L10 FePt Films Research Articles

Synthesis of L1-FePt perpendicular films with controllable coercivity and intergranular exchange coupling by interfacial microstructure control

A series of FePtBi/Au multilayers were fabricated by magnetron sputtering. The interfacial microstructure control of Bi and Au atoms and its effect on comprehensive properties of L10-FePt perpendicular films were carefully studied. Results show that: perpendicular magnetic anisotropy of the L10-FePt film can be remarkably enhanced with the epitaxial inducement of Au atoms. On the other hand, intergranular exchange coupling (IEC) of the film is greatly decreased due to the isolation of FePt particles by nonmagnetic Au particles. Moreover, the controllable coercivity of the film can be realized by adjusting ordering degree of the film through diffusion of Bi atoms. Thus, an L10-FePt perpendicular film with controllable coercivity and no IEC is realized with the interfacial microstructure control of surfactant Bi and Au atoms.

Magnetic properties of perpendicularly orientated L10 FePt nanoparticles

L10 FePt films were deposited on MgO (001) substrates heated to 700°C by magnetron sputtering. Assisted by the misfit of lattice between film and substrate, strong (001) texture was formed. The film at nominal thickness tN = 5 nm was composed of nanoparticles with a size of ∼70 nm, and showed a high coercivity of ∼105 kOe at 4.2 K. At tN =∼50 nm, as the film changed from discontinuous to continuous, the coercivity dropped about one order of magnitude. Micromagnetic simulation implies that the magnetization reversal is a vortex-like nuclear type. The ideal coercivity of a separated single-domain L10 FePt nanoparticle with a size of 70 nm× 70 nm× 5 nm is ∼121 kOe. This tells us that the experimental coercivity has nearly reached the limit of ideal single crystalline nanoparticles.

Ultrafast heating effect on transient magnetic properties of L1 0-FePt thin films with perpendicular anisotropy

Laser-induced ultrafast magnetization dynamics and transient coercivity behavior in perpendicular magnetized L1 0-FePt films are investigated using the time-resolved polar magneto-optical Kerr technique. The magnetization after photo-excitation shows a dramatic reduction on the picosecond time scale. In contrast, the coercivity shows a weak decrease, accompanied by a skewed Kerr loop shape for low and intermediate fluences. The results can be interpreted by the laser-induced non-uniform demagnetization due to the weakened coupling between FePt grains of different size and/or components. The remaining coercivity vanishes when the film is fully demagnetized at higher fluence. We claim that the remaining coercivity can be manipulated by employing appropriate laser energy and film thickness, which may be helpful for application in heat-assisted magnetic recording.

Single laser pulse induced dynamic magnetization reversal mechanism of perpendicularly magnetized L1 FePt films

In order to measure photoinduced dynamic magnetization reversal by single laser pulse, alternating magnetic field is synchronized with the femtosecond laser pulse such that the magnetization state is reset before each single laser pulse. For perpendicularly magnetized L10 FePt films, the dynamic magnetization reversal process is accompanied by the nucleation of reversed domains and the barrierless transient domain wall motion at low magnetic fields and subsequent pinned domain wall motion at the switching field. The switching field does not change with the pump-probe delay time.

Fabrication of (200)-Textured NiO Films on Glass Substrate Using Radio Frequency Sputtering

The experimental result shows that the preferred orientations of NiO thin films are closely related to the working pressure of argon. All of NiO(111), NiO(200), and NiO(220) diffraction peaks are observed in the XRD patterns and exhibited random orientation of NiO film when the film is deposited in low Ar pressure of 5 mTorr. As the Ar pressure is increased to 20 mTorr, only NiO(200) peak appears and shows (200)-textured NiO films. However, the lattice parameter of NiO film deposited in high Ar pressure of 20 mTorr is 0.424 nm, which is much larger than that of NiO bulk (0.417 nm). The lattice parameter can be decreased by post-annealing the film due to the interstitial Ar atoms are released from the NiO lattice and it reduces continuously from 0.423 to 0.417 nm as the NiO films are annealed from 300 to 600 °C. Therefore, post-annealing the NiO film deposited in higher Ar pressure will reduce the lattice misfit between the NiO and L10 FePt film that is beneficial to improve the perpendicular magnetic anisotropy of FePt film.

Influence of an Fe cap layer on the structural and magnetic properties of Fe49Pt51/Fe bi-layers

The influences of an Fe cap layer on the structural and magnetic properties of FePt/Fe bi-layers are investigated. Compared with single FePt alloy films, a thin Fe layer can affect the crystalline orientation and improve the chemical ordering of L10 FePt films. Moreover, the coercivity increases when a thin Fe layer covers the FePt layer. Beyond a critical thickness, however, the Fe cover layer quickens the magnetization reversal of Fe49Pt51/Fe bi-layers by their exchange coupling.

Effect of Ni doping on the microstructure and magnetic properties of FePt films

FePtNi films with different Ni contents were deposited on glass substrates by RF magnetron sputtering, and the L10-FePtNi films were obtained after the as-deposited samples were subjected to vacuum annealing. The magnetic properties, structures, and orientations of the L10-FePt films with Ni doping were studied. The results show that with increasing Ni content, the (001) peak position of the FePtNi films shifts to a higher angle in comparison with FePt, which suggests that there is partial Ni substitution in the L10 lattice. The perpendicular coercivity decreases from 661 to 142 kA/m and magnetization decreases from 512 to 433 kA/m with increasing Ni content. In comparison with FePt films, the FePtNi films can effectively reduce the Curie temperature, which makes FePtNi films promising media candidates in thermally assisted recording (TAR).

Microstructure and magnetic properties of L1-FePt thin films prepared under high pressures

The ultrafine grain size with a narrow size distribution is a critical issue for the development of L10-FePt thin films as ultrahigh density magnetic recording media. In this study, we succeeded in reducing both the grain size and the ordered domain size of L10-FePt thin films and simultaneously improving the size uniformity by annealing disordered FePt thin films at a high pressure of 1 GPa. This is attributed to the fact that high pressure promotes the nucleation of the L10 ordered domains and inhibits the growth of both the grains and the ordered domains during the L10 ordering transition.

Electric-field control of surface magnetic anisotropy: a density functional approach

In a recent experiment, Weisheit et al (2007 Science 315 349) demonstrated that the coercivity of thin L10 FePt and FePd films can be modified by the external electric field in an electrochemical environment. Here, this observation is confirmed by density functional calculations for the intrinsic magnetic anisotropy. The origin of the effect is clarified by means of a general and simple method to simulate charged metal surfaces. It is predicted that the coercivity of thin CoPt films is much more susceptible to electric field than that of FePt films.

Interplay between structural and magnetic properties of L1-FePt(001) thin films directly grown on MgO(001)

We present a detailed study of the magnetic and structural properties of L10-FePt thin films. The films are prepared via molecular beam epitaxy directly onto MgO(001) substrates, i.e., without buffer layer. Despite the large lattice misfit between the in-plane lattice parameters of L10 FePt and MgO, highly ordered thin films are obtained with the easy magnetization c axis perpendicular to the film plane. Via high resolution transmission electron microscopy and Rutherford backscattering measurements we focus on the FePt/MgO interface to study the misfit relaxation and the defect density. Further, the influence of elevated substrate temperatures and of postgrowth high temperature annealing on the structural and magnetic properties is discussed.

Structure and magnetic properties of L10 FePt film with Ag heat sink layer

In this work, the L10 FePt films with a Ag heat sink layer for the application of heating assisted magnetic recording (HAMR) were developed. It was found that the crystalline orientation of the FePt layer depended on the texture of the fcc Ag layer, which was strongly affected by the deposition conditions. The (200) textured Ag layer can induce the L10 FePt (001) texture films due to epitaxial growth with the epitaxial growth relationship of FePt (001) [100]//Ag (001) [100]//CrRu (002) [110] and thus can act as the heat sink layer for HAMR application,. The magnetic properties of the L10 FePt films were maintained with the thickness of the Ag heat sink layer from 30 to 100 nm.

Coercivity of ledge-type L1-FePt/Fe nanocomposites with perpendicular magnetization

Exchange-coupled ledge-type L10-FePt/Fe composite systems with out-of-plane anisotropy composed of nanostructured L10-FePt films covered by Fe are prepared to analyze the influence of the soft magnetic layer thickness on the magnetic properties. By the soft magnetic layer thickness dFe the coercivity can be tailored according to a 1/dFe1.38 relation. This result can be used to realize recording media with coercivities in the range which are afforded by conventional write heads.

Preparation of L10-FePt film for perpendicular magnetic recording media by using FePt/Au mutilayers

FePt/Au mutilayers were deposited on 100℃ MgO（001） single crystal substrate by magnetron sputtering. The influence of the FePt/Au multilayer structure on the ordering temperature, coercivity （ＨC）, perpendicular magnetic anisotropy （PMA）, FePt grain size and interparticle exchange coupling （IEC） of films were studied. Results show that the film possesses high ＨC, excellent PMA, small grain size and weak interparticle exchange coupling after annealing. The results of HRTEM reveal that Au can relieve mismatch between MgO and FePt lattices, leading to excellent PMA. Moreover, the FePt/Au interface energy, the stress energy due to the mismatch between FePt and Au lattices and the diffusion of Au atoms promote the ordering of FePt film. This leads to the decrease of ordering temperature and considerable increase of ＨC. Au atoms partly diffuse to the boundaries of FePt phase and function as grain finer and particle isolator, which decrease the FePt grain size and IEC.

Microstructure and magnetic properties of Ag/FePt double layer films with different cooling processes

Ag/FePt double layer films with different Ag thicknesses were deposited on Corning glass substrates by RF magnetron sputtering and L10-FePt films were prepared after the as-deposited samples were subjected to vacuum annealing with different cooling processes. Experimental results show that the thickness of the Ag layer is a very important factor in preparing the film with a high perpendicular anisotropy. The 40 nm Ag/15 nm FePt film exhibits a high perpendicular coercivity of 9053 Oe and a low in-plane coercivity of 6059 Oe. The Ag/FePt double layer films with natural cooling have larger Mr/Ms values than those with fast cooling processes. The angular dependence of the coercivity profile for the FePt film with a thickness of 15 nm shows a typical domain wall motion behavior, while both the simulated curve of FePt film of 15 nm and the one of 40 nm Ag/15 nm FePt double layer film revealed the rotation mode, which indicates that the grains are weakly coupled and their reversals of magnetizations are independent in the 40 nm Ag/15 nm FePt film.

Nucleation-promoted L10 ordering in FePt thin films with ultrasmall grain size

FePt thin films with an ultrasmall grain size of ∼3.4 nm were successfully prepared by employing a relatively high sputtering pressure of 4 Pa that produces deposited target atoms or clusters with low energy. The L10 ordering in the FePt thin films is dominantly dependent on the nucleation of ordered domains rather than their growth which is usually observed in the films with relatively large grain size. The nucleation-promoted L10 ordering makes a reduction in grain growth and thus yields the L10-FePt thin films with a uniform and small grain size below 10 nm.

Reactive ion etching of FePt using inductively coupled plasma

We propose a reactive ion etching (RIE) process of an L1 0-FePt film which is expected as one of the promising materials for the perpendicular magnetic recording media. The etching was carried out using an inductively coupled plasma (ICP) RIE system and an etching gas combination of CH 4/O 2/NH 3 was employed. The L1 0-FePt films were deposited on (1 0 0)-oriented MgO substrates using a magnetron sputtering system. The etching masks of Ti were patterned on the FePt films lithographically. The etch rates of ∼16 and ∼0 nm/min were obtained for the FePt film and the Ti mask, respectively. The atomic force microscopy (AFM) analyses provided the average roughness ( R a) value of 0.95 nm for the etched FePt surface, that is, a very flat etched surface was obtained. Those results show that the highly selective RIE process of L1 0-FePt was successfully realized in the present study.

Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth

This paper presents a detailed study of structure, morphology, and magnetic properties in FePt thin films epitaxially grown on MgO(100) at intermediate temperatures (≅380°C). It focuses on the effects obtained by in situ annealing the FePt films after deposition. We have demonstrated that the annealing allows one to complete the ordering, thus obtaining an epitaxial L10-FePt film with large perpendicular anisotropy (up to 3×107erg∕cm3) and high perpendicular squareness and remanence ratio (both higher than 0.85). At the same time, we have found a peculiar morphology evolution by increasing the annealing time: the interconnected mazelike grains evolve towards interconnected dotlike grains of reduced size (down to 10nm). Due to the interconnection at the grains basis, the increase in the film perpendicular coercivity with the annealing process is moderate (up to 4.1kOe), leading to an increasing anisotropy/coercivity ratio with the annealing time. As a further step towards the understanding of the properties evolution with annealing, we have investigated the magnetic domain pattern and analyzed the domain-period dependence on the anisotropy constant in comparison with the behavior expected by the Kooy–Enz theory [C. Kooy and U. Enz, Philips Res. Rep. 15, 7 (1960)]. The main magnetic properties and reduced grain size obtained after the in situ annealing are promising to realize perpendicular recording media with reduced bit size, high thermal stability, and suitable switching field.

Evolution of granular to particulate structure of (001) FePt on amorphous substrates (invited)

Well-aligned (001)-oriented granular or particulate L10 FePt films on amorphous SiO2 or glass substrates were obtained by depositing atomic-scale Fe∕Pt∕SiO2 multilayers followed by various postannealing conditions. After 350°C annealing, (001) FePt–SiO2 granular films were obtained with an average grain size of 5.14nm, perpendicular coercivity of 7.7kOe, and squareness of 0.95. The diffusion of SiO2 to grain boundaries and free surface of FePt accelerated the diffusion of Fe and Pt atoms to form the L10 phase; therefore, low-temperature ordering of FePt was achieved. By further raising the annealing temperature to 700°C, isolated particles of aligned (001) single crystals were achieved. The nucleation of voids occurred at low temperatures due to the existence of low-surface-energy SiO2, and then an agglomeration process proceeded, leading to a particulate structure. The isolated particles showed a perpendicular coercivity of 60kOe and squareness of unity.

Improvement of magnetic property of L1-FePt film by FePt∕Au multilayer structure

We have prepared L10-FePt films by sputtering FePt∕Au mutilayers on glass substrates followed by a vacuum annealing process. The magnetic measurement and x-ray diffraction indicate that the ordering temperature of the FePt films is reduced to 350°C and the coercivity (HC) is greatly enhanced by using the FePt∕Au multilayer structure. The interface energy and the stress energy due to the misfit between FePt and Au crystal lattices provide extra driving force to the ordering process of FePt films. Additionally, partial Au atoms are supposed to diffuse to the FePt grain boundaries during annealing, which consequently pins the magnetic domain wall and leads to the high HC.

High coercivity L10 FePt films with perpendicular anisotropy deposited on glass substrate at reduced temperature

The microstructures and magnetic properties of FePt films grown at 350°C on glass substrates with MgO (200) intermediate layer and CrRu (200) underlayer were investigated. The film with 1nm MgO intermediate layer showed higher degree of chemical ordering than that with 4nm MgO layer due to the compression of the lattice constant of 1nm MgO intermediate layer along the [100] direction. Isolated FePt particles were formed when nominal thickness of FePt was 4nm. The room temperature coercivity of isolated FePt particles with 1nm MgO intermediate layer was as high as 12kOe, significantly larger than that with 4nm MgO intermediate layer (6.3kOe).