谱都只检测到了Co₃O₄晶相的存在．与Co₃O₄体相相比,XPS结果表明在200 oC焙烧的(Co₃O₄(200)/SiO₂)催化剂中Co₃O₄表面上存在着过量的Co²⁺．与XPS的结果一致,TPR结果表明Co₃O₄(200)/SiO₂催化剂中Co₃O₄表面上存在氧缺陷, 并且XAFS结果也表明Co₃O₄(200)/SiO₂催化剂中Co₃O₄具有更多的Co²⁺．提高焙烧温度使得过量的Co²⁺进一步氧化为Co³⁺,同时降低了表面氧缺陷浓度,从而得到计量比的Co₃O₄4/SiO₂催化剂．在所有的负载催化剂中Co₃O₄(200)/SiO₂催化剂表现出了最好的CO氧化催化性能,表明过量Co²⁺和表面氧缺陷的存在能够促进Co₃O₄催化CO氧化反应的活性.     

Ground-state magnetism of chromium-substituted LiMn2O4 spinel

Comparative crystal structure and magnetic properties studies have been conducted on quaternary powder spinel samples LiMn_1.82Cr_0.18O₄ obtained by two different synthesis methods, glycine-nitrate (GN) and ultrasonic spray-pyrolysis (SP). Although both samples possess the same spinel structure of the cubic space group Fd3¯m, their low-temperature magnetic properties display significant differences. While the SP sample undergoes only spin-glass transition at the freezing temperature T_f=20 K, the GN sample possesses more complicated low-temperature magnetic behavior of the reentrant spin-glass type with the Néel temperature T_N=42 K and freezing temperature T_f=22 K. High-temperature magnetic susceptibility of both samples is of the Curie–Weiss type with the effective magnetic moments in agreement with the nominal compositions. This fact together with the results of the chemical analysis discards the existence of the diversity in chemical compositions as a possible cause for the observed differences in the low-temperature magnetism. On the other hand, the crystal structure analysis done by the Rietveld refinement of the X-ray powder diffraction data points to the strong influence of the cation distribution on the ground-state magnetism of these systems. An explanation of this influence is proposed within the framework of a collective Jahn–Teller effect.     

Magnetic structure of TbNiAl4

Magnetisation and specific heat measurements, in the range 2 K to room temperature, demonstrate that three magnetic phases exist for the intermetallic compound TbNiAl₄. Powder neutron diffraction, also carried out over a wide temperature range, establishes that the intermediate magnetic phase is incommensurate, and confirms that the lowest temperature phase has a linear antiferromagnetic structure with a (0 1 0) propagation vector. The respective transition (Néel) temperatures, in zero applied magnetic field are 34.0 and 28.0 K.     

Study of the quasi two-dimensional CoxNi1-xTa2O6 compounds by X-ray diffraction and magnetic susceptibility measurements

We report results on the structural and magnetic properties of the Co_xNi_1−xTa₂O₆ series of compounds by X-ray powder diffraction, magnetic susceptibility and magnetization measurements. X-ray refinements carried out by the Rietveld method show that these compounds crystallize in a P4₂/mnm tetragonal structure. Magnetic susceptibility curves show a broadened maximum witnessing that these compounds exhibit two-dimensional antiferromagnetic behaviors. All the Co_xNi_1−xTa₂O₆ compounds order below 10 K and present a large ion anisotropy. The magnetic properties have been determined in both the paramagnetic and antiferromagnetic state. In the hypothesis of two dimensional AF ordering, the near neighbor exchange constants (J₁) and the next near neighbor exchange constants for two different paths (J₂ and J'₂) were determined. The composition dependence of the magnetic properties including ordering temperature, exchange constants and anisotropy factors are discussed. The drastic reduction of the ordering temperature for x=0.20 for Co_xNi_1−xTa₂O₆, suggest the hypothesis of a peculiar magnetic behavior for this composition.     

Neutron diffraction study of the (1-x)αFe2O3xAl2O3 system

The magnetic structure of the (1-x) α Fe₂O₃xAl₂O₃ system with x= 0–0.1 has been investigated on polycrystalline samples by neutron diffraction method. The Morin transition temperature and the Néel temperature are observed to decrease on increasing x. The angle by which the magnetic moments turn out of the basal plane in the Morin transition also decreases with increasing x. The Morin transition does not occur above x = 0.09.     

Structural and magnetic properties of Sc1.1Fe3.9Al8 alloy

The crystallochemical and magnetic nature of ternary Sc_1+δFe_4−δAl₈ intermetallic with a small Sc excess δ=0.1 was investigated by scanning electron microscope, X-ray powder diffraction, neutron diffraction, Mössbauer effect and superconducting quantum interference device techniques. The sample crystallizes in a tetragonal ThMn₁₂ type structure. The excess of Sc atoms substitute Fe at the (8f) positions and have a pronounced effect on the magnetic properties. The experiments carried out in temperature range 4-320 K show that below 120 K the magnetic structure of the alloy forms a double cycloid with magnetic moments rotating according to the incommensurate in-plane wave vector, which is temperature independent up to 160 K. The value of Fe magnetic moment is close to 0.9 μ_B atom⁻¹ at 4 K. Temperature dependence of unit cell dimensions can be explained within the Debye-Grüneisen approximation.     

Magnetic and transport properties of SmCoAsO1−xFx

A series of SmCoAsO_1−xF_x (with x=0, 0.05, 0.1, and 0.2) samples have been prepared by solid state reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has been doped into the lattice. The magnetic and transport properties of the samples have been investigated. Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism, ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Néel temperatures were almost independent of the F content and metamagnetic transitions were observed below antiferromagnetic Néel temperatures. With increasing F content, high temperature (below 142 K) ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction in the region of 2-300 K and Fermi liquid behavior at low temperatures were shown in all samples. Transport properties at applied magnetic fields showed anomalies at low temperatures.     

Comparison of magnetic properties in LixCoO2 and its decomposition products LiCo2O4 and Co3O4

Thermal stability of cathode material in the charged state is an important aspect for the safety of rechargeable batteries. It is well known that layered Li_xCoO₂ decomposes to a mixture of LiCoO₂ and Co₃O₄ at elevated temperatures. However, not many experimental evidences exist on intermediate phases those may form during the decomposition. Using magnetic measurements we show that it is possible to distinguish between the spinels LiCo₂O₄ and Co₃O₄ and thereby follow the decomposition of Li_xCoO₂. We characterize the magnetic behavior of thermally aged Li_xCoO₂ (x = 0.98, 0.76, 0.55) with increasing annealing time. Our results reveal the appearance of magnetic ordering in the thermally degraded products. The detailed analysis illustrates that the formation of Co₃O₄ is preceded by the formation of a meta stable LiCo₂O₄ phase.