Polarized-neutron-diffraction study of the microscopic magnetic structure in ${\alpha }^{\prime \prime }\text{−}\mathrm{Fe}{}_{16}{\mathrm{N}}_2$ nanoparticles

Polarized-neutron-diffraction (PND) measurements were carried out using a pseudo-single-phase powder sample of ferromagnetic ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ nanoparticles. For the well-identified ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ phase, sizes of the magnetic moments at the three crystallographic Fe sites were determined in the absolute scale. The agreement between the magnetization value deduced from the present PND and that measured by a magnetometer $\left(M_{\mathrm{VSM}}\right)$ supports the hypothesis that $M_{\mathrm{VSM}}$ is primarily caused by the magnetization value in the target ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$; thus there is no evidence for macroscopic giant saturation magnetization, at least for ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$ nanoparticles. On the basis of the large magnetic moment size at one of the Fe sites, a possible coexisting state of localized spins and itinerant electron spins is inferred. Drawing a distinction between thin films and nanoparticles is currently necessary because of their divergent magnetic evolutions.

Figure 1

(a) Raw $D$ profiles of $I^{+}$ and $I^{-}$. The series of bars at the bottom indicate $D_{hkl}$ positions calculated from the crystal parameters of ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2,\alpha$-Fe, and Al. The non-negligible Al scattering comes not from the sample itself but possibly from an Al attachment above the sample holder. (b) Difference plot of $I^{+}-I^{-}$. The inset shows a magnification of the large $D$ region. Thirteen Bragg reflections, denoted by green indices, are used for analysis (see Supplemental Material [22]), and the resultant fit is shown by the green curve.

Figure 2

Prefactor-corrected net integrated intensity $A_{hkl}^{\mathrm{obs}}$ versus the calculated interference term $\left(\propto F_{\mathrm{N}}{F}_{\mathrm{M}}\right)$. In this plot, $m_1=0.8m_2,m_3=1.5m_2,B_{\mathrm{Fe}}=4.8$ Å${}^2$, and $B_{\mathrm{N}}=2.7$ Å${}^2$ are used. The lower inset magnifies the crowd portion around the origin. The upper inset shows a unit cell of ${\alpha }^{\prime \prime }\text{−}{\mathrm{Fe}}_{16}{\mathrm{N}}_2$, drawn by vesta [23].