APPLIED PHYSICS LETTERS 107, 032403 (2015)
Ferromagnetic MnGaN thin films with perpendicular magnetic anisotropy for spintronics applications Hwachol Lee,1 Hiroaki Sukegawa,1,a) Jun Liu,1,2 Tadakatsu Ohkubo,1 Shinya Kasai,1 Seiji Mitani,1,2 and Kazuhiro Hono1,2 1
National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
2
(Received 9 May 2015; accepted 8 July 2015; published online 20 July 2015) Perpendicularly magnetized flat thin films of antiperovskite Mn67Ga24N9 were grown on an MgO(001) substrate by reactive sputtering using an argon/1% nitrogen gas mixture and a Mn70Ga30 target. The films showed a saturation magnetization of 80 –100 kA/m, an effective perpendicular magnetic anisotropy (PMA) energy of 0.1–0.2 MJ/m3, and a Curie temperature of 660–740 K. Upon increasing the N composition, the films transformed from ferromagnetic to antiferromagnetic as expected in the stoichiometric Mn3GaN phase. Point contact Andreev reflection spectroscopy revealed that the ferromagnetic MnGaN has a current spin polarization of 57%, which is comparable to D022-MnGa. These findings suggest that MnGaN is a promising C 2015 AIP Publishing LLC. PMA layer for future spintronics devices. V [http://dx.doi.org/10.1063/1.4927097]
Thin perpendicular magnetic anisotropy (PMA) films have become important for use as ferromagnetic (FM) electrodes for spintronics devices such as a spin transfer torque (STT)-magnetic random access memory (MRAM). Recently, D022 and L10 MnGa alloys have been intensively explored due to their high magnetocrystalline anisotropy energy densities (Ku), low magnetic damping constants (a), and potentially high spin polarizations (P) (88% in theory1), which can lead to reducing the critical currents for STT switching of magnetic tunnel junctions (MTJs).1–9 Moreover, one can tune the saturation magnetization by varying the Mn-Ga composition (Mn3xGa, 0 x 2) while keeping PMA.6 Searching for half-metallic spin-compensated ferrimagnets has also been attempted by third element (Ru and Co) additions in MnGa.10–12 Such spin-compensated ferrimagnetic Mn-Ga alloys may open up a new path for finding promising ferromagnetic layers for future spintronics applications such as MRAMs. However, high magnetocrystalline anisotropy Mn3xGa films tend to show island morphology,3,8,13 because they must be grown on heated substrates to produce the highly ordered D022 structure. N-doped Mn-Ga has been of great interest particularly for the cubic Mn3GaN antiperovskite structure14–16 with functional and unique properties such as negative thermal expansion.17 Mn3GaN is known as a non-collinear antiferromagnet (AFM) and has a Neel temperature of TN 298 K;14 however, some experimental studies of Mn3GaN have demonstrated a ferromagnetic transition at low temperature.14,17,18 In Mn3MN antiperovskites (M: transition metal), magnetic properties and magnetic transition temperatures strongly depend on the atomic number and valence electron concentration of the M atom. The various attracting properties of Mn3MN are mainly induced by their unique band structure arising from Mn6N octahedra in the Mn3MN lattice.14 This means that slight composition modification a)
Electronic mail:
[email protected]
0003-6951/2015/107(3)/032403/5/$30.00
or lattice distortion enables us to control the magnetic properties widely. Recently, a tetragonally distorted Mn3(Sb1xSnx)N antiperovskite was reported to have an FM phase with a large coercivity and a Curie temperature (Tc) above room temperature (RT).19 Therefore, if the FM phase of Mn3GaN is stabilized above RT by controlling its composition and lattice distortion, the MnGa nitride film could be a good candidate for an FM layer in future MTJ structures. In this letter, we demonstrate FM antiperovskite MnGaN epitaxial thin films with unexpectedly high PMA of 0.1–0.2 MJ/m3 and high Tc of 660–740 C using an N-deficient composition. In addition, the surface roughness of these films is substantially lower than D022 MnGa films that grow in the Volmer-Weber mode at elevated substrate temperatures. Using point contact Andreev reflection spectroscopy (PCAR), we confirmed a relatively high spin polarization of current up to 57% at 4.2 K, which is comparable to that reported for the D022 MnGa films. Here, we prepared 50 nm thick MnGa nitride epitaxial films on an MgO(001) single crystal substrate at substrate temperatures (Ts) of 480 C and 580 C by RF reactive magnetron sputtering under ultra-high vacuum base pressure (
