Enhancement of coercivity with reduced grain size in CoCrPt film grown by pulsed laser deposition

We report a pulsed laser deposition (PLD) growth of VMn/CoCrPt bilayer with a magnetic coercivity (H_c) of 2.2 kOe and a grain size of 12 nm. The effects of VMn underlayer on magnetic properties of CoCrPt layer were studied. The coercivity, H_c, and squareness, S, of VMn/CoCrPt bilayer, is dependent on the thickness of VMn. The grain size of the CoCrPt film can also be modified by laser parameters. High laser fluence used for CoCrPt deposition produces a smaller grain size. Enhanced H_c and reduced grain size in VMn/CoCrPt is explained by more pronounced surface phase segregation during deposition at high laser fluence.     

Ru-O/Ru hybrid type of underlayer for CoPtCr-SiO2 perpendicular recording media on flexible tape

Ru/CoPtCr-SiO₂ bilayer prepared at 4 and 26 mTorr of Ar gas pressure for the deposition of Ru and CoPtCr-SiO₂ layers, respectively, exhibits better magnetic properties suitable for perpendicular magnetic recording media when they are deposited at room temperature on a Pt seed layer prepared at 450 °C. The Ru-O seed layer fabricated by a reactive sputtering method improves the Ru (0 0 1) texture deposited on a Ru-O layer. The Ru-O/Ru hybrid type of underlayer causes the improvement of the c-axis orientation of CoPtCr crystallites in the CoPtCr-SiO₂ layer deposited on it. Fine granulation of magnetic grains in the CoPtCr-SiO₂ layer is also attained when they are deposited on the Aramid type of flexible tape substrates.     

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.     

Effect of Ta buffer and NiFe seed layers on pulsed-DC magnetron sputtered Ir20Mn80/Co90Fe10 exchange bias

A systematic investigation has been done on the correlation between texture, grain size evolution and magnetic properties in Ta/Ni₈₁Fe₁₉/Ir₂₀Mn₈₀/Co₉₀Fe₁₀/Ta exchange bias in dependence of Ta buffer and NiFe seed layer thickness in the range of 2-10 nm, deposited by pulsed DC magnetron sputtering technique. A strong dependence of 〈1 1 1〉 texture on the Ta/NiFe thicknesses was found, where the reducing and increasing texture was correlated with exchange bias field and unidirectional anisotropy energy constant at both NiFe/IrMn and IrMn/CoFe interfaces. However, a direct correlation between average grain size in IrMn and H_ex and H_c was not observed. L1₂ phase IrMn₃ could be formed by thickness optimization of Ta/NiFe layers by deposition at room temperature, for which the maximum exchange coupling parameters were achieved. We conclude finally that the coercivity is mainly influenced by texture induced interfacial effects at NiFe/IrMn/CoFe interfaces developing with Ta/NiFe thicknesses.     

Magnetic properties and characterization of CoCrM/TiCr/Si films produced by pulsed laser deposition

A series of nanogranular CoCrM/TiCr thin films have been fabricated by pulsed-laser deposition on Si(1 1 1) substrates at 450–500 °C. The crystal structure and magnetic properties of these films were investigated. The transmission electron microscope images with selected area diffraction and X-ray diffraction showed that the structure of as-prepared films is dependent on laser energy and deposition temperature. It was found that the microstructure of CoCrM/TiCr films consisted of fine dispersive crystal grains, while the preferential c-axis orientation of films deteriorated when the thickness of CoCr-alloy layer increased along with metal doping into the CoCr films. The magnetic properties of CoCrM/TiCr films formed on Si are strongly dependent on the thickness of magnetic layer, grain structure, and grain shape. Enhancement of coercivity and squareness of the laser-deposited film is probably due to the improvement in the magnetocrystalline anisotropy energy and the reduction in the thickness of magnetic layer.     

Investigation of structural and magnetic properties of Ni, NiFe and NiFe2O4 thin films

Soft magnetic thin films of Ni, NiFe and NiFe₂O₄ were prepared using reactive magnetron sputtering in various deposition conditions. Experimentally observed soft magnetic property was compared and correlated with nanocrystalline structure evolution. Ni and NiFe deposited films are textured with fcc(111) phase preferred orientation. Accordingly, grain size and lattice parameter were calculated from X-ray diffraction (111) peak line width and 2θ peak position. Addition of reactive gas oxygen in deposition process has substantial effect on crystalline structure of film. There is phase transition from the ordered NiFe (111) structure to the NiFe₂O₄ nanocrystalline phase. The resulting film has shown small X-ray diffraction intensity peak corresponding to (311) and (400) orientation, indicating small amount of existing NiFe₂O₄ phase. The mechanism has been discussed to be responsible for nanocrystallization and amorphization of NiFe₂O₄ films. Magnetic measurement (M-H) loop reveal soft magnetic nature of films with magnetic anisotropy. The coercivity (H_c) of films is in accordance with random anisotropy model, where H_c reduced with grain size. The structural transformation was supported by Fourier transforms infrared spectroscopy measurement. The films are highly smooth with surface roughness in the range of ∼0.53-0.93 nm. NiFe₂O₄ films have shown lowest surface roughness with highest electrical resistivity values. The structural, surface, magnetic and infrared spectroscopy results are observed and analyzed.     

Synthetic Co50Pt50/Ru/CoFe hard–soft trilayer in spin valves

The influence of deposition power and seedlayer on the properties of hard magnet Co₅₀Pt₅₀ was studied. Co₅₀Pt₅₀(/Co₉₀Fe₁₀)/Ru/Co₉₀Fe₁₀ trilayer was used as pining/pinned layer in spin valves. The influences of different hard layer, soft layer and free layer on exchange bias, interlayer coupling, and magnetoresistance (MR) ratio were studied. Weak antiferromagnetic interlayer coupling was obtained by adjusting the thickness of hard and soft layers. MR of a spin valve with structure Cr2/CoFe0.5/CoPt4/CoFe0.5/Ru0.8/CoFe2.2/Cu2.05/CoFe2.6/Cu1.1/Ta1 reached 10.68% (unit in nm), which is comparable to those of IrMn-based synthetic spin valves. The increment of the coercivity of the free layer is mainly due to the static magnetic interaction between the hard layer and the free layer.     

Magnetic properties of Pr-Fe-B/Mn films with perpendicular anisotropy

The influence of a Mn layer on the magnetic properties of sputtered Pr-Fe-B/Mn films with Cu spacer layer has been investigated for various Mn layer thicknesses. The Pr-Fe-B/Mn films all possess perpendicular anisotropy. An enhancement of the intrinsic coercivity _iH_c is observed for suitable Mn layer thickness and _iH_c exhibits an oscillatory dependence on the thickness of the Mn layer with a period of about 60 nm. The average size of the columnar Pr₂Fe₁₄B grains is about 100 nm. A highest _iH_c value of 22.1 kOe and an optimal (BH)_max value of 18.2 MGOe are reported for these Pr-Fe-B/Mn films.     

Growth temperature dependence of the hysteretic behavior of Ni0.5Zn0.5Fe2O4 thin films

Herein, a discussion of the effect of deposition temperature on the magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ thin films. The thin films were grown by r.f. sputtering technique on (1 0 0) MgO single-crystal substrates at deposition temperatures ranging between 400 and 800 °C. The grain boundary microstructure was analyzed via atomic force microscopy (AFM). AFM images show that grain size (φ∼70-112 nm) increases with increasing deposition temperature, according to a diffusion growth model. From magneto-optical Kerr effect (MOKE) measurements at room temperature, coercive fields, H_c, between 37and 131 Oe were measured. The coercive field, H_c, as a function of grain size, reaches a maximum value of 131 Oe for φ ∼93 nm, while the relative saturation magnetization exhibits a minimum value at this grain size. The behaviors observed were interpreted as the existence of a critical size for the transition from single- to multi-domain regime. The saturation magnetization (21 emu/g<M_s<60 emu/g) was employed to quantify the critical magnetic intergranular correlation length (L_c≈166 nm), where a single-grain to coupled-grain behavior transition occurs. Experimental hysteresis loops were fitted by the Jiles-Atherton model (JAM). The value of the k-parameter of the JAM fitted by means of this model (k/μ_o∼50 A m²) was correlated to the domain size from the behavior of k, we observed a maximum in the density of defects for the sample with φ∼93 nm.     

Grain size reduction and recording performance improvement by using NiW as a partial interlayer in CoCrPt/SiO2 recording medium

We report the effect of NiW, as an interlayer to partially replace Ru, on the microstructure, magnetic properties, and recording performance of CoCrPt/SiO₂ perpendicular recording media. It was found that the full width at half maximum of the rocking curves of the Co (0002) peak changed little with NiW thickness up to 10 nm. However, further increase of NiW thickness caused a larger c-axis dispersion. The grain size of the CoCrPt/SiO₂ recording layer was reduced from 10.9±1.8 nm for the films without NiW to 7.7±1.5 nm for films with 10-nm NiW as a partial interlayer. The coercivity, H _c, nucleation field, H _n, and the reverse overwrite, Rev_OV, of the CoCrPt/SiO₂ layer did not change much (less than 15%) with increased NiW thickness. However, it did affect the switching field distribution of the CoCrPt/SiO₂ layer (more than double). The recording performance was improved by using NiW as a partial interlayer, which was mainly attributed to the reduced grain size.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.     

Investigation on upconversion photoluminescence of Bi3TiNbO9:Er:Yb thin films

Bi₃TiNbO₉:Er³⁺:Yb³⁺ (BTNEY) thin films were fabricated on fused silica by pulsed laser deposition. It was demonstrated that different laser fluence and substrate temperature during growth of BTNEY upconversion photoluminescence (UC-PL) samples control the film’s grain size and hence influences the UC-PL properties. The average grain size of BTNEY thin films deposited on fused silica substrates with laser fluence 4, 5, 6, and 7 J/cm² are 30.8, 35.9, 40.6, and 43.4 nm, respectively. The 525 nm emission intensities increase with the deposition laser fluence and the emission intensities of BTNEY thin film deposited under 700 and 600 °C are almost 24 and 4 times, respectively, as strong as those of samples under 500 °C. The grain size of BTNEY thin film increases with the increasing temperature. UC-PL of BTNEY films is enhanced by increasing grain size of the films.     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

Magnetic properties of strontium hexaferrite films prepared by pulsed laser deposition

The magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films fabricated by pulsed laser deposition on the Si(100) substrate with Pt(111) underlayer have been studied as a function of film thickness (50–700 nm). X-ray diffraction patterns confirm that the films have c-axis perpendicular orientation. The coercivities in perpendicular direction are higher than those for in-plane direction which indicates the films have perpendicular magnetic anisotropy. The coercivity was found to decrease with increasing of thickness, due to the increasing of the grain size and relaxation in lattice strain. The 200 nm thick film exhibits hexagonal shape grains of 150 nm and optimum magnetic properties of M_s=298 emu/cm³ and H_c=2540 Oe.     

The microstructure and magnetic properties of electrodeposited Co–Pt thin films on Ru buffer layer

For high-density magnetic recording media, this study examined the crystal structure and the texture of electrodeposited cobalt–platinum (Co–Pt) films on Ru buffer layer. A 15-nm-thick Co–Pt film exhibited very high out-of-plane coercivity and squareness, which were 6248 Oe and 0.89, respectively. The coercivity, H_c, of Co–Pt films grown on Ru buffer layer decreased significantly with increasing thickness, possibly due to the lattice misfit of 5.4% between Co–Pt and Ru, leading to the decrease of perpendicular magnetic anisotropy (PMA) of Co–Pt films as indicated by the observed hexagonal-closed-packed (HCP) (1 1¯ 0 1) plane of Co–Pt films. According to nano beam diffraction pattern (NBDP), however, Co–Pt film grown on Ru layer of HCP exhibited mixed HCP and FCC phases. Also, cross-sectional TEM image suggests that the high PMA may result from the columnar structure of physically isolated Co–Pt grains with the c-axis perpendicular to the film plane.     

Effect of continuous layer in CGC perpendicular recording media

The effect of continuous layer on CoCrPt–SiO₂ granular layer is studied in coupled granular continuous (CGC) perpendicular recording media. In the cross-section transmission electron microscope (TEM) observation, magnetic grain in the granular layer shows columnar structure, while Co/Pd multilayer shows continuous layer. The plane-view TEM image of the granular layer shows well-isolated grain structure with average grain size of around 6 nm, and grain-to-grain separation width of around 2 nm. Therefore, the interactions among the grains are negligible (J∼0). By depositing a continuous layer on a CoCrPt–SiO₂ granular layer, the grains in the granular layer are magnetically coupled through capping layer that leads to the suppression of magnetic anisotropy dispersion. This CGC structure reduces the coercivity dispersion (Δ_Hc/_Hc$\mathrm{\Delta }{H}_{\mathrm{c}}/H_{\mathrm{c}}$) from 0.26 to 0.15 and saturation field (H_s) from 10.4 to 6.7 kOe. The reduction of H_s and Δ_Hc/_Hc$\mathrm{\Delta }{H}_{\mathrm{c}}/H_{\mathrm{c}}$ improves the OW by 21.3 dB. The small Δ_Hc/_Hc$\mathrm{\Delta }{H}_{\mathrm{c}}/H_{\mathrm{c}}$ also maintains SNR of CGC media with strong magnetic exchange coupling. Furthermore, the coupling of grains through continuous layer enlarges the magnetic nucleation field (H_n) from 0.4 to −1.7 kOe. Consequently, CGC media shows better thermal stability compared to non-CGC media.     

Computer simulation of magnetization flop in magnetic tunnel junctions exchange-biased by synthetic antiferromagnets

Computer simulation in a single domain multilayer model is used to investigate magnetization flop in magnetic tunnel junctions, exchange-biased by pinned synthetic antiferromagnets with the multilayer structure NiFe/AlO_x/Co/Ru/Co/FeMn. The resistance to magnetization flop increases with decreasing cell size due to increased shape anisotropy and hence increased coercivity of the Co layers in the synthetic antiferromagnet. However, when the synthetic antiferromagnet is not or weakly pinned, the magnetization directions of the two layers sandwiching AlO_x, which mainly determine the magnetoresistance, are aligned antiparallel due to a strong magnetostatic interaction, resulting in an abnormal MR change from the high MR state to zero, irrespective of the direction of the free layer switching. This emphasizes an importance of a strong pinning of the synthetic antiferromagnet at small cell dimensions. The threshold field for magnetization flop is found to increase linearly with increasing antiferromagnetic exchange coupling between the two Co layers in the synthetic antiferromagnet. The restoring force from magnetization flop to the normal synthetic antiferromagnetic structure is roughly proportional to the resistance to magnetization flop. Irrespective of the magnetic parameters and cell sizes, the state of magnetization flop does not exist near H_a=0, indicating that magnetization flop is driven by the Zeeman energy.     

Room temperature ferromagnetism in Zn1−xCoxS thin films with wurtzite structure

The magnetic properties of Zn_1−xCo_xS (x=0.025 and 0.05) thin films grown on α-quartz substrates at different temperatures (T_S) of 200, 400 and 600 °C by means of pulsed laser deposition are presented. The films are crystallized with wurtzite structure. Optical absorption and transmission electron microscopy measurements indicate that Co ions are substituted to Zn on tetrahedral sites. Their magnetic response is composed of ferromagnetic and paramagnetic components of which respective strengths depend on T_S and Co concentration. This behavior is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or in crystal cores.     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

Einfluß der Bestrahlung mit hochenergetischen Sauerstoffionen auf die kritische Stromdichte supraleitender Nb3Sn-Schichten

Nb₃Sn diffusion layers were irradiated with 24 MeV oxygen ions at fluences from 3.2×10¹³ up to 1.6×10¹⁵ cm^?2. The enhancement of the superconducting critical current density Δj _c has been measured as a function of fluence and of the external magnetic fieldH _a(j _c⊥H _a). The thermal annealing treatment of the defects concerning thej _c and induced by irradiation, has been investigated in the temperature region from 200 to 800 °C. The results are compared with the measurements of irradiation of Nb₃Sn with protons and deuterons. The measured data are discussed in connection with size of defects, cluster distance, fluxline distance and pinning-force.