Dependence of microstructure and magnetic properties of FePt films on Cr90Ru10 underlayers

The effect of the thickness of Cr₉₀Ru₁₀ underlayers on the microstructure and magnetic properties of FePt films has been studied. Experimental results showed that the FePt films grown on the Cr₉₀Ru₁₀ underlayers exhibited a (0 0 1) preferred orientation with out-of-plane magnetic anisotropy. The degree of the FePt(0 0 1) preferred orientation was closely related to that of the Cr(0 0 2) preferred orientation. The degree of ordering of the FePt films increased with the thickness of the Cr₉₀Ru₁₀ underlayers. The angular dependence of the coercivity curves suggested that an incoherent curling rotational mode be a dominant magnetic reversal mechanism in the FePt films at various Cr₉₀Ru₁₀ underlayer thicknesses.     

Use of film thickness and Cu additive to improve (0 0 1) texture in MgO/FePtCu(C) bilayers

A multilayer structure has been proposed that demonstrates improved (0 0 1) texture for FePt-based L1₀ perpendicular media. Achieving a strong perpendicular magnetic anisotropy requires aligning the L1₀ crystallographic c-axis along the film normal. The ordered L1₀ FePt structure is tetragonal with a c/a ratio close to 0.965. This makes discriminating between the three crystallographic variants ([1 0 0], [0 1 0], and the desired [0 0 1]) difficult. Alloying FePt with Cu to reduce the c/a ratio and using a multilayer approach to keep the magnetic layers thin results in a structure with an adjustable M_rt and a strong (0 0 1) texture (rocking curve widths around 2°). This is a remarkable improvement in texture from pure FePt multilayered films or monolithic FePt(X) films. The proposed [MgO(2 nm)/Fe_50−xPt₅₀Cu_x(5 nm)]_×n structure limits grain size in the vertical (perpendicular) direction albeit not in the plane of the film. Carbon can be added to the FePtCu layer to reduce the grain size with minimal degradation of the (0 0 1) orientation.     

Barrier height imaging of magnetic films: Use for studying the initial growth of Co films and the surface structure of FePt films

Barrier-height (BH) imaging using scanning tunneling microscopy (STM) was used to study the growth of Co films on Au(001) surfaces. We have observed BH of metastable bcc Co film (> 1 ML) for the first time, and that showed a large BH value (~ 6 eV), whereas the observed BH of the Au(001) surface (~ 3.5 eV) was consistent with the previous results. The origin of the large BH was qualitatively understood by considering that 3d electrons for tunneling are dominant for the Co(001) surface. We have observed numerous islands with different sizes and heights after 0.15 ML Co coverage and successfully obtained, from the BH imaging, an element-specific contrast, i.e. recognizing aggregated Au islands and Co islands, and information about inhomogenities of BH with proper consideration of the artifacts near the step edges. The height modification by the large BH difference is discussed.STM/BH studies of FePt films revealed two kinds of monolayer heights, the sum which was equal to the c axis lattice constant of L1₀ FePt. Two different dI/dz signal levels were observed on atomically flat terraces.     

Microstructure refinement and significant improvements of magnetic properties in Pr2Fe14B/α-Fe nanocomposites

Exchange coupled (Pr,Tb)₂(Fe,Nb,Zr)₁₄B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c-axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10 m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17 m/s and annealed at 700°C for 20 min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6 kOe and 14.7 MGOe for the (Nb,Zr)-free sample to 8.2 kOe and 20.3 MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20 nm, leading to a high remanence of 0.73 M_s. Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.     

Rare earth free exchange spring magnet FeCo/FePt(001): Giant magnetic anisotropy and energy product

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the thickness dependent magnetic properties of rare earth free exchange spring magnet FeCo/FePt(001). The FeCo adlayer thickness is increased from one monolayer (ML) to four ML coverage. It is observed that the FeCo adlayers and Fe atoms in FePt substrate show almost half metallic behavior, while an ordinary metallic feature is found in Pt atoms. The average magnetization increases with FeCo thickness and the estimated maximum energy product reaches 66 MGOe in FeCo(4 ML)/FePt(001). A giant perpendicular magnetocrystalline anisotropy (MCA) energy of 18.20 meV/cell is found in pure FePt(001) and it becomes 17.35 meV/cell even in FeCo(4 ML)/FePt(001). In addition, we find very large coercivity field in FeCo/FePt(001) systems. For instance, the calculated maximum coercivity field in FeCo(4 ML)/FePt(001) is about 188 kOe. Both energy product and coercivity field calculations may imply that the FeCo/FePt can be utilized for potential rare earth free exchange spring magnet material.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Giant magneto-caloric effect around room temperature at moderate low field variation in La0.7(Ca1−xSrx)0.3MnO3 perovskites

Among the perovskite manganites, a series of La_1?xCa_xMnO₃ has the largest magneto-caloric effect (MCE) (|ΔS_m|_max=3.2–6.7 J/kg K at ΔH=13.5 kOe), but the Curie temperatures, T_C, are quite low (165–270 K). The system of LaSrMnO₃ has quite high T_C but its MCE is not so large. The manganites La_0.7(Ca_1?xSr_x)_0.3MnO₃ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25) have been prepared by solid state reaction technique with an expectation of large MCE at room temperature region. The samples are of single phase with orthorhombic structure. The lattice parameters as well as the volume of unit cell are continuously increased with the increase of x due to large Sr²⁺ ions substituted for smaller Ca²⁺ ions. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic measurements at low field and low temperatures indicate that there is a spin-glass like (or cluster glass) state occurred. The Curie temperature T_C increases continuously from 258 K (for x=0) to 293 K (for x=0.25). A large MCE of 5 J/kg K has been observed around 293 K at the magnetic field change ΔH=13.5 kOe for the sample x=0.25. The studied samples can be considered as giant magneto-caloric materials, which is an excellent candidate for magnetic refrigeration at room temperature region.     

Hot-pressed and die-upset Pr-Co magnets produced from mechanically activated alloys

Anisotropic die-upset PrCo₅-based magnets were produced by hot pressing and subsequent die-upsetting of alloy powders activated by high-energy ball milling. In addition to the PrCo₅ major phase the magnets contained Pr₂Co₁₇, even when prepared from a stoichiometric 1:5 alloy. Hard magnetic properties can be improved by partial Sm substitution for Pr and partial Cu substitution for Co. A higher cobalt/rare-earth ratio as well as the addition of Fe was found to be unfavorable for the deformation-induced anisotropy. The largest values of remanence, intrinsic coercivity and maximum energy product in hot-pressed magnets were obtained for Pr_0.7Sm_0.3Co_5.5 (7.8 kG, 14.1 kOe and of 13.1 MGOe) and in die-upset magnets for Pr_0.9Sm_0.1(Co_0.98Cu_0.02)₅ (9.2 kG, 10.5 kOe and 16.7 MGOe).     

Superconducting characteristics and microstructure of polycrystalline Zn-doped In2O3 films

In order to investigate the relation among the superconducting transition T_c, carrier density n, resistivity ρ and the microstructure in the polycrystalline (In₂O₃)_1?x–(ZnO)_x films, we prepared specimen films by post annealing of amorphous films with x = 0.025 at various annealing temperature T_a and for annealing time t_a = 1 h and 4 h. As for microstructures, we have investigated the distribution of elements by scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). We have found followings: (1) The annealed films clearly show the superconductivity of which T_c depends on T_a, t_a and n. This indicates that the superconductivity is determined by the combination of crystallinity and carrier density. (2) The data on STEM–EELS spectra mapping of indium plasmon indicate that droplets of the pure indium phase exist inside a film, where the distribution of these droplets dispersed. Therefore, it seems that droplets do not form an electrical conducting path, that is, it is possible that observed superconductivity is due to intrinsic characteristic of polycrystalline (In₂O₃)_1?x–(ZnO)_x films.     

Thickness dependence of Ic and Jc of LTG-SmBCO coated-conductor on IBAD-MgO tapes

We have reported SmBa₂Cu₃O_y (SmBCO) films on single crystalline substrates prepared by low-temperature growth (LTG) technique. The LTG-SmBCO films showed high critical current densities in magnetic fields compared with conventional SmBCO films prepared by pulsed laser deposition (PLD) method. In this study, to enhance critical current (I_c) in magnetic field, we fabricated thick LTG-SmBCO films on metal substrates with ion-beam assisted deposition (IBAD)-MgO buffer and estimated the I_c and J_c in magnetic fields.All the SmBCO films showed c-axis orientation and cube-on-cube in-plane texture. T_c of the LTG-SmBCO films were 93.1–93.4 K. J_c and I_c of a 0.5 μm-thick SmBCO film were 3.0 MA/cm² and 150 A/cm-width at 77 K in self-field, respectively. Those of a 2.0 μm-thick film were 1.6 MA/cm² and 284 A/cm-width respectively. Although I_c increased with the film thickness increasing up to 2 μm, the I_c tended to be saturated in 300 A/cm-width. From a cross sectional TEM image of the SmBCO film, we recognized a-axis oriented grains and 45° grains and Cu–O precipitates. Because these undesired grains form dead layers, I_c saturated above a certain thickness. We achieved that I_c in magnetic fields of the LTG-SmBCO films with a thickness of 2.0 μm were 88 A/cm-width at 1 T and 28 A/cm-width at 3 T.     

Structural and magnetic properties of HCP-CoCrPt–SiO2 granular media

Granular HCP-(CoCrPt)_100−x(SiO₂)_x thin films with Cr underlayers have been fabricated by sputtering multilayers followed by post-deposition annealing. Magnetic and structural properties of the films for potential applications in magnetic recording media have been investigated in detail. In as-deposited films coercivities exceeding 2.5 kOe have been obtained with SiO₂ varying from 8 to 16 vol%; high coercivity of 5.6 kOe and anisotropy of 4.6×10⁶ erg/cm³ have been achieved at low M_rt value (about 0.4 memu/cm²) in the post-annealed films. VSM measurements showed that the magnetic moment lies well in the film plane under proper preparation conditions. Grain isolation in the magnetic layer was improved by segregating SiO₂ into grain boundaries and further enhanced by post-deposition annealing. The rapid increase of the coercivity upon annealing is most likely due to the significant decrease in intergranular exchange coupling, as shown by the δM measurement in which the peak value of δM curves changed from a positive value to a negative value upon annealing. Magnetic reversal properties of the films have also been systematically studied. These results show that the HCP-CoCrPt–SiO₂ granular film is a promising candidate for ultra-high-density recording media up to 100 Gbit/in² or beyond because of its low Pt content and desirable properties.     

Effect of Boron substitution on the superconductivity of non-oxide perovskite MgCNi3

We report synthesis, structural and magnetic (DC and AC) properties of Boron substituted MgCNi₃ superconductor. A series of polycrystalline bulk samples Mg_1.2C_1.6?xB_xNi₃ (x=0.0, 0.08 and 0.16) is synthesized through standard solid-state reaction route, which are found to crystallize in cubic perovskite structure with space group Pm3m. Rietveld analysis of observed XRD data show that lattice parameters expand from a=3.8106 (4) Å for pure, to 3.8164 (2) Å and 3.8173 (5) Å for 5% and 10% Boron substituted samples respectively. DC magnetization exhibited superconducting transition (T_c) at around 7.3 K for pure sample, and the same decreases slightly with Boron substitution. The lower critical field (H_c1) at 2 K is around 150 Oe for pure sample, which increases slightly with Boron substitution. For pure sample the upper critical field (H_c2) being determined from AC susceptibility measurements is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively, which decreases to 5.57 kOe and 42.5 kOe respectively for 10% Boron substituted sample. 10% Boron substitution at Carbon site has decreased both the H_c2 and T_c. On the other hand lower critical field (H_c1) at 2 K is slightly increased from around 150 Oe for pure sample, to 200 Oe for 10% Boron substituted sample. Seemingly, the Carbon site Boron substitution induced disorder though has increased slightly the H_c1 but with simultaneous decrease in superconducting transition temperature (T_c) and upper critical field (H_c2). The high relative proportion of Ni in studied MgCNi₃ suggests that magnetic interactions are important and non-oxide perovskite structure make it interesting.     

High coercivity Nd2(Fe,Co)14C-type ribbons prepared by melt spinning

Melt-spun Nd₁₃Dy₂Fe_77−xCo_xC₆B₂ (x=0, 5, 10, 15, 20) ribbons with a high coercivity more than 2 T have been obtained. It was found that the ribbons quenched at the optimum wheel speed 15 m/s (as-spun ribbons) mainly consist of ferromagnetic 2 : 14 : 1 phase and paramagnetic NdC₂ phase, and the ribbons spun at 25 m/s and subsequently annealed at 973 K for 15 min (as-annealed ribbons) are primarily composed of the magnetic 2 : 14 : 1 and 2 : 17 phases. The magnetization process of as-spun ribbons controlled by a pinning of the domain wall is different from that of as-annealed ribbons determined by a nucleation of the reverse domain. This significant difference originates possibly from the existence of paramagnetic NdC₂ phase acting as a pinning center in as-spun ribbons. In the as-annealed ribbons, the substitution of Co for Fe leads to increase of remanence (μ₀M_r), maximum energy product ((BH)_max) from 0.67 T, 9.7 MGOe for x=0 to 0.84 T, 14.4 MGOe for x=10, respectively. A coercivity of 2.74 T is obtained for as-quenched Nd₁₃Dy₂Fe_77−xCo_xC₆B₂ (x=0) ribbons.     

Structure and magnetic properties of co-sputtered Co–C thin films

We studied the structure and magnetic properties of co-sputtered Co_1−xC_x thin films using a transmission electron microscope (TEM) and a SQUID magnetometer. These properties were found to depend critically on deposition temperature, T_S, and composition, x. Generally, phase separation into metallic Co and graphite-like carbon phases proceeds with increasing T_S and decreasing x. Plan view and cross-sectional TEM images of the films prepared showed that Co grains about 10–20 nm in diameter and 30–50 nm in height are three-dimensionally separated by graphite-like carbon layers 1–2 nm thick. Optimum magnetic properties with saturation magnetization of 380 emu/cc and coercivity of 400 Oe were obtained for a film with x=0.5 and T_S=350°C.     

Homoepitaxial growth of MOD-YBCO thick films on evaporated and MOD templates

We have prepared metal organic deposition (MOD)-YBCO thick films by repeating the coating-pyrolysis-crystallization procedure onto ～100-nm-thick evaporated and MOD templates. Surface morphology of the template was found to strongly affect the homoepitaxial growth of MOD-YBCO layers on the template; namely, the epitaxial growth of MOD-YBCO on the evaporated template was much easier than that on the MOD template. A 220-nm-thick epitaxial MOD-YBCO film was successfully prepared on the 100-nm-thick evaporated-YBCO template to obtain a 320-nm-thick YBCO film, which exhibited J_c = 2.44 MA/cm² and I_c = 78 A/cm. The I_c value has significantly increased from 37 A/cm for the evaporated-template.     

Dielectric properties of PIN–PT ceramics under compressive stress

Lead indium niobate, Pb(In_1/2Nb_1/2)O₃ or (PIN), is an interesting ferroelectric material, because it can be changed from a disordered state to ordered state by long-time thermal annealing. However, the temperature related to the maximum dielectric constant (T_max) of PIN in relaxor phase is low (at 1 kHz, T_max = 66 °C). In this study, to increasing T_max of PIN, lead titanate, PbTiO₃(PT) was thus added to PIN with compositions (1 − x)PIN–xPT (for x = 0.1–0.5). The influence of stress on the dielectric properties of (1 − x)PIN–xPT ceramics was then investigated. The dielectric properties were measured under various uniaxial compressive stress up to 400 MPa. The results showed that the superimposed compression load reduced the dielectric constant in 0.9PIN–0.1PT. For the other compositions, the dielectric constants first increased with the compressive stress, and then decreased when the stress was further increased up to 400 MPa. The dielectric loss tangent of all composition was found to decrease with increasing compressive stress.     

A high (0 0 1)-oriented CoPt/Ag film deposited on glass substrate

The magnetic properties and structures of CoPt_15 nm/Ag_0−100 nm films deposited by DC magnetron sputtering on glass substrates have been studied. The (0 0 1) texturing was improved by introducing an Ag underlayer. As the Ag underlayer thickness was 100 nm, a nearly perfect (0 0 1)-textured CoPt film was obtained.     

Structural and magnetic properties of CoCrPt perpendicular media grown on different buffer layers

The influence of buffer layer characteristics on the structural and magnetic properties of CoCrPt perpendicular media has been investigated. Thin (∼10–15 nm) buffer layers consisting of Ta/Ru, Ta/Hf, or amorphous (CoCrPt)Ta₂₅ produced media layers having high perpendicular coercivity of nearly 3 kOe, but the coercivity was only 1.7 kOe when using a Ta/Ti buffer. X-ray diffraction rocking curves showed the highest degree of (0 0 0 2) texture for the Ta/Ru buffer. In-plane diffraction indicated that the Ta/Ru buffer also had the smallest lattice mismatch (5.6%) with the CoCrPt. Cross-sectional transmission electron microscopy showed that the Ta/Ru buffer promoted local epitaxy with the media layer. Amorphous transition regions were observed at the interfaces between the media and the Ta/Hf and Ta/Ti buffer layers. Some small CoCrPt grains were observed at the interface with the amorphous CoCrPtTa buffer.     

Magnetic and electrical properties of Gd7Rh3 single crystal

Magnetic and electrical properties of hexagonal Gd₇Rh₃ single crystals have been studied by measuring magnetization, magnetic susceptibility, and electrical resistivity. Gd₇Rh₃ shows antiferromagnetic order below T_N=141 K as reported by Loebich et al; magnetic anisotropy is small in paramagnetic region. Metamagnetic transitions were observed at 4 K in the external magnetic field up to 130 kOe along the c-axis and in the c-plane. Electrical resistivity shows a characteristic hump just below T_N due to the super-zone gap formation along the new Brillouin zone boundaries. High-temperature resistivity indicates that Gd₇Rh₃ has a semi-metallic band structure; the band gap energy 4.9 meV was obtained.     

Magnetic properties of nanocrystalline Fe86.7Zr3.3B4Ag6 thin film

The changes of magnetic properties with annealing temperature were studied in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film. The thin films were deposited by a DC magnetron sputtering method, annealed at 300–700°C for 1 h in vacuum under a field of 1.5 kOe parallel to the film plane, and then furnace-cooled. As a result, it has been found that the Ag addition to Fe–Zr–B amorphous thin films resulted in the decrease of crystallization temperature to 400°C due to promoted crystallization ability. Also, it gave rise to formation of fine BCC α-Fe crystalline precipitates with a grain size smaller than 10 nm in the amorphous matrix near 400°C, and led to prominent enhancement in the magnetic properties of the Fe_86.7Zr_3.3B₄Ag₆ thin films. Significantly, excellent magnetic properties such as a saturation magnetization of 1.7 T, a coercive force of 1 Oe and a permeability of 7800 at 50 MHz were obtained in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film containing 7.2 nm-size BCC α-Fe, which was annealed at 400°C. Also, core loss of 1.4 W cm⁻³ (B_m=0.1 T) at 1 MHz in the thin film was obtained, and it is a much lower value than had been obtained in any existing soft magnetic materials. Such excellent properties are inferred to originate from the uniform dispersion of nano-size BCC α-Fe in the amorphous matrix.