Effect of Ga irradiation on the magneto-optic spectra of Pt/Co/Pt sandwiches

Interfacial changes in rf sputtered Pt/Co(2.6 nm)/Pt sandwiches grown onto sapphire (Al₂O₃) substrates induced by irradiation of 30 keV Ga⁺ ions at low dose (10¹⁴ ions/cm²) have been investigated by magneto-optic polar Kerr rotation (PKR) spectroscopy between 1 and 5 eV. The irradiation resulted in an increase of PKR over the whole spectral range. The measured PKR spectra were compared with those computed from the transfer matrix formalism using known polar Kerr rotation and ellipticity spectra for Co and five Co_xPt_1 − x alloys. The comparison between measured and computed PKR spectra provided an in-depth profile of Co and Pt ion distributions across the sandwich and confirmed that irradiation favors alloying in the vicinity of the two interfaces. These results are in a good agreement with the profile evaluated independently by TRIDYN simulations. Our results evidence an asymmetry in the irradiation effect due to an excess of Pt-Co alloying at the upper interface. Moreover, the observation of a negative PKR peak around 3.2 eV states definitively the presence of a chemically ordered Co_0.75Pt_0.25 alloy phase inside the irradiated film structure.     

Magnetic and magneto-optical properties of amorphous boron-cobalt and silicon-cobalt alloys

Amorphous alloys of the type B_1?xCo_x and Si_1?xCo_x were prepared by vapour deposition.Their magnetic properties were determined in the range 4.2 - 300 K. The polar Kerr rotation was measured at two wavelengths (λ = 633 nm and 830 nm) at room temperature. The magnetic properties of these alloys were compared with amorphous alloys in which Co is combined with Sn, P, Mg, Y, La or Zr. The magnetic properties were analysed in terms of a model in which proper account is taken of possible differences in chemical short-range ordering of the atoms.     

The effect of Ta on structure and magnetic properties in Fe–N films

Microstructure and magnetic properties of Fe–Ta–N alloy films near the eutatic composition were studied. The four systematic alloy films with different Ta content were prepared by reactive sputtering. The dependence of structures and magnetic properties on Ta and annealing were investigated by VSM and X-ray diffraction. It is found that Ta atoms replace Fe in α-Fe lattice and have strong affinity for nitrogen, which inhibits the formation of γ-Fe₄N phase in Fe–Ta–N films. The TaN phase precipitates in grain boundaries and suppresses the growth of α-Fe(N) crystalline during annealing. Coercivity varies with the change of microstructure.     

Effect of chemical composition and Co-core size on the magnetic and magneto-transport properties of CoyAg100−y core–shell nanocrystallites

This paper describes the applicability of a modified polyol process in conjunction with a transmetallation reaction in synthesis of Co_yAg_100?y nanocrystallites of a core–shell structure. The substitution Co → Ag is varied as 20 ≤ y ≤ 95 by tuning the microstructure with functional magnetic and giant magneto resistance properties. The existence of core–shell structures was confirmed through transmission electron microscope and the size of Co-core increased from 8 to 50 nm with decrease in Ag-content of the sample. The magnetic behaviour of the sample changed from a pseudo-superparamagnetic nature to ferromagnetic with increase in Co-core size. Normalised saturation magnetisation values increased from 89.5 to 128.6 emu/g with increase in size of Co-core; however, none of the ferromagnetic samples exhibited any magneto resistance (MR). A value of 2.0% MR was observed in case of Co_20.6Ag_79.4 which increased to a maximum of 3.6% MR for Co_36.6Ag_63.4 sample when measured at 5 K.     

Selective sensitivity of ellipsometry to magnetic nanostructures

Magneto-optic (MO) ellipsometry of ferromagnetic materials is extremely sensitive to ultra-thin films, multilayers, and nanostructures. It gives a possibility to measure all components of the magnetization vector in the frame of the magneto-optic vector magnetometry and enables us to separate magnetic contributions from different depths and materials in nanostructures, which is reviewed in this article. The method is based on ellipsometric separation using the selective MO Kerr effect. The figure of merit used to quantify the ellipsometric selectivity to magnetic nanostructures is defined on the basis of linear matrix algebra. We show that the method can be also used to separate MO contributions from areas of the same ferromagnetic materials deposited on different buffer layers. The method is demonstrated using both: (i) modeling of the MO ellipsometry response and (ii) MO measurement of ultra-thin Co islands epitaxially grown on self-organized gold islands on Mo/Al₂O₃ buffer layer prepared using the molecular beam epitaxy at elevated temperatures. The system is studied using longitudinal (in-plane) and polar (perpendicular) MO Kerr effects.     

Ab-initio study for the correlation effect on the magneto-optical properties of Co-based full Heusler alloys

Magneto-optical (MO) properties of ferromagnetic Co₂YGe full-Heusler alloys (Y = Fe and Mn) are investigated by the first principles electronic structure calculations using the highly accurate FLAPW method within GGA + U approach. The polar magneto-optical Kerr angles are calculated by solving Kubo's linear response formula using the FLAPW eigenstates and eigenfunctions to obtain the optical conductivity by interband transitions. The typical features of polar Kerr rotation of transition metals with two major peaks were obtained while the low energy spectra are enhanced largely at 1-2 eV region compared to GGA results. Detailed electronic structures analysis revealed that this enhancement is due to the decreased interband transitions by reduced diagonal optical conductivity. Results indicate that the MO spectra can be used to determine the appropriate correlation level for the present alloys.     

Effect of rapid recurrent thermal annealing on the soft magnetic behaviour of Co85.5Nb8.9Zr5.6 nanocrystalline thin films

The rapid recurrent thermal annealing (RRTA) method has been used to crystallize-amorphous Co_85.5Nb_8.9Zr_5.6 soft magnetic thin films, which were fabricated by DC magnetron sputtering onto glass-ceramic substrates directly at room temperature. As a result, crystalline grains with diameter of about 10 nm were formed and a partiall nanocrystallization of the films was obtained. The soft magnetic properties of the Co-based nanocrystalline thin films were largely improved after RRTA. The resistivity is decreased by a quarter and the coercivity is also decreased. The magnetic and electrical properties were investigated using the RRTA method with varied parameters such as annealing temperature, annealing time and repeat cycle. The experimental results revealed that the RRTA is an effective method to control the magnetic and electrical properties of Co_85.5Nb_8.9Zr_5.6 thin films.     

Ferromagnetism and Morphology of Annealed Fe2O3/CoXOY/ZnO Thin Films

We have studied thermal solid‐state reactions in the Fe₂O₃/Co_XO_Y/ZnO thin film systems grown using the atomic layer deposition technique. The compound produced after annealing at 700 °C is found to be a complex mixture of three different spinel phases: ZnCo₂O₄, CoFe₂O₄, and ZnFe₂O₄. The magnetic properties of the compound strongly depend on the atomic ratio of Fe³⁺ and Co²⁺ atoms, which can be set by choosing the corresponding thicknesses of the Fe₂O₃ and Co_XO_Y films. In addition, we also find a formation of 100 nm voids at the interface between Fe–Co–Zn–O compound and remaining ZnO film after 1h annealing at 700 °C in argon atmosphere. The formation of these voids shows indirectly the preferential outward diffusion of Zn²⁺ ions from ZnO into the Fe₂CoO₄ phase layer what we prove via our magnetic measurements.     

Microstructure and giant magnetoresistance of FeCo–Cu nanogranular films

A series of (Fe₅₀Co₅₀) _x Cu_1−x granular films were prepared using a magnetron controlled sputtering system. The microstructure and giant magnetoresistance (GMR) of FeCo–Cu films deposited at room temperature and then annealed at various temperatures were investigated through transmission electron microscope and conventional four probes method under room temperature, respectively. The results revealed that all FeCo–Cu films consisted of fine FeCo granules uniformly dispersed in the Cu matrix and formed fcc structure. By a non-linear least-squares method, the size distribution of FeCo granules in all as-deposited films satisfied a log-normal function. Upon varying the magnetic volume fraction (x), the GMR of as-deposited FeCo–Cu films reached a maximum of about 0.7% at the volume fraction of 31% FeCo, corresponding to the fact that the GMR has a non-monotonic relationship with the granule size. With increasing the annealing temperature, the GMR of films with lower volume fraction reached a peak at higher temperature, while for films with higher volume fraction the GMR reached a peak at lower temperature. In addition, the relationships between the full width at half maximum (FWHM) or the sensitivity of the GMR and the volume fraction were discussed in detail.     

Deposition and characterization of magnetic Ti-Fe-C nanocomposite thin films

Nanocomposite (Ti_1−xFe_x)C_y films with different compositions have been deposited by dc magnetron sputtering at 450 °C. The sputtered films could dissolve as much as 20-30 at.% of Fe on the Ti sites which is far above the maximum solid solubility at equilibrium. The solubility was dependent on the carbon content and more carbon-rich films could dissolve more Fe without the formation of Fe-precipitates. The addition of Fe also reduced the grain size of the carbide particles. Upon annealing, α-Fe starts to precipitate and the amount and size of these precipitates can be controlled by the annealing procedure and from the total composition of the as-deposited films. Mechanical and tribological studies show that some compositions of the (Ti_1−xFe_x)C_y films have very good wear-resistant properties. These results together with magnetization measurements suggest that Ti-Fe-C films can be used as a wear-resistant magnetic thin film material.     

Magnetic properties and microstructures of sputtered epitaxial Co-rich Co-Pt films

Co₃Pt films of various thicknesses were deposited on Pt underlayers by conventional sputtering in order to investigate the effects of Pt underlayers and annealing temperatures on their microstructure and the magnetic properties. XRD and HRTEM analyses reveal perpendicular magnetic anisotropy in films of good epitaxial growth of Co₃Pt (002) on the Pt (111) underlayer when annealed at 300 °C. However, Pt atoms in the Pt underlayer will diffuse seriously into the Co₃Pt layer when the annealing temperature is increased to 375 °C. This changes the compositions to approach equiatomic CoPt, and shows in-plane magnetic anisotropy with soft magnetic properties.     

Theory of the Non-linear Longitudinal Kerr Magneto-optic Effect in Ferromagnetic Metals

A theory of the non-linear longitudinal Kerr magneto-optic effect in ferromagnetic metals is developed. The material model is based on the classical equation of motion for a free electron with a finite relaxation frequency under the action of a Lorentz force. A second harmonic current density is found of the form: J₂ = αE₂ + β _h (H ₁)E₁ + σ (E₁? · E₁), where α, β_h (H ₁) and σ are non-liner conductivity tensors, E₁ and H₁ are the fundamental electric and magnetic fields, and E₂ is the induced second harmonic electric field. Results of this theory reduce, in the limit of a vanishing ferromagnetic state, to results obtained by Jha [1] from the Boltzmann transport equation for conduction electrons subject to a potential barrier at the metal surface. As required, the theory reduces to the linear longitudinal Kerr magneto-optic effect in the absence of second harmonic generation. The second harmonic reflection coefficients are derived. To the degree of approximations made, all four coefficients vanish at grazing incidence. Unlike the non-linear polar Kerr effect, they do not vanish at normal incidence.     

Pulsed laser ablated off-stoichiometric thin films of the Heusler alloy Co2TiSn on Si (100) substrate

We report the growth of thin films of ferromagnetic Heusler alloy Co₂TiSn on Si (100) substrate using KrF excimer pulsed laser ablation. Films of thicknesses ranging from 8 to 220 nm were deposited on Si (100) substrate heated up to 200 ± 10 °C, with an aim to study the structural, morphological and magnetic properties. The grown films are off-stoichiometric, polycrystalline, having single-phase with high degree of (220) texturing. Angle dependent fluorescence measurements suggest no segregation of alloying elements as a function of depth. X-ray reflectivity measurements indicate that all the films are having low density layer at the top as well as at the film-substrate interface. Magneto optical Kerr effect measurements at room temperature reveal clear hysteresis loops suggesting ferromagnetic behavior of the films. Thermal annealing at temperature ≥ 220 °C suggest transformation of Co₂TiSn phase into cobalt silicide phase, which confirms the necessity of low substrate temperature (< 220 °C) to produce such single-phase Co₂TiSn films.     

The enhanced magneto-optical Kerr effect in Co/TiO2 multilayer films

Spectral dependences of the polarization plane rotation angle (??_k) in the polar Kerr effect in Co/TiO₂ multilayer nanocomposite films have been studied in the 400?C1000 nm wavelength range. It is established that the sign, magnitude, and shape of the magneto-optical spectrum depend on the dielectric spacer thickness and the number of layers in the structure. The Kerr rotation angle in Co/TiO₂ multilayers is significantly greater than that in homogeneous Co films of the same thickness. The angle of rotation of the polarization plane reaches a record high value of 2??_k = 7.3° in the Co(5 nm)/TiO₂(17 nm) multilayer structure with number of layers n = 8 at a wavelength of 540 nm.     

Structural, magnetic, and magneto-optical properties of nanocrystalline face centered cubic Co70Cr30/Pt multilayers with perpendicular magnetic anisotropy

Co70Cr30 alloyed layers are combined with extremely thin Pt layers in order to produce novel face-centered-cubic multilayered films to be considered as a potential perpendicular magnetic recording medium. The films were grown on Si, glass and polyimide substrates by e-beam evaporation at a temperature slightly higher than room temperature. The multilayered structure of the films was verified by X-ray diffraction experiments. Plane-view transmission electron microscopy images have revealed the formation of very small grains in the range of 7-9 nm. Hysteresis loops as a function of temperature were recorded via the magneto-optic Kerr effect in the polar geometry configuration. The system exhibits perpendicular magnetic anisotropy, which enhances with decreasing temperature. Hysteresis loops with a squareness of 1 and a coercivity of 1.45 kOe were obtained at 10 K. Furthermore, complete magneto-optic spectra of the films are recorded, showing a strong magneto-optic enhancement in the ultraviolet region at around 4.5 eV.     

Magnetic Features of Boron Doping on Hf<Subscript>2</Subscript>Co<Subscript>11</Subscript> Thin Films

The effect of boron (B) doping on the structural and magnetic properties is studied systematically in the Hf₂Co₁₁ thin films. The modified crystal microstructure and annealing process enhanced the coercive field up to 1754 Oe. In order to obtain superior magnetic properties, the optimum doping ratios should be properly controlled attentively. To identify the structure, B-doped Hf₂Co₁₁ thin films were investigated by temperature-dependent X-ray diffraction analysis for the first time. The hard magnetic phase of Hf₂Co₁₁ B _x thin films was formed to orthorhombic phase with some Co impurities and had a crystallization temperature around 823 K.     

Structures in superconducting YBa2Cu3O7−δ thin films investigated by magneto-optic technique

Using a reflection magneto-optic technique we have investigated natural inhomogeneities and artificial structures in YBCO thin films exposed to an external magnetic field. The artificial structures were mechanically scratched by scanning a diamond tip with different loading over the film surface. Alternatively planar structures with reduced oxygen content could be patterned by heating the YBCO film with a focused laser beam in nitrogen atmosphere. Depending on the laser annealing parameters different screening properties concerning the applied magnetic field could be achieved.As a magneto-optically active layer we used EuS films evaporated on glass as well as bismuth- and gallium-doped lutetium-iron-garnet films grown onto (111) oriented gadolinium-gal lium-gar net substrates by liquid phase epitaxy. In contrast to measurements with EuS films that show only weak faraday rotation for temperatures higher than 20 K the magneto-optic studies have been expanded to about 60 K by using the garnet films.     

Increase of Fe solubility in ZnO induced by the grain boundary adsorption

Nanograined (grain size 6–15 nm) ZnO films with various Fe content (between 0 and 40 at.%) were synthesized by the novel liquid ceramics method. The films with 0, 0.1, 5 and 10 at.% Fe contain only ZnO-based solid solution with wurtzite structure. The films with 20 at.% Fe contain mainly amorphous phase. The peaks of the second phase (ZnFe₂O₄ with cubic lattice) become visible in the X-ray diffraction spectra at 30 at.% Fe. Therefore, the overall solubility of Fe in nanograined ZnO films at 550 °C is about 20 at.% Fe. The solubility limit in the bulk is about 1.5 at.% Fe. The recently published papers on the structure and magnetic behaviour of Fe-doped ZnO allowed us to obtain the dependence of Fe solubility in ZnO on the grain size. The overall Fe solubility drastically increases with the decreasing grain size. The quantitative estimation shows that, close to the bulk solubility limit, the thickness of a Fe-enriched layer in grain boundaries is that of several monolayers.     

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.     

Annealing effects on the structural and magnetic properties of Fe–Al silica nanocomposites prepared by sequential ion implantation

The nanostructural and magnetic properties of FeAl–SiO₂ granular solids prepared by sequential ion implantation have been investigated as a function of the annealing atmosphere (either oxidizing or reducing) and implantation order. Nanoscopic particles with a bcc structure were found in both as-implanted samples. In the sample Al–Fe prepared by implanting first the Al ions and later the Fe ions, the lattice parameter indicates the presence of practically pure iron nanoparticles. On the other hand, in sample Fe–Al with the implantation order inverted, the lattice parameter is consistent with the presence of an iron rich iron–silicon alloy. The magnetic data confirm the presence of the pure Fe and the Fe–Si alloy in the as-implanted samples and the absence of FeAl intermetallic compounds. The annealing in Ar/H₂ promotes the growth of the clusters and increases the Si content in the particles in both samples. In Fe–Al sample, this induces a disorder–order phase transition from the disordered Fe–Si solid solution to the Fe₃Si phase and the coprecipitation of the ordered FeSi phase. The magnetic moment increases after the annealing in Ar/H₂ due to the incorporation of the iron atoms dispersed in the matrix and to the higher crystalline order. The annealing in air is responsible essentially of the growth of the Fe–Si clusters in both samples. On the other hand, in sample Al–Fe the oxygen interacts with the pure iron clusters by promoting the Fe₂O₃ formation.