| Author(s) |
Keita Ito, GeunHyoung Lee, Kazunori Harada, Mao Ye, Yukiharu Takeda, Yuji Saitoh, Takashi Suemasu, Akio Kimura, Hiro Akinaga |
| Abstract Scope |
Ferromagnetic γ’-Fe<sub>4</sub>N has potential interest for application to spintronics devices because the spin polarization of electrical conductivity at the Fermi level was calculated to be -1.0. In addition, γ’-Fe<sub>4</sub>N has a relatively small lattice mismatch of 1.3% to Si(100). Thus, γ’-Fe<sub>4</sub>N is considered an appropriate material for application in spin injection electrodes to Si(100). However, the magnetic moments of γ’-Fe<sub>4</sub>N remain unclear from an experimental point of view. The saturation magnetization per volume (<i>M</i><sub>S</sub>) in γ’-Fe<sub>4</sub>N on LAO(100), STO(100) and MgO(100) substrates by sputtering was reported to increase with decreasing lattice mismatch between γ’-Fe<sub>4</sub>N and the substrate used, that is 0, 3, and 11%, respectively. It is also still under discussion whether the <i>M</i><sub>S</sub> value of γ’-Fe<sub>4</sub>N is really larger than that of α-Fe, that is approximately 2.2 <i>μ</i><sub>B</sub> per Fe atom. In order to get the accurate <i>M</i><sub>S</sub> value of γ’-Fe<sub>4</sub>N, we think high-quality γ’-Fe<sub>4</sub>N epitaxial films should be employed, and the <i>M</i><sub>S</sub> value should be evaluated by x-ray magnetic circular dichroism (XMCD), because the <i>M</i><sub>S</sub> value can be obtained free from its volume with XMCD. There have been several reports on XMCD measurements of γ’-Fe<sub>4</sub>N. However, <i>M</i><sub>S</sub> values were not systematically evaluated for γ’-Fe<sub>4</sub>N epitaxial films grown on different substrates. We prepared 10-nm-thick γ’-Fe<sub>4</sub>N films epitaxially by molecular beam epitaxy (MBE) using solid Fe and a raido-frequency NH<sub>3</sub> plasma on LAO(100) (sample A) and MgO(100) (sample B) substrates. After the growth of γ’-Fe<sub>4</sub>N layers, 3-nm-thick Au capping layers were subsequently deposited at RT in the same MBE chamber to prevent oxidation of the surfaces. X-ray diffraction patterns showed that the γ’-Fe<sub>4</sub>N films were not strained. The magnetic moments were deduced from XMCD measurements at 300 K. Circularly polarized x-rays were incident perpendicular to the sample surface with an external <i>H</i> of ±3 T, applied perpendicular to the sample surface. We confirmed that the magnetic moments of samples were saturated under <i>H</i> = 3 T. The total magnetic moments of γ’-Fe<sub>4</sub>N in samples A and B were calculated to be 2.44 ± 0.06 <i>μ</i><sub>B</sub> and 2.47 ± 0.06 <i>μ</i><sub>B</sub> per Fe atom, respectively. These are almost the same, and are much closer to the theoretically predicted value of 2.59 <i>μ</i><sub>B</sub>. Therefore, we conclude that the <i>M</i><sub>S</sub> value in γ’-Fe<sub>4</sub>N does not change depending on lattice mismatch between γ’-Fe<sub>4</sub>N and the substrate used as long as γ’-Fe<sub>4</sub>N is not strained. We should also note that the <i>M</i><sub>S</sub> value of γ’-Fe<sub>4</sub>N is significantly larger than that of α-Fe. |