An Hu

Symmetry-induced perfect transmission of light waves in quasiperiodic dielectric multilayers

Resonant transmission of light has been observed in symmetric Fibonacci TiO2/SiO2 multilayers, which is characterized by many perfect transmission peaks. The perfect transmission dramatically decreases when the mirror symmetry in the multilayer structure is deliberately disrupted. Actually, the feature of perfect transmission peaks can be considered as general evidence for dielectric multilayers with symmetric internal structure. It opens a unique way to control light propagation.

Transformations in sputter-deposited thin films of NiTi shape memory alloy

Abstract Amorphous NiTi films deposited by magnetron sputtering have been studied. The results show that it is easier for precipitation to occur in crystallized films and that, in the films aged at 550 °C for 1 h, the parent phase transforms to rhombohedral phase at 27 °C and then to martensitic phase at 2 °C.

X-ray diffraction pattern of quasiperiodic (Fibonacci) Nb-Cu superlattices

Abstract 1-D quasiperiodic Nb-Cu metallic superlattices have been prepared by magnetron sputtering. The X-ray diffraction pattern may be indexed by the projection method from the high-dimensional periodic structure.

Selectable-frequency and tunable-Q perfect transmissions of electromagnetic waves in dielectric heterostructures

Multiple perfect transmissions of electromagnetic waves are found in the photonic band gap of the symmetric dielectric heterostructures (SDH) constructed as (AB)nBm(BA)n, where A and B stand for different dielectric materials, and m and n are the repeating numbers of the units. The photonic frequency and the mode number of resonant transmissions therein can be manipulated by varying m, and the quality factor Q of the perfect transmission peak increases exponentially with increasing n. These features are experimentally demonstrated in a SDH of TiO2/SiO2 for visible and near infrared light. The possible applications of SDH for the wavelength division multiplexing system are discussed.

Magnetization dependence of training effect of exchange coupling in ferromagnet/FeMn bilayers

The exchange coupling and its training effect are studied as a function of the ferromagnetic layer magnetization by using various ferromagnet/FeMn bilayers with ferromagnetic materials Ni, Ni81Fe19, Ni50Fe50, Co, and Fe. The exchange coupling energy Jex increases with increasing MFM as Jex∝MFM. The training effect of the exchange field is related to both the ferromagnet magnetization and the magnetization reversal mechanism. For ferromagnet/FeMn bilayers with similar magnetization reversal mechanisms, the relative change of the exchange field decreases with increasing magnetization in an exponential manner.

Current-induced domain wall motion in permalloy nanowires with a rectangular cross-section

We performed micromagnetic simulations of the current-induced domain wall motion in permalloy nanowires with rectangular cross-section. In the absence of the nonadiabatic spin-transfer term, a threshold current, Jc is required to drive the domain wall moving continuously. We find that Jc is proportional to the maximum cross product of the demagnetization field and magnetization orientation of the domain wall and the domain wall width. With varying both the wire thickness and width, a minimum threshold current in the order of 106 A/cm2 is obtained when the thickness is equivalent to the wire width. With the nonadiabatic spin-transfer term, the calculated domain wall velocity ν equals to the adiabatic spin transfer velocity u when the current is far above the Walker limit Jw. Below Jw, ν=βαu, where β is the nonadiabatic parameter and α is the damping factor. For different β, we find the Walker limit can be scaled as Jw=α|β-α|Jc. Our simulations agree well with the one dimensional analytical calculation, sug...

Exchange bias in Co/Co3O4 bilayers

Exchange biasing effect in Co/Co3O4(t) bilayers was carefully investigated by a vibrating sample magnetometer. It was surprising that a significant exchange bias effect appeared when the temperature varied from 90 to 220 K. In this temperature range bulk Co3O4 should be in a paramagnetic state because its Neel temperature is about 40 K. The exchange bias field (HE) of Co/Co3O4 fluctuated in the range of 110–130 Oe with various thickness of Co3O4 at 90 K and HE was zero when the temperature was above 220 K. X-ray photoelectron spectroscopy results indicated that a thin CoO layer formed near the interface, which may be due to the interdiffusion between Co and Co3O4 layers. Therefore, the exchange bias effect in Co/Co3O4 bilayers is thought to be dominated by the exchange coupling between the Co and CoO layers. Using the finite-size effect theory, we evaluated the average thickness of interfacial CoO layer to be 39 A. The fluctuation of HE in Co/Co3O4 is probably due to thickness fluctuations of the CoO. Ass...

Bias dependent tunneling in ferromagnetic junctions and inversion of the tunneling magnetoresistance from a quantum mechanical point of view

In the framework of the free-electron approximation, we have developed a quantum mechanical treatment for describing the bias dependent tunneling in FM/I/FM ferromagnetic junctions. In our theory, the Slonczewski model is extended to include the bias effect. In the barrier region, the Wentzel–Kramers–Brillouin wave function is used following Harrison. The main point of our treatment is to match the wave functions at both sides of the electrode/barrier interfaces quantum mechanically. We find that apart from the usual density of states effect, there exists a quantum coherent factor D(Ex,V)=κR2(Ex,V)−kR↑(Ex,V)kR↓(Ex,V), which decreases monotonously with the increasing applied bias and could change its sign at a sufficiently high bias. The characteristic of this coherent factor can explain the observed rapid decrease of tunneling magnetoresistance (TMR) with increasing bias and the sign change of TMR in some ferromagnetic junctions. Furthermore, numerical results for asymmetry barrier junctions provide a goo...

Positive isotropic resonance field shift of exchange coupled wedged-permalloy/FeMn bilayers

Exchange-coupled wedged-permalloy/uniform-FeMn bilayers are studied by ferromagnetic resonance and magnetometry measurements with applied field parallel to the film plane. An additional weak resonance peak was observed besides an intense resonance, indicating the existence of interfacial diffusion. For all samples, the exchange field measured by the ferromagnetic resonance is close to that by magnetometry measurements. For the wedged-Py/FeMn bilayers, the in-plane isotropic resonance field shift is positive and inversely proportional to the ferromagnetic layer thickness. It is originated from either specific geometry of Py layer thickness or interfacial diffusion.

Thermal stability of exchange bias in FeMn based bilayers

The thermal stability of the exchange bias field HE in samples of Cu (30 nm)/[ferromagnetic (FM) FeMn]/Cu (30 nm) and Cu (30 nm)/(FeMn/FM)/Cu (30 nm) prepared with an ultrahigh vacuum magnetron sputtering system was studied experimentally and by model calculations. The samples were heated to various temperatures and then kept for 10 min in an applied field of 1000 Oe antiparallel to the cooling field. It was found that HE decreased when the annealing temperature was increased and became zero after annealing at about 100 °C. When the annealing temperature was further increased, HE changed sign. Although the exchange bias field and the coercivity are strongly dependent on various factor, e.g., the stacking order of the layers, the thickness and the magnetization of FM layer, the thermal stability of the exchange bias field is influenced only by the thickness of antiferromagnetic (AFM) layer. A magnetic training effect appeared and became more apparent after annealing in a field antiparallel to the cooling f...