Thin films of iron and permalloy (Ni80Fe20) were prepared using an Ar+N2 mixture with magnetron sputtering technique at ambient temperature. The nitrogen partial pressure, during sputtering process was varied in the range of 0 ≤ RN2 ≤ 100%, keeping the total gas flow at constant. At lower nitrogen pressures (RN2 ≤ 33%) both Fe and NiFe, first form a nanocrystalline structure and an increase in RN2, results in 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 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.
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 has been evidenced experimentally, higher (than pure Co) magnetic moment is evidenced in this work.
The self-diffusion of iron and nitrogen is measured in nm range non-magnetic iron nitride thin films. Two non-magnetic iron nitrides, Fe 2.23 N 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/ 57 Fe-N] 10 and [Fe-N/Fe-15 N] 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. V
In this work we investigate the process of iron nitride (Fe-N) phase formation using 2 at.% Al or 2 at.% Ti as additives. The samples were prepared with a magnetron sputtering technique using different amount of nitrogen during the deposition process. The nitrogen partial pressure (\pn) was varied between 0-50% (rest Argon) and the targets of pure Fe, [Fe+Ti] and [Fe+Al] were sputtered. The addition of small amount of Ti or Al results in improved soft-magnetic properties when sputtered using \pn $\leq$ 10\p. When \pn is increased to 50\p non-magnetic Fe-N phases are formed. We found that iron mononitride (FeN) phases (N at% $\sim$50) are formed with Al or Ti addition at \pn =50% whereas in absence of such addition \eFeN phases (N\pat$\sim$30) are formed. It was found that the overall nitrogen content can be increased significantly with Al or Ti additions. On the basis of obtained result we propose a mechanism describing formation of Fe-N phases Al and Ti additives.Comment: 9 Pages, 7 Figure
In this work, we studied cobalt nitride (Co-N) thin films deposited using a dc magnetron sputtering method at a substrate temperature (T s ) of 523 K. We find that independent of the reactive gas flow (R N 2 ) used during sputtering, the phases of Co-N formed at this temperature seems to be identical having N at.% ∼5. This is contrary to Co-N phases formed at lower T s . For T s ∼300 K, an evolution of Co-N phases starting from Co(N)→Co 4 N→Co 3 N→CoN can be seen as R N 2 increases to 100%, whereas when the substrate temperature increases to 523 K, the phase formed is a mixture of Co and Co 4 N, independent of the R N 2 used during sputtering. We used x-ray diffraction (XRD) to probe long range ordering, x-ray absorption spectroscopy (XAS) at Co absorption edge for the local structure, Magneto-optical Kerr e ffect (MOKE) and polarized neutron reflectivity (PNR) to measure the magnetization of samples. Quantification of N at.% was done using secondary ion mass spectroscopy (SIMS). Measurements suggest that the magnetic moment of Co-N samples deposited at 523 K is slightly higher than the bulk Co moment and does not get affected with the R N 2 used for reactive sputtering. Our results provide an important insight about the phase formation of Co-N thin films which is discussed in this work.
In this work, we measured N self-diffusion in the Co-N system and found an unexpected result that N diffuses out almost completely around 500 K, leaving behind fcc Co irrespective of the amount of N used to deposit Co-N. On the other hand, in previous attempts the Co 4 N phase has always been grown at 550 K or above. In view of our finding, it appears that fcc Co could have been mistaken for Co 4 N, probably due to the closeness of their lattice parameters (LP; fcc Co = 3.54 Å, Co 4 N= 3.74 Å). Therefore, Co 4 N -an interesting material for its high spin-polarization ratio and high magnetic moment remained unexplored. By bringing down the growth temperature, we report the growth of stoichiometric Co 4 N epitaxial thin films. Films were grown using a direct current reactive magnetron sputtering process on LaAlO 3 (LAO mismatch 1.4%) and MgO (mismatch 11.3%) substrates and their structural and magnetic properties were studied. Precise magnetic moment (M s ) of Co 4 N samples were measured using polarized neutron reflectivity and compared with bulk magnetization results. We found that the M s of Co 4 N is higher due to a magnetovolume effect. Unlike previous findings, we observed that substrates induce misfit strain and strain inhomogeneity is the cause of modifications in magnetic ensemble such as coercivity, saturation magnetization, and magnetic anisotropy. A consequence of incoherent strain present in our samples is also reflected in the magnetic anisotropy leading to a superposition of strong fourfold and a small fraction of uniaxial magnetic anisotropy. Obtained results are presented and discussed in this work.
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