Face-centered Tetragonal Research Articles

A new type of FCT martensite phase in single-crystalline Fe 3Pt Invar alloy

Martensitic transformation in a highly ordered Fe 3Pt has been investigated by magnetization and X-ray diffraction measurements. We confirmed that a new type of face-centered tetragonal (FCT) martensite phase appears below 60 K in Fe 3Pt with a degree of order S = 0.88. The tetragonality c/ a gradually increases with decreasing temperature, and is approximately 1.005 at 10 K. This is in contrast to the tetragonality c/ a < 1 generally observed in the FCT martensite with a degree of order less than 0.8. The spontaneous magnetization increases in association with the transformation.

Age-hardening by grain interior and grain boundary precipitation in an Au-Ag-Pt-Zn-In alloy for multipurpose dental use

The complex precipitation mechanisms related to the age-hardening of Cu-free Au-Ag-Pt-Zn-In alloy for multipurpose dental use was studied by means of hardness test, X-ray diffraction (XRD) studies, field emission scanning electron microscopic (FE-SEM) observations, energy dispersive spectrometer (EDS) analysis, and electron probe microanalysis (EPMA). The early diffusion and then clustering of the In-concentrated phase in the grain interior, together with the early diffusion and then ordering of the PtZn phase in the grain boundary, introduced the internal strains in the Au-Ag-rich A 1 matrix, resulting in the hardening process. As the Au-Ag-rich A 1 a and PtZn B lamellarforming grain boundary reaction progressed, the phase boundaries between the solute-depleted face-centered cubic (FCC) A 1 a matrix and the face-centered tetragonal (FCT) PtZn B precipitate reduced, resulting in softening. In the particlelike structures composed of the major Pt-Au-rich A 2 phase and the minor Pt-Zn-rich A 3 phase, the separation of In and Zn progressed producing the In-increased Pt-Au-rich A 2 a phase and the

Fabrication of three-dimensional (3D) woodpile structure photonic crystal with layer by layer e-beam lithography

Photonic crystal based superprism offers a way to design new optical components for beam steering and DWDM (Dense Wavelength Division Multiplexing) application. Three-dimensional (3D) photonic crystals are especially attractive as they could offer more control of the light beam. A FCT (Face-Centered-Tetragonal) woodpile structure has been fabricated using layer by layer stacking techniques with E-Beam lithography. Special planarizations and processes have been introduced to ensure the survivability and good alignment of the fabricated nanostructures. Scanning electron microscopy results proved the structure uniformity. With the proper design, the structure exhibits superprism effects around 1550 nm, and such effects have been observed in the experiments.

Effects of transformation twin on Hall–Petch relationship in MnCu alloy

MnCu high damping alloys bear the transforms from face centred cubic (FCC) phase to face centred tetragonal (FCT) phase with formation of the {1 0 1} twin boundaries. In the present work, a commercial M2052 (Mn–20Cu–5Ni–2Fe) high damping alloy was adopted, in order to characterize the features of transformation twins in the specimens with different grain sizes. For this purpose, the yield strengths, the phase transformation points (Tp) and the axes ratio (c/a) of M2052 with different grain sizes were determined by tensile tests, dynamic mechanical analysis (DMA) and X-ray diffraction (XRD), respectively. The results indicated that the effects of twinning structure on the yield strength of the MnCu alloy can be divided into two terms; a grain-size-independent term and a grain-size-dependent term. The grain-size-independent term influences the nominal friction stress (σ∞) and the grain-size-dependent term influences the Hall–Petch (HP) slope (k) of the specimens significantly. Analysis of the experimental data shows that the formation of twins in M2052 is affected by not only aging treatment prior to phase transformation, but also the grain size of the FCC phase. Higher nominal friction stress observed in the specimens with longer time aging treatment would be due to the higher twin density, and the decrease of HP slope would be due to the grain-size dependence of twin density and axes ratio. It indicated that even under the same aging condition, the twin densities in larger grain-sized specimens would be higher than those in smaller grain-sized specimens.

Realization of magnetic anisotropy andL10 CoPt orderedphase by Pt+ ion irradiation on a Co/Pt bilayer film

Ion-beam synthesis of a CoPt layer was achieved byPt+ ionirradiation on Pt/Co bilayer films, and further growth and ordering of the CoPt layer was seen upon4 MeV Si+ ionirradiation at 300 °C. Rutherford backscattering (RBS) results show that the ion-beam mixing increases as a function ofPt+ ion fluence, andat a fluence of 1015 ions cm−2 the formation of an ordered face-centered tetragonal (FCT) CoPt phase in the mixed regionwas clearly observed from the grazing-incidence x-ray diffraction (GIXRD) measurements.The magneto-optic Kerr effect results show that the coercivity increases with increase inPt+ ion fluence.

The Twin and Twin System in FCT L1&lt;SUB&gt;0&lt;/SUB&gt; &amp;theta;-MnNi Phase in an Equiatomic Mn-Ni Alloy

This study examined the twinning mechanism and operating twin system of a face centered tetragonal (FCT) L1 0 0-MnNi phase in an equiatomic Mn-Ni alloy using detailed TEM analysis (electron diffraction, high resolution imaging, and HAADF (Z-contrast) imaging techniques) of the structure of the twins formed during the β (B2, bcc) to 0 (L1 0 , fct) phase transformation. In addition to the well-documented {111}1/6<112] deformation twinning system in L1 0 crystals, {111}1/6<211]* type pseudo-twin systems were also observed. The Z-contrast image of the twins showed that no atomic shuffling occurred during twinning in the 0-MnNi phase and atomic order was maintained in the {111}1/6(112] type true-twins. *in <ijk], i and j are interchangeable in the tetragonal system but k is not.

Synthesis and Fabrication of a Thin Film Containing Silica‐Encapsulated Face‐Centered Tetragonal FePt Nanoparticles

Self-assembly of monodisperse, silica-encapsulated, face-centered tetragonal FePt nanoparticles forms closely packed 2D arrays (see figure). Placing monodisperse FePt nanoparticles in silica nanocapsules allows the transition from a disordered face-centered cubic phase to a ferromagnetic crystalline face-centered tetragonal structure at elevated temperature without severe sintering. These materials are potential candidates for the generation of ultrahigh-density magnetic recording media.

NiMn/FeNi exchange biasing systems–magnetic and structural characteristics after short annealing close to the phase transition point of the AFM layer

This work addresses the issue of acceleration of the long post-deposition annealing procedures typically used for obtaining the antiferromagnetic (AFM) state of the pinning film in NiMn-based spin-valve systems. It presents results on exchange biasing (EB) bi-layers NiMn(50 nm)/Ni 19Fe 81(5 nm) grown on a permalloy seed layer after annealing for a very short time at temperatures in the vicinity of the phase transition of the NiMn film. Both the magnetic and the structural characteristics measured after the short annealing procedure reveal clear differences between the samples annealed below and above a specific temperature T * which is lower than the phase transition temperature in the equilibrium NiMn phase diagram. The absence of an EB in the samples short annealed at T an< T * is related to the dominance of the face-centered cubic (FCC) paramagnetic NiMn phase. In contrast, the significant EB effect in the samples annealed at T an> T * and the saturation found in H EB for T an⩾375°C are connected with the existence of a dominant stable face centered tetragonal (FCT) AFM NiMn phase. This shows that for the investigated sputter-deposited NiMn pinning films a specific temperature T * exists above which a fast FCC⇒FCT (paramagnetic⇒AFM) phase transition occurs. The unidirectional EB effect surprisingly found in samples cooled without magnetic field can be explained with a two-step generation model: (i) the formation of specific magnetic anisotropy in the as-deposited permalloy films strongly depending on the phase composition of the NiMn pinning film, on its surface morphology and the stresses in the AFM/FM interface, respectively, and (ii) recording (“burning in”) of the existing magnetic anisotropy through the described fast paramagnetic⇒AFM phase transition.

Magnetic and structural characteristics of exchange biasing systems based on NiMn antiferromagnetic films

We have investigated the magnetic and the structural characteristics of bi-layer exchange biasing (EB) systems NiMn–Co as function of the antiferromagnetic (AFM) film deposition parameters and of the post-deposition annealing and field-cooling procedures. The effects of sputtering pressure, growth rate, seed layer and of the annealing parameters were studied by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Magneto-Optical Kerr (MOKE) effect measurements. The results yielded optimised deposition conditions and permitted us to establish efficient annealing and field-cooling procedures. The best obtained EB systems with 5 nm Co film reveal coercive field H c⩾35 Oe and exchange bias field H EB⩾95 Oe and have good stability in air up to at least 200°C. The obtained magnetic characteristics are closely correlated with the crystalline structure of the as-deposited samples and with the phase transformation effects. According to our results, the as-deposited samples are a mixture of a non-equilibrium FCC NiMn phase (quenched by the sputtering deposition process) and a variable amount of the equilibrium face centred tetragonal (FCT) phase. The ratio of these two phases depends on the deposition conditions—the amount of the FCT NiMn phase is larger if deposition conditions closer to equilibrium have been used, especially at higher sputtering gas pressure and higher deposition rates. The annealed samples contain dominantly the FCT AFM phase as confirmed by XRD and TEM-diffraction analyses. The transition to this AFM state depends on the initial structure of the as-grown samples and, respectively, on their deposition conditions.

CoPt : B granular thin films for high density magnetic recording media

Granular CoPt : B films, consisting of high anisotropy, face centered tetragonal (FCT) CoPt particles embedded in a B matrix have been successfully prepared. Their structural and magnetic properties are investigated as a function of annealing temperature and boron content, to examine their suitability for magnetic recording media. Grain size and isolation can be adjusted by the boron concentration and annealing process: boron is found to lower the required processing temperature. A minimum grain size is obtained for the 20 vol% B samples for all annealing temperatures. The activation volume as estimated by magnetic relaxation and remanence measurements approaches the physical particle size only for the nanocomposites with grain sizes below 20 nm, indicating a coherent rotation mechanism in these films.

Influence of phase transformation on the permanent-magnetic properties of Fe–Pt based alloys

We have studied the effect of the atomic disorder–order transformation on remanence enhancement and coercivity in Fe–Pt-based materials by isothermal annealing at temperatures well below the transformation point. We also investigated the effect of the annealing temperature and the effect of various types of additives. The relative amount of the low-temperature hard-magnetic face-centered-tetragonal (FCT) phase precipitated in the high-temperature magnetically soft face-centered-cubic (FCC) phase was determined by means of X-ray diffraction. As a function of annealing time and annealing temperature, particle size and relative amount of the FCT phase increased at the cost of the FCC phase. These changes were followed by means of magnetic measurements. We observed a continuous increase in coercivity with increasing annealing time, eventually reaching a maximum. The Kneller–Hawig model was used to explain the occurrence of remanence enhancement and the continuously changing degree of exchange coupling between the magnetically soft and hard phase. The suitability of Fe–Pt based alloys in dental applications is discussed.

Structural and magnetic properties of epitaxial Fe 0.5Pd 0.5 thin films studied by Mössbauer spectroscopy

The Mössbauer spectra of Fe 0.5Pd 0.5 thin films in a disordered, partially ordered and ordered L1 0 structure were analyzed on the basis of three main considerations. First, the hyperfine field acting on a resonant atom depends on the number of Fe in its first coordination sphere and this relation gives information on the homocoordination tendency in the alloy. Second, the uniaxial symmetry related to the chemical ordering induces a dipolar field: this contribution was evaluated in ordered face-centered-tetragonal (FCT) films. Third, the hyperfine field direction is in the film plane of a disordered sample, along the normal to the film plane in an ordered L1 0 structure and isotropic like in the magnetic domain walls. The contributions from disordered and ordered FCT Fe 0.5Pd 0.5, magnetic domain walls and small FCT iron rich clusters were clearly identified.

In-situ observation of preparatory untwinning in reverse martensitic transition in an In-22.5at%Tl shape memory alloy

In the present study, in-situ observation of preparatory untwinning during reverse martensitic transition in an In-22.5at%Tl alloy is discussed. It is for the first time that the phenomenon of preparatory untwinning is being reported in SMAs. It forms an important observation, and its origins are explored which helps shed light on the mechanism of reverse martensitic transitions, and gives insight into the phenomenon of shape memory effect. Shear stresses arise on the twin-planes of the face-centered tetragonal (FCT) phase due to thermoelastic coupling, giving rise to preparatory untwinning in reverse martensitic transition in an In-22.5at%Tl SMA. Due to pretransition C{prime} elastic softening, the shear modulus on twin systems almost approaches zero, causing preparatory untwinning to occur spontaneously. The shear stresses arising due to pretransition thermoelastic coupling are not precluded by Neumann`s principle, and the entropy generated due to this coupling appears at new main-bands.

Magnetic and crystallographic characteristics of the order-disorder phase transformation in Pt/sub 1/Co/sub 1/

The atomic order-disorder transformation mechanism was examined in the Pt/sub 1/Co/sub 1/ magnetic binary alloy by magnetization and X-ray measurements. The results suggest that the ordering mechanism occurs in two stages. The initial, transient stage involves the transformation of the high-temperature, relatively strain free, disordered FCC (face-centered-cubic) lattice to a nonuniformly strained, disordered FCC lattice; the associated increase in the strain energy of the lattice is accompanied by a decrease in its magnetic energy. The highly strained regions then apparently serve as the nuclei for the second-stage transformation to the low-temperature, ordered, FCT (face-centered-tetragonal) structure. >