L10 FePt Research Articles

Temperature dependence of the anisotropy field of L1 FePt near the Curie temperature

Near the Curie temperature, the anisotropy field of magnetically uniaxial L10 FePt is expected to follow the scaling law (1 – T/Tc)β, where T is the temperature and Tc is the Curie temperature. In the literature, β values between 0.36 and 0.65 have been reported. Based on recording measurements and micromagnetic analysis, we show that only the values of β near the low end of the reported range are compatible with the data. We also conclude that thermally activated magnetization reversal at temperatures near Tc cannot be ignored, even at time scales smaller than 1 ns. We demonstrate that thermally activated magnetization reversal at temperatures close to Tc is well described by conventional theory with a frequency factor f0 of the order of 1012 Hz. It is reasoned that the unusually high value for f0 is a consequence of the temperature-induced reduction of the degree of alignment of the micro-spins within the grains.

Magnetic and Mössbauer studies of L10–FePt/Fe/Ta multilayer structures

Magnetic L10–FePt(10 nm)/Fe(t, nm)/Ta(2 nm) (t is the Fe film thickness that is varied from 0 to 15 nm) multilayer structures have been prepared by magnetron codeposition. The 2-nm-thick Ta layer is a corrosion protection. The magnetization reversal processes and the magnetic interactions have been studied. The hysteresis loops measured in the plane of a single-layer L10–FePt films demonstrate a near-linear behavior. N the magnetic multilayer FePt(10 nm)/Fe(t, nm)/Ta(2 nm) system, in which the Fe layer thickness is smaller than 3 nm, the FePt/Fe system behaves as a single-phase magnetic material and the coercivity is close to the values determined by the Zeeman energy. In the case when the Fe layer thickness in the magnetic multilayer FePt(10 nm)/Fe(t, nm)/Ta(2 nm) structure is larger than 3 nm, the hysteresis loops measured in the structure plane indicate that the FePt/Fe film possesses the properties analogous to the properties of a soft magnetic material. The Mossbauer studies showed that the minimal deviation of the magnetic moments on the normal to the multilayer structure surface was observed as the Fe layer thickness is 1 nm. The increase in the Fe layer thickness to values higher than 1 nm led to the increase in the angle of deflection θ to ~40° at t = 15 nm. In this case, the coercivity of the multilayer structure slowly decreased, because of the limitations of the exchange bond length between the FePt and Fe layers. The measured values of the coercivity were optimized using relationship 1/t Fe 1.15.

Thermal switching probability distribution of L10 FePt for heat assisted magnetic recording

Transition jitter is one of the most important parameters for evaluating the potential areal density of Heat Assisted Magnetic Recording. However, obtaining this either experimentally or through micro-magnetic simulation is time consuming. Here, we show that the more easily calculated thermal switching probability distribution (SPD) can serve as a substitute. In particular, we provide an equation relating the standard deviation of write temperature to the jitter. The equation is verified for three critical noise sources: Tc variance, HK variance, and grain size distribution. In addition, the SPD subject to cooling at various cross-track positions is investigated. A method to determine the magnetic grain size is proposed.

Magnetic anisotropy phase-graded A1/L10-FePt films on amorphous glass substrates

Magnetic anisotropy phase-graded A1/L10-FePt films deposited on amorphous glass substrates were investigated combining ultra-high resolution electron microscopy and angular-dependent magnetic measurements. A highly textured (001) L10 FePt film was first deposited at the relative low temperature of 625K using an MgO/Cr underlayer stack, hence a second layer was grown while continuously decreasing the deposition temperature down to a final value ranging from 515K to 365K depending on the layer thickness (tg). This procedure leaded to the formation of a phase-graded system consisting of hard and soft magnetic phases separated by a rough nanometer-size interphase boundary, where the magnetic anisotropy gradually changes due to the variation of the relative amount of hard and soft phases across the whole film thickness. Electron microscopy analysis allowed the structure of the samples to be investigated at an atomic level. The A1 and L10-FePt phases were localized inside the film and the orientation relationships between their lattices were determined. The samples show a preferential perpendicular anisotropy up to tg=15nm and a significant reduction of the coercive field with the increase of the graded layer thickness (~30% for tg=5nm), suggesting their potential application as magnetic recording media.

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.

Kerr microscopy study of exchange-coupled FePt/Fe exchange spring magnets

Magnetization reversal and magnetic microstructure of top soft magnetic layer (Fe) in exchange spring coupled L10 FePt/Fe is studied using high resolution Kerr microscopy. With remnant state of the hard magnetic layer (L10 FePt) as initial condition, magnetization loops along with magnetic domains are recorded for the top soft magnetic layer (Fe) using Kerr microscopy. Considerable shifting of Fe layer hysteresis loop from center which is similar to exchange bias phenomena is observed. It is also observed that one can tune the magnitude of hysteresis shift by reaching the remanent state from different saturating fields (HSAT) and also by varying the angle between measuring field and HSAT. The hysteresis loops and magnetic domains of top soft Fe layer demonstrate unambiguously that soft magnetic layer at remanent state in such exchange coupled system is having unidirectional anisotropy. An analogy is drawn and the observations are explained in terms of established model of exchange bias phenomena framed for field-cooled ferromagnetic - antiferromagnetic bilayer systems.

Calculated dependence of FePt damping on external field magnitude and direction

Near the Curie temperature (Tc), magnetic parameters including magnetization, anisotropy, and damping depend strongly on both temperature and length scale. This manifestation of renormalization theory is most readily seen in the case of magnetization where the magnitude of the atomic spin is largely unaffected by temperature, but the bulk magnetization vanishes at Tc. It has been previously argued that the Landau-Lifshitz-Gilbert damping parameter alpha exhibits a similar effect owing to its dependence on both atomic effects and magnon-magnon scattering, the latter having a strong length dependence. Here, we calculate, using an anisotropic exchange description of L10 FePt (Tc = 705 K), the damping (and other magnetic properties) dependence on temperature for FePt at length scales around 1.0 nm as appropriate for high temperature micromagnetic simulation. While the damping reduces as the applied field along the easy direction increases, it tends to increase as the field direction is changed to in-plane. The renormalized parameters are also calculated for higher and lower Tc (770K and 630K) by invoking the linear relationship between the exchange stiffness parameter and Curie temperature. This corresponds to doped and/or non-stoichiometric FePt and allows better understanding of the effects of varying anisotropy to exchange ratio.

Thickness dependence of magnetization reversal mechanism in perpendicularly magnetized L10 FePt films

We have studied the magnetic switching behavior of L10-ordered FePt films with varying thickness. It was found that coercivity is strongly dependent on the film thickness. The obvious variations of the coercivity in the thin films are confirmed by the measurements of structural and magnetic properties. With increasing thickness, the degree of L10 chemical ordering increased, while the magnetization reversal process transforms from a pinned two-steps magnetization reversal to a comparatively smooth domain wall motion behavior. Although considering anisotropy, exchange interaction and applied magnetic field, the switching behavior in films is quite complex, the main features of the magnetization reversal mechanism can be understood by performing detailed investigation on the effect of the deposition temperature and the angle of magnetic field.

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.

Exchange hardening in FePt/Fe3Pt dual exchange spring magnet: Monte Carlo modeling

The FePt/Fe3Pt bilayer, a model dual exchange spring system with intergranular exchange hardening between A1 and L10 FePt and interfacial exchange hardening between L10 FePt and Fe3Pt is investigated systematically for varying thickness of soft layer (Fe3Pt). Optimal exchange hardening with an appreciable coercivity (Hc) is observed for the bilayer system with a thickness of 2 nm of the soft layer. Recoil studies reveal coherent magnetization up to a reverse field of 7 kOe in this sample, with possible application for memory device. Monte Carlo simulation is performed to analyze the effect of magnetic anisotropy and interactions on the reversal mechanism at the interface of the bilayer system. The angular remanence of bilayered system reveals that the easy axis of L10 FePt and Fe3Pt are mutually perpendicular.

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.

On the advantages of spring magnets compared to pure FePt: Strategy for rare-earth free permanent magnets following a bottom-up approach

Nanostructured magnets benefiting from efficient exchange-coupling between hard and soft grains represent an appealing approach for integrated miniaturized magnetic power sources. Using a bottom-up approach, nanostructured materials were prepared from binary assemblies of bcc FeCo and fcc FePt nanoparticles and compared with pure L10-FePt materials. The use of a bifunctional mercapto benzoic acid yields homogeneous assemblies of the two types of particles while reducing the organic matter amount. The 650°C thermal annealing, mandatory to allow the L10-FePt phase transition, led to an important interdiffusion and thus decreased drastically the amount of soft phase present in the final composites. The analysis of recoil curves however evidenced the presence of an efficient interphase exchange coupling, which allows obtaining better magnetic performances than pure L10 FePt materials, energy product above 100kJm−3 being estimated for a Pt content of only 33%. These results clearly evidenced the interest of chemically grown nanoparticles for the preparation of performant spring-magnets, opening promising perspective for integrated subcentimetric magnets with optimized properties.

Fe-Based Magnetic Alloy Electrodeposition for Thin Films and Template Based Nanostructures

Iron based alloys exhibit extraordinary material and magnetic properties. They are of fundamental scientific interest, e.g. as smart and intelligent materials for magnetic storage, recording, sensing or actuating. There is an increasing importance of technical applications due to ongoing omnipresent miniaturization and integration in MEMS/NEMS systems. The dimension and geometry of the material can be tailored towards thin films, nanowires or advanced nanostructures generating high functionality for various purposes. To meet demands in miniaturization and performance of electronic devices, often low dimensional structures with highly complex geometries are requested. Electrodeposition is the most appropriate, cost and time efficient method for the fabrication of well-ordered nanoscaled structures and in particular to produce high aspect ratio nanowires. The preparation of nanoscaled magnetic materials such as Co, Fe and Ni in porous templates is a well known standard procedure [1-3]. Magnetic alloys or multilayers with specific magnetic properties are still of increasing interest. Compared to single metal deposition a more precise control of the respective electrolyte composition and deposition parameters is needed. Alloy deposition is often restrained by a strong difference in deposition potentials of the respective components. In reduced dimensions transport conditions and therefore, deposition kinetics are crucial for a successful plating process, too. This contribution focusses on the electrochemical preparation of Fe-based magnetic alloys such as Co-Fe, Fe-Ga, Fe-Pd and Fe-Pt. CoFe alloys are well known for tremendous soft magnetic properties with high saturation polarization of up to 2.4 T. A high magnetostriction is achieved in Fe-Ga alloys with 80 at.% Fe. The Fe70Pd30 alloy exhibits the magnetic shape memory effect. The Fe-Pt L10 phase shows good hard magnetic behavior due to high magnetocrystalline anisotropy and a high saturation magnetization. Therefore these alloys are very promising candidates for smart and intelligent materials in the fields of actuating or sensing or as micromagnets in MEMS. For the Co-Fe system simple sulfate based electrolytes and potentiostatic deposition [4] as well as alternating potential regimes are most efficient. The more challenging systems Fe-Ga, Fe-Pd [5, 6] and Fe-Pt require advanced complexed electrolytes. Deposition mechanisms are discussed based on quartz microbalance experiments. Deposition parameters are systematically investigated with regard to compact defect free deposits with homogeneous composition distribution. The correlation of electrolyte composition and deposition regime with deposit structure, morphology and magnetic properties is discussed. [1] Schlörb et al., Phys. Status Solidi B 247, 2010, 2463 [2] Haehnel et al., Acta Mater. 58, 2010, 2330 [3] Nielsch et al., Adv. Mater. 12, 2000, 582 [4] Vock et al., J. Appl. Phys. 118, 2015, 233901 [5] Iselt et al., J. Electrochem. Soc. 159, 2012, H633 [6] Konczak et al., Mater. Chem. Phys. 174, 2016, 150

Suppressing FePt order-disorder structural phase transition temperature by Ag doping: First principles investigation for enhancing magnetic recording density

ABSTRACTIn this work, we used density functional theory to study the structures, formation energy and order–disorder structural transition temperature (To) of L10 FePt with Ag additive. The results show that an Ag atom substituting for a Fe site is more energetically preferable than that of substituting for a Pt, thus Ag atom should replace Fe site. We found that To is significantly suppressed by Ag impurity because of the increasing in configurational entropy, whereas the magnetism tends to decrease due to the absence of some Fe atom in doping processes. The decrease in To could open the opportunity to prepare FePt with lower temperature and small grain size for the high density magnetic recording media application.

Influence of chemical ordering on the thermal conductivity and electronic relaxation in FePt thin films in heat assisted magnetic recording applications

We report on the out-of-plane thermal conductivities of tetragonal L10 FePt (001) easy-axis and cubic A1 FePt thin films via time-domain thermoreflectance over a temperature range from 133 K to 500 K. The out-of-plane thermal conductivity of the chemically ordered L10 phase with alternating Fe and Pt layers is ~23% greater than the thermal conductivity of the disordered A1 phase at room temperature and below. However, as temperature is increased above room temperature, the thermal conductivities of the two phases begin to converge. Molecular dynamics simulations on model FePt structures support our experimental findings and help shed more light into the relative vibrational thermal transport properties of the L10 and A1 phases. Furthermore, unlike the varying temperature trends in the thermal conductivities of the two phases, the electronic scattering rates in the out-of-plane direction of the two phases are similar for the temperature range studied in this work.

Synthesis and Characteristics of FePt Nanoparticle Films Under In Situ-Applied Magnetic Field

In situ external magnetic field was applied during the synthesis of FePt nanoparticles via a chemical solution method. FePt nanoparticle films were prepared on Si by a drop-coating method with and without a magnetic field. Annealing at 700 °C in reductive atmosphere was explored to obtain ferromagnetic FePt L10 phase. The effect of in situ-applied magnetic field on the structure, morphology, and magnetic properties of FePt nanoparticle films was characterized. It is found that the applied magnetic field during the chemical synthesis of FePt nanoparticles plays a key role in the crystallinity and magnetic property of FePt nanoparticle films. As-synthesized FePt nanoparticles under the magnetic field are monodispersed and can be self-assembled over a larger area by a dropping method. The applied magnetic field during the synthesis of FePt nanoparticles not only significantly improves the nanoparticles’ c-axis preferred orientation but also benefits the phase transition of FePt nanoparticles from face-centered cubic to face-centered tetragonal structure during the annealing process. The FePt nanoparticle films derived under magnetic field also show some magnetic anisotropy.

Correlation between structural order parameter of epitaxial Fe/Pt multilayer and corresponding perpendicular magnetic anisotropy and orbital magnetic moment upon annealing

A strong correlation has been noted between the structural order parameter of an epitaxial L10 Fe/Pt thin film and the corresponding perpendicular magnetic anisotropy (PMA) and orbital magnetic moment. This relationship is studied in detail here. An epitaxial L10 FePt thin film grown from an [Fe(10 Å)/Pt(10 Å)]15 multilayer with (001) orientation was prepared using ion beam sputtering deposition. The Fe/Pt multilayer was transformed into L10 alloy at an annealing temperature of about 500 °C. When annealed at 700 °C, the out-of-plane order parameter of L10 FePt reached 0.95, with the squareness close to 1 and the magnetocrystalline anisotropy constant reaching 3 × 107 erg/cm3. This indicates that the ordered FePt film has a strong PMA effect. The plane-normal orbital-to-spin magnetic moment ratios are 0.088, 0.149 and 0.174 for the samples of Fe/Pt multilayer without annealing, after annealing at 500 °C and the ordered FePt film after 700 °C annealing, respectively. The results suggest that the higher order parameter results in a stronger hybridization and spin–orbital coupling between Fe and Pt atoms. Also, the higher orbital magnetic moment anisotropy implies that the plane-normal orbital-to-spin magnetic moment ratio increases.

Effect of TiON–MgO intermediate layer on microstructure and magnetic properties of L10 FePt–C–SiO2 films

The microstructure and magnetic properties of L10 FePt–C–SiO2 films grown on TiON–MgO intermediate layer were studied. TiON–MgO layer was deposited by co-sputtering TiN and MgO–TiO2 targets at 380°C. With increasing MgO–TiO2 doping concentration, the contact angle between FePt grains with intermediate layer gradually increased, and it was close to 90° when the volume percentage of MgO–TiO2 reached 30%. At this condition, a high out-of-plane coercivity of 19.1kOe was obtained, while the opening-up of in-plane M-H loop was very narrow. Moreover, it was found that the out-of-plane coercivity can be further improved to 21.6kOe, by slightly increasing the percentage of MgO–TiO2 to 35vol%.

Correlation between crystal planes and disordering of ordered L10 FePt structure caused by ion irradiation

In this study, we investigated the correlation between the crystal planes of L10 FePt and the structure disordering caused by ion irradiation. As part of the experiment, Ar ions were irradiated onto ordered L10 FePt films at several angles, corresponding to different crystal planes. The efficiency of the L10 FePt structure disordering depended on the areal atomic density of the crystal planes facing the ion irradiation direction. Thus, the efficiency of disordering on the (101) crystal plane, which has a lower areal atomic density, was higher than that of the (111) crystal plane, which has a higher areal atomic density. Based on the results of the dose dependence of the L10 structure disordering, the disordered regions seemed to propagate from the surfaces to the bottoms of the films.

Effect of Copper on the Formation of Ordered L10(FePt) Phase in Nanosized Fe50Pt50/Cu/Fe50Pt50 Films on SiO2/Si (001) Substrates

The effect from thickness of an intermediate copper layer in nanosized Fe50Pt50 (15 nm)/Cu (x)/Fe50Pt50 (15 nm) (x = 7.5, 15, and 30 nm) composite films on SiO2 (100 nm)/Si(001) substrates on the diffusion-controlled phase formation processes—transformation of the disordered magnetically soft A1(FePt) phase into the ordered magnetically hard L10(FePt) phase during annealing in vacuum—is studied by physical materials science methods: X-ray diffraction and measurement of magnetic properties. The A1(FePt) phase forms in all films during deposition. Annealing in vacuum in the temperature range 300–900°C is accompanied by thermally activated diffusion processes between the Cu and FePt layers. When thickness of the intermediate Cu layer increases from 7.5 nm up to 15 nm, the onset temperature of A1(FePt) → L10(FePt) phase transformation raises by 100°C, i.e., to 800°C. Simultaneously, the coercivity in films decreases since Cu dissolves in the FePt lattice.