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An efficient contrast agent for magnetic resonance imaging (MRI) is essential to enhance the detection and characterization of lesions within the body. In this study, we described the development of biodegradable nanoparticles with a core-shell structure to formulate superparamagnetic iron oxide (CSNP-SPIO) for MRI. The developed nanoparticles were composed of a hydrophobic PLGA core and a positively-charged glycol chitosan shell. The results obtained by transmission electron microscopy, energy dispersive X-ray analysis, electron energy loss spectroscopy, and X-ray diffraction measurement confirmed that the prepared nanoparticles had a core-shell structure with SPIO in their core area. No aggregation of nanoparticles was observed during storage in water, as a result of the electrostatic repulsion between the positively-charged nanoparticles. The magnetic properties of nanoparticles were examined by a vibrating sample magnetometer and a superconducting quantum interference device; the results showed that the superparamagnetism of SPIO was preserved after the CSNP-SPIO formulation. In tracking their cellular internalization pathway, we found that CSNP-SPIO accumulated in lysosomes. In the biodistribution study, a high level of radioactivity was observed in the liver shortly after administration of the (99m)Tc-labeled CSNP-SPIO intravenously. Once taken up by the liver cells, the liver turned dark on T(2)* images. Following cellular internalization, CSNP-SPIO were broken down gradually; therefore, with time increasing, a significant decrease in the darkness of the liver on T(2)* images was found. The aforementioned results indicate that the developed CSNP-SPIO can serve as an efficient MRI contrast agent and could be degraded after serving their imaging function.

Low-temperature-ordering of L10 ordered FePt films have been extensively studied in recent years due to its potential for future application on magnetic perpendicular media. The predominant issue of the ordering process is the diffusion of iron and platinum atoms from a disordered to an ordered phase. The diffusion can be enhanced by adjusting diffusivity, providing extra energy, or reducing energy barriers. In addition to reducing the ordering temperature of FePt, (001)-oriented granular films require perpendicular media because the magnetic easy axis of the ordered phase is [001]. Atomic-scale multilayer deposition is proposed to achieve designed film structures.

We demonstrate an effective room-temperature chemical solution treatment, by using thioacetamide (S treatment) or thioacetamide-InCl3 (In-S treatment) solution, on Cu(In,Ga)Se2 (CIGSe) surface to engineer the ZnS(O,OH)/CIGSe interface and junction quality, leading to enhanced efficiency and minimized metastability of flexible solar cells. The control device without treatment reveals a relatively low efficiency of 8.15%, which is significantly improved to 9.74% by In-S treatment, and 10.39% by S treatment. Results of X-ray photoelectron spectroscopy suggest that S is incorporated into CIGSe surface forming CIGSSe by S treatment, whereas a thin In-S layer is formed on CIGSe surface by In-S treatment with reduced amount of S diffusing into CIGSe. PL spectra and TRPL lifetime further reveal that S incorporation into CIGS surface may substitute the OSe and/or directly occupy the vacant anion site (VSe), resulting in the effective passivation of the recombination centers at CIGSe surface. Moreover, reducing the concentrations of VSe may thereby decrease the density of (VCu-VSe) acceptors, which can minimize the metastability of ZnS(O,OH)/CIGSe solar cells. With S treatment, the light soaking (LS) time of ZnS(O,OH)/CIGSe device is reduced approximately to one-half of control one. Our approach can be potentially applied for alternative Cd-free buffer layers to achieve high efficiency and low metastability.

A design for the fabrication of metallic nanoparticles is presented by thermal dewetting with a chemically heterogeneous nano-template. For the template, we fabricate a nanostructured polystyrene-b-polydimethylsiloxane (PS-b-PDMS) film on a Si|SiO2 substrate, followed by a thermal annealing and reactive ion etching (RIE) process. This gives a template composed of an ordered hexagonal array of SiOC hemispheres emerging in the polystyrene matrix. After the deposition of a FePt film on this template, we utilize the rapid thermal annealing (RTA) process, which provides in-plane stress, to achieve thermal dewetting and structural ordering of FePt simultaneously. Since the template is composed of different composition surfaces with periodically varied morphologies, it offers more tuning knobs to manipulate the nanostructures. We show that both the decrease in the area of the PS matrix and the increase in the strain energy relaxation transfer the dewetted pattern from the randomly distributed nanoparticles into a hexagonal periodic array of L10 FePt nanoparticles. Transmission electron microscopy with the in situ heating stage reveals the evolution of the dewetting process, and confirms that the positions of nanoparticles are aligned with those of the SiOC hemispheres. The nanoparticles formed by this template-dewetting show an average diameter and center-to-center distance of 19.30 ± 2.09 nm and 39.85 ± 4.80 nm, respectively. The hexagonal array of FePt nanoparticles reveals a large coercivity of 1.5 T, much larger than the nanoparticles fabricated by top-down approaches. This approach offers an efficient pathway toward self-assembled nanostructures in a wide range of material systems.

This study demonstrates the wireless magnetostrictive type inductive sensing CMOS-MEMS pressure sensors using the TSMC 0.18μm 1P6M process. Features of this study are: (1) High sensitivity pressure sensors are realized based on the inverse-magnetostrictive sensing principle; (2) metal and dielectric layers of CMOS process are employed to form the magnetic coil and sensing diaphragm respectively; (3) additional magnetostrictive CoFeB film was deposited and patterned by the shadow sputtering; (4) wireless sensing is available by the external reading coil. Experiments show sensors with gauge-factor ranging 480∼1100 are achieved, and wireless sensing capability is also demonstrated.

The tunnel magnetoresistance (TMR) in the magnetic tunnel junction (MTJ) with embedded nanoparticles (NPs) was calculated in range of the quantum-ballistic model. The simulation was performed for electron tunneling through the insulating layer with embedded magnetic and nonmagnetic NPs within the approach of the double barrier subsystem connected in parallel to the single barrier one. This model can be applied for both MTJs with in-plane magnetization and perpendicular one. We also calculated the in-plane component of the spin transfer torque (STT) versus the applied voltage in MTJs with magnetic NPs and determined that its value can be much larger than in single barrier system (SBS) for the same tunneling thickness. The reported simulation reproduces experimental data of the TMR suppression and peak-like TMR anomalies at low voltages available in literature., Comment: 9 pages, 6 figures

In the past decades, mesocrystal, a kind of nanocrystals with specific crystallographic orientation, has drawn a lot of attention due to its intriguing functionalities. While the research community keeps searching for new mesocrystal systems, it is equally crucial to develop new approaches to tune the properties of mesocrystals. In this work, a self-organized two-dimensional mesocrystal composed of highly oriented CoFe2O4 (CFO) nano-crystals with assistance of different perovskite matrices is studied as a model system. We have demonstrated that the strain state and corresponding magnetic properties of the CFO mesocrystal can be modulated by changing the surrounding perovskite matrix through their intimate structural coupling. Interestingly, this controllability is more strongly correlated to the competition of bonding strength between the matrices and the CFO mesocrystals rather than the lattice mismatch. When embedded in a matrix with a higher melting point or stiffness, the CFO mesocrystal experiences higher out-of-plane compressive strain and shows a stronger magnetic anisotropy as well as cation site-exchange. Our study suggests a new pathway to tailor the functionalities of mesocrystals.

Magnetic patterning, with designed spatial profile of the desired magnetic properties, has been a rising challenge for developing magnetic devices at nanoscale. Most existing methods rely on locally modifying magnetic anisotropy energy or saturation magnetization and thus post stringent constraints on the adaptability in diverse applications. We propose an alternative route for magnetic patterning: by manipulating the local intergranular exchange coupling to tune lateral magnetic properties. As demonstration, the grain boundary structure of Co/Pt multilayers is engineered by thermal treatment, where the stress state of the multilayers and thus the intergranular exchange coupling can be modified. With Ag passivation layers on top of the Co/Pt multilayers, we can hinder the stress relaxation and grain boundary modification. Combining the pre-patterned Ag passivation layer with thermal treatment, we can design spatial variations of the magnetic properties by tuning the intergranular exchange coupling, which diversifies the magnetic patterning process and extends its feasibility for varieties of new devices.

The Cu2ZnSn(S,Se)4 (CZTSSe) absorbers were fabricated by two step process which consists of deposited stacking precursor followed by selenization. We investigated the influence of adding sodium into CZTSSe absorbers via sputtering NaF layer during the fabrication of precursor, or evaporating NaF layer on top of CZTSSe absorbers followed by vacuum annealing. We discovered that the NaF layer at the bottom of precursor can change the morphological properties of CZTSSe absorbers; on the contrary, the NaF layer on the top of precursor show no significant change. The increasing thickness of NaF layer revealed positive dependence on crystallinity of CZTSSe absorbers and increased the carrier concentration. However, we found device performance to be worse than those without NaF addition.

The effect of Ag incorporation on the kesterite CZTSSe thin film by vacuum process has been investigated aiming for the band engineering of the kesterite solar cells. Phase stability, crystalinity, morphology and optical properties of Ag2ZnSnSe thin film were firstly reported and measured by XRD, SEM and UV-visible spectrometer in this work. We have successfully fabricated the ACZTSe thin films over a wide range of composition and the optical band gap could be controlled by the Ag/(Cu+Ag) ratio in a range from 1.06eV to 1.3 eV. Finally, the cell performance around 4% was demonstrated by the ACZTSSe devices.

Spray pyrolysis is a simple way to fabricate CZTSSe and has potential for low-cost industrial manufacturing. Doping of elements can be easily conducted using spray pyrolysis by addition of dopant salts into solution. Aluminum doping in CZTSSe might be an alternative way to adjust bandgap for application to tandem cell or bandgap grading. In this paper, we demonstrate the Al-doing of CZTSSe by spray pyrolysis. Aluminum is introduced by adding AlCl3•6H2O into the precursor solution of CZTSSe. With aluminum in CZTSSe, the crystal structure remained kesterite, but c/a ratio is slightly changed. However, small grain layer was observed when aluminum was incorporated in this film, but this layer might be reduced by optimizing selenization condition.

Sputtering from a single CIGS compound target is a promising process to fabricate CIGS absorbers for mass production. However, only few reports demonstrate efficient CIGS cells without post-selenization. The need for the additional post-selenization step may be possibly due to the low Se supply during deposition, which originates from the low Se content in the target. In this study, we proposed a method to supply extra Se, sputtering from a Se target. With this, the Se supply during the quaternary sputtering process is adjusted nominally from 1 to 1.24. The effects of Se supply during the quaternary sputtering process on the film and device properties were studied. Efficiency of 11% was obtained with medium Se flux.

FePt has drawn lots of attention for the next-generation perpendicular magnetic recording media because of its large magnetocrystalline anisotropy. Different segregants have been studied to achieve well-isolated grains of L1 0 FePt films with reduced grain sizes. C addition was reported as a promising segregant to obtain a small grain size of 6 nm with good L1 0 ordering and (001)-orient-ed FePt grains [1]. However, it is quite challenging to achieve columnar grains because extra C segregation forms on the top of FePt grains, leading to a second layer of FePt grains with (111) orientation. Columnar grains with elongated shape along the film thickness enhance the thermal stability and improve SNR. To eliminate the growth of the second layer, film thickness of FePt:C films is limited to less than 7 nm with grain size less than 10 nm. On the other hand, B as a segre-gant was demonstrated to achieve a grain size of 3 nm and good columnar growth with aspect ratio of 4 [2]; however, the B additions deteriorated the ordering of FePt grains, which led to a small coecivity of less than 1 kOe. The deterioration of L1 0 ordering is attributable to the formation of Fe-B bonding in the FePt grains [3].

Epitaxial core-shell CoO-CoFe2 O4 nanocrystals are fabricated by using pulsed laser deposition with the aid of melted material (Bi2 O3 ) addition and suitable lattice mismatch provided by substrates (SrTiO3 ). Well aligned orientations among nanocrystals and reversible core-shell sequence reveal tunable magnetic anisotropy. The interfacial coupling between core and shell further engineers the nanocrystal functionality.

Spin valves composed of TbCo/CoFe/Cu/CoFe/TbFeCo were fabricated with perpendicular magnetization and GMR ratios of 4.5%. The (TbCo/CoFe) layers and (CoFe/TbFeCo) layers are referred to the free and the pinned layers, respectively. The compositions of two layers were chosen to have a lower Curie temperature (130 °C) but higher coercivity (13.2 kOe) of the free layer at room temperature than those of the pinned layer; therefore, the free layer is quite stable at room temperature but its magnetization can be easily switched at a relatively low temperature. Spin valves were patterned into 100-μm-wide cells and their coercivity was reduced with increasing writing current due to the temperature rise by current-heating. When the current density of the writing current was increased to 2.1×106 A/cm2, the required switching field for the free layer was only 10 Oe.

Epitaxial-(1 0 0), textured-(1 0 0) and epitaxial-(1 1 0) Fe/Si multilayers (MLs) were grown by a sputtering system, and the orientation and temperature dependences of magnetoresistance (MR) were studied. The MR of Fe/Si MLs did not strongly depend on the interfacial roughness. Instead, the MR depended on the kind of silicides formed in the spacer. The temperature dependence of MR and the saturation field in these MLs showed the similar tendencies, implying that the kind of silicides in the spacer determined the temperature dependence of MR and interlayer exchange coupling.

The magnetic circular dichroism in X-ray absorption is employed to study the element-specific magnetization in a TbFe/Co bilayer system and patterned Ni 80 Fe 20 film. Taking advantage of the energy tunability as well as the penetration power of synchrotron radiation, the magnetization directions of Co and Fe deposited in different layers were examined as a function of Co film thickness. For patterned films, the photoemission electron microscope reveals a clear correlation between the magnetic configurations and geometries of the patterns.

In this paper, we attempt the theoretical modeling of the magnetic tunnel junctions with embedded magnetic and nonmagnetic nanoparticles (NPs). A few abnormal tunnel magnetoresistance (TMR) effects, observed in related experiments, can be easily simulated within our model: we found, that the suppressed TMR magnitudes and the TMR sign-reversing effect at small voltages are related to the electron momentum states of the NP located inside the insulating layer. All these TMR behaviors can be explained within the tunneling model, where NP is simulated as a quantum well (QW). The coherent (direct) double barrier tunneling is dominating over the single barrier one. The origin of the TMR suppression is the quantized angle transparency for spin polarized electrons being in one of the lowest QW states. The phenomenon was classified as the quantized conductance regime due to restricted geometry., Comment: 4 pages, 5 figures

NiFe/NiMn exchange field was manipulated by an underlayer NiFeCr or by C ion irradiation. Underlayer NiFeCr significantly promoted (1 1 1) texture growth, leading to a higher exchange field. However, the threshold annealing temperature required to reach a large value of exchange field was raised. This can be attributed to well aligned orientations between NiFe and NiMn at the interface, which stabilizes the FCC γ phase of NiMn and increases the nucleation energy. C ion irradiation with the dosage of 3.7×1015/cm2 prior to the annealing increases the exchange field by 21% without altering the blocking temperature (>400°C). The enhancement of the exchange field by irradiation can be related to the increase of defect sites in NiMn or (and) to the new phase NiMn(C) formation having c/a ratio significantly less than NiMn.

Fe/Si multilayers with Cr buffer layers have been grown on various substrates to manipulate the orientations by DC magnetron sputtering. The strength of interlayer coupling is determined by the orientation of the Fe layers, which can be attributed to the various silicide formations. The antiferromagnetic interlayer coupling is strongest in the multilayers with Fe(2 0 0) textures, which may be due to the formation of e-FeSi silicides in spacers. α-FeSi 2 or mixed spacers may exist in Fe/Si multilayers with Fe(1 1 0) textures and polycrystalline films, which show large M r / M s ratios and small saturation fields.

This paper reports on a magnetic device with a [Co40Pd60/Cu]10/Fe structure for use in the detection of H2. In a magneto-optical, transport, and gas-detection system, the proposed device presented sharp, reproducible H2-dependent magnetic/electrical properties. The device's saturation magnetization (induced resistance change) dropped (increased) by a factor of ∼5 when a H2 pressure of 75 kPa is given. Besides, the electrical signal-to-noise ratio of the device can be restored ∼50% by exposing it to a magnetic field of 1000 Oe, even when the sensitivity of the device dropped at a low H2 pressure (0.7 KPa). This demonstrates the applicability of the device for use as a low-pressure H2 detector. Operando x-ray spectroscopy revealed that changes in H2-induced magnetism arose from a Co-Pd charge transfer effect coupled to changes in the local-structure symmetry.

Nanopatterned FePt nano-dots often exhibit low coercivity and a broad switching field distribution, which could arise due to edge damage during the patterning process causing a reduction in the $L1_{0}$ ordering required for a high magnetocrystalline anisotropy. Using an atomistic spin model, we study the magnetization reversal behavior of $L1_{0}$ FePt nanodots with soft magnetic edges. We show that reversal is initiated by nucleation for the whole range of edge widths studied. For narrow soft edges the individual nucleation events dominate reversal; for wider edges, multiple nucleation at the edge creates a circular domain wall at the interface which precedes complete reversal. Our simulations compare well with available analytical theories. The increased edge width further reduces and saturates the required nucleation field. The nucleation field and the activation volume manipulate the thermally induced switching field distribution. By control of the properties of dot edges using proper patterning methods, it should be possible to realize exchange spring bit patterned media without additional soft layers.

The A1- L10 phase transformation has been investigated in (001) FeCuPt thin films prepared by atomic-scale multilayer sputtering and rapid thermal annealing (RTA). Traditional x-ray diffraction is not always applicable in generating a true order parameter, due to non-ideal crystallinity of the A1 phase. Using the first-order reversal curve (FORC) method, the A1 and L10 phases are deconvoluted into two distinct features in the FORC distribution, whose relative intensities change with the RTA temperature. The L10 ordering takes place via a nucleation-and-growth mode. A magnetization-based phase fraction is extracted, providing a quantitative measure of the L10 phase homogeneity., 17 pages, 5 figures, 4 page supplementary material (4 figures)

Intriguing functionalities at nano-sized domain walls have recently spawned a new paradigm for developing novel nanoelectronics due to versatile characteristics. In this study, we explore a new scenario to modulate the local conduction of ferroic domain walls. Three controlling parameters, i.e., external electrical field, magnetic field and light, are introduced to the 90° domain walls (90° DWs) of BiFeO3. Electrical modulation is realized by electrical transport, where the mobility of 90° DWs can be altered by gating voltage. We further use the ferromagnetic/antiferromagnetic coupling to reveal the inherent magnetism at the DWs. With an established magnetic nature, magnetotransport has been conducted to introduce magnetic controlling parameter, where a giant positive magnetoresistance change can be observed up to 200%. In addition, light modulated conduction, a core factor for multifunctional applications, is successfully demonstrated (current enhancement by a factor of 2 with 11 W white lamp). These results offer new insights to discover the tunability of domain wall nanoelectronics.

Non-toxic, earth abundant absorber material CZTSSe thin film was successfully fabricated by spray pyrolysis, which is a low-cost, convenient and non-vacuum process. In this work, CZTSSe thin film is made from spray-deposited precursor stacks with subsequent sulfurization and selenization. The effect of the selenization time on the CZTSSe growth was investigated. X-ray diffraction, Raman spectrum and scanning electron microscope are used to characterize the precursors and CZTSSe thin films. The device is characterized by current-voltage measurements. By this process, the power conversion efficiency of the fabricated device reaches 3.1%.

Ultrathin ferromagnetic biaxial films exhibit uniaxial anisotropy induced by underlayer and double shifted magnetization curves. Two possible origins of the double shifted curves are identified from experimental data, namely, the step-induced anisotropy and the exchange-coupled induced interfacial anisotropy. Micromagnetic study of modified astroids with starlike shapes indicates that a large uniaxial anisotropy induced by the interface plays a key role in the formation of double shifted magnetization waves. An additional bias field along the easy axis may enhance the shifted field. The present research status, both experimental and theoretical, is briefly discussed.

Polycrystalline La1−xCaxMnO3 films were rf-sputtered on Si substrates coated with SiO2, and the effects of deposition parameters on the temperature coefficient of resistance (TCR) values were investigated. For the temperature higher than the metal–insulator transition temperature (Tp), the temperature dependence of resistance can be described by using the small polaron model, and the TCR value increases with the activation energy Ea for the polaron hopping. For the temperature higher than Tp, increasing the oxygen or Ca content of the film results in the decrease in resistivity, Ea, and TCR value. The resistivity and TCR value are also reduced in large-grained films. Applying a substrate bias can enhance the TCR value.

High magnetization (Mr) and enhanced coercivity magneto-optical (MO) recording media TbFeCo (TFC) were investigated for hybrid magnetic flux reading/optical recording due to their large magnetic flux. To increase the stability of high Mr TFC media, dual-layer disks composed of TM-rich readout layer with saturation magnetization (MS)=370 emu/cm/sup 3/ and RE-rich memory layer were fabricated. Coercivity and interface energy between double layers as a function of active layer thickness were examined. By simulating thermo-magnetic writing, the dual-layer media were shown to be applicable for optical writing/magnetic flux reading recording.

The high-frequency characteristics of the RF integrated inductors with antiferromagnetic/ferromagnetic (AF/F) multilayers (MLs) are studied. Each AF layer is 8-nm IrMn and F is Co50Fe50 with a 200-nm splitting into N = 1 to 5 repeats of MLs. This exchange coupled {IrMn/CoFe(d nm)}N MLs are deposited on top of the two-port inductors with five-turn square spiral coils, a dimension of 100 mum times 100 mum and line/space of 5 mum/2 mum. The inductor surface and magnetic layer is separated by 1-mum-thick SiO2. The enhancement of inductance (DeltaL) is 20% compared to an air-core of the same coil size. The resonance peak gradually shifted to a higher frequency with increasing N, and reached at a maximum of 4.3 GHz when N = 5. This is in good agreement with our magnetic data which revealed that the anisotropy field (Hk) and ferromagnetic resonance frequency (fFMR) of {IrMn/CoFe(d nm)}N MLs are increased with increasing N. The quality factor, Q is improved by 6.8% at 1.5 GHz for the {IrMn/CoFe(40 nm)}N=5 integrated inductors compared to air-core inductors.

A simple approach is proposed for fabrication of percolated perpendicular media (PPM). Nano pore array with pore diameter of 12-15 nm and different pore densities has been prepared by anodizing aluminum on silicon wafers. The pore density increases from 3.2times1010 cm-2 to 11.0times1010 cm-2 with the decrease in anodizing voltage from 30 to 10 V. Then, Pt (7 nm)/{Co (0.5 nm)/Pt (2) nm}5 multilayers are deposited onto this porous anodized alumina (AAO) by sputtering. The pore size is reduced to 10-12 nm after deposition of magnetic layers. The Co/Pt multilayers on AAO exhibit perpendicular magnetic anisotropy, squareness ratios of unity, and negative nucleation fields. The perpendicular coercivity increases linearly with the increase in pore density due to the pinning effect imposed by the pores, which is consistent with theoretical calculation for PPM