FePt Phase Research Articles

Comparative Studies on the Microstructure and Magnetic Properties of FePt/ (Ag, MgO) Nanocomposite thin Films on Glass Substrate

A comparative study on the microstructure and magnetic properties of FePt-MgO and FePt-Ag thin film sputtered onto glass substrate has been carried out systematically in this paper. FePt-MgO thin film shows better (001) orientation of L10-ordered FePt phase, narrower c-axis orientation distribution, higher out-of-plane coercivity, higher remanence ratio and larger hysteresis slope α than the FePt-Ag thin film. The results of angular variation of coercivity indicate that the magnetization reversal behavior of FePt-MgO thin film is closer to the Stoner–Wohlfarth rotation model than that of FePt-Ag thin film. The magnetic domain size in FePt-MgO thin film (~80nm) is smaller than that in FePt-Ag thin film (~160nm) due to the isolated grain structure. From this point of view, the FePt-MgO thin film is more suitable than the FePt-Ag thin film for the application of high density recording media. Keywords: FePt thin film, magnetic recording media, magnetic properties, magnetization reversal, magnetic coupling, Stoner–Wohlfarth rotation, Glass Substrate

FePt as Read Sensor Hard Bias Material

A low temperature chemical ordering approach was presented for L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase FePt alloy's application in read sensor hard bias. We show that platinum seed and cap layers together can help the iron-rich FePt layer to achieve A1 to L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase transformation at temperatures as low as 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. For platinum-rich FePt alloy iron seed and cap layers are effective in promoting its phase transformation. Element cap and seed together are more effective to obtain high coercivity than either of them alone. In order to implement FePt permanent magnet in transducer product, new thermal processes and sequences are needed.

Magnetization reversal in a preferred oriented (111) L10 FePt grown on a soft magnetic metallic glass for tilted magnetic recording

L10 FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L10 FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L10 FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L10 FePt/glassy Fe71Nb4Hf3Y2B20/SiO2/Si) in which the magnetic easy axis of L10 FePt is tilted by ∼36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L10 FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L10 FePt with the soft magnetic Fe71Nb4Hf3Y2B20 glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ∼10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L10 FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.

Aligned and exchange-coupled FePt-based films

Two-phase nanostructures of hard L10-ordered FePt and soft iron-rich fcc Fe-Pt are investigated experimentally and by model calculations. The Fe-Pt thin films were produced by epitaxial co-sputtering onto MgO and have a thickness of about 10 nm. They form two-phase dots that cover a large fraction of the surface but are separated from each other. X-ray diffraction and TEM show that the c-axis of the phase FePt is aligned in the direction normal to the film plane. The experimental and theoretical hysteresis loops indicate archetypical exchange coupling, and excellent magnetic properties are obtained. The largest values of coercivity, saturation magnetization, and nominal energy product obtained in the samples studied are 51 kOe, 1287 emu/cc, and 54 MGOe, respectively.

Rapid Thermal Annealing Induced Metastable Phase in FePt Thin Films

Room-temperature-deposited FePt thin films with thickness ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> ) ranged from 5 to 100 nm treated by rapid-thermal annealing (RTA) were studied. With annealing condition of 900 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 60 seconds at heating rate of 80 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C/sec., a metastable phase of FePt was observed in the films with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> ≥ 40 nm. The phase is chemically ordered with a face-centered-cubic (fcc) structure. The lattice parameter of it is found identical to the planar spacing of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> 1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (100). The metastable structure is dominant in the film with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> = 40 nm and gradually replaced by <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> 1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase with increasing <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> . The fcc phase is soft magnetic with saturation magnetization similar to that of disordered FePt. Drastic changes were also observed in surface morphology. The results infer the connection between the metastable transformation and internal strain. The formation of the fcc structure is sensitive to processing parameters except <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> . Slower heating rate, lower annealing temperature, and longer annealing time, tend to suppress the formation of it. The interesting findings provide additional knowledge for FePt thin films.

Direct formation of L10 FePt in as-cast FePt-based magnetic nanocomposite ribbons without post-synthesis annealing

A novel nanocomposite FePt-based exchange-coupled magnet has been synthesized and structurally and magnetically characterized. We report for the first time the direct formation of the L10 FePt phase without the need for post-synthesis annealing procedures in Fe-Pt-based melt-spun ribbons, obtained by a conventional melt spinning method. The structure and magnetic properties are investigated and the occurrence of the L10 ordered phase in the as-cast state of Fe–Pt–Ag–B melt-spun ribbons is confirmed by XRD and magnetic measurements. A microstructure consisting of fine, uniformly dispersed, 22–24 nm FePt grains dispersed within a soft magnetic matrix is observed by scanning transmission electron microscopy imaging. Coercive fields as high as 727 kA m−1, saturation magnetization of about 1.2 T and energy product around 87 kJ m−3 are determined from 270 K hysteresis loops of the as-cast ribbons, making one of the best FePt-based nanocomposite magnet ribbons even without further annealing treatments.

Interfacial structure of ferromagnetic Fe-Pt thin films grown on a Si substrate

In order to understand the Fe-Pt phase transformation, Fe-Pt films were sputtered on HF-treated, untreated Si substrates, and SiO 2 substrates and the magnetic and structural properties were analyzed by XRD, VSM, and XPS. The formation of Pt 2Si on the interface between Fe-Pt films and a Si substrate could be clearly observed with a shift of Pt4f 7/2 binding energy. Native oxide on the surface of Si substrate slowed the transformation of the disordered Fe-Pt phase to the ordered phase. The Pt 2Si phase was easily formed during sputtering on an HF treated Si substrate; moreover, a formation of a very small amount of Pt 2Si phase could be observed on the interface of Fe-Pt films and a SiO 2 substrate. Thus, the formation of Pt silicide assumes an important role in the transformation of Fe-Pt films grown on Si substrates.

Magnetic properties and microstructure of Ag2Se/FePt particulate films

Multilayer Ag/[Ag2Se(t)/FePt(1nm)]10 (thickness t = 0.1–0.4 nm) were alternately deposited on a glass substrate and subsequently annealed by rapid thermal process (RTP) at 800 °C for 3 min. After RTP, the interface between FePt and Ag2Se was intermixed to form particulate films. The grains size of the L10 FePt decreased from 9.8 to 7.7 nm when the total thickness of Ag2Se intermediate layer increases to 1 nm. The Ag/(Ag2Se/FePt)10 particulate film showed perpendicular magnetization and a slight increase in out-of-plane coercivity over that of the original thickness of Ag2Se. The Ag2Se atoms were immiscible with FePt phase but the FePt grains were refined and separated well from the Ag2Se phase. Compared with Ag/FePt bilayer, the grains of the FePt were refined and uniformly separated in the Ag/[Ag2Se(t)/FePt]10 multilayer with t = 0.1 nm.

L1 FePt(111)/glassy CoFeTaB bilayered structure for patterned media

We report on the use of CoFeTaB metallic glass thin film as a soft magnetic underlayer which promotes the growth of L10 FePt along the preferred (111) crystallographic direction. The preferred oriented phase of L10 FePt is obtained by either in situ deposition upon a heated CoFeTaB/SiO2/Si or at room temperature (FePt/CoFeTaB/SiO2/Si) followed by annealing at 400–450 °C for 15 min. The latter process is shown to be advantageous in terms of the fabrication of patterned media. Pattern sizes ranging from 25–100 nm made from L10 FePt (111)/glassy CoFeTaB were fabricated by photo/electron beam lithography. The structural and magnetic characterizations strongly suggest the applicability of the present bilayered structure in the fabrication of high density bit-patterned magnetic recording media.

Evolution of structural and magnetic properties of FePt/C granular films with thermal annealing

We report the structural and magnetic properties of as-deposited and thermally annealed FePt/C granular multilayer films. The as-deposited system exhibits a disordered fcc FePt phase with an average grain size of 3 nm. Thermal annealing at 650 °C results in partial L 1 0 ordering and an associated grain growth to 7 nm. Mössbauer measurements show that there is no non-magnetic component present, suggesting that carbon resides only in the grain boundary region. The ferromagnetic grains are magnetically decoupled.

Enhanced Structural and Magnetic Ordering of FePt/Mn-Oxide Bilayers by Ion-Beam Bombardment and Annealing

Structural and magnetic properties of FePt thin films were affected strongly by capped MnOx layers prepared by ion-beam bom-bardment and post-annealing. As-deposited FePt/MnOx bilayer exhibited a magnetically soft fee phase, and it turned to an ordered fct FePt phase with large coercivity (~8 kOe) after annealing at 550°C. Increasing the %02/Ar in capped MnOx layer during de position resulted in smaller ordered FePt grains separated by grain boundaries of MnOx. We found that the superlattice (001) peak is broadened considerably with larger amount of MnOx incorporated into FePt, likely due to the hindered formation of hard phase. Our results indicate that FePt/MnOx films deposited with lower %02/Ar, the oxygen atoms may occupy the interstitial positions in the FePt lattice to induce a local strain thus enhancing the FePt ordering. Further increased %02/Ar in capped MnOx layer, the excess oxygen atoms act a diffusion barrier effectively to inhibit the FePt grain growth and ordering.

Magnetic and structural properties of Fe x Pt100−x on MgO(110) substrates

FexPt100−x (70.1 ≤ x ≤ 83.4) thin films with ordered Fe3Pt phase were grown successfully onto MgO(110) substrates by electron beam evaporation. The unit cell of ordered Fe3Pt phase is elongated along c-axis direction and the thin films become more chemically ordered with decreasing Fe content. The magnetization of thin films shows a decrease when Fe content is around 79 at.%. The relationship between magnetic anisotropy and structural parameters suggests that the change of magnetic anisotropy in ordered Fe3Pt thin films with different compositions most likely stems from the magnetocrystalline origin.

Block Copolymer Directed One-Pot Simple Synthesis of L10-Phase FePt Nanoparticles inside Ordered Mesoporous Aluminosilicate/Carbon Composites

A "one-pot" synthetic method was developed to produce L1(0)-phase FePt nanoparticles in ordered mesostructured aluminosilicate/carbon composites using polyisoprene-block-poly(ethylene oxide) (PI-b-PEO) as a structure-directing agent. PI-b-PEO block copolymers with aluminosilicate sols are self-assembled with a hydrophobic iron precursor (dimethylaminomethyl-ferrocene) and a hydrophobic platinum precursor (dimethyl(1,5-cyclooctadiene)platinum(II)) to obtain mesostructured composites. The as-synthesized material was heat-treated to 800 °C under an Ar/H(2) mixture (5% v/v), resulting in the formation of fct FePt nanocrystals encapsulated in ordered mesopores. By changing the quantities of the Fe and Pt precursors in the composite materials, the average particle size of the resulting fct FePt, estimated using the Debye-Scherer equation with X-ray diffraction patterns, can be easily controlled to be 2.6-10.4 nm. Using this simple synthetic method, we can extend the size of directly synthesized fct FePt up to ∼10 nm, which cannot be achieved directly in the colloidal synthetic method. All fct FePt nanoparticles show hysteresis behavior at room temperature, which indicates that ferromagnetic particles are obtained inside mesostructued channels. Well-isolated, ∼10 nm fct FePt have a coercivity of 1100 Oe at 300 K. This coercivity value is higher than values of fct FePt nanoparticles synthesized through the tedious hard template method by employing SBA-15 as a host material. The coercivity value for FePt-1 (2.6 nm) at 5 K is as high as 11 900 Oe, which is one of the largest values reported for FePt nanoparticles, or any other magnetic nanoparticles. The fct FePt nanoparticles also showed exchange-bias behavior.

Microwave-Assisted Chemical Reduction Routes for Direct Synthesis of (fct) L10 Phase of Fe-Pt

Microwave-assisted chemical reduction route has been explored for the direct synthesis of fct L10-phase of Fe-Pt nanoparticles in the present work. Effects of microwave power and irradiation time on the growth process are systematically studied. Using this facile and high yield technique we could tune particle size from 7 to 17 nm. Prepared Fe-Pt NPs exhibited ordered face centered tetragonal (fct) L10 phase without any post-synthesis treatment. The particle size and magnetic properties of the prepared Fe-Pt were found to be very sensitive to the microwave irradiation power, while influence of exposure time was insignificant. The hysteresis measurements were performed at 300 K to study magnetic properties of the synthesized Fe-Pt as a function of crystallite size. Coercivity and saturation magnetization were observed to be decreasing with diminishing particle size. The microwave-assisted route is found to be a simple technique for direct synthesis of metal alloys and may prove to be a potential tool of high density data storage materials such as Fe-Pt.

Microstructures and Magnetism of Different Oxides Separating FePt Grains via Ion-Beam Bombardment and Annealing

The effects of the fabrication methods and different capped oxide (SiO2 and TiO2) layers on the microstructure and magnetism of FePt thin films were studied. Both structural ordering (S ∼0.7) from the fcc FePt phase to the fct FePt phase and magnetic hardening were observed in the annealed FePt/SiO2 thin films with a low substrate rotation speed (Sr = 1 rpm). However, only the annealed FePt/SiO2 thin films prepared with a high Sr (10 rpm) exhibited isolated FePt grains separated by the grain boundary SiO2, as revealed by transmission electron microscopy and magnetometry. Furthermore, similar results in microstructures and magnetic properties were obtained after replacing the capped layer with TiO2. However, an enhanced order parameter (S ∼0.85) and a smaller FePt grain size (∼6.8 nm), which are promising characteristics for ultrahigh-density magnetic recording, were achieved in the annealed FePt/TiO2 thin films; however, the annealed FePt/SiO2 thin films exhibited a larger grain size (∼15 nm). This indicates that TiO2 inhibits the grain growth of FePt more effectively than SiO2.

The influence of microstructure on magnetic properties of nanocrystalline Fe–Pt–Nb–B permanent magnet ribbons

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mössbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range.

Arrangement of FePt Nanocubes Utilizing Chemical Binding Selectivity

This study on future ultrahigh density magnetic recording devices aims to develop a technique for arranging FePt nanocubes on a substrate by using organosilane as an interlayer. An array of FePt nanocubes was formed by chemical synthesis, utilizing the Pt–S binding between the SH functional group in (3-mercaptopropyl)trimethoxysilane and Pt in the FePt nanocubes. The morphology of the FePt nanocube array on the substrate was characterized using atomic force microscopy and plan-view scanning electron microscopy, which revealed that the array was a monolayer. After was coated by chemical vapor deposition as a protective layer for sintering prevention, annealing was carried out at for 3 h in a reducing atmosphere to transform the crystal structure of the FePt nanocubes to the phase. X-ray diffraction results revealed that the annealed FePt nanocube array had a FePt(110) orientation and phases such as (001) and (110). The Pt–S binding did not obstruct the ordering of FePt. Consequently, a method for arranging nanoparticles by using the organic molecules with functional groups that can interact with nanoparticle selectivity as an interlayer can be applied as a formation technique for arranging a wide range of nanoparticles.

The Effect of Cobalt Addition on Structural and Magnetic Properties of Electrodeposited Iron-Platinum Nanocrystalline Thin Films

Iron-Platinum (Fe-Pt) and Cobalt-Iron-Platinum (Co-Fe-Pt) nanocrystalline thin films were deposited on brass substrates using a single bath electrodeposition system. The effects of addition of various concentrations of cobalt towards the composition, crystallographic structure, microstructure and magnetic properties of the Fe-Pt alloy films were investigated. Ammonium tartrate and ammonia solution were used as complex forming additives to stabilize the Fe2+ and Co2+ ions and to enhance co-deposition with platinum complexes. The as-synthesized thin films were characterized by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and alternating gradient magnetometry (AGM). EDS results showed that cobalt was preferentially deposited compared to iron during Co-Fe-Pt film electrodeposition. SEM results showed that the microstructure of the films consisted of spherical granules. XRD showed formation of disordered face-centered cubic (fcc) Fe-Pt phase and all peaks were shifted to slightly higher diffraction angles with increasing cobalt content. The average crystallite size calculated from XRD peak broadening varied from 2.33nm to 6.54nm. The saturation magnetization and coercivity increased with increasing cobalt content.

Enhanced L10 Ordering and (001) Orientation in FePt: Ag Nanocomposite Films by Monatomic Layer Deposition

The magnetic and structural properties of atomic-scale [Fe/Pt]18 and [Fe/Pt/Ag]18 multilayer films annealed at 350 and 400°C have been investigated. The results indicate that, by introducing thin Ag addition layers, the ordering temperature of FePt with nominal 7.4-nm thickness (excluding Ag thickness) can be significantly reduced to 350°C, at which a relatively large coercivity is achieved. When annealing temperature increases to 400°C, the atomic-scale [Fe/Pt]18 and [Fe/Pt/Ag]18 multilayer films show (001) orientation. The reduced diffusion length of atoms in atomic-scale multilayer films and an enhanced atomic diffusion coefficient produced by Ag doping led to the low-temperature ordering and (001) orientation of L10 FePt phase.

$L1_{0}$ FePt/Fe Exchange Coupled Composite Structure on MgO Substrates

A core-shell type exchange-coupled composite (ECC) structure is developed with an L10 phase FePt core and an Fe shell. Coercivity decreases from 30.6 kOe for the FePt granular film with a very thin thickness of 3.2 nm, to 7.85 kOe for the FePt/Fe core-shell type granular film with a 3-nm-thick Fe. The energy barriers of the FePt/Fe films are measured by using Sharrock equation. A modified formula of gain factor considering the effect of films' thickness is proposed. Core-shell type FePt/Fe ECC granular film has a large advantage over a conventional granular film for media application.