A Two-Dimensional Manganese Gallium Nitride Surface Structure

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A Two-dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature Yingqiao Ma, Abhijit V Chinchore, Arthur Reed Smith, Maria Andrea Barral, and Valeria Ferrari Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03721 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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A Two-dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature Yingqiao Ma1 , Abhijit V. Chinchore1† , Arthur R. Smith1∗ , Mar´ıa Andrea Barral2 , and Valeria Ferrari2 1

Nanoscale and Quantum Phenomena Institute,

Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA 2

Departamento de F´ısica de la Materia Condensada,

GIyA, CAC-CNEA, 1650 San Mart´ın, Buenos Aires, Argentina and Consejo Nacional de Investigaciones Cient´ıcas y Técnicas - CONICET, Argentina ∗

To whom correspondence should be addressed; E-mail: [email protected].

Abstract Practical applications of semiconductor spintronic devices necessitate ferromagnetic behavior at or above room temperature. In this paper we demonstrate a two-dimensional manganese gallium nitride surface structure (MnGaN-2D) which is atomically thin and shows ferromagnetic domain structure at room temperature as measured by spin-resolved scanning tunneling microscopy and spectroscopy. Application of small magnetic fields proves that the observed magnetic domains follow a hysteretic behavior. Two initially oppositely-oriented

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MnGaN-2D domains are rotated into alignment with only 120 mT and remain mostly in alignment at remanence. The measurements are further supported by first-principles theoretical calculations which reveal highly spin-polarized and spin-split surface states, with spin polarization of up to 95% for manganese partial density of states. Keywords: spin-polarized STM; manganese gallium nitride; spintronics; magnetic semiconductor; two-dimensional material


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The field of spintronics has generated great interest worldwide both scientifically and technologically over the past twenty years, since spin-based electronics has potential advantages over conventional charge-based electronics by exploiting the spin degree of freedom of the electron, such as better data processing performance, lower energy consumption, higher integration densities, and nonvolatility [1]. A very promising approach is to blend ferromagnetism with semiconductors to either control the spin-polarization within the semiconductor by injecting polarized electrons or to manipulate ferromagnetism via control of electron concentration [2]. However, one important issue is that typically, ferromagnets and semiconductors are incompatible, having very different crystal structures and chemical compositions. A good ferromagnetic semiconductor spintronic material system should first of all, solve this problem of crystal structure and chemical bonding mismatch. It should also have a very high spin polarization (ideally close to 100%), and it should work well above cryogenic temperatures (ideally room temperature) [1, 2]. Many compound materials and layer combinations have been investigated to date, including ferromagnetic metal on semiconductor layered systems [3,4], intrinsic ferromagnetic semiconductors (FS’s) [5], and dilute (doped) magnetic semiconductors (DMS’s) [6,7]. However, many of these do not achieve the requirements already mentioned, an important problem being that magnetic elements typically do not bond coherently on top of, nor dissolve well within, layers of semiconductor crystals. And when they do, often the concentrations are too low to achieve ferromagnetism at room temperature [8]. Ferromagnetism was discovered in Mn-doped GaAs with a relatively high Curie temperature TC of 110 K [9], and subsequent theoretical work explained its origin and predicted that the ferromagnetism could persist above room temperature in several classes of III-V DMS, especially wide band-gap p-type nitrides and oxides [10]. Among these host semiconductors, GaN is one of the most promising ones [11], since it has extraordinary electronic and photonic properties including very long theoretically predicted electron spin lifetimes (several thousands of times longer than in GaAs), very large band-gap (3.4 eV) [12, 13], and widespread technological applications. After many years of intense research from diverse groups, various studies reported ferromagnetic behavior at or above room temperature in GaN-based DMS systems [14], but the possible alternate sources of ferromagnetism such as the presence of secondary metallic phases or magnetic element segregation must be carefully considered and ruled out, which is not straightforward [8, 15–17]. And in the case of (Mn,Ga)As, which is the most studied DMS material [18–22], the highest TC reported (200K) is still much lower than room temperature (295K) [23], which hinders possible future realistic applications. 3 ACS Paragon Plus Environment

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Broad interest in low-dimensional magnetism began in the 1970’s, and since then many ultrathin magnetic film systems have been studied as approximations to true 2D magnets [24]. This has led to a fundamentally deeper understanding of 2D magnetism in terms of physics and important technological applications such as the GMR effect [25, 26]. After the discovery of graphene in 2004 [27, 28], many different freestanding 2D materials with a wide range of electronic properties have also been studied [29–31]. And recently, freestanding ferromagnetic 2D materials have been discovered, including chromium germanium telluride (Cr2 Ge2 Te6 ) and chromium triiodide (CrI3 ) [32, 33]. With these come the possibility of new fundamental physics discoveries in 2D systems, but so far the reported Curie temperatures of these materials (45 K in the case of CrI3 ) are well below room temperature which limits their practical applications. In this paper, in contrast to randomly doped bulk DMS systems or layered 2D magnetic materials, we demonstrate a two-dimensional (2D) MnGaN surface alloy which has almost all the desir√ √ able properties, including a unique well-ordered, hexagonal-like 3× 3−R30◦ bonding structure (hereafter referred to as MnGaN-2D), high spin polarization, and in particular, ferromagnetism at room temperature. Furthermore, the MnGaN-2D is grown on and coupled to a semiconducting GaN substrate. Theoretically, as the dimensionality of a physical system is reduced from 3D to 2D, magnetic ordering tends to be quenched at finite temperature because of the reduced bonding coordinations at the surface and enhanced thermal and quantum fluctuations [34]. Therefore, it is very interesting to observe long-range ferromagnetic ordering persisting on a monolayer-thick MnGaN surface alloy at room temperature, especially when neither Mn nor GaN are ferromagnetic. Moreover, this unique system is characterized using the ultimate spin-resolving surface technique of spin-polarized scanning tunneling microscopy/spectroscopy (SP-STM/STS) which is a direct measurement technique with spin resolution down to the atomic scales [35–38]. Hysteretic behavior upon application of small out-of-plane magnetic fields lends further proof of the room-temperature ferromagnetism. The experimental findings are furthermore consistent with first-principles theoretical calculations which predict almost fully spin-split and spin-polarized Mn states within the surface. In previous work, we have shown that depositing ∼0.4 ML Mn on the GaN(0001) 1×1 surface √ √ at ∼ 200 ◦ C forms a 3 × 3 − R30◦ surface reconstruction [39]. The successful preparation of the MnGaN-2D reconstruction is indicated by its signature RHEED pattern immediately appearing after the Mn deposition (Supporting Information Figure S1E,F). To achieve the GaN(0001) 1×1 surface reconstruction, the as-grown GaN(0001) 3×3 surface 4 ACS Paragon Plus Environment

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reconstruction is annealed to ∼700 ◦ C to remove the excess Ga adatoms on the surface (Supporting information Figure S1). However, usually after annealing, the GaN(0001) surface will begin to roughen due to partial surface decomposition, and it is impossible for 100% of the surface to achieve the ideal GaN 1×1 surface reconstruction. Therefore after the Mn deposition, areas with 1×1 surface reconstruction will form the MnGaN-2D structure and one can identify the MnGaN2D areas on the STM images through their featureless and epitaxial appearance. Whereas, other decomposed surface areas or areas which did not achieve the ideal GaN 1×1 surface reconstruction will accumulate clusters and have a poor surface quality. The overall surface quality varies for different samples and highly depends on the growth condition especially the annealing procedure of each sample. Usually, the less excess Ga adatoms which remain on the as-grown surface, the less time will be required for annealing, and the higher quality the MnGaN-2D surface will have after Mn deposition. As a result, this can lead to the appearance of MnGaN-2D islands surrounded by rough areas as shown in Figs. S4 and S6. But, usually the MnGaN-2D areas and the surrounding rough areas are not located on the same terrace (Fig. S4E). And since the MnGaN-2D structure is only monolayer thick and the GaN underneath is nonmagnetic, there should not be any magnetic or electrical couplings between the MnGaN-2D areas and the surrounding disordered areas in general. A constant current STM topograph of two atomically-smooth MnGaN-2D terraces is shown in Fig. 1A. A topography line profile along the black arrow across the two terraces is shown in the left bottom inset of Fig. 1A. As expected, it shows a terrace height of 0.259 nm which is the interplanar spacing of GaN. One-hundred twenty degree upper terrace corners also clearly indicate the 3-fold GaN hexagonal-like symmetry. The bright extended features on the two terraces are referred to as long-range topographical distortions (LTDs) and are most likely caused by ionized impurities underneath the surface within the GaN layers [40]. A zoom-in of the blue box surrounded areas in the top part of Fig. 1A is shown in its right bottom inset and expanded in Fig. 1B, in which the atomically resolved MnGaN-2D reconstruction on both terraces is revealed. Aside from there being a few small defects (dark sites), the surface is very well ordered overall. Line sections drawn along [1010] directions (Fig. 1B, top inset) √ ˚ (Fig. 1C, a being the GaN lattice constant). show the atomic periodicity equal to 3a = 5.52 A However, the corrugation measured along [1010] varies locally, and along one direction (blue line section) it oscillates with smaller amplitude than along two similar directions (red and black line sections), resulting in a row-like appearance. As seen from Fig. 1B that these LTD’s also have the MnGaN-2D structure, which indicates the local MnGaN-2D structure or properties are unchanged. 5 ACS Paragon Plus Environment

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The model structure, found using density functional theory (DFT), reveals the chemical bonds between Mn and Ga, Mn and N, as well as Ga and N atoms, as shown in Fig. 1D. The bonding forms the unique and stable MnGaN-2D surface alloy which is resistant to heating up to as high as 700 ◦ C. Its 25% Mn concentration is much higher than the solubility limit for (Mn)GaN dilute magnetic semiconductors but quite low for Heusler MnGa alloys [41]. The surface bonding is highly anisotropic so that both Ga-N and Mn-N bonds tilt away from [0001] (surface normal direction), thus removing the full 3-fold symmetry and contributing to a structure with a uniaxial symmetry. Intermixing of different rotated uniaxial domains leads to the non-uniform appearance seen in the STM images (Fig. 1B). In the following, to explore the spin ordering of the MnGaN-2D structure, we perform SPSTM/STS measurements at room temperature with an Fe-coated W tip (Supporting Information Experimental Methods). Figures 2A and 2B are simultaneously acquired topographical (presented in derivative mode) and dI/dV conductance mode images, respectively, obtained from the MnGaN-2D surface in its spontaneous magnetic state with no external field applied, revealing dI/dV contrast on different areas labeled 1 to 5. The dI/dV conductance signal contains a spin component directly proportional to the sample magnetic density of states multiplied by the tip spin polarization which contains a factor of cosine(θ) (θ being the angle between sample and tip magnetization vector directions), as well as a component proportional to the electronic density of states [37]. If different areas with relative dI/dV contrast have the same local (electronic) structure, which we can see is the case here by looking just at the topographical image where all areas look the same (Fig. 2A), it indicates that the observed contrast is not electronic, but instead is magnetic in origin, coming from the ferromagnetic domain structure of the MnGaN-2D surface. Also on the same MnGaN-2D surface, with a plain W tip, we did not observe any dI/dV contrasts (Supporting information Figure S6), which further proves the magnetic nature of the dI/dV contrast observed in Fig. 2B. Since our tip spin vector direction (from which the angle theta is measured) may have both in-plane and out-of-plane components (canted tip vector), therefore the sample spins could be in-plane, out-of-plane, or canted as well, which can not be determined at this point. The dI/dV contrast is more quantitatively seen in line profiles 1-2-4 and 4-2-3 drawn across Fig. 2B and shown in Fig. 2C. They reveal the same relative lower dI/dV value for areas 1 and 4 and higher dI/dV value for areas 2, 3, and 5. We interpret the origin of the dI/dV contrast here as a difference in cosine(θ), indicating that the spin direction of areas 1 and 4 are more antiparallel to the tip spin direction, as compared to regions 2, 3, and 5 which are more parallel 6 ACS Paragon Plus Environment

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to the tip spin direction. An additional effect is that, in order to maintain the tunneling current constant, when scanning on areas 1 and 4 which have a relatively lower integrated dI/dV , the tip will approach closer toward the sample surface as compared to areas 2, 3, and 5 [37], leading to a slight difference in the step heights. This consideration is confirmed by the height profile 4-5˚ between areas 4 and 5 as 3 drawn across Fig. 2A showing the enhanced step height ≈ 2.84 A ˚ (GaN c/2 value) between areas 3 and 5 (Fig. 2C). compared to the normal step height ≈ 2.59 A More evidence that the observed spin contrast corresponds directly to MnGaN-2D surface regions is presented via a zoom-in scan of a different area of the sample surface (Supporting information Figure S7), revealing atomic resolution (similar to that shown in Fig. 1B) as well as spin contrast via its simultaneously acquired dI/dV spin map. In order to explore the electronic structure of the MnGaN-2D surface, normalized tunneling conductance spectra (dI/dV )/(I/V ) (Fig. 3A), considered to be proportional to the surface local density of states (LDOS) [42, 43], are measured above MnGaN-2D area. The normalized dI/dV curves exhibit several features including two pronounced peaks, one located at ≈ -1.25 eV below the Fermi level EF and the other located at ≈ +1.75 eV above EF . It may be assumed that these 2 most prominent peaks correspond to spin-polarized Mn states, as suggested by the SP-STM results shown in Fig. 2. Two smaller peaks and a dip are also observed at ≈ -0.55 eV, +0.25 eV, and between -0.25 and 0 eV, respectively. Spin-polarized DFT calculations are performed for the MnGaN-2D structure (Supporting information). Different theoretical magnetic configurations are considered, and we found that the ferromagnetic configuration between neighboring Mn atoms has the lowest energy with a large magnetic moment of 3.8 µB for each Mn atom. A Heisenberg-type model suggests that the ferromagnetism originates from a superexchange interaction between neighboring Mn atoms mediated by Ga atoms. Two important coupling constants can be determined in this model, J1 and J2 , each corresponding to indirect Mn-Mn interaction (see Fig.3C). Here, J1 represents the coupling mediated by Ga adatoms not bonded to N, while J2 is the one mediated by Ga adlayer atoms bonded to N. It is found that J1 is stronger than J2 , with J1 = 3.5 meV and J2 = 0.6 meV. The calculated total LDOS of the MnGaN-2D structure including Mn adatom, Ga adatom, and Ga surface atoms is shown in Fig. 3A inset (Note: the 2nd layer N and Ga states are not included), and it shows an overall shape very similar to our experimental normalized dI/dV spectra, also revealing two robust surface DOS peaks, one being the filled states peak at -1.55 eV (below EF ), and the other being the empty states peak at +1.45 eV (above EF ). Aside from a small shift of ≈ 0.3 eV, these two peaks agree well with the two most prominent experimental peaks seen 7 ACS Paragon Plus Environment

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in Fig. 3A (Note: the small Fermi level shift can be easily explained, for example, by a small upward band bending which theory does not account for). Furthermore, Fig. 3B plots the spinpolarized total LDOS as well as the Mn states only, which confirms that the two main DOS peaks are spin-polarized and spin-split, dominated by the strongly spin-polarized Mn states. The Mn partial density of states are highly spin-polarized and spin-split with the -1.55 eV filled states peak 95% spin-up-polarized and the +1.45 eV empty states peak 94% spin-down-polarized. It can also be seen from Fig. 3B (and Supporting Information Figure S3B) that there is a strong net spinpolarization for the total surface states for energies going over a wide energy range from -2.7 eV to -1.4 eV with an average spin-up spin-polarization of 53% (filled states ), and +1.4 eV to +2.7 eV with an average spin-down spin-polarization of 42% (empty states), so that the voltage bias need not to be precisely at the prominent peak energies to detect strong spin-polarized surface states. As shown by the MnGaN-2D density of states plot with a wider energy range (Supporting Information Figure S3), the weakly spin-polarized N-states (2nd layer, bulk states) begin increasing rapidly near -0.9 eV (-0.6 eV based on the Fermi level shift seen in experiment), indicating the onset of the bulk valence band edge. The filled Mn surface state peak at -1.55 eV (-1.25 eV expt) therefore lies 0.65 eV inside the valence band, whereas the empty Mn states peak at +1.45 eV (+1.75 eV expt) lies about 1 eV below the conduction band edge near +2.5 eV, thus inside the GaN band gap (3.4 eV). It should be noted however, that since the sub-surface N and Ga atoms are in the second layer (about 2 angstroms below the surface Mn and Ga layer), the STM/STS is largely insensitive to the bulk N and Ga states. Therefore, the total spin polarization within the MnGaN-2D surface top layer is as shown in Fig. 3B and does not include the bulk N and Ga states. Next, to prove the spin-splitting nature of the filled and empty MnGaN-2D surface states, we investigate the bias-voltage dependence of two MnGaN-2D islands marked as 1 and 2 as shown in Fig. 4A. The dI/dV spin maps of the same area for both surface states with no magnetic field applied are plotted in Fig. 4B (filled states) and Fig. 4C (empty states), showing the magnetic domain structure of these two islands. For the filled states (Fig. 4B), the rim of island 1 appears to be bright, whereas the inner part is dark; on the other hand, the contrast is reversed for island 2 which shows a bright inner part with a dark rim. Fig. 4D shows the topography (the black curve) and dI/dV (the blue curve) line profiles taken at the same location indicated by the black and blue arrow across the topography image Fig. 4A and its simultaneously acquired dI/dV map Fig. 4B, which reveals clearly the rim states of the island domain. The rim states have a typical width of ≈ 3.5 nm, i.e. 6−7 atomic rows wide, as shown in Fig. 4D. The rim state’s onset and maximum position is indicated by the dashed black 8 ACS Paragon Plus Environment

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lines in the topography image Fig. 4A, and it is clear that the rim states are located on top of the flat island and not off of the edge of the island, which proves the dI/dV enhancement of the rim states is part of the magnetization state of the island. So we find that the filled states spin of the inner part of island 1 is relatively antiparallel to the tip spin versus that of island 2, which is relatively parallel to the tip spin, and that there are spin-polarized rim states with opposite spin character to the inner part, spatially localized at the edges of both islands. Similar magnetic rim states were also reported for other magnetic island systems based on SP-STM measurements, such as Co islands on Cu(111) surfaces [44, 45]. On the other hand, we do not find any rim states at the step edges of the magnetic terraces in Fig. 2A, which is also consistent with observations in other magnetic systems. The second level seen on the lower end of island 1 is a common phenomenon for the MnGaN2D islands. These second levels also have the MnGaN-2D structure, but they are defective (Supporting Information Figure S4), unlike most well-ordered MnGaN-2D areas, which might affect their electronic or magnetic properties. A magnetic switch occurring at the dashed black lines shown in Fig. 4B during scanning, where the inner part of island 1 is initially bright and then becomes dark whereas its rim state is initially dark and then becomes bright, is further proof of the magnetic nature of the observed dI/dV contrast and may indicate superparamagnetism for these MnGaN-2D nanoislands, which again confirms that room-temperature is still below the Curie temperature of the MnGaN-2D structure. Similar superparamagnetism has been observed in several magnetic nanoislands systems. Electronic tip switching can be ruled out since the dI/dV signal surrounding the island is unchanged across the switching line. This indicates that either the tip or sample magnetic state switched. The dI/dV line profile taken along the cyan arrow in Fig. 4B across island 1 and island 2 is shown in Fig. 4E as the cyan curve, which shows again the magnetic rim states as well as the dI/dV magnetic switch. In separate STM experiments at low temperature (4.2 K), we have observed a similar and clear magnetic switch on a MnGaN-2D island domain (Supporting Information Figure S5D). In that case however, a neighboring MnGaN-2D domain along the same line showed no switching whereas it should have also switched if there had been a tip change of any type; therefore we can conclude it was the sample island domain which switched, and this could have been induced by the tip magnetic stray field, or by the spin-polarized tunneling current. For the empty states, the spin of island 1 should be relatively parallel to the tip spin versus that of island 2, which should be relatively antiparallel to the tip spin, since the filled and empty 9 ACS Paragon Plus Environment

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MnGaN-2D surface states are theoretically spin-split according to Fig. 3B. This should result in a reversed dI/dV contrast between island 1 and island 2 compared to the filled states situation [46], which is exactly what is observed in Fig. 4C, where island 1 is bright in dI/dV while island 2 is dark. This spin-split contrast reversal between filled and empty states is shown clearly in line profiles in Fig. 4E where for filled states (cyan curve) island 1 is low with island 2 high, versus for empty states (red curve) island 1 is high with island 2 low, thus nicely verifying the theoretical expectation. The rim states, on the other hand, are not obvious on the empty states side. Evolution of the MnGaN-2D magnetic domain structure of Fig. 4 as a function of the applied out-of-plane magnetic field is shown in a series of dI/dV spin maps (Fig. 5) taken with different applied magnetic field. Fig. 5A is the topography image of the two MnGaN-2D island domains and Fig. 5B is its spontaneously acquired dI/dV image with no extremal magnetic fields applied, exhibiting the same contrast as Fig. 4A that island 1 is dark with a bright rim whereas island 2 is bright with a dark rim, indicating that the spin of island 1 is antiparallel to the tip spin direction whereas that of island 2 is parallel to the tip spin direction. As the applied magnetic field is increased first to 2.5 mT and then to 8.0 mT (Fig. 5C-D), the dI/dV contrast of islands 1 and 2 changes clearly. The interior of island 1 gets slightly darker while its rim state is strongly reduced. The inner part of the island 2 changes from bright to dark clearly and gradually starting from the center, while its rim states remain dark. This proves the magnetic nature of the observed dI/dV contrast between the two MnGaN-2D islands as well as the rim states, and indicates that as the applied magnetic field increases, the MnGaN-2D sample spin orientation begins to align with the applied field direction. Due to the applied field being out-of-plane, the MnGaN-2D sample spins are assumed to be out-of-plane sensitive. The tip spin vector should be unchanged at this moment given the small applied magnitude fields used (8.0 mT compared to fields normally required to fully rotate Fe-coated tip spin vectors of 800-2000 mT. [47, 48]). When the applied field is increased to 15 mT (Fig. 5E), the contrast between the two magnetic domains is significantly less visible, indicating the spins of both islands are mostly aligned with the applied out-of-plane magnetic field. After the magnetic field is further increased up to 120 mT, the MnGaN-2D island domains are fully saturated as the dI/dV contrast between island 1 and 2 completely disappears (Fig. 5F). Then the magnetic field is decreased to 58 mT and then to 0 mT, and the MnGaN-2D island domains remain nearly saturated, exhibiting high magnetic remanence in the out-of-plane direction, as shown in Fig. 5G-H. It is very unlikely for a 2D system with in-plane spin-polarization to be highly sensitive to small out-of-plane magnetic fields and to exhibit strong out-of-plane magnetic remanence. Therefore we believe the MnGaN-2D sample has 10 ACS Paragon Plus Environment

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an out-of-plane spin polarization, which is also supported by our low temperature spin-polarized STM data (Supporting Information Figure S5). Shown in Fig. 6 is a plot of the dI/dV asymmetry defined as: AdI/dV = (dI/dV1 − dI/dV2 )/(dI/dV1 + dI/dV2 ) between the magnetic island domains 1 and 2 as a function of the applied out-of-plane magnetic field B [49]. The AdI/dV has the convenient feature of subtracting away the (normal) electronic component of the LDOS and leaving the remainder of only magnetic origin. Also, AdI/dV is closely related to the spin polarization of the tip PT multiplied by that of the sample PS as well as the relative angles between the tip and sample magnetic vectors of the 2 domains. In the special case that the magnetic vectors of the 2 island domains are oppositely oriented to each other, then AdI/dV = −PT PS cosθ, where θ is the sample spin angle relative to the tip spin angle. Although the expression for the case of non-collinear sample vectors is slightly more complicated, in general and assuming the tip vector is constant, AdI/dV is a quantity closely related to how well the 2 domains are magnetically aligned. Therefore a plot of AdI/dV versus B corresponds to a plot of magnetic hysteresis, with AdI/dV going to 0 at 100% magnetic saturation. This occurs at B = 120 mT, and after reducing B back to zero, we find AdI/dV decreases slightly to -0.1 (the remanent state). As for the initial, unpolarized state, assuming the simplest case of 2 anti-aligned sample spins which are each in relative close alignment with the tip spin, and assuming a PT around 0.45 (near the accepted value for Fe coatings) [37], then we obtain an estimate for PS = -AdI/dV /PT = (0.44 ± 5%)/(0.45 ± 20%). So we get PS = 77-100% (cannot exceed 1). This is in reasonable agreement with (or even exceeds) the theoretical value, considering the very large spin polarization of the surface (Fig. 3B) in which Mn (Mn+Ga) surface spin-DOS polarization is 89% (46%) at -2.0 eV. In conclusion, the atomically thin, MnGaN-2D surface alloy discussed here displays highly spin-polarized, ferromagnetic behavior at room temperature, which is revealed directly using SPSTM, opening a new pathway for integrating magnetism into semiconductor systems. The wellordered MnGaN-2D structure overcomes the problem of chemical bonding mismatch at the magnetic/semiconductor interface, and its high spin polarization opens new possibilities for future practical applications. Such possible applications include ultimately thin magnetic memories at room temperature, room-temperature spin injection into semiconducting GaN with long spin lifetime, and thus quantum computation at room temperature, along with multi-functional nanodevices with reduced dimensions and increased integration densities [1, 2]. From the fundamental science point of view, its 2-D nature holds promise for discovery of exotic new physical phenomena in 11 ACS Paragon Plus Environment

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nanoscale magnetism and nanospintronics. ASSOCIATED CONTENT Supporting Information: Additional information about experimental methods, surface morphology, sample electronic structure, and more STM data at both room temperature and liquid helium temperature with different tips. Author Information ∗

Corresponding Author; E-mail: [email protected]. † Present address: Intel Corporation, Hillsboro, OR 97123. Notes The authors declare no competing financial interest. Acknowledgments Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DE-FG02-06ER46317 (SP-STM studies of magnetic and spintronic nitride systems). Y.M. thanks Dr. Jeongihm Pak for helping with MBE/STM setup at the early stage. The authors thank J.P. Corbett for developing the tip etching process used for making SP-STM tips having specific radius of curvature as well as for assisting with tip selection based on SEM tip imaging. The authors would like to acknowledge RHK for the R9 STM electronics as well as RHK data acquisition & image processing software. Authors also acknowledge use of the WSxM software for STM data analysis and image processing [50].


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Fig. 1. Structural surface properties of the MnGaN-2D reconstruction. A Large-scale constantcurrent STM image of the MnGaN-2D surface, sample bias voltage Vs = -2.5 V, tunneling current I = 100 pA. A line profile along the black arrow in A is shown in its left bottom inset. A zoom-in of the solid blue rectangular surrounded area is shown in the right bottom inset with the same scale bar as (A), Vs = +1.8 V, I = 100 pA, which is further expanded in the derivative STM image B showing atomic resolution, Vs = +0.6 V, I = 100 pA. The insets with a blue and black frame are further zoom-ins of the dashed blue and black square enclosed regions. The black, red, and blue topography line profiles shown in C are measured along the corresponding lines shown in the inset of (B). D Relaxed MnGaN-2D model structure based on density functional theory.


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Fig. 2. Evidence for room-temperature ferromagnetic domains on the MnGaN-2D surface with SP-STM data acquired using an Fe-coated W tip. A Large area derivative mode STM topography image of the MnGaN-2D surface, and its simultaneously acquired dI/dV image is shown in B with the conductance scale shown at the right, the very high (magenta) and very low (cyan) dI/dV intensity which are observed at the step edges are due to scanning direction (right to left) caused edge effect and adsorbates which adsorb onto the step edges, Vs = -2.5 V, I = 100 pA. Three line profiles across different labeled regions of the image are shown in C including dI/dV line profile 124 crossing regions 1, 2, and 4, dI/dV profile 423, and height profile 453.


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Fig. 3. Electronic structure of the MnGaN-2D surface. A Normalized dI/dV tunneling spectra of MnGaN-2D structure, Vstab = -0.9 V which is the bias voltage to stabilize the tip before the feedback loop is turned off. Shown plotted here is an average over many such normalized spectroscopy curves. The inset shows the calculated total LDOS of the MnGaN-2D surface (the contribution from the second layer Ga and N atoms are not taken into account) B Calculated spin-polarized total LDOS plot of the MnGaN-2D surface structure together with the spin-polarized DOS plot of Mn states only. The positive DOS represents spin-up states and the negative DOS represents spin-down states. C Ferromagnetic spin configuration of the MnGaN-2D surface structure.


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Fig. 4. A Topography image of two ferromagnetic MnGaN-2D island domains marked as 1 and 2, the boundaries of both islands are indicated by the dashed cyan contour and its simultaneously acquired dI/dV conductance map is shown B, (scanning bottom to top, Vs = -2.0 V, I = 100 pA). C Consecutive dI/dV conductance maps of the same area of B showing the empty surface states of the MnGaN-2D structure, (scanning top to bottom, Vs = +2.0 V, I = 100 pA). A sudden magnetic dI/dV contrast reversal as observed in (B) is indicated by a dashed black horizontal line. D shows a topography height profile (the black curve) and a dI/dV line profile (the blue curve), which are taken at the same spatial location. The topography line profile is taken along the black arrow across (A) and the dI/dV profile is taken along the blue arrow across (B). Two dI/dV line profiles measured along the cyan, and red arrows (the dashed parts are neglected) cutting across (B) and (C) are shown in E.


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Fig. 5. The development of the MnGaN-2D ferromagnetic domains in an applied magnetic field. A the topography image of the two MnGaN-2D island domains marked as 1 and 2 (Vs = -2.0 V, I = 100 pA). B to H dI/dV STM images of the same two island systems taken at different applied out-of-plane magnetic fields with the same scanning parameters Vs = -2.0 V, I = 100 pA, exhibiting different dI/dV magnetic contrast between island 1 and island 2.


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Fig. 6. Hysteresis curve obtained from from Figs. 5B-H, by calculating dI/dV asymmetry AdI/dV = (dI/dV1 − dI/dV2 )/(dI/dV1 + dI/dV2 ), where dI/dV1 (dI/dV2 ) is the average dI/dV value of the inner part of island 1 (island 2).


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A Two-Dimensional Manganese Gallium Nitride Surface Structure

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