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1.
The pure single phase of Zn0.95 Co0.05 O bulks is successfully prepared by solid-state reaction method. The effects of annealing atmosphere on room-temperature ferromagnetic behaviour for the Zn0.95 Co0.05 O bulks are investigated. The results show that the air-annealed samples has similar weak ferromagnetic behaviour with the as-sintered samples, but the obvious ferromagnetic behaviour is observed for the samples annealed in vacuum or Ar/H2 gas, indicating that the strong ferromagnetism is associated with high oxygen vacancies density. High saturation magnetization Ms = 0.73μg/Co and coercivity Hc = 233.8 Oe are obtained for the Ar/H2 annealed samples with pure single phase structure. The enhanced room-temperature ferromagnetic behaviour is also found in the samples with high carrier concentration controlled by doping interstitials Zn (Zni).  相似文献   

2.
Zero-field-cooled(ZFC) magnetization,field-cooled(FC) magnetization,ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160K.An empirical model is proposed to calculate the measured ZFC magnetization.The result indicates that the calculated ZFC magnetization compares well with the measured one.Based on the generalized Preisach model.both the ZFC and FC curves are reproduced by numerical simulations.The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.  相似文献   

3.
王旭飞  施立群 《中国物理 B》2010,19(10):107502-107502
Based on a lognormal particle size distribution, this paper makes a model analysis on the polydispersity effects on the magnetization behaviour of diluted ferrofluids. Using a modified Langevin relationship for the lognormal dis- persion, it first performs reduced calculations without material parameters. From the results, it is extrapolated that for the ferrofluid of lognormal polydispersion, in comparison with the corresponding monodispersion, the saturation magnetization is enhanced higher by the particle size distribution. It also indicates that in an equivalent magnetic field, the lognormally polydispersed ferrofluid is magnetically saturated faster than the corresponding monodispersion. Along the theoretical extrapolations, the polydispersity effects are evaluated for a typical ferrofluid of magnetite, with a dispersity of σ = 0.20. The results indicate that the lognormal polydispersity leads to a slight increase of the saturation magnetization, but a noticeable increase of the speed to reach the saturation value in an equivalent magnetic field.  相似文献   

4.
钱萍  刘九丽  胡耀文  白丽君  申江 《中国物理 B》2011,20(7):76104-076104
The effects of Fe substitution for Co on the structural stability and the site preference of intermetallics Nd 2 Co 7-x Fe x with a hexagonal Ce 2 Ni 7-type structure are studied by using a series of interatomic pair potentials.In Nd 2 Co 7-x Fe x,Fe atoms are substituted for Co atoms with a strong preference for the 6h sites and the order of site preference is 6h,4e,4f,2a,and 12k.Calculated lattice parameters are found to be consistent with the reported results in the literature.The variation behaviour of the Curie temperature of Nd 2 Co 7-x Fe x is explained qualitatively by the exchange interaction model.The properties related to lattice vibration,such as phonon density of states and Debye temperature,are first evaluated for the Nd 2 Co 7-x Fe x compounds.  相似文献   

5.
An iron film percolation system is fabricated by vapour-phase deposition on fracture surfaces of α-Al2O3 ceramics. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurement reveals that the magnetic phase of the film samples evolve from a high-temperature ferromagnetic state to a low-temperature spin-glass-like state, which is also demonstrated by the temperature-dependent ac susceptibility of the iron films. The temperature dependence of the exchange bias field He of the iron film exhibits a minimum peak around the temperature T=5 K, which is independent of the magnitude of the cooling field Hcf. However, for T 〉 10K, (1) He is always negative when Hcf=2kOe and (2) for Hcf= 20 kOe (1Oe≈80 A/m), He changes from negative to positive values as T increases. Our experimental results show that the anomalous hysteresis properties mainly result from the oxide surfaces of the films with spin-glass-like phase.  相似文献   

6.
Based on Monte Carlo method, the oscillatory behaviour of the average magnetic moment as a function of the cluster sizes and the temperature dependences of magnetic moment with different sizes have been studied. It is found that the oscillations superimposed on the decreasing moment are associated with not only the geometrical structure effects but also the thermal fluctuation. The hystereses and thermal coercivities for free clusters with zero and finite uniaxial anisotropies have been calculated. The simulated thermal dependence of the coercivity is consistent with the experimental result, but does not fit the Tα law in the whole temperature range. It is evident that an easy magnetization direction and an anisotropy resulting from the spin configurations exist in the free clusters with the pure exchange interaction, which is also proved by the natural angle and energy distribution of clusters. A systematic theoretical analysis is also made to establish the relationship between natural angle and coercivity.  相似文献   

7.
Structural relaxation through isothermal annealing at temperature below glass transition is conducted on Zr46.75 Ti8.25 CU7.5Ni11Be27.5 (Vitreloy-4) bulk metallic glass. Defect concentration is correlated with the annealing time t according to differential scanning calorimetry thermalgrams. The effects of structural relaxation on mechanical properties and deformation behaviour are investigated by using instrumented nanoindentation. It is found that the as-cast alloy exhibits pronounced serration flow during the loading process of nanoindentation, and the size and number of the serrations decrease with the annealing time. The change of the deformation behaviour with structural relaxation is explained using a free volume model.  相似文献   

8.
In this paper, the preparation of 0.08BiGaO_3–0.90BaTiO_3–0.02LiNbO_3 is investigated at pressure 3.8 GPa and temperature 1100–1200?C. Experimental results indicate that not only is the sintered rate more effective, but also the sintered temperature is lower under high pressure and high temperature than those of under normal pressure. It is thought that the adscititious pressure plays the key role in this process, which is discussed in detail. The composition and the structure of the as-prepared samples are recorded by XRD patterns. The result shows that the phases of Ba TiO_3, BaBiO_(2.77), and Ba_2Bi_4Ti_5O_(18) with piezoelectric ceramic performance generate in the sintered samples. Furthermore, the surface morphology characteristics of the typical samples are also investigated using a scanning electron microscope. It indicates that the grain size and surface structure of the samples are closely related to the sintering temperature and sintering time. It is hoped that this study can provide a new train of thought for the preparation of lead-free piezoelectric ceramics with excellent performance.  相似文献   

9.
Powder mixtures of Zn, Ni O, and Fe2O3 are mechanically alloyed by high energy ball milling to produce Ni–Zn ferrite with a nominal composition of Ni0.36Zn0.64Fe2O4. The effects of milling atmospheres(argon, air, and oxygen),milling time(from 0 to 30 h) and heat treatment are studied. The products are characterized using x-ray diffractometry,field emission scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy, and transmitted electron microscopy. The results indicate that the desired ferrite is not produced during the milling in the samples milled under either air or oxygen atmospheres. In those samples milled under argon, however, Zn/Ni O/Fe2O3 reacts with a solid-state diffusion mode to produce Ni–Zn ferrite nanocrystalline in a size of 8 nm after 30-h-milling. The average crystallite sizes decrease to 9 nm and 10 nm in 30-h-milling samples under air and oxygen atmospheres, respectively. Annealing the 30-h-milling samples at 600°C for 2 h leads to the formation of a single phase of Ni–Zn ferrite, an increase of crystallite size, and a reduction of internal lattice strain. Finally, the effects of the milling atmosphere and heating temperature on the magnetic properties of the 30-h-milling samples are investigated.  相似文献   

10.
Nanometre-sized (hereafter nano-) Pb particles embedded in an Al matrix are prepared by ball milling. It is found that the size of nano-Pb particles was decreased with increasing milling time. The melting behaviour of nano-Pb particles embedded in the Al matrix is studied by means of dynamic mechanical analysis, and a single internal friction peak in the vicinity of Pb melting temperature is observed. The onset temperature of the peak moves to lower temperature with the decrease of particles size and the internal friction peak height is increased, which indicates a size-dependent melting behaviour of nano-Pb particles. It is suggested that the size-dependent melting behaviour is associated with surface melting.  相似文献   

11.
Fe3O4 nanowire arrays with different diameters of D=50, 100, 150 and 200 nm were prepared in anodic aluminum oxide (AAO) templates by an electrodeposition method followed by heat-treating processes. A vibrating sample magnetometer (VSM) and a Quantum Design SQUID MPMS magnetometer were used to investigate the magnetic properties. At room temperature the nanowire arrays change from superparamagnetism to ferromagnetism as the diameter increases from 50 to 200 nm. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the blocking temperature TB increases with the diameter of nanowire. The ZFC curves of D=50 nm nanowire arrays under different applied fields (H) were measured and a power relationship between TB and H were found. The temperature dependence of coercivity below TB was also investigated. Mössbauer spectra and micromagnetic simulation were used to study the micro-magnetic structure of nanowire arrays and the static distribution of magnetic moments of D=200 nm nanowire arrays was investigated. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in nanowires.  相似文献   

12.
The effect of tetravalent Ti+4 substitution in Mg0.95Mn0.05Fe2O4 on its magnetic and electrical properties has been studied using X-ray diffraction, Mössbauer spectroscopy, isothermal dc magnetization and dielectric measurements. X-ray diffraction studies have shown the structural transformation from cubic to tetragonal with the Ti+4 substitution. The Mössbauer spectra of Mg0.95Mn0.05Fe1.0Ti1.0O4 recorded in the temperature range 20-300 K shows the presence of the magnetic as well as quadrupole interactions. The isothermal hysteresis loop infers that the system exhibits a ferrimagnetic ordering at room temperature. The Zero-field-cooled (ZFC) and field-cooled (FC) magnetization studies support ferrimagnetic ordering of Mg0.95Mn0.05Fe1.0Ti1.0O4 at room temperature. Signatures of ferroelectric transition have been observed in the temperature range 200-300 K from dielectric measurements. The observed magnetic and dielectric behaviour indicate that this material exhibits multiferroic behaviour.  相似文献   

13.
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.  相似文献   

14.
Well-dispersed Fe3O4 nanoparticles are synthesized via an oxidization method with NANO2 as oxidant. The microwave magnetic properties of the composites are studied with different volume fractions of fe3O4 nanoparticles. It is found that a lower volume fraction corresponds to a higher magnetic resonance frequency. This could be ascribed to the enhancement of exchange interaction with a weakened dipolar interaction when the volume fraction decreases.  相似文献   

15.
Magnetization of the ZnFe2O4 sample of average size 4 nm measured with SQUID in the temperature range 5–300 K shows anomalous behaviour in field cooled (FC) and zero-field-cooled (ZFC) conditions. The FC and ZFC curves measured in 50 Oe field cross each other a little before the peaks. No such anomaly is observed with samples of 6 nm particle size made with the same procedure. The characteristics of the FC and ZFC curves are very different in ZnFe2O4 samples of the same size (6 nm) made via two different chemical routes. The genesis of these differences are suggested to be in cationic configuration and spin disorder. Fe-extended X-ray absorption fine structure (EXAFS) studies show that there is around 80% inversion in case of zinc ferrite (ZnFe2O4) with the particle size 4 nm, whereas ZnFe2O4 of size 6 nm shows 40% inversion. The samples with an average particle size of 7 nm and more show negligible inversion. Theoretical simulations suggest that the electrostatic energy of the system plays a crucial role in deciding the cationic configuration of spinel ferrites.  相似文献   

16.
The effect of Gd-doping on the charge ordering (CO) state in perovskite-type manganates Bi0.3−xGdxCa0.7MnO3 with x=0, 0.02, 0.05, 0.1, 0.3 has been investigated by transport and magnetic property measurements. It is found that CO temperature (TCO) and antiferromagnetic (AFM) ordering temperature TN occurring below TCO decrease obviously with increasing Gd-doping level. Accompanying the variation of TCO, the increased magnetization and the decreased resistivity are observed. In addition, the increased magnetic inhomogeneity has been also observed in the samples based on the difference between the zero-field-cooling (ZFC) magnetization MZFC and field-cooling (FC) magnetization MFC, which is ascribed to the competition between ferromagnetic (FM) phase induced by Gd-doping and CO AFM phase. The experimental results indicate that the Bi3+ lone pair electron with 6s2 character plays a dominating role on the CO state of Bi0.3Ca0.7MnO3.  相似文献   

17.
Co0.8Fe2.2O4 ferrite thin films have been prepared on Si(0 0 1) substrates by the chemical solution deposition. Structural characteristics indicate all films are single phase with spinel structure and the space group and the mean grain size increases from 8 to 30 nm with the increase of annealing temperature. The magnetic properties of Co0.8Fe2.2O4 thin films are highly dependent on annealing temperature. The sample annealed at 800 °C possesses high saturation magnetization, moderate coercivity and squareness ratio, making it a promising application candidate in high-density record and magneto-optical materials.  相似文献   

18.
SmCo5/Fe65Co35 and SmCo5/Fe spring exchange magnets are fabricated by dc magnetron sputtering on MgO substrates and 100-nm-thick Si3N4 membranes, respectively. The base pressure of sputtering chamber is kept below 10^-7 Tort, and Ar pressure is 3 to 8mTorr. The samples are characterized by an x-ray diffractometer, a superconducting quantum interference magnetometer, and high resolution magnetic soft x-ray microscopy. We obtain the complete exchange coupling and single phase behaviour of composite magnets. The (BH)max value achieved is 28.8 MGOe.  相似文献   

19.
Thin films of (111)-oriented spinel ferrite Al0.5Fe2.5O4 have been prepared by a pulsed-laser deposition (PLD) technique on α-Al2O3 (0001) substrates. The films exhibit cluster-glass behaviors with a spin-freezing temperature, Tg, near or above room temperature. The magnetization was found to increase following light irradiation below Tg, which indicates the photoinduced melting of cluster-glass states. An analysis comparing the dynamic behavior of magnetic response to light irradiation between zero-field-cooled (ZFC) states and field-cooled (FC) states at 10 K under various light intensities, I, revealed that the direct photoexcitation of spins occurs when I≤0.78 mW/mm2, while the thermal heating effect following the light absorption of the samples also contributes to the enhancement of magnetization when I≥1.22 mW/mm2. The magnetization of the films could be controlled by light irradiation even at room temperature. This suggests the possibility of utilizing these films in the development of novel magneto-optical memory devices.  相似文献   

20.
Magnetic properties of zero field cooled (ZFC) and field cooled (FC) sample of (Mn,Fe)2O3−t nanograins have been investigated by magnetometry (up to 70 kOe) and Mössbauer spectroscopy (up to 60 kOe) in the temperature interval 4.2–300 K. Large horizontal (up to 0.8 kOe) and vertical (up to 80%) shifts of the magnetization hysteresis loops are observed in the FC regime. The obtained results are discussed in terms of exchange interaction between an antiferromagnetic core and a spin-glass-like state of the nanograins boundaries. It is shown that hysteresis loop shifts (horizontal and vertical) depend on the field cooling magnitude, an effect that can be understood by the change of the boundary magnetic structure induced by the external magnetic field. The vertical magnetization shift is described by a phenomenological model, which takes into account the magnetic interaction between the spin-glass like boundary spins and the applied field.  相似文献   

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