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.     

FePt L10 ordering and grain growth using millisecond pulse laser processing

The structural and magnetic properties of ∼12 nm thick FePt thin films grown on Si substrates annealed using a 1064 nm wavelength laser with a 10 ms pulse have been examined. The A1 to L1₀ ordering phase transformation was confirmed by electron and X-ray diffraction. An order parameter near 50% and a maximum coercivity of 12 kOe were obtained with laser energy densities of 25-32 J/cm². Grain growth, quantified by dark field transmission electron microscopy, occurred during chemical ordering at the laser pulse widths studied.     

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.     

L10 FePt thin films with [0 0 1] crystalline growth fabricated by SiO2 addition—rapid thermal annealing and dot patterning of the films

FePt films that have a high degree of order S in their L1₀ structure (S>0.90) and well-defined [0 0 1] crystalline growth perpendicular to the film plane were fabricated on thermally oxidized Si substrates by the addition of an oxide and successive rapid thermal annealing (RTA). The mechanism of L1₀ ordering and [0 0 1] crystalline growth perpendicular to the film plane arising through the oxide addition and RTA process is also discussed. The L1₀ ordering (S>0.90) and the [0 0 1] crystalline growth were achieved by (1) lowering the activation energy due to in-plane tensile stress and the initiation of L1₀ ordering at a low temperature, (2) [0 0 1] crystalline growth through in-plane tensile stress, and (3) enhancement of atomic diffusion via the addition of an oxide and the resultant lowering of the ordering temperature. Effect (1) was observed in the case of SiO₂ addition, effect (2) was generally observed in the case of oxide addition and the RTA process, and effect (3) was prominent in the case of ZnO addition. With the addition of ZnO, the L1₀ ordering started at below 400 °C and was completed at 500 °C. Finally, dot patterns were successfully fabricated down to a diameter of 15 nm using electron beam lithography, and the magnetic state of the dot pattern was observed by magnetic force microscopy.     

垂直取向FePt/Ag纳米颗粒薄膜的结构和磁性

用磁控溅射法在单晶MgO(100)基片上制备了［FePt 2 nm/Ag dnm］₁₀多层膜， 经真空热处理后，得到具有高矫顽力的垂直取向L1₀-FePt/Ag颗粒膜.x射线衍射结 果表明，在250 ℃的热基片上溅射，当Ag层厚度d=3—11 nm时，FePt颗粒具有很好的［001］取向，随着Ag层厚度的增加，FePt颗粒尺寸减小.［FePt 2 nm/Ag 9 nm］₁₀经过6 00 ℃真空热处理15 min后，颗粒大小仅约8 nm，垂直矫顽力达到692 kA/m.这种无磁耦合作用的颗粒膜，适合用作超高密度的垂直磁记录介质. 关键词： 磁控溅射 垂直磁记录 纳米颗粒膜 0-FePt/Ag')" href="#">L1₀-FePt/Ag     

Synthesis and magnetic properties of ordered barium ferrite nanowire arrays in AAO template

BaFe₁₂O₁₉ nanowire arrays having single magnetic domain size (≤460 nm) in anodic aluminum oxide (AAO) templates were prepared by sol-gel and self-propagating high-temperature synthesis techniques. The diameter of the nanowire arrays is approximately 70 nm and the length is about 2-4 μm. The specimens were characterized using X-ray diffraction, vibrating sample magnetometer, field emission scan electron microscope, atomic force microscopy and microwave vector network analyzer. The magnetic properties of BaFe₁₂O₁₉ nanowire arrays embedded in AAO templates were measured by VSM with a field up to 1274 KA/m at room temperature. The results indicate that the nanowire arrays exhibit large saturation magnetization and high coercivity in the range of 6000 Oe and an obvious magnetic anisotropy with the easy magnetizing axis along the length of the nanowire arrays, probably due to the shape anisotropy and magneto-crystalline anisotropy. Finally the microwave absorption properties of the nanowires were discussed.     

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.     

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.     

Granular L10 FePt (0 0 1) thin films for Heat Assisted Magnetic Recording

Granular L1₀ FePt (0 0 1) thin films were deposited on a Si substrate with Ta/MgO underlayers by rf sputtering. The effects of in-situ heating temperatures (350-575 °C), pressures (2-40 mTorr), and sputtering powers (15-75 W) on texture and microstructure were investigated for the FePt films. We obtained films with grain densities approaching 50 teragrains per in.², grains sizes down to 2.2 nm with center-to-center spacing of 4.2 nm and coercivity of 24 kOe. The order parameters for the L1₀ FePt thin films were derived and calculated to be as high as 0.91. Although the grain size is small, the spacing between grains is too large for practical heat assisted magnetic recording media. To reach the desired results, we propose that layer-by-layer growth should be promoted in the FePt layer by inserting another underlayer that provides a better lattice match to L1₀ FePt.     

Synthesis, structural and magnetic properties of the nanocomposite Fe63B23Nd7Y3Nb3Cr1 magnets

The Fe₆₃B₂₃Nd₇Y₃Nb₃Cr₁ nanocomposite magnets in the form of sheets have been prepared by copper mold casting technique. The phase evolution, crystal structure, microstructural and magnetic properties have been investigated in the as-cast and annealed states. The as-cast sheets show magnetically soft behaviors which become magnetically hard by thermal annealing. The optimal annealed microstructure was composed of nanosize soft magnetic α-Fe (19-29 nm) and hard magnetic Nd₂Fe₁₄B (45-55 nm) grains. The best hard magnetic properties such as intrinsic coercivity, _jH_c of 1119 kA/m, remanence, B_r of 0.44 T, magnetic induction to saturation magnetization ratio, M_r/M_s=0.61 and maximum energy product, (BH)_max of 55 kJ/m³ was obtained after annealing at 680 °C for 15 min. The annealing treatment above 680 °C results in non-ideal phase grains growth, which degrade the magnetic properties.     

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.     

Very high coercivities of top-layer diffusion Au/FePt thin films

The Au/FePt samples were prepared by depositing a gold cap layer at room temperature onto a fully ordered FePt layer, followed by an annealing at 800 °C for the purpose of interlayer diffusion. After the deposition of the gold layer and the high-temperature annealing, the gold atoms do not dissolve into the FePt Ll₀ lattice. Compared with the continuous FePt film, the TEM photos of the bilayer Au(60 nm)/FePt(60 nm) show a granular structure with FePt particles embedded in Au matrix. The coercivity of Au(60 nm)/FePt(60 nm) sample is 23.5 kOe, which is 85% larger than that of the FePt film without Au top layer. The enhancement in coercivity can be attributed to the formation of isolated structure of FePt ordered phase.     

Influence of flat milling on vortex matching effect in Bi2Sr2CaCu2O8+y with antidot array

Significant enlargements of antidot diameter by Ar-ion milling were observed in Bi₂Sr₂CaCu₂O_8+y single-crystal films with antidot arrays as well as the thinning of the films. In an original sample with triangular array of antidots, whose diameter is about 200 nm, a few dip structures by the matching effect were observed in the vortex-flow resistance as a function of magnetic field. With increasing the milling time, the number of the dips increases and the appearance of the flow resistance becomes periodic oscillations. These features can be explained mainly by the increase of the antidot diameter.     

The crystallographic texture and magnetic anisotropy of melt-spun CeCo5.4 ribbon

CeCo_5.4 ribbons have been prepared by melt spinning at wheel speeds v=5, 15, 25 and 35 m/s. The ribbons are essentially single 1:5 phase, and have significant crystallographic texture and magnetic anisotropy. The ribbon's longitudinal direction is easy direction, and normal to the ribbon plane direction is hard direction. For v=5 m/s [1 1 1]-axes of the grains near non-contact surface of the ribbon are normal to the ribbon plane. With increase of v, [1 1 0] and [2 0 0]-axes of the grains near non-contact surface rotate toward the normal direction and the c-axes parallel to the ribbon plane. The anisotropy increases up to v=25 m/s and then decreases. The grains near contact-wheel surface are randomly oriented for all v. The coercivity increases with increase of v due to decrease of the grain size. The values of coercivity are smaller in the easy direction and are larger in the hard direction, meaning that the coercivity mechanism is mainly characterized by domain wall pinning.     

Influence of Si-N interlayer on the microstructure and magnetic properties of γ′-Fe4N films

The (γ′-Fe₄N/Si-N)_n (n: number of layers) multilayer films and γ′-Fe₄N single layer film synthesized on Si (1 0 0) substrates by direct current magnetron sputtering were annealed at different temperatures. The structures and magnetic properties of as-deposited films and films annealed at different temperatures were characterized using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The results showed that the insertion of Si-N layer had a significant influence on the structures and magnetic properties of γ′-Fe₄N film. Without the addition of Si-N lamination, the iron nitride γ′-Fe₄N tended to transform to α-Fe when annealed at the temperatures over 300 °C. However, the phase transition from γ′-Fe₄N to ?-Fe₃N occurred at annealing temperature of 300 °C for the multilayer films. Furthermore, with increasing annealing temperature up to 400 °C or above, ?-Fe₃N transformed back into γ′-Fe₄N. The magnetic investigations indicated that coercivity of magnetic phase γ′-Fe₄N for as-deposited films decreased from 152 Oe (for single layer) to 57.23 Oe with increasing n up to 30. For the annealed multilayer films, the coercivity values decreased with increasing annealing temperature, except that the film annealed at 300 °C due to the appearance of phase ?-Fe₃N.     

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.     

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.     

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.     

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.     

Effect of sintering temperature and the particle size on the structural and magnetic properties of nanocrystalline Li0.5Fe2.5O4

Sintering temperature and particle size dependent structural and magnetic properties of lithium ferrite (Li_0.5Fe_2.5O₄) were synthesized and sintered at four different temperatures ranging from 875 to 1475 K in the step of 200 K. The sample sintered at 875 K was also treated for four different sintering times ranging from 4 to 16 h. Samples sintered at 1475 K have the cubic spinel structure with a small amount of α-Fe₂O₃ (hematite) and γ-Fe₂O₃ (maghemite). The samples sintered at≤1275 K do not show hematite and maghemite phases and the crystals form the single phase spinel structure with the cation ordering on octahedral sites. Particle size of lithium ferrite is in the range of 13-45 nm, and is depend on the sintering temperature and sintering time. The saturation magnetization increased from 45 to 76 emu/g and coercivity decreases from 151 to 139 Oe with an increase in particle size. Magnetization temperature curve recorded in ZFC and FC modes in an external magnetic field of 100 Oe. Typical blocking effects are observed below about 244 K. The dielectric constant increases with an increase in sintering temperature and particle size.