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Ferroelectric HfO2-based materials hold great potential for widespread integration of ferroelectricity into modern electronics due to their robust ferroelectric properties at the nanoscale and compatibility with the existing Si technology. Earlier work indicated that the nanometer crystal grain size was crucial for stabilization of the ferroelectric phase of hafnia. This constraint caused high density of unavoidable structural defects of the HfO2-based ferroelectrics, obscuring the intrinsic ferroelectricity inherited from the crystal space group of bulk HfO2. Here, we demonstrate the intrinsic ferroelectricity in Y-doped HfO2 films of high crystallinity. Contrary to the common expectation, we show that in the 5% Y-doped HfO2 epitaxial thin films, high crystallinity enhances the spontaneous polarization up to a record-high 50 µC/cm2 value at room temperature. The high spontaneous polarization persists at reduced temperature, with polarization values consistent with our theoretical predictions, indicating the dominant contribution from the intrinsic ferroelectricity. The crystal structure of these films reveals the Pca21 orthorhombic phase with a small rhombohedral distortion, underlining the role of the anisotropic stress and strain. These results open a pathway to controlling the intrinsic ferroelectricity in the HfO2-based materials and optimizing their performance in applications.

Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DWs density using super-coercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs. The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multi-level memories and neuromorphic computing applications.

In this work, we exploit the ferroelectric-dielectric layer thickness ratio r as an effective tuning parameter to control the ferroelectric polarization and transient negative capacitance (NC) state in epitaxial ${\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ bilayer heterostructures. The remnant polarization decreases monotonically with decreasing r, with the system exhibiting an abrupt transition from ferroelectric to dielectric dominated behavior at a critical ratio ${r}_{c}$ of 8 to 7, which is consistent with the evolution of the free-energy profile modeled via Landau theory. For samples with large r, the polarization switching dynamics during the transient NC regime can be well described by the nucleation and growth model, with the narrow distribution of the characteristic switching time (${t}_{0}$) pointing to a domain-wall-motion-limited behavior. Right below ${r}_{c}$, we observe a significantly broadened distribution of ${t}_{0}$, which can be attributed to the emergence of a multidomain state. The transient NC mode is quenched in samples with r 6, confirming that the ferroelectric order is suppressed. Our study provides critical information for optimizing the materials design for complex oxide-based NC transistors, paving the path for their application in low-power nanoelectronics.

Laser processing of metal surfaces by ultrafast pulse lasers is a developing technology with many potential uses including applications in heat transfer, medical implants, and tribology. Laser processed silver surfaces has several potential applications such as biomedical devices, antibacterial surfaces, and chemical sensors. However, there is a lack of previous research on laser processing of silver is more difficult to process compared with other metals. A newly investigated dual-pulse femtosecond laser surface processing technique was capable of producing self-organized, micro/nanoscale surface features on silver where single-pulse techniques had previously failed. Three-dimensional (3D) scanning electron microscopy (SEM) was used to examine mound-like structures produced by this new method to determine their composition and formation processes. The interior microstructure revealed that the mounds were comprised mostly of resolidified Ag grains with approximately 1% porosity. Hydrodynamically-driven fluid flow was the primary process that forms these surface structures without significant oxidation.

Author(s): Yin, Y; Yin, Y; Jiang, X; Koten, MA; Shield, JE; Chen, X; Yun, Y; N'Diaye, AT; Hong, X; Xu, X | Abstract: We report a spin-rectification effect in a spin-valve structure consisting of ferroelectric croconic acid (C5H2O5) sandwiched between ferromagnetic electrodes La0.7Sr0.3MnO3 and Co, which can be switched between a high-resistance (off) and a low-resistance (on) state by a poling voltage. In the off state, the magnetoresistance (MR) sign reverses with the measurement voltage with a 0.1-V offset, suggesting a spin-rectification behavior, while in the on state the MR remains negative. These observations can be understood in terms of electrically controlled interfacial energy-band alignment either from the electrostatic effect or from the interfacial redox process. The observed spin-rectification effect suggests the possibility of diodelike devices for spin-polarized current.

The effect of adding a low melting point Pr-Cu-Al alloy during spark plasma sintering of melt-spun Nd-Fe-B ribbons is investigated. Regions of coarse grains were reduced and overall grain refinement was observed after the addition of Pr68Cu25Al7, leading to an enhancement of coercivity from 12.7 kOe to 20.4 kOe. Hot deformation of the samples in the spark plasma sintering system resulted in the formation of platelet-like grains, producing crystallographic alignment and magnetic anisotropy. The hot deformation process improved the remanence and energy product but reduced the coercivity. The decrease of coercivity resulted from grain growth and aggregation of Pr and Nd elements at triple-junction phases.

Fe-Si alloys ranging from 3 to 8 wt% Si were rapidly solidified using melt spinning. Wheel speeds of 30 m/s and 40 m/s were employed to vary cooling rates. Mossbauer spectroscopic studies indicated the Si content significantly influenced the number of Fe sites, relative abundance of various Fe species, and internal magnetic fields/structural environments. Wheel speed altered Fe speciation only in the 3 wt% sample. Scanning electron microscopy confirmed that increasing the wheel speed refined both the ribbon thickness and grain size. Electron backscatter diffraction results suggest tailoring melt spinning process parameters and alloy chemistry may offer the ability to manipulate {001} texture development. Electrical resistivity measurements were observed to increase in response to elevated Si content. Increased hardness was correlated to elevated Si content and wheel speed.

Pool boiling heat transfer with the use of femtosecond laser surface processing (FLSP) on copper surfaces has been studied. FLSP creates a self-organized micro/nanostructured surface. In the previous pool boiling heat transfer studies with stainless steel FLSP surfaces, enhancements in critical heat flux (CHF) and heat transfer coefficients (HTCs) were observed compared to the polished reference surface. However, this study shows that copper FLSP surfaces exhibit reductions in both CHF and HTCs consistently. This reduction in heat transfer performance is a result of an oxide layer that covers the surface of the microstructures and acts as an insulator due to its low thermal conductivity. The oxide layer was observed and measured with the use of a focused ion beam milling process and found to have thickness of a few microns. The thickness of this oxide layer was found to be related to the laser fluence parameter. As the fluence increased, the oxide layer thickness increased and the heat transfer performance decreased. For a specific test surface, the oxide layer was selectively removed by a chemical etching process. The removal of the oxide layer resulted in an enhancement in the HTC compared to the polished reference surface. Although the original FLSP copper surfaces were unable to outperform the polished reference curve, this experiment illustrates how an oxide layer can significantly affect heat transfer results and dominate other surface characteristics (such as increased surface area and wicking) that typically lead to heat transfer enhancement.

A comparison is made between the self-organized surface features that form on copper using femtosecond versus picosecond laser pulses. Modification of material surfaces (like metals and dielectrics) have shown to affect the properties for applications including enhancing heat transfer and producing anti-corrosive or antibacterial surfaces. Femtosecond laser surface processing is a precise and repeatable technique to produce uniform self-organized quasi-periodic structures on the surface for most metals. However, it is difficult to produce uniform quasi-periodic structures on noble metals (e.g. copper). Using femtosecond pulses in the processing of copper results in non-uniform structure formation with areas that have little to no mound development. Lengthening the pulses from femtoseconds to picoseconds is shown to result in uniform quasi-periodic structure formation with high repeatability. The subsurface microstructure formed using femtosecond versus picosecond pulses while keeping other processing parameters the same was investigated, providing insight into mound formation processes. Picosecond pulses resulted in finer-grained regions within the mounds, and after a critical pulse count threshold was reached, onion-like layers with alternating high-density copper and highly porous copper oxide regions formed near the surface and blanketed the mounds. Multiple formation mechanisms are proposed to help explain the microstructures that formed using picosecond pulses.

The structural and magnetic properties of Nd12Fe82B6 and Nd10M2Fe82B6 (M = Nb, Ti, Zr, Cr) alloys prepared using arc melting and melt spinning have been investigated. All the samples are found to crystallise with a tetragonal Nd2Fe14B phase without any alloy or elemental impurities. There is a small decrease in the unit cell volume of Nd2Fe14B due to transition metal (M) addition. The substitution of Nb and Ti refines and homogenises the nanostructure of the alloys, promoting intergrain exchange coupling leading to an increase in the remanence and energy product. For example, the remanence and energy product of Nd12Fe82B6 and Nd10Nb2Fe82B6 are 8.4 kG and 15 MGOe, and 9.9 kG and 20 MGOe, respectively. The J(T) curves are similar to those of a single phase ferromagnetic material suggesting no segregation of ferromagnetic impurities. The observed structural and magnetic properties are consistent with the fact that the substitutional transition metal atoms occupy the Nd site of the tetragonal Nd2Fe14B...

The effect of Hf and B additions on the phase stability, microstructure and magnetic properties of the metastable SmCo7 (1:7H) ribbons has been investigated with a combined approach of experimental measurements and first principle DFT (density functional theory) calculations. A series of (Sm0.12Co0.88)95Hf5−xBx (x = 0, 1, 2, 3, 4 and 5) alloys were arc-melted in a TIG (tungsten inert gas) arc melting furnace, followed by melt-spinning onto a copper roller at a wheel velocity of 40 m/s. Characterization based on X-ray diffraction indicates that the major phase is SmCo7 having meta-stable (TbCu7-type) structure. From the total energy calculations using DFT, the phase stability of (Sm0.12Co0.88)95Hf5−xBx ribbons have been confirmed. Moreover, Hf and B addition results in an effective grain refinement; average grain size being as low as ~ 80 nm. The reduction in grain size leads to significant changes (increase or decrease) in magnetic properties depending on the Hf/B ratio. The coercivity value (Hc) varies between 7 kOe and 12 kOe as x (at.% B) increases from 0 to 5 at.%. The experimental coercivity values have been compared with the computed anisotropy energies. The saturation magnetization (Ms) increases from ~ 54 emu/g to 77 emu/g with increasing B concentration (x).

An experimental investigation of the effects of layers of nanoparticles formed during femtosecond laser surface processing (FLSP) on pool boiling heat transfer performance has been conducted. Five different stainless steel 304 samples with slightly different surface features were fabricated through FLSP, and pool boiling heat transfer experiments were carried out to study the heat transfer characteristics of each surface. The experiments showed that the layer(s) of nanoparticles developed during the FLSP processes, which overlay FLSP self-organized microstructures, can either improve or degrade boiling heat transfer coefficients (HTC) depending on the overall thickness of the layer(s). This nanoparticle layer thickness is an indirect result of the type of microstructure created. The HTCs were found to decrease with increasing nanoparticle layer thickness. This trend has been attributed to added thermal resistance. Using a focused ion beam milling process and transmission electron microscopy (TEM), the physical and chemical properties of the nanoparticle layers were characterized and used to explain the observed heat transfer results. Results suggest that there is an optimal nanoparticle layer thickness and material composition such that both the HTCs and critical heat flux (CHF) are enhanced.

Core–shell nanoparticles are known to form in binary systems using a one-step gas-condensation deposition process where a large, positive enthalpy of mixing provides the driving force for phase separation and a difference in surface energy between component atoms creates a preferential surface phase leading to a core–shell structure. Here, core–shell nanoparticles have been observed in systems with enthalpy as low as −5 kJ mol−1 and a surface energy difference of 0.5 J m−2 (Mo–Co). This suggests that surface energy dominates at the nanoscale and can lead to phase separation in nanoparticles. The compositions and size dependence of the core–shell structures are also compared and no core–shell structures are observed below a critical size of 8 nm.

A detailed structural and chemical analysis of a class of self-organized surface structures, termed aggregated nanoparticle spheres (AN-spheres), created using femtosecond laser surface processing (FLSP) on silicon, silicon carbide, and aluminum is reported in this paper. AN-spheres are spherical microstructures that are 20-100 μm in diameter and are composed entirely of nanoparticles produced during femtosecond laser ablation of material. AN-spheres have an onion-like layered morphology resulting from the build-up of nanoparticle layers over multiple passes of the laser beam. The material properties and chemical composition of the AN-spheres are presented in this paper based on scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX) analysis. There is a distinct difference in the density of nanoparticles between concentric rings of the onion-like morphology of the AN-sphere. Layers of high-density form when the laser sinters nanoparticles together and low-density layers form when nanoparticles redeposit while the laser ablates areas surrounding the AN-sphere. The dynamic nature of femtosecond laser ablation creates a variety of nanoparticles that make-up the AN-spheres including Si/C core-shell, nanoparticles that directly fragmented from the base material, nanoparticles with carbon shells that retarded oxidation, and amorphous, fully oxidized nanoparticles.

Amorphous Fe32Ni52Zr3B13, prepared by rapid solidification, undergoes crystallization during high-energy mechanical milling. The resulting structure consists of face-centered cubic (fcc) FeNi and Zr3Ni20B6 nanocrystallites. Structural evolution and defect analysis of as-solidified Fe32Ni52Zr3B13 ribbons with different milling times is investigated. From the differential thermal analysis (DTA) curve of amorphous ribbons, exothermic peaks were observed at 415 °C and 475 °C corresponding to the crystallization of fcc Zr3Ni20B6 phase and fcc FeNi phase, respectively. However, high-energy mechanical milling induces the formation of FeNi within the first 2 h of mechanical milling. Further milling induces the crystallization of Zr3Ni20B6. Doppler broadening positron annihilation spectroscopy (DBPAS) was used to investigate vacancy-type defects. The milling-induced crystallization appears to be related to enhanced vacancy-type defect concentrations allowing growth of pre-existing Fe(Ni) nuclei. The milling and enhanced vacancy concentration also de-stabilizes the glass, leading to decreased crystallization temperatures for both phases, and ultimately eliminating the glass transition altogether.

Electronic and magnetic properties of L10-ordered FexMn1−xAl alloys (x=0, 0.0625, 0.125, 0.1875, 0.5) are investigated by first-principle supercell calculations. Pristine MnAl exhibits robust ferromagnetism involving the dense-packed Mn atoms in (001) planes of the tetragonal structure. Iron substitution for Mn significantly deteriorates the magnetization of the alloy. The reduction is a dilution effect, caused by the relatively small Fe moment of about 1.9 μB per atom, as compared to the Mn moment, which exceeds 2.4 μB. By contrast, 50% Fe substituted for Mn (x=0.5) yields a magnetocrystalline anisotropy enhancement of about 40%. Both the magnetization reduction and the anisotropy enhancement mainly arise due to the change in dxy and dzx minority density of states (DOS) at the Fermi level.

The magnetocaloric properties of Gd–Ga alloys were examined with the aim to explore their potential application as magnetic refrigerants near room-temperature. A series of Gd 100− x Ga x ( x =0, 5, 10, 15, and 20 at%) alloys were prepared by conventional melting and rapid solidification. Both as-cast and melt-spun alloys are comprised of α-Gd and Gd 5 Ga 3 with differing microstructural characteristics. While the as-cast alloys have proeutectic α-Ga and eutectic α-Ga+Gd 5 Ga 3 (grain size ~50–100 µm), the melt-spun alloys have nanostructured (~50–100 nm) α-Ga and Gd 5 Ga 3 . The Curie temperature ( T C ) derived from M–T curves indicates that alloying of Ga with Gd decreases T C . The decrease in T C for melt-spun alloys is gradual—~0.5 K per at% of Ga, and the as-cast alloys (5≤ x ≤20) have similar T C (~286 K). The saturation magnetization ( M S ) derived from M–H curves (260 K), show that alloying of Ga with Gd decreases M S . M S for melt-spun alloys decreases from ~108 emu/g for Gd to ~42 emu/g for Gd 80 Ga 20 and for as-cast alloys M S decreases from ~120 emu/g for Gd to ~62 emu/g for Gd 80 Ga 20 . Both maximum magnetic entropy change (−Δ S M ) max and refrigerant capacity ( RC ) for as-cast and melt-spun alloys are lower than Gd. (−Δ S M ) max decreases by ~1.55% and ~3.0% per atomic percent of Ga in as-cast and melt-spun alloys, respectively. A steeper decrease in RC is observed in melt-spun alloys than is observed for as-cast alloys—~433 J/kg for Gd to ~235 J/kg for Gd 80 Ga 20 for melt spun alloys and ~462 J/kg for Gd to ~306 J/kg for Gd 80 Ga 20 for as-cast. As-cast alloys appear to have better magnetocaloric properties than the melt-spun alloys. Alloying of diamagnetic Ga with Gd provides superior magnetocaloric properties compared to other diamagnetic elements in Group 13. The near room-temperature magnetocaloric properties of Gd–Ga alloys were comparable to a few first-order-transition based magnetic refrigerants.

The nanoscale structural, compositional, and magnetic properties are examined for annealed MnAu nanoclusters. The MnAu clusters order into the L1(0) structure, and monotonic size-dependences develop for the composition and lattice parameters, which are well reproduced by our density functional theory calculations. Simultaneously, Mn diffusion forms 5 Å nanoshells on larger clusters inducing significant magnetization in an otherwise antiferromagnetic system. The differing atomic mobilities yield new cluster nanostructures that can be employed generally to create novel physical properties.

The magnetocaloric properties and microstructure of melt-spun Gd100-xMnx (x=0, 5, 10, 15, and 20 at%) alloys were investigated to explore their potential application as room-temperature magnetic refrigerants. The alloys were composed of nanostructured hexagonal α-Gd phase with grain sizes between 40 and 150 nm. Increasing the Mn content resulted in a decrease in the lattice parameters (a and c) of Gd, suggesting the formation of a substitutional solid solution. The Curie temperature (TC), derived from M–T curves and Arrot plots, decreased monotonically from ∼293 K (melt-spun Gd) to ∼278 K (melt-spun Gd80Mn20). The saturation magnetization at 260 K decreased from ∼108 emu/g (melt-spun Gd) to ∼78 emu/g (melt-spun Gd80Mn20). The peak value of magnetic entropy change, (−ΔSM)max, decreased from ∼8.6 J/kgK for melt-spun Gd to ∼5.9 J/kgK for melt-spun Gd80Mn20 alloy and the refrigeration capacity (RC) initially increased from ∼433 J/kg for melt-spun Gd to ∼452 J/kg for melt-spun Gd95Mn5 and then gradually decreased to ∼321 J/kg for melt-spun Gd80Mn20, for a magnetic field varying from 0 to 5 T. The near-room temperature magnetocaloric properties of melt-spun Gd100-xMnx (0≤x≤20) alloys were found to be comparable to other magnetic refrigerants based on first-order phase transitions.

First-principle supercell calculations are used to determine how 3d elemental additions, especially Fe additions, modify the magnetization, exchange and anisotropy of L10-ordered ferromagnets. Calculations are performed using the VASP code and partially involve configurational averaging over site disorder. Three isostructural systems are investigated: Fe-Co-Pt, Mn-Al-Fe, and transition metal-doped Fe-Ni. In all three systems the iron strongly influences the magnetic properties of these compounds, but the specific effect depends on the host. In CoPt(Fe) iron enhances the magnetization, with subtle changes in the magnetic moments that depend on the distribution of the Fe and Co atoms. The addition of Fe to MnAl is detrimental to the magnetization, because it creates antiferromagnetic exchange interactions, but it enhances the magnetic anisotropy. The replacement of 50% of Mn by Fe in MnFeAl2 enhances the anisotropy from 1.77 to 2.5 MJ/m3. Further, the substitution of light 3d elements such as Ti, V, Cr into L10-ordered FeNi is shown to substantially reduce the magnetization.

Nanostructured Mn3-x Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D019 and cubic L21 Heusler crystal structures based on the concentration of Mn in Mn3-x Ga. Upon vacuum-annealing the samples at 450 °C for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D022 structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn1.9 Ga and Mn2.1 Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn2.6 Ga and Mn3.0 Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn3-x Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn2.6 Ga and Mn3.0 Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn3-x Ga. The maximum magnetization of 57 emu/g (300 emu/cm3) and the coercivity of 6.5 kOe were measured in the Mn1.9 Ga and Mn3.0 Ga ribbons, respectively.

Nanostructured MnBi ribbons doped with impurity elements including B, C, Fe, Hf, Sm and Tb were prepared using the arc melting and melt-spinning techniques. The melt-spun ribbons were annealed in vacuum furnace at 350°C to obtain the intended hexagonal structure. The external impurity doping made a significant change in the magnetic properties of the nanostructured MnBi ribbons including a decrease in saturation magnetization (Ms) and anisotropy energy (K) and an increase in coercivity (Hc). However, Hf and C co-doping showed the opposite effect with a small increase in both Ma and K. Interestingly, the anisotropy energy of the boron doped sample increased by about 15% irrespective of the small decrease in magnetization. A significant increase in Hc of MnBi ribbons was found due to Hf, Tb and Sm doping. Hc as high as 13 kOe was achieved in Hf-doped sample after the sample was aligned in a magnetic field. A thermal hysteresis was observed at the structural phase transition of MnBi, which shifts by about 5 K towards higher temperatures due to impurity doping. The observed magnetic properties of the impurity doped MnBi ribbons are explained as the consequences of the disorder and the competing ferromagnetic and antiferromagnetic interactions.

The effects of Cr and C additions on the structures and magnetic properties of Sm–Co alloys have been investigated. x-ray diffraction revealed that the metastable TbCu 7 -type structure was formed during melt spinning at 40 m/s. The lattice parameters varied with Cr and C content, suggesting dissolution of the alloying additions in the SmCo 7 structure. Increasing Cr and C content also promoted the formation of an amorphous phase, as indicated by both x-ray diffraction and differential thermal analysis. The alloying additions significantly refined the microstructural scale from ∼1 μm for the simple binary Sm 12 Co 88 alloy to approximately 30 nm for the sample with additions of 6 at% C and 9 at% Cr. The coercivity of the as-solidified samples increased from 0.5 kOe for the simple binary alloy to 9.8 kOe for the alloy modified with 3 at% C and 4.5 at% Cr. The coercivity was also found to be strongly dependent on the temperature of thermal treatments. The coercivity initially increased, and then dramatically decreased before again increasing at higher annealing temperatures. x-ray diffraction and TEM analysis suggest that the development of the ordered Sm 2 Co 17 structure upon annealing may be influencing the coercivity. Differences in initial susceptibility of the annealed samples suggest a combination of domain wall pinning and nucleation-controlled magnetization mechanisms.

Femtosecond laser surface processing is a technology that can be used to functionalize many surfaces, imparting specialized properties such as increased broadband optical absorption or superhydrophilicity/superhydrophobicity. In this study, two unique classes of surface structures, below surface growth (BSG) and above surface growth (ASG) mounds, were formed by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40 with two different grain sizes. Cross sectional imaging of these mounds revealed thermal evidence of the unique formation processes for each class of surface structure. BSG mounds formed on all three substrates using the same laser parameters had similar surface morphology. The microstructures in the mounds were unaltered compared with the substrate before laser processing, suggesting their formation was dominated by preferential valley ablation. ASG mounds had similar morphology when formed on the polycrystalline Ni60Nb40 substrates with 100 nm and 2 [H9262]m grain size. However, the ASG mounds had significantly wider diameter and higher peak-to-valley heights when the substrate was amorphous Ni60Nb40. Hydrodynamic melting was primarily responsible for ASG mound formation. On amorphous Ni60Nb40 substrates, the ASG mounds are most likely larger due to lower thermal diffusivity. There was clear difference in growth mechanism of femtosecond laser processed BSG and ASG mounds, and grain size does not appear to be a factor.

The magnetization and magnetic anisotropy of iron-rich Fe-W and W-Co intermetallics and nanostructures are investigated by first-principle calculations. The W atom is antiferromagnetically coupled to nearest neighbor Co atom and interface Fe atom in the case of Fe-W multilayers. The first anisotropy constants are positive, nearly zero, or negative, depending on the atomic structures. Typical anisotropy energies per atom are -0.513 meV for a dilute W-Co alloy, -0.06 meV for W2Fe2, and 0.44 meV for W2Fe4 (0.1nmeV/atom ~1MJ/m3). These values are of interest for permanent-magnet applications but also indicate the need for careful structural control and for a more detailed investigation of structure-property relationships in Fe-W nanostructures.

The magnetocaloric properties of melt-spun Gd–B alloys were examined with the aim to explore their potential application as magnetic refrigerants near room temperature. A series of Gd 100− x B x ( x =0, 5, 10, 15, and 20 at%) alloys were prepared by melt spinning. With the decrease in Gd/B ratio, Curie temperature ( T C ) remains constant at ∼293 K, and saturation magnetization, at 275 K, decreases from ∼100 to ∼78 emu/g. Negligible magnetic hysteresis was observed in these alloys. The peak value of magnetic entropy change, (−Δ S M ) max , decreased from ∼9.9 J/kg K (0–5 T) and ∼5.5 J/kg K (0–2 T) for melt-spun Gd to ∼7.7 J/kg K (0–5 T) and ∼4.0 J/kg K (0–2 T), respectively for melt-spun Gd 85 B 15 and Gd 80 B 20 alloys. Similarly, the refrigeration capacity ( q ) decreased monotonously from ∼430 J/kg (0–5 T) for melt-spun Gd to ∼330 J/kg (0–5 T) for melt-spun Gd 80 B 20 alloy. The near room temperature magnetocaloric properties of melt-spun Gd 100− x B x (0≤ x ≤20) alloys were found to be comparable to few first-order transition based magnetic refrigerants.

Rare-earth transition-metal (R-TM) alloys show superior permanent magnetic properties in the bulk, but the synthesis and application of R-TM nanoparticles remains a challenge due to the requirement of high-temperature annealing above about 800 °C for alloy formation and subsequent crystalline ordering. Here we report a single-step method to produce highly ordered R-TM nanoparticles such as YCo(5) and Y(2)Co(17), without high-temperature thermal annealing by employing a cluster-deposition system and investigate their structural and magnetic properties. The direct ordering is highly desirable to create and assemble R-TM nanoparticle building blocks for future permanent-magnet and other significant applications.

High-temperature order–disorder transformations in R2T17 and R2T17-M-C intermetallics with R = Pr, Sm, Dy, Tb; T = Co, Fe; and M = Zr, Nb were studied utilizing time-resolved synchrotron x-ray diffraction at the Advanced Photon Source (APS) at the U.S. Department of Energy’s Argonne National Laboratory (Argonne, IL). High-energy synchrotron radiation provides intense, highly penetrating x-rays, which are ideal for in situ studies of phase transformations. Alloying additions are used to stabilize formation of metastable phases; their influence on order recovery was investigated. The experimental setup utilized Debye–Scherrer geometry; specimens were heated at a rate of 10 K/min. Full-profile diffraction patterns collected every 10 s were refined in sequence using the Rietveld method to track changes of lattice parameters and phase assemblages during heating. Sharp changes observed in the evolution of temperature-dependent lattice parameters suggested formation of ordered structure via nucleation and growth. Both 2-17 polymorphs co-existed in light and heavy rare-earth systems at high temperatures. The presence of alloying additions in the solid solution greatly influenced long-range order formation.

The structural transformation from the metastable, disordered TbCu 7 -type SmCo 7 structure to the equilibrium, ordered Th 2 Zn 17 -type Sm 2 Co 17 structure was revealed by X-ray diffraction analysis using Rietveld refinement. The magnetic properties depended strongly on the stage of the transformation, as the coercivity depended on the annealing temperature. The as-solidified alloys in the TbCu 7 -type structure exhibited coercivity as high as 7.85 kOe, which increased to greater than 9 kOe after heat treatment. The magnetization processes were also strongly influenced by the structural state. Initially it was dominated by nucleation processes, which gave way to domain wall pinning-controlled magnetization processes with the development of the Sm 2 Co 17 structure. Transmission electron microscopy revealed the development of antiphase domains during heat treatment, which apparently served as the domain-wall pinning sites during magnetization reversal.

In this article, we report the microstructural evolution of rapidly solidified Sm-Co alloys from 4 to 16 at. pct Sm. We have observed a wide variety of phase formation and microstructures, ranging from primary Co dendrite formation to eutectic structures to the formation of the metastable SmCo7 compound. In particular, we observed nonequilibrium formation of Co along with SmCo7, whose presence caused a decrease in coercivity from ∼10 kOe to 500 Oe. Alloying elements reduced the scale of the microstructure, effectively offsetting the detrimental effects of the Co phase formation and leading to a recovery of the coercivity. The eutectic structure with Co rods surrounded by SmCo7 provides a natural path to nanoscale hard/soft magnetic nanocomposites, where control of scale and phase content is critical.

The relatively low magnetocrystalline anisotropy (MCA) in strongly correlated manganites $(\mathrm{La},\mathrm{Sr}){\mathrm{MnO}}_{3}$ has been a major hurdle for implementing them in spintronic applications. Here we report an unusual, giant enhancement of in-plane MCA in 6 nm ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ (LSMO) films grown on (001) ${\mathrm{SrTiO}}_{3}$ substrates when the top 2 nm is patterned into periodic stripes of 100 or 200 nm width. Planar Hall effect measurements reveal an emergent uniaxial anisotropy superimposed on one of the original biaxial easy axes for unpatterned LSMO along $⟨110⟩$ directions, with a 50-fold enhanced anisotropy energy density of $5.6\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\mathrm{erg}/{\mathrm{cm}}^{3}$ within the nanostripes, comparable to the value for cobalt. The magnitude and direction of the uniaxial anisotropy exclude shape anisotropy and the step edge effect as its origin. High resolution transmission electron microscopy studies reveal a nonequilibrium strain distribution and drastic suppression in the $c$-axis lattice constant within the nanostructures, which is the driving mechanism for the enhanced uniaxial MCA, as suggested by first-principles density functional calculations.

In this paper, we report on the production of Fe cluster/FePt matrix nanocomposite permanent magnets. Monodispersed Fe clusters with sizes below 10 nm were formed by gas aggregation techniques. These Fe clusters were imbedded in an FePt matrix by alternate deposition from two sources. Specimens with a range of Fe cluster phase content from 0 to 30 vol% were produced by controlling deposition times from each source. As-deposited FePt formed in the A1 structure; thus, post-deposition heat treatment was necessary to form the hard magnetic L10 FePt compound. A single-step heat treatment at 600 °C for 10 min leads to nanocomposite structures with excellent magnetic properties. The coercivity decreased with increasing Fe cluster content, while the energy product initially increased, reaching a maximum of almost 18 MGOe, and then decreased at higher Fe cluster content. Secondary heat treatment at 500 °C significantly improved the magnetic properties when compared with the single-step heat treatment at 600 °C. Increased coercivity and remanence was observed, resulting in energy products of 21 MGOe. The energy products are close to 70 percent greater than expected for uncoupled systems.

Iron and cobalt nanoparticle fluids have been prepared by inert-gas condensation into an oil/surfactant mixture. Superparamagnetic iron fluids (mean particle size=11.6±0.4 nm) and ferromagnetic cobalt fluids (mean particle size=51.6±3.4 nm) produced by this technique are promising candidates for magnetic targeting and hyperthermia applications.

High-remanent Sm–Co permanent magnetic alloys modified with Nb and C have been produced with isotropic grain structures consisting of 100–200 nm SmCo7 and 10–15 nm fcc Co phases, with the Co phase comprising only 2–4 vol%. Remanence ratios of 0.7 have been consistently observed, with coercive forces approaching 10 kOe.

The effect of Zr addition on nanostructure and magnetic properties in nanocrystalline Sm1-xZrxCo5 (x = 0-0.6) has been investigated. (Sm, Zr)Co5 with the CaCu5 structure was synthesized by melt spinning. The lattice parameters a and b decrease with x, whereas c increases. Thus, the unit cell volume of (Sm, Zr)Co5 shrinks because the smaller Zr atoms occupy the sites of the larger Sm atoms. Zr addition decreases the grain size and induces the formation of planar defects. The coercivity decreases with x, due to weakening of magnetocrystalline anisotropy energy and effective intergrain exchange coupling. A very high coercivity of 39 kOe and energy product of 13.9 MGOe are obtained for x = 0. The remanence of (Sm, Zr)Co5 increases with x. For x ≤ 0.4, the energy product slightly decreases with x. The results show that 40% of the Sm can be replaced by the less expensive Zr, with an energy-product reduction of only 10%. In addition, the planar defects are responsible for the change of coercivity mechanism from the nucleation-type of reverse domain for the x = 0 to the pinning-type of domain wall for the x = 0.4.

Microalloying of Pr–Fe–B nanocomposite permanent magnets with refractory metals have resulted in the development of very fine two-phase nanostructures. Energy-filtered transmission electron microscopy has been used to characterize the phase and chemistry distributions in the nanostructures. With Hf and Ti additions, α -Fe and Pr 2 Fe 14 B form as discrete phases, forming a granular nanocomposite. However, with Nb additions grains with a discrete chemical segregation pattern were formed. The grain interiors were found to be poor in Fe and rich in Pr, while the grain shells were found to be poor in Pr and rich in Fe, Co and B. An increased coercivity was attributed to this unique structure.

The metastable TbCu 7 -type structure has been formed over a range of compositions ranging from SmCo 5.67 to SmCo 8 . The stabilization was attributed to the addition of Nb combined with rapid solidification processing. As a result, the concentration of transition metal dumbbells varied, while the placement in the structure remained random. The coercivity varied linearly with composition and was highest in Sm-rich compositions (17.5 kOe ) and decreased to ∼3 kOe as Sm content decreased. The variation in coercivity was attributed to a decrease in magnetocrystalline anisotropy, with estimations predicting that the anisotropy varies linearly with composition between values reported for the compounds SmCo 5 and Sm 2 Co 17 .

A Nd–Dy–Fe–B alloy was designed to exhibit high coercivity and optimum loop shapes utilizing a combination of intrinsic and extrinsic alloy design principles. After achieving these initial goals, conventional structural analysis using X-ray and TEM could explain the salient features of the observed hysteresis behavior. However, greater understanding could be gained on the complex dynamic behavior of hysteresis through the exploitation of a new recoil technique that was developed, which focuses on switching field distributions arising from irreversible magnetic behavior. Utilizing this approach, we have successfully linked specific magnetic reversal behavior with distinct magnetic phases that have characteristic microstructural length scales. This new approach may become a powerful tool for the study of the structure–property–hysteresis behavior of hard magnetic materials.

Minor alloying additions to Nd–Fe–B significantly alter the crystallization kinetics of the alloy. The crystallization behavior of stoichiometric Nd 2 Fe 14 B displayed conventional behavior, with an Avrami exponent, n , close to 3. TiC additions stabilized the glassy phase and contributed to a more uniform crystallized grain structure. The time to complete crystallization was extended, implying a change in crystallization kinetics. The kinetic behavior and microstructural evolution implied growth-controlled crystallization. However, substitution additions to form a nine-component alloy displayed anomalous crystallization behavior (large n ). EDS results, which showed partitioning of Ti and C, interfacial enrichment of the rare-earth elements, and interfacial depletion of Fe, suggested that diffusion processes may be important in this system, while microstructural development suggested that the transformation was dominated by a rapidly changing nucleation rate.

The effect of combined alloying additions on the structure and scale of rapidly solidified Sm–Fe alloys was investigated. Transition metal additions tend to promote the formation of the disordered TbCu7-type structure in Sm2Fe17 alloys, as determined by monitoring the long-range order parameter. Essentially no order was observed for M = Ti, Zr, V, or Nb. Thus, the structure was close to the prototypical TbCu7-type structure. With M = Si, a large amount of order was observed (S = 0.62), resulting in a structure closer to the well-ordered Th2Zn17-type. The microstructural scale was also affected by alloying. In this case, refinement depended on the substituent and also on carbon for microstructural refinement. The scale of the as-solidified grain structures ranged from 100 nm for SiC-modified alloys to 13 nm for NbC-modified alloys. The degree of refinement was directly related to the atomic size of the M addition. The refinement was the result of solute partitioning to grain boundaries, resulting in a solute drag effect that lowered the growth rates.

In this paper, the effect of alloy composition on the microstructural development of devitrified Nd–Fe–B-based alloys is investigated. While crystallization of simple ternary Nd2Fe14B resulted in a highly variable microstructure and relatively poor demagnetization behavior, alloying additions, notably Ti and C, were found to significantly refine and homogenize the crystallized microstructure. High spatial resolution energy dispersive X-ray spectroscopy revealed that partitioning to the grain boundaries led to the grain refinement by slowing interface kinetics. The more homogeneous grain structure resulted in improved demagnetization behavior, but segregating elements isolated grains and limited intergranular exchange coupling.

It is investigated how magnetostatic interactions affect the coercivity of alnico-type magnets. Starting from exact micromagnetic relations, we analyze two limits, namely cooperative reversal processes operative on short lengths scales and noncooperative reversal processes on long length scales. In alnicos, intrawire interactions are predominantly cooperative, whereas interwire effects are typically noncooperative. However, the transition between the regimes depends on feature size and hysteresis-loop shape, and interwire cooperative effects are largest for nearly rectangular loops. Our analysis revises the common shape-anisotropy interpretation of alnicos.

Formation of chemically ordered compounds of Fe and Au is inhibited in bulk materials due to their limited mutual solubility. However, here we report the formation of chemically ordered L12-type Fe3Au and FeAu3 compounds in Fe-Au sub-10 nm nanoparticles, suggesting that they are equilibrium structures in size-constrained systems. The stability of these L12-ordered Fe3Au and FeAu3 compounds along with a previously discovered L10-ordered FeAu has been explained by a size-dependent equilibrium thermodynamic model. Furthermore, the spin ordering of these three compounds has been computed using ab initio first-principle calculations. All ordered compounds exhibit a substantial magnetization at room temperature. The Fe3Au had a high saturation magnetization of about 143.6 emu/g with a ferromagnetic spin structure. The FeAu3 nanoparticles displayed a low saturation magnetization of about 11 emu/g. This suggests a antiferromagnetic spin structure, with the net magnetization arising from uncompensated surface spins. First-principle calculations using the Vienna ab initio simulation package (VASP) indicate that ferromagnetic ordering is energetically most stable in Fe3Au, while antiferromagnetic order is predicted in FeAu and FeAu3, consistent with the experimental results.

We present simple, self-assembled, and robust fabrication of ultrahigh density cobalt nanowire arrays. The binary Co-Al and Co-Si systems phase-separate during physical vapor deposition, resulting in Co nanowire arrays with average diameter as small as 4.9 nm and nanowire density on the order of 10(16)/m(2). The nanowire diameters were controlled by moderating the surface diffusivity, which affected the lateral diffusion lengths. High resolution transmission electron microscopy reveals that the Co nanowires formed in the face-centered cubic structure. Elemental mapping showed that in both systems the nanowires consisted of Co with undetectable Al or Si and that the matrix consisted of Al with no distinguishable Co in the Co-Al system and a mixture of Si and Co in the Co-Si system. Magnetic measurements clearly indicate anisotropic behavior consistent with shape anisotropy. The dynamics of nanowire growth, simulated using an Ising model, is consistent with the experimental phase and geometry of the nanowires.

Partially ordered Sm 2 Fe 17 , closer to the SmFe 7 structure, formed upon melt spinning of Sm 11 Fe 89 . A strong dependence of grain size on the wheel speed was also observed, with wheel speeds less than 40 m/s generating grain sizes above the single domain limit for Sm 2 Fe 17 N x . The addition of Ti and C resulted in an order of magnitude grain refinement compared to the binary alloys. The Ti and C also effectively inhibited grain growth during annealing. The nitrided Sm–Fe–Ti–C alloys also displayed a rapid decrease in coercivity with decreasing grain size, which was attributed to increased exchange interactions.

The deformation characteristics of icosahedral Al–Cu–Fe quasicrystals were determined by high temperature creep experiments between 680 and 720 °C and 15 and 41 MPa. The deformation process was determined to be controlled by grain boundary mechanisms. Both the stress and grain size sensitivity exponents were found to be 2, suggesting that grain boundary sliding was the rate-controlling deformation mechanism. Microstructural analysis supported this conclusion, as no intragranular defects were produced during the deformation experiments.

Planar defects were observed in a hot isostatically pressed Al–Cu–Fe alloy. The defects were determined to be perpendicular to the fivefold direction. They were also found to be regions where the quasiperiodicity in the five-fold direction was disrupted, resulting in a periodicity along this direction of 1.06 nm. The defects were stable up to approximately 750 °C. The defects were determined to be formed by the applied shear forces associated with the consolidation process. The displacement vector of the defects was determined to be in a twofold direction perpendicular to the periodic direction.

We report a one-pot synthesis of urchin-like FePd-Fe3O4 nanocomposites, spherical clusters of FePd nanoparticles (NPs) with spikes of Fe3O4 nanorods (NRs), via controlled thermal decomposition of Fe(CO)5 and reduction of Pd(acac)2. The FePd NPs with sizes between 6 and 9 nm self-aggregate into 60 nm superparticles (SPs), and Fe3O4 NRs grow on the surface of these SPs. Reductive annealing at 500 °C converts the FePd-Fe3O4 into exchange-coupled nanocomposites L1(0)-FePd-Fe with their Hc tunable from 0.8 to 2.6 kOe and Ms controlled from 90 to 190 emu/g. The work provides a general approach to L1(0)-FePd-Fe nanocomposite magnets for understanding exchange coupling at the nanoscale. The concept may be extended to other magnetic nanocomposite systems and may help to build superstrong magnets for magnetic applications.