Origin of anomalous diffusion in iron mononitride thin films

Gupta

et al. 2019

Phys. Rev. B

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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.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

See 2 more Smart Citations

Structural and magnetic properties of stoichiometric Co4N epitaxial thin films

Gupta

et al. 2019

Phys. Rev. B

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“…4(b)] accompanied by a Gaussian distribution in all three samples. This linear tail indicates La diffusion through grain boundaries [44][45][46] beyond the interface into the Fe 4 N films. It is known that the slope of the linear relation between ln(SIMS counts) and Z 6/5 yields the grain boundary diffusion (D g ), if volume diffusion (D v ) is known [44][45][46].…”

Section: B Structural and Magnetic Depth Profiles Of Fe 4 N Filmsmentioning

confidence: 99%

Effect of interfacial interdiffusion on magnetism in epitaxial Fe4N films on LaAlO3 substrates

Pütter

Amir

et al. 2019

Phys. Rev. Materials

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Epitaxial Fe 4 N thin films grown on LaAlO 3 (LAO) substrate using sputtering and molecular beam epitaxy techniques have been studied in this work. Within the sputtering process, films were grown with conventional direct current magnetron sputtering (dcMS) and using a high power impulse magnetron sputtering (HiPIMS) process. Surface morphology and depth profile studies on these samples reveal that HiPIMS deposited film has the lowest roughness, the highest packing density, and the sharpest interface. We found that the substrate-film interface and the microstructure play a vital role in affecting the electronic hybridization and magnetic properties of Fe 4 N films. La from the LAO substrate and Fe from the film interdiffuse and form an undesired interface. The magnetic moment (M s) was compared using bulk, element-specific and magnetic depth profiling techniques. We found that M s was the highest when the thickness of the interdiffused layer was lowest and such conditions can only be achieved in the HiPIMS grown samples. The presence of a small moment at the N site was also evidenced by element-specific x-ray circular dichroism measurement in the HiPIMS grown sample. A large variation in the M s values of Fe 4 N films found in the experimental works carried out so far could be due to such an interdiffused layer which is generally not expected to form in otherwise stable oxide substrate at a low substrate temperature ≈675 K. In addition, a consequence of substrate-film interdiffusion and microstructure resulted in the different kinds of magnetic anisotropies in Fe 4 N films grown using different techniques. A detailed investigation of the substrate-film interface and microstructure on the magnetization of Fe 4 N film is presented and discussed in this work.

Structure, Thermal Stability, and Magnetism of Ni₄N Thin Films

Physica Rapid Research Ltrs

Gupta

Stahn

2020

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Herein, the synthesis, structure, thermal stability, and magnetic properties of Ni 4 N thin films are studied. Ni 4 N is difficult to synthesize in the correct chemical order and stoichiometry due to unfavorable thermodynamics. During the synthesis of Ni-N thin films, it is found that the substrate temperature (T s) is a critical parameter affecting the growth of a Ni 4 N phase. The Ni 4 N phase transforms into an amalgamation of [NiþNi 3 N] phases even if the T s rises just above 300 K. These results elucidate a correlation between atomic diffusion and T s. N self-diffusion measurements carried out using secondary-ion mass spectroscopy indicate that substantial N self-diffusion is occurring at low T s. A comparison of N self-diffusion coefficients in Fe-N, CoN , and Ni-N indicates that N self-diffusion in Ni-N lies between that of Fe-N and CoN and exhibits a greater correlation with the enthalpy of formation. A transformation from the cubic-to-tetragonal deformation in the Ni 4 N crystal lattice as a function of increasing N concentration is evident from the X-ray diffraction and X-ray absorption near-edge spectroscopy measurements. Magnetization measurements confirm a nonferromagnetic state of Ni 4 N at 300 K, which transforms into a ferromagnetic state at 15 K.