The creation of large magnetic fields is a necessary component in many technologies, ranging from magnetic resonance imaging, electric motors and generators, and magnetic hard disk drives in information storage. This is typically done by inserting a ferromagnetic pole piece with a large magnetisation density MS in a solenoid. In addition to large MS, it is usually required or desired that the ferromagnet is magnetically soft and has a Curie temperature well above the operating temperature of the device. A variety of ferromagnetic materials are currently in use, ranging from FeCo alloys in, for example, hard disk drives, to rare earth metals operating at cryogenic temperatures in superconducting solenoids. These latter can exceed the limit on MS for transition metal alloys given by the Slater-Pauling curve. This article reviews different materials and concepts in use or proposed for technological applications that require a large MS, with an emphasis on nanoscale material systems, such as thin and ultra-thin films. Attention is also paid to other requirements or properties, such as the Curie temperature and magnetic softness. In a final summary, we evaluate the actual applicability of the discussed materials for use as pole tips in electromagnets, in particular, in nanoscale magnetic hard disk drive read-write heads; the technological advancement of the latter has been a very strong driving force in the development of the field of nanomagnetism.

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A review of high magnetic moment thin films for microscale and nanotechnology applications

Magnetic nanoparticles (NPs) are prominent in various fields of scientific research and applications such as magnetic energy storage, magnetic fluids, biomedical fields, and catalysis. Metallic Fe NPs are difficult to protect from air oxidation, and iron oxide NPs suffer from reduced magnetization. Iron nitrides have the advantage of retaining high saturation magnetization (MS); for example, α″-Fe16N2 and γ′-Fe4N have average magnetic moments of 2.9 and 2 μB/Fe, respectively, which are comparable to 2.2 μB/Fe for α-Fe. The iron nitrides are ferromagnetic (FM) up to a maximum lattice dilution of 25% nitrogen for e-Fe3N. Even though Fe–N NPs are extremely attractive as magnetic materials, the focus has been majorly on the bulk powders and thin films. This review provides a comprehensive overview of the crystal structures, nitriding kinetics, and synthesis methodologies of binary, doped, and ternary nanostructures, thin films, and bulk materials and their magnetism. Substitution of Fe by any other metal atom...

Iron Nitride Family at Reduced Dimensions: A Review of Their Synthesis Protocols and Structural and Magnetic Properties

Nitride materials are of considerable interest due to their fundamental importance and practical applications. However, synthesis of transition metal nitrides often requires extreme conditions, e.g., high temperature and/or high pressure, slowing down the experimental discovery. Using global structure search methods in combination with first-principles calculations, we systematically explore the stoichiometric phase space of iron–nitrogen compounds on the nitrogen-rich side at ambient and high pressures up to 100 GPa. Diverse stoichiometries in the Fe–N system are found to emerge in the phase diagram at high pressures. Significantly, FeN4 is found to be stable already at ambient pressure. It undergoes a polymerization near 20 GPa which results in a high energy density. Accompanying the polymerization, FeN4 transforms from a direct band gap semiconductor to ferromagnetic metal. We also predict several phase transitions in FeN and FeN2 at high pressure, and the results explain the previous experimental obse...

Prediction of Stable Iron Nitrides at Ambient and High Pressures with Progressive Formation of New Polynitrogen Species

The exchange interactions of Cr4N, Mn4N, Fe4N, Co4N, and Ni4N compounds with perovskite structure were calculated to obtain the Curie temperatures for these compounds from Monte Carlo calculations. Contrary to naive expectation, the exchange interactions vary markedly among these five compounds. In Fe4N, the intra-sublattice interaction of the Fe 3c atoms is strongly negative, leading to a significant reduction of the Curie temperature. The calculated Curie temperatures are 291 K (Cr4N), 710 K (Mn4N), 668 K (Fe4N), 827 K (Co4N), and 121 K (Ni4N), in good agreement with experimental observations where available. The much lower Curie temperature of Ni4N compared to fcc Ni is explained on the basis of the exchange interactions.

https://iopscience.iop.org/article/10.1088/0953-8984/28/5/056006/pdf

Exchange interactions and Curie temperatures of the tetrametal nitrides Cr4N, Mn4N, Fe4N, Co4N, and Ni4N

Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath-core cobalt oxynitride (CoOx@CoNy) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoOx@CoNy nanorods, composed of metallic Co4N cores and oxidized surfaces, exhibit excellent OER activity (E = 1.69 V at 10 mA cm-2) in an alkaline medium. Although pristine NCNF or CoOx@CoNy alone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E = 0.78 V at -3 mA cm-2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoOx@CoNy nanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoOx@CoNy nanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoOx@CoNy/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn-air batteries.

Hierarchically Assembled Cobalt Oxynitride Nanorods and N-Doped Carbon Nanofibers for Efficient Bifunctional Oxygen Electrocatalysis with Exceptional Regenerative Efficiency.

Thin films of iron and permalloy $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ were prepared using an $\mathrm{Ar}+{\mathrm{N}}_{2}$ mixture with a magnetron sputtering technique at ambient temperature. The nitrogen partial pressure during the sputtering process was varied in the range of $0\ensuremath{\leqslant}{R}_{{\mathrm{N}}_{2}}\ensuremath{\leqslant}100%$, keeping the total gas flow at constant. At lower nitrogen pressures $({R}_{{\mathrm{N}}_{2}}\ensuremath{\leqslant}33%)$, both Fe and NiFe first form a nanocrystalline structure, and an increase in ${R}_{{\mathrm{N}}_{2}}$ results in the formation of an amorphous structure. At intermediate nitrogen partial pressures, nitrides of Fe and NiFe were obtained, while at even higher nitrogen partial pressures, nitrides themselves became nanocrystalline or amorphous. The surface, structural, and magnetic properties of the deposited films were studied using x-ray reflection and diffraction, transmission electron microscopy, polarized neutron reflectivity, and using a dc extraction magnetometer. The growth behavior for amorphous film was found to be different as compared with poly or nanocrystalline films. The soft-magnetic properties of FeN were improved on nanocrystallization, while those of NiFeN were degraded. A mechanism inducing nanocrystallization and amorphization in Fe and NiFe due to reactive nitrogen sputtering is discussed in the present article.

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Nanocrystallization and amorphization induced by reactive nitrogen sputtering in iron and permalloy

Thermal stability of nanometer range Ti/Ni multilayers

Cobalt nitride (Co-N) thin films prepared using a reactive magnetron sputtering process are studied in this work. During the thin film deposition process, the relative nitrogen gas flow (RN2) was varied. As RN2 increases, Co(N), Co4N, Co3N and CoN phases are formed. An incremental increase in RN2, after emergence of Co4N phase at RN2 = 10%, results in a linear increase of the lattice constant (a) of Co4N. For RN2 = 30%, a maximizes and becomes comparable to its theoretical value. An expansion in a of Co4N, results in an enhancement of the magnetic moment, to the extent that it becomes even larger than pure Co. Such larger than pure metal magnetic moment for tetra-metal nitrides (M4N) have been theoretically predicted. Incorporation of N atoms in M4N configuration results in an expansion of a (relative to pure metal) and enhances the itinerary of conduction band electrons leading to larger than pure metal magnetic moment for M4N compounds. Though a higher (than pure Fe) magnetic moment for Fe4N thin films ...

Phase formation, thermal stability and magnetic moment of cobalt nitride thin films

The self-diffusion of iron and nitrogen is measured in nm range non-magnetic iron nitride thin films. Two non-magnetic iron nitrides, Fe2.23N and FeN, were studied using neutron reflectivity. Neutron reflectivity with a depth resolution in the sub-nm range has a different scattering cross section for isotopes, providing a unique opportunity to measure very small diffusivities. The isotope heterostructure in thin film multilayers [Fe-N/57Fe-N]10 and [Fe-N/Fe-15N]10 were prepared using magnetron sputtering. It was observed that nitrogen diffuses slower than iron although the atomic size of iron is larger than that of nitrogen. It was found that a significantly larger group of N atoms participates in the diffusion process than of Fe, making N diffusion slower than that of Fe.

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Iron and nitrogen self-diffusion in non-magnetic iron nitrides

Iron self-diffusion in nanocomposite FeZr alloy has been investigated using a neutron reflectometry technique as a function of applied compressive stress. A composite target of $\mathrm{Fe}+\mathrm{Zr}$ and $^{57}\mathrm{Fe}+\mathrm{Zr}$ was alternatively sputtered to deposit chemically homogeneous multilayer (CHM) structure ${[^{\mathit{\text{natural}}}\mathrm{Fe}_{75}{\mathrm{Zr}}_{25}∕^{57}\mathrm{Fe}_{57}{\mathrm{Zr}}_{25}]}_{10}$. The multilayers were deposited onto a bent Si wafer using a three-point bending device. Post-deposition, the bending of the substrate was released which results in an applied compressive stress on to the multilayer. In the as-deposited state, the alloy multilayer forms an amorphous phase, which crystallizes into a nanocomposite phase when heated at $373\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Bragg peaks due to isotopic contrast were observed from CHM, when measured by neutron reflectivity, while x-ray reflectivity showed a pattern corresponding to a single layer. Self-diffusion of iron was measured with the decay of the intensities at the Bragg peaks in the neutron reflectivity pattern after thermal annealing at different temperatures. It was found that the self-diffusion of iron slows down with an increase in the strength of applied compressive stress.

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Rachana Gupta

Papers

Nanocrystallization and amorphization induced by reactive nitrogen sputtering in iron and permalloy

Thermal stability of nanometer range Ti/Ni multilayers

Phase formation, thermal stability and magnetic moment of cobalt nitride thin films

Iron and nitrogen self-diffusion in non-magnetic iron nitrides

Iron self-diffusion in Fe Zr ∕ Fe 57 Zr multilayers measured by neutron reflectometry: Effect of applied compressive stress