Nanostructured MnGa films on Si / SiO 2 with 20 . 5 kOe room temperature coercivity

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INTRODUCTION
Magnetic thin films and nanostructured magnetic materials exhibiting a high room temperature coercivity have attracted considerable attention for permanent magnet applications, 1 nano-and micro-electromechanical systems (NEMS-MEMS), 2 new types of recording media, 3 and as reference materials in pseudo-spin valves (e.g., in magnetoresistive random access memory (MRAM)). 4Apart from classical permanent magnetic materials such as Co-based alloys (e.g., Co 3 Pt, CoCrPtX, etc.), L1 0 alloys (e.g., FePt, MnAl, CoPt, FePd, etc.), and rare earth-transition metal alloys (e.g., SmCo 5 , Nd 2 Fe 14 B), 5 novel alternative materials such as e-Fe 2 O 3 , 6 hexagonal MnBi, 7 and DO 22 Mn 2-3 Ga (Ref.8) are currently being investigated.DO 22 Mn 2À3 Ga has recently garnered particular interest since it was theoretically predicted to be a half-metallic-like ferrimagnet with 88% spin polarization 9 and experimentally shown to exhibit spin polarization as high as 58%. 10 Moreover, it has been reported to exhibit very low damping constant. 11Therefore, it has been proposed to have great potential for applications in spin transfer-torque devices, 9 such as spin-transfer-torque MRAM, 12 and spin torque nano-oscillators. 13ulk DO 22 Mn 2-3 Ga has been reported to have a large coercivity, H C , and moderate saturation magnetization, M S , of 13.5 kOe and 300 emu/cm 3 , respectively. 8 $ 110 erg/cm 3 ). 10Moreover, Wu et al. also reported electrical transport properties of perpendicularly magnetized DO 22 MnGa epitaxial films, consistent with a high spin polarization. 16In our previous study we have demonstrated the utility in using DO 22 MnGa as a reference layer in pseudo spin valves. 17Although progress has been made in bulk and thin films, little is known about the behavior of nanostructured DO 22 MnGa.
In this paper we report on the growth of nanostructured, discontinuous, Mn 67 Ga 33 thin films containing the ferrimagnetic DO 22 Mn 3 Ga phase by magnetron sputtering on inexpensive, thermally oxidized, Si substrates.After proper tuning of the sputtering parameters a record high room temperature coercivity, H C ¼ 20.5 kOe, and moderate saturation magnetization, M S ¼ 140 emu/cm 3 , have been achieved.As the temperature is lowered the coercivity further increases to H C ¼ 28.2 kOe at 5 K. Nanostructuring, by properly tuning the growth conditions, is therefore a simple approach to significantly enhance the coercivity in these materials.

EXPERIMENTAL
All film stacks were deposited on thermally oxidized Si (100) substrates using a magnetron sputtering system with base pressure of better than 5 Â 10 À8 Torr.The 200 nm thick MnGa films were sputtered using a Mn 60 Ga 40 alloy target.During deposition the substrates were held at 400 C and subsequently annealed in situ at the same temperature for an additional 15 min.Two different growth rates are presented here.The high-rate (0.11 nm/s) sample is deposited at a DC power of 75 W and Ar working pressure of 5.0 mTorr while the low-rate (0.08 nm/s) sample was deposited at 30 W and a) Electronic mail: chaolinzha@gmail.com.b) Electronic mail: akerman1@kth.se.0021-8979/2011/110(9)/093902/4/$30.00 V C 2011 American Institute of Physics 110, 093902-1 1.5 mTorr.Ta (6 nm) buffer and capping layers were deposited at room temperature to provide a fresh growth surface for the MnGa film and to prevent oxidation of its top surface, respectively.
Top and cross-section views of the microstructure were studied using high-resolution field-emission scanning electron microscopy (HR-SEM) and high-resolution field-emission transmission electron microscopy (HR-TEM).Electron energy-loss spectra (EELS) acquired in scanning TEM (STEM) mode provided the local Ga and Mn distribution.The global composition of the films were characterized by energy dispersive x-ray spectrometry and found to be Mn 67 Ga 33 , i.e., within the 66-74 at.%Mn content range required to achieve the DO 22 phase. 8The surface topography was studied using atomic force microscopy (AFM).Crystallographic structures were investigated by x-ray diffraction (XRD) using Cu Ka radiation in symmetric scan geometry.The top 100 nm of the films were studied by glancing incidence XRD (GIXRD) with a fixed incident angle of 0.85 .The XRD and GIXRD patterns were analyzed using the FullProf code. 18Magnetic properties were characterized using vibrating sample magnetometer (VSM) with a maximum field of 90 kOe.The magnetic domain structure was observed using a magnetic force microscope (MFM).

RESULTS AND DISCUSSION
The XRD patterns (Fig. 1) for the two MnGa films are similar and show the multiphase nature of both samples.The Rietveld refinement of the patterns reveals the presence of two phases, b-Mn(Ga) with a cubic P4 1 32 structure and stoichiometric DO 22 Mn 3 Ga with a tetragonal I4/mmm structure.The volume ratio of both phases depends on the deposition rate with 12(1)% of DO 22 phase for the low rate sample and 14(1)% for the high rate one.The lattice parameters for the DO 22 phase are a ¼ 0.38994(6) nm and c ¼ 0.7116(2) nm with c/a ¼ 1.8248 for the low rate sample and a ¼ 0.39032(4) nm and c ¼ 0.7142(2) nm with c/a ¼ 1.8300 for the high rate sample, respectively.The lattice parameters and the c/a ratios for the DO 22 Mn 3 Ga phase are consistent with literature values. 8,14,15The crystallite sizes, D, were found to be D ¼ 49.1(1.7)and D ¼ 41.7(1.1)nm, for the low and high rate samples.Interestingly, the GIXRD results show a relative increase of the DO 22 (112) peak with respect to the b-Mn(Ga) (221) one (Fig. 1  The roughness exponent of the two samples [0.62(1) -low rate-and 0.70(1) -high rate-] further confirms the rougher character of the low rate films. 19,20This implies that in the low-rate sample the islands are more isolated and consequently the tip can penetrate deeper in between the islands.The high-rate film is more prone to form a continuous film due to the higher kinetic energy and mobility of the sputtered atoms.However, the low-rate films, which lack the necessary kinetic energy, tend to accumulate and form the island-like structure. 21he microstructure of the low-rate sample was also investigated by TEM. Figure 2(e) is a bright-field image showing a typical island.Though not evident from this image, lattice fringes may be seen from thinner parts of such islands at higher magnifications.Selected area electron diffraction showed that these islands are predominately single crystals.The inset of Fig. 2(e) is a diffraction pattern from one such island.The top of Fig. 2(f) is a STEM image of two well separated islands.The rectangular box in this image indicates a segment where chemical distribution of Mn and Ga was mapped with electron energy-loss spectra.Using the L 2-3 edge of Mn and Ga, we generated the chemical profile of these two elements along the length of the rectangular box, which is showed in the bottom of Fig. 2(f).Clearly, the concentration of Ga and Mn drops off rapidly as the electron beam scans near the island boundaries and the trench in between.
Hysteresis loops measured both parallel and perpendicular to the film plane are shown in Figs.4(a) and 4(b) for the high-and low-rate films, respectively.Remarkably, the inplane and out-of-plane hysteresis loops are virtually identical in both samples, suggesting a random orientation of the easy magnetic axis in both films.Note that the rather high squareness of the loops, compared to M R /M S ¼ 0.5 for randomly oriented uniaxial single domain particles, 22 implies some degree of dipolar coupling between the islands.The highrate film has H C ¼ 18.3 kOe and M S ¼ 151 emu/cm 3 , respectively.The H C of the low-rate sample is found to increase to 20.5 kOe while a small drop in M S , M S ¼ 140 emu/cm 3 , is also observed.As the temperature is lowered H C of the lowrate sample further increases to 28.2 kOe (see inset in Fig. 4(b)).Importantly, it should be emphasized that this may not be the intrinsic maximum H C for this material, since the considerable anisotropy field, H K ¼ 2 K u /M S $ 96 kOe (calculated using Wu et al. data 15 ), for this material certainly leaves room for even larger coercivities.
The moderate M S observed in our films is consistent with previously reported values 8,14,15 and the multiphase character of our samples.The large coercivity of both samples is primarily linked to the presence of the high anisotropy DO 22 Mn 3 Ga phase although slightly lower degree of ordering in our samples may imply a somewhat smaller anisotropy.Therefore, the increased coercivity of the low-rate sample over the high-rate sample is ascribed to the more pronounced island-like morphology of the films, as observed for other systems. 23Nevertheless, given the multiphase-polycrystalline nature of our films, other sources for the different coercivity between the high-rate and low-rate samples, such as small differences in stoichiometry or ordering parameter, cannot be ruled out.
The impact of the sample morphology on the enhanced coercivity can be explained by its influence on the magnetic domain structure and the likelihood of magnetic domain wall propagation.In the low-rate sample, each island is magnetically isolated from all other islands, and the global magnetic properties of the film correspond to the sum of all individual islands.We can estimate the critical size, d C , for single domain behavior using d C ¼ 24 ffiffiffiffiffiffiffiffi ffi AK u p =NM 2 S , 24 where K u , A, and N are the uniaxial anisotropy, the exchange stiffness constant, and the demagnetization factor, respectively.Using K u ¼ 1.2 Â 10 7 erg/cm 3 , M S $ 250 emu/cm 3 , in DO 22 MnGa from Wu et al., 15 and assuming A ¼ 1 Â 10 À6 erg/cm and N $ 4p/3, leads to d C $ 3.2 lm.While this calculation is only a rough estimation, since d C is dominated by M S À2 and we use M S values among the highest reported in the literature, we are confident that the observed island size of only a few hundred nm is certainly smaller than any reasonable d C for this material.Indeed, the MFM image of the low-rate sample clearly confirms the single domain character of the islands (Fig. 3(e)).The islands, from Fig. 3(b), are outlined in the MFM image and the arrows indicate some of the single domain structures which exhibit strong black/white contrast typical of a magnetic dipole.From the MFM and GIXRD results we can infer that the DO 22 Mn 3 Ga should be on the top part of the islands.In the high-rate sample the islands are more interconnected and thus may be larger than d C .This leads to multidomain structures and reversal by domain propagation, and consequently a reduction in H C . 25 However, the still considerably large coercivity of the high-rate sample can be explained by the rough microstructure, grain boundaries, and other imperfections (e.g., nonmagnetic phases) which inevitably lead to considerable domain wall pinning. 26s is typically found in hard magnetic systems, 1,5 the presence of non-magnetic or weakly magnetic phases is crucial for both strong domain wall pinning and magnetic separation of individual grains, both of which contribute to a large coercivity.Unfortunately, there is usually a trade-off between high coercivity and high saturation magnetization in such systems.The specific energy product E ¼ H C Â B r , where B r is the remanence, can be used as a figure of merit for permanent magnets where both a high H C and remanence are desired. 14In analyzing data from the literature we find a maximum E of 25 and 20 MGOe for bulk 8 and thin film 15 DO 22 MnGa material systems, respectively.Using nanostructuring as a morphology based approach to decouple grains into single domains, we simultaneously achieve high coercivity and a substantial saturation magnetization.The 30 MGOe energy product realized in our low-rate film is 20% larger than that reported in bulk and 50% larger than that previously reported in thin films.

CONCLUSIONS
In summary, by properly adjusting the sputtering parameters we have demonstrated nanostructured MnGa films on Si/SiO 2 with record high room temperature coercivity in excess of 20 kOe without a significant reduction in the saturation magnetization.Consequently, the energy product E is improved compared to the previously reported values.The high coercivity and high energy product of our nanostructured MnGa films are attributed to both the presence of the high anisotropy DO 22 Mn 2-3 Ga phase and the magnetic single domain character of the isolated, exchange-decoupled single crystal islands (interacting through dipolar forces).
FIG. 1. (Color online) XRD pattern of the low-rate (0.08 nm/s) MnGa film, where the symbols are the experimental data and the thick line is the calculated curve.The thin line below the main pattern is the difference between the observed and calculated patterns.Shown in the inset is the GIXRD pattern for the same sample.The vertical stripes mark the position of the b-Mn(Ga) and DO 22 Mn 3 Ga Bragg reflections.

FIG. 3 .
FIG. 3. (Color online) AFM image (3 Â 3 lm 2 scan area) and line scans (white dashed lines) for the (a), (c) high-rate and (b), (d) low-rate MnGa films.(e) MFM image of the low-rate MnGa film where the island from (b) are outlined for guidance.The arrows indicate the single domain contrast with an in-plane component found in many of the islands.Measurements were carried out on thermally demagnetized samples.
Somewhat smaller values have been recently reported by Winterlik et al. (H C ¼ 5.7 kOe, M S $ 190 emu/cm 3 ). 14By using pseudoepitaxial growth on single crystal substrates, Wu et al. were able to achieve a high chemical ordering degree of the DO 22 phase with a giant magnetocrystalline anisotropy of K eff $ 1.2 Â 10 7 erg/cm 3 .The resulting thin films exhibited a comparable coercivity and magnetization (H C $ 6.6 kOe, M S $ 250 emu/cm 3 ) (Ref.15) to the bulk values.However, Kurt et al. have reported epitaxial films with large H C but somewhat low M S (H C $ 19.0 kOe, M S