Ag Granular Films Research Articles

Highly effective antibacterial properties of self-formed Ag nanoparticles/Zr-Ag granular films

Zr-Ag granular films are prepared by magnetron sputtering, on which monodisperse Ag nanoparticles are controllably fabricated through thermal annealing the thin films. The self-formation behaviors of Ag nanoparticles are temperature and Ag content dependent. At the annealing temperature of 360 ℃, the Zr-41.0 wt% Ag granular films have the highest number density of Ag nanoparticles. In vitro antibacterial experiments show that the Ag nanoparticles/Zr-Ag granular films exhibit an excellent antibacterial property, even more than 99.99% for both Escherichia coli and Staphylococcus aureus. The remarkable antibacterial activity can be ascribed to the release of Ag+ from Ag nanoparticles/Zr-Ag granular films and mechanical antibacterial effect of Ag nanoparticles. The results are helpful to the flexible fabrication and development of granular films with high antibacterial performances on different substrates.

Oxygen Effects on Magnetic Properties and Microstructure of FePt(B, Ag) Granular Films

Multilayers [FePt(1 nm)/(B0.7Ag0.3)(0.1 nm)]10 were alternately deposited on a glass substrate and subsequently annealed using a rapid thermal process (RTP) at 800°C for 3 min. Plasma oxidation process was performed on a sputtered (B, Ag) layer. Thereafter, granular FePt(B, Ag) films with perpendicular magnetization were formed. The immiscible (B, Ag) elements were used as a segregant to isolate FePt grains and maintain c-axis alignment. After plasma oxidation and posting annealing, the iron-oxide and boron-oxide were indexed in granular FePt(B, Ag) films. When the oxygen flow ratio was within 1%, the FePt(B, Ag) films show maze-like structures with interconnected grains after plasma oxidation. The oxygen may create vacancies that improve the atomic diffusion during ordering and enhance the grain growth. When the oxygen flow ratio was above 3%, the FePt grain size was furthered reduced to around 4 ~ 5 nm due to the formation of boron oxide. However, the perpendicular magnetization and coercivity of Fe(B, Ag) film deteriorated due to the formation of soft magnetic iron-oxide during postannealing.

Influence of the Interactions on the Magnetotransport Properties of Fe–Ag Granular Thin Films

Fe(x)Ag(100-x) granular thin films, 25 < or = x < or = 40, have been prepared by DC-Magnetron sputtering deposition. These samples are composed of small Fe nanoparticles (2.5-3 nm) embedded in an Ag matrix, and their size remains nearly constant with increasing Fe content. A crossover in the collective magnetic behaviour of the samples near x = 35, had been previously reported for these samples. In this article, we show that the transport and magnetotransport properties are clearly correlated with these magnetic behaviours, and that the resistivity, the magnetoresistance and the Hall resistivity depend on the nature of the most relevant interactions, dipolar or direct exchange, as a function of the thermal evolution and the Fe content.

Giant magnetoresistance in electrodeposited Co–Ag granular films

We report on the electrodeposition of Co–Ag granular films with GMR from a chloride-based electrolyte. Two different electrochemical techniques (chronoamperometry and pulse plating) were used to prepare the films. Both electrochemical and TEM analysis have revealed the heterogeneity of the as-deposited samples. GMR values up to 7% at room temperature were measured. The longitudinal (LMR) and transverse (TMR) magnetoresistance were practically indistinguishable indicative of real granular systems. An appropriate numerical analysis of the magnetoresistance curves showed high superparamagnetic contribution to the total MR.

Relevant GMR in As-Deposited Co−Ag Electrodeposits: Chronoamperometric Preparation

Electrodeposited Co−Ag granular films were prepared by potentiostatic deposition from a chloride-based electrolyte. Different Co(II)/Ag(I) ratios were tested to study the influence of the composition bath on the magnetotranport properties. All as-deposited samples showed GMR effect because both longitudinal and transverse magnetoresistances were negative in the whole field range. GMR values up to 5.85% were measured at room temperature. Electrochemical and structural techniques (XRD, TEM, and electron diffraction) as well as magnetoresistance measurements confirmed the heterogeneity of the deposits with a size distribution from superparamagnetic to ferromagnetic particles. Fcc-Ag and a mixture of hcp-Co and fcc-Co were present in the coatings. An analysis of the magnetotransport properties based on the decomposition of the total magnetoresistance into its ferromagnetic and superparamagnetic contributions revealed a high superparamagnetic contribution in all samples.

Temperature dependence of GMR and effect of annealing on electrodeposited Co–Ag granular films

The magnetoresistance of Co–Ag granular films composed of superparamagnetic and ferromagnetic particles was studied at different temperatures. The increase in the GMR values while decreasing temperature down to 20 K was quantified. The non-saturating behaviour of the MR( H ) curves was retained even at the lowest measurement temperature, which was mainly attributed to the dipolar interaction among the superparamagnetic particles. The influence of the annealing conditions on the magnetoresistance was also studied. In all conditions, a decrease in the GMR values was measured being attributed to an increase in the particle size.

Microstructure and magnetoresistance effect of Fe–Ag granular films without any heat treatment

Fe–Ag granular films were prepared using magnetron sputtering. The magnetoresistance value of Fe–Ag film prepared at the sputtering pressure of 0.1 Pa is about 24% at room temperature. X-ray diffraction and vibrating sample magnetometer measurement indicate that some magnetic particles were separated from the Fe–Ag supersaturated solid solution without any heat treatment. The phase separation and the stronger texture of film may result in the higher giant magnetoresistance value.

Photoinduced magnetic softening of perpendicularly magnetized L1-FePt granular films

Ultrafast spin dynamics has for the first time been studied in perpendicular magnetized granular films. For FePt continuous films and FePt–MgO and FePt–Ag granular films with femtosecond laser excitations, the coercivity HC and the saturation Kerr rotation θKS are dramatically reduced, accompanied by a sharp increase in the reflectivity R. Afterward, these physical quantities are slowly recovered. The changes in HC, θKS, and R are all different among FePt, FePt–MgO, and FePt–Ag films. The difference is caused by different film thicknesses and in particular by the surface plasmon resonance of metallic nanoparticles.

Nanogranular Fe-Cu-Ag Thin Films: Structure, Microstructure and Giant Magnetoresistance

Fe(x)Cu(y)Ag(z) granular thin films with several compositions were prepared by dc magnetron sputtering. These films consist of small Fe magnetic particles embedded in a nonmagnetic CuAg matrix. Structure, microstructure, morphology and magnetotransport properties were studied. The compositions of these samples were determined by energy-dispersive X-ray analysis. X-ray diffraction results showed strong Ag(111) peaks and broad Cu(111) peaks in all the samples. The variation of the (111) lattice spacings indicates a partial intermixing of Fe, Cu and Ag atoms. Microstructural studies using transmission electron microscopy (TEM) on a selected sample showed only Ag reflections and no reflection from Cu and Fe. Both XRD and TEM studies did not reveal any diffraction peak due to Fe and Cu for this sample. The fitting of the experimental grain size data obtained from TEM micrograph to the lognormal distribution function has allowed an estimation of the average grain diameter of 3.7 nm. The surface image of the Fe22Ag78 film observed using a scanning electron microscope showed the presence of droplet like Ag particles on the film surface. The Cu substitution results in smooth films without any Ag particles on the surface. Surface morphology by atomic force microscopy shows that the Fe39Cu13Ag48 film has a surface roughness of 0.75 nm. Finally, we have obtained a maximum giant magnetoresistance ratio of 3.2% in these films measured at 300 K for an in-plane magnetic field of 20 kOe.

Inverse Magnetoresistance Loops in Nearly Percolating Co–Ag Nanostructures and Two-Phase Model

We observed unusual inverse magnetoresistance (MR) loops in the transverse configuration for nearly percolating Co–Ag granular thin films deposited by a coevaporation technique onto a room-temperature glass substrate. When the substrate was heated above about 60°C during evaporation, the inverse MR loops disappeared and converted into normal loops. On the other hand, when the sample was annealed below about 120°C after deposition onto a room-temperature substrate, the inverse MR loops remained, whereas when annealed from about 120 to 300°C, the inverse MR loops were converted into “normal+inverse”-type loops and finally into totally normal loops when annealed above about 350°C. The unusual MR loops may have originated from magnetic nanostructures in nearly percolating Co–Ag granular films. A two-phase model incorporating uniaxial anisotropy, exchange interaction and spin-dependent scattering is presented, which is able to qualitatively explain some of the observed results.

Magnetic properties and giant magnetoresistance in electrodeposited Co–Ag granular films

Abstract Small cobalt particles embedded in a silver matrix have been prepared using the electrodeposition technique. The size of the clusters is controlled by the deposition potential and the Co growth time. Structural, magnetic and magneto-transport properties of Co–Ag samples have been investigated as a function of the Co concentration between 2 and 40 at% cobalt. Superparamagnetic behavior is evidenced for the low contents of cobalt while long-range magnetic order appears at higher Co concentrations. The particles size has been determined from magnetic properties and from the X-ray diffraction technique, and varies between 3.5 and 9 nm. Magnetoresistance passes through a maximum as a function of the cobalt concentration. A maximum of ∼4% GMR is obtained at room temperature while GMR reaches a value of 14% at 10 K.

Structure and magnetic properties of FePt/Ag nano-granular films with perpendicular magnetic anisotropy

［FePt 2 nm/Ag d nm］10 (d=0—11) multilayers were prepared on single crystal MgO(100) substrates kept at room temperature and 250 ℃ by means of magnetosputtering. L10-FePt/Ag granular films with perpendicular magn etic anis otropy and high coercivity have been obtained after vacuum annealing the multila yers deposited on substrates kept at 250 ℃ for 15 min. X-ray diffraction result s indicate that FePt particles have a strong ［001］ orientation. The size of Fe Pt particles decreases with the increase of the thickness of Ag layer. After vac uum annealing the ［FePt 2 nm/Ag 9 nm］10 films at 600 ℃ for 15 min, FePt /Ag granular films are obtained with only 8 nm of particle size and 692 kA/m of coercivity. Isothermal magnetization remanence and dc-demagnetization remanence measurements show that no magnetic exchange coupling between magnetic particles can be observed. Such a kind of films is suitable for future ultra-density perp endicular magnetic recording medium.

Structural and magnetic properties of Al doped Co–Ag granular films: temperature dependence of magnetoresistance

The correlation between the structure, the magnetic and magnetotransport properties of sputter-deposited (Co 90Al 10) x Ag 100− x granular films (19⩽ x⩽45) are investigated. For an (Co 90Al 10) content of about 28% the magnetoresistance (MR) ratio of the films reach its highest value of 6%. Al doping reduces the MR ratio in CoAg films, however, leads to a almost constant MR ratio against post annealing up to 773 K in (Co 90Al 10) 28Ag 72 granular films, although the film structure is affected by annealing, respectively. According to magnetization measurements, superparamagnetic cobalt grains in the as-deposited (Co 90Al 10) 28Ag 72 sample show after annealing up to 773 K both superparamagnetic and ferromagnetic behaviour. Annealing at 823 K causes a domain formation of large ferromagnetic grains with strong correlation.

A phenomenological theory of the granular size effect on the giant magnetoresistance of granular films

Based on the existence of size distribution of magnetic granules in granular films, the magnetic granules can be divided into three categories: superparamagnetic, single domain ferromagnetic and multidomain ferromagnetic granules. In the present paper we take respectively the critical sizes D 1( T) and D 2( T) as phenomenological parameters at different temperatures distinguishing single domain ferromagnetic granules from superparamagnetic granules and from multidomain granules and use the log-normal distribution function describing the size distribution of magnetic granules to investigate the different effects of three kinds of magnetic granules on giant magnetoresistance (GMR) by combining effective medium theory with a two-channel conducting model. In the calculation of Co–Ag granular films as an example, only when single domain ferromagnetic granules possess larger spin-asymmetric scattering factor as weight with GMR, the calculated results are qualitatively in agreement with the various experiments reported, from which it indicates that single domain ferromagnetic granules play a key role in GMR.

Giant magnetoresistance in Co–Ag granular films prepared by electrodeposition

Preparation of granular magnetic Co–Ag films produced by pulsed electrodeposition from a chloride bath containing both cobalt ions and a low concentration of silver has been investigated. Deposition of cobalt on Ag is performed by a double pulse method. Combining in situ electrochemical and microgravimetric measurements, the kinetics of silver and cobalt reduction are presented. The thickness and deposition rates are monitored using an electrochemical quartz crystal microbalance (EQCM) during the growth of each material. Magnetic measurements have shown a superparamagnetic behavior in agreement with the existence of very small cobalt particles. Giant magnetoresistance (GMR) of ∼2% at room temperature is observed for the Co 5Ag 95 sample.

Microstructures of FePt–Al–O and FePt–Ag nanogranular thin films and their magnetic properties

The microstructure and magnetic property relationship in L10 ordered FePt–Al–O and FePt–Ag granular thin films have been studied. As-sputtered FePt–Al–O films, composed of isolated fine spherical FePt particles of ∼2 nm diameter with a disordered face-centered-cubic (fcc) structure, exhibit superparamagnetism. Annealing above 650 °C induces a transformation from a disordered fcc structure to ordered L10 phase and the films become magnetically hard. The microstructures of these films change greatly depending on the film compositions and annealing conditions, which are correlated with the magnetic properties. It was found that FePt particles smaller than 5 nm do not order at 500 °C, while the continuous FePt film orders perfectly at the same temperature, suggesting that the ordering temperature, Tc, decreases significantly when the particle size becomes less than 5 nm. In the FePt–Ag granular thin film, when the Ag composition is around 50 at. %, high coercivity (∼10 kOe) and fine uniform microstructure are obtained by annealing the films at 550 °C for 1 h. The changes in the magnetic properties of these granular films are discussed based on microstructural observation results.

Magnetic properties and domains in Co–Ag granular thin films

The intrinsic perpendicular magnetic anisotropy and the domain patterns in granular CoxAg1−x films with 17%⩽x⩽62% are studied. Magnetic stripe domains are found in samples with x⩾45% and to change to in-plane domain patterns with increasing annealing temperature. This is consistent with the fact that perpendicular magnetic anisotropy becomes negative with annealing.

Structural and magnetic properties of PLD Co–Ag granular thin films

An investigation of static magnetic properties of Co x Ag 1− x ( x=0.25, 0.40) granular films grown by the pulsed laser deposition technique is reported for samples heat treated at different temperatures. Different terms have been found to contribute to the observed temperature dependence of the magnetization: isolated clusters, superparamagnetic particles and larger blocked particles. The increase of the deposition temperature up to 250°C for x=0.40 leads to an increase of the fraction of superparamagnetic particles. An annealing treatment at 250°C for x=0.25 determines the growth of particles size.

GMR in co-evaporated CoAg granular thin films

We report the magnetic and magnetoresistive behavior of Co x Ag 100− x granular films over a wide range of compositions ( x = 20–80 at%), deposited by oblique co-evaporation at room temperature. The average size of magnetic particles estimated from the magnetization data, assuming a log-normal distribution function, varies with x from 20 to about 50 Å. The GMR variation with x shows a broad maximum at x max = 34 at% at T = 300 K, which increases ∼ 3 times at 10 K while x max shifts to 25 at%.

Characterization and giant magnetoresistance effect in cobalt–silver granular films formed by MEVVA implantation

Cobalt–silver granular thin films exhibiting giant magnetoresistance (GMR) effect were formed by Co implantation into Ag using a metal vapor vacuum arc (MEVVA) ion source. The magnetic field dependence and the temperature variation of the GMR effect and their relation with the processing conditions were studied and discussed in conjunction with results of Rutherford backscattering spectrometry, atomic force microscopy and magnetic force microscopy (MFM). For the best sample obtained in this study, the magnitude of the GMR effect measured at a magnetic field strength of 7.6 kOe increases from about 1% at room temperature to over 7% at 20 K. The coercive field H C in the perpendicular-to-film direction determined from GMR measurements showed an anomalous temperature variation that shows a maximum value at around 240 K and decreases with decreasing temperature from 240 to 20 K. The domain structures of the implanted granular films revealed by MFM images exhibit a very different feature compared with those of sputter deposited Co–Ag granular films.