Influence of magnetic field sweep rate on the hysteresis loops of Ni0.8Fe0.2/Fe0.5Mn0.5 exchange bias bilayer

The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.     

Fermi Surface Topology of Na0.5CoO2 from the Hybrid Density Functional

The Fermi surface topology of Na0.5CoO2 is studied using the hybrid density functional theory. We first study a single （CoO2）^0.5- layer model with the percentage of the nonlocal Hartree-Fock exchange changing from 0% to 20%. The results show that only when the mixed nonlocal Hartree-Fock exchange is between 1% and 5%, the Fermi surface topology is similar to the experimental one. With 3% HF exchange in the hybrid density functional, considering the effects of Na ions in the Na0.5CoO2 system, we find that the Fermi surface is split to double holes and small gaps open near the intersections between the Brillouin zone and the Fermi surface. Our results show that both the amounts of the nonlocal Hartree-Fock exchange in the hybrid density functional and the Na ions have much influence on the Fermi surface topology.     

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.     

Effects of exchange interaction and spin-fluctuation on the magnetic and magneto-optic properties of NdF3

The exchange interaction between the electrons in the different magnetic ions and the spin-fluctuation of the magnetic ions exist in the paramagnetic media NdF3. The exchange interaction between the electrons in the different magnetic ions may be equivalent to an effective field Hin that is in direct proportion to the magnetization M. The spin-fluctuation of the magnetic ions leads the coefficient of the effective field to vary with temperature. The effective field is given as Hin = -（0.75 ＋ 0.22T） × 10^-5M in NdF3. When the secondary crystal field effect is taken into account, the magnetic susceptibility and Verdet constant are calculated for NdF3 by means of the effective field Hin and the applied field He. The calculated results are in agreement with the measured ones.     

Theoretical investigation of exchange bias

The exchange bias of bilayer magnetic films consisting of ferromagnetic (FM) and antiferromagnetic (AFM) layers in an uncompensated case is studied by use of the many-body Green's function method of quantum statistical theory. The effects of the layer thickness and temperature and the interfacial coupling strength on the exchange bias H_E are investigated. The dependence of the exchange bias H_E on the FM layer thickness and temperature is qualitatively in agreement with experimental results. When temperature varies, both the coercivity H_C and H_E decrease with the temperature increasing. For each FM thickness, there exists a least AFM thickness in which the exchange bias occurs, which is called pinning thickness.     

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic bilayers

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic （FM/AF） bilayers has been investigated in detail by extending Slonczewski＇s ＇proximity magnetism＇ idea. Here three important parameters are discussed for FM/AF bilayers, i.e. interracial bilinear exchange coupling J1, interracial biquadratic （spin-flop） exchange coupling J2 and antiferromagnetic layer thickness tAF. The results show that both the occurrence and the variety of the exchange bias strongly depend on the above parameters. More importantly, the small spin-flop exchange coupling may result in an exchange bias without the interracial bilinear exchange coupling. However, in general, the spin-flop exchange coupling cannot result in the exchange bias. The corresponding critical parameters in which the exchange bias will occur or approach saturation are also presented.     

Effect of Fe impurity on the optical loss of Nd-doped phosphate laser glass

The optical loss coefficient at 1053-nm wavelength, influenced by Fe ions in N31-type Nd-doped phosphate laser glass, was determined precisely and analyzed in detail. It is found that the optical loss coefficient per unit of Fe concentration (cm-1/ppmw) increases with Fe concentration in the range of 0-300 ppmw, but it approaches a constant as the Fe concentration is larger than 300 ppmw. Such a concentration effect is due to a shift in the redox equilibrium between Fe3+ and Fe2+ ions in the glass. The effect of oxygen pressure, temperature, and variable valence states of other metal ions in glass samples on the optical loss is also discussed.     

Electron-impact ionization cross section calculations for lithium-like ions

The electron-impact ionization of lithium-like ions C³⁺,N⁴⁺,O⁵⁺,Ne⁷⁺,and Fe2³⁺is studied using a combination of two-potential distorted-wave and R-matrix methods with a relativistic correction.Total cross sections are computed for incident energies from 1 to 10 times of ionization energy and better agreements with the experimental results are obtained in comparison with the theoretical data available.It is found that the indirect ionization processes become significant for the incident energy larger than about four times of the ionization energy.Contributions from the exchange effects along the isoelectronic sequence are also discussed and found to be important.The present method can be used to obtain systematic ionization cross sections for highly charged ions across a wide incident energy range.     

Effective Field Based on the Exchange Interaction in Gd3Ga5O12 under Low and Moderate Magnetic Field at Low Temperature

The roles of the exchange interaction between different magnetic ions in the gadolinium gallium garnet Gd3 Ga5O12 are systematically investigated. The effective field based on the exchange interaction is expressed as functions of the temperature T and applied field He. Furthermore, the effects of T and He on the magnetic moment and the entropy of GdsGa5O12 are studied in the ranges of 0 〈 He 〈 9 T and 3 〈 T 〈 40K. The computational results are consistent with the experimental data. Our study reveals that the applied field and temperature have important influences on the exchange interaction of Gd3 Ga5 O12 at low temperature.     

Influence of Ga+ ion irradiation on the magnetisation reversal process and magnetoresistance in CoFe/Cu/CoFe/IrMn spin valves

Ga+ion irradiation is performed on the surfaces of IrMn-based spin valves and the effects of ion irradiation on the magnetisation reversal process and magnetoresistance(MR) are investigated.The results show that the exchange bias field and magnetoresistance ratio of the spin valve decrease with the increase of ion dose.The width of the forward step between the free layer and the pinned layer becomes gradually smaller with the increase of ion dose whilst the recoil step tends to be narrower with ion dose increasing up to 6×10 13 ions/cm 2 and the step disappears afterwards.Two peaks in the R-H curve are found to be asymmetric.     

Unified nonequilibrium dynamical theory for exchange bias and training effects

We have investigated the exchange bias and training effect in the ferromagnetic/antiferromagnetic (FM/AF) heterostructures using a unified Monte Carlo dynamical approach. The magnetization of the uncompensated AF layer is still open after the first field cycling is finished. Our simulated results show obvious shift of hysteresis loops (exchange bias) and cycling dependence of exchange bias (training effect) when the temperature is below 45~K. The exchange bias field decreases with decreasing cooling rate or increasing temperature and the number of the field cycling. Essentially, these two effects can be explained on the basis of the microscopical coexistence of both reversible and irreversible moment reversals of the AF domains. Our simulations are useful to understand the real magnetization dynamics of such magnetic heterostructures.     

Influence of Ga+ ion irradiation on thermal relaxation of exchange bias field in exchange-coupled CoFe/IrMn bilayers

This paper reports that the CoFe/IrMn bilayers are deposited by magnetron sputtering on the surfaces of thermallyoxidized Si substrates.It investigates the thermal relaxations of both non-irradiated and Ga + ion irradiated CoFe/IrMn bilayers by means of holding the bilayers in a negative saturation field.The results show that exchange bias field decreases with the increase of holding time period for both non-irradiated and Ga + ion irradiated CoFe/IrMn bilayers.Exchange bias field is also found to be smaller upon irradiation at higher ion dose.This reduction of exchange bias field is attributed to the change of energy barrier induced by ion-radiation.     

Electron spin resonance investigation of the substitution of Fe3^＋ for Ti4^＋ ions in rutile TiO2 single crystal

A Fe doped rutile TiO 2 single crystal is grown in an O 2 atmosphere by the floating zone technique.Electron spin resonance (ESR) spectra clearly demonstrate that Fe 3+ ions are substituted for the Ti 4+ ions in the rutile TiO 2 matrix.Magnetization measurements reveal that the Fe:TiO 2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K.The Fe 3+ ions possess weak magnetic anisotropy with an easy axis along the c axis.The annealed Fe:TiO 2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.     

Magnetic properties of Nd0.5Pb0.5-xSrxMnO3 materials

The structure and magnetic properties of Nd0.5Pb0.5-xSrxMnO3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state.Curie temperature Tc and the MI transition temperature Tp increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn-Teller electron-phonon coupling.     

H3+团簇离子的实验研究

报道了H+ 3 离子通过碳膜后 ,在其中产生的各种微量产物的测量结果 ,讨论了离子与碳膜作用中的电荷交换过程 ,分析了这些微量产物的形成机理 ,证明在产物的形成中电荷交换过程起关键作用 ,研究了离子与碳膜作用中的团簇效应和尾流效应. The yields of H -, H, H 2, H + 2 and H + 3 for H + 3 ions passing through carbon foils have been measured respectively.The results show that the charge exchange process in the interaction of Hydrogen ions with carbon foils plays a key role in the formation of these products. The cluster effects and wake effect of H + 2 and H + 3 ions in carbon foils are also discussed.     

Fluorescence properties of divalent and trivalent europium ions in CdWO4 single crystals grown by the Bridgman method

Optical absorption,excitation,and fluorescence were investigated in Eu ion-doped CdWO4 single crystal grown by a modified Bridgman method.The results indicate that Eu2+ and Eu3+ ions coexist in CdWO4 crystal and an energy transfer occurs between these Eu2+ and Eu3+ ions.When the crystal is excited by 266-nm light,the energy corresponding to the 4f65d to 8S7/2 transition of Eu2+ ions results in the excitation of the Eu3+ ions to the 5DJ level.The effect on fluorescence of annealing in oxygen at various temperatures was investigated.The excitation intensity of Eu2+ ions at 266 nm decreases as annealing temperature increases from 300 K to 1073 K,but it remains at a certain equilibrium level when the annealing temperature is further increased.     

Influence of temperature on thermal relaxation of exchange bias field in CoFe/Cu/CoFe/IrMn spin valve

A multilayered spin valve film with a structure of Ta(5 nm)/Co_(75)Fe_(25)(5 nm)/Cu(2.5 nm)/Co_(75)Fe_(25)(5 nm)/Ir_(20)Mn_(80)(12 nm)/Ta(8 nm) is prepared by the high-vacuum direct current(DC) magnetron sputtering. The effect of temperature on the spin valve structure and the magnetic properties are studied by x-ray diffraction(XRD), atomic force microscopy(AFM), and vibrating sample magnetometry. The effect of temperature on the exchange bias field thermomagnetic properties of multilayered spin valve is studied by the residence time of samples in a reverse saturation field. The results show that as the temperature increases, the Ir Mn(111) texture weakens, surface/interface roughness increases, and the exchange bias field decreases. Below 200℃, the exchange bias field decreases with the residence time increasing, and at the beginning of the negative saturation field, the exchange bias field Hex decreases first quickly and then slowly gradually. When the temperature is greater than 200℃, the exchange bias field is unchanged with the residence time increasing.     

Quantum Statistical Calculation of Exchange Bias

The phenomenon of exchange bias of ferromagnetic (FM) films, which are coupled with an antiferromagnetic (AFM) film, is studied by Heisenberg model by use of the many-body Green‘s function method of quantum statistical theory for the uncompensated case. Exchange bias HE and coercivity Hc are calculated as functions of the FM film thickness L, temperature, the strength of the exchange interaction across the interface between FM and AFM and the anisotropy of the FM. Hc decreases with increasing L when the FM film is beyond some thickness. The dependence of the exchange bias HE on the FM film thickness and on temperature is also qualitatively in agreement with experiments.     

The synthesis and exchange bias effect of monodisperse NiO nanocrystals

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.     

Optical spectra and local structure of Eu3+ ions doped in Nb2O5-La2O3-B2O3-BaO glasses

The xNb2O5-(15-x)La2O3-40B2O3-45BaO (x = 5, 7.5, 12.5 mol%) glasses doped with Eu3+ ions in 1mol% are fabricated by the melting method. The Fourier transform infrared (FTIR) spectra, phonon sideband spectra, emission and excitation spectra of the glasses are measured. The crystal field parameter and coordination number of Eu3+ ions in the glasses are obtained according to the splitting of their 5D0 - 7F1levels. The intensity parameters Ω2 and Ω4 of Eu3+ ions for optical transition are calculated from their emission spectra in terms of reduced matrix U(t) (λ= 2,4,6) character for optical transitions. The results indicate that the intensity parameters Ω2 and Ω4 increase with the increase of Nb2O5 content, suggesting that the symmetry becomes lower, the band of Eu and O atoms becomes stronger and the covalence increases with the increase of Nb2O5 content.