Superlattice CoCrPt/Ru/CoFe structure fabricated by pulsed laser deposition

The synthetic antiferromagnets (SAF) have been used in spin-valve sensor in data storage industry [1]. We report a new hard/Ru/soft sandwich structure (SHBL) fabricated by pulsed lased deposition to replace current single layer structure for information recording application. SHBL consists of two magnetic layers separated by thin nonmagnetic layers, typically with Ru layers of 0.7-1.2 nm, through which antiferromagnetic coupling is induced. Varying the relative thickness of the magnetic layers, the spacer layers, and the type of magnetic materials can alter magnetic properties of CoCrPt/Ru/CoFe superlattice. The coercivity H_c and grain size of magnetic layer is also dependent on the laser fluence. High laser fluence results in both small grain size and high H_c. The observed phenomena are related to high quenching and deposition rates during PLD at high fluence, resulting in more pronounced phase segregation.     

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.     

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.     

In-plane magnetization with high coercivity in terbium iron garnet thin films deposited on Pt/Si substrate by PLD

We report on the growth of terbium iron garnet (TbIG, Tb₃Fe₅O₁₂) thin films having anomalously large coercivity and in-plane easy axis of magnetization. The TbIG thin films were prepared at room temperature (RT) on Pt/Si(1 0 0) substrates by pulsed laser deposition technique. The films deposited at RT were X-ray amorphous and do not show any magnetic order. Annealing of the RT deposited film at 900 °C resulted into fully textured (532) TbIG film. Atomic force microscopy and cross-sectional scanning electron microscopy studies of the TbIG films showed good surface quality with an average surface roughness of 5.0 nm and thickness of about 300 nm, respectively. The M-H loops measured at 20 K for TbIG films, exhibit about an order of magnitude enhancement in the coercivity value (H_c) than the single crystal. In-plane and out-of-plane M-H loops revealed that the easy axis of the magnetization lies within the film’s plane. In-plane magnetization combining with large H_c value of the TbIG thin film may be of scientific interest for the possible applications.     

Fabrication, magnetism and high frequency application of exchange-coupled Fe65Co35±2-SiO1.7±0.2 granular films

A series of (Fe₆₅Co_35±2)_x-(SiO_1.7±0.2)_1−x nano-granular films with various metal volume fractions (x) were fabricated by rf sputtering. In a wide range, excellent soft magnetic properties have been achieved. In the x range from 0.7 to 0.48, the films exhibit small coercivity H_c not exceeding 4 Oe and high electrical resistivity ρ up to 1.15 × 10⁴ μΩ cm. And a minimum H_c value of 1.65 Oe was obtained for the sample of x = 0.57 with ρ = 2.86 × 10³ μΩ cm. At a frequency lower than 2.0 GHz, the real part μ′ of complex permeability of this sample is more than 170 and the FMR frequency is as high as 2.6 GHz, implying a high cut-off frequency for high frequency applications. With decreasing Fe₆₅Co_35±2 volume fraction, the resistivity of films increases remarkably and the grain size decreases obviously. At the same time, the coercivity H_c decreases with grain size decreasing, which is consistent with the conclusion resulted from random anisotropy model quoted by Herzer. Study on Henkel plots shows that intergranular ferromagnetic exchange coupling exists among grains and is important for realizing soft magnetic properties.     

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.     

Pulsed laser deposition and annealing of Dy-Fe-B thin films on melt-spun Nd-Fe-B ribbons for improved magnetic performance

Pulsed laser deposition of 250-nm thick, amorphous Dy₂Fe₁₄B layers on 40-μm thick Nd₂Fe₁₄B melt-spun ribbons was conducted to improve coercivity and energy product. The coated ribbons were subsequently annealed by two methods: (1) furnace annealing in an inert-gas controlled quartz furnace using tantalum foil at 1173 K for 2 h; (2) laser annealing using a continuous wave CO₂ laser with power varying from 10 to 20 W for 0.2 s (estimated temperatures using a thermal model were 993-1528 K). X-ray diffraction was used to identify the microstructural phases and grain size. Magnetic hysteresis tests were conducted at 300 K using a SQUID magnetometer with a maximum field of 5.0 T. Results showed a 10% increase in coercivity and 30% increase in energy product in coated over uncoated samples that were furnace-annealed. However, the coated and laser-annealed samples exhibited soft magnetic behavior with almost zero coercivity. The incomplete crystallization of amorphous phase and precipitation of α-Fe during laser annealing are found to be responsible for the observation of poor magnetic performance.     

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.     

Effect of oxygen pressure on microstructure and magnetic properties of strontium hexaferrite (SrFe12O19) film prepared by pulsed laser deposition

The effects of oxygen pressure during deposition on microstructure and magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films grown on Si (100) substrate with Pt (111) underlayer by pulsed laser deposition have been investigated. X-ray diffraction pattern confirms that the films have c-axis perpendicular orientation. The c-axis dispersion (Δθ₅₀) increases and c-axis lattice parameter decreases with increasing oxygen pressure. The films have hexagonal shape grains with diameter of 150-250 nm as determined by atomic force microscopy. The coercivities in perpendicular direction are higher than those in in-plane direction, which shows the films have perpendicular magnetic anisotropy. The saturation magnetization and anisotropy field for the film deposited in oxygen pressure of 0.13 mbar are comparable to those of the bulk strontium hexaferrite. Higher oxygen pressure leads to the films having higher coercivity and squareness. The coercivity in perpendicular and in-plane directions of the film deposited in oxygen pressure of 0.13 mbar are 2520 Oe and 870 Oe, respectively.     

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.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.     

Magnetic properties of CoFeP films prepared by electroless deposition

Structure and magnetization of CoFeP films prepared by the electroless deposition were systematically investigated by varying the bath composition and deposition parameters to optimize soft magnetic properties. The cobalt content in the CoFeP films varies from 40.4 to 94.9 wt% by controlling the bath composition. Increase of the metallic ratio FeSO₄·7H₂O/(CoSO₄·7H₂O+FeSO₄·7H₂O) affects the films’ microstructure, which switches from amorphous to crystalline structure. The magnetic properties of CoFeP films reveal that the coercivity (H_c) values range from 80 up to 185 A/m and the saturation magnetization (M_s) from 82 to 580 eum/g depending on the bath composition, deposition parameters and heat-treatment conditions. Increase of M_s and remanent magnetization (M_r) as well as decrease of H_c are observed for the CoFeP films with bath pH, temperature and the metallic molar ratio increasing. It is also found that the H_c is enhanced with the increase of NaH₂PO₂·H₂O concentration. CoFeP films showing good soft magnetic properties with coercivities less than 140 A/m and M_s close to 600 emu/g can be obtained in high pH bath and thereafter heat treatment. The deposit is found to be suitable as soft magnetic materials for core materials.     

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.     

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.     

Preparation of strontium hexaferrite film by pulsed laser deposition with in situ heating and post annealing

Strontium hexaferrite (SrFe₁₂O₁₉) films have been fabricated by pulsed laser deposition on Si(1 0 0) substrate with Pt(1 1 1) underlayer through in situ and post annealing heat treatments. C-axis perpendicular oriented SrFe₁₂O₁₉ films have been confirmed by X-ray diffraction patterns for both of the in situ heated and post annealed films. The cluster-like single domain structures are recognized by magnetic force microscopy. Higher coercivity in perpendicular direction than that for the in-plane direction shows that the films have perpendicular magnetic anisotropy. High perpendicular coercivity, around 3.8 kOe, has been achieved after post annealing at 500 °C. Higher coercivity of the post annealed SrFe₁₂O₁₉ films was found to be related to nanosized grain of about 50–80 nm.     

Influence of rare earth Ce on structural, electrical and magnetic properties of Sr based W-type hexagonal ferrites

A series of single phase W-type Sr_3−xCe_xFe₁₆O₂₇ (x=0, 0.02, 0.04, 0.06, 0.08, 0.10) hexagonal ferrites prepared by the Sol-Gel method was sintered at 1050 °C for 5 h. The X-ray diffraction analysis reveals that all the samples belong to the family of W-type hexagonal ferrites. The c/a ratio falls in the range of W-type hexagonal ferrites. The grain size was measured by SEM varies from 0.7684 to 0.4366 μm which shows that the Ce³⁺ substituted samples have smaller grain size than pure ferrite Sr₃Fe₁₆O₂₇ which results from the difference in ionic radii of Ce³⁺ (1.034 Å) and Sr²⁺ (1.12 Å). The room temperature resistivity of the present samples varies from 6.5×10⁸ to 272×10⁸ Ω-cm. The coercivity increases from 1370 to 1993 Oe which is consistent with the decrease in grain size. The coercivity values indicate that the present samples fall in the range of hard ferrites. The large value of H_c may be due to domain wall pinning at the grain boundaries.     

Effects of the Hf content on the microstructure and magnetic properties of Co-Hf-Ta thin films

Effects of the Hf content in Co-Hf-Ta thin films on the microstructure and magnetic properties were investigated in this study. It was found that appropriate Hf addition can effectively refine the Co grain size. Co grain sizes sharply decreased from 50 nm down to 2.3 nm with increasing the Hf content from 1.02 at.% to 2.81 at.%, leading to the reduced magneto-crystalline anisotropy. The Co-Hf-Ta thin films with small Co grains reveal low anisotropy field, low coercivity, and high resistivity. By optimizing the Hf content, the film with Hf concentration of 2.81 at.% exhibits excellent soft magnetic properties: high saturation magnetization (4πM_S ∼ 13.6 kG), and low coercivity (H_C ∼ 0.6 Oe). The effective permeability of the film reaches 800 and remains constant up to 1 GHz.     

Magnetic characteristics of HPT deformed soft-magnetic materials

Five sets of soft-magnetic metals, such as pure Fe, pure Ni, Fe-3 wt% Si, Fe-6.5 wt% Si and Fe-17 wt% Co, were subjected to high pressure torsion (HPT) up to strain levels where a saturation of the microstructural refinement is observed. Following HPT at 77, 293 and 723 K, transmission electron microscopy (TEM) was used to study the grain size and grain shape of the severely deformed metals. The coercivity H_C was characterized in a magnetic closed system by using ring shaped samples. Magnetic measurements obtained on ring shaped samples give a much higher accuracy for determining the coercivity. Depending on the material the mean microstructural sizes in the steady state vary from 300 nm at 723 K to 30 nm at 77 K, respectively. The coercivity of the deformed materials first increases with decrease in grain size. Once the crystallite size is far below 100 nm the coercivity shows a strong decrease.     

Investigation on magnetic properties of orientated nanocomposite Pr2Fe14B/α-Fe permanent magnets by micromagnetic finite-element method

Demagnetization curves for nanocomposite Pr₂Fe₁₄B/α-Fe permanent magnets with different hard grain alignment are calculated by a micromagnetic finite-element method. The results show that both remanence and coercivity increase with improving hard grains alignment. The demagnetization curves show a single-phase demagnetization behavior for the samples with grain size d of 10 nm and two-phase behavior for the samples with d of 20 and 30 nm. H_ex (reflecting the magnetic hardening of α-Fe) and H_irr (expressing the irreversible reversal of hard phase) are both enhanced with improving the hard grain alignment. The magnetic reversal in orientated nanocomposite permanent magnets is mainly controlled by inhomogeneous pinning of the nucleated type.