Antiferromagnet IrMn thickness dependence in exchange-biased perpendicular magnetic anisotropy based on CoFe/Pt/CoFe/[IrMn(tIrMn)] multilayers

The effect of the antiferromagnetic IrMn thickness upon the magnetic properties of CoFe/Pt/CoFe/[IrMn(t_IrMn)] multilayers is studied. An oscillatory interlayer coupling (IEC) has been shown in pinned CoFe/Pt(t_Pt)/CoFe/IrMn multilayers with perpendicular anisotropy. The period of oscillation corresponds to about 2 monolayers of Pt. The oscillatory behavior of IEC depends on the nonmagnetic metallic Pt thickness and is thought to be related to the antiferromagnetic ordering induced by the IrMn layer. From the extraordinary Hall voltage amplitude (EHA) curves as function of IrMn thickness, we report that the oscillation dependence of IEC for the [CoFe/Pt/CoFe] multilayer system induced by IrMn with spacer-layer thickness is a important features of perpendicular exchange biased system.     

Interlayer coupling between out-of-plane magnetized multilayers across a thin antiferromagnetic spacer

The interlayer exchange coupling between Co/Pt perpendicular-to-plane magnetized layers across a thin IrMn spacer layer was experimentally studied. In contrast to earlier studies on interlayer coupling through antiferromagnetic NiO, which revealed an oscillatory coupling behavior as a function of NiO thickness, a ferromagnetic coupling was observed here in the range of IrMn thickness between 0.6 and 1.5 nm and antiferromagnetic between 1.5 and 2.5 nm. The antiferromagnetic coupling is attributed to an orange peel magnetostatic mechanism whereas the ferromagnetic coupling is attributed to an out-of-plane polarization of the antiferromagnetic IrMn layer induced by the interfacial exchange interaction with the adjacent out-of-plane ferromagnetic layers. Measurements of hysteresis loops versus temperature show that the coupling vanishes at 510 K for t_IrMn=1 nm. This critical temperature is far below the Néel temperature of bulk IrMn, but above the blocking temperature of IrMn/Co bilayers at such thickness. Using a one-dimensional model describing a partial domain wall in the antiferromagnet, we explain the coupling in terms of an out-of-plane tilt of the Mn moments at the IrMn/(Co/Pt) interfaces yielding a weak net polarization of the IrMn. Finally, the non-oscillatory decay of the coupling was attributed to the compensated spin structure of the IrMn in the parallel to the interfaces.     

Interface characterization of a [Pt/Co)]3/IrMn multilayer by laser-assisted tomographic atom probe

A sputtered Ta_3 nm/[[Pt_2 nm/Co_0.4 nm)]₃/IrMn_7 nm]₇/Pt_10 nm multilayer has been analyzed by laser-assisted tomographic atom probe, allowing to perform three-dimensional reconstructions of the layers and to determine their chemical composition at the atomic scale. From the concentration profiles, we show that the Co layer in contact with IrMn is strongly mixed. The Co/IrMn and IrMn/Pt interfaces are thus non-symmetric, the Co/IrMn interface being more diffuse than the IrMn/Pt one. This study demonstrates that the LATAP technique is extremely well suited for atomic scale structural and chemical characterizations of magnetic multilayers in relationship with their magnetic properties.     

Effect of Ta buffer and NiFe seed layers on pulsed-DC magnetron sputtered Ir20Mn80/Co90Fe10 exchange bias

A systematic investigation has been done on the correlation between texture, grain size evolution and magnetic properties in Ta/Ni₈₁Fe₁₉/Ir₂₀Mn₈₀/Co₉₀Fe₁₀/Ta exchange bias in dependence of Ta buffer and NiFe seed layer thickness in the range of 2-10 nm, deposited by pulsed DC magnetron sputtering technique. A strong dependence of 〈1 1 1〉 texture on the Ta/NiFe thicknesses was found, where the reducing and increasing texture was correlated with exchange bias field and unidirectional anisotropy energy constant at both NiFe/IrMn and IrMn/CoFe interfaces. However, a direct correlation between average grain size in IrMn and H_ex and H_c was not observed. L1₂ phase IrMn₃ could be formed by thickness optimization of Ta/NiFe layers by deposition at room temperature, for which the maximum exchange coupling parameters were achieved. We conclude finally that the coercivity is mainly influenced by texture induced interfacial effects at NiFe/IrMn/CoFe interfaces developing with Ta/NiFe thicknesses.     

Phase-Transition and Magnetic Moment of the Gd3+ Ion in the Gd2Fe17 Compound

The structure and magnetic phase transitions of the Gd2Fe17 compound are investigated by using a differential thermal/thermogravimetric analyzer, x-ray diffraction, and magnetization measurements. The result shows that there are two phase structures for the Gd2Fe17 compound： the hexagonal Th2Nilr-type structure at high temperatures （above 1243℃）, and the rhombohedral Th2Zn17-type structure, respectively. A method to measure the magnetic moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound is presented. The moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound from 77 to 500 K are measured in this way with a vibrating sample magnetometer. A detailed discussion is presented.     

Vacuum Annealing Induced Room-Temperature Ferromagnetism in Co0.1Ti0.9O2-δ Films Prepared by Sol-Gel Method

The Co-doped TiO2 films （Co0.1Ti0.9O2-δ） are prepared on silicon substrates by sol-gel method and post annealing. TheCo0.1Ti0.9O2-δ film annealed in air is non-ferromagnetic at room temperature. After further annealed in a vacuum, the room-temperature ferromagnetism （RTFM） is observed. Experimental evidences show that the RTFM in the Co0.1Ti0.9O2-δ film may come from the Co-doped TiO2 matrix and is related to the oxygen vacancies created by vacuum annealing.     

Scalar Statistics along Inertial Particle Trajectory in Isotropic Turbulence

The statistics of a passive scalar along inertial particle trajectory in homogeneous isotropic turbulence with a mean scalar gradient is investigated by using direct numerical simulation. We are interested in the influence of particle inertia on such statistics, which is crucial for further understanding and development of models in non-isothermal gas-particle flows. The results show that the scalar variance along particle trajectory decreases with the increasing particle inertia firstly; when the particle＇s Stokes number St is less than 1.0, it reaches the minimal value when St is around 1.0, then it increases if St increases further. However, the scalar dissipation rate along the particle trajectory shows completely contrasting behavior in comparison with the scalar variance. The mechanical-to-thermal time scale ratios averaged along particle, （r）p, are approximately two times smaller than that computed in the Eulerian frame r, and stay at nearly 1.77 with a weak dependence on particle inertia. In addition, the correlations between scalar dissipation and flow structure characteristics along particle trajectories, such as strain and vorticity, are also computed, and they reach their maximum and minimum, 0.31 and 0.25, respectively, when St is around 1.0.     

Phase Diagram of Antiferromagnetically Exchange-Coupled Bilayer

Magnetic hysteresis properties of antiferromagnetically exchange-coupled bilayer structures, in which the two magnetic layers have different magnetic parameters and thicknesses, are studied within the framework of the Stoner-Wohlfarth model. Analytical expressions for the switching fields corresponding to the linear magnetic states are obtained. By adjusting the magnetic parameters or thicknesses of layers, nine different types of easy-axis hysteresis loops may exist. The phase diagram of easy-axis hysteresis loops is mapped in the k1 and k2 plane, where k1 and k2 are the ratios of magnetic anisotropy to the interlayer exchange coupling of the two magnetic layers, respectively.     

Amorphization Induced High Magneto-Caloric Effect of Gd55Al20Ni25 Ternary Alloy

We report the amorphization induced high magneto-caloric effect （MCE） of recently developed Gd55Al15Ni30 bulk metallic glass （BMG）. The magnetic properties of the Gd55Al15Ni30 BMG are investigated in comparison with that of its crystalline counterpart. It is found that amorphization can increase the saturation magnetization and decrease the hysteresis of Gd55Al15Ni30 alloys, which indicate the possible enhancement of MCE. The magnetic entropy changes and the refrigerant capacity of the BMG as well as the crystalline samples is calculated directly from isothermal magnetic measurements. The results show the amorphization induced high MCE of the alloy and the excellent refrigerant efficiency of Gd55Al15Ni30 bulk metallic glass.     

Structural and Magnetic Properties of [Fe/Ni]N Multilayers

[Fe/Ni]N multilayered structure grows epitaxially on the single crystalline MgO substrate. Due to the different directions of magnetic easy axes of Fe and Ni and the strong strain, large anisotropy dispersion is assumed. According to the layer model, the magnetization of Fe and Ni layers cannot follow each easy axis because of exchange coupling, and then the anisotropies are averaged out. The reduction of the effective anisotropy enhances with the decrease of periodic thickness. Thus, the coercivity of [Fe/Ni]N multilayers reduces with decreasing periodic thickness.     

Fabrication and Characterization of SmCo5/Fe65Co35 and SmCo5/Fe Composite Spring Exchange Magnets

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.     

A Modified Free Volume Model for Characterizing of Rate Effect in Bulk Metallic Glasses

We investigate the plastic deformation and constitutive behaviour of bulk metallic glasses （BMGs）. A dimensionless Deborah number DeiD = tr/ti is proposed to characterize the rate effect in BMGs, where tr is the structural relaxing characteristic time of BMGs under shear load, ti is the macroscopic imposed characteristic time of applied stress or the characteristic time of macroscopic deformation. The results demonstrate that the modified free volume model can characterize the strain rate effect in BMGs effectively.     

Magnetic Properties and Spin State Transition of Gallium Doped Perovskite Cobaltite Oxide

A series of Ga doping perovskite cobaltite La2/3Sr1/3 （Co1-y Gay）03 （y = 0, 0.1, 0.2, 0.3 and 0.4） are prepared by the standard solid-state reaction method. Their magnetic properties and Co ions spin state transitions are studied. Upon doping, no appreciable structure changes can be found. However, the corresponding Curie temperature sharply decreases and the magnetization is greatly reduced, indicating that Ga doping destroys the ferromagnetic interaction in the system. In addition, the high temperature magnetization data follow the Curie-Weiss law. At least one kind of Co ions （Co^3＋ or Co^4＋） favours the mixed spin state, and most Co ions are at the lower spin state （low and intermediate state）. With increasing Ga content, more Co ions transit to the higher spin state.     

Development of Simple Dip-Stick-Type Uniaxial Stress Actuator for Alternating-Current Susceptibility Measurements

A simple dip-stick type uniaxial stress actuator for ac-susceptibility measurement is designed. Target pressure can be achieved by smooth and continues work carried out using a combination of light weight micrometer and spring. The magnitude of the pressure is directly calculated from the force constant of the spring and the surface atea of the sample. Benchmark on the quality of the data under uniaxial pressure is confirmed by the Piezo resistance measurements on [100] oriented n-type Si. The system is examined and calibrated with the standard paramagnetic Gd2O3. Further, the device performance, generation of constant uniaxial pressure against temperature variations, is assured by investigating the ac-magnetic susceptibility measurements on highly anisotropic La1.25Sr1.75Mn2O7 bilayer single crystal.     

Bianisotropy Picture of Higher Permeability at Higher Frequencies

With an effective bianisotropy picture, high-frequency behaviours of different magnetic materials can be reconciled, and the higher permeability and higher resonance frequencies are achieved even in the GHz range. The validity of the bianisotropy picture is quantitatively verified by the in-plane anisotropic Fe₃₄Co₅₅Zr₁₁ thin films. A prolate elliptical precession of the magnetization about its equilibrium direction is the key point, which can be induced by an artificial or an intrinsic bianisotropy system.     

Measurement of the coupling strength distribution in ferromagnetic (F)/antiferromagnetic (AF) exchange bias systems

We propose a method for determination of the distribution function P(j) of the coupling energy density j in polycrystalline textured ferromagnetic (F)/antiferromagnetic (AF) film systems. P(j) governs the entire film coupling J and the exchange bias field H_e and was not measurable until now. The method is verified by torquemetry in a high magnetic field and by reversing its rotation sense. The transition to a new magnetic steady state after rotation reversal is analyzed within a Stoner–Wohlfarth model including thermal relaxation. This transition is completed earlier for strongly coupled grains than for grains with smaller j, which is reflected in the torque curves. We determined P(j) for a sputtered NiFe(16 nm)/IrMn(0.8 nm) film at T=50 K in the hysteretic range of coupling energies and found that P strongly decreases for increasing j.     

Synthesis and Characteristics of Electrodeposited CoxZn1-x Nanorods

CoxZn1-x nanorod arrays were fabricated by electrodeposition in porous anodic aluminum oxide templates at different electric potentials. X-ray diffraction and transmission electron microscopy indicate that highly-ordered and uniform nanorods have been fabricated. The amounts of Co and Zn contents are investigated using energy dispersive spectroscopy, which demonstrates that the atom ratio of the alloy nanorods changes with the deposition potential. In addition, magnetic measurements show that the magnetic isotropy Co-rich CoZn nanorods will change to magnetic anisotropy nanorods with the easy axis parallel to the rod long axis with decreasing Co content.     

Correlation between Zero-Field Splitting and Site Distortions of Cr^3＋ Ions in NH4Cl：Cr^3＋ System： a Complete Energy Matrix Study

A theoretical method for investigating the inter-relation between the electronic and molecular structures of 3d^3 configuration ions in a tetragonal ligand field is established on the basis of the 120 × 120 complete energy matrices. Using this method, the local structure parameters of two tetragonal Cr^3＋ centers in the NH4 Cl：Cr^3＋ system are determined, Furthermore, the relations between the molecular symmetry and the ligand field symmetry are discussed.     

Properties of a New Nonlinear Optical Crystal CdZn2B2O6

Cadmium dizinc diborate （CdZn2B2O6） single crystals have been grown for the first time. The crystal structure of CdZn2B2O6 is the same as that of the Cd3Zn3B4O12. The x-ray diffraction, infrared and Raman spectra, differential scanning calorimetry analysis and density indicate that the physical and chemical properties of both crystals are very similar. Especially, the nonlinear optical coefficients of CdZn2B2O6 and Cd3Zn3B4O12 crystals are 2.6 and 2.4 times as large as that of KH2PO4 crystal respectively. Chemical etching experiments indicated that these crystals are very stable in neutral solution and not hygroscopic in air at room temperature.