GMR enhancement in spin valves structures with nano-semiconducting layer

We report on the giant magnetoresistance enhancement in Co/Ru/Co-based spin valve structures with nano-semiconducting layer. The films were grown by ion beam sputtering on glass substrate at room temperature. The soft layer is composed of Fe/Co bilayers, while the hard layer is ensured by the Co/Ru/Co artificial antiferromagnetic subsystem (AAF) as follows: Fe_5nm/Co_0.5nm/Cu_3nm/Co_3nm/Ru_0.5nm/Co_3nm/Cu_2nm/Cr_2nm. This structure shows a giant magnetoresistance (GMR) signal of about 1.7%. To confine the electrons inside the spin valve structure, a 1.5 nm thick ZnSe semiconducting layer has been grown on the top of the AAF. This induces a strong GMR increase, up to 4%, which can be attributed to a dominant potential step at the Co/ZnSe interface.     

Structural and magnetic properties of BCC Fe/Co (0 0 1) superlattices

In thin layered Fe/Co (0 0 1), grown on MgO (0 0 1), both Fe and Co crystallize in the body-centered cubic (BCC) structure, as seen in a series of superlattices where the layer thickness of the components is varied from two to twelve atomic monolayers. These superlattices have novel magnetic properties as observed by magnetization and polarized neutron reflectivity measurements. There is a significant enhancement of the magnetic moments of both Fe and Co at the interfaces. Furthermore, the easy axis of the system changes from [1 0 0] for films of low cobalt content to [1 1 0] for a Co content exceeding 33%. No indication of a uniaxial anisotropy component is found in any of the samples. The first anisotropy constant (K₁) of BCC Co is found to be negative with an estimated magnitude of 110 kJ/m³ at 10 K. In all cases, the magnetic moments of Fe and Co have parallel alignment.     

The effect of thickness and annealing condition on the microstructure and magnetic properties of Fe/Pt multilayer thin films

[Fe(0.5 nm)/Pt(0.5 nm)]₄₀, [Fe(1 nm)/Pt(1.5 nm)]₂₀ and [Fe(3 nm)/Pt(3 nm)]₁₀ multilayer were prepared by DC magnetron sputtering. By conventional furnace annealing (CA) at 270–600 °C for various time, all of the films still remained the disordered structure with the soft magnetic phase. By rapid thermal annealing (RTA) at 500 °C for various time, we obtained the [Fe(1 nm)/Pt(1.5 nm)]₂₀ and [Fe(3 nm)/Pt(3 nm)]₁₀ films with L1₂ ordered FePt₃ phase which was almost ferromagnetic at room temperature. However, the [Fe(0.5 nm)/Pt(0.5 nm)]₄₀ films was still disordered state even under RTA. Compared with CA, RTA exposed an outstanding effect on accelerating the phase transition when the film thickness is over [Fe(0.5 nm)/Pt(0.5 nm)]₄₀.     

Hard magnetic CoCr layer in ferromagnetic tunnel junctions

In magnetic tunnel junctions a highly spin-polarizing layer is usually exchange biased by an antiferromagnetic layer, an artificial antiferromagnetic layer system or a combination of both, while the magnetically soft layer is free to rotate. The use of a single layer of a hard magnetic material is rarely investigated up to now. In this paper, we present the electric and magnetic properties of tunnel junctions with a hard magnetic Co₈₃Cr₁₇ layer. The soft magnetic electrode consists of either a single Co layer or a Co/Ni₈₀Fe₂₀ bilayer. The magnetic anisotropy and coercive field H_C of the CoCr layer depend on its thickness and the kind of the bottom layer (Cu or Ta) and can vary from H_C=50–700 Oe. It is found that a thin Co cap layer also influences the hysteretic behavior. Furthermore, only small changes after annealing up to 450°C promise a high thermal stability for the application in magnetic tunnel junctions. Measurements of the tunnel magnetoresistance on large area junctions, however, show a strong magnetic coupling of the hard and soft electrodes.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Influence of stress and unidirectional field annealing on structural and magnetic performance of PtMn bottom spin-filter spin valves

The influence of unidirectional field annealing (UDA) and subsequent stress annealing (SA) on the structure and magnetic response of PtMn bottom spin-filter spin-valve sensors with a NiFeCr/NiFe seed layer was investigated. A time constant of 1.17 h for the FCC–FCT phase transition of the PtMn layer was obtained upon UDA at 265°C in an external magnet field of 13 kOe, which is consistent with 1.1 h observed for the GMR. A lower time constant of 0.7 h was determined for the exchange interaction field. The SA with similar conditions as in the manufacturing environment leads to a UDA time-dependent alteration of sensor performance. A SA up to 38 h causes a GMR value reduction, which becomes more pronounced with increasing UDA duration prior to SA. Furthermore, SA causes a progressive oxidation of the Ta cap layer; in contrast, the exchange field remains nearly unaffected, particularly for the sensors with 5 or 6 h UDA. From the point view of application, an UDA treatment of (5–6) h yields the optimal sensor performance for the finished read-write head.     

The effect of annealing under hydrostatic pressure on the visible photoluminescence from si-ion implanted SiO2 films

We have studied the influence of the hydrostatic pressure during annealing on the intensity of the visible photoluminescence (PL) from thermally grown SiO₂ films irradiated with Si⁺ ions. Post-implantation anneals have been carried out in an Ar ambient at temperatures T_a of 400°C and 450°C for 10 h and 1130°C for 5 h at hydrostatic pressures of 1 bar–15 kbar. It has been found that the intensity of the 360, 460 and 600 nm PL peaks increases with rising hydrostatic pressure during low-temperature annealing. The intensity of the short-wavelength PL under conditions of hydrostatic pressure continues to rise even at T_a=1130°C. Increasing T_a leads to a shift in the PL spectra towards the ultraviolet range. The results obtained have been interpreted in terms of enhanced, pressure-mediated formation of ≡Si–Si≡ centres and small Si clusters within metastable regions of the ion-implanted SiO₂.     

Growth of β-MnO2 films on TiO2(110) by oxygen plasma assisted molecular beam epitaxy

We have used oxygen plasma assisted MBE to grow epitaxial films of pyrolusite (β-MnO₂) on TiO₂(110) for thicknesses of one to six bilayers (BL). We define a bilayer to be a layer of Mn and lattice O and an adjacent layer of bridging O within the rutile structure. The resulting surfaces have been characterized in situ by reflection high-energy electron diffraction, low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and atomic force microscopy. Well-ordered, pseudomorphic overlayers form for substrate temperatures between 400 and 500°C. Mn–Ti intermixing occurs over the time scale of film growth (1 BL/min) for substrate temperatures in excess of 500°C. Films grown at 400–500°C exhibit island growth, whereas intermixed films grown at temperatures of 500–600°C are more laminar. 1 BL films grown at 450°C are more laminar than multilayer films grown at the same temperature, and form a well-ordered surface cation layer of Mn on the rutile structure with at most 10% indiffusion to the second cation layer.     

Nanoparticle-induced multi-functionalization of silicon: A plug and play approach

Here, we demonstrate a “plug and play” approach to achieve multi-functionalization of Si. In this approach, externally synthesized functional nanoparticles are introduced onto device quality Si wafers and the surface chemical bonds are manipulated. Sonochemically synthesized Fe₂O₃ nanoparticles are introduced onto Si from an alcohol suspension. On annealing this sample in ultra-high vacuum, the oxygen atoms change the bonding partner from Fe to Si and desorb as SiO at 750 °C. This results in the formation of nanoparticles of Fe on the surface and exhibits ferromagnetic behavior. Deposition of a thin layer (2 nm) of Si onto the sample containing the metallic Fe nanoparticles followed by annealing at 560 °C leads to optically active Si. Photoluminescence measurements show that this sample emits light at three different wavelengths, namely 1.57, 1.61 and 1.63 μm, when excited by He–Ne or Ar lasers. Oxidation of this material results in the formation of a selective capping layer of SiO₂. Thus we obtain multi-functional Si in an “all in one” form and we believe that this approach is universal.     

Observation of triangular terraces and triangular craters of CaF2 film on Si(1 1 1) substrate

We have investigated the growth mode and surface morphology of CaF₂ film on Si(1 1 1)7×7 substrate by reflection high-energy electron diffraction (RHEED) using very weak electron beam and atomic force microscopy (AFM). It was found by RHEED intensity oscillation measurements and AFM observations that three-dimensional (3D) islands grow at RT; however, rather flat surface appears with two-dimensional (2D) islands around 300 °C. Especially, at high temperature of 700 °C, characteristic equilateral triangular terraces (or islands) with flat and wide shape grow with the tops directed toward [1 1 −2] of substrate Si(1 1 1). On the other hand, the desorption process of the CaF₂ film due to electron stimulated desorption (ESD) was also examined. It was found that the ESD process at 300 °C forms characteristic equilateral triangular craters on the film surface with the tops (or corners) directed toward [−1 −1 2] of substrate Si(1 1 1), provided that the film was grown at 700 °C.     

Structural comparison of gadolinium and lanthanum silicate films on Si(1 0 0) by HRTEM, EELS and SAED

High-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED) were used to study gadolinium and lanthanum silicate films deposited on Si(1 0 0) substrates using electron-beam evaporation from pressed-powder targets. As-deposited films consist of an amorphous silicate layer without an interfacial layer. After annealing at 900 °C in oxygen for 2 min, an interfacial SiO₂ layer is formed in the gadolinium silicate film, while this interfacial layer is a SiO₂-rich lanthanum silicate layer in the lanthanum silicate film. The formation of interfacial silicate layers is thermodynamically more favorable for the lanthanum films than for the gadolinium films. The gadolinium silicate films crystallize at a temperature between 1000 and 1050 °C, while the crystallization temperature for the lanthanum silicate films is between 900 and 950 °C.     

Synthesis and physical behaviour of In2S3 films

In₂S₃ layers have been grown by close-spaced evaporation of pre-synthesized In₂S₃ powder from its constituent elements. The layers were deposited on glass substrates at temperatures in the range, 200–350 °C. The effect of substrate temperature on composition, structure, morphology, electrical and optical properties of the as-grown indium sulfide films has been studied. The synthesized powder exhibited cubic structure with a grain size of 63.92 nm and S/In ratio of 1.01. The films grown at 200 °C were amorphous in nature while its crystallinity increased with the increase of substrate temperature to 300 °C. The films exhibited pure tetragonal β-In₂S₃ phase at the substrate temperature of 350 °C. The surface morphological analysis revealed that the films grown at 300 °C had an average roughness of 1.43 nm. These films showed a S/In ratio of 0.98 and a lower electrical resistivity of 1.28 × 10³ Ω cm. The optical band gap was found to be direct and the layers grown at 300 °C showed a higher optical transmittance of 78% and an energy band gap of 2.49 eV.     

Soft X-ray emission study of thermally treated Ni(film)/4H–SiC(substrate) interface

Deposition of Ni as contact on 4H–SiC has been investigated. Ni/4H–SiC samples were annealed at temperatures of 600, 800 and 950 °C for 30 min and were non-destructively characterized by soft X-ray emission spectroscopy (SXES) using synchrotron radiation as excitation. Si L_2,3 SXE showed the formation of Ni₂Si for all annealing temperatures. The C K SXE indicated the formation of graphite and graphitic carbons at annealing temperatures of 950 °C and below 800 °C, respectively.     

Structural and magnetic properties of RF sputtered FePt/Fe multilayers

Series of [FePt(4min)/Fe(t_Fe)]₁₀ multilayers have been prepared by RF magnetron sputtering and post-annealing in order to optimize their magnetic properties by structural designs. The structure, surface morphology, composition and magnetic properties of the deposited films have been characterized by X-ray diffractometer (XRD), Rutherford backscattering (RBS), scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDX) and vibrating sample magnetometer (VSM). It is found that after annealing at temperatures above 500 °C, FePt phase undergoes a phase transition from disordered FCC to ordered FCT structure, and becomes a hard magnetic phase. X-ray diffraction studies on the series of [FePt/Fe]_n multilayer with varying Fe layer thickness annealed at 500 and 600 °C show that lattice constants change with Fe layer thickness and annealing temperature. Both lattice constants a and c are smaller than those of standard ones, and lattice constant a decreases as Fe layer deposition time increases. Only a slight increase in grain size was observed as Fe layer decreased in samples annealed at 500 °C. However, the increase in grain size is large in samples annealed at 600 °C. The coercivities of [FePt/Fe]_n multilayers decrease with Fe layer deposition time, and the energy product (BH)_max reaches a maximum in the samples with Fe layer deposition time of 3 min. Comparison of magnetic properties with structure showed an almost linear relationship between the lattice constant a and the coercivities of the FePt phase.     

Amorphous Si/insulator multilayers grown by vacuum deposition and electron cyclotron resonance plasma treatment

a-Si/insulator multilayers have been deposited on (0 0 1) Si by electron gun Si evaporation and periodic electron cyclotron resonance plasma oxidation or nitridation. Exposure to an O or N plasma resulted in the formation of a thin SiO₂ and SiN_x layer whose thickness was self-limited and controlled by process parameters. For thin-layer (2 nm) Si/SiO₂ and Si/SiN_x multilayers no visible photoluminescence (PL) was observed in most samples, although all exhibited weak “blue” PL. For the nitride multilayers, annealing at 750°C or 850°C induced visible PL that varied in peak energy with Si layer thickness. Depth profiling of a-Si caps on thin insulating layers revealed no detectable contamination for the SiN_x layers, but substantial O contamination for the SiO₂ films.     

Surface deformation phenomena induced by electron-beam irradiation in strained InGaAs/AlGaAs layers on GaAs (1 0 0) and (3 1 1)B substrates

The strained InGaAs/AlGaAs layer structures have been grown on GaAs ( 10 0) and (3 1 1)B substrates in a horizontal low-pressure metalorganic vapor-phase epitaxy system at a temperature of 800°C. In the surface observation using a high-resolution scanning electron microscope, we have found that surface deformation phenomena induced by electron-beam irradiation in strained In_0.36Ga_0.64As,/Al_0.3Ga_0.7As layers on GaAs (1 0 0) and (3 1 1)B substrates. The change of the surface morphology was observed in real time on the display of SEM with the accelerating voltage of 30 kV and the irradiated time of 60–120 s. The surface deformation through mass transport seems to be the cause of the residual strain relaxation due to electron-beam irradiation.     

The origin of photoluminescence in Ge-implanted SiO2 layers

Ge ions were implanted at 100 keV with 3×10¹⁶ cm⁻² into a 300  nm thick SiO₂ layer on Si. Visible photoluminescence (PL) around 2.1 eV from an as-implanted sample is observed, and faded out by subsequent annealing at 900°C for 2 h. However, PL shows up again after annealing above 900°C at the same peak position. Compared with the as-implanted sample, significant increase of Ge–Ge bonds is measured in X-ray photoelectron spectroscopy, and the formation of Ge nanocrystals with a diameter of 5 nm are observed in transmission electron microscopy from the sample annealed at 1100°C. We conclude that the PL peak from the sample annealed above 900°C is caused by the quantum confinement effects from Ge nanocrystals, while the luminescence from the as-implanted sample is due to some radiative defects formed by Ge implantation.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

Structural effects in the growth of giant magnetoresistance (GMR) spin valves

An investigation has been made of the thin-film structure and interface morphology of giant magnetoresistance (GMR) spin valves of the cobalt/copper/cobalt (Co/Cu/Co) type that were grown on polycrystalline NiO substrates at three different temperatures (150, 300 and 450 K). Sputter-depth-profile analyses indicate that the quality of the layering in the Co/Cu/Co structure was only slightly better for the 150 K sample than for the 300 K sample. For the 450 K sample, however, the Co/Cu/Co structure showed extensive disruption. The similarity in the depth-profiles for the 150 and 300 K samples indicates the sensitivity of the GMR to subtle structural differences.     

Spin-axis-dependent magnetic properties of FePt and CoPt

A theoretical study of the magnetic properties of the CoPt and FePt ordered alloys has been performed. The calculation is done as a function of the spin-quantization axis by means of both the local spin density and the generalized-gradient approximations in conjunction with the full-potential linear muffin–tin orbital method. Both approximations produced similar results for the FePt and CoPt compounds. The band structure and the total density of states have been calculated and it was confirmed that all electronic states contribute to the magneto-crystalline anisotropy energy; the magnetization axis is along the [0 0 1] direction. The Fe and Co orbital magnetic moments decrease with respect to the angle γ between the [0 0 1] axis and the spin quantization axis, but for the [1 0 0] axis the orbital moment is comparable to the [0 0 1] moment. The Pt orbital moments are of the same order of magnitude as those of Fe and Co moments due to the large spin–orbit splitting parameter of Pt and show a similar behavior with the angle γ.