Granular Thin Films Research Articles

Magnetotransport study of granular chromium dioxide thin films prepared by the chemical vapor deposition technique

The temperature-dependent magnetotransport properties of granular chromium dioxide (CrO 2) films with grain size about 1 μm grown by chemical vapor deposition on Si(0 0 1) substrates has been studied over a temperature range from 2 to 300 K. The magnetoresistance (MR) of about 20% at T=2 K decreases very fast with increasing temperature as a result of the rapid increase of the spin-independent hopping channel, whilst the temperature-dependent MR is almost the same for samples after 100 days annealing in air at room temperature, and its electrical resistivity at room temperature increases from roughly 50 to >75 μΩ m. The contribution to the conductance at T<6 K can be attributed to the intergranular tunneling and that at 6 K<T<25 K is the second order inelastic hopping, which is amenable to σ∼ T 1.33. It becomes a poor metal at temperatures >160 K and its resistance follows R∼T 2 exp(−Δ/T) . We have shown experimentally that the room temperature annealing results in the decomposition of CrO 2 into Cr 2O 3 mainly near the surface of the films.

GMR in Granular CuFe with a Face Centered Tetragonal Structure of Iron

Granular thin CuFe films with a face centered tetragonal (fct) structure of Fe clusters in Cu matrix show a giant magnetoresistance effect. The samples were prepared by co-deposition of 57 Fe and Cu metal in Ultra High Vacuum. The role of the scattering of conduction electrons at the interface between Fe ultrafine particles and Cu matrix has been studied by Mossbauer spectroscopy and magnetoresitivity measurements. The ratio of Fe atoms located at Cu/Fe interface and in the ultrafine particles (bulk) was resolved by Mossbauer spectroscopy. In the system consisting of ultrafine fct-Fe particles imbedded in Cu matrix the giant magnetoresistance effect is not correlated to the ratio of interface/bulk in contrast to observations in FeAg granular system. The spin-dependent scattering of conduction electrons occurs within the ferromagnetic fct-Fe particles.

ＣｏＰｔ‐（Ｃ４Ｆ８）ｎグラニュラー薄膜の磁気特性

We studied CoPt-(C4F8)n granular thin films for use in high-density magnetic recording media. The (C4F8)n matrix in granular film is very hard and flat, and these properties are advantageous for recording media. CoPt-(C4F8)n granular films were fabricated by the facing targets sputtering method with plasma polymerizing reaction. The CoPt alloy was deposited on substrate and the (C4F8)n matrix was polymerized at the same time. After annealing at 300°C, the CoPt-(C4F8)n thin film had a high coercivity of 4.1 kOe. AFM images of the CoPt-(C4F8)n granular thin films indicated that the grain size was decreased by addition of (C4F8)n matrix. It is considered that the CoPt-(C4F8)n granular films has the potential to be used in high-density recording media.

On the bulk averaging approach for obtaining the effective parameters of thin magnetic granular films

An analytical model for the effective magnetic properties of a thin film consisting of a few layers of particles with arbitrary permeability and the stochastical initial magnetization is presented. The model is magnetostatic and implies the explicit averaging of the true field. It is shown that it works for all possible thicknesses of the film (arbitrary number of particles across the film) and there is no need for surface-to-bulk transition. The surface effect leading to the change of the reflective properties of the thin film compared to the prediction of the traditional theory is significant for a sparse arrangement of the grains and vanishes when these are touching. The method is verified by comparison with the model of the lattice of magnetic scattering centers.

Electrical Conduction and Tunneling Magnetoresistance of Fe,Co/MgF2 Granular Multilayers

Electrical transport was investigated in Fe/MgF2 and Co/MgF2 granular thin films prepared by alternating deposition in ultrahigh vacuum. Temperature dependence of the resistivity changed from metallic to semiconductor-like when Fe (Co) layer thickness was less than 45 Å (40 Å) with a fixed MgF2 layer of 30 Å. Discontinuous ferromagnetic layers were formed in films with semiconductor-like conduction and were confirmed with transmission electron micrography. The largest room-temperature magnetoresistance of 8.5% at 15 kOe (11% at 50 kOe) was found in a [Fe(15 Å)/MgF2(20 Å)] film, and that of 5.4% was found in a [Co(15 Å)/MgF2(30 Å)] film. The resistivity of the Fe,Co/MgF2 films, which show large tunneling magnetoresistance (TMR), was one order higher than those of other systems prepared with co-deposition. Tunnelling activation energy, estimated to be from 200 to 1700 K, did not markedly differ from that of other systems.

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 and electrical properties of Fe–MoO x (2⩽ x⩽3) granular thin films

We have synthesized for the first time the new granular system Fe–MoO x (2⩽ x⩽3) by using a sputtering technique. The magnetic measurements reveal that the granular films exhibit superparamagnetism and the interface of the iron granules is oxidized partially. Negative magnetoresistance is observed at low temperatures for the Fe–MoO x ( x∼2) granular thin films synthesized under reducing atmosphere. The behavior of the negative magnetoresistance strongly depends on the ratio of the Fe to the MoO x . It is suggested that the electronic conduction is mainly due to the spin-dependent tunneling through the oxidized interface between the granules and the matrix.

(Fe–Co)–(Mg-fluoride) insulating nanogranular system with enhanced tunnel-type giant magnetoresistance

Insulating nanogranular-type tunnel magnetoresistive thin films made of (Fe or Fe–Co)–(Mg-fluoride) have been investigated. The films were prepared by a tandem deposition method, using Fe, Co, or Fe+Co metal and MgF2 insulator targets. The granular structure was found to consist of Fe or Fe–Co based nanogranules surrounded by thin intergranules of Mg based fluoride with the MgF2 crystal structure. A magnetoresistance value of 13.3% at room temperature and 10 kOe, the largest values ever reported, were obtained at the compositions of 32 vol %(Fe0.51Co0.49)–(Mg–F). To increase the magnetic field sensitivity of the magnetoresistance, a granular-in-gap film consisting of an (Fe–Co)–(Mg–F) granular thin film filling a narrow gap in a soft magnetic Permalloy thin film was prepared. A remarkably high magnetoresistance of 4% or more at 1–2 Oe was obtained.

Zero field viscosity in magnetic thin films and tapes

We have measured the relaxation of the remanent magnetization in a number of granular thin films in which delta-M measurements suggest the presence of exchange coupling. The relaxation measurements were made at zero field after application of a saturation field followed by a reverse field. The films had varying compositions (CoPtCrB, Co/CN, CoPt/SiO/sub 2/) and had thermal stability factors, KV/k/sub B/T, ranging from 29 to 670. The most general behavior of the zero field relaxation, observed for the smaller thermal stability factors, was an initial increase in the remanent magnetization followed by a decrease at longer times. The maximum initial zero field viscosity is strongly correlated with the inverse thermal stability factor, k/sub B/T/KV, and generally increases with the area under delta-M curve, normalized by the remanent coercivity. A small zero field relaxation was also observed in barium ferrite and CrO/sub 2/ tapes.

Magnetic domain structures and magnetotransport properties in Co-Ag granular thin films

The magnetic microstructures and magnetotransport properties in granular CoxAg1-x films with 17%≤x≤62% were studied. Magnetic force microscopy (MFM) observations showed the presence of magnetic stripe domains in as-deposited samples with x≥45% and the evolution of the magnetic domain patterns to in-plane domains with annealing. A perpendicular magnetic anisotropy as high as about 8×105 ergs/cc for as-deposited Co62Ag38 and about 6×105 ergs/cc for as-deposited Co45Ag55 was observed by magnetization and torque measurements. With increasing annealing temperature, the perpendicular magnetic anisotropy became negative. The origin of the perpendicular magnetic anisotropy may be attributed to a rhombohedral distortion of the cubic cell due to residual substrate-film stresses. The magnetic stripe domains are the consequence of the interplay of the indirect or direct exchange, perpendicular magnetic anisotropy and dipolar interactions. Finally, magnetoresistance (MR) curves displayed training behaviours and different shapes when measured with different configurations (parallel, transverse and perpendicular). It is proposed that the existence and the evolution of the magnetic domain structures strongly affect the magnetotransport properties due to the extra contribution of the electron scattering at the domain walls. Furthermore, an anisotropic MR also contributes to the overall MR curves.

Nanoscale protection for CoCrPt thin film magnetic recording media

In this study, CrMn, CoCrPt-C multilayers and carbon overcoat were sequentially deposited on the NiAl coated glass substrate as underlayer, granular media layer, and protect layer by DC sputtering. Samples with various interlayer carbon concentration and top-layer carbon were tested for corrosion resistance in the 0.5 N HCl saturated vapor environment. The results show that surface concentration of cobalt and nickel varies with the carbon content in the magnetic layer. An nonannealed top-layer carbon and low temperature annealed interlayer carbon can provide grain-sized or nanoscale protection to the CoCrPt magnetic media. Carbon concentration within/above the top of CoCrPt-C granular thin film plays an important roles in the suppression of corrosion.

Controlling the preferred orientation in granular CoCrPt:C thin films for high-areal density magnetic recording

Magnetic and structural properties of hcp-(CoCrPt):C granular thin films for potential applications in high areal density recording media have been investigated. CrX (X=I, Ru, Mn) underlayers with and without NiAl seedlayer has been applied to improve the in-plane preferred orientation of the Co alloy grains. The addition of Ru or Mn to the Cr underlayer promotes Co (11.0) in-plane orientation; using a NiAl seedlayer promotes Co (10.0). For films with CrMn underlayer and NiAl seedlayer, H/sub c/ reaches 3360 Oe and M/sub r/t decreases to 0.33 memu/cm/sup 2/. Thermal stability and reversal properties have been studied by moment-decay measurements. These granular films possess good in-plane preferred orientation of the magnetic moment and c-axis, which is desirable for high areal density longitudinal recording.

Magnetotransport and Mössbauer study of Fe3O4/γFe2O3 granular thin films

Magnetotransport nature of Fe3O4/γFe2O3 granular thin films is examined in the transformation process from Fe3O4 to γFe2O3. The electrical and magnetoresistive features systematically change around the percolation threshold of ferrimagnetic Fe3O4 metal. The electrical conduction can be described on the basis of small-polaron hopping and variable range hopping with coulomb gap in the Fe3O4/γFe2O3 thin films. Possible causes for the change in the magnetoresistance are also addressed.

Tunnel magnetoresistance (TMR) in ferromagnetic metalinsulator granular films

We review the recently discovered tunnel-type giant magnetoresistance (GMR) in ferromagnetic metalinsulator granular thin films, which is the magnetoresistance (MR) associated with the spin-dependent tunneling between two ferromagnetic metal particles. The theoretical and experimental results including electrical resistivity, magnetoresistance and their temperature dependence are described. Limitations to the applications of the ferromagnetic metalinsulator granular films are also discussed. Additionally, a brief survey of another two magnetic properties, high-frequency property and giant Hall effect (GHE) associated strongly with the granular structures is also presented.

Tunnel-type Giant Magnetoresisitance in Co–Al–Ta–O Insulated Granular System

We have investigated the tunnel-type giant magnetoresistance (TMR), the related magnetic and electric properties and the microstructure of Co–Al–Ta–O granular thin films which were prepared by the reactive sputter-deposition technique with oxygen and argon. The TMR attained a maximum value of 12.5% at room temperature under a magnetic field of 1 T for Co40Al13Ta3O44 films. The MR ratio is the largest in a granular system similar to Co–Al–O. During the sputtering, O2 gas flow was introduced at 2.2 sccm into the chamber filled with Ar at a total pressure higher than 1×10-4 Torr. The specific electric resistance of the film that showed the maximum TMR is 1.46×105 µΩ·cm. Ta forms an oxide compound similar to Ta2O5 in the insulator matrix. The effect of Ta addition on the TMR appears at around 2.8 at.%. The MR ratio increases with an increase of the fraction of fcc-Co, accompanying the decrease of the Co–O fraction and the decrease in ferromagnetic interaction between Co particles. The specimen that shows large TMR consists of nanometer-sized Co particles which are completely isolated by amorphous Al2O3 and Ta2O5 intergranular layers. The standard free energy of oxidation of Co, Al and Ta may play an important role in the formation of the microstructure of Co–Al–Ta–O granular films and in TMR improvement.

Characterization of Perpendicular Recording Media of CoPtCrO Granular Thin Films.

Thermal stability of CoPtCrO granular thin films with perpendicular magnetic anisotropy is investigated. Magnetization decay with time M(t) exhibits almost a linear dependence with ln(t), similar to that for longitudinal recording media. In comparison with amorphous rare earth-transition metal alloys media which shows a non-linear decay with ln(t), the differences in magnetization reversal process and energy barrier distribution are discussed. From magnetic viscosity measurements, activation volume is determined and shows a minimum of about 2x10-18 cm3 for CoPtCrO at fields near its coercivity, leading to a coupled region of 10 nm average diameter. Also recording performance is investigated using a conventional merged GMR head and magnetic force microscope. The linear recording density dependence of signal amplitude reveals that the 50% rolloff density D50 is about 300 kfci.

Fine structure and possible growth mechanisms of some electrodeposited CuCo granular films

Granular thin-films of the alloy Cu100−xCox within the composition range 6<x<35 have been grown by electrodeposition. The films have been investigated both magnetically and structurally and a comparison made between both types of measurements. Low field susceptibility measurements have been made which, in principle, allow us to determine the distribution of blocking temperatures and thus make an estimate of particle sizes. Before structural examination, the films were thinned either electrochemically or by ion-etching; both standard electron diffraction and TEM techniques have been employed. We believe that we are able to see Co-rich regions in the vicinity of sub-grain boundaries and that it is these regions that account for the magnetic behaviour of the samples. Whereas electron diffraction patterns from thin sections correspond to the FCC phase only, in thicker sections, additional reflections corresponding to interplanar spacings that cannot be indexed in terms of an FCC phase are observed. The possible origins of these extra reflections are discussed in terms of either double-diffraction at microtwins or, alternatively, in terms of extended, Co-rich HCP phases formed at matrix/twin interfaces. A possible mechanism for the growth of the HCP phase, which is a consequence of the electrodeposition technique, is discussed.

The influence of oxygen flux on the tunnelling magnetoresistanceeffect of Co-Al-O granular thin films

A series of \\mbox{Co-Al-O} granular thin films were fabricated by RF reactivesputtering at various oxygen fluxes, and the tunnelling magnetoresistance(TMR), phase composition, microstructure and the conductance mechanismof the films were systematically investigated. X-ray diffraction andtransmission electron microscopy revealed that the metal granules in the filmsgradually change from an AlCo alloy phase to a pure Co phase, and the granulesbecome smaller with increasing oxygen flux. The TMR also varied withan increase of the oxygen flux and exhibited a maximum of -7% at 1.7 vol% oxygenflux. A crossover of the temperature coefficient of resistivity (TCR) frompositive to negative with the increase of the oxygen flux is observed, andzero TCR is obtained at 1.0 vol% oxygen flux, implying a change of the conductionmechanism. These behaviours of the TMR and the TCR can be ascribed to the influence ofthe oxygen flux on the phase composition and the microstructure.

Nature of critical current and coherent phenomena in granular MoNx thin films

Investigations of the critical current versus temperature and applied magnetic field are carried out for granular MoNx films. All samples display a two-stage superconducting transition and can be treated as a percolating network of SNS contacts with a Josephson coupling between grains. The temperature behavior of the critical current for the films studies is the same as the Ic(T) dependence for a SNS junction in the diffusive limit. The value of critical current in a magnetic field is governed by the pinning of Josephson vortices.

Domain structures and training effects in granular thin films

Magnetic force microscopy (MFM) evidenced that sputtered CoFe-Ag(Cu) granular films displayed long-range magnetic domain structures (magnetic percolation) at ferromagnetic (FM) volume contents, x v, well below the volume percolation threshold. A variety of remanent magnetic microstructures was observed as a function of the thermomagnetic history, which strongly condition both magnetic and transport properties. These microstructures are formed due to the competition between perpendicular anisotropy and dipolar and matrix-mediated exchange interactions. All as-deposited samples displayed a high degree of 1 1 1 texture perpendicular to the film plane and a rhombohedral distortion induced by film–substrate strains, which are suggested to be at the origin of the observed perpendicular uniaxial anisotropy. Structural data indicate a 2% of CoFe is alloyed to the metallic matrix, which seems to be enough to couple the FM particles when they are closer than 1.5 nm at x v⩾25, leading to magnetic percolation. These FM correlations among particles were well evidenced in the low-field susceptibility measurements, which supports the hypothesis that exchange correlations propagate due to the CoFe spins diluted in the matrix. Annealing relaxed the rhombohedral distortion, leading to a cubic anisotropy along the 1 0 0 axis of the CoFe cell. Besides, annealing caused the segregation of the CoFe spins alloyed to the matrix, and consequently, the domain structure was lost.