Epitaxial Growth of Hard Ferrimagnetic Mn 3 Ge Film on Rhodium Buffer Layer

Mn3Ge has a tetragonal Heusler-like D022 crystal structure, exhibiting a large uniaxial magnetic anisotropy and small saturation magnetization due to its ferrimagnetic spin structure; thus, it is a hard ferrimagnet. In this report, epitaxial growth of a Mn3Ge film on a Rh buffer layer was investigated for comparison with that of a film on a Cr buffer layer in terms of the lattice mismatch between Mn3Ge and the buffer layer. The film grown on Rh had much better crystalline quality than that grown on Cr, which can be attributed to the small lattice mismatch. Epitaxial films of Mn3Ge on Rh show somewhat small coercivity (Hc = 12.6 kOe) and a large perpendicular magnetic anisotropy (Ku = 11.6 Merg/cm), comparable to that of the film grown on Cr.


Introduction
Magnetism in Mn-based alloys and compounds has been studied, and various types of Mn-based alloys and compounds are known to exist.Among them, to our knowledge, are four types of hard magnetic Mn-based alloys and compounds that show strong ferro/ferrimagnetism with both a high Curie temperature and high uniaxial magnetic anisotropy in their bulk forms.These are categorized in terms of their crystal structures as follows: (a) Tetragonal L1 0 -type crystal structure, for example, MnAl [1,2] and MnGa [3] (Figure 1a); (b) Tetragonal D0 22 -type crystal structure, for example, Mn 3 Ga [4,5] and Mn 3 Ge [6-8] (Figure 1b); (c) Tetragonal C38-type crystal structure, for example, MnAlGe [9] (Figure 1c); and (d) Hexagonal B8 1 -type crystal structure, for example, MnBi [10,11] (Figure 1d).The distinct difference between Mn-based magnets and others such as Fe-Pt and Nd-Fe-B is that the former are composed of III and/or IV main group elements, and the latter are composed of heavy transition or rare-earth elements.This fact is important in today's magnetism-based technology for two reasons.One is related to the developmentof permanent magnets without critical elements, and the other is related to spintronics applications.Hereafter, we focus on the use of hard magnetic Mn-based alloys for spintronics applications; the other topic regarding Mn-based alloys and compounds is discussed in the reviews [12,13].[11,14] are also shown as solid circles (•).Dashed line indicates the boundary between materials with the magnetic easy axis parallel to the film surface and those with the easy axis perpendicular to the film surface (perpendicular magnetization).References for the data are as follows.[a]: [15]; [b]: [16]; [c]: [17], [d]: [18], [e]: [19], [f ]: [17], [g], [h]: [20], [i]: [21], [j]: [22], [k]: [23].
Mn-based hard magnets are very good candidates for magnetic random access memory (MRAM) applications [24,25] because some of them show a small saturation magnetization as well as a large uniaxial magnetic anisotropy.The Gilbert magnetic damping is expected to be smaller than that of hard magnets containing heavy elements because the addition of heavy elements or rare-earth elements to magnetic materials tends to increase the Gilbert damping [25].For this reason, it is crucial to investigate c-axis-oriented Mn-based hard magnetic films with a high degree of chemical order, as the exchange interaction between Mn atoms is very sensitive to the environment and the nearest-neighbor distance.This situation is distinct from that of other ordered alloys, such as Fe-Pt, which are ferromagnetic even in disordered states.
Figure 1e shows typical K u and M s data from our work and from other groups for c-axis-oriented epitaxial films of several Mn-based alloys and the values for conventional hard magnetic materials [14].The L1 0 phase of the Mn-Al binary system is metastable, but epitaxial films were reported recently ([a], [b] in Figure 1e) [15,16].The L1 0 phase of the Mn-Ga binary system is thermodynamically stable at room temperature (RT), and high-quality films have been grown on several types of buffer layers using various techniques ([c], [d] in Figure 1e) [17,18].The reported values of M s and K u for the films are 190-600 emu/cm 3 and 13.65-21.7Merg/cm 3 , respectively.The D0 22 phase of the Mn-Ga binary system is also metastable according to recent reports [26], but high-quality epitaxial films can be obtained, as we reported for the first time [19].The reported values of M s and K u for D0 22 Mn-Ga films are 140-470 emu/cm 3 and 12.4-23.5Merg/cm 3 , respectively ([e], [f ], [g], [h] in Figure 1e) [17,19,20].Epitaxial films of C38 MnAlGe can be easily grown; their M s value is 250 emu/cm 3 , but K u is small ([k] in Figure 1e) [23].
Quite recently, (001)-oriented epitaxial films of D0 22 Mn 3 Ge have also been reported to show small magnetization of about 73 emu/cm 3 and a K u value of 9.1 Merg/cm 3 ([i] in Figure 1e) [21].Higher-quality films were obtained using a Cr buffer layer and sophisticated control of the Mn composition and growth temperature [22,27].Films with stoichiometric compositions exhibited a K u value of 11.8 Merg/cm 3 and an M s value of ∼110 emu/cm 3 ([j] in Figure 1e) [22].D0 22 Mn 3 Ge is considered to have the properties most suitable for MRAM [28,29], although few examples of epitaxial growth have been reported.In particular, it is unclear how lattice matching between Mn 3 Ge and the buffer or substrate affect the magnetic properties of films.For example, films grown on the (001) SrTiO 3 substrate, with negligible lattice mismatch, show very broad hysteresis curves with a large coercive force [21], whereas those grown on a Cr buffer layer showed well-squared out-of-plane hysteresis loops and smaller coercivity, even though the lattice mismatch is much greater than that with SrTiO 3 [22,27].In this article, the epitaxial growth of Mn 3 Ge films on a Rh buffer layer was investigated, as the lattice constant of Rh is identical to that of Mn 3 Ge and we can compare the result with previously reported films grown on a Cr buffer layer in terms of the lattice mismatch between Mn 3 Ge and the buffer.

Experimental Methods
Samples were prepared on single-crystal MgO (001) substrates by a UHV sputtering apparatus (MPS-3000, Ulvac, Chigasaki, Japan) whose base pressure and work pressure are <2×10 −7 Pa and ∼0.1 Pa, respectively.We prepared two types of stacked films having a buffer layer consisting of a Cr single layer or a Cr/Rh bilayer.Before film preparation, the substrates were thermally flushed at 700 • C in the sputtering chamber.Cr layers 40 nm thick were deposited on the substrates at RT and subsequently annealed at 700 • C to form a flat surface.A 20-nm-thick Rh layer was deposited at RT on the Cr layer.Mn 3 Ge layers 130 nm thick were deposited by a co-sputtering technique from simple Mn and Ge targets on the substrates with the buffer layers, which were heated at 400 • C. Finally, 3-nm-thick MgO layers were deposited as cap layers.Schematic illustrations of the films are shown in Figure 2. The samples were characterized according to their surface roughness, crystal structure, magnetization hysteresis curves, and magnetic anisotropy by an ex-situ atomic force microscope (AFM) (SII NanoTechnology, SPI-3800N/SPA400, Chiba, Japan), X-ray diffractometer (XRD) (Rigaku, SmartLab, Akishima, Japan), vibrating sample magnetometer, and magnetic torquemeter, respectively.All the measurements were conducted at RT and ambient pressure.Figure 4a shows the XRD profiles of the films with Cr and Cr/Rh buffer layers.Peaks marked by asterisks are the background signals, such as those from the substrate and X-ray tube.Only the (200) peaks from Cr and Rh are observed for the films, indicating that both buffer layers are (001)-oriented as a result of epitaxial growth on their MgO (001) substrates.For the Mn 3 Ge layer, only the (002) and (004) peaks from D0 22 -structured Mn 3 Ge were observed, which indicated that the Mn 3 Ge layers has the D0 22 structure and a completely (001)-preferred crystallographic orientation.Figure 4b-d show pole figures (φ-scans) of Mn 3 Ge, Rh, and MgO substrate, and those exhibit the four-fold symmetry.The φ value of the (111) peak from the MgO (001) substrate was the same as that of the (112) peak from D0 22 -Mn 3 Ge and the (111) peak from Rh but was shifted by 45 • from the (222) peak of Cr (not shown here), indicating the following epitaxial relationship: MgO[100](001)||Cr[110](001)||Rh[100](001)||D0 22 -Mn 3 Ge[100](001).Figure 4e-g show magnified profiles of the Mn 3 Ge (002), Mn 3 Ge (004), and Cr (200) peaks, respectively.Note that the vertical axis is on a linear scale, whereas that of Figure 4a is on a logarithmic scale.The (002) and (004) peaks from Mn 3 Ge for the film with the Cr/Rh buffer layer are significantly more intense than those from the film with the Cr buffer layer.This indicates that the crystallinity of Mn 3 Ge on the Cr/Rh layer is better than that on the Cr buffer layer.Because the intensity of the Cr (200) peak is comparable, the effect of interdiffusion between the Cr and Mn 3 Ge layers is absent in the film.Hence, the Cr/Rh buffer layer intrinsically improves the crystallinity of the Mn 3 Ge above.This is possibly the result of the reduction in the lattice mismatch between the buffer layer and Mn 3 Ge layer due to the presence of Rh.Assuming that the epitaxial distortion in the Rh layer due to the lattice mismatch between Cr (a = 2.885 Å in bulk) and Rh (a = 3.805 Å in bulk) is sufficiently relaxed, the lattice mismatch between the buffer layer and the Mn 3 Ge (a = 3.80 Å in bulk) layer is reduced from 6.86% to 0.13% by the Rh layer.Although a flat surface is expected as the result of layer-by-layer growth due to the absence of distortion energy for such a small lattice mismatch, the Mn 3 Ge on Rh shows a rough surface whose Ra value is comparable with that of the film on Cr, as shown in the AFM images.The large roughness may be the result of the poor wettability of Rh for Mn 3 Ge.The use of a surfactant may be effective for obtaining a smooth surface of Mn 3 Ge on the Cr/Rh buffer layer [30,31].

Experimental Results and Discussion
Figure 5 shows magnetization field (M -H) curves of Mn 3 Ge on Cr and Cr/Rh buffer layers.The saturation magnetization (M s ) for the film with the Cr/Rh buffer layer (115 emu/cm 3 ) is comparable with that for the film with the Cr buffer layer (100 emu/cm 3 ), whereas the former has a lower coercivity (H c = 12.6 kOe) than the latter (14.9 kOe).This is consistent with the previously reported experimental results, which suggested that highly crystalline Mn 3 Ge has a comparatively high M s and low H c [22,27]. Figure 6a shows the magnetic torque curve of Mn 3 Ge with the Cr/Rh buffer layer measured at various applied fields.The curves have a sawtooth-like shape, which indicates that the Mn 3 Ge is not magnetically saturated during the measurements.In this case, a special method is needed to estimate the anisotropy constant.We employed the Miyajima method (45 • method) [32].Figure 6b shows plots based on the Miyajima method for the films on the Cr and Cr/Rh buffer layers; their x intercepts give the torque values at an infinite field (completely saturated state).From these plots, the anisotropy constants (K u s) are estimated to be 13.2 and 11.6 Merg/cm 3 for the films on the Cr and Cr/Rh buffer layers, respectively.
Interestingly, the film on the Cr/Rh buffer layer has a lower K u value than that on the Cr buffer layer despite its better crystallinity.The origin of this low K u is not yet clear.One possibility is the difference between the optimal composition of the Mn-Ge layer on the Cr and Cr/Rh layers.Mn-Ge has different optimal compositions for obtaining D0 22 structure and the best magnetic properties, e.g., Mn 3.4 Ge in the bulk state [33], Mn 3.55 Ge in the thin film state on MgO (001) [28], and Mn 3 Ge in the thin film state on Cr (001) [22,27].It may be necessary to optimize the composition of the Mn 3 Ge layer to obtain the best magnetic properties on a Cr/Rh buffer layer.

Conclusions
We prepared Mn 3 Ge layers on Cr and Cr/Rh buffered MgO (001) substrates and characterized their surface roughness, crystal structure, magnetization field curves, and magnetic anisotropy constants.The crystallinity of the epitaxially grown Mn 3 Ge layer on Cr/Rh was better than that of the layer grown on the Cr buffer layer.It was demonstrated that the epitaxial relationship among MgO, Cr, Rh, and Mn 3 Ge is MgO[100](001)||Cr[110](001)||Rh[100](001)||D0 22 -Mn 3 Ge[100](001).The saturation magnetization and coercivity obtained from magnetization field curves also suggested that Mn 3 Ge on Cr/Rh had better crystallinity than that on Cr.Magnetic torque measurements exhibited that the anisotropy constant of Mn 3 Ge on the Cr/Rh buffer layer is comparable to that of the film on the Cr buffer layer despite the better crystallinity of the former.

Figure 2 .
Figure 2. Schematic illustrations of films with (a) Cr and (b) Cr/Rh buffer layer.

FigureFigure 3 .Figure 4 .
Figure3a,b show AFM images of the surfaces of the Cr and Cr/Rh buffer layers, respectively.Both show atomically flat surfaces whose Ra roughness is smaller than 0.2 nm.Figure3c,dshow AFM images of Mn 3 Ge layers on the Cr and Cr/Rh buffer layers, respectively.Their roughness is comparable, although the grain sizes differ significantly, which implies a difference in their crystallinity.

Figure 5 .
Figure 5. Magnetization field curves for films with Cr (blue) and Cr/Rh (red) buffer layer.

Figure 6 .
Figure 6.(a) Magnetic torque curves measured at various applied fields for the film with a Cr/Rh buffer layer; (b) Miyajima plot for films with Cr (blue) and Cr/Rh (red) buffer layers.