Perpendicular FePt Research Articles

Switching Field Distribution in BN/FePtCAg/MgTiON and FePtCAg/MgTiOBN Films.

BN is the currently required segregant for perpendicular FePt media. We found that BN can be diffused from the MgTiOBN intermediate layer during a high temperature process. The FePtCAg film sputtered on MgTiOBN layers illustrates higher perpendicular magnetocrystalline anisotropy (Ku) (1.43 × 107 erg/cm3) and coercivity (normal to film surface) (17 kOe) at 350 K compared to BN/FePtCAg/MgTiON film. From the microstructure, the FePtCAg film shows the granular structure on the MgTiOBN intermediate layer, but parts of the irregular FePt grains are agglomerated and partially separated in the matrix, with grains size being, on average, 26.7 nm. Cross-sectional imaging showed that the FePt grains have a truncated pyramid shape with a lower wetting angle, which is influenced by the surface energy of MgTiOBN. BN segregation at FePt grains or boundaries is still not clear. Using the electron energy loss spectrum (EELS), we found that part of the BN atoms were clearly observed in the FePt lattice and iron-boride oxide was indexed in the x-ray photoelectron spectroscopy (XPS) spectra. To determine the effects of BN segregant (from capping layer or intermediate layer) on the magnetic switching behavior of FePtCAg film, the intrinsic-(ΔHint = 6.17 kOe, 6.54 kOe) and extrinsic- (ΔHext = 0.80 kOe, 0.39 kOe) switching field distribution (SFD) were measured by plotting saturated major- and unsaturated minor- hysteresis loops to evaluate the crystal orientation and microstructure (grains volume and distribution) for BN/FePtCAg/MgTiON and FePtCAg/MgTiOBN films, respectively. The main contribution of intrinsic SFD is the c-axis misalignment for the BN/FePt/MgTiON sample; however, the dispersed magnetic anisotropy has a higher input to intrinsic SFD for FePtCAg/MgTiOBN/CrRu film.

Controlled Magnetic Isolation and Decoupling of Perpendicular FePt Films by Capping Ultrathin Cu(002) Nano-Islands

This study investigated the ultrathin Cu(002) capping nano-island effects on the magnetic characterizations and microstructure of epitaxial FePt(001) films directly fabricated on MgO(001) substrates at the relatively low temperature of 300 °C via electron-beam deposition. The enhancement of the coercivity is attributed to the lowered exchange coupling of FePt magnetic grains that begun from Cu atom behavior of spreading in many directions mainly along grain boundaries due to its lower surface energy than that of pure Fe or Pt. The measurement of angular-dependent coercivity shows a tendency of a domain-wall motion shift toward the rotation of the reverse-domain type upon the thickness of the Cu capping nano-island layer atop the FePt films. The intergranular interaction was clarified by the Kelly–Henkel plot, which indicated that there was strong exchange coupling (positive δM) between neighboring grains in the FePt continuous films without Cu capping nano-islands. On the other hand, a negative δM value was gained when the FePt films were capped with a Cu(002) single layer, indicating that the Cu capping layer can be used to control the strength of intergrain exchange coupling between the adjacent FePt grains and thicker Cu(002) capping nano-islands toward magnetic isolation; thus, there was an existence of dipole interaction in our designed Cu/FePt composite structure of stacked films.

High resolution imaging based on photo-emission electron microscopy excited by deep ultraviolet laser

Magnetic imaging technology based on photo-emission electron microscopy (PEEM) has become an important and powerful tool for observing the magnetic domain in spintronics. The PEEM can get access to real-time imaging with high spatial resolution and is greatly sensitive to the spectroscopic information directly from the magnetic films and surfaces through photoemission process with variable excitation sources. Moreover, the breakthrough in the deep ultraviolet (DUV) laser technology makes it possible to realize domain imaging without the limitation of synchrotron radiation facilities or the direct excitation of photoelectrons due to the high enough photon energy of the source in the current threshold excitation study. In this review article, the deep ultraviolet photo-emission electron microscopy system is first introduced briefly. Then, a detailed study of the magnetic domain observation for the surface of L1<sub>0</sub>-FePt films by the DUV-PEEM technique is presented, where a spatial resolution as high as 43.2 nm is successfully achieved. The above results clearly indicate that the DUV-PEEM reaches a level equivalent to the level reached by X-ray photoemission imaging technique. Finally, a series of recent progress of perpendicular FePt magnetic thin films obtained by the DUV-PEEM technique is provided in detail. For example, a stepped Cr seeding layer is used to form the large-area epitaxial FePt films with (001) and (111) two orientations, where magnetic linear dichroism (MLD) with large asymmetry is observed in the transition area of two phases. The signal of MLD is 4.6 times larger than that of magnetic circular dichroism. These results demonstrate that the magnetic imaging technology based on DUV-PEEM with excellent resolution ability will potentially become an important method to study magnetic materials in the future.

High frequency out-of-plane oscillation with large cone angle in mag-flip spin torque oscillators for microwave assisted magnetic recording

We investigated spin torque induced magnetization dynamics in the mag-flip spin torque oscillators (STOs) of diameters D from 29 to 96 nm comprising of an in-plane magnetized field generation layer (FGL) Fe67Co33 (7 nm) with high saturation magnetization, μ0Ms ∼ 2.3 T, and perpendicular FePt(10 nm)/Co2FeGa0.5Ge0.5(3 nm) highly spin polarized spin injection layers. Out-of-plane high frequency, f ∼ 21–26 GHz, spin torque induced oscillation with a large cone angle in FGL was observed under nearly perpendicular external magnetic field μ0Hext of 1.1 T for the pillar D of 29 and 42 nm. Our micromagnetic simulation results indicated that ac magnetic fields of about 0.15 to 0.2 T are obtainable from the STOs having the same stacking structure and size as the experiment, which is large enough for the applications to microwave assisted magnetic recording technology.

Electrical control of fast ordering process in as-deposited non-perpendicular and perpendicular FePt films

Abstract Here, DC power supplies were applied on as-deposited non-perpendicular FePt single films or perpendicular FePt/Au multilayer films to control momentum exchanges between rapid moving electrons and Fe or Pt atoms. Moreover, lattice defects in the films provide abundant diffusion pathways for atomic rearrangement. Through the current driven atomic motion and rearrangement, fast LOP and hard magnet properties were realized in the FePt alloy films without heat treatment. The electromigration effect appears to be general and is applicable for the ordering in other magnetic materials with a L1 0 structure.

Electromigration induced fast L1 ordering phase transition in perpendicular FePt films

Realizing fast L10 ordering phase transition (LOPT) in L10 structured magnetic materials without heat treatment is crucial for their applications in spintronic devices. This article reports on the electromigration controlled momentum transfer and rapid ordering of Fe and Pt atoms in the as-deposited FePt films. Lattice defects in the films provide enough diffusion pathways and allow the Fe and Pt atoms rearranging. Through the current driven atomic motion and rearrangement, fast LOPT can result in the establishment of perpendicular magnetic anisotropy of the FePt films at room temperature. This effect is expected to work with other L10 typed magnetic materials for spintronic devices development.

Columnar grain growth of FePt(L1) thin films

An experimental approach for obtaining perpendicular FePt-SiOx thin films with a large height to diameter ratio FePt(L10) columnar grains is presented in this work. The microstructure for FePt-SiOx composite thin films as a function of oxide volume fraction, substrate temperature, and film thickness is studied by plan view and cross section TEM. The relations between processing, microstructure, epitaxial texture, and magnetic properties are discussed. By tuning the thickness of the magnetic layer and the volume fraction of oxide in the film at a sputtering temperature of 410 °C, a 16 nm thick perpendicular FePt film with ∼8 nm diameter of FePt grains was obtained. The height to diameter ratio of the FePt grains was as large as 2. Ordering at lower temperature can be achieved by introducing a Ag sacrificial layer.

Continuity of perpendicular FePt film with a compositional gradient design

By magnetron sputtering, [Fe(0.6 nm)/Fe30.5Pt69.5(1.9 nm)]10 continuous multilayer films with the nominal composition of Fe50Pt50 and total thickness of 25 nm were deposited on MgO(001) substrates heated to 400°C, and then subjected to a vacuum annealing at temperatures in the range of Ta = [500, 900]°C. The magnetocrystalline anisotropic energy exceeded 2.5 × 107 erg/cc at Ta ≥ 700°C, and the morphology kept continuous at Ta ≤ 800°C. Though the interlayer diffusion during sputtering was fairly sufficient, a weak periodical compositional gradient was left in the films, which is critical for the morphological continuity of film subjected to subsequently annealing. The slight periodical compositional gradient might suppress the segregation of Pt and sufficiently prevent from the Pt enrichment that would result in the discontinuous boundaries. A modified diffusion-recombination model of carrier was adopted to discuss the morphology continuity of annealed FePt films. The continuous film would fit the need for microfabrication of patterned arrays of L10 FePt.

The Magnetic Properties of Perpendicular FePt thick Films and the Written Patterns for a T-ray Source Undulator

The Magnetic Properties of Perpendicular FePt thick Films and the Written Patterns for a T-ray Source Undulator

Modeling and characterization of irreversible switching and viscosity phenomena in perpendicular exchange-spring Fe-FePt bilayers

A numerical model has been developed for simulating magnetic domain configurations, remanence, and viscosity curves in systems with strong perpendicular anisotropy and strong disorder, starting from internal switching field distributions for pinning and nucleation processes in the slow dynamics regime. In the considered systems, the domains expand in a percolationlike manner and domain configuration displays fractal properties. The simulations show that even in the case of pinning not governed by nucleation an abrupt avalanche propagation of reversed domains occurs. It is moreover evidenced that a decrease of viscosity coefficients can coexist with lowered energy barrier heights. The model has been applied to perpendicular FePt thin films with granular morphology and corresponding exchange-coupled Fe/FePt bilayers, by exploiting magnetic force microscopy, dc demagnetization (DCD), isothermal remanence, and viscosity data. The comparison between magnetic viscosity phenomena in thin films and exchange-coupled bilayers has been achieved by the definition of an adimensional viscosity coefficient. The addition of the Fe layer causes a decrease of the maximum viscosity coefficient, thus demonstrating that viscosity measures can be utilized to verify the coupling of hard/soft layers. From experiments it is inferred that our samples are mainly influenced by the pinning energy barrier law, the linearity of which allows reconstructing universal viscosity curves starting from DCD data. The calculated activation volumes are comparable to the average grain volume of the FePt layer. The obtained results also demonstrate that the addition of the Fe layer leads to a widening and a shift to lower fields of the pinning field distribution, determining a decrease of both the maximum viscosity and the pinning energy barrier heights and an increase of the demagnetizing effective field.

Strong coercivity reduction in perpendicular FePt∕Fe bilayers due to hard/soft coupling

We have prepared perpendicular hard/soft bilayers made of a 10nm L10-FePt layer, which has been epitaxially grown on MgO(100) and a Fe layer with thicknesses of 2 and 3.5nm. The control of the interface morphology allows to modify the magnetic regime at fixed Fe thickness (from rigid magnet to exchange-spring magnet), due to the nanoscale structure effect on the hard/soft coupling and to tailor the hysteresis loop characteristics. Despite the small thickness of the soft layer, the coercivity is strongly reduced compared to the hard layer value, indicating that high anisotropy perpendicular systems with moderate coercivity can be easily obtained.

Magnetic Properties of Percolated Perpendicular FePt–MgO Films

Percolated perpendicular FePt-MgO films with a (Fe48 Pt 52)100-x-(MgO)x/Pt(001)/Cr(002) structure were prepared by conventional dc magnetron sputtering (x=0-6.13). Magnetic measurements demonstrate that the coercivity of the magnetic film drastically increases from 169 to 285 kA/m as the MgO content is increased from 0 to 0.15 vol.%. However, the grain sizes of the FePt phase do not significantly varying upon doping with MgO. MgO does not appear at the grain boundaries of the FePt phase, but is present as crystalline dots that are uniformly precipitated in the FePt matrix. The MFM images revealed that the domain structure transformed from extending to isolate when the MgO dots precipitated into the FePt grains. Consequently, the MgO dots serve as pinning sites of the domain wall and enhance perpendicular coercivity. Percolated perpendicular magnetic recording is thus regarded as a solution to the problem of thermal instability in ultrasmall grains

Approach to enhance the coercivity in perpendicular FePt∕Ag nanoparticle film

[ FePt 2 nm ∕ Ag d nm ] 10 multilayers were prepared on 100 °C MgO (100) single-crystal substrates with magnetron sputtering. The ordering and perpendicular texture of L10-FePt∕Ag particle films with high coercive force were realized at a lower postannealing temperature and by controlling accurately the atomic ratio of Fe to Pt of L10-FePt films. The results of the x-ray-diffraction pattern show that the FePt particles have excellent (001) texture when the multilayers were sputtered on 100 °C substrates with the thickness of the Ag layer d=3–9nm, and the mean size of L10-FePt particles reduces with increasing thickness of the Ag layer. After the [FePt2nm∕Ag5nm]10 was vacuum annealed at 600 °C for 15 min, the mean size of L10-FePt particles is roughly about 8 nm and its perpendicular coercive force reaches 979kA∕m(12.3kOe). This particle film has little magnetic exchange coupling and is a candidate for future perpendicular magnetic recording media with ultrahigh density.

Promotion of L1 ordered phase transformation by the Ag top layer on FePt thin films

The influence of the deposition of an ultrathin Ag top layer on the structure and magnetic properties of FePt thin films has been investigated. Crystallographic ordering of the L10 face-centered-tetragonal (fct) FePt phase was significantly promoted when a Ag layer was deposited on top of the film. The increased fraction of hard magnetic L10 fct FePt phase led to a dramatic increase of coercivity from 1 to about 6.0 kOe. The δM curves suggested exchange decoupling of magnetic grains. A perpendicular FePt film with a coercivity of 5.0 kOe and remanent squareness of 0.98 was achieved with a CrRu underlayer and the Ag top layer.