Effects of annealing time on the structural and magnetic properties of L10 FePt thin films

Thermal annealing of [Fe 1.65 nm/Pt 1.84 nm]₅₀ multilayers at 673 K for various annealing times between 60 and 12000 s leads to the direct formation of the fully ordered L1₀ FePt phase with (111) texture. The average grain sizes, determined from X-ray diffraction size-strain analysis, are smaller than the critical size for multi-domain FePt particles, suggesting the presence of single-domain (SD) grains. The coercivity increases with annealing time and increasing grain size and reaches values of about 955 kA/m. The remanence values are typical for randomly oriented weakly-interacting particles. A decrease of the remanence with annealing time suggests a decrease of the intergrain exchange interactions with annealing time. Analysis of minor loops and the initial magnetization curves shows the presence of a broad distribution of critical fields, which the individual SD particles have to overcome for the magnetization reversal.     

Structure and properties of nanoporous FePt fabricated by dealloying a melt-spun Fe_(60)Pt_(20)B_(20) alloy and subsequent annealing

A nanoporous FePt alloy has been fabricated by dealloying a melt-spun Fe(60)Pt(20)B(20)alloy composed of nanoscale amorphous and face-centered-cubic FePt(fcc-FePt)phases in H2 SO4 aqueous solution.The nanoporous alloy consists of single fcc-FePt phase with an Fe/Pt atomic ratio of about 55.3/44.7,and possesses a uniform interpenetrating ligament-channel structure with average ligament and pore sizes of 27 nm and 12 nm,respectively.The nanoporous fcc-FePt alloy shows soft magnetic characteristics with a saturation magnetization of 37.9 emu/g and better electrocatalytic activity for methanol oxidation than commercial Pt/C in acidic environment.The phase transformation from disordered fcc-Fe Pt into ordered face-centered-tetragonal FePt(L10-FePt)in the nanoporous alloy has been realized after annealing at823-943 K for 600 s.The volume fraction of the L10-FePt phase in the alloy increases with the rise of annealing temperature,which results in the enhancements of coercivity and saturation magnetization from 0.14 kOe and 38.5 emu/g to 8.42 kOe and 51.4 emu/g,respectively.The ligament size of the samples is increased after annealing.     

Effect of Zn Addition on the Reduction of the Ordering Temperature of FePt Nanoparticles

Monodisperse FePt nanoparticles with an average size of 4.11 nm were successfully synthesized via chemical co-reduction of iron acetylacetonate, Fe(acac)₃, and platinum acetylacetonate, Pt(acac)₂, by 1,2hexadecanediol as a reducing agent. Also (FePt)₈₇Zn₁₃ nanoparticles with average size of 4.24 nm were synthesized using the same method. The structural and magnetic properties of the prepared samples were respectively studied by XRD, TEM and VSM. L1₀ FePt ordered phase is formed at lower annealing temperature by addition of Zn. The (FePt)₈₇Zn₁₃ nanoparticles starts ordering after annealing at 400 °C, whereas FePt nanoparticles at 400 °C are still disordered alloys with superparamagnetic behavior. Additive Zn is very effective in decreasing the ordering temperature and enhancing the chemical ordering in (FePt)₈₇Zn₁₃ particles, So that coercivity 5200 Oe was measured for (FePt)₈₇Zn₁₃ nanoparticles annealed at 500 °C, compared with 1800 Oe for samples without Zn. This reduction in ordering temperature significantly reduces FePt particle coalescence and loss in positional order.     

Effect of annealing in external magnetic field on the microstructure and magnetic properties of FePt films

We have studied the influence of annealing in an external magnetic field on the microstructure and magnetic properties of a multilayer Si/Fe(2 nm)/Fe₅₀Pt₅₀(20 nm)/Pt(2 nm) structure synthesized by means of sequential RF magnetron sputtering of the components. The magnetic field was oriented perpendicular to layers of the structure. It is established that annealing in the external magnetic field leads to the formation of predominant (001) texture in the multilayer structure with L1₀-FePt phase. Thus, a method of obtaining multilayer structures based on FePt films required for the perpendicular magnetic recording has been developed.     

The effect of composition and structural ordering on the magnetism of FePt nanoparticles

Spherical 4 nm FePt nanoparticles were synthesized by the simultaneous decomposition of Fe(CO)5 and the polyol reduction of Pt(acac)2. The final Fe-to-Pt composition was tuned between 15-55 at.% by varying the ingredient precursor ratios. The effect of composition and structural ordering on the macroscopic magnetic features of final FePt nanoparticles was examined via post-synthetic annealing stages at different conditions. Structural ordering is promoted in all cases, though samples approximating equiatomic Fe/Pt ratios eventually transform to fct-FePt phase while the FePt3-phase is favored for the Pt-richer samples. Consequently, the magnetic features of the annealed nanoparticles may be categorized; the hard magnetic FePt region dominating for Fe content between 40-55 at.% and the soft magnetic FePt3 region dominating in the region 20-30 at.% while Fe content less than 20 at.% results in Pt-richer phases with diminishing ferromagnetic behavior.     

Effects of Magnetic Annealing on Structure and Magnetic Properties of L10-FePt/Ag Films

The structural and magnetic properties of L1₀-FePt/Ag films were studied by X-ray diffraction and a vibrating sample magnetometer. The FeAg/Pt films were obtained by depositing FeAg thin films on thermally oxidized Si (001) substrates via magnetron sputtering and Pt layers on their surface after annealing FeAg thin films at 400 °C with and without an out-of-plane magnetic field of 10 kOe. These films were further annealed at various temperatures to obtain L1₀-FePt phase. The results indicated that the pre-annealing of FeAg thin films under 10 kOe magnetic field caused (001) orientation of Fe particles, and the deposition of Pt layer on such orientated underlayers reduced the ordering temperature of FePt in FeAg/Pt films, realizing the L1₀-FePt phase at 400 °C. The higher coercivity and ordering degree were also observed in the samples, compared with those pre-annealed without magnetic field at the same annealing condition.     

Effect of Oleic Acid and Oleylamine Surfactants on the Size of FePt Nanoparticles

FePt magnetic nanoparticles have been synthesized by superhydride reduction of FeCl₂ and Pt(acac)₂ at high temperature. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl₂ and Pt(acac)₂ in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 190?^°C, followed by refluxing at 245?^°C, led to monodisperse 3.5?nm FePt nanoparticles. The effect of oleylamine and oleic acid surfactants on the nucleation and growth of FePt nanoparticles were studied. The size of Pt was controlled by oleylamine surfactant in nucleation stage. To prevent sintering of the FePt nanoparticles, oleic acid surfactant was used in growth stage. The energy dispersive spectroscopy results revealed that the particle composition was first Fe₁₁Pt₈₉ in nucleation stage and after adding superhydride the composition changed to Fe₆₃Pt₃₇ in growth stage. The structural and magnetic measurements indicated that the L1₀ structure of FePt nanoparticles is formed after annealing and the coercivity of superlattice FePt nanoparticles increases to 7.5?kOe after heat treatments.     

Effects of interfacial structure on the magnetic properties of ultrathin Fe/Pt(111) films with Ag buffer layer

For Fe/Pt(111) grown at 180 K, the easy axis of magnetization changes from out-of-plane to in-plane while the surface structure changes from fcc(111) to bcc(110). The Curie temperature for 1 ML Fe/Pt(111) is close to room temperature. After insertion of submonolayer Ag, no significant change for the Curie temperature is observed. By thermally annealing Ag/Pt(111) at 550 K to form a Ag-Pt surface alloy, we demonstrate that the Curie temperature of the deposited Fe films is elevated to be higher than room temperature. For an ultrathin Fe/Ag/Fe/Pt(111) sandwich system, the strong interaction between the two Fe layers is superior due to the effect of the double Fe/Ag interfaces. This causes that the easy axis of the magnetization switches back to the in-plane direction during the deposition of Fe overlayer on the surface of a Ag/Fe/Pt(111) film.     

Effect of carbon additive on microstructure evolution and magnetic properties of epitaxial FePt (001) thin films

FePt:C thin films were deposited on CrRu underlayers by DC magnetron co-sputtering. The effects of C content, FePt:C film thickness and substrate temperature on the microstructural and magnetic properties of the epitaxial FePt (001) films were studied. Experimental results showed that even with 30 vol.% C doping, the FePt films could keep a (001) preferred orientation at 350 °C. When a FePt:C film was very thin (< 5 nm), the film had a continuous microstructure instead of a granual structure with C diffused onto the film surface. With further increased film thickness, the film started to nucleate and formed a column microstructure over continuous FePt films. A strong exchange coupling in the FePt:C films was believed to be due to the presence of a thin continuous FePt layer attributed to the carbon diffusion during the initial stage of the FePt:C film growth. Despite the presence of a strong exchange coupling in the FePt:C (20 vol.% C) film, the SNR ratio of the FePt:C media was about 10 dB better than that of the pure FePt media. The epitaxial growth of the FePt:C films on the Pt layers was observed from high resolution TEM cross sectional images even for the films grown at about 200 °C. The TEM images did not show an obvious change in the morphology of the FePt:C films deposited at different temperatures (from 200 °C to 350 °C), though the ordering degree and coercivity of the films increased with increased substrate temperature.     

Onset of hard magnetic L10 FePt phase with (001) texture

The perpendicular anisotropic magnetic properties of in-situ deposited FePt/Pt/Cr trilayer films were elucidated as functions of the deposition temperature and the sputtering rate of the FePt magnetic layer. Ordered L1₀ FePt thin films with perpendicular anisotropy and a (001) texture can be developed at a temperature as low as 300 °C with the sputtering of a FePt layer at a low rate. The larger Pt(001)[100] lattice induced an expansion of the FePt a- and b-axis, leading to the contraction of the FePt c-axis, enabling the epitaxial growth of the L1₀ FePt(001) texture to occur. A low rate of sputtering of the FePt thin film promotes the formation of the magnetically hard FePt(001) texture on the surface of the Pt(001) buffer layer at low temperature, while the high sputtering rate of FePt layer suppresses the phase transformation.     

Properties of thin FePt films synthesized by sequential sputtering of components

Thin (001)-oriented FePt films in the form of multilayer [Pt/Fe] _n structures have been synthesized by means of sequential RF magnetron sputtering of the components. The dependence of the microstructure, magnetic properties, and magnetic anisotropy of the [Pt/Fe] _n system on the substrate temperature during deposition, the type of the first deposited layer, the thicknesses of the partial Fe and Pt layers, and the total film thickness has been studied.     

Coercivity variation in exchange-coupled Fe/FePt bilayer with perpendicular magnetization

The soft/hard Fe/FePt film with perpendicular magnetization has been deposited on a glass substrate. The (001) oriented L1₀ FePt film was obtained when annealed by rapid thermal process at 800 °C and a Fe layer was deposited at room temperature with thicknesses of 2 nm to 20 nm. Controlling the Fe layer thickness allowed modification of the hysteresis loops from out-of-plane rigid magnet to in-plane exchange-spring like magnet due to the nanometer scale interface coupling. When the Fe layer thickness increased to 2 nm, the out-of-plane coercivity is reduced to 5.9 kOe but the remanence ratio (0.98) is still high. The Fe (2 nm)/FePt film shows perpendicular magnetization with linear in-plane hysteresis loop. The remanence ratio is reduced to 0.85 when the Fe layer thickness increased to 5 nm. When the Fe layer thickness was varied up to 10-20 nm, the in-plane hysteresis loop shows exchange-spring like behavior with two-step magnetization reversal processes. The films with perpendicular coercivity were moderated by the thickness of soft magnetic layer.     

Magnetic and Morphological Properties of Electrodeposited Thick FePt Films on Metallic (Au, Ag, Cu) Underlayers

Electrodeposited thick films of FePt (with the nominal composition 50 % Fe/50 % Pt) on three metallic (Au, Ag, Au) underlayers were annealed at various temperatures. The magnetic and morphological properties of the resulting films were then monitored. The Au and Ag underlayers promoted the growth of the (bct) L1₀ FePt phase. The greater growth of this phase in the films deposited on the Ag underlayer led to the crystallographic texturing in the (001) direction. This was accompanied by a significant magnetic anisotropy and a negative shift of the remanent magnetization in the presence of an applied field. The coercivity of the Ag underlayer films increased to 18 kOe while the coercivity of the Au underlayer films decreased to ～2 kOe when the annealing temperature was increased to 800 ^°C.     

Microstructural evolution of FINEMET type alloys with chromium: An electron microscopy study

Microstructural changes in Fe_73.5–xCr_xCu₁Nb₃Si_13.5B₉ (0x5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe(Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is enhanced with the Cr content of the alloy. In a second stage, around 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type unknown crystal phase with a lattice parameter of a=1.52 nm, and recrystallization of the previous Fe(Si) nanophase also occurs. No qualitative differences were found between dynamic and isothermal crystallization. The size effect for thin samples is limited to a lowering of crystallization temperatures. For isothermal nanocrystallization in the temperature range 775–900 K, the mean grain size of the nanocrystals increases for short annealing times to stabilize at a constant value of about 10–15 nm for long annealing times. The stabilized grain size increases with increasing annealing temperature and slightly decreases with the Cr content of the alloy.     

Phase relationship in the Fe-Pt-Pr ternary system at 500°C

The phase relationship in the Fe-Pt-Pr ternary system at 500°C was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) energy dispersion spectroscopy techniques. The 500°C isothermal section consists of 13 single-phase regions, 23 two-phase regions and 11 three-phase regions. At 500°C, the maximum solid solubility of Pt in α-Fe is about 10 at.% and Fe in Pt is about 18 at.%; the maximum solubilities of Pr in α-(Fe,Pt), Fe₃Pt, FePt, FePt₃ and (Pt,Fe) (the solid solution of Fe in Pt) are about 6 at.% 1.5 at.% 2 at.%, 2.5 at.% and 1.5 at.%, respectively. No Pr₃Pt₄ binary compound and new ternary compounds were found.     

Microstructure and magnetic properties of (0 0 1)-oriented L10 FePt films: Role of Ag underlayer and Fe/Pt ratio

FePt multilayer films were deposited on Si(1 0 0) substrate with thermally grown SiO₂ film and sputtered Ag underlayer at room temperature by dc magnetron sputtering and subsequently annealing in vacuum. Experimental results suggest that proper thickness of Ag underlayer and slightly rich of Fe content can effectively induce the (0 0 1) texture of FePt films. A Fe_57.4Pt_42.6 thin film on the 8 nm Ag underlayer exhibits a large perpendicular coercivity of 7.6 kOe with magnetic remanence close to 1.     

Changes of morphology of tungsten microsized monocrystal induced by Pt submonolayer

The morphology of tungsten tip (microsized monocrystal) adsorbed with Pt at coverages up to around 2 ML and subsequently annealed at temperatures ranging from 1000 K to 2100 K was studied using Field Emission Microscopy (FEM) technique. The sharpening of the [111] pole region was manifested by significant increase of field emission current density. This effect was observed for Pt coverages ranging from 0.1 ML to 1.1 ML and full range of examined annealing temperatures, but was most pronounced for annealing at temperatures exceeding 1300 K. Formation of the sharp apex was accompanied by growth of {112} facets for the annealing temperatures lower than 1900 K. Annealing at higher temperatures resulted in formation of sharp apex of the W tip and complete disappearance of {112} planes when the Pt coverage exceeded approximately 0.25 ML.     

Effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers

FePt (50 nm) and [FePt(xnm)/AlN(1, 2, 3 nm)]₁₀ (x=2, 3 nm) films were prepared by RF magnetron sputtering technique, then were annealed at 550 °C for 30 min. This work investigates the effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers. Superlattice (0 0 1) peaks can be found in the grazing incidence X-ray diffraction of FePt and [FePt (3 nm)/AlN (1, 2, 3 nm)]₁₀ films, which indicate that the FCC phase has been partially transformed into ordered L1₀ phase. Compared with the single layer FePt film, superlattice (0 0 1) peaks of FePt/AlN multilayers are weak and wide, which indicates that the introducing of AlN hinders the growth of FePt particle, and also shows the introducing of AlN is not beneficial to the transformation from FCC phase to L1₀ phase. In addition, the low-angle XRD spectra show the layered structure of FePt/AlN has been broken after annealing. The coercivities, particle size, intergrain exchange interactions of FePt/AlN films are decreased with increasing AlN layer thickness.     

一步法制备L10相FePt纳米颗粒

以乙酰丙酮铁(Fe(acac)3)和氯铂酸(H2PtCl6.6H2O)分别作Fe源和Pt源,三缩四乙二醇(TEG)作溶剂和还原剂,聚乙烯基吡咯烷酮(PVP)作表面活性剂,通过多元醇还原法制备出单分散的FePt纳米颗粒。通过X射线衍射仪(XRD)及透射电子显微镜(TEM)分析表明,所制备的FePt纳米颗粒形状近似球形,分散性较好,平均颗粒粒径约为5.5nm。通过振动样品磁强计(VSM)分析显示所制备FePt纳米颗粒矫顽力为37.64kA/m,这意味着FePt纳米颗粒部分转变为面心四方相(L10相)。     

Grain Refining and Decoupling in FePt/SiO2 Nanogranular Films for Magnetic Recording

FePt/SiO₂ nanogranular thin films have been prepared by molecular-beam epitaxy system on MgO (001) substrates with the method of insertion dual SiO₂ layers into Fe/Pt multilayer films. We report the relationships between the inserting thickness of SiO₂ layers and the microstructural and magnetic properties of FePt thin films. It indicated the nanogranular FePt thin films were successfully formed by inserting amorphous SiO₂ layers into the Fe/Pt films. The reduction of grain/domain size and isolation of FePt particles can be achieved by such insertion and maintain (001) texture. The average grain size of FePt films with 5-nm SiO₂ insert layers is estimated to be around 8 nm, while domain rotation is enhanced depicting a decoupling of intergrain interaction. The isolated grains are less magnetically coupled in the rotation mode and the reversal of magnetization is more independent