FePt Alloy Films Research Articles

Effect of additional Au layer on ordering processes in FePt alloy films on and without substrates during high-temperature annealing

Effect of additional Au layer on ordering processes in FePt alloy films on and without substrates during high-temperature annealing

Comparison of microstructures and magnetic properties in FePt alloy films deposited by direct current magnetron sputtering and high power impulse magnetron sputtering

Single-layered FePt films of 30 nm thickness with random orientation are produced by direct current magnetron sputtering (DCMS) deposited directly onto Corning 1737 glass substrates and then post-annealed at 700 °C for 30 min. In films produced by high power impulse magnetron sputtering (HiPIMS) however, a strong (001) texture of L10 FePt film with isolated island structure is achieved and presents high perpendicular magnetic anisotropy after post-annealing under the same conditions. This is due to the compressive stress in as-deposited FePt films being enhanced greatly by HiPIMS. The films achieved by HiPIMS possess perpendicular coercivity, saturation magnetization and perpendicular squareness of 14.2 kOe, 620 emu/cm3 and 0.98, respectively, which reveal HiPIMS as a good technique to produce single-layered FePt films with high perpendicular magnetic properties.

Formation of $L1_{0}$ -FePt(001) Ultra-Thin Films With Flat Surfaces Using VC and VN Underlayers

FePt alloy films of 2 to 10 nm thicknesses are prepared on (001) single-crystal underlayers of MgO, VN, and VC by employing a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. MgO, VN, and VC crystals are selected as underlayer materials, since the crystals have the same NaCl-type crystal structure and similar lattice constants of about 0.42 nm but different surface energies. The single-crystal underlayers are prepared by RF sputtering on the single-crystal substrates of MgO and SrTiO3 at 600 °C. The influences of underlayer material and FePt film thickness on the $L1_{0}$ ordering degree, the magnetic properties, and the surface morphology are systematically investigated. Epitaxial films involving $L1_{0}$ (001) crystal structure with the $c$ -axis normal to the substrate surface are formed on all the underlayers. As the film thickness decreases, the ratio of out-of-plane to in-plane lattice constant $c/a$ decreases and the ordering degree $S$ increases. Stronger perpendicular magnetic anisotropy is observed for an FePt film with a higher $S$ value. The film thickness trends of $c/a$ , $S$ , and magnetic properties are similar to those for FePt films thicker than 5 nm formed on MgO underlayers. However, when the thickness is decreased to 2 nm, de-wetting of deposited atoms occurs on MgO underlayers during annealing at 600 °C. The surfaces of 2-nm-thick films formed on MgO underlayers are composed of isolated islands. On the contrary, continuous surfaces are realized for the investigated film thickness range of 2–10 nm in the cases of VC and VN underlayers. The surface roughness of FePt film formed on VN underlayer ( $R_{a}$ : 0.1–0.2 nm) is lower than that of film formed on VC underlayer ( $R_{a}$ : 0.1–0.4 nm). This paper shows that the employment of a VN underlayer is effective in the preparation of $L1_{0}$ ordered FePt ultra-thin film with flat surface.

Enhancement of L1 ordering with the c-axis perpendicular to the substrate in FePt alloy film by using an epitaxial cap-layer

FePt alloy thin films with cap-layers of MgO or C are prepared on MgO(001) single-crystal substrates by using a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. The FePt film thickness is fixed at 10 nm, whereas the cap-layer thickness is varied from 1 to 10 nm. The influences of cap-layer material and cap-layer thickness on the variant structure and the L10 ordering are investigated. Single-crystal FePt(001) films with disordered fcc structure (A1) grow epitaxially on the substrates at 200 °C. Single-crystal MgO(001) cap-layers grow epitaxially on the FePt films, whereas the structure of C cap-layers is amorphous. The phase transformation from A1 to L10 occurs when the films are annealed at 600 °C. The FePt films with MgO cap-layers thicker than 2 nm consist of L10(001) variant with the c-axis perpendicular to the substrate surface, whereas those with C cap-layers involve small volumes of L10(100) and (010) variants with the c-axis lying in the film plane. The in-plane and the out-of-plane lattices are respectively more expanded and contracted in the continuous-lattice MgO/FePt/MgO structure due to accommodations of misfits of FePt film with respect to not only the MgO substrate but also the MgO cap-layer. The lattice deformation promotes phase transformation along the perpendicular direction and L10 ordering. The FePt films consisting of only L10(001) variant show strong perpendicular magnetic anisotropies and low in-plane coercivities. The present study shows that an introduction of epitaxial cap-layer is effective in controlling the c-axis perpendicular to the substrate surface.

Relaxation effects of magnetization modulus and precession axis on damping time of precession in L10-ordered FePt alloy film

We investigate the relaxation effects of magnetization modulus and precession axis on the damping time of the precession in L10-ordered FePt alloy film using a time-resolved magneto-optical Kerr effect. It is revealed that their fast relaxation processes during the precession bring about the overestimation of the damping time of the precession. We found that this error becomes more significant as the pump intensity increases. We propose that these corrections need to be considered for materials with the fast relaxation dynamics during the precession.

Electrochemical Synthesis and Characterization of Fe-Ni-Pt Alloy Films

Fe-Pt alloy films and nanopatterns have been intensively investigated for magnetic recording applications, but their practical use may be limited by the maximum magnetic field available from the write head. Heat Assisted Magnetic Recording (HAMR)1 overcomes this limitation by locally heating the magnetic medium close at or above the Curie temperature to decrease its anisotropy, thus overcoming the limitations brought by the write field. The possibility to modulate magnetic anisotropy by tailoring composition on the other hand could provide close control over coercivity, which could then be matched to the available write field. Partial substitution of Ni for Fe, leading to Fe50-xNixPt50 alloys, could modulate the magnetic anisotropy in a wide range, varying ideally from the full Fe-Pt value (6.6 MJ/m3) down to zero. Fe-Ni-Pt films were grown using an electrolyte modified from a slightly alkaline citrate-glycinate electrolyte previously used for Fe-Pt synthesis2, with the addition of NiSO4. Electrodeposition was performed at 75°C onto Ru films sputtered on Si wafers; the thickness of Fe-Ni-Pt was kept between 10 and 80 nm. In order to achieve a wide range of Ni fractions in the alloy while maintaining a sufficiently fast deposition rate, it was necessary to use different Ni++ concentrations in the electrolyte (2- 20 mM). For a given [Ni++], the Ni fraction in the alloy generally decreased with increasing overvoltage. On the other hand, the Ni fraction at a given potential did not increase monotonously with [Ni++], suggesting the occurrence of an anomalous deposition process3. Annealing in forming gas (5% H2, 95% Ar) at 650°-700°C was necessary to observe the (100) reflection of the L10 phase, evidencing the phase transformation. The resulting films were unoriented and showed an approximately isotropic magnetic behavior. Film coercivity after annealing increased with Fe fraction, from about 2 to 10 kOe, demonstrating the possibility to tailor coercivity via composition. Finally, temperature-dependent magnetic measurements between 100-300 K showed a monotonous increase both in magnetization and coercivity, as expected.

Demagnetization dynamics of C-doped FePt film

Magneto-optical information storage has been a hot research subject for several years. FePt exhibits abundant physical properties and has received much attention as a candidate material. Its alloy film with perpendicular anisotropy and small grain size has important applications in magnetic recordings due to the large intrinsic magnetic anisotropy which ensures long-time thermal stability of nanometer sized bits. However, the large coercive field of FePt is a significant factor that hinders its application. As is well known, the magnetic anisotropy in FePt alloy can be precisely modulated by carbon-doping, and as a result, the coercive field of FePt film can be modified effectively with the carbon dopant. On the other hand, the microscopic mechanism of magnetic storage relies on the motion of spin system. Ultrashort femtosecond laser has been demonstrated to be a very effective tool to investigate the dynamical coupling among different degrees of freedom, such as electron, spin and lattice in a ferromagnetic film. The research on spin dynamics has become a new frontier of condensed matter physics, which is crucial for ultrafast magnetic recording materials. In this work, by using the time-resolved magneto-optical Kerr effect spectroscopy, we study the ultrafast spin dynamics of two FePt alloy films with different carbon dopants under the applied magnetic field along the film surface. The FePt alloy films with different carbon dopants are fabricated on silicon substrates by the sputtering method. The main experimental findings in this work are as follows. (i) The transient Kerr signal is linearly proportional to the magnetization with the magnetic field up to 0.8 T, while the transient reflectivity of the film is independent of the applied magnetic field. (ii) For FePt alloy films with different coercive fields, it is found that the demagnetization time of the film with smaller coercive field is significantly faster than that of the larger counterpart: the former shows 0.8 ps demagnetization time, and the latter has a magnitude of 1.2 ps. The demagnetization times for both soft and hard magnetic films are independent of the applied magnetic field. (iii) With ultrafast laser pulse radiation, we observe the propagation of acoustic phonon with a resonance frequency of ～ 49 GHz, and the frequency of the acoustic phonon is independent of the applied magnetic field. From the above, the spin dynamics of the samples shows strong correlation with carbon-doping. Our experimental findings are desired for basic research as well as for the design and development of novel magneto-optical devices.

高密度垂直磁存储FePt颗粒薄膜媒质的研究

本文采用磁控溅射法在硅基片上生长FePt颗粒薄膜。FePt膜层下面为MgO籽层，以此引发薄膜的L10相fct织构。在FePt薄膜中掺入C可减小其颗粒尺寸。采用X射线衍射仪(XRD)、超导量子干涉仪(SQUID)和透射电镜(TEM)对FePt颗粒进行表征，结果表明制备的样品具有优良的L10相结构，其磁滞曲线表明方形度很好，而且垂直矫顽力能有26 kOe，颗粒大小为8.3 nm。高分辨透射电镜表征表明其优良的fct超晶格结构。该磁性薄膜有望应用于下一代高密度磁存储媒质。 Magnetron sputtering was applied to grow FePt granular film on a silicon substrate. A MgO inter-layer was in-between the film and substrate to induce the L10 structure in the FePt alloy film with fct texture. The carbon addition helps reduce the grain size of the film. XRD, SQUID and TEM were applied to measure its structure, magnetic properties, and microstructures, respectively. Results show that the film has excellent L10 order, and the squareness of MH loop is close to unity, with a high perpendicular coercivity of 26 kOe. The microstructure shows that it has small grain size of 8.3 nm with uniform distribution. High-resolution TEM image shows extremely excellent super- lattice structure in the L10 phase. This film is expected to be a promising candidate for ultra-high density magnetic recording in the future.

(Invited) Electrodeposition of Fe-Pt Magnetic Films and Multilayers with Largemagnetic Anisotropy for Magnetic Recording and Microsystems

Alloys of Pt with metals of the Fe group form various phases with large magnetic anisotropy (above 1 MJ/m3), in particular the L10 phases of equiatomic Fe-Pt and Co-Pt. These alloys are promising candidates for permanent magnet components in magnetic MEMS or magnetic storage media. Electrochemical growth of magnetic films presents unique advantages, such as the close control of the driving force for film growth to finely tune properties via various deposition modalities, and the ability to form micro- or nano-structures of high aspect ratio via purely additive processes. This talk will discuss the electrochemical synthesis, structural characterization and magnetic properties of equiatomic Fe-Pt alloy films, highlighting the unique features afforded by electrochemical processing. As-deposited equiatomic Fe-Pt alloys are obtained by an underpotential codeposition process; these films exhibit a metastable face-centered cubic structure in the as-deposited condition, but formation of the tetragonal L10 phase and coercivities of up to 14 kOe can be achieved via thermal annealing of the films at only 450C [1, 2]. The composition of Fe-Pt alloys can be easily varied during growth by potential pulsing, enabling the formation of multilayers with a compositional gradient. Thermal annealing of Fe75Pt25/Fe50Pt50 bi- and multi-layers in forming gas results in a significant acceleration of the phase transformation to the L10 phase, enabling the formation of highly coercive films at lower annealing temperature. The quality of the multilayers is inferred by the observation of exchange-coupled behavior at about 5 nm sublayer thickness. [1] D Liang, JJ Mallett, G Zangari, ACS Appl Mater Interfaces 2 (4) 961-964 (2010); [2] D. Liang, S. Ge, G. Zangari, IEEE Trans Magn. DOI: 10.1109/TMAG.2015.2394326 (2015)

Control of <inline-formula> <tex-math notation="LaTeX">${\boldsymbol {c}}$ </tex-math></inline-formula>-Axis Orientation of <inline-formula> <tex-math notation="LaTeX">${\boldsymbol {L}}\textbf {1}_{\mathbf{0}}$ </tex-math></inline-formula> Ordered FePt, CoPt, and FePd Alloy Thin Films Deposited on MgO(001) Substrates

L1 0 films formed on (001)-oriented underlayers or substrates may involve (100) crystal with the c-axis lying in the film plane in addition to (001) crystal with the c-axis normal to the film surface. In this paper, FePt, CoPt, and FePd alloy films with L1 0 structure are prepared on MgO(001) single-crystal substrates. The c-axis orientation is accurately controlled to be perpendicular by introducing a thin MgO cap layer on magnetic films with disordered structure prepared by low-temperature deposition at 200 °C and by annealing the bilayer films at 600 °C. The phase transformation preferentially occurs along the perpendicular direction in the MgO/(FePt, CoPt, FePd)/MgO sandwich structures. Furthermore, the cap layer promotes L1 0 ordering. The order degrees of FePt, CoPt, and FePd films with and without cap layers, (S w/ , S w/o ), are (0.82, 0.58), (0.36, 0.30), and (0.72, 0.62), respectively. The FePt film with cap layer shows a strong perpendicular magnetic anisotropy and a very low in-plane coercivity because of which the film does not include L1 0 (100) crystal and possesses a high-order degree.

Perpendicular magnetization reversal mechanism of functional FePt films for magnetic storage medium

Magnetization reversal mechanism and related surface morphology of functional FePt(001) alloy films with large perpendicular magnetic anisotropy have been explored by alternate-atomic-layer deposition onto Pt/MgO(100) substrates via electron beam evaporation, and all evaporated films have been kept at in-situ substrate heating temperature of 400 °C. The FePt alloy film was composed of ultrathin [Fe (0.5 nm)/Pt (0.5 nm)]n Fe/Pt multilayer structures. The corresponding thickness of multilayer films was controlled by the periodic bilayer numbers (n) and varied in the range from 15 nm (n = 15) to 30 nm (n = 30). The surface topography was observed and varied from granular-like island to continuous microstructures with increasing the periodic numbers of Fe/Pt bilayer films. The measurement of angular dependent coercivity showed a tendency of the near rotation of reverse-domain type (n = 15) shift towards the domain-wall motion as a typical peak behavior (n = 30) with increasing the periodic bilayer numbers of Fe/Pt multilayers. On the basis of all magnetic measurements and corresponding magnetization analysis, indicating that the perpendicular magnetization reversal mechanism and related surface morphology of ordered FePt(001) alloy films could be systematically controlled by varying the periodic bilayer numbers accompanied with the thickness dependence.

The Effect of Dy-Additive on Phase Transformation and Magnetic Properties of Fe-Pt Alloy Films

A series of Fe-Pt based alloy films were deposited on glass substrates by DC magnetron sputtering. It is found that the Fe3Pt and FePt3 phases appear in Dy-addition films at first and then a composition reaction Fe3Pt + FePt3 FePt occurs at 500°C as annealing time increase. It suggested that Dy element can restrain the fct phase forming but help to form Fe3Pt and FePt3 phases. Coercivity and remanence ratio of DyxFe50-xPt50 alloy films annealed at 500°C for 200 h achieve the maximum 11kOe and 0.89 as x =1.5, respectively.

Quantitative Kinetic Models of the A1 to ${\rm L}1_{0}$ Transformation in FePt and Related Ternary Alloy Films

Quantitative kinetic models for the A1 to L10 transformation in FePt and related ternary alloy films are derived for three nucleation conditions for both isothermal and constant heating rate annealing. The nucleation conditions are: 1) continuous nucleation, 2) burst nucleation, and 3) pre-existing nuclei. For the first nucleation condition, new nuclei are formed continuously throughout the transformation. For the second nucleation condition under isothermal annealing, nucleation is constrained to occur at a constant rate over only a short period of time. In the limit of this time period going to zero, the nucleation sites are saturated very rapidly and nucleation can be considered to have occurred as a burst. For the third nucleation condition, the nuclei are pre-existing, having formed prior to the start of the transformation. No new nuclei form during the transformation. To identify the nucleation condition that best describes the transformation in FePt and related ternary alloy films, model predictions must be compared with experimental kinetic results.

FePt-C颗粒薄膜的制备和表征

本文采用磁控溅射法在热氧化硅基片上生长FePt颗粒薄膜。利用MgO籽层来引发FePt合金薄膜的fct织构，采取加入C的方法来减小其颗粒尺寸。采用X射线衍射仪(XRD)、超导量子干涉仪(SQUID)和透射电镜(TEM)对FePt颗粒进行表征，结果表明制备的样品具有优良的L10相结构，其磁滞曲线表明方形度很好，而且垂直矫顽力能有12 kOe，颗粒大小为8 nm。该磁性薄膜非常适合用做下一代高密度磁存储媒质，能大幅度提高磁存储密度。FePt-C granular thin film was fabricated on a thermally-oxidized silicon substrate by magnetron sputtering method. A MgO interlayer was applied to induce the L10 structure in the FePt alloy film. The addition of carbon can reduce the grain size of FePt. XRD, SQUID and TEM were applied to measure its structure, magnetic properties, and microstructures, respectively. Results show that the film has excellent L10 order, and the squareness of MH loop is close to unity, with a high per-pendicular coercivity of 12 kOe. The microstructure shows that it has small grain size of 8 nm with uniform distribution. This magnetic film is a promising candidate for magnetic recording media with ultra-high areal density.

FePd, FePt, and CoPt alloy epitaxial thin films with flat surface grown on MgO(111) substrate

FePd, FePt, and CoPt alloy epitaxial films of 40 nm thickness are prepared on MgO(111) singlecrystal substrates by employing two different methods. One is a one-step method consisting of hightemperature deposition at 600 °C and the other is a two-step method consisting of low-temperature deposition at 200 °C followed by annealing at 600 °C. Although the preparation method is different, similar final crystal structures are realized for all the film materials. FePd and FePt alloy films grow on the substrates with six L 10 (111) variants, whose c -axes are about 35° canted from the substrate surface and rotated around the film normal by 60° each other. The order degrees of FePd and FePt films prepared by one- and two-step methods, (S one-step , S two-step ), are estimated to be (0.25, 0.33) and (0.08, 0.15), respectively. On the contrary, L 10 ordered phase formation is not recognized for CoPt alloy films. The films prepared by one-step method have rough surfaces surrounded by side facets, whereas the films prepared by two-step method have very flat surfaces with the arithmetical mean roughness of 0.3 nm. The two-step method is useful for preparation of L 10 ordered films with flat surface.

Thickness dependence of the anomalous Hall effect in disordered face-centered cubic FePt alloy films

The anomalous Hall effect in disordered face-centered cubic (fcc) FePt alloy films is experimentally studied. The longitudinal resistivity independent term of the anomalous Hall conductivity (AHC) increases and approaches saturation with increasing film thickness. The contribution of side jump scattering is suggested to decrease monotonically with increasing film thickness, which can be ascribed to the variation of the surface scattering with the film thickness. The sign of the skew scattering contribution to the AHC is opposite to that of the intrinsic contribution in the system.

Magnetic force microscope tips coated with FePd, FePt, and CoPt alloy films

Magnetic force microscope (MFM) tips are prepared by coating Si tips of 4nm radius with FePd, FePt, and CoPt films of 80nm thickness at room temperature followed by annealing at 600°C. The spatial resolution and the switching field are investigated. The resolution of about 9nm is realized for all the tips. On the contrary, the switching fields of FePd-, FePt-, and CoPt-coated tips are 117.4, 37.8, and 268.6 kA/m, respectively. The crystal structure and the magnetic properties of coated magnetic thin films prepared on flat Si substrates are measured and the interrelationships with the MFM tip characteristics are briefly investigated.

Electrical control of fast ordering process in as-deposited non-perpendicular and perpendicular FePt films

Abstract Here, DC power supplies were applied on as-deposited non-perpendicular FePt single films or perpendicular FePt/Au multilayer films to control momentum exchanges between rapid moving electrons and Fe or Pt atoms. Moreover, lattice defects in the films provide abundant diffusion pathways for atomic rearrangement. Through the current driven atomic motion and rearrangement, fast LOP and hard magnet properties were realized in the FePt alloy films without heat treatment. The electromigration effect appears to be general and is applicable for the ordering in other magnetic materials with a L1 0 structure.

Electrode processes and in situ magnetic measurements of FePt films in a LiPF6 based electrolyte

Reversible control of magnetism at the metal/electrolyte interface offers great application potential for multifunctional magnetic devices. Here, the electrode–electrolyte interactions and the charging of FePt alloy films in LiPF6 in dimethyl carbonate(DMC)–ethylene carbonate (EC) are analysed in comparison to the pure metals Fe and Pt. The main electrode processes are Li deposition and dissolution, Fe dissolution and redeposition, and reduction as well as oxidation processes of impurities and electrolyte. Fe is found to be more stable against anodic dissolution in FePt films. Pt attenuates electrode passivation and catalyses impurity reduction reactions. This results in a cathodic potential limit of 1.8V vs. Li/Li+ for charging FePt films without side reactions. Subsequently, ultrathin textured FePt films promising large voltage induced magnetic surface effects where used as electrodes. Magnetic properties during charging were monitored by an in situ setup based on the anomalous Hall effect. Marginal Fe dissolution at the surface above 3V vs. Li/Li+ is detected by the in situ measurement and limits the upper charging potential. In consequence, a potential window of 1.2V for charging the FePt films is obtained. In this potential range a large reversible change of magnetic moment is detected, which is attributed to reversible redox processes in the native iron oxide surface layer.

Perpendicular magnetic anisotropy and spin reorientation transition in L1 FePt films

We investigated the thickness and composition dependence of perpendicular magnetic anisotropy (PMA) in L10 Fe1−xPtx (x = 0.4, 0.5, and 0.55) films. The FePt films with different thicknesses of 35 and 70 Å were grown at the substrate temperature Ts = 300 °C by molecular beam epitaxy coevaporation technique. A (001)-oriented epitaxial L10 FePt film was grown on the thin (001)-oriented fcc Pt layer, while a poorly crystallized FePt film was formed on the (111)-textured Pt layer. Our results showed that, at a fixed thickness of 70 Å, the PMA of FePt alloy films is enhanced as Pt content increases from 40% to 55%.