Guohong Yun

Band structures of two-dimensional magnonic crystals with different shapes and arrangements of scatterers

The spin-wave band structures of two-dimensional magnonic crystals with different shapes and arrangements of scatterers are investigated numerically. It is shown that the magnonic band gap of spin waves in magnonic crystals can be optimized by changing the shape or the arrangement of scatterers in background materials. The largest absolute gap can be achieved when the scatterer has the same symmetry as that of the coordination polygon of lattice point, such as the case of square rods in square lattices. For a given shape of scatterer, the gap width is the largest when the lattice has the largest coordination number if the scatterer does not reduce the symmetry.

Surface elasticity effect on the size-dependent elastic property of nanowires

A modified core-shell (MC-S) model is proposed to investigate the effect of surfaceelasticity on the elastic properties of nanowires under bending and tension loading modes. The continuous exponential function based on bulk elasticity is applied to the surface region of nanowires to better describe the elasticity in the surface layer. Two parameters related to the surface, namely, the inhomogeneous degree constant α , and the transition region of this inhomogeneous state r s (i.e., surface layer thickness), are introduced for examining the size effects of the elastic modulus of the overall nanowires. A strong size dependence of elasticity is revealed under both bending and tension loads. Furthermore, the theoretical solution for an effective Young’s modulus with relevant experiments, as well as the results of a molecular statistical thermodynamics (MST) method for zinc oxide (ZnO)nanowires, and a molecular dynamics (MD) simulation for silicon (Si) nanowires, are compared. It is shown that the theoretical curves not only agree well with the experimental data, but also fit the computational results (MST or MD) approximately below 20 nm. As a result, our model can predict the behavior of surfaceelasticity, with respect to the lateral size of nanostructures at a relatively small scale, no matter how stiff or soft the surface of the nanomaterials.

Spin-wave band gaps created by rotating square rods in two-dimensional magnonic crystals

Absolute spin-wave band gaps can be substantially opened and tuned by rotating noncircular rods in two-dimensional magnonic crystals. Spin-wave band structures of two-dimensional magnonic crystals composed of Fe (EuO) square rods squarely arranged in a EuO (Fe) matrix are numerically calculated using the plane-wave method. The results show that it is possible to increase the width of the band gaps or to create band gaps by rotating the noncircular rods. For the system of EuO rods in Fe matrix, the largest absolute spin-wave gap in the structure of square rods is 227% of the size of that in the corresponding structure of circular rods. Such an approach may open up a new scope for engineering band gaps of two-dimensional magnonic crystals.

The jump phenomenon in the angular dependence of the off-aligned exchange bias

Based on the principle of minimal energy, the angular dependence of exchange bias has been investigated in detail with noncollinear easy axes of unidirectional and uniaxial anisotropies in the ferromagnetic/antiferromagnetic bilayers. The competition between unidirectional and uniaxial anisotropies divides the initial magnetization state of the bilayers into monostable state and bistable state, which determine the angular dependence of exchange bias directly. When the external field is applied along the intrinsic easy axes and intrinsic hard axes, it is found that one of the coercive fields at the descending or ascending branch of the hysteresis loop makes an abrupt change, whereas the other coercive field keeps continuity by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show the jump phenomenon in the angular dependence of exchange bias. This jump phenomenon of the exchange bias is found to be an intrinsic property of the bilayers which is dependent on the relative magnitudes and orientations between unidirectional and uniaxial anisotropies. The numerical calculations indicate that both the exchange bias field and the coercivity are larger in the magnitude at the points of the jumps. At the jumping points of the intrinsic easy axes, the coercivity reaches the maximum; at the jumping points of the intrinsic hard axes, the exchange bias field reaches the maximum, at the meantime the coercivity can vanish itself suddenly. These features of the jump phenomenon are useful in the technological applications to achieve the largest exchange bias field.

Point defect states of exchange spin waves in all-ferromagnetic two-dimensional magnonic crystals

Using the plane-wave expansion method under supercell approximation, band structures of exchange spin waves propagating in all-ferromagnetic two-dimensional magnonic crystals with point defects are calculated. The results indicate that the point defects in these structures can create localized states inside the bandgap. The characteristics and the magnetization distributions of these localized states are studied. The results show that the group velocities of the localized states almostequal zero in the whole first Brillouin zone, and numbers of localized states and their frequency-positions in the bandgap are related to the size of the point defect. Magnonic crystalswith such defect states can be used as fabricatingmaterials of narrow bandpass spin-wave filters.

The jump phenomenon in the angular dependence of exchange bias for ferromagnetic/antiferromagnetic bilayers

Based on the principle of minimal energy, the angular dependence of exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated in detail. The competition between unidirectional and uniaxial anisotropies divides the initial magnetization state of the bilayer into monostable state and bistable state, which determine the angular dependence of exchange bias directly. When the bilayer is in the bistable state, the exchange bias field and the coercivity will display a jump phenomenon at the orientation angles of the intrinsic hard axes. The jump phenomenon in the angular dependence of exchange bias has been explained by analyzing the magnetization reversal processes. It is found that both the exchange bias field and the coercivity are larger in the magnitude at the points of the jumps. This jump phenomenon is an intrinsic property of the bilayers which is dependent on the interfacial exchange-coupling constant, the thickness, and the uniaxial anisotropy constant of the ferromagnetic layer.

Thermal Stress Analysis for Octagonal Quasicrystals

The complex variable method for solving the two-dimensional thermal stress problem of octagonal quasicrystals is stated. The closed-form solutions for octagonal quasicrystals containing an elliptical hole subjected to a remote uniform heat flow are obtained. When the hole degenerates into a crack, the explicit solutions for the stress intensity factors and energy release rate are presented.

Properties of perfect confined modes and interface modes of spin-waves in a ferromagnetic bilayer system

Abstract The exact solutions of spin-wave modes in a ferromagnetic bilayer system with a periodic boundary condition are investigated by using the interface-rescaling approach. The dispersion relations of the modes in the sublayers are obtained. The perfect confined modes and the interface spin-wave modes are given and discussed.

Effect of Surface Elasticity on the Piezoelectric Potential of a Bent ZnO Nanowire

The influence of surface elasticity on the piezoelectric potential distribution of a deformed ZnO nanowire is investigated by the effective Youngs modulus based upon elastic and piezoelectric theory. When the nanowire in radius 25 nm subjects to an lateral applied force 5 nN, the maximum piezoelectric potential of the nanowire we derived is about 13.8 mV, which approaches much more closely to the experiment measurement value (~10 mV) [Z. L. Wang and J. H. Song: Science 312 (2006) 242]. Moreover, a comprehensive analysis of maximum piezoelectric potential between the cases with and without the effect of surface elasticity is analyzed. The results show that the values of piezoelectric potential generated in ZnO nanowires are decreased due to the surface stiffening. From the theoretical analysis, the effect of surface elasticity has a significant impact on the piezoelectric potential for a bent ZnO nanowire, actually it reduces the gap between theoretical estimation and experiment measurements.

Modelling of the magnetostrictive trilayer cantilever for actuators

The basic mechanical equilibrium equation is employed to model the cantilever system which consists of two giant magnetostrictive films (GMSFs), one with positive and the other with negative magnetostriction, and a non-magnetic substrate (NMS) for actuators. The bending and loading characteristics of the cantilever system are discussed systematically, and the optimal condition for actuator application is presented. The results show that a thicker substrate is favourable for larger force exerted by the cantilever when one of its sublayer thickness is kept constant. But a thinner substrate is required when the total thickness of the cantilever needs to be kept constant so that a proper balance is needed in choosing the sublayer and substrate thicknesses. It is also found that, to obtain the maximum exerted force, the thickness configuration of GMSFs should be fixed at the optimal value. The GMSF/NMS/ GMSF cantilever is generally superior in loading characteristics to the GMSF/NMS and GMSF/GMSF cantilevers. Our discussion may be helpful to the designing and fabricating of the giant magnetostrictive cantilever actuators with more realistic and optimal geometry.