Substrate heating effects on the microstructure and magnetic properties of MnAlGe films grown on Si/SiO₂ substrate by sputtering system were investigated. The MnAlGe film fabricated by low-temperature substrate heating demonstrated amorphous phase and paramagnetic property. The film of c-axis orientation associated with the Cu₂Sb-type structure was obtained by sputtering at a substrate heating temperature of 270°C and it exhibited perpendicular magnetic anisotropy. From the magnetization curves measured at room temperature, the uniaxial magnetic anisotropy energy, K_u, was evaluated to be in the order of 10⁶ erg/cm³, which is consistent with the literature, although the heating processing and temperature are slightly different. Microstructural observation indicated that the c-axis oriented grains were isolated in the matrix of the amorphous phase. The film lost the c-axis orientation at elevated substrate heating temperature, resulting in loss of the anisotropic magnetic property.

SEM image (upper left) and XTEM bright field images of MnAlGe films on Si/SiO₂ substrate sputtered at 270°C. Upper right panel is the low magnification image, and lower panels are the high magnification images.

MnAlGe layered compounds are known to exhibit large magnetic anisotropy (MA) due to their Cu₂Sb-type anisotropic crystal structure. As shown in Fig. 1, the crystal structure of the MnAlGe compound has a C38-type and a space group number of 129 (P4/nmm). Mn atoms occupy the 2a Wyckoff position in the (001) basal plane, Al atoms the 2c (0, 1/2, u) and (1/2, 0, $\bar{u}$), and Ge atoms also the 2c (0, 1/2, v) and (1/2, 0, $\bar{v}$), here the values of u and v were reported to be about 0.273 and 0.720, respectively.^1,2) Neutron powder diffraction results revealed that the magnetic moments of Mn atoms were directed along the c-axis and its value was 1.40 μ_B at 300 K.¹⁾ From the temperature dependence of the magnetization, the spontaneous magnetic moment at 0 K was expected to be 1.70 μ_B by the extrapolation,³⁾ which is comparable to 1.81 μ_B obtained from the theoretical calculation.^4,5) The layers constructed on non-magnetic elements, such as Al and Ge, are stacked in the c-axis direction, and the distance between the Mn layers is much larger than that of the nearest neighbor Mn–Mn in the c-plane. Therefore, the compound is considered as a pseudo-two-dimensional compound. The uniaxial MA energy, K_u, estimated by the magnetization curve using a single crystal was 9.7 × 10⁶ erg/cm³ at 4.2 K²⁾ and 5.3 × 10⁶ erg/cm³ at room temperature,⁶⁾ and the Curie temperature, T_C, was about 503 K.²⁾ Nuclear magnetic resonance experiments were also conducted, and the hyperfine fields at Mn and Al nuclei were reported to be 177.4 and 52.6 kOe, respectively.⁷⁾ Although the value of K_u is smaller than that for the L1₀-type FePt (Fe₆₀Pt₄₀: 55 × 10⁶ erg/cm³ at 298 K),⁸⁾ CoPt (45 × 10⁶ erg/cm³ at 298 K),⁹⁾ and FePd (17 × 10⁶ erg/cm³ at 300 K)¹⁰⁾ alloys, MnAlGe have an advantage owing to its comparatively high MA energy without novel metals.

Fig. 1

Crystal structure of the MnAlGe compound with the Cu₂Sb-type tetragonal structure.^1,2)

Previous investigations for fabricating MnAlGe films have been performed. Magnetic domain was observed for the film on NaCl single crystal substrate,¹¹⁾ and (001) textured film was deposited on KCl crystal.¹²⁾ Recently, MnAlGe epitaxial film on MgO substrate was reported to exhibit clear perpendicular MA with a comparatively large K_u of 4.7 × 10⁶ erg/cm³.¹³⁾ Such ferromagnetic materials with low saturation magnetization and high MA are promising candidates as magnetic electrodes in magnetic tunnel junction (MTJ) devices in spin-transfer-torque applications for random access memory.¹⁴⁾ In addition, for domain wall memory devices, high perpendicular MA is also needed to obtain sufficiently narrow domain walls in high density.^15,16) The low spin relaxation of Mn-based ferromagnetic materials, i.e., a low value of the Gilbert damping constant is also considered favorable in applications.¹³⁾ From the theoretical calculations, it was predicted that the low value of the Gilbert damping constant is common feature among Mn-based compounds.^13,16–18) Therefore, Mn-based materials with high MA are of interest in the field of spintronics.

Recently, Kubota et al. reported that c-axis oriented film was obtainable by controlling the annealing temperature of the Si/SiO₂ substrate.^19,20) They also systematically investigated the Ge composition dependence and Cr doping effects on the perpendicular MA properties of the MnAlGe film. It is advantageous for industrial applications if the c-axis oriented film is easily fabricated without using the single crystal substrate. In the present investigations, MnAlGe films were deposited on the Si/SiO₂ heating substrate, and the substrate temperature effects on the microstructure, morphology and magnetic properties were investigated.

MnAlGe films with 100 nm thickness were sputtered on a Si/SiO₂ substrate via a dual-ion-beam deposition system (Millatron 820, Hakuto Co. Ltd.) at several substrate heating temperatures (T_sub) of 200, 270, 340, and 400°C. The accelerating voltage and beam current were 950 V and 80 mA, respectively, the gas pressure was 5.9 × 10⁻² Pa, and the deposition rate was 0.25 nm/s. Although the composition of the starting target was Mn27–Al36.5–Ge36.5 (at%), some amount of pure Mn and Al tips were finally added for controlling the composition of the deposited films. The composition of the finally obtained films evaluated by energy dispersive X-ray spectroscopy (EDX) (S4800, Hitachi Co. Ltd.) was Mn32.9–Al32.4–Ge34.7 (at%). The microstructure was observed using scanning electron microscope (SEM) (S4800, Hitachi Co. Ltd.) and cross-sectional transmission electron microscopy (XTEM) (JEM-2000EX, JEOL Co. Ltd.). The sample for the XTEM observation was prepared as follows: thin slices were cut out perpendicularly to the film layer of the sample, then the film layers were tightly glued. Next, the glued sandwich was pre-thinned by mechanical polishing using SiC abrasive paper and finished by the standard ion milling polisher (PIPS II System, Gatan Co. Ltd.) using Ar ion beam of 3.0 keV.²¹⁾ The crystal structure was confirmed by X-ray diffraction (Rint-Ultima III, Rigaku Co. Ltd.) analyses at room temperature with Cu-K_α radiation. The magnetization was measured by a vibrating sample magnetometer at room temperature (VSM-5, Toei Industry Co. Ltd.).

The XRD patterns of MnAlGe films sputtered on Si/SiO₂ substrate at 200, 270, 340, and 400°C are shown in Fig. 2. All the observed peaks can be indexed as the Cu₂Sb-type crystal structure except that of the Si/SiO₂ substrate. The diffraction pattern at the bottom in Fig. 2 indicates simulated patterns of MnAlGe with the Cu₂Sb-type crystal structure as the reference. The XRD pattern of the MnAlGe film sputtered at T_sub = 200°C demonstrates no peak except that of the Si/SiO₂ substrate, suggesting this film is an amorphous phase. The MnAlGe films sputtered at T_sub = 270°C and above demonstrate crystallinity, as shown in Fig. 2. The observed peaks for the film sputtered at T_sub = 270°C are indexed as (0, 0, n), thereby confirming the c-axis orientation. However, the intensity reduces for the specimen obtained at T_sub = 340°C, and other indexed peaks are also revealed in the XRD pattern sputtered at T_sub = 400°C. The lattice parameters a and c obtained from the XRD patterns at T_sub = 400°C are 0.391 and 0.591 nm, respectively, which are consistent with the reported values of 0.3914 and 0.5933 nm for bulk crystal.¹⁾

Fig. 2

XRD patterns for MnAlGe films sputtered on Si/SiO₂ substrates at different substrate heated temperatures. The observed peaks can be indexed as the Cu₂Sb-type crystal structure. The diffraction pattern at the bottom is simulated as the crystal structure with the lattice parameters of a = 0.391 and c = 0.591 nm.

Figure 3 shows SEM images of MnAlGe films sputtered at T_sub = 200, 270, 340, and 400°C. Significant changes in the morphology of the film surface are observed with elevating sputtering temperatures. The MnAlGe film sputtered at T_sub = 200°C does not reveal any surface feature, which corresponds to the XRD patterns indicating an amorphous phase. The MnAlGe film sputtered at T_sub = 270°C exhibits partial crystallization with crystalline grains of about 1 µm in the amorphous matrix. At T_sub = 340°C, two kinds of crystallized grains with different morphologies appear. One exhibits a similar microstructure of the film sputtered at T_sub = 270°C, and the other has a narrow and fine structure with a grain size of several hundred nanometers. There are further changes in the features of the film surfaces of the MnAlGe film sputtered at T_sub = 400°C; the overall surface is completely and densely crystallized. These changes are consistent with the XRD results, i.e., the film sputtered at T_sub = 200°C demonstrates an amorphous phase, that at T_sub = 270°C exhibits a c-axis oriented phase, and those at T_sub = 340 and 400°C comprise mixed oriented phases. Because of the anisotropic crystal structure, the surface energy difference of the crystal orientations results in a change of the morphology depending on the substrate temperature. As shown in the Fig. 1, Mn atoms lie on the c-plane and the structure has stacking layer along to the c-axis, so (001) plane becomes to be densest. It has been known that the oriented films can be easy fabricated by controlling the sputtering condition when there is a large surface energy difference as like this case.

Fig. 3

SEM images of MnAlGe films on Si/SiO₂ substrate sputtered at different substrate heating temperatures. (a) at 200°C, (b) at 270°C, (c) at 340°C, and (d) at 400°C.

The crystallization from the amorphous phase is generally discussed by two step processes, such as nucleation process and crystal growth process. In the low temperature region, the frequency of the nucleation is low, so the crystal growth cannot be basically occurred. With elevating temperature, the frequency of the nucleation becomes to be higher, then, the crystals grow. Although the orientation of the nuclei is random, the orientation with higher rate in crystal growth becomes dominant. Such behavior is phenomenologically explained as the evolutionary selection rule.^22,23) With elevating higher temperature, the frequency of the nucleation further becomes to be dense, and the distance between the nuclei is narrower. Another crystal orientation becomes dominant or crystals grow with random orientation. In Structure Zone Model (SZM) predicted by Movchan and Demchishin,²⁴⁾ and revised by Thornton,²⁵⁾ the morphology and structure of the crystallized film is categorized with the ratio of T_s/T_m, here, the T_s and T_m are the substrate heating temperature and the melting point of the deposited film, respectively. Although there are other important factors for the sputtering conditions relating to the morphology and structure of the films, it has been known that the SZM well follows the various types of the films made by metals or oxides. It is said from the model that the threshold changing from oriented columnar structure to equiaxed grains structure is T_sub/T_m = 0.5∼0.6.²⁶⁾ That is, above the temperature, the structure is like to the bulk poly-crystalline state. The melting point of the MnAlGe bulk poly-crystal determined by differential thermal analysis is 790°C (see the Appendix), and the temperature that the morphology suddenly changes is 400°C. Therefore, the value of the rate of T_sub/T_m = 673/1063 (in kelvin) = 0.63 roughly consists with the threshold value.

The magnetization curves obtained at room temperature for the MnAlGe films are presented in Fig. 4(a)–(d). Here, a magnetic field was applied in the out-of-plane (⊥) and in-plane (//) directions relative to the film. The MnAlGe film at T_sub = 200°C does not crystallize into a ferromagnetic Cu₂Sb-type structure, thus, the film does not exhibit magnetization. The film at T_sub = 270°C, which is oriented in the c-axis, clearly demonstrates perpendicular MA. The value of K_u was defined as K_u = K_u^eff + 2π(M_s)². Here, K_u^eff and M_s are the effective perpendicular MA energy and saturation magnetization, respectively. K_u^eff was evaluated from the area enclosed by the out-of-plane and in-plane volume magnetization curves. To evaluate the enclosed area, the magnetization curves were extrapolated linearly to a higher magnetic field. From the definition, K_u of the film in Fig. 3(b) is obtained as 2.7 × 10⁶ erg/cm³, which corresponds with the value of 2.5 × 10⁶ erg/cm³ reported by Kubota et al.¹⁹⁾ In the literature, the annealing processing was different because the film was annealed after sputtering in the chamber. However, it has also been reported that the c-axis oriented film was obtained after the optimization of the annealing temperature. In detail, there is a possibility that the presented K_u would be underestimated than that of the actual value of the MnAlGe with Cu₂Sb-type structure because the sputtered film exhibits mixed phase state of non-magnetic amorphous phase and the MnAlGe compound phase. However, as indicated from the above formula for evaluating the K_u, the magnitude of the anisotropic energy is still on the order of 10⁶ erg/cm³ even if the volume magnetization is underestimated.

Fig. 4

Out-of-plane (⊥) and in-plane (//) magnetization curves measured at room temperature for MnAlGe films sputtered at different substrate temperatures.

The MA decreases with an increase in the sputtering temperature and completely disappears in the MnAlGe film at T_sub = 400°C. These behaviors correspond well with the XRD results, i.e., c-axis oriented structure was observed in the film at T_sub = 270°C, but they exhibit mixed oriented structures at higher sputtering temperatures. Therefore, the sputtering conditions must be further optimized to improve the MA in the MnAlGe film at T_sub = 270°C in the present work. However, we can demonstrate that the c-axis oriented film is easily obtained by controlling the sputtering temperature in the MnAlGe film on Si/SiO₂ substrate.

Theoretical investigations related to the origin of MA in different transition metal alloys have been performed. For FePt and CoPt with L1₀-type structure, where the alloys include a heavy element of Pt, it is generally understood that the large spin-orbit coupling is associated with the large MA. In contrast, MnAl and MnGa exhibit large MA energy despite the absence of a heavy metal element. In this case, the enhanced MA energy is attributed to the hybridization between the occupied and unoccupied state of Mn-3d bands around the Fermi level.^20,27) Recently, Okabayashi et al. revealed that orbital magnetic moments and anisotropies for Mn₃Ga are low by combining the X-ray magnetic circular/linear dichroism (XMCD/XMLD) and theoretical calculations, whereas there is a band mixing between the Mn-3d up- and down-spin states, resulting a significant MA.²⁸⁾ Because the MnAlGe has a simple ferromagnetic structure and only one kind of Mn atomic site, the investigations on the origin of the large MA energy will be an interesting research subject as one of the Mn-based alloy systems.

For further microstructural observations, XTEM observations were performed for the MnAlGe films on Si/SiO₂ substrate. Figure 5(a) presents an overview of a bright field (BF) image of MnAlGe film at T_sub = 200°C obtained from a low magnification XTEM. It is confirmed that a MnAlGe film with a thickness of approximately 100 nm is formed on the substrate. Figure 5(b) shows a selected area electron diffraction (SAED) extracted from the film region circled in Fig. 5(a). A halo-type diffraction pattern is evident in Fig. 5(b). This indicates that the MnAlGe film has an amorphous structure, which is confirmed from a high magnification XTEM BF image shown in Fig. 5(c).

Fig. 5

(a) Overview of low magnification XTEM bright-field (BF) image of MnAlGe film sputtered at a heating substrate temperature of 200°C on Si/SiO₂. (b) Selected area electron diffraction (SAED) pattern. (c) High magnification XTEM BF image.

Figure 6(a) shows an overview of a low magnification XTEM BF image of the MnAlGe film at T_sub = 270°C. From Fig. 6(a), contrast differences between some regions in the film are observed. Figures 6(b) and (c) are the high magnification XTEM BF images, while (d) and (e) are the SAED patterns corresponding to the Area-1 and Area-2 in Fig. 6(a), respectively. Considering the images of (b) and (d), Area-1 exhibits an amorphous phase, while Area-2 demonstrates a crystalizing phase as shown in (c) and (e). Especially in (c), the stripe structure becomes parallel to the substrate, and SAED spots in (e) can be indexed as the Cu₂Sb-type tetragonal structure. The lattice parameters were roughly measured to be a = 0.391 nm and c = 0.591 nm from the spacing of SAED spots, which are well-agree with that measured from XRD pattern shown in Fig. 2. It is evident that the amorphous MnAlGe film is partially crystallized at T_sub = 270°C and the crystallized MnAlGe is preferentially oriented in a perpendicular direction to the Si/SiO₂ substrate plane.

Fig. 6

(a) Overview of low magnification XTEM BF image of MnAlGe film sputtered at a heating substrate temperature of 270°C on Si/SiO₂. (b), and (c) High magnification XTEM BF images for the Area-1 and Area-2 in (a). (d) and (e) SAED patterns for the Area-1 and Area-2. The SAED pattern shown in (e) was indexed by crystalline MnAlGe with the Cu₂Sb-type tetragonal structure. The incident beam was close to [010] direction of the MnAlGe phase.

Figure 7(a) and (b) show the low magnification XTEM BF image of MnAlGe film at T_sub = 400°C and the corresponding SAED pattern extracted from the red circle, respectively. In contrast to Fig. 5(a) and 6(a), the entire film comprises crystalized regions, which are suggested by the diffracted spots identified as a Cu₂Sb-type structure of MnAlGe, as similar to the Fig. 6(e). All the crystallized MnAlGe regions are preferentially oriented in a perpendicular direction to the Si/SiO₂ substrate plane, although there are small misorientations among them as shown in the high magnification XTEM BF images, Figs. 7(c)–(e). These results from XTEM observations are consistent with those from XRD analysis. Relating to SZM mentioned in the previous section for the SEM observations, it is pointed out that the morphology changes at the film of T_sub = 400°C. The prediction of the model that the ratio of T_sub/T_m = 0.5∼0.6 is the threshold changing from oriented columnar structure as seen at the film T_sub = 270°C to equiaxed grains structure also follows these observations.

Fig. 7

(a) A low magnification XTEM BF image of MnAlGe film sputtered at a heating substrate temperature of 400°C on Si/SiO₂. (b) The SAED pattern taken from $[\bar{1}10]$ direction of the MnAlGe phase. (c)–(e) High magnification XTEM BF images. (d) and (e) are higher magnification images taken from areas I and II in (c), respectively.

Thus, a c-axis oriented MnAlGe film with Cu₂Sb-type crystal structure can be fabricated on Si/SiO₂ substrate by controlling the substrate heating temperature, T_sub. At low T_sub, the film demonstrated an amorphous phase, and a paramagnetic property with low magnetization. In this study, the c-axis oriented grain appeared in the amorphous phase matrix at T_sub = 270°C. The fabricated film indicated perpendicular MA, and the evaluated K_u was 2.7 × 10⁶ erg/cm³ from the magnetization curves measured at room temperature. At higher T_sub, entire film was completely crystalized into mixed grains of different orientations. In this phase, the film did not exhibit perpendicular MA which is evident in the magnetization curves. In literature, Kubota et al. also reported that the c-axis oriented film of MnAlGe was grown on Si/SiO₂ substrate,¹⁹⁾ although the heating process and temperature were different in our investigations. The directly grown c-axis oriented film on Si/SiO₂ substrate without single crystalline substrate and/or adjustment of the under-layer will be advantageous in industrial applications. Furthermore, it is said the MnAlGe has good potential for perpendicular MA with low magnetization.

In this study, MnAlGe films with Cu₂Sb-type structure were directly deposited on a Si/SiO₂ substrate without using single crystalline substrate and/or nucleating a seed layer. The effects of substrate heating temperature on the microstructure, morphology and magnetic properties were investigated. The main experimental findings are highlighted as follows:

The work was supported by a Grant-In-Aid for Science Research in a Priority Area “Creation of Life Innovation Materials for Interdisciplinary and International Researcher Development” from the Ministry of Education, Sports, Culture, Science and Technology, Japan.

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In order to investigate the melting temperature of MnAlGe, differential thermal analysis was performed. Poly-crystalline MnAlGe was fabricated by induction melting in an argon gas atmosphere. The data was correcting in the heating rate of 20°C/min. A large endothermic peak associated with melting is observed. The melting temperature is defined as the onset of the peak and evaluated to be 790°C from the figure.

Fig. A1

Differential thermal analysis curve of the MnAlGe bulk poly-crystal.