CoPt Films Research Articles

Enhanced Coercivity in $L1_{1}$ CoPt Thin Film on Glass Substrate by Fine-Tuning Pt Underlayer

Perpendicularly magnetized CoPt films with L11 rhombohedral lattice were deposited on glass substrates with a Pt underlayer. The results show that the magnetic properties of CoPt films are substantially affected by post-annealing time (ta) and temperature (Ta) of Pt underlayer, as well as the thickness of Pt underlayer (tPt). Soft magnetic phase is formed as CoPt is directly deposited on the glass substrate at 350 °C. Besides, a very low coercivity (Hc) of 103 kA/m is obtained when ta, Ta, and tPt are 5 min, 300 °C, and 20 nm, respectively. Further varying ta, Ta, and tPt to 15 min, 350 °C, and 25 nm increases Hc to about 207 kA/m. The microstructural studies indicate that the size of CoPt grain is the key factor to determine the magnetic properties, which could be controlled by the formation conditions of Pt underlayer. In this study, the optimum deposition conditions for Pt underlayer to obtain L11 CoPt phase with high Hc are ta = 15 min, Ta = 350 °C, and tPt = 20-25 nm. Our study demonstrates that using a Pt underlayer/glass substrate can effectively replace the single-crystal substrate and also enhance Hc of CoPt film, which may increase the application potential of L11 CoPt film in the future.

$L1_{0}$ Ordered FePd, FePt, and CoPt Thin Films With Flat Surfaces Prepared on MgO(110) Single-Crystal Substrates

FePd, FePt, and CoPt alloy thin films are prepared on MgO(110) single-crystal substrates by using two different methods. One is one-step method consisting of deposition at an elevated substrate temperature of 600°C and the other is two-step method consisting of low-temperature deposition at 200°C followed by annealing at 600°C. The crystal structure, the crystallographic orientation, and the surface morphology are compared. L10 ordered FePd, FePt, and CoPt films epitaxially grow on the substrates with three variants, (110), (101), and (011), when deposited at 600°C. The c-axis of L10(110) variant is lying in the film plane, whereas those of L10(101) and L10(011) variants are 45° canted from the substrate surface. Disordered FePd, FePt, and CoPt single-crystal films of (110) orientation are formed by deposition at 200°C. An A1-L10 phase transformation occurs along A1[001], A1[010], and A1[001] by annealing the disordered films at 600°C. The annealed films thus consist of three L10 variants. Similar final crystal structure and orientation are realized in the FePd, the FePt, and the CoPt films prepared by employing the two different methods. The L10 ordered films prepared by one-step method have island-like surfaces involving facets, whereas those prepared by two-step method have very flat surfaces with the arithmetical mean roughness lower than 0.3 nm. The two-step method is useful for preparation of L10 ordered films with flat surfaces.

Batch Patterning of SubMillimeter Features in Hard Magnetic Films Using Pulsed Magnetic Fields and Soft Magnetizing Heads

In this paper, we report a method to impart spatially defined magnetic patterns into hard magnetic films using pulsed magnetic fields. Differing from prior investigations on magnetic patterning, a single magnetizing head is used rather than a sandwich structure with two magnetizing heads. Arrays of 250μm-wide and 125μm-wide magnetic stripes with alternating out-of-plane polarity (perpendicular magnetization) are patterned in a 10μm-thick Co-rich Co-Pt alloy (Co80Pt20) film using three different microstructured, 2-mm-thick, Fe-Co magnetizing heads and pulsed magnetic fields ranging from 0.1 to 1 T. A magneto-optical imaging film is used to view the magnetization patterns at the surface of the Co-Pt film. Scanning Hall probe measurements of the resulting magnetic structures indicate a ~5mT peak magnetic field 100 m above the surface which compares favorably with numerical simulations of the field patterns at the same height.

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.

Electrochemically driven variation of magnetic properties in ultrathin CoPt films

Controlled variation of magnetism in ultrathin metallic films by external electric fields is a promising route towards advanced multifunctional devices. In situ magnetic measurements of ultrathin CoPt films in a liquid electrolyte (LiClO4 in dimethyl carbonate-ethylene carbonate) have been performed, dependent on the applied external voltage. Huge hysteretic changes of coercivity (above 200%) and smaller variations of saturation magnetisation (∼4 %) have been identified. The commonly considered concept of changing the electron density of states by double layer charging is not applicable as the changes observed are mainly irreversible. Rather, a strong influence of redox processes, namely surface oxide reduction and Co dissolution, is considered. We argue that by exploiting these redox reactions much higher changes of the magnetic properties are achievable than for double layer charging.

Microstructure study of CoPt thin film with phase change from A1 → L11 → A1 → L10

In this study, 50-nm-thick CoPt film was sputtered on MgO(111) substrate at substrate temperature (Ts) from room temperature (RT) to 750 °C. Within the temperature range, the phase evolves in the sequence of A1 (disordered) at RT, L11 (ordered), A1, and L10 (ordered) at 750 °C. L11-ordering occurred in the range of 250 °C ≦ Ts ≦ 500 °C induces strong perpendicular magnetic anisotropy. Microstructure results indicate that the CoPt grain size increases gradually from about 10 to 50 nm as Ts increases from RT to 600 °C. Grain size significantly increases to larger than 70 nm as Ts > 600 °C, which is attributed to the occurrence of L10-ordering. Further increase Ts to 750 °C, isotropic L10-(111) textured CoPt forms with a higher Hc of 5.0 kOe. Detailed discussion of relation between magnetic behaviors and microstructure is given.

Metastable L11 and B h Ordered Phase Formation in CoPt Alloy Thin Films Epitaxially Grown on Metal Underlayers

Co50Pt50 (at. %) alloy epitaxial thin films with the close-packed plane parallel to the substrate surface are prepared on fcc-Pt, fcc-Pd, fcc-Cu, bcc-Cr, hcp-Ti, and hcp-Ru underlayers hetero-epitaxially grown on MgO(111) single-crystal substrates at 300 {degree sign}C by using a radio-frequency magnetron sputtering system. Ordered phase formation is recognized for CoPt films grown on all the underlayers. The ordered phase is determined from a view-point of stacking sequence of close-packed plane. The CoPt films formed on fcc underlayers consist of L11 ordered phase, whereas those grown on bcc and hcp underlayers involve B h ordered phase in addition to L11 ordered phase. These ordered films show strong perpendicular magnetic anisotropies reflecting the magnetocrystalline anisotropies of L11- and/or B h-CoPt crystals.

Preparation ofL10ordered FePd, FePt, and CoPt thin films with flat surfaces on MgO(001) single-crystal substrates

FePd, FePt, and CoPt alloy thin films are prepared on MgO(001) single-crystal substrates by employing a two-step method consisting of sputter deposition at low-temperature (room temperature) followed by high-temperature annealing at 600 °C. FePd, FePt, and CoPt single-crystal films with disordered structure (A 1) grow epitaxially on MgO substrates at room temperature. By annealing these disordered films at 600 °C, a transformation from A 1 to L 10 -ordered phase occurs. The order degrees of annealed FePd, FePt, and CoPt films are 0.63, 0.38, and 0.16, respectively. The films before annealing have very flat surfaces and the flat surfaces are kept after annealing. The arithmetical mean surface roughness values of annealed FePd, FePt, and CoPt films are 0.3, 0.1, and 0.2 nm, respectively. The two-step method is considered to be useful for preparation of FePd, FePt, and CoPt alloy thin films with flat surfaces, which are necessary for practical applications.

Metastable Ordered Phase Formation in CoPt and Co<sub>3</sub>Pt Alloy Thin Films Epitaxially Grown on Single-Crystal Substrates

CoPt and Co3Pt alloy thin films are prepared on MgO(111), SrTiO3(111), and Al2O3(0001) single-crystal substrates by varying the substrate temperature in a range from room temperature to 600°C by using an ultra-high vacuum radio-frequency magnetron sputtering system. The formation of metastable ordered phase and the structural thermal stability are briefly investigated. CoPt and Co3Pt films with the close-packed plane parallel to the substrate surface grow epitaxially on these oxide single-crystal substrates. CoPt epitaxial films are also formed by employing Pt, Pd, Cu, Cr, Ti, and Ru underlayers hetero-epitaxially grown on MgO(111) substrates. The crystal structure is evaluated by considering the order degree and the atomic stacking sequence of close-packed plane. Metastable ordered phases of L11, Bh, and D019 are preferentially formed in the CoPt and the Co3Pt films deposited around 300°C. Metastable ordered phase formation is influenced by the substrate temperature, the film composition, and the underlayer material. With increasing the substrate temperature up to around 300°C, the order degree increases. As the substrate temperature further increases, the order degree decreases. Annealing a disordered film at 300°C does not effectively enhance ordering. The CoPt and the Co3Pt films which include metastable ordered phases have flat surfaces and show strong perpendicular magnetic anisotropies reflecting the magnetocrystalline anisotropies of ordered crystals.

Thermal stability ofL11ordered phase in CoPt-alloy thin films formed on MgO(111) single-crystal substrates

Metastable L 11 ordered CoPt(111) epitaxial thin films with the order degrees around 0.3 are formed on MgO(111) single-crystal substrates at 300 °C by using an ultra-high vacuum radio-frequency magnetron sputtering system. The effects of annealing after film formation on the structure and the magnetic properties are investigated. As the annealing temperature increases, the order degree gradually decreases. At 600 °C, the ordered phase disappears and the film changes to disordered structure. CoPt(111) epitaxial films with disordered structure are also prepared at room temperature and 600 °C and then these films are annealed at 300 °C. However, ordered phase formation is not recognized in the films. The metastable L 11 ordered phase is preferentially formed when the film is deposited at a temperature of 300 °C. Annealing a film with disordered structure at 300 °C does not promote L 11 ordering. The CoPt films involving L 11 ordered structure show perpendicular magnetic anisotropies, whereas the disordered films have in-plane anisotropies. The magnetic properties are influenced by the order degree.

Fabrication of 30-nm-Pitched CoPt Magnetic Dot Arrays Using 30-keV-Electron Beam Lithography and Ion Milling for Patterned Media

CoPt magnetic dot arrays with a fine pitch of 30 nm have been fabricated using electron beam (EB) lithography and ion milling. The possibility to ion-mill CoPt film using EB drawn calixarene resist pattern as a mask has been studied. We formed 30 nm pitch resist dot arrays with a dot diameter of 20 nm using 30-keV-EB lithography with calixarene resist. The resist dot arrays were ion-milled for 4 min using 200-eV Ar ion milling to fabricate CoPt dot arrays on a Si substrate. We fabricated fine pitched CoPt magnetic dot arrays with a diameter of 22-35 nm and a pitch of 30-150 nm. Results show that the ion-milled CoPt dot diameter increased with the dot pitch while the resist dot had a similar diameter of 20 nm.

Structure and Magnetic Properties of CoPt, CoPd, FePt, and FePd Alloy Thin Films Formed on MgO(111) Substrates

CoPt, CoPd, FePt, and FePd alloy thin films are deposited on MgO(111) single-crystal substrates by using a radio-frequency magnetron sputtering system. The effects of magnetic material and substrate temperature on ordered phase formation are investigated. CoPt, FePt, and FePd epitaxial films with the close-packed plane parallel to the substrate surface are obtained at substrate temperatures ranging between room temperature (RT) and 600 °C. CoPd films grow epitaxially at temperatures higher than 200 °C, whereas CoPd amorphous films are formed below the temperature. Metastable L11 ordered phase formation is recognized for the CoPt films prepared at temperatures ranging between RT and 400 °C. As the temperature increases from RT to 300 °C, the order degree of L11 phase increases. With further increasing the temperature, the degree declines. The CoPt films formed above 500 °C are with A1 disordered structure. L10 ordered FePt and FePd films with the c-axis 55 ° canted from the perpendicular direction are obtained at 600 °C, whereas A1 disordered FePt and FePd films are formed below the temperature. No ordered phase is formed in CoPd film. The L11 ordered CoPt films show perpendicular magnetic anisotropies. As the order degree of L11 phase increases, perpendicular anisotropy is enhanced. On the contrary, the L10 ordered FePt and FePd films have in-plane anisotropies. The magnetic properties are influenced by the kind of ordered phase and the order degree.

Preparation of CoPt-Alloy Thin Films With Perpendicular Magnetic Anisotropy on MgO(111), ${\rm SrTiO}_{3}(111)$, and ${\rm Al}_{2}{\rm O}_{3}(0001)$Single-Crystal Substrates

Co50Pt50 (at. %) alloy thin films are prepared on MgO(111), SrTiO3(111), and Al2O3(0001) single-crystal substrates by ultra-high vacuum radio-frequency magnetron sputtering by varying the substrate temperature from room temperature to 600°C . Effects of substrate material and substrate temperature on the crystallographic and magnetic properties are investigated. CoPt-alloy epitaxial thin films with the close-packed plane parallel to the substrate surface are obtained on MgO and SrTiO3 substrates. On Al2O3 substrates, CoPt epitaxial films are obtained at temperatures higher than 500°C, whereas CoPt films consisting of epitaxial and polycrystalline crystals are formed below 400°C. From the view points of order degree and stacking sequence of close-packed plane along the perpendicular direction, the crystal structure is determined. L11 ordered phase is preferentially formed and is stabilized around 300°C for all the employed substrate materials. A higher order degree is observed for the films prepared on MgO >; SrTiO3 >; Al2O3. The magnetic properties are influenced by the crystal structure and the order degree.

Electrochemically deposited cobalt/platinum (Co/Pt) film into porous silicon: Structural investigation and magnetic properties

A nanostructured CoPt magnetic film was deposited from a single electrolyte into porous silicon layer by an electrochemical technique, followed by annealing at 600°C in Ar atmosphere during which the CoPt alloy was converted to L10 ordered phase. Porous silicon with pore diameter between 5 and 100nm was firstly fabricated by galvanostatic anodization of n-type silicon wafer in the presence of CrO3 as oxidizing agent and ethanol or sodium lauryl sulfate as surfactants. The role of the surfactant on the produced pore size and morphology was investigated by means of UV–vis spectra. As-formed porous silicon was consequently used as a template for the electrodeposition of magnetic CoPt film. The phase formation, microstructure and the magnetic properties were fully analyzed by XRD, FE-SEM, EDS and VSM measurements. It was found that, upon annealing the coercivity was significantly increased due to the transformation to the L10 ordered structure. The saturation magnetization and remanence ratio were also found to increase, indicating no loss of Co content or oxidation reaction after the annealing. Results of synthesis and characterization of CoPt/porous silicon nanocomposite are addressed and thoroughly discussed.

Reversible Change of Magnetism in FePt and CoPt Films by Electrochemical Charging

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Curie temperatures of CoPt ultrathin continuous films

The effects of layer thickness and thermal annealing on Curie temperature have been studied for CoPt ultrathin continuous layers in AlN/CoPt multilayer structures. It is found that there exists a critical thickness below which Curie temperature rapidly decreases due to the loss of spin-spin interactions in the vicinity of interface. After high temperature annealing, the in-plane lattice constant of CoPt film is increased and the exchange coupling parameter is decreased. Consequently, Curie temperatures decrease for some films with large thickness, compared with as-deposited state. Upon annealing at 600 ∘C, CoPt undergoes ordering transformation, which also contributes to the degradation of the Curie temperature. However, when the CoPt film thickness is below 2 nm, the Curie temperature is increased after annealing. Especially for 1 nm thick film, the Curie temperature is strikingly increased from 173 ∘C to 343 ∘C after annealing at 600 ∘C. This effect is attributed to the out-of-plane lattice deformation of CoPt thin layers in AlN/CoPt multilayer structures.

L1 ordered phase formation in FePt, FePd, CoPt, and CoPd alloy thin films epitaxially grown on MgO(001) single-crystal substrates

The FePt, FePd, CoPt, and CoPd epitaxial thin films are prepared on MgO(001) single-crystal substrates by ultrahigh vacuum RF magnetron sputtering. The effects of the magnetic material and the substrate temperature on the film growth, the film structure, and the magnetic properties are investigated. The L10 ordered phase formation is observed for FePt, FePd, and CoPt films prepared at temperatures higher than 200, 400, and 600 °C, respectively, whereas that is not recognized for CoPd films. The L10-FePd(001) single-crystal films with the c-axis normal to the substrate surface are formed, whereas the FePt and CoPt epitaxial films include L10(100) crystals whose c-axis is parallel to the substrate surface, in addition to the L10(001) crystals. Upon increasing the substrate temperature, the ordering degree increases. A higher ordering parameter is observed in the order of FePd > FePt > CoPt. The magnetic properties are influenced by the crystal structure, the crystallographic orientation of the L10 crystal, and the ordering degree.

Temperature‐Driven Spin Reorientation Transition in CoPt/AlN Multilayer Films

Spin reorientation transition phenomena from out‐of‐plane to in‐plane direction with increasing temperature are observed for the 500°C annealed CoPt/AlN multilayer films with different CoPt layer thicknesses. CoPt‐AlN interface and volume anisotropy contributions, favoring out‐of‐plane and in‐plane magnetization, respectively, are separately determined at various temperatures. Interface anisotropy exhibits much stronger temperature dependence than volume contribution, hence the temperature‐driven spin reorientation transition occurs. Interface anisotropy in this work consists of Néel interface anisotropy and magnetoelastic effect. Magnetoelastic effect degrades rapidly and changes its sign from positive to negative above 200°C, because of the involvement of stress state in CoPt films with temperature. By contrast, Néel interface anisotropy decays slowly, estimated from a Néel mean field model. Thus, the strong temperature dependence of CoPt‐AlN interface anisotropy is dominated by the change of magnetoelastic effect.

Effect of Ag Additive on Magnetic Properties and Microstructure of the Copt-Sinx/Ag Granular Thin Films

Abstract When the Ag layer was introduced under the CoPt alloy film annealed at 700 °C, the CoPt/Ag films had great out-of-plane squareness (S ⊥ ), out-of-plane coercivity (H c⊥ ), and saturation magnetization (M s ). The SiN x ceramic materials were co-sputtered with CoPt on the Ag under layer then annealed to reduce the grain size of CoPt films. The particle size of CoPt was about 10 nm with 46.2 vol. % SiN x content. Furthermore, Ag was added into the CoPt-SiN x layer of CoPt-SiN x /Ag films to reduce the ordering temperature of L1 0 CoPt (fct phase) films.

전기도금법으로 제작한 두께가 다른 CoPt 자성막의 자화역전과 자기역전 부피

두께가 다른 CoPt 자성막을 전기도금법으로 제작하여 자화역전 기구와 자기상호작용이 자기역전 부피에 미치는 영향을 연구하였다. 자기모멘트 감쇠의 시간의존성과 잔류자기화 곡선 측정결과로부터 모든 시료의 지배적인 자화역전 기구는 자벽고착에 의한 자벽이동임을 알 수 있었다. 자기점성계수 및 비가역 자화율로부터 구한 자기역전 부피는 시료의 두께 및 인가 자기장의 세기에 거의 무관하게 일정하였으나, 자기역전 부피로부터 구한 역전 직경의 크기는 두께가 두꺼울수록 감소하는 경향을 보였다. 이와 같은 현상은 자벽고착에 의해 제어되는 자벽운동과 쌍극자 상호작용에 의한 것으로 판단된다. The thickness dependence of the magnetic switching volumes in electrodeposited CoPt films was investigated from the magnetization reversal and the magnetic interaction behavior. As the sample thickness is increased, the field difference between the wall pinning field (<TEX>$H_{DW}$</TEX>) and the nucleation field (<TEX>$H_N$</TEX>) as well as the absolute value of <TEX>${\Delta}$</TEX>area are increased. Therefore, the decrement tendency of the switching diameter with increasing sample thickness can be well explained by the domain wall motion controlled by the domain wall pinning and the strength of dipolar interaction.