FePt Phase Research Articles

Studies of Exchange Coupling in FeCo/L10-FePt Bilayer Thin Films

FeCo/FePt bilayer thin films were synthesized by RF/dc sputtering on Si <100> substrate, and characterized for structural and magnetic properties. Synchrotron X-Ray diffraction revealed L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase of FePt and amorphous FeCo after annealing. An attempt was made to increase the strength of interfacial exchange coupling in FeCo/FePt bilayer thin films by varying the thickness of the soft layer. The in-depth analysis of reversal process at the interface was carried out using recoil curves. A giant energy product 47 MGOe was obtained for the FeCo(6 nm)/FePt(27 nm) bilayer thin film. The system was modeled by using micromagnetic simulations to visualize the magnetization reversal at the interface. Quantitative estimation of exchange energy was made and its variation with the reverse field was studied in order to understand the coupling between the hard and soft layers.

Magnetic Anisotropy in Thin Films of FePt Detected by the Ferromagnetic Resonance Method

FePt thin films are made via the heat treatment of structures containing Pt (001) and Fe (001) epitaxial layers. X-ray spectral analysis and field-emission scanning electron microscopy show that the samples contain the cubic L12–Fe3Pt phase and the tetragonal L10–FePt phase. The magnetic anisotropy of the thin films is studied by ferromagnetic resonance and magnetometric methods. The contributions from the cubic and tetragonal phases to the ferromagnetic-resonance spectrum of the sample are distinguished. The key characteristics of thin films which determine their practical applicability are determined: the Gilbert damping parameter G ~ 0.4, the first and second crystal anisotropy constants K1⊥ = –4.08 × 106 erg/cm3, K2⊥ = 1.34 × 106 erg/cm3, K2|| = –9.76 × 104 erg/cm3.

A Controllability Investigation of Magnetic Properties for FePt Alloy Nanocomposite Thin Films.

An appropriate writing field is very important for magnetic storage application of L10 FePt nanocomposite thin films. However, the applications of pure L10 FePt are limited due to its large coercivity. In this paper, the ratios of L10 and non-L10 phase FePt alloy nanoparticles in FePt/MgO (100) nanocomposite thin films were successfully tuned by pulsed laser deposition method. By adjusting the pulsed laser energy density from 3 to 7 J/cm2, the ordering parameter initially increased, and then decreased. The highest ordering parameter of 0.9 was obtained at the pulsed laser energy density of 5 J/cm2. At this maximum value, the sample had the least amount of the soft magnetic phase of almost 0%, as analyzed by a magnetic susceptibility study. The saturation magnetization decreased with the increase in the content of soft magnetic phase. Therefore, the magnetic properties of FePt nanocomposite thin films can be controlled, which would be beneficial for the magnetic applications of these thin films.

Enhancement of L1 transformation in Fe/Pt multilayer by Cu addition

Enhancement in L10 transformation kinetics in FePt is achieved by incorporating an optimum concentration of ternary element Cu, which has limited solubility in the fcc FePt phase, into the FePt multilayer stack. Two different multilayer structures were deposited. In first multilayer Cu is deposited at one interface of Fe/Pt and in other Cu is alloyed with Fe and Pt layers by co-sputtering. One Fe42.5Pt42.5Cu15 alloy film is also prepared and detailed study of evolution of structural and magnetic properties as a function of isochronal annealing is done using XRD and Magneto Optic Kerr Effect (MOKE) measurements respectively. Annealing up to 200oC results only in intermixing in the multilayer structure, with no sign of L10 transformation. Annealing at 300oC for 1h results in partial transformation to L10 phase as evidenced by appearance of (001) superlattice peak as well as large increase in the coercivity. It is found that in the Fe(Cu)/Pt(Cu) multilayer exhibits significantly faster L10 transformation as compared to Fe/Pt/Cu multilayer or FePtCu alloy film. Inter-diffusion study using x-ray reflectivity measurements reveals that constant for interdiffusion in Fe(Cu)/Pt(Cu) is only marginally higher than that in Fe/Pt/Cu multilayer. The observed enhancement in L10 transformation rate in Fe(Cu)/Pt(Cu) multilayer is discussed in terms of possible enhancement of diffusivities of constituent species in fcc FePt phase.

A novel direct reduction method to synthesize ordered Fe-Pt alloy nanoparticles

In this work, a direct green solid-phase reduction method for the fabrication of large yield of ordered phase Fe-Pt alloy nanoparticles was reported, in which inorganic salts were used as metal precursors and H2-containing atmosphere was used as reducer. Utilizing this method, the composition and chemical ordered phase, such as L12-Fe3Pt, L12-FePt3, and L10-FePt phases can be easily achieved by one step reaction. The synthesized nanoparticles have clean surface because no organic precursors, no organic solutions or organic surfactants/ligands were used. Their magnetic performance and the formation mechanism of Fe-Pt alloy nanoparticles were also investigated. This strategy can be applied to synthesize many other types of alloy nanoparticles with desired composition and necessary crystal structure, which can be used for a variety of practical applications, such as in magnetism and catalyst research fields.

Matrix effect on order-disorder phase transition of FePt nanoparticles

A theoretical model is developed to systematically study the effect of the matrix on the order-disorder phase transition of FePt nanoparticles, which shows a significant role of the matrix or substrate to the ordering kinetics of FePt nanoparticles during the annealing process. It is shown that there exists a temperature window where one can obtain ordered FePt phase, while the high temperature limit with the largest nucleation rate is favorable for the formation of ordered FePt phase (L10). The matrix or substrate not only reduces particle agglomeration and sintering, but also increases the high temperature limit as well as nucleation rate. Furthermore, the effect of the matrix to the ordering kinetics of FePt nanoparticles relies on its surface energy, the higher the surface energy of the matrix is, the more pronounced the effect is. The validity of the present model is supported by as well the explanations of the contradiction results as the good agreement between the model predictions and experimental data. Those findings are very interesting and crucial for practical application when synthesizing ordered FePt nanoparticles.

Investigations of the Magnetic Perpendicular Exchange Bias in L10 FePt/NiO Bilayer Thin Films

ABSTRACTWe report on the exploration of perpendicular exchange bias in iron platinum/nickel oxide (FePt/NiO) bilayer thin films grown using pulsed laser deposition (PLD) on MgO (100) substrates. Exchange bias is an important property for giant magnetoresistance, and, as such has promise for applications in spin valves, magnetic sensors and magnetic random access memory. The magnetic L10 phase of FePt is known for having high perpendicular magnetic anisotropy, tunable coercivity/grain size and large magnetic storage density. The FePt layer was first deposited directly on MgO, followed by the deposition of the NiO layer on top of the FePt layer. The coercivity of the L10 FePt layer was tuned during growth to form a hard or soft magnetic layer. The FePt/NiO thin films grown for this study exhibit perpendicular exchange bias at 5K, as quantified using our SQUID measurements. XRD confirms parallel plane ordering between the MgO (200), FePt (002) and NiO (111) atomic planes while cross sectional TEM confirms the epitaxial growth of L10-FePt(001)&lt;100&gt;//MgO(100)&lt;001&gt; and the preferential growth of NiO on top of the FePt. Films of only FePt were grown to examine the surface architecture of the ferromagnetic layer and thus the interface of the FePt/NiO bilayer. The results from our XRD, TEM and magnetometry characterization of the FePt films and FePt/NiO bilayer thin films will be discussed.

Study of exchange bias effect in a patterned Fe/Pt multilayer with the thermal annealing

The present work reports the preparation of 5 μm wide and 10 μm grating periodicity patterned [Fe(2.0nm)/Pt(2.5nm)]X10 multilayer and its thermal annealing behavior. Intermixing across the Fe and Pt layers is observed with vacuum annealing at 650 K resulting in the formation of disordered face centered cubic (FCC) and ordered L10 face centered tetragonal (FCT) FePt phases. The longitudinal magneto-optical Kerr effect (L-MOKE) hysteresis loop, measured from the single 5 μm FePt stripe shows double loop behavior indicating the presence of two magnetic phases, one with the low coercivity (HC) and the second one with the higher HC values. This could be due to the presence of disordered FCC and ordered L10 FCT phases of FePt, known to be soft and hard magnetic phases. Conversion electron Mössbauer spectroscopy of the annealed sample shows the presence of FCC and FCT FePt phases corroborating the x-ray diffraction and MOKE measurements. The hysteresis loop of the soft phase shows the exchange bias effect with the remnant state of the hard magnetic phase as initial condition akin to the exchange bias as observed in conventional ferromagnetic - antiferromagnetic bilayers.

The mechanism of texture evolution in annealed L10–FePt thin films

The L10–FePt thin films have been prepared by magnetron sputtering method and annealed at 400, 550, 600, and 700 °C for 30 min. The texture and microstructure of annealed L10–FePt thin films have been investigated in this work. The texture was quantitatively investigated by X-ray diffraction (XRD). The microstructure was characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results indicated that the L10–FePt phase was obtained at 550 °C via the disorder–order transformation, which has a great influence on the development of (001) fiber texture in FePt thin films. It is worth noting that the intensity of (001) fiber texture enhanced with the increase of temperature, but it weakened when the temperature raised to 700 °C. Correspondingly, there were a large number of twins in the 700 °C-annealed L10–FePt thin films. Twinning will relax the in-plane strain and deflect the grain growth direction, which is one of the main reason for the decrease of (001) fiber texture at 700 °C.

Self-Stabilized Carbon-$L{{1}}_{0}$ FePt Nanoparticles for Heated Dot Recording Media

Heated dot magnetic recording (HDMR) technology promises to achieve ultrahigh storage density well above 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> by making use of L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt bimetallic islands. HDMR media require the development of thermally stable, noninteracting magnetic patterns to circumvent the signal noise and writability challenges. We prepared self-stabilized carbon-L10 FePt nanoparticles (NPs) with average diameter of about 7.2 nm for the HDMR media. FePt NPs synthesized by chemical reduction using oleic acid and oleylamine transform to the carbon-L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt phase with optimized heat treatment at 873 K. The high-temperature annealing not only helps to achieve the desired L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase but also helps in the formation of a carbon coating on the FePt NPs due to degradation of the organic cap. We investigated the effect of carbon coating on the electronic states of Fe using X-ray absorption measurements and corroborated with high-resolution transmission electron microscopy and Fourier-transform infrared spectroscopy. X-ray magnetic circular dichroism reveals the presence of magnetocrystalline anisotropy in the carbon-L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt NPs, which is also supported by the structural and magnetic measurements. A magnetization-field loop at 300 K shows high coercivity of ≅ 1.6 T. The synthesized carbon-L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> FePt NPs can be used as FePt islands and should be suitable for high-density HDMR media.

Magnetic hardening of Fe-Pt and Fe-Pt- M (M=B, Si) microwires

We have prepared Fe-Pt and Fe-Pt- M (M = B, Si) microwires using Taylor-Ulitovsky technique and studied their magnetic properties. Magnetic properties considerably depend on the metallic core composition and annealing conditions. As-prepared microwires present either amorphous or mixture of amorphous and nanocrystalline phases with a presence of BCC FePt, FCC PtFe and small amount of tetragonal FePt phase. After annealing at 500 °C Fe50Pt40Si10 microwires we observed a remarkable magnetic hardening (coercivity increasing from 5 Oe up to 500 Oe) related to crystallization of as-prepared amorphous Fe50Pt40Si10 microwires. Annealed Fe50Pt50 sample present coercivity up to 800 Oe at 5K, but the magnetization of Fe50Pt50 sample is low and rapidly decreases with temperature. We discussed peculiarity of Fe50Pt50 microwires considering the influence of internal stresses on magnetic ordering of Fe-atoms.

Manipulating the magnetic properties and interphase coupling in FePt/Pt/Fe multilayer films by Pt spacer layer

Single-layer FePt (10 nm) film and trilayer FePt (10 nm)/Pt (t)/Fe (3 nm)/Pt (3 nm) (t = 0 nm, 0.5 nm, 1 nm, 2 nm and 4 nm) films were prepared on MgO (1 0 0) substrate. Their cross-sectional morphologies, ordering and interphase coupling behaviors were studied in detail. The results showed that the interphase coupling stiffness (J) increased at first, and then decreased with increasing thickness of the spacer layer. An exponential fitting of J ~ t dependence was obtained for t ⩾ 1 nm. A discrepancy between experiments and exponential fitting, mainly caused by interfacial diffusion, was seen for t ⩽ 1 nm. With respect to the proposed idea of using a spacer layer to modify the interface, FePt (8 nm)/Pt (0.5 nm)/Fe (3 nm)/Pt (3 nm) anisotropic nanocomposite film exhibited the best magnetic properties, with coercivity 0.87 T and (BH)max 370 kJ m−3, accounting for 95% of the theoretical (BH)max of L10-ordered FePt phase.

Columnar structural FePt films with good perpendicular anisotropy induced by tuning the crystal structure of doping materials

Crystalline ZrO2 doping would degrade the perpendicular texture of FePt films due to the epitaxial growth of FePt (200) textured phase on the tetragonal (002) textured ZrO2. In the present paper, crystalline HfO2 was used to replace crystalline ZrO2 as the crystalline doping material and FePt media with columnar structure and good perpendicular anisotropy were achieved. It is found that HfO2 crystallized on TiON intermediate layer presented the coexistence of both monoclinic and tetragonal phase. The extremely large lattice mismatch strain (around 25–28%) between FePt and HfO2 would prohibit the (200) or (001) epitaxial growth of FePt phase on HfO2. This would cause FePt-HfO2 film to have better (001) texture and perpendicular anisotropy compared to FePt-ZrO2 films. By tuning the crystal structure of doping crystalline phase material, columnar structural FePt films with better perpendicular anisotropy could be obtained, which may offer a method for the application of FePt media in heat assisted magnetic recording.

Exploring the potential of remote plasma sputtering for the production of L10 ordered FePt thin films

Lowering the temperature at which the desirable L10 phase forms in FePt thin films is a key requirement in the development of next generation high-density data storage media and spintronic devices. Remote plasma sputtering offers a higher degree of control over the sputtering parameters, allowing the properties of films to be tailored, and potentially can affect the ordering kinetics of the L10 phase of FePt. Here, we report a comprehensive study of FePt thin films deposited under a range of temperatures and sputtering conditions. X-ray diffraction and magnetometry investigations show that whilst FePt thin films ordered in the L10 phase with high perpendicular anisotropy can be produced using this technique, there is no significant reduction in the required ordering temperature compared with films produced using conventional DC sputtering. Optimally ordered L10 FePt films were fabricated when the film was deposited at a substrate temperature of 200 °C, followed by post annealing at 750 °C.

Preparation of FePt Nanoparticles by Pulsed Plasma in Liquid Method

FePt alloys are an important class of materials in permanent magnetic applications because of their large uniaxial magnetocrystalline anisotropy and good chemical stability. We have applied the pulsed plasma in liquid method to synthesis nanoparticles. Short duration of pulse and quenching effects of the surrounding liquid limit the size of crystal. That enable synthesis of small size and metastable nanoparticles. In this study, ferromagnetic FePt nanoparticles were successfully synthesized. Face-centered-cubic (FCC) A1-type phase was synthesized from FePt (Fe:PT=50:50, 55:45 in atomic %) alloy rod electrodes using the pulsed plasma in ethanol. The ordered face-centered-tetragonal (FCT) L10-type phase FePt was obtained by annealing the A1-type phase at 400oC for 1 h. The average diameter of L10-type FePt nanoparticles was less than 10 nm. Vibrating Sample Magnetometer (VSM) analysis showed that the coercivity of L10-type nanoparticles was much larger than that of the A1-type phase nanoparticles.

Fabrication and electrocatalytic properties of ferromagnetic nanoporous PtFe by dealloying an amorphous Fe 60 Pt 10 B 30 alloy

A nanoporous PtFe (np-PtFe) alloy composed of a single face-centered-cubic FePt phase has been fabricated by electrochemically dealloying an amorphous Fe60Pt10B30 alloy in 0.1 mol L−1 H2SO4 solution. The np-PtFe alloy possesses a fine bicontinuous ligament/channel structure with average ligament and pore sizes of about 7 and 5 nm, respectively. The np-PtFe alloy exhibits superior electrocatalytic activity for methanol oxidation reaction in the acid media to the commercial Johnson Matthey Pt/C catalyst and soft magnetic characteristics with a saturation magnetization of 19.09 emu/g.

Magnetic and structural properties of Fe87Pt13–Al2O3 composite thin films synthesized by solid-state reactions

The structural and magnetic properties of Fe87Pt13 films synthesized by solid-state reactions and Fe87Pt13–Al2O3 composite films fabricated by aluminothermy are investigated. It is shown that the synthesized samples of both types are characterized by the rotational magnetic anisotropy, when the easy magnetization axis in the film plane can be set by a magnetic field. It is established that the value of rotational magnetic anisotropy in the Fe87Pt13–Al2O3 composite films is higher than in the Fe87Pt13 samples by an order of magnitude. The rotational magnetic anisotropy is assumed to be caused by the exchange coupling of the L10–FePt phase with the L12–Fe3Pt phase in the Fe87Pt13 films and magnetic iron oxides in the Fe87Pt13–Al2O3 samples.

Luminomagnetic Silica-Coated Heterodimers of Core/Shell FePt/Fe3O4 and CdSe Quantum Dots as Potential Biomedical Sensor

We report the synthesis of a new multifunctional nanomaterial based on silica-coated FePt/Fe3O4-CdSe heteronanostructures, combining luminescent and magnetic properties in a promising bifunctional sensor for biomedical applications. Spherical Fe3O4-coated FePt (FePt/Fe3O4) superparamagnetic nanoparticles (10.8 ± 1.5 nm) with high saturation magnetization and controlled size and shape were obtained using thermal decomposition coupled with seed-mediated growth method. Luminescent property was added to the nanomaterial by using the FePt/Fe3O4 magnetic core as seed and growing the CdSe quantum dots (2.7 ± 0.6 nm) onto its surface in a heterodimer-like structure using the hot-injection approach. The FePt/Fe3O4-CdSe luminomagnetic heteronanostructures were coated with silica shell using the reverse-micelle microemulsion route to avoid solvent-quenching effects. After silica coating, the water-dispersible heteronanostructures showed a diameter of 25.3 ± 2 nm, high colloidal stability, magnetic saturation of around 11 emu g−1, and photoluminescence in the blue-green region, as expected for potential bifunctional platform in biomedical applications. The saturation magnetization of heteronanostructures can be increased to 28 emu g−1 by annealing at 550°C due to the presence of the FePt phase.

Large coercivity FePt nanoparticles prepared via a one-step method without post-annealing

L10 FePt nanoparticles were synthesized by a one-step sol-gel autocombustion method, using nontoxic ferric nitrate, hexachloroplatinic acid, and glycine as starting materials. In contrast to common syntheses, high-temperature post-annealing was not required to form the L10 FePt phase. The entire ignition and combustion process lasted no more than one minute. The L10 FePt phase could form in the presence of the high temperature caused by the exothermic combustion reaction. Adjusting the glycine-to-metal ion molar ratio from 0.5 to 6.0 allowed its effects on the phase transformation and magnetic properties of the products to be investigated. X-ray diffraction indicated that pure phase L10 FePt was obtained at a glycine-to-metal ion molar ratio of 1.5. Transmission electron microscopy indicated that the monodisperse L10 FePt nanoparticles had an average particle size of about 20 nm. The reasons why the as-synthesized L10 FePt nanoparticles were not aggregated and sintered could be attributed to the large amount, a gas is being released and the short duration of heat treatment during this combustion. This finding constituted a significant improvement in the synthesis of L10 FePt nanoparticles. Magnetic measurements showed that the L10 FePt nanoparticles had a coercivity of 15.8 kOe at 300 K, and 23.2 kOe at 5 K. Thus, the L10 FePt nanoparticles had a very large coercivity.

Colloidal Nanoparticle Clusters to produce large FePt nanocrystals

The present work seeks to enlarge the FePt nanocrystal size by submitting Colloidal Nanoparticle Clusters (CNCs) coated by a FeOx shell (FePt@FeOx) to a thermal treatment. The nanostructures were prepared by thermal decomposition in organic media under limited surfactant protection. By adjusting the molar ratio of surfactants it is possible to control the degree of oxidation and aggregation of the CNCs. The FeOx coating of the FePt CNCs resulted to be crucial during the thermal treatment to achieve a controlled crystal growth and induce a structural transition to tetragonal L10 FePt phase producing 31nm nanocrystals with high magnetization and coercivity. Clustering and controlled oxidation is proposed here as a new method to overcome size limitation in the synthesis of FePt nanoparticles.