La-doped BiFeO_3:Synthesis and multiferroic property study

A new sol-foam-gel method was developed to fabricate La-doped BiFeO3multiferroic materials. It was demonstrated that a gradual increase in the content of La-doped into BiFeO3results in its structure changing from rhombohedral to orthorhombic. A study of other property changes indicates that La-doping in BiFeO3enhances its ferromagnetism and ferroelectricity. A temperature-dependent magnetization study suggests that the magnetic property of La-doped BiFeO3samples varied from antiferromagnetic to ferromagnetic as the content of La-doped into BiFeO3increased from 0 to 20%.Unique temperature-dependent zero field cooling(ZFC) and field cooling(FC) magnetization behaviors were observed in 15% La-doped BiFeO3— its ZFC temperature-dependent magnetization being ferromagnetic and its FC temperaturedependent magnetization being antiferromagnetic. A possible mechanism of such an interesting M–T behavior is discussed.     

Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca_3CoNb_2O_9

We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca3 Co Nb2O9, in which the effective spin of Co2+is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant θCW ~-55 K and the low Neel temperature TN~ 1.45 K give a frustration factor f =| θCW/TN|≈ 38, suggesting that Ca3 Co Nb2O9resides in strong frustration regime. Slightly below TN, deviation between the susceptibility data under zero-field cooling(ZFC)and field cooling(FC) is observed. A new magnetic state with 1/3 of the saturate magnetization Ms is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca3 Co Nb2O9is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.     

Zero—field—cooled and field—cooled magnetizations and magnetic susceptibility of itinerant ferromagnet SrRuO3

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.     

Anomalous magnetic properties of an iron film system deposited on fracture surfaces of α-Al2O3 ceramics

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.     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

Investigation on dependence of BiFeO3 dielectric property on oxygen content

The influence of oxygen content on the dielectric property of BiFeO3 ceramics is studied by experiment and firstprinciples calculation.The experimental result demonstrates that the dielectric constant of BiFeO3 is strongly dependent on introduced oxygen and oxygen vacancies.By comparison with BiFeO3,the introduced oxygen and oxygen vacancies can lead to a reduction in dielectric constant of BiFeO δ at a lower frequency.The first-principles calculation also shows a similar result when photon energy is in a range of 2.0-4.1 eV.A likely explanation is that this oxygen content dependence may be ascribed to the distortion of Fe-O octahedron structure due to oxygen vacancies or excess oxygen ions in the crystal structure of BiFeO3.     

Mode-II Crack Problem for a Long Rectangular Slab of Superconductor under an Electromagnetic Force

We present a theoretical analysis to the fracture parameters of the large single domain YBCO superconductor with a tangential line crack under electromagnetic force. The mode-II fracture parameters are obtained due to coupled finite element and infinite element method, and the numerical results are conducted for two activation processes. For a zero-field cooling （ZFC） magnetization process, in the process of magnetic field descent, the larger the applied field is, the larger the stress intensity factors. In the case of field cooling （FC） magnetization process, the stress intensity factors have obvious differences between the two cases of bfc 〉 1 and bic ≤1. Additionally, J-integral characteristic is obtained, and according to these results, the mode-Ⅱ crack growth trend is predicted. These results are benefit for us to understand the fracture mechanism of superconductor both in theory and application.     

Double spin-glass-like behavior and antiferromagnetic superexchange interaction between Fe~(3+) ions in α-Ga_(2-x)Fe_xO_3(0≤x≤0.4)

Single phase of Fe3+-doped α-Ga2-xFexO3(α-GF x O, x = 0.1, 0.2, 0.3, 0.4) is synthesized by treating the β-Ga2-x Fe x O3(β-GF x O) precursors at high temperatures and high pressures. Rietveld refinements of the X-ray diffraction data show that the lattice constants increase monotonically with the increase of Fe3+content. Calorimetric measurements show that the temperature of the phase transition from α-GF x O to β-GF x O increases, while the associated enthalpy change decreases upon increasing Fe3+content. The optical energy gap deduced from the reflectance measurement is found to decrease monotonically with the increase in Fe3+content. From the measurements of magnetic field-dependent magnetization and temperature-dependent inverse molar susceptibility, we find that the superexchange interaction between Fe3+ions is antiferromagnetic. Remnant magnetization is observed in the Fe3+-doped α-GF x O and is attributed to the spin glass in the magnetic sublattice. At high Fe3+doping level(x = 0.4), two evident peaks are observed in the image part of the AC susceptibility χ ac. The frequency dependence in intensity of these two peaks as well as two spin freezing temperatures observed in the DC magnetization measurements of α-GF0.4O is suggested to be the behavior of two spin glasses.     

Magnetization of Co-Fe-Ta-B-O Amorphous Thin Films

An amorphous magnetic material system(Co_(20)Fe_(47)Ta_(20)B_(13))_(1-x)O_x is fabricated by magneto sputtering. Three stages of magnetization behavior exist when oxygen content changes in the system. As the oxygen increases, the absence of percolation effect of magnetic nano-particles makes the multi-domain structure broken so that high coercivity appears in the samples with proper oxygen content. A temperature-dependent Stoner–Wohlfarth model is used to explain the magnetization properties at relatively high temperature. Magnetizations with magnetic field in and out of the sample plane are also investigated to prove the mechanisms. This work provides a systematic study of a new kind ofv amorphous magnetic system and is helpful for us to know more about this type of material.     

Multiple sign reversals of the exchange bias field in polycrystalline SmCr_（0.9）Fe_（0.1）O_3

We synthesize the perovskite compound Sm Cr0.9Fe0.1O3 by the sol–gel method and investigate its exchange bias properties through thermomagnetic and isothermal magnetization measurements. The sign reversals of the exchange bias field are observed at the magnetization compensation temperatures 29.6 K and 96.2 K. It is demonstrated that the occurrence of the exchange bias originates from the antiferromagnetic coupling between the Cr-rich and Fe–Cr regions, of which the net magnetization is temperature-dependent. These results imply that there are potential applications in single systems with sign reversals of both magnetization and exchange bias.     

Effectively Blocked Mechanism in Quantum Tunnelling of n-Coupled Single-Molecular Magnets

We present theoretical study on quantum tunnelling in s-coupled single-molecule magnets （SMMs） by spincoherent-state path integral. It is found that, due to weak coupling between SMMs, the tunnelling process involving more than one-spin-flip is effectively blocked and the main contribution to the relaxation of the mag- netization comes from the tunnelling processes involving just one-spin-flip. Starting from the negative saturated magnetization, the effect of the antiferromagnetic on tunnelling coupling is found to be qualitatively different from the ferromagnetic coupling. A criterion is developed to determine both the nature and the strength of the exchange coupling from the position of the first resonance of a spherical sample with homogeneous magnetization.     

Coexistence of positive and negative magnetic entropy changes in CeMn_2(Si_(1-x)Ge_x)_2 compounds

A series of CeMn2(Si1-xGex)2(x = 0.2, 0.4, 0.6, 0.8) compounds are prepared by the arc-melting method. All the samples primarily crystallize in the Th Cr2Si2-type structure. The temperature dependences of zero-field-cooled(ZFC) and FC magnetization measurements show a transition from antiferromagnetic(AFM) state to ferromagnetic(FM) state at room temperature with the increase of the Ge concentration. For x = 0.4, the sample exhibits two kinds of phase transitions with increasing temperature: from AFM to FM and from FM to paramagnetic(PM) at around TN~197 K and T C~300 K,respectively. The corresponding Arrott curves indicate that the AFM–FM transition is of first-order character and the FM–PM transition is of second-order character. Meanwhile, the coexistence of positive and negative magnetic entropy changes can be observed, which are corresponding to the AFM–FM and FM–PM transitions, respectively.     

Lateral magnetic stiffness under different parameters in a high-temperature superconductor levitation system

Magnetic stiffness determines the stability of a high-temperature superconductor(HTS) magnetic levitation system.The quantitative properties of the physical and geometrical parameters that affect the stiffness of HTS levitation systems should be identified for improving the stiffness by some effective methods. The magnetic stiffness is directly related to the first-order derivative of the magnetic force with respect to the corresponding displacement, which indicates that the effects of the parameters on the stiffness should be different from the relationships between the forces and the same parameters.In this paper, we study the influences of some physical and geometrical parameters, including the strength of the external magnetic field(B0) produced by a rectangular permanent magnet(PM), critical current density(Jc), the PM-to-HTS area ratio(α), and thickness ratio(β), on the lateral stiffness by using a numerical approach under zero-field cooling(ZFC)and field cooling(FC) conditions. In the first and second passes of the PM, the lateral stiffness at most of lateral positions essentially increases with B0 increasing and decreases with β increasing in ZFC and FC. The largest lateral stiffness at every lateral position is almost produced by the minimum value of Jc, which is obviously different from the lateral force–Jc relation. The α-dependent lateral stiffness changes with some parameters, which include the cooling conditions of the bulk HTS, lateral displacement, and movement history of the PM. These findings can provide some suggestions for improving the lateral stiffness of the HTS levitation system.     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

Structural properties of Bi1-xLaxFeO3 studied by micro-Raman scattering

This paper reports that La-doped BiFeO 3(Bi1-x La x FeO3,x = 0,0.1,0.2,0.3,0.6,0.8 and 1.0) were studied by using micro-Raman spectroscopy and x-ray diffraction(XRD).The XRD patterns indicate that the structure of Bi1-xLaxFeO 3 changes from rhombohedral BiFeO3 to orthorhombic LaFeO3.The results of Raman spectroscopy show good agreement with the XRD results.Strikingly,the phonon peak at around 610 cm-1 and the two-phonon peaks in the high frequency range exist in all compounds and enhance with increasing La substitution.The increasing intensity of the 610 cm-1 peak is attributed to the changes in the FeO 6 octahedron during the rhombohedral-orthorhombic phase transition.The enhancements of the two-phonon peaks are associated with the breakdown of the cycloid spin configuration with the appearance of the orthorhombic structure.These results indicate the existence of strong spin-phonon coupling in Bi1-xLax FeO3,which may provide useful information for understanding the effects of La content on the structural and magnetic properties of Bi1-xLaxFeO3.     

The physics of spin rectification and its application

A coherent nonlinear coupling between the charge and spin dynamics of electrons results in the rectification of microwaves,which is enhanced through resonant magnetization dynamics such as ferromagnetic resonance.This property,known as the spin rectification effect,enables the spin dynamics within a material to be electrically detected with a high sensitivity.Techniques utilizing this property have been widely used to study the magnetization dynamics of various ferromagnetic materials and structures in the past decade.Additionally,the coherent nature of spin rectification opens the door for spintronic devices to be used in phase-resolved microwave sensing techniques.In this work we review the physics of spin rectification and its applications in several interesting topics of magnetism and spintronics.     

Degradation of ferroelectric and weak ferromagnetic properties of BiFeO_3 films due to the diffusion of silicon atoms

Crystalline BiFeO3 （BFO） films each with a crystal structure of a distorted rhombohedral perovskite are characterized by X-ray diffraction （XRD） and high-resolution electron microscopy （HRTEM）. The diffusion of silicon atoms from the substrate into the BiFeO3 film is detected by Rutherford backscattering spectrometry （RBS）. The element analysis is per- formed by energy dispersive X-ray spectroscopy （EDS）. Simulation results of RBS spectrum show a visualized distribution of silicon. X-ray photoelectron spectroscopy （XPS） indicates that a portion of silica is formed in the diffusion process of silicon atoms. Ferroelectric and weak ferromagnetic properties of the BFO films are degraded due to the diffusion of silicon atoms. The saturation magnetization decreases from 6.11 down to 0.75 emu/g, and the leakage current density increases from 3.8 × 10^-4 upto7.1 × 10^-4 A/cm-2.     

Phase separation in Sr doped BiMnO3

Phase separation in Sr doped BiMnO3 （Bil_xSrxMnO3, x = 0.4-0.6） was studied by means of temperature-dependent high-resolution neutron powder diffraction （NPD）, high resolution X-ray powder diffraction （XRD）, and physical property measurements. All the experiments indicate that a phase separation occurs at the temperature coinciding with the reported charge ordering temperature （Tco） in the literature. Below the reported TCO, both the phases resulting from the phase separation crystallize in the orthorhombically distorted perovskite structure with space group Imma. At lower temperature, these two phases order in the CE-type antiferromagnetic structure and the A-type antiferromagnetic structure, respectively. However, a scrutiny of the high-resolution NPD and XRD data at different temperatures and the electron diffraction exper- iment at 300 K did not manifest any evidence of a long-range charge ordering （CO） in our investigated samples, suggesting that the anomalies of physical properties such as magnetization, electric transport, and lattice parameters at the TCO might be caused by the phase separation rather than by a CO transition.     

Enhancement of Field Emission Properties in La-Doped ZnO Films Prepared by Magnetron Sputtering

Field emissions (FE) from La-doped zinc oxide (ZnO) films are both experimentally and theoretically investigated. Owing to the La-doped effect, the FE characteristic of ZnO films is remarkably enhanced compared with an undoped sample, and a startling low turn-on electric field of about 0.4V/μm (about 2.5V/μm for the undoped ZnO films) is obtained at an emission current density of 1μA/cm² and the stable current density reaches 1mA/cm² at an applied field of about 2.1V/μm. A self-consistent theoretical analysis shows that the novel FE enhancement of the La-doped sample may be originated from its smaller work function. Due to the effect of doping with La, the Fermi energy level lifts, electrons which tunnelling from surface barrier are consumedly enhancing, and then leads to a huge change of field emission current. Interestingly, it suggests a new effective method to improve the FE properties of film materials.