Nian Ji

N site ordering effect on partially ordered Fe16N2

Partially ordered Fe16N2 thin films have been fabricated on Fe (001)-buffered GaAs (001) single-crystal substrates by a facing target sputtering process. The saturation magnetization has been systematically investigated as a function of N site ordering in partially ordered Fe16N2 thin films, which is found to be increased monotonically with the increase in the N site ordering parameter, reaching up to 2.68 T at high ordering case. A model discussion is provided based on the partial localization of 3d electron states in this material system, which successfully rationalizes the formation of the giant saturation magnetization in chemically ordered Fe16N2. We further demonstrate that the average magnetic moment of partially ordered Fe16N2 sensitively depends on the special arrangement of Fe6N clusters, which is the key to realize high magnetic moment in this material system.

Theory of giant saturation magnetization in α″-Fe16N2: role of partial localization in ferromagnetism of 3d transition metals

A new model is proposed for the ferromagnetism associated with partially localized electron states in the Fe16N2 system that explains its giant saturation magnetization. It is demonstrated that an unusual correlation effect is introduced within the Fe–N octahedral cluster region and the effective on-site 3d–3d Coulomb interaction increases due to a substantial 3d electron charge density difference between the cluster and its surroundings, which leads to a partially localized electron configuration with a long-range ferromagnetic order. The first-principles calculation based on the LDA+U method shows that giant saturation magnetization can be achieved at sufficiently large Hubbard U values. The feature of the coexistence of the localized and itinerant electron states plays a key role in the formation of giant saturation magnetization.

Strain induced giant magnetism in epitaxial Fe16N2 thin film

We report a direct observation of giant saturation magnetization in Fe16N2. By exploiting thin film epitaxy, which provides controlled biaxial stress to create lattice distortion, we demonstrate that giant magnetism can be established in Fe16N2 thin film coherently grown on MgO (001) substrate. Explored by polarized neutron reflectometry, the depth-dependent saturation magnetic induction (Bs) of epitaxial Fe16N2 thin films is visualized, which reveals a strong correlation with the in-plane lattice parameter and tensile strain developed at near substrate interface. With controlled growth process and dimension adjustment, the Bs of these films can be modulated over a broad range, from ∼2.1 Tesla (T) (normal Bs) up to ∼3.1 T (giant Bs).

Fabrication of Films by Sputtering Process and Experimental Investigation of Origin of Giant Saturation Magnetization in

We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe16N2 and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe16N2 is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe16N2+Fe8N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed “cluster+atom” theory provides promising directions on designing novel magnetic materials with unique performances.

The effect of strain induced by Ag underlayer on saturation magnetization of partially ordered Fe16N2 thin films

Partially ordered Fe-N thin films were grown by a facing target sputtering process on the surface of a (001) Ag underlayer on MgO substrates. It was confirmed by x-ray diffraction that the Ag layer enlarged the in-plane lattice of the Fe-N thin films. Domains of the ordered α″-Fe16N2 phase within an epitaxial (001) α′-FexN phase were identified by electron diffraction and high-resolution aberration-corrected scanning transmission electron microscopy (STEM) methods. STEM dark-field and bright-field images showed the fully ordered structure of the α″-Fe16N2 at the atomic column level. High saturation magnetization(Ms) of 1890 emu/cc was obtained for α″-Fe16N2 on the Ag underlayer, while only 1500 emu/cc was measured for Fe-N on the Fe underlayer. The results are likely due to a tensile strain induced in the α″-Fe16N2 phase by the Ag structure at the interface.

Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings

It was demonstrated that Fe–N martensite (α′) films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe16N2 films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe8N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe8N phase measured by VSM was evaluated in the range of 2.0–2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in α″-Fe16N2 phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 0630...

Strain effect of multilayer FeN structure on GaAs substrate

Overly doped FeN multilayer structure on GaAs substrate was fabricated. After the post-annealing process, FeN martensite in each Fe/FeN layer formed partially chemically ordered Fe16N2, which was observed by X-ray diffraction. To detect the saturation magnetization (Ms) depth profile, polarized neutron reflectivity was conducted. Fe/FeN layer showed a significant improvement of Ms for each layer compared to Ms of Fe. More importantly, different FeN layers showed different Ms according to the physical distance to the substrate GaAs. The most enhanced Ms (exceeding the limit of Fe65Co35 Ms) observed at the bottom part of the film, consistent with previous reports, should be attributed to the lattice strain by GaAs substrate. In order to detect the lattice constant, In-plane X-ray Diffraction was done and a large in-plane lattice constant was determined.

GROWTH AND DEPTH-DEPENDENCE OF SATURATION MAGNETIZATION OF IRON NITRIDE THIN FILMS ON MgO SUBSTRATE

We used polarized neutron reflectometry (PNR) to investigate the depth-dependent saturation magnetization (4πMs) of bi-layer structured Fe–N/Fe grown on MgO substrates, which were prepared by a facing target sputtering method followed by a subsequent annealing process to purposely study the N inter-diffusive effect. It is observed that by tuning the Fe layer thickness from 2 nm to 5 nm, the magnetic properties of the resulting product varies substantially. According to X-ray diffraction, an additional peak, indexed to Fe16N2(004), was developed in the sample with thinner (2 nm) underlayer. Its corresponding PNR study shows a 4πMs of up to 2.82 T towards the substrate interface, which is 14% higher than the known limit (Fe65Co35 ~ 4πMs = 2.45 T). We attribute this giant magnetization to the presence of chemically ordered Fe1N2. We have seen evidence that the high Ms is favorably stabilized due to the lattice misfit. The thickness range is consistent with the strained region of the films. This depth-dependent saturation magnetization analysis can shed a light to help understand the previous magnetization reports by classical magnetometry methods for the samples prepared using the same underlayer.

Fabrication of Fe16N2 Films by Sputtering Process and Experimental Investigation of Origin of Giant Saturation Magnetization in Fe16N2

We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe16N2 and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe16N2 is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe16N2+Fe8N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed “cluster+atom” theory provides promising directions on designing novel magnetic materials with unique performances.

Fabrication of

We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe16N2 and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe16N2 is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe16N2+Fe8N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed “cluster+atom” theory provides promising directions on designing novel magnetic materials with unique performances.