Self-assembled L10 FePt thin films for high-density magnetic recording

A new technique, which utilizes the interlayer diffusion, for preparation of self-assembled nanodot magnetic structures has been proposed. L1₀-phase Pt/FeCu and Pt/FeAg films have been successfully synthesized by this technique. Both the coercivity of Pt/FeCu and Pt/FeAg films exhibited, respectively 4.1 and 8.0 kOe in perpendicular direction. Pt/Fe and Pt/FeAg films show positive values, while Pt/FeCu shows negative value in δm plot. The results indicate that the exchange coupling between the grains has been decoupled in the self-assembled nanodot structure in Pt/FeCu film.     

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.     

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.     

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.     

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.     

Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

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.     

Perpendicular magnetic anisotropy of FePt film on Si substrate with SiO2 underlayer and B4C interlayers

FePt multilayer composite films with and without B₄C interlayer have been prepared by magnetron sputtering, respectively, and subsequent annealing in vacuum. It was found that the B₄C layers effectively serve as spacers to separate the FePt layers, enhancing (0 0 1) orientation of FePt alloy. Our results show that highly (0 0 1) oriented FePt/B₄C films have significant potential as perpendicular recording media.     

Epitaxial growth of Co(0 0 0 1)hcp/Fe(1 1 0)bcc magnetic bi-layer films on SrTiO3(1 1 1) substrates

Co(0 0 0 1)_hcp/Fe(1 1 0)_bcc epitaxial magnetic bi-layer films were successfully prepared on SrTiO₃(1 1 1) substrates. The crystallographic properties of Co/Fe epitaxial magnetic bi-layer films were investigated. Fe(1 1 0)_bcc soft magnetic layer grew epitaxially on SrTiO₃(1 1 1) substrate with two type variants, Nishiyama–Wasserman and Kurdjumov–Sachs relationships. An hcp-Co single-crystal layer is obtained on Ru(0 0 0 1)_hcp interlayer, while hcp-Co layer formed on Au(1 1 1)_fcc or Ag(1 1 1)_fcc interlayer is strained and may involve fcc-Co phase. It has been shown possible to prepare Co/Fe epitaxial magnetic bi-layer films which can be usable for patterned media application.     

The role of the dopant in the magnetism of Fe-doped SnO2 films

Transparent pure and Fe-doped SnO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. X-ray diffraction shows that the films are polycrystalline and have the rutile structure. Surprisingly, the pure film presents magnetic-like behavior at room temperature with a saturated magnetization of almost one-third of the doped film (∼3.6 and 11.3 emu/g, respectively) and its magnetization could not be attributed to any impurity phase. Taking into account the magnetic moment measured in the pure film, the effective contribution of the impurity in the doped one can be inferred to be ∼2 μ_B per Fe atom. A large magnetic moment was also predicted by an ab initio calculation in the doped system, which increases if an oxygen vacancy is present near the Fe impurity.     

Fabrication of L10 FePtAg nanoparticles and a study of the effect of Ag during the annealing process

L1₀ ferromagnetic phase FePt nanoparticles containing Ag atoms (FePtAg) were synthesized by means of a liquid phase process, followed by annealing. The addition of Ag to FePt nanoparticles permits annealing to be conducted at a lower temperature (350 °C). This is further accompanied by a subsequent transformation in the crystal phase from the FCC superparamagnetic phase to the FCT (L1₀) ferromagnetic phase. The effects of annealing temperature and the Ag atoms inside the nanoparticles on the magnetic properties of the FePt nanoparticles have been studied. Using electron spectroscopy for the chemical analysis (ESCA), Ag atoms in the L1₀ phase FePtAg nanoparticles were found to be localized on the surface region of the annealed nanoparticles. The Ag atoms function to inhibit the oxidation of FePt, causing the particles to become more stable and to have ferromagnetic properties.     

Magnetic and structural study of Cu-doped TiO2 thin films

Transparent pure and Cu-doped (2.5, 5 and 10 at.%) anatase TiO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. The samples were structurally characterized by X-ray absorption spectroscopy and X-ray diffraction. The magnetic properties were measured using a SQUID. All films have a FM-like behaviour. In the case of the Cu-doped samples, the magnetic cycles are almost independent of the Cu concentration. Cu atoms are forming CuO and/or substituting Ti in TiO₂. The thermal treatment in air promotes the CuO segregation. Since CuO is antiferromagnetic, the magnetic signals present in the films could be assigned to Cu substitutionally replacing cations in TiO₂.     

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.     

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.     

Optical and magnetic properties of (Ga1−xMnx)N/GaN digital ferromagnetic heterostructures

(Ga_1−xMn_x)N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga_1−xMn_x)N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga_1−xMn_x)N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga_1−xMn_x)N/GaN DFHs were much larger than those in (Ga_1−xMn_x)N thin films. These results indicate that the (Ga_1−xMn_x)N/GaN DFHs hold promise for potential applications in spintronic devices.     

Band-edge exciton transitions in (Ga1−xMnx)N diluted magnetic semiconductor films with above room temperature ferromagnetic transition

(Ga_1−xMn_x)N thin films grown on GaN buffer layers by using molecular beam epitaxy were investigated with the goal of producing diluted magnetic semiconductors (DMSs) with band-edge exciton transitions for applications in optomagnetic devices. The magnetization curve as a function of the magnetic field at 5 K indicated that ferromagnetism existed in the (Ga_1−xMn_x)N thin films, and the magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature of the (Ga_1−xMn_x)N thin film was above room temperature. Photoluminescence and photoluminescence excitation spectra showed that band-edge exciton transitions in (Ga_1−xMn_x)N thin films appeared. These results indicate that the (Ga_1−xMn_x)N DMSs with a magnetic single phase hold promise for potential applications in spin optoelectronic devices in the blue region of the spectrum.     

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.     

The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy₂O₃ and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy₂O₃ or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy₂O₃ additions result in the increase in the H_ci and temperature dependence due to the increase of T_c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy₂O₃ and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N_eff and the smaller Φ phase.     

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.