Effect of magnetic field annealing on microstructure and magnetic properties of FePt films

FePt (20 nm) films were annealed in a magnetic field (along the normal direction of the films) at a temperature around the Curie temperature of L1₀ FePt. The influence of magnetic filed annealing on texture and magnetic properties of FePt films were investigated. The results indicate that preferential (0 0 1) orientation and perpendicular anisotropy can be obtained in L1₀ FePt films by using magnetic field annealing around the Curie temperature of L1₀ FePt. This is one of the potential methods to obtain (0 0 1) orientation and thus to improve the perpendicular anisotropy in FePt films.     

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.     

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 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.     

Ordering temperature of L10-type FePt films reduced by CuO addition

Effects of addition of CuO layers in L1₀-type FePt thin films are investigated. The ordering temperature of L1₀-type FePt films can be reduced by CuO addition. The coercivities of 0.78 and 0.82 T are achieved in [Pt(10 Å)/Fe(14 Å)/CuO(2 Å)]₁₀ film annealed at 550 °C for 20 min and [Pt(10 Å)/Fe(15 Å)/CuO(3 Å)]₁₀ film annealed at 600 °C for 20 min, respectively, and these values are compared to the coercivity of 0.8 T in [Pt(10 Å)/Fe(13 Å)]₁₀ film annealed at 650 °C. The thickness of Fe and CuO layers strongly influences the ordering temperature of L1₀-type FePt and the magnetic properties of the films. The addition of CuO not only brings microstructure and surface morphology changes of FePt film, but also lowers the ordering temperature.     

Assembly of high-anisotropy L10 FePt nanocomposite films

In this paper we report results on the synthesis and magnetic properties of L1₀ FePt nanocomposite films. Three fabrication methods have been developed to produce high-anisptropy FePt films: non-epitaxial growth of (0 0 1)-oriented FePt:X (X=Ag, C) composite films that might be used for perpendicular media; monodispersed FePt(CF_x) core–shell nanocluster-assembled films grown with a gas-aggregation technique and having uniform cluster size and narrow size distribution; and template-mediated self-assembled FePt clusters prepared with chemical synthesis by a hydrogen reduction technique, which has a high potential for controlling both cluster size and orientation. The magnetic properties are controllable through variations in the nanocluster properties and nanostructure. Analytical and numerical simulations have been done for these films, providing better understanding of the magnetization reversal mechanisms. The films show promise for development as magnetic recording media at extremely high areal densities.     

各向同性纳米结构Fe-Pt薄膜的结构和磁性

采用直流溅射和热处理技术制备了两个各向同性的纳米结构Fe-Pt永磁合金薄膜系列,并研究了它们的结构和磁性.研究表明,在富Fe双相纳米结构Fe-Pt永磁合金薄膜中,仅由硬磁的FePt相与软磁的Fe₃Pt相组成;在同一系列中,随Fe层厚度的增加,饱和磁极化强度和剩磁明显增大.由Kelly-Henkel图研究指出,在上述Fe-Pt纳米结构永磁合金薄膜中,磁相互作用主要由近邻纳米晶粒间的铁磁交换相互作用控制. 关键词： 磁性薄膜 纳米结构 矫顽力     

Magnetic viscosity effects in epitaxial L10 FePt thin films and exchange spring Fe–FePt bilayers

We have studied the thermal stability of L1₀ FePt thin films and Fe–FePt exchange-spring (ES) bilayers grown on (1 0 0) MgO by RF sputtering. The viscosity curves showed both for FePt films and bilayers a clear logarithmic decay of magnetization. Moreover, it was possible to evaluate the viscosity coefficient S for different applied reverse fields and the activation volumes at the coercivity. The latter values were then related to structural, magnetic and morphological measurements performed on the samples.     

Morphology, magnetic, magnetoresistance and optical properties of Co-Ni-Mo alloys thin films

We present in this paper several results concerning the preparation by means of electrolysis and characterization of Co-Ni-Mo thin films. Co-Ni-Mo thin films with different molybdenum content in the range 0-25 at% Mo were prepared from a complex solution containing ions of Co, Ni and Mo, using galvanostatic control, on aluminum substrates. The effects of applied current density on the morphology, magnetic, magnetoresistance, and optical properties of the electrodeposited Co-Ni-Mo films were investigated. The applied current density significantly influenced the film composition and their magnetic properties. The increase of molybdenum content in Co-Ni films (up to 25 at% Mo) enhances the resistivity, but it reduces the magnetoresistance effect. We report the first observation of magnetoresistance as high as 8% in Co-Ni-Mo thin films.     

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.     

Enhancement of L10 ordered FePt by Pt buffer layer

L₁₀-ordered FePt thin films prepared by molecular-beam epitaxy on MgO (0 0 1) substrate at 320 °C with different thickness of Pt buffer layer have been investigated. The out-of-plane coercivity increases with increasing thickness of Pt buffer. The maximum values of the long-range order parameter and uniaxial magnetic anisotropy energy are 0.72 and 1.78×10⁷ erg/cm³, respectively, for films with 12 nm thick Pt buffer layer, where the c/a ratio (0.976) shows the minimum value. The reason for the enhancement in ordering is due to the proper lattice strains Pt buffer bestows on FePt layer, these strains are equal to the contraction in lattice parameter c and the expansion in a. Studies of angular-dependent coercivity revealed that the magnetization reversal behaviour shifts from a domain-wall motion dominated case towards a near rotational mode with increasing thickness of Pt buffer layer.     

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.     

Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition

The antimony doped tin oxide (SnO₂:Sb) (ATO) thin films were prepared by oblique angle electron beam evaporation technique. X-ray diffraction, field emission scanning electron microscopy, UV-vis-NIR spectrophotometer and four-point probe resistor were employed to characterize the structure, morphology, optical and electrical properties. The results show that oblique angle deposition ATO thin films with tilted columns structure are anisotropic. The in-plane birefringence of optical anisotropy is up to 0.035 at α = 70°, which means that it is suitable as wave plate and polarizer. The electrical anisotropy of sheet resistance shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane and it can be changed from 900 Ω/□ to 3500 Ω/□ for deposition angle from 40° to 85°, which means that the sheet resistance can be effectively tuned by changing the deposition angle. Additionally, the sandwich structure of SiO₂ buffer layer plus normal ATO films and oblique angle deposition ATO films can reduce the resistance, which can balance the optical and electrical anisotropy. It is suggested that oblique angle deposition ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display.     

Pulsed magnetic field study of the spin gap in intermediate valence compound SmB6

In this work, we report the behavior of electrical resistivity of SmB₆ at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at T_C~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB₆ proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.     

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.     

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.     

The study of magnetic properties and microstructures of nano-size FePt islands on amorphous carbon film

This work focuses on the formation mechanisms of nano-island FePt film on commercial copper grids covered with an amorphous carbon film. FePt films of different thickness (1-7.5 nm) were deposited on amorphous carbon film and then post-annealed at 700 °C for 30 min. The configuration of the film was changed during the annealing process due to the surface energy difference between the amorphous carbon films and FePt alloy. We have prepared nanometer-size island-shaped FePt films on the amorphous carbon films and investigated their magnetic properties and microstructures. A discontinuous nano-size island magnetic film can reduce the exchange coupling of the media and increase the recording density.     

Electrical transport and magnetic properties of perovskite-type electron-doped La–Pr–Mn–O epitaxial films

La_1−xPr_xMnO₃ (LPrMO) thin films have been epitaxially grown on (1 0 0)SrTiO₃ single-crystal substrates by pulsed-laser deposition. The films have a perovskite structure and give rise to the colossal magnetoresistance effect with the maximum magnetoresistance ratio of 10³% (at 240 K and 5 T). The electrical transport and magnetic properties have been investigated for the La_0.8Pr_0.2MnO₃ film with thickness 3000 Å. The results indicate that the films have quite a distinctive magnetotransport behavior compared to the bulk. The analysis of X-ray photoemission spectroscopy suggests that the valence state of Pr is 4+ in LPrMO film. Therefore, the epitaxial film is most likely an electron-doped colossal magnetoresistance system.     

Effects of Cr underlayer and Pt buffer layer on the interfacial structure and magnetic characteristics of sputtered FePt films

This work develops a new method for growing L1₀ FePt(0 0 1) thin film on a Pt/Cr bilayer using an amorphous glass substrate. Semi-coherent epitaxial growth was initiated from the Cr(0 0 2) underlayer, continued through the Pt(0 0 1) buffer layer, and extended into the L1₀ FePt(0 0 1) magnetic layer. The squareness of the L1₀ FePt film in the presence of both a Cr underlayer and a Pt buffer layer was close to unity as the magnetic field was applied perpendicular to the film plane. The single L1₀ FePt(1 1 1) orientation was observed in the absence of a Cr underlayer. When a Cr underlayer is inserted, the preferred orientation switched from L1₀ FePt(1 1 1) to L1₀ FePt(0 0 1) and the magnetic film exhibited perpendicular magnetic anisotropy. However, in the absence of an Pt intermediate layer, the Cr atoms diffused directly into the FePt magnetic layer and prevented the formation of the L1₀ FePt(0 0 1) preferred orientation. When a Pt buffer layer was introduced between the FePt and Cr underlayer, the L1₀ FePt(0 0 1) peak appeared. The thickness of the Pt buffer layer also substantially affected the magnetic properties and atomic arrangement at the FePt/Pt and Pt/Cr interfaces.     

Field dependence of spin and orbital moments of Fe in L10 FePt magnetic thin films

The field dependence of spin and orbital magnetic moments of Fe in L1₀ FePt magnetic thin films was investigated using X-ray magnetic circular dichroism (XMCD). The spin and orbital moments were calculated using the sum rules; it was found that the spin and orbital moment of Fe in L1₀ FePt films are ∼2.5 and 0.2 μ_B, respectively. The relative XMCD asymmetry at Fe L₃ peak on the dependence of applied field suggested that the majority magnetic moment of L1₀ FePt films resulted from Fe.