The carbon (C) ratios, namely the atomic ratios of C/(C + M), in nano-sized coherent MC precipitates (M = Ti, Nb) with the NaCl-type (B1) structure in ferritic steels, which had been isothermally aged at 580 °C, were investigated using atom probe tomography (APT). Considering the influences of the trajectory aberration, detection loss, and peak overlap, we determined the C ratios to be ~0.40 and ~0.45 for an equivalent volume diameter of 1.5–5 nm and 1–5 nm for the TiC and NbC precipitates, respectively, suggesting that there is a considerable fraction of C vacancies in both nano-sized precipitates. The apparent C ratios show significant scatter with decreasing particle size, while the apparent mean C ratios of very fine TiC particles, smaller than 1.5 nm, decreased with decreasing particle size. With the use of one of the latest APT instruments with a high detection efficiency, the scattering in the apparent C ratios was reduced because the counting statistics were improved; however, the artificial enrichment of C atoms to particular crystallographic directions of ferrite hindered the determination of the C ratio for very fine TiC particles smaller than 1.5 nm.

Spin-transfer-torque magnetoresistive random-access memory (STT-MRAM) is an emerging nonvolatile memory that uses magnetic tunnel junctions (MTJs) to store information. MTJs with a crystalline MgO(001) tunnel barrier sandwiched between ferromagnetic layers, such as CoFeB, exhibit giant tunnel magnetoresistance, which is used to readout the STT-MRAM. Writing of STT-MRAM is based on current-induced magnetization reversal, called STT switching. STT-MRAM with perpendicular magnetization is especially important for high-density and low-power-consuming memory applications such as embedded memory for large-scale integrated circuit. For STT-MRAM to replace ultrahigh-density dynamic random-access memory, however, there are still technological challenges concerning the materials and fabrication processes of MTJs. This article reviews the physics and materials science of MTJs for STT-MRAM. We also discuss the importance of new MTJ materials and processes for next-generation ultrahigh-density MRAM.

This article reviews progress in magnetoresistive (MR) read sensor technology for hard-disk drives (HDDs). MR reader technology has progressed from the anisotropic magnetoresistance sensor, to the current-in-plane giant magnetoresistive (CIP-GMR) sensor, to today’s current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) sensor. This evolution has driven the continuous growth of the areal density of HDDs from 2 Gbpsi (gigabits/in2) in early longitudinal recording to ∼1 Tbpsi (terabits/in2) currently in perpendicular magnetic recording. For further increases in the areal density, a transition to energy-assisted recording is expected in the near future. Further technical challenges for the read sensor technology toward 2 Tbpsi and then 5 Tbpsi areal densities are discussed based on recent promising experimental work on CPP-GMR using Heusler alloys, and CPP-GMR’s laterally expanded version, the lateral spin valve (LSV). To realize large MR output and narrow shield-to-shield spacing requirements for higher density recording, materials selection and optimization of interface structures of CPP-GMR and LSV devices are critical.

This article focuses on four topics that demonstrate the importance of atom probe tomography for obtaining nanostructural information that provides deep insights into the structures of metallic alloys, leading to a better understanding of their properties. First, we discuss the microstructure–coercivity relationship of Nd-Fe-B permanent magnets, essential for developing a higher coercivity magnet. Second, we address equilibrium segregation at grain boundaries with the aim of manipulating their interfacial structure, energies, compositions, and properties, thereby enabling beneficial material behavior. Third, recent progress in the search to extend the performance and practicality of the next generation of advanced high-strength steels is discussed. Finally, a study of the temporal evolution of a Ni-Al-Cr alloy through the stages of nucleation, growth, and coarsening (Ostwald ripening) and its relationship with the predictions of a model for quasi-stationary coarsening is described. This information is critical for understanding high-temperature mechanical properties of the material.

We review microstructures and properties of metal matrix composites produced by severe plastic deformation of multiphase alloys. Typical processings are wire drawing, ball milling, roll bonding, equal-channel angular extrusion, and high-pressure torsion of multiphase materials. Similar phenomena occur between solids in frictional contact such as in tribology, friction stir welding, and explosive joining. The resulting compounds are characterized by very high interface and dislocation density, chemical mixing, and atomic-scale structural transitions at heterointerfaces. Upon straining, the phases form into nanoscaled filaments. This leads to enormous strengthening combined with good ductility, as in damascene steels or pearlitic wires, which are among the strongest nanostructured bulk materials available today (tensile strength above 6 GPa). Similar materials are Cu-Nb and Cu-Ag composites, which also have good electrical conductivity that qualifies them for use in high-field magnets. Beyond the engineering opportunities, there are also exciting fundamental questions. They relate to the nature of the complex dislocation, amorphization, and mechanical alloying mechanisms upon straining and their relationship to the enormous strength. Studying these mechanisms is enabled by mature atomic-scale characterization and simulation methods. A better understanding of the extreme strength in these materials also provides insight into modern alloy design based on complex solid solution phenomena.

A recent report on the “room temperature superplasticity” in the Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass [Y.H. Liu et al., Science315, 1385 (2007)] was ascribed to the distinctive micrometer-sized structural heterogeneity. To verify the microstructure in this alloy, transmission electron microscopy (TEM) and anomalous small-angle x-ray scattering experiments were conducted. The results show that no micrometer-sized or nanometer-sized structural heterogeneities can be found. The micrometer-sized dark and bright regions that were previously reported as the reason for the plasticity are artifacts caused by TEM specimen preparation, rather than the intrinsic structure feature of this alloy. This finding is important for further studying the unique properties of this alloy.

This article reviews investigations of the growth and reactions within thin metal and oxide films using atom-probe tomography. Included in this review are (1) studies of interfacial and growth reactions in magnetoresistive metallic, metal/oxide, and magnetic magnetostrictive multilayers; (2) comparison of selected portions of these results to simulated film growth using molecular dynamics; and (3) study of the origin of room-temperature ferromagnetism in dilute magnetic semiconductors. Information of this type is useful in order to understand the formation and thermal evolution of thin films (and to compare to theory and modeling) and, ultimately, to permit further optimization of devices based on thin films.

The effect of melt temperature on the structure and mechanical properties of three Zr-based bulk metallic glasses (BMGs)—Zr62Cu17Ni13Al8, Zr55Cu20Ni10Al10Ti5, and Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105)—has been studied. The results show that the BMGs cast from higher melt temperature exhibit large plastic strains despite their amorphous structure. The samples become macroscopically brittle when the quenched-in crystals form an interconnected microstructure. In contrast to previous studies, there is no notable effect on the Poisson’s ratio (ν) and other elastic constants.

Reduction of structural defects in III-nitride based optical devices is of critical importance for high efficient and high reliable optoelectronic performance. Here, three different types of structural defects such as threading dislocations, Mg-related pyramidal defects and columnar defects, observed in GaN-related epitaxial films are described and their relation to reliability of GaN-based LDs is discussed. Composition fluctuations of GaInN MQWs with different In concentrations by analyzed by a laser assisted 3D atom probe are also described.

A Yb:KGW femtosecond laser (400 fs) with 3rd and 4th harmonic generators (wavelength = 343 and 258 nm) was adopted to a locally built 3DAP instrument to assist field evaporation from ceramics tips that were bonded on tungsten wire. Using this setting, we have demonstrated that quantitative atom probe tomography is possible from Y2O3-ZrO2-MgAl2O4, (Ce,Y)O2, Li(Co,Ni,Mg,Al)O2 sintered bulk ceramics, which are all insulators.