(0 0 1) Exchange-coupled FCC/L10 FePt bilayers

Exchange coupling between hard and soft magnetic materials has implications for both permanent magnet and magnetic recording technologies. This paper looks at exchange coupling of FCC FePt epitaxially grown onto (0 0 1) oriented L1₀ FePt deposited on (0 0 1) MgO substrates at elevated temperature. By varying the thickness of the FCC layer there is a relaxation of the single crystal FCC layer that produces a polycrystalline microstructure as evidenced by the development of rings in the electron diffraction pattern. A concurrent decoupling of the layers is apparent from magnetization curves with two distinct switching fields as the FCC layer thickness is increased above 20 nm. The results shown here confirm the importance of the epitaxial relationship between materials of disparate anisotropies in maintaining strong exchange coupling.     

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.     

Structural and magnetic properties of FePt films grown on Cr1-xMox underlayers

FePt films were deposited on Cr_1-xMo_x underlayers by dc magnetron sputtering. The effects of the Mo content in the underlayers, underlayer thickness, substrate temperature, and FePt film thickness on the structural and magnetic properties of the FePt films were studied. Experimental results showed that the (200) textured Cr₉₀Mo₁₀ film was a promising underlayer for promoting the growth of the L1₀ FePt films with (001) preferred orientation at relatively low temperatures. With the Cr₉₀Mo₁₀ underlayers, the ordering process of the FePt films could start at 200 °C. Both the ordering degree and the out-of-plane coercivity (H_c) of the FePt films increased with an increase in substrate temperature. When the substrate temperature was ≥250 °C, the FePt films grown on the Cr₉₀Mo₁₀ underlayers could have the (001) preferred orientation. The FePt films grown on the Cr₉₀Mo₁₀ underlayers at different temperatures showed a continuous microstructure. The out-of-plane coercivities H_c decreased while the ordering degree increased with increased FePt film thickness, which could be due to the variation of the magnetic reversal mechanism from rotation predominant mode to domain wall motion predominant mode. PACS 68.55.Jk; 75.50.Ss     

Strongly in-plane magnetized FePt thin film on ultrathin Fe underlayer

The effect of ultrathin Fe underlayer on the strong in-plane magnetization of FePt magnetic thin film was investigated. This FePt thin film could be attained using the ultrathin Fe underlayer with 1 nm thickness. The in-plane coercivity of FePt film with 20 nm thickness grown on ultrathin Fe underlayer was high up to 7400 Oe. However, its out-of-plane coercivity was extremely low to 350 Oe compared to those of FePt thin films in other conventional studies. This result indicates that FePt thin film was strongly in-plane magnetized by ultrathin Fe underlayer. The strong ordering phase transformation kinetics and the high texturing to in-plane direction of the FePt thin film by ultrathin Fe underlayer were confirmed by Kinetics Monte Carlo (KMC) simulation and XRD measurement result, respectively. It is also supposed that they are associated with the reduction of an interface free energy between the film and the substrate with an introduction of ultrathin underlayer.     

Effect of Ag underlayer thickness on the microstructure and magnetic properties of L10-FePt films

FePt/Ag films were deposited on thermally oxidized Si(100) substrates by magnetron sputtering at room temperature and then the as-deposited films were annealed at 500 ^°C. The microstructure and magnetic properties of the films have been investigated by X-ray diffraction and vibrating sample magnetometry. The results indicate that introduction of the Ag underlayer promotes an ordering transformation of the FePt phase due to thermal tensile stress between the Ag underlayer and the FePt film. The in-plane tensile stress induced by the Ag underlayer should stretch the horizontal lattice parameter of FePt; thus, it is helpful for the ordering transformation. With increasing Ag underlayer thickness, the ordering parameter and coercivity first increase and then decrease. When the Ag underlayer thickness is 12 nm, the ordering parameter and coercivity of the film reach the maximum values, respectively. The Ag underlayer thickness also affects the magnetization reversal mechanism.     

L1_0-FePt合金单层磁性薄膜的微磁学模拟

具有四方结构的L10-FePt合金因其具有高磁晶各向异性和良好的化学稳定性而成为超高密度薄膜磁记录介质的最佳选择.对实验制备得到的磁性能良好的垂直取向L10-FePt合金单层膜进行了微磁学分析.在传统微磁学模型的基础上,根据晶体的对称性,引入了四角磁晶各向异性能密度的唯象表达形式;又依据薄膜生长过程中晶格对称性的破坏,考虑了薄膜面内的应力,并引入了磁弹性能.以四角磁晶各向异性能和磁弹性能为重点,对L10-FePt合金单层膜的磁滞回线进行了详细的分析,并且用微磁学方法确定了薄膜面内应力的大小.     

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     

Structure and magnetic properties of 300-nm-thick FePt films with Hf underlayer

Structure, microstructure, magnetic properties of 300-nm-thick FePt films with 10-nm-thick Hf underlayer have been studied. The experimental results showed that the very thin Hf underlayer could promote the ordering at reduced temperatures by facilitating the nucleation of the order phase, leading to refined grain size and magnetic domain size. Therefore, the permanent magnetic properties of FePt films were enhanced. (BH)_max and H_c of FePt films were greatly enhanced from 5.0–21.0 MGOe and 1.4–11.0 kOe for single layer to 10.2–23.6 MGOe and 4.5–13.2 kOe for Hf-underlayered films annealed in T_a region of 400–600 °C, respectively. Nevertheless, the severe interdiffusion between the Hf and FePt layers at T_a=800 °C resulted in the decreased S, coarsened surface morphology, grain and magnetic domain sizes, and therefore the slightly decreased (BH)_max to 18.0 MGOe.     

Low-temperature deposition of L10 FePt films for ultra-high density magnetic recording

A method based on strain-induced phase transformation was used to lower the ordering temperature of FePt films. The strain resulted from the lattice mismatch between the FePt film and the substrate or underlayer favored the ordering. The relationships between the lattice mismatch, the ordering of FePt film, and the corresponding magnetic anisotropic constant were investigated. A critical lattice mismatch near 6.33% was believed to be most suitable for improving the chemical ordering of the FePt films. CrX (X=Ru, Mo, W, Ti) alloys with (2 0 0) texture was used to control the easy axis and ordering temperature of FePt films on glass substrate. Large uniaxial anisotropy constant K_u?1×10⁷ erg/cm³, good magnetic squareness (∼1) and FePt(0 0 1) texture (rocking curve −5°) were obtained at the temperature T_s?250 °C when using CrRu underlayer. The diffusion from overlying layers of Ag and Cu and an inserted Ag pinning layer were effective in reducing the exchange decoupling and changing the magnetization reversal. The media noise was effectively reduced and the SNR was remarkably enhanced when a 2 nm Ag was inserted.     

Ag和Ti底层对[Fe/Pt]n多层膜有序化的影响

利用磁控溅射的方法，在热玻璃基片上制备了以Ag，Ti，Cu，Cr，Pt和Ta为底层的［Fe/Pt］_n多层膜，后经不同温度真空热处理，得到L1₀有序结构的FePt 薄膜（L1₀-FePt）.实验结果表明，以Ag和Ti为底层，通过采用基片加温，同时 利用［Fe/Pt］_n多层膜结构，可以促进FePt薄膜的有序化过程，使FePt-L1_０有序化温度从500℃降低到350℃.在较高的温度下退火，以Ag为底层对薄膜的磁性 能影响较小，而以Ti为底层在高于500℃退火后，矫顽力明显下降.在400℃退火20min后，以 Ag和Ti为底层的样品平行膜面的矫顽力分别达到597kA/m和645kA/m，剩磁比分别达到0.81和 0.94，为将来FePt-L1_０有序相合金薄膜用于未来超高密度磁记录介质打下基础. 关键词： ０-FePt薄膜')" href="#">L1_０-FePt薄膜 有序化温度 底层 多层膜结构     

L10 FePt films epitaxially grown on MgO substrates with or without a Cr underlayer

FePt and FePt/Cr films were epitaxially grown on MgO (2 0 0) substrates at 350 °C by DC magnetron sputtering. The structural properties and epitaxial relationship are investigated by high-resolution X-ray diffraction (XRD). The XRD spectra revealed that both FePt and FePt/Cr films had a (0 0 1) preferred orientation. However, FePt films with Cr underlayers had a larger a and a smaller c than those of the samples without Cr underlayers. Furthermore, the FePt (0 0 1) peak characterized by its rocking curves became less pronounced when the Cr underlayer was applied. The off-spectra from the MgO (1 1 1), Cr (1 0 1) and FePt (1 1 1) demonstrated that the epitaxial relationship between the FePt film, Cr underlayer and MgO substrate was confirmed to be FePt (0 0 1)<100> || Cr (1 0 0)<1 1 0> || MgO (1 0 0)<0 0 1>. The domain size and M_s decreased when the Cr underlayer was applied due to the diffusion of Cr and the existence of the initial layer between Cr and FePt layers.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

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.     

TiN underlayer and overlayer for TbFeCo perpendicular magnetic recording media

TiN thin film is prepared by DC reactive sputtering in Ar+N₂ atmosphere and its suitability as underlayer and overlayer for TbFeCo perpendicular recording media as well as its effect on the magnetic properties of the latter have been studied. Only 5 nm TiN overlayer and 20 nm under layer can successfully protect the TbFeCo film from oxidation. Initially the coercivity is increased sharply from about 2 to 6 kOe for an increase of underlayer thickness to 60 nm then the increasing rate of coercivity becomes very slow. The saturation magnetization remains almost constant with the underlayer thickness. The remanent squareness ratio remains constant at 1.0 with the underlayer thickness up to 60 nm then decreases.     

Temperature dependent magnetic anisotropy in metallic magnets from an ab initio electronic structure theory: L1(0)-ordered FePt

Using a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy K with magnetization M in metallic ferromagnets. We apply the theory to the high uniaxial K material, L1(0)-ordered FePt, and find its magnetic easy axis perpendicular to the Fe/Pt layers for all M and K to be proportional to M2 for a broad range of values of M. For small M, near the Curie temperature, the calculations pick out the easy axis for the onset of magnetic order. Our ab initio results for this important magnetic material agree well with recent experimental measurements, whereas the single-ion anisotropy model fails to give the correct qualitative behavior.     

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.     

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.     

Magnetization reversal dynamics in Au/Co/Au(111) ultrathin films: Effect of roughness of the buffer layer

We present a study of the magnetization reversal dynamics in ultrathin Au/Co/Au films with perpendicular magnetic anisotropy, for a Co thickness of 0.5, 0.7 and 1 nm. In these films, the magnetization reversal is dominated by domain nucleation for t_Co=0.5, 0.7 nm and by domain wall propagation for t_Co=1 nm. The prevalence of domain nucleation for the thickness range 0.5-0.7 nm is different from results reported in the literature, for the same system and for the same thickness range, where the magnetization reversal took place mainly by domain wall motion. We attribute this difference to the effect of roughness of the Au buffer layer on the morphology of the magnetic layer.     

Sputtered amorphous Co-Pt-P thin films for soft underlayer of perpendicular magnetic recording

In this paper, a novel amorphous Co-Pt-P thin film fabricated by DC sputtering was proposed for soft underlayer in perpendicular recording. The structural and magnetic properties of the films were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. The results show that maximum permeability of Co-12 at%Pt-P films sputtered at room temperature (RT) is quite low when the phosphorus content is less than 5.2 at%, but Co-12 at%Pt-7 at%P thin film, is of amorphous state and has a high permeability of ∼500. The excellent soft magnetic properties of maximum permeability (500), coercivity (18 Oe) and saturation magnetization (760 emu/cm³) indicate that the amorphous Co-12 at%Pt-7 at%P film is expected to be a promising soft magnetic underlayer of perpendicular magnetic recording. The Co-12 at%Pt-7 at%P thin film is gradually crystallized with increasing substrate temperatures. The crystallization temperature of the Co-12 at%Pt-7 at%P thin film is ∼200 °C.     

Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.