Basic Concepts of Nitride Magnetic Materials and Their Applications

Transition metal nitrides are technologically important because of their unique physical and chemical properties. Research on metal nitrides is scanty, due to their synthetic difficulty. Also, the stability of the metal nitrides is a matter of concern, as most nitrides are sensitive to oxygen and form oxides in the air atmosphere. Therefore, studies on metal nitrides are not explored much. This chapter deals with a general introduction to transition metal nitrides including their classification, crystal structure and bonding. The current chapter investigates novel synthesis and magnetic properties study of various binary and ternary transition metal nitrides such as: ε-CoxFe3-xN (0 ≤ x ≤ 0.4), γ′-NixFe4-xN (x = 0.2, 0.4, 0.6, 0.8), γ-Mo2N, γ-CoxMo2-xN (x = 0.25), FeMoN2, β-W2N and CoWN2. A general discussion on magnetism in bulk and nano form has been presented. Various novel synthesis routes have been developed and adopted to obtain the above-mentioned nitrides as pure-phase nano-structures. The observed properties have been correlated to the size, shape and structure of the materials. Size and surface effects become predominant at nano scale leading to variation in intrinsic properties, which is evident in our research findings. ε-Fe3N and γ′-Fe4N materials have been synthesized via the citric acid-assisted sol–gel method. The effect of Co substitution on the structural and magnetic properties of ε-phase iron nitride has been studied in detail. With a gradual increase of Co content (x) to 0.2 at wt%, the ε-Fe3N phase was stabilized. However, with further increase of Co content (x) to 0.3 and 0.4 at wt%, a small amount of α-Fe precipitation in the nitride product as an impurity was observed. The values of saturation magnetization, Ms and coercivity, Hc, of ε-CoxFe3-xN (0 ≤ x ≤ 0.4) system of nitrides were found to be in the range of 8–39 emu/g and 199–219 Oe, respectively. The values of Ms are found to increase with a slight initial decrease with a progressive increase in Co content. The effect of Ni substitution on the structural and magnetic properties of γ′-Fe4N nitrides has been investigated. Insertion of Ni atoms into the host lattice expands the lattice due to nano size and strain effect, dominantly. A maximum value of Ms, i.e. 181 emu/g, was obtained for γ′-Ni0.6Fe3.4N material. The observed values of Hc are in the range of 76–109 Oe. γ-Mo2N and γ-Co0.25Mo1.75N have been synthesized by using various precursor routes and using solid urea and gaseous ammonia as nitridation agents. Hydrazine and potassium borohydride have been used for the synthesis of precursors of γ-Mo2N and γ-Co0.25Mo1.75N, respectively for the nitridation using the ammonia route. Ethylenediamine has been used as an organic source for the precursor synthesis of γ-Mo2N obtained via the urea route. M versus H hysteresis study indicates weak ferromagnetic behaviour of γ-Mo2N and γ-Co0.25Mo1.75N (ammonia route) and γ-Mo2N (urea route) materials having coercivity values of 2838 Oe, 296 Oe and 880 Oe, respectively. Pure phase β-W2N and ternary CoWN2 materials have been synthesized using the citric acid-based sol–gel method followed by nitridation using urea and ammonia route, respectively. The values of saturation magnetization, Ms and coercivity, Hc, are estimated to be 0.031 emu/g and 759 Oe, respectively for crystalline β-W2N synthesized at 750 °C. The values of Ms and Hc of CoWN2 synthesized at 750 °C are estimated to be 1.71 emu/g and 375Oe, respectively.