AF Layer Research Articles

Fe3O4@CNTs/WPU@CNF aerogel/Ag foam composites with a double-layer structure for absorption-dominated electromagnetic shielding performance

With the popularity of electronic devices, absorption-dominated multilayered electromagnetic shielding materials are increasingly attractive due to the effective decrease of second pollution compared to the traditional homogeneous electromagnetic shielding materials. However, the design of multilayered structure and strong interface adhesion of functional composite while keeping light weight and practical use is not readily to realize. Therefore, we propose herein a double-layer aerogel/foam composite electromagnetic shielding composite. The absorption layer is made by a nanocellulose (CNF)@waterborne polyurethane (WPU) aerogel, in which magnetic nano iron tetraoxide (Fe3O4) joined with carbon nanotubes (CNTs) to optimize the impedance matching. The reflection layer is a chemically silver-plated foam (AF) used to reflect the transmitted electromagnetic waves through its strong electrical conductivity, improving the overall ability of electromagnetic waves shielding and achieving enhanced electromagnetic shielding performance. The two layers are strongly combined by the adhesion of WPU without the use of adhesives. When the mass ratio of CNTs to Fe3O4 of 2:1 and the total mass fraction of 5 wt%, the EMI shielding efficiency of the composites reaches 62.81 dB in the X-band (8.2–12.4 GHz) at the thickness of 4 mm and the maximum value reaches a remarkable 76.63 dB at 1.17 GHz. The specific SE value of SSE/t reaches 1744.72 dB cm2 g−1. Thanks to the effect of AF layer, the average reflectivity is only 0.141, demonstrating an absorption-dominated shielding mechanism. In addition, the as-prepared composite material is remarkably flexible and deformation-resistant, with a density of only 0.0913 g cm−3 and can withstand approximately 85,000 times its own weight.

Influence of grain boundaries, interface roughness and non-collinear magnetic configurations in the AF layer on the exchange bias properties of F/AF nanodots: A numerical investigation

Motivated by the insufficient understanding of the drastic effects of the disordered interfacial phases at the interface in F/AF bilayers on the exchange bias properties, we consider a realistic model for F/AF nanodots which accounts for roughness at the interface, grain boundaries (GB) and magnetic frustration in the AF layer. Then, we investigate the effect of the magnetic disorder at the F/AF interface on the exchange field by means of Monte Carlo simulations. First, using a simplified model based on a perfect crystal structure (without GB) and with an ideal interface, our results indicate that non-collinear ordered magnetic configuration at the F/AF interface (due to frustration) causes a small decrease of the exchange field (about few dozen percent). Including GB in the model, we show that this kind of structural disorder produces disordered magnetic configurations with domains at the F/AF interface. This magnetic disorder does not lead to a decrease of the exchange field compared to the perfect crystal but makes the AF layer more stable. Our study also reveals that adding roughness in the form of a percentage of mixed F and AF moments in the interfacial layer results in a more significant decrease of the exchange field (two orders of magnitude) leading to values which are comparable to experimental values.

Electrowetting on polyimide and silicon substrates with high hysteresis

Electrowetting-on-dielectric (EWOD) with atypical liquid-substrate combinations has growing utility for a range of microfluidic devices. However, there are challenges to understanding and utilizing the EWOD behavior on rougher substrates with potentially heterogeneous wetting conditions that cause high contact angle (CA) hysteresis. Accurate and consistent EWOD responses can be obtained for these substrates by measuring advancing and receding CAs at constant applied potential. This method was used to characterize the EWOD (DC) responses of two ionic liquids and an aqueous solution on several conductive polyimide and silicon substrates coated with Parylene C/Teflon AF. The results showed Lippmann–Young behavior with liquid-dependent zero-voltage CAs and 13°–40° control authority, 11° average hysteresis, and CA saturation at approximately 80° for all liquids and substrates. Surface roughness and substrate material had little effect on EWOD response and CA hysteresis. The hysteresis was surprisingly high for a Teflon AF-coated substrate. O2 plasma etching of the Parylene prior to Teflon coating may have contributed to the high hysteresis conditions by forming Teflon AF islands or embedding charges into the substrate that affected wetting through the thin Teflon AF layer. CA “ratcheting” was observed during advance corresponding to stick–slip of the droplet triple-line. The stick–slip behavior occurred with all liquid-substrate combinations at sufficiently high voltages and on previously polarized surfaces at lower voltages. Experiments with smooth (“ideal”) Si substrates showed ratcheting only at saturation-level voltages suggesting a link between ratcheting and saturation.

Modulated exchange bias in NiFe/CoO/α-Fe2O3 trilayers and NiFe/CoO bilayers

While the exchange bias in ferromagnetic/antiferromagnetic (FM/AF) bilayer and FM1/AF/FM2 trilayer configurations has been widely investigated, the role of an AF2 layer in FM/AF1/AF2 trilayer configurations is still not well understood. In this work, the magnetic properties of NiFe/CoO, NiFe/α-Fe2O3 bilayers, and NiFe/CoO/α-Fe2O3 trilayer were studied comparatively. The microstructure and chemical composition were characterized. Temperature dependent magnetometry reveals increased irreversibility temperature in NiFe/CoO/α-Fe2O3 trilayer compared with NiFe/CoO bilayer. The magnetic hysteresis loops show that the exchange bias (Hex) and coercivity (Hc) depend strongly on the anisotropy of AF layer (CoO, α-Fe2O3 and CoO/α-Fe2O3). Our work shows that the AF1/AF2 interfacial interactions can be used effectively for tuning the exchange bias in FM/AF1/AF2 trilayers.

Low Voltage Electrowetting on Ferroelectric PVDF-HFP Insulator with Highly Tunable Contact Angle Range.

We demonstrate a consistent electrowetting response on ferroelectric poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) insulator covered with a thin Teflon AF layer. This bilayer exhibits a factor of 3 enhancement in the contact angle modulation compared to that of conventional single-layered Teflon AF dielectric. On the basis of the proposed model the enhancement is attributed to the high value of effective dielectric constant (εeff ≈ 6) of the bilayer. Furthermore, the bilayer dielectric exhibits a hysteresis-free contact angle modulation over many AC voltage cycles. But the contact angle modulation for DC voltage shows a hysteresis because of the field-induced residual polarization in the ferroelectric layer. Finally, we show that a thin bilayer exhibits contact angle modulation of Δθ (U) ≈ 60° at merely 15 V amplitude of AC voltage indicating a potential dielectric for practical low voltage electrowetting applications. A proof of concept confirms electrowetting based rapid mixing of a fluorescent dye in aqueous glycerol solution for 15 V AC signal.

Durable Superhydrophobic Surfaces via Spontaneous Wrinkling of Teflon AF.

We report the fabrication of both single-scale and hierarchical superhydrophobic surfaces, created by exploiting the spontaneous wrinkling of a rigid Teflon AF film on two types of shrinkable plastic substrates. Sub-100 nm to micrometric wrinkles were reproducibly generated by this simple process, with remarkable control over the size and hierarchy. Hierarchical Teflon AF wrinkled surfaces showed extremely high water repellence (contact angle 172°) and very low contact angle hysteresis (2°), resulting in droplets rolling off the surface at tilt angles lower than 5°. The wrinkling process intimately binds the Teflon AF layer with its substrate, making these surfaces mechanically robust, as revealed by macroscale and nanoscale wear tests: hardness values were close to that of commercial optical lenses and aluminum films, resistance to scratch was comparable to commercial hydrophobic coatings, and damage by extensive sonication did not significantly affect water repellence. By this fabrication method the size of the wrinkles can be reproducibly tuned from the nanoscale to the microscale, across the whole surface in one step; the fabrication procedure is extremely rapid, requiring only 2 min of thermal annealing to produce the desired topography, and uses inexpensive materials. The very low roll-off angles achieved in the hierarchical surfaces offer a potentially up-scalable alternative as self-cleaning and drag-reducing coatings.

Study of angular dependence of exchange bias and misalignment in uniaxial and unidirectional anisotropy in NiFe(111)/FeMn(111)/CoFeB(amorphous) stack

We report the investigation of the in-plane azimuthal angular dependence of the magnetization reversal in the ion beam sputtered exchanged biased NiFe(111)/FeMn(111)/CoFeB(amorphous) stack. Compared to the as-deposited case, the magnetic annealing resulted in 3 fold enhancement in exchange bias but decrease in coercivity. The observed cosine dependence of exchange biased CoFeB layer on the in-plane azimuthal angle of applied field is corroborated with Meiklejohn and Bean model. The training effect associated with the exchange bias showed unconventional increase in coercivity after first cycle of hysteresis loop, while the exchange bias decreases sharply, and for subsequent cycles the exchange bias follows the empirical relation based on the energy dissipation in the AF layer. The ferromagnetic resonance (FMR) measurements also exhibited the in-plane azimuthal angle dependence of the magnetic resonance field indicating that the uniaxial and unidirectional anisotropies are not collinear, although they lie in the same plane. However, no misalignment between the unidirectional anisotropy and the exchange bias direction is observed. The misalignment angle between the uniaxial and unidirectional anisotropy, as measured by FMR, is found to be 10° and 14° for CoFeB and NiFe, respectively. This misalignment is attributed to the interface roughness as revealed by x-ray reflectance measurements.

Layered Compounds AFBiS2: Superior Birefringent Crystals

First-principles calculations predict that SrFBiS2 has a superior birefringence of 1.28, which is larger than the giant birefringence of LaOBiS2 [Wang H, Chin. Phys. Lett. 31 (2013)047802]. An AF layer (i.e., SrF) is shown to be important by the further investigations on AFBiS2 (A=Mg, Ca, Ba). Here MgFBiS2 exhibits the largest inherent birefringence (∼1.66) among single-crystal compounds. The origin of SBF is also discussed based on the electronic structure and refractive index.

Tuning the Granular Contribution to Exchange Bias by Varying the Antiferromagnetic Material but Without Affecting the Ferromagnetic/Antiferromagnetic Interface

The exchange bias properties of ferromagnetic/antiferromagnetic (F/AF) bilayers depend strongly on both the F and AF bulk properties, and the interfacial uncompensated AF spins that magnetically couple the F and the AF materials. Whether it is possible to adjust the bulk properties of the AF layer by changing the nature of the AF material but without affecting the interface is one of the challenges raised by the development of some spintronic devices. In this context, we engineered composite AF materials whose basic composition is: (FeMn/Pt/AF). These structures are made of FeMn and IrMn alloys with the insertion of a thin Pt diffusion/trap barrier to Mn in order to ensure as much as possible the integrity of the AF layer at the interface. Magnetic measurements evidenced that, by changing the nature of the AF material, it remains possible to tune the thermal stability of the AF grains without affecting much the interface.

Dependence of the Exchange Bias on the Thickness of Antiferromagnetic Layer in the Trilayered NiFe/IrMn/NiFe Thin-films

The influence of the antiferromagnetic layer thickness on the magnetic properties of the trilayered thin-film structures with the exchange bias phenomenon was studied in the temperature range from 100 up to 295K. Dependences of the coercive force and exchange bias field on AF-layer thickness were analyzed for all structures with AF layer thickness from 2nm up to 50nm at room and low temperatures. The explanation in terms of the anisotropy and thermal energies competition was given for disclosure of an exchange bias for structures with 2nm AF layer thickness at low temperatures.

Enhancement of the CIGS solar cell’s efficiency by anti-reflection coating with Teflon AF

An anti-reflection (AR) layer of Teflon AF was deposited on the front surface of a Cu(In,Ga)Se2 (CIGS) solar cell with a structure of grid/TCO/ZnO/CdS/Cu(In,Ga)Se2/Mo/glass by using the spin coating method. This AR layer reduced the front-surface reflection, which resulted in high efficiency for the CIGS solar cell. The thickness of the Teflon AF layer was varied to determine the thickness that gave the highest transmittance of incident light into the active absorber of the CIGS solar cell. The optimum thickness of the Teflon AF layer was found to be 105 nm. CIGS solar cells with a Teflon AF layer of 105 nm were constructed, and their efficiencies were compared with those of solar cells without a Teflon AF layer. The average increase in the relative efficiency of the solar cells was 2.63% due to the inclusion of an anti-reflection layer of Teflon AF.

Temperature dependence of the exchange bias properties of ferromagnetic/antiferromagnetic polycrystalline bilayers

Monte Carlo simulations using a granular level model of coupled ferromagnetic/antiferromagnetic (F/AF) layers that accounts for thermal effects on both F and AF grains are performed to calculate the temperature dependence of the exchange bias properties. The influence of the coupling between F grains and of the grain size dispersion is also investigated. The predicted thermal variation of the exchange bias and coercive fields are in good qualitative agreement with experiment. For monodispersive grains, it is shown that the exchange field vanishes below the blocking temperature of the AF grains, i.e. when the AF grains become sufficiently unstable, so that they switch but after the F grains. At the same temperature, the coercive field exhibits a maximum due to the coupling with the AF layer. It is also evidenced that a grain size distribution increases the stability of each layer, thus improving the exchange bias properties.

Exchange bias in Ferromagnetic/Antiferromagnetic bilayer systems with varying microstructure and sequence of layer deposition

Correlations of the exchange bias and surface roughness in bilayer structures of a ferromagnetic (Ni80Fe20)/antiferromagnetic (Ir25Mn75) type (F/AF structures) with varying thickness and sequence of deposition of the AF layer are studied using the angular dependence of the resonance field of the ferromagnetic resonance (FMR). It is found that the roughness of the samples in structures with an AF layer deposited on top of an F layer (TS type) is higher than in structures with an AF layer located at the bottom (BS type). The possible causes of differences in the behavior of the exchange bias as a function of surface roughness in samples with the different location of the AF layer are discussed.

Influence of surface roughness and deposition order on exchange bias in bilayer structures NiFe/IrMn

Using ferromagnetic resonance (FMR) method, magnetic properties of bilayer structures consisting of ferromagnetic (F, Ni80 Fe20 ) and antiferromagnetic (AF, Ir25 Mn75 ) layers with alternative order of F and AF deposition were studied. It was found that the surface roughness is larger for the samples with AF on the top of F layers. The influence of AF layer thickness on the surface roughness and exchange bias for different types of structures was discussed.

Tailoring the Hysteresis Loop of the Si/Cu(10nm)/FeMn(10-30nm)/CoFeB(10nm) Bilayer System

The dependence of hysteretic properties of FeMn/CoFeB (AF/FM) thin bilayers on AF thickness and magnetic annealing temperature is systematically investigated for possible applications in spin-valve devices. Inducement of an easy axis after magnetic annealing is established. The anisotropy is studied by investigating the coercivity along and perpendicular to the induced easy axis using Magneto-Optic Kerr Effect (MOKE) and the AF texture is ascertained by XRD. The existence of an optimum annealing temperature, for which the maximum change in coercivity occurs for a particular AF thickness, is evidenced. The maxima in HC of the bilayer with thickness of AF layer were correlated with FCC FeMn (111) texture of the sample.

Magnetization and Magnetoresistance First-Order-Reversal-Curves Analysis in Spin Valves

This work describes the study on the magnetization reversal for two different spin valves: the first one is characterized by in-plane magnetic anisotropy, and the second by a perpendicular anisotropy. Magnetoresistance-first-order-reversal-curve analysis was used in order to map the magnetoresistance of each valve as a function of the coercive fields and the interaction fields of the particles in the layers. For the valve with in-plane anisotropy, it was possible to identify an exchange-spring behavior in the pinned layer induced by the contact with the AF layer, and mean-field effects produced by interparticle interactions. Also, it was determined that the rotation of the magnetization of the pinned layer cannot happen by coherent rotation. For the second valve, it was possible to identify the distribution of coercivities and interaction fields of each layer, as a function the different magnetic orientations that they may have.

A polycrystalline model for magnetic exchange bias

This work introduces a realistic model for the magnetic behavior of polycrystalline ferromagnet/antiferromagnet (FM/AF) systems with granular interfaces. It considers that, for strong enough interface exchange coupling, the AF layer breaks the adjacent FM into small-sized domains and that at the interface there exist grains with uncompensated spins interacting with the FM magnetizations; the classification of these grains as unstable (rotatable, responsible for a coercivity enhancement) or stable (adding to the bias) depends on both the anisotropy and the magnetic coupling with the adjacent FM. The distinctive characteristic of the model is that the effective rotatable anisotropy changes when the external magnetic field is varied resulting in a non-zero hard-axis coercivity, a feature commonly observed, though little understood and often ignored. The applicability of this model was checked on a typical magnetron-sputtered IrMn/Co bilayer and excellent agreement between experiment and simulation was achieved.

Symmetry and the macroscopic dynamics of antiferromagnetic materials in the presence of spin-polarized current

Antiferromagnetic (AFM) materials with zero or vanishingly small macroscopic magnetization are nowadays the constituent elements of spintronic devices. However, possibility to use them as active elements that show nontrivial controllable magnetic dynamics is still discussible. In the present paper we extend the theory [A.F.Andreev, V.I.Marchenko, Sov. Phys. --- Uspekhi, 23 (1980), 21] of macroscopic dynamics in AFMs for the cases typical for spin-valve devices. In particular, we consider the solid-like magnetic dynamics of AFMs with strong exchange coupling in the presence of spin-polarized current and give an expression for the current-induced Rayleigh dissipation function in terms of the rotation vector for different types %generalized potential of AFMs. Basing on the analysis of linearized equations of motion we predict the current-induced reorientation and AFM resonance, and found the values of critical currents in terms of AFMR frequencies and damping constants. We show the possibility of current-induced spin-diode effect and second-harmonic generation in AF layer. The proposed approach is generalized for the description of current-related phenomena in inhomogeneous AFMs.

Temperature-dependent Dielectric Properties in ITO/AF/Al Device

Temperature-dependent dielectric properties were studied in a device with a structure of ITO/amorphous fluoropolymer (AF)/Al. The AF was thermally deposited at a deposition rate of 0.1 ˚ to a thickness of 20 nm under a pressure of 5 ◊ 10 6 Torr. From the dielectric properties of the device, an equivalent circuit for and the equivalent complex impedance Zeq of the device were obtained. The interfacial resistance was found to be approximately 38 . As the temperature was increased, the radius of the Cole-Cole plot and also increased for a constant applied voltage. However, as the applied voltage was increased, those values decreased at a constant temperature. These behaviors are thought to be due to an orientational polarization eect of the molecules inside the AF layer.

Measurement of the antiferromagnet activity in exchange bias systems

In this work, we describe the results of a recent set of experimental measurements on samples of CoFe/IrMn ferromagnet/antiferromagnet (AF) bilayer exchange bias materials. Careful magnetic measurement protocols and detailed particle size measurements have enabled us to confirm the link between the exchange field (Hex) and the distribution of grain volumes in the IrMn AF layer. This can be achieved by fitting the blocking temperature curve to obtain an effective value for the Néel temperature (TN). The value of (TN) is then used to determine the stable and set fraction of the grain volume distribution. Through a comparison of samples with different seed layer structures, we demonstrate control of the exchange bias where almost all the AF grains contribute to Hex. These samples have an AF with a mean grain diameter of 8.7 nm and an anisotropy constant of 7.8×106 ergs/cc. Our results indicate a reduction in the value of the Néel temperature (TN) of up to 100 K.