Preparation and evaluation of Mn3GaN1-x thin films with controlled N compositions

Thin films of antiperovskite Mn3GaN1-x were grown on MgO (001) substrates by reactive magnetron sputtering, and their structural, magnetic, and magneto-optical properties were systematically investigated. It was found that the combination of the deposition rate and the N2 gas partial pressure could produce epitaxial films with a wide range of N composition (N-deficiency) and resulting c/a values (0.93 - 1.0). While the films with c/a = 0.992 - 1.0 were antiferromagnetic, the films with c/a = 0.93 - 0.989 showed perpendicular magnetic anisotropy (PMA) with the maximum PMA energy up to 1.5×106 erg/cm3. Systematic dependences of the energy spectra of the polar Kerr signals on the c/a ratio were observed, and the Kerr ellipticity was as large as 2.4 deg. at 1.9 eV for perpendicularly magnetized ferromagnetic thin films with c/a = 0.975. These results highlight that the tetragonal distortion plays an important role in magnetic and magneto-optical properties of Mn3GaN1-x thin films.Thin films of antiperovskite Mn3GaN1-x were grown on MgO (001) substrates by reactive magnetron sputtering, and their structural, magnetic, and magneto-optical properties were systematically investigated. It was found that the combination of the deposition rate and the N2 gas partial pressure could produce epitaxial films with a wide range of N composition (N-deficiency) and resulting c/a values (0.93 - 1.0). While the films with c/a = 0.992 - 1.0 were antiferromagnetic, the films with c/a = 0.93 - 0.989 showed perpendicular magnetic anisotropy (PMA) with the maximum PMA energy up to 1.5×106 erg/cm3. Systematic dependences of the energy spectra of the polar Kerr signals on the c/a ratio were observed, and the Kerr ellipticity was as large as 2.4 deg. at 1.9 eV for perpendicularly magnetized ferromagnetic thin films with c/a = 0.975. These results highlight that the tetragonal distortion plays an important role in magnetic and magneto-optical properties of Mn3GaN1-x thin films.


I. INTRODUCTION
Antiperovskite compounds exhibit unique combinations of interesting and potentially useful structural, electrical and magnetic properties.From the view point of spintronic applications, there have been an increasing interest on antiperovskite nitrides such as antiferromagnetic (AFM) Mn 3 GaN, 1 ferromagnetic Co 3 FeN with a half-metallic nature, 2,3 and ferromagnetic Mn 3 GaN 1-x with perpendicular magnetic anisotropy (PMA). 4In particular, much attention was paid to the Mn 3 GaN 1-x (0 x 1) system.Mn 3 GaN with a cubic structure is known to be a noncollinear (Γ 5g ) antiferromagnet with the Néel temperature of 298 K. 5 We have reported preparation of AFM Mn 3 GaN thin films and the current-induced magnetization switching in AFM-Mn 3 GaN/FM-Co 3 FeN exchange-coupled bilayers. 6Recently, ferromagnetic Mn 3 GaN 1-x thin films with giant PMA, which is a nitride material of D0 22 MnGa with a tetragonal structure of c/a = 0.92, have been reported. 4Mn 3 GaN 1-x shows N-deficient structures compared with the stable antiferromagnet (AFM) E2 1 Mn 3 GaN antiperovskite structure.Thus, Mn 3 GaN 1-x , which locates the phase between MnGa (tetragonal) and Mn 3 GaN (cubic), is interesting because of changes of magnetic properties from the FM into the AFM by N-doping concentration.However, there is no report on systematic investigations of Mn 3 GaN 1-x thin films with respect to the N-doping concentration and/or the tetragonal distortion c/a.In order to achieve and examine exchange-coupled AFM/PMA-FM bilayers using Mn 3 GaN 1-x system, which is one of the promising candidate for future magnetoresistive random access memory devices, it is necessary to understand the relationship between the N-doping (resulting tetragonal distortion c/a) and the magnetic properties of Mn 3 GaN 1-x system.In this paper, we will report controllable preparation and magnetic and magneto-optical properties of Mn 3 GaN 1-x thin films with a wide range of c/a values.

II. EXPERIMENTS
The Mn 3 GaN 1-x films were deposited on a MgO (001) single crystal substrate at 400 O C by RF magnetron sputtering using a Mn 70 Ga 30 target in an Ar + N 2 mixture gas atmosphere.Lattice relaxation occurred due to the large lattice mismatch (about 8 %) between the substrate and films.The precise N-doping was controlled by varying the N 2 gas percent (0.0 -3.0 %) and the deposition rate was controlled by RF power.When the deposition rate changes, that is, amount of MnGa emitted from metal target changes, N-deficiency changes relatively and N compositions changes.The crystal structure was analyzed using out-of-plane and in-plane X-ray diffraction (XRD) measurements using a Cu Kα radiation source.Magnetization versus magnetic-field (M-H) curves were measured at room temperature using a superconducting quantum interference device.Photon energy dependencies of ellipticity in the Magneto-optical Kerr effect were determined using polar Kerr effect.

III. RESULTS AND DISCUSSION
Figure 1(a) shows the out-of-plane XRD profiles for Mn 3 GaN 1-x thin films for each N 2 gas concentration.The Mn 3 GaN 1-x (002) peak was observed.As the N 2 gas content decreases, the peak position decreases and approaches the MnGa (002) peak position.In the case of an N 2 gas content of 2.25 % and over, the Mn 3 GaN (111) peak was observed and a polycrystalline phase forms.The (111) orientation is the preferred orientation of Mn 3 GaN.Figure 1(b) shows the out-of-plane XRD profiles for Mn 3 GaN 1-x thin films for each deposition rate.As the deposition rate increases, the Mn 3 GaN 1-x (002) peak position shifts to higher angles.The variation of the peak position was small compared to case when the N 2 gas content was varied.
Figure 2(a) shows the N 2 gas content dependencies of the c and a lattice constants.As shown in Fig. 2(a), Mn 3 GaN 1-x thin films of a wide range of c lattice constant (0.3604 -0.3872 nm) are obtained by changing the N 2 gas percent (0 -3 %).On the other hand, a systematic change of the a lattice constant was not observed.Even though there is same dispersion, the a lattice constants of the all samples are almost the same as that of bulk Mn 3 GaN (0.3898 nm 1 ), irrespective of the c lattice constant.These results indicate that the N-deficiency does not affect the a lattice constant.Therefore, variation of the lattice constant by N-deficiency means that lattice diminishes only perpendicularly to thin film surface.In other words, it is thought that a tetragonal distortion arises.N-deficiency becomes bigger, so that the c/a gradually decreases by tetragonal distortion from the point of view of structural changes.Figure 2(c) shows that deposition rate dependency of the c and a lattice constants.As shown in Fig. 2(c), at the N 2 gas content of 1 %, Mn 3 GaN 1-x thin films with a c lattice constant between 0.3812 and 0.3880 nm are obtained by changing the deposition rate (1.24 -3.30 nm/min.).The c lattice constant exhibits a constant value (0.388 nm) when the deposition rate is below 2.0 nm/min.and decreases with increasing deposition rate, and then, exhibits a constant value 0.381 nm above 2.8 nm/min.Figure 2(d) shows the deposition rate dependence of the c/a values.As shown in Fig. 2(d), as the deposition rate increases, the c/a value decreases, indicating that the N-deficiency increases.These results highlight that FM and AFM Mn 3 GaN 1-x thin films can be obtained by changing the N 2 gas content during growth or the deposition rate.from cubic to tetragonal so that the magnetic properties change from AFM to FM.A large change appears around c/a = 0.99, which may be related to the AFM spin alignment becoming unbalanced by the slight tetragonal distortion (neighborhood c/a = 0.99) so that the Mn 3 GaN 1-x thin films exhibit PMA.
Figure 4(a) shows perpendicular and in-plane M-H curves for a Mn 3 GaN 1-x thin film with c/a = 0.975.As shown in Fig. 4(a), PMA is confirmed.The film shows an effective PMA energy of 1.5×10 6 erg/cm 3 .Perpendicular M-H curves of Mn 3 GaN 1-x thin films with different c/a values are shown in Fig. 4(b).The shapes of the hysteresis loops are systematically changed by c/a value.
In order to clarify the relationship between structural change and electronic structure, the photon energy dependent ellipticity of Magneto-optical Kerr effect measurements were performed.Figure 5 shows photon energy dependent polar Kerr ellipticity of Mn 3 GaN 1-x thin films with different c/a.Four ellipticity peaks are observed between 1.4 and 3.4 eV.As shown in Fig. 5, c/a decreases, the ellipticity peaks become larger and their peak positions are changed.Since a large change appears between c/a = 0.985 and 0.992, the electronic structure may be modified around c/a = 0.99.

IV. CONCLUSIONS
In summary, epitaxial Mn 3 GaN 1-x thin films were grown on MgO (001) substrates by reactive magnetron sputtering.It was found that the tetragonal distortion of the films could be controlled by the proper choice of deposition rate and the N 2 gas partial pressure.Even the slight distortion of c/a = 0.989 could result in the formation of the perpendicularly magnetized FM thin films.These results highlight that the tetragonal distortion plays an important role in determining the magnetic and magneto-optical properties of Mn 3 GaN 1-x films.

Figure 2 (
Figure1(a) shows the out-of-plane XRD profiles for Mn 3 GaN 1-x thin films for each N 2 gas concentration.The Mn 3 GaN 1-x (002) peak was observed.As the N 2 gas content decreases, the peak position decreases and approaches the MnGa (002) peak position.In the case of an N 2 gas content of 2.25 % and over, the Mn 3 GaN (111) peak was observed and a polycrystalline phase forms.The (111) orientation is the preferred orientation of Mn 3 GaN.Figure1(b)shows the out-of-plane XRD profiles for Mn 3 GaN 1-x thin films for each deposition rate.As the deposition rate increases, the Mn 3 GaN 1-x (002) peak position shifts to higher angles.The variation of the peak position was small compared to case when the N 2 gas content was varied.Figure2(a) shows the N 2 gas content dependencies of the c and a lattice constants.As shown in Fig.2(a), Mn 3 GaN 1-x thin films of a wide range of c lattice constant (0.3604 -0.3872 nm) are obtained by changing the N 2 gas percent (0 -3 %).On the other hand, a systematic change of the a lattice constant was not observed.Even though there is same dispersion, the a lattice constants of the all samples are almost the same as that of bulk Mn 3 GaN (0.3898 nm 1 ), irrespective of the c lattice constant.These results indicate that the N-deficiency does not affect the a lattice constant.Therefore, variation of the lattice constant by N-deficiency means that lattice diminishes only perpendicularly to thin film surface.In other words, it is thought that a tetragonal distortion arises.Figure2(b)shows the N 2 gas content dependency of the c/a values.As shown in Fig.2(b), as the N 2 gas content is lowered, the c/a value becomes smaller, indicating that as N 2 gas percent becomes smaller and

FIG. 2 .
FIG. 2. (a) N 2 gas content dependencies of the c lattice constant and the a lattice constant.(b) N 2 gas content dependency of the c/a values.(c) Deposition rate dependence of the c lattice constant and the a lattice constant.(d) Deposition rate dependence of the c/a values of Mn 3 GaN 1-x thin films.The c/a values of blue squares of panel (d) are evaluated from the average of experimental a lattice constant (black squares of panel (c)).

Figure 3
FIG.3.M s and H c as a function of c/a values for Mn 3 GaN 1-x thin films.