Magnetic properties and microstructure study of high coercivity Au/FePt/Au trilayer thin films

High-coercivity Au(60 nm)/FePt(δ nm)/Au(60 nm) trilayer samples were prepared by sputtering at room temperature, followed by post annealing at different temperatures. For the sample with δ=60 nm, L1₀ ordering transformation occurs at 500 °C. Coercivity (H_c) is increased with the annealing temperature in the studied range 400–800 °C. The H_c value of the trilayer films is also varied with thickness of FePt intermediate layer (δ), from 27 kOe for δ=60 nm to a maximum value of 33.5 kOe for δ=20 nm. X-ray diffraction data indicate that the diffusion of Au atoms into the FePt L1₀ lattice is negligible even after a high-temperature (800 °C) annealing process. Furthermore, ordering parameter is almost unchanged as δ is reduced from 60 to 15 nm. Transmission electron microscope (TEM) photos indicate that small FePt Ll₀ particles are dispersed amid the large-grained Au. We believe that the high coercivity of the trilayer sample is attributed to the small and uniform grain sizes of the highly ordered FePt particles which have perfect phase separation with Au matrix.     

Low-temperature ordering of L10 FePt phase in FePt thin film with AgCu underlayer

FePt (20 nm) films with AgCu (20 nm) underlayer were prepared on thermally oxidized Si (0 0 1) substrates at room temperature by using dc magnetron sputtering, and the films annealed at different temperature to examine the disorder–order transformation of the FePt films. It is found that the ordered L1₀ FePt phase can form at low annealing temperature. Even after annealing at 300 °C, the in-plane coercivity of 5.2 kOe can be obtained in the film. With increase in annealing temperature, both the ordering degree and coercivity of the films increase. The low-temperature ordering of the films may result from the dynamic stress produced by phase separation in AgCu underlayer and Cu diffusion into FePt phase during annealing.     

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.     

Effect of GePt buffer layer on magnetic properties and microstructure of FePt films

We have explored the interlayer diffusion effect of Ge/FePt, GePt/FePt bilayer on the formation of ordered L1₀ FePt phase. In Ge/FePt bilayer, the Ge₃Pt₂ compound was formed during post annealing at 400^oC for 1.0 h. Diffusion between Ge and FePt layer suppres the formation of ordered L1₀ FePt phase. With Ge₂Pt₃ underlayer, the FePt film was ordered at 400 °C and the in-plane coercivity was 9.3 kOe. The ordering temperature was reduced about 50 °C compared to the single layer FePt film.     

Effect of interfacial diffusion on microstructure and properties of FePt/B4C multifunctional multilayer composite films

FePt/B₄C multilayer composite films were prepared by magnetron sputtering and subsequent annealing in vacuum. By changing Fe layer thickness of [Fe/Pt]₆/B₄C films, optimal magnetic property (8.8 kOe and remanence squareness is about 1.0) is got in [Fe(5.25 nm)/Pt(3.75 nm)]₆/B₄C sample whose composition is Fe rich and near stoichiometric ratio. The characterizations of microstructure demonstrate that the diffusion of B and C atoms into FePt layer depends strongly on B₄C interlayer thickness. When B₄C interlayer thickness of [Fe(2.625 nm)/Pt(3.75 nm)/Fe(2.625 nm)/B₄C]₆ films is bigger than 3 nm, stable value of grain size (6-6.5 nm), coercivity (6-7 kOe) and hardness (16-20 GPa) is observed. Finally, the multifunctional single FePt/B₄C composite film may find its way to substitute traditional three-layer structure commonly used in present data storage technology.     

Nanocomposite L10 FePt–SiNx and FePt–SiNx–C films with large coercivity and small grain size on a TiN intermediate layer

FePt–SiN_x–C films with high coercivity, (001) texture and small grain size were obtained by co-sputtering FePt, Si₃N₄ and C on TiN/CrRu/glass substrate at 380 °C. Without C doping, FePt–SiN_x films with good perpendicular anisotropy and a single layer structure were obtained. However, the grain size was still too large and the grain isolation was poor. When C was doped into the FePt–SiN_x films, the out-of-plane coercivity increased due to the decrease of the exchange coupling. In addition, the grain size of the FePt films decreased, and well-separated FePt grains with uniform size were formed. The microstructure of [FePt–SiN_x 40 vol%]−20 vol% C films changed from a single layer structure to a multiple layer structure when the FePt thickness was increased from 4 to 10 nm. By optimizing the sputtering process, the [FePt (4 nm)–SiN_x 40 vol%]−20 vol% C (001) film with coercivity higher than 21.5 kOe, a single layer structure, and small average FePt grain size of 5.6 nm was obtained, which makes it suitable for ultrahigh density perpendicular recording.     

Effect of annealing in an external magnetic field on the microstructure and magnetic properties of the Fe/FePt/Pt multilayer system

The effect of annealing in an external magnetic field applied perpendicular to the plane of the film on the kinetics of Ll ₀ phase transformation of the microstructure and the magnetic properties of the Fe(2 nm)/FePt(20 nm)/Pt(2 nm) multilayer system has been investigated. The relations between the hysteresis loop shape, magnetic correlation length, and structural disorders, which are characteristic of magnetic information carriers, have been analyzed. It has been found that the annealing of the Fe(2 nm)/FePt(20 nm)/Pt(2 nm) multilayer system at a temperature of 470°C in an external magnetic field of 3500 Oe, which is applied perpendicular to the film plane, leads to the formation of a face-centered tetragonal structure of the Ll ₀ phase in the FePt film, which is characterized by the high coercivity H _c , the (001) preferred texture, the magnetic anisotropy perpendicular to the film plane, small sizes of FePt grains in the film, and weak exchange coupling between the particles. The energy of the external magnetic field encourages the process of transformation of the FePt film into the Ll ₀ phase. Thus, a method has been developed for fabricating multilayer films based on the FePt Ll ₀ phase with the parameters necessary for information carrier materials with perpendicular-type magnetic recording.     

Magnetic properties and microstructure of x/FePt (x=Ge,GePt) bilayer

We have investigated the effect of Ge, GePt underlayers on the formation of ordered L1₀ FePt films. With Ge underlayer, the Ge₃Pt₂ compound was formed during post-annealing at 400 °C for 1 h. Interlayer diffusion of Ge and FePt layer suppress the formation of ordered L1₀ FePt phase. With Ge₂Pt₃ underlayer, the FePt film was ordered at 350 °C and the in-plane coercivity was 5.1 kOe. The ordering temperature was reduced to about 50 °C compared to the single-layer FePt film.     

Effect of Cr100−xTix (x = 0-19 at.%) alloy underlayers on the high coercivity of FePt thin film below the substrate temperature of 250 °C

The effect of Cr_100−xTi_x underlayer on orderd-L1₀ FePt films was investigated. A low-temperature ordering of FePt films could be attained through changing the Ti content of Cr_100−xTi_x underlayer. The ordering temperature of the 30 nm FePt film grown on 20 nm Cr₉₀Ti₁₀ underlayer was reduced to 250 °C which is practical manufacture process temperature. An in-plane coercivity was very high to 6000 Oe and a ratio of remnant magnetization (M_r) to saturation magnetization (M_s) was as large as 0.85. This result indicates that the coercivity obtained at 250 °C by the effect of CrTi underlayer is significantly higher than those obtained at 250-275 °C by the effect of underlayers in other conventional studies. The prominent improvement of the magnetic properties of ordered FePt thin films at low temperature of 250 °C could be understood with considering the strain-induced ordering phase transformation associated with lattice mismatch between Cr underlayer and FePt magnetic layer due to an addition of Ti content.     

Low-temperature ordering and enhanced coercivity of L10-FePt thin films with Al underlayer

FePt multilayer films with and without Al underlayer were prepared by magnetron sputtering on SiO₂ substrate and subsequently annealed in vacuum. Experimental results suggest that the existence of Al underlayer can effectively reduce the ordering temperature and increase the coercivity of FePt films. Due to the slight larger lattice constant of Al underlayer than that of FePt films, [Fe (0.66 nm)/Pt (0.84 nm)]₃₀ films begin to order at 350 °C and the coercivity of them reach to 5.7 kOe after annealing at 400 °C for half an hour.     

Understanding magnetic properties of arrays of small FePt dots with perpendicular anisotropy

FePt dot arrays with dot size down to 15 nm are fabricated by film annealing and patterning. The array coercivity shows an increase with dot size decreasing from 100 to 30 nm, and a slight reduction for the 15 nm dot sample. Annealing these dot arrays at higher temperatures results in large enhancements in the coercivities, except the 15 nm dot array where the coercivity increases a little. Micromagnetic models of a 15 nm FePt dot with uniform and nonuniform edges of soft magnetic defects and with inside defects are calculated to reveal the microstructure origins of the dot magnetic properties. It is found that the volume fraction of the L1₀-phase FePt with perpendicular c-axis orientation is about 50% in the dot and the switching field distribution of the dot array can be influenced significantly by the defect arrangement in the dots.     

Exchange interaction effect of TbCo/FePt bilayers

Interlayer exchange coupling in dc-magnetron sputtered Tb_29.6Co_70.4/FePt bilayers with different annealing temperatures of the FePt film have been investigated. The dependence of ordering degree on perpendicular magnetic properties of the FePt film was studied. The Tb_29.6Co_70.4/FePt film has high perpendicular coercivity and high saturated magnetization about 7.5 kOe, and 302 emu/cm³, respectively as the substrate temperature is 500 °C and annealing at 500 °C for 30 min. It also shows a strong exchange coupling between this FePt layer and Tb_29.6Co_70.4 layer. We also examined the interface wall energy in the exchange coupled Tb_29.6Co_70.4/FePt double layers.     

Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films

The L1₀ ordered FePt films have been prepared at 300 °C with a basic structure of CrRu/MgO/FePt, followed by a post-annealing process at temperatures from 200 to 350 °C. The magnetic properties and the microstructure of the films were investigated. It is found that coercivity of FePt films increases greatly from 3.57 to 9.1 kOe with the increasing annealing temperature from 200 to 350 °C. The loop slope of the M–H curves decreases with the increasing annealing temperature, which is due to the grain isolation induced by MgO underlayer diffusion during the annealing process. The underlayer diffusion could be a useful approach to prepare the FePt-based composite films for high-density recording media.     

Microstructural and magnetic studies of L10 FePt–SiO2 nano-composite thin film with columnar structure for perpendicular magnetic recording

(Fe₅₀Pt₅₀)_100−x-(SiO₂)_x films (x=0–30 vol%) were grown on a textured Pt(0 0 1)/CrRu(0 0 2) bilayer at 420 °C using glass substrates. FePt(0 0 1) preferred orientation was obtained in the films. Interconnected microstructure with an average grain size of about 30 nm is observed in the binary FePt film. As SiO₂ is incorporated, it precipitates as particles are dispersed at FePt grain boundaries. When the content of SiO₂ is increased to 13 vol%, columnar FePt with (0 0 1) texture separated by SiO₂ is attained. The FePt columns have a length/radius ratio of 2:1. Additionally, the mean grain size is reduced to about 13 nm. The development of this well-isolated columnar structure leads to an enhancement in coercivity by about 44% from 210 to 315 kA/m. As the SiO₂ content exceeds 20 vol%, a significant ordering reduction is found accompanied by a transformation of preferred orientation from (0 0 1) to (2 0 0) and the columnar structure disappears, resulting in a drastic degradation in magnetism. The results of our study suggest that isolated columnar, grain refined, (0 0 1)-textured FePt film can be achieved via the fine control of SiO₂ content. This may provide useful information for the design of FePt perpendicular recording media.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

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 Ni-Al underlayer-accelerated L10 ordering transition of FePt thin films

In the present study, we succeeded in accelerating the L1₀ ordering transition of FePt thin films by employing amorphous Ni-Al as underlayers. The coercivity H_c = 5 kOe and ordering parameter S = 0.67 of FePt thin films deposited on a Ni-Al underlayer with a thickness of ∼5 nm after 380 °C annealing for 30 min are significantly higher than those H_c = 0.4 kOe and S = 0.35 of the films without the Ni-Al underlayer. The L1₀ ordering process of and the coercivity of FePt thin films can be significantly tuned by varying the thickness of the Ni-Al underlayer.     

Control of crystallographic orientation in L10 FePt films by using Cr and CrW underlayers

The microstructure and magnetic properties of FePt films grown on Cr and CrW underlayers were investigated. The FePt films that deposited on Cr underlayer show (2 0 0) orientation and low coercivity because of the diffusion between FePt and Cr underlayer. The misfit between FePt magnetic layer and underlayer increases by small addition of W element in Cr underlayer or using a thin Mo intermediate layer, which is favorable for the formation of (0 0 1) orientation and the transformation of FePt from fcc to fct phase. A good FePt (0 0 1) texture was obtained in the films with Cr₈₅W₁₅ underlayer with substrate temperature of 400 °C. The FePt films deposited on Mo/Cr underlayer exhibit larger coercivity than that of the films grown on Pt/Cr₈₅W₁₅ because 5 nm Mo intermediate layer depressed the diffusion of Cr into magnetic layer.     

Comparative evaluation of performance of percolated perpendicular media using FePt–MgO/Pt/Cr trilayer and FePt–MgO/MgO bilayer

(Fe₄₈Pt₅₂)_100−x–(MgO)_x films were used to examine the performance of a perpendicular percolated medium. Two underlayers, Pt(0 0 1)/Cr(0 0 2) and MgO(0 0 2), were used for comparison. The (Fe₄₈Pt₅₂)_100−x–(MgO)_x film with the MgO underlayer exhibits a strong preference to segregate at FePt grain boundaries. The microstructure with small closely packed MgO particles (2–4 nm) dispersed uniformly in the L1₀ FePt matrix was achieved in the Pt/Cr underlayered sample. Structural data reveal that the precipitate is crystallographically coherent with the surrounding L1₀ FePt phase and preserves good lattice alignment. Magnetic results indicate significant pinning behavior for those introduced non-magnetic columns with an enhanced coercivity of about 70%—much greater than that of the MgO underlayered samples. Percolated perpendicular medium can be realized in the FePt system and a Pt(0 0 1)/Cr(0 0 2) underlayer promotes the formation of pinning sites within the FePt grains.     

L10-ordered high coercivity (FePt)Ag-C granular thin films for perpendicular recording

We report (FePt)Ag-C granular thin films for potential applications to ultrahigh density perpendicular recording media, that were processed by co-sputtering FePt, Ag, and C targets on MgO underlayer deposited on thermally oxidized Si substrates. (FePt)_1−xAg_x-yvol%C (0<x<0.2, 0<y<50) films were fabricated on oxidized silicon substrates with a 10 nm MgO interlayer at 450^oC. We found that the Ag additions improved the L1₀ ordering and the granular structure of the FePt-C films with the perpendicular coercivity ranging from 26 to 37 kOe for the particle size of 5-8 nm. The (FePt)_0.9Ag_0.1-50vol%C film showed the optimal magnetic properties as well as an appropriate granular morphology for recording media, i.e., average grain size of D_av=6.1 nm with the standard deviation of 1.8 nm.