Research Center for Magnetic and Spintronic Materials

(ESICMM-G8 Symposium on Next Generation Permanent Magnets, Tsukuba, 2015)

Magnetic properties of NdFe₁₂N_x thin film

Y. Hirayama, Y.K. Takahashi, S. Hirosawa and K. Hono

Elements Strategy Initiative Center for Magnetic Materials (ESICMM),
National Institute for Materials Science, Sengen 1-2-1, Tsukuba 305-0047, Japan

Abstract:

I. INTRODUCTION
A higher saturation magnetization M_S is expected for R-Fe compounds (R : rare-earth element) with ThMn₁₂ structure since the ratio of Fe is the highest in R-Fe system. However, the RFe₁₂ compounds do not form spontaneously in normal methods since RFe₁₂ compounds are unstable. To stabilize the ThMn₁₂ structure in R-Fe system, Fe needs to be substituted partially with structure stabilized elements M [1] i.e. R(Fe,M)₁₂. Recently, the M_S of 2.06 T for NdFe₁₂N was estimated by using the first principle calculation [2,3]. In this work, we studied the possibility of obtaining the NdFe₁₂N_x phase without any M to achieve a higher M_S than that of Nd₂Fe₁₄B.

II. EXPERIMENTAL
The NdFe₁₂ film of ~70 and ~360 nm were epitaxially grown at 650°C on the 20 nm (001) oriented W under layer deposited on the MgO(001) single crystal substrate using UHV DC-magnetron co-sputtering systems. Then, NdFe₁₂N_x films were obtained by nitriding in the N₂ atmosphere of 1.0 Pa at 500°C for 1 hour. XRD (RIGAKU, SmartLab), TEM (FEI, TitanG2 80-200) and VSM (Quantum Design, Inc. MPMS SQUID VSM) were used to evaluate the crystal structure, micro structure and magnetic properties, respectively.

Figure 1, STEM/EDX image and nano
beam diffraction of The NdFe₁₂N_x with an
average thickness of 357 ± 2 nm [3].

III. RESULT AND DISCUSSION
XRD spectra indicated that epitaxial NdFe₁₂N_x film with ThMn₁₂ structure has been prepared although the small amount of α-Fe phase formed. The c-axis was perpendicular to the MgO(100) plane. According to the hysteresis curve at room temperature, the intrinsic NdFe₁₂N_x M_S of 1.66 ± 0.08 T was determined by subtracting the contribution of the α-Fe phase which was estimated by the STEM/EDS map and the magnetization curve at room temperature. T_C of 550 °C was estimated. The NdFe₁₂N_x with an average thickness of 357 ± 2 nm shown in figure 1 were also obtained, indicating that there are possibilities to obtain the bulk compound with ThMn₁₂ structure without structure stabilized element once ThMn₁₂ structure is formed. The NdFe₁₂N_x phase is a new promising hard magnetic compound for high-performance permanent magnets [3, 4].

REFERENCES
[1] K H. J. Buschow, J. Appl.Phys. 63 (1988) 3130.
[2] T. Miyake et al., J. Phys. Soc. Japan. 83 (2014) 043702.
[3] Y. Hirayama et al., JOM. (2015), DOI:10.1007/s11837-015-1421-9.
[4] Y. Hirayama et. al., Scripta Materialia. 95 (2015) 70.