Thesis Supervisor:  Prof. N. S. Gajbhiye

Month and year of thesis submission: October, 2005

            The magnetism of nanostructured metal nitrides continues to receive attention from both fundamental and technological point of view. Metal nitrides are commercially important because of their technological applications as refractory, superconducting, semiconducting, catalytic and magnetic materials. Many of these nitrides are difficult to synthesize because they are unstable, air sensitive and reactive in atmospheric conditions and therefore, metal nitrides are not explored much. The present study investigates the synthesis, characterization and magnetic interactions of the following nitrides: e-Fe₃N, e-Fe_3-xNi_xN (0.0 £ x £ 0.8), e-Fe₃N-GaN and e-Fe₃N-CrN nanocomposite systems. These nitrides are synthesized by precursor method and characterized for their nanocrystalline nature, superparamagnetic/ferromagnetic behavior and in particular for their spin structure. The intrinsic magnetic properties of nanosize magnetic nitride materials depend on the microstructure, interparticle magnetic interactions and particle size. The consequences of nanostructured nature for the nitride materials are studied for the following representative systems: e-Fe_2.6Ni_0.4N nanoparticles, e-Fe₃N/GaN, 54/46-composite nanowires and e-Fe₃N/CrN, 53/47-composite nanorods. These nanoparticles, nanowires and nanorods are made with core-shell morphology/structure and explored extensively for their magnetic properties. These aspects form the basis for the present study and the results are discussed in detail and presented in seven chapters of the thesis.

The introduction chapter 1 reviews the classification, crystal structure and bonding of nitride materials, which are important and relevant for the present study. Also, the salient features of nanostructured magnetic materials, new aspects of magnetism in nanocrystalline solids and important areas of conventional magnetism are discussed. The particle size effect and surface effects including magnetization reversal mechanism, structural and magnetic disorder, random exchange anisotropy, superparamagnetism and interparticle interactions for the nitride nanoparticles are presented. The intrinsic magnetic properties (saturation magnetization (s_s), Curie temperature (T_c), magnetic hyperfine field (H_hf) and magnetic anisotropy (K)) are found to change from the bulk. The spin glass, exchange bias and superparamagnetic phenomena are briefly highlighted from the experimental point of view. Also, the basic concepts and definitions on photoluminescence measurements for optical materials are presented. The available literature on metal nitrides and the scope of the present work are also discussed.    

In chapter 2, the citrate precursor technique and its significance for the synthesis of nanostructured nitrides, the chemical analyses methods for the determination of stoichiometry are given. The brief description of techniques: XRD, SEM/TEM and AFM that are used for the structural characterization, morphology, size and distribution are presented. XRD-Rietveld analysis program is described to calculate the lattice parameters, space group and 3D-structures. The primary details about X-ray photoelectron spectroscopy (XPS) and ⁵⁷Fe Mössbauer spectroscopy techniques, which are used for the study of microscopic structural aspects are highlighted. The details of various magnetometers like VSM, SQUID and AC susceptibility that are used to measure the magnetization of nanostructured nitride systems as a function of magnetic field, temperature and frequency are discussed. The photoluminescence measurement technique is also outlined.

In chapter 3, the details of synthesis and structural characterization of nanostructured metal nitride systems: e-Fe₃N, e-Fe_3-xNi_xN (0.0 £ x £ 0.8), e-Fe₃N-GaN and e-Fe₃N-CrN are presented. Ferromagnetic e-Fe₃N nanoparticles (50-70 nm) are prepared at 973 K for 4 h in NH₃ (g) atmosphere with flow rate of 120 cm³/min. XRD-Rietveld analysis shows the hexagonal structure with space group P6₃/mmc. The nanostructured e-Fe_3-xNi_xN (0.0 £ x £ 0.8) system is prepared by simultaneous decomposition and nitridation of the precursor, in the temperature range 673 K – 823 K for 12 h and flow rate of 200 cm³/min. XRD-Rietveld analysis shows that pure hexagonal structure corresponding to e-Fe₃N is formed for x = 0.0-0.4 compositions; whereas a mixture of hexagonal e-Fe_3-xNi_xN and fcc g'-Fe_4-yNi_yN phases are obtained for x = 0.5-0.8 compositions. The space groups of e-Fe_3-xNi_xN and g'-Fe_4-yNi_yN phases are obtained as P6₃/mmc and P3m respectively. The crystallite size is found to be in the range of 19-25 nm and the unit cell volume remains nearly constant with the increase of Ni concentration. e-Fe₃N-GaN nanoparticles and nanowires are prepared by nitridation of the oxide precursors at 823 K – 1023 K for 24 h and flow rate of 200 cm³/min. For e-Fe₃N/GaN, 85/15 system, single phase e-Fe_2.8Ga_0.2N nanoparticles is formed. For e-Fe₃N/GaN, 73/27, 62/38 and 54/46 systems, e-Fe₃N-GaN nanocomposites, with core-shell structure are formed; where GaN forms the shell and e-Fe₃N core. For e-Fe₃N/GaN, 54/46 nanocomposite, core-shell nanowires are obtained. XRD-Rietveld analysis shows that e-Fe₃N and GaN crystallize in the space group P6₃/mmc and P6₃mc respectively. The average size of the particles is 25-28 nm, while the diameter and length of the nanowires are 25 nm and 1 mm respectively. e-Fe₃N-CrN nanoparticles and nanorods are prepared by nitridation of the oxide precursors at 983 K – 1103 K for 24 h and flow rate of 240 cm³/min. For e-Fe₃N/CrN, 81/19 composition, single phase e-Fe_2.8Cr_0.2N is formed. The systems with e-Fe₃N/CrN, 68/32, 58/42 and 53/47 compositions, the nanocomposites are formed, whereas the e-Fe₃N/CrN, 53/47 composition forms the nanocomposites and nanorods. XRD-Rietveld analysis shows that e-Fe₃N and CrN crystallize in the space group P6₃/mmc and Fmm respectively. The average size of the particles is found 25-30 nm, while the average diameter and length of the nanorods is 30 nm and 400 nm respectively.

In chapter 4, the magnetic properties of e-Fe_3-xNi_xN (0.0 £ x £ 0.8) system are discussed. XPS studies reveal the presence of oxynitride/oxide surface layer (shell) at the metal nitride core, for compositions x = 0.0 and 0.4 systems. ⁵⁷Fe Mössbauer experiments showed superparamagnetic doublets for x = 0.0-0.4 systems and for x = 0.5-0.8 systems, the mixture of superparamagnetic doublet and ferromagnetic sextets are observed. For e-Fe₃N nanoparticles (70 nm), the s_s value measured at 5 K and 3.0 T is 128.0 emu/g, which is very close to the literature reported value of 133.0 emu/g. However, for             e-Fe_3-xNi_xN (0.0 £ x £ 0.8) system, the s_s values at 5 K are found in the range 15.96 emu/g to 66.03 emu/g. The reduction of the s_s values is because of the presence of spin pairing effect, superparamagnetic fractions and spin canting at the surface layer. The ZFC magnetization curves exhibit the blocking of superparamagnetic nanoparticles close to 300 K. The strong dipolar interactions are evident from the weak cusp observed at ~130 K. A critical temperature or spin freezing temperature (T_f) is identified with the spin-glass like ordering process below 30 K, in ZFC and FC magnetization curves. The peak observed in ZFC curves at T_f1 is mainly attributed to the random field exchange anisotropy at the non-uniform interface of antiferromagnetic oxide/oxynitride surface layer and ferromagnetic nitride core. However, the peak in FC curves (T_f2) is attributed to the reentrant spin-glass behavior, due to the coexistence of both spin-glass and ferromagnetic behavior. In these nanocomposites, the incorporation of ferromagnetic nickel provides the percolation path for the reentrant spin-glass nature. The frequency and field dependent ac-susceptibility studies confirmed the spin-glass like ordering. The analyses of data fittings in dynamic scaling equations yields the parameters, t₀ = 10^-8 s, T_c = 31 K, and zn = 11.0. The value of zn is obtained in the range 4 to 12, which is also typically observed in other spin glass systems. The magnetic phase diagram is constructed from the dc magnetization studies and that showed the magnetic transitions for the entire composition range.

            In chapter 5, the magnetic and photoluminescence properties of e-Fe₃N-GaN system are discussed. The core-shell morphology of the e-Fe₃N/GaN, 54/46-composite nanowires is probed by XPS studies. In-depth profile analysis showed that e-Fe₃N phase forms the core and GaN, the shell with a large interface region in nanowires. Room temperature ⁵⁷Fe Mössbauer experiments show the presence of superparamagnetic doublets. At 5 K, the s_s values are observed in the range 14.68-31.76 emu/g. The reduction of s_s values from 133.0 emu/g of e-Fe₃N is attributed to the superparamagnetic fractions, spin pairing effect and the random distribution of the ferromagnetic e-Fe₃N in the diamagnetic GaN matrix within the interface region. The exchange bias effects, though smaller are observed for e-Fe₃N/GaN, 85/15 and 73/27-nanocomposite systems. The ZFC/FC magnetization curves show that the superparamagnetic blocking occurs near 300 K (T_B). The strong dipolar interactions are evident from the weak cusp in ZFC curve around 118-130 K (T_D). Below 50 K, the peak in ZFC magnetization is attributed to the spin-glass like ordering (T_f). The mechanism that causes the spin freezing phenomenon is due to the random field exchange anisotropy of localized ferromagnetic e-Fe₃N clusters precipitated in GaN matrix and also, the exchange coupling between antiferromagnetic Fe-O-N and ferromagnetic e-Fe₃N at the interface. The spin-glass phase is confirmed by the thermo-remnant magnetization and the ac susceptibility measurements for the representative e-Fe₃N/GaN, 54/46-composite nanowires. The spin freezing temperature, T_f is found dependent on frequency and external applied field in ac susceptibility study. The dynamic scaling equations and the data fitting analysis yield the parameters:             t₀ = 10^-12 s, T_c = 54.1 K, and zn = 8.0. The room temperature photoluminescence (PL) spectra showed the absorptions at 412-420 nm and 428-437 nm and showed the presence of surface states and defects in the GaN shell. The weak parasitic yellow luminescence band (525-532 nm) is observed due to the parasitic effects for e-Fe_2.8Ga_0.2N and e-Fe₃N/GaN, 73/27 and 62/38-nanocomposites; but it disappears for e-Fe₃N/GaN, 54/46-composite nanowires.

            In chapter 6, the magnetic properties of e-Fe₃N-CrN nanocomposite/nanorods are discussed. The XPS study of nanocomposites/nanorods shows unreacted metallic impurity of Cr, as CrN/Cr with antiferromagnetic nature. ⁵⁷Fe Mössbauer experiments showed superparamagnetic doublet, for all the systems. At 10 K, the s_s values are in the range 12.63 to 14.64 emu/g, which is lower as compared to pure e-Fe₃N phase (133.0 emu/g). The reduced s_s values are attributed to the presence of superparamagnetic fractions, spin pairing effects, surface spin non-colinearity and the magnetic interactions between antiferromagnetic CrN and ferromagnetic e-Fe₃N throughout the volume of the nanoparticles/nanorods. The exchange bias effects are studied with cooling fields (zero and 1 T) for all the systems and as a function of temperature and cooling fields for e-Fe₃N/CrN, 53/47-composite nanorods. The exchange bias (hysteresis loop shift) is observed significantly at low temperatures and found highest (78 Oe) for the e-Fe_2.8Cr_0.2N composition as compared to other nanocomposite systems. The results obtained are corroborated with the roughness of AF-FM interface that induces disorder due to the random exchange anisotropy. The temperature dependent magnetization curves showed T_B near 300 K. The weak shoulder in ZFC curves around 115-134.3 K, indicates T_D and the strong dipolar interactions. The Néel temperature (T_N) of antiferromagnetic CrN phase is observed in the range 280 K to 286 K. Also, T_N is found to decrease with the increase in applied field due to the presence of uncompensated spins in the CrN/Cr phase in e-Fe₃N-CrN nanorods. The strong/broad overlapping peaks in ZFC (T_E @ T_f), below 65 K for all e-Fe₃N-CrN systems are attributed to the enhancement of unidirectional anisotropy (T_E), arising from the exchange bias between the AF-FM interfaces and the spin-glass like ordering (T_f) arising from the localized disordered interactions of the randomly oriented spins. The temperatures, T_E @ T_f decrease drastically from 48.56 K to 20.49 K with the increase in applied field from 0.002 T to 0.010 T for e-Fe₃N/CrN, 53/47-composite nanorods. The ac susceptibility curves plotted at different applied fields and frequencies confirm the existence of the spin-glass phase in e-Fe₃N-CrN nanorods, and exhibit two different cusps: the overlapping sharp/broad peaks (T_E @ T_f), around 28 K, arising from the random exchange anisotropy and spin-glass like ordering. However, the cusp (T_D), around 133 K is due to the strong dipolar interactions. The thermo-remnant magnetization studies confirm the presence of antiferromagnetic ordering. The ac susceptibility studies, at different frequencies confirm the spin-glass like ordering. The dynamic scaling equations and the data fitting analyses yield parameters as: t₀ = 10^-8 s, T_c = 30.8 K, and zn = 10.0 and the zn value obtained is in the range of 4 to 12 confirms the spin glass nature that is typically observed in other similar spin glass systems too.

            In chapter 7, the summary of the present work and the future prospects are presented.