Effect of Thermal Annealing on the Stoichiometry and Magnetism of

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Effect of Thermal Annealing on Stoichiometry and Magnetism of Mn-Ga Thin Films Crislaine da Cruz, Ronei Cardoso de Oliveira, Itamar T Neckel, Dante H Mosca, and Jose Varalda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08893 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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The Journal of Physical Chemistry

Effect of Thermal Annealing on Stoichiometry and Magnetism of Mn-Ga Thin Films C. da Cruz,† R. C. de Oliveira,‡ I. T. Neckel,¶ D. H. Mosca,‡ and J. Varalda∗,‡ †Programa de Pós-Graduação em Ciências – Física, Universidade Estadual de Ponta Grossa, CEP 84030-900, Ponta Grossa, PR, Brasil ‡Laboratório de Superfícies e Interfaces, Universidade Federal do Paraná, C. P. 19044, CEP 81531-990 Curitiba PR, Brasil ¶Laboratório Nacional de Luz Síncrotron (LNLS), Centro Nacional de Pesquisa em Energia e Materiais (CNPEM), CEP 13083-970, Campinas, São Paulo, Brasil E-mail: [email protected] Phone: +55 413361 3423

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Abstract Mnx Ga thin films (1.64 ≤ x ≤ 2) were grown at room temperature by molecular beam epitaxy on the native SiO2 layer of Si(100) commercial wafers. After growth, MnxGa films were thermally annealed under different conditions (200, 300 and 400 °C). The X-ray diffraction results reveal a D022 -Mn2 Ga as the main phase and an improvement of the crystalline quality as a function of the annealing temperature. The samples were also investigated using X-ray photoelectron spectroscopy, atomic force microscopy, and vibrating sample magnetometry techniques. The magnetization curves suggest that the magnetic behavior is strongly dependent on the annealing time and temperature and that the saturation magnetization decreases with an increase in the Mn concentration in the alloy. These magnetic properties are related to the morphology and crystallinity of the samples. The magnetic moment distributions of the films were calculated by the density functional theory, bringing a better understanding for the origin of the observed magnetic anisotropy.

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Introduction Mn-Ga alloys have recently attracted the attention of the scientific community because of its technological potential for usage as free-rare-earth magnets 1 as well as spin-transfer torque 2–4 and spintronic devices 5,6 . Furthermore, Mn-Ga alloys gather some of the most important properties for modern magnetic applications such as tunable magnetic properties from ferrimagnetic to ferromagnetic passing by antiferromagnetic ordering by varying Mn:Ga stoichiometry 7,8 , high magnetic critical temperature (Tc) 3,9 , strong magnetic anisotropy 1,2,10 , properties which are envisaged for high-density magnetic storage 5,6 , nonvolatile magnetic memory and permanent magnets 6,11 . Mn-Ga alloys also demonstrate the potential for spintronic applications because of the high spin polarization at Fermi level and low Gilbert damping constant 2 , both of which are interesting for spin valves, perpendicular magnetic tunnel junctions 7 , high-density spin-transfer-torque magnetoresistive random access memories 2,12 , and magneto-optical applications like ultra-fast modulators 13 . Additionally, these alloys are interesting from an economic viewpoint since the materials (Mn and Ga) are relatively abundant and, therefore, comparatively low-cost to replace noble-metal and rare-earth metals as well as expensive Ir-based antiferromagnets. Manganese-gallium alloys (such as Mn3 Ga, Mn2 Ga, and MnGa) were already grown by molecular beam epitaxy (MBE) which is a very controllable and reproducible technique to obtain high quality thin films 14 . These alloys are quite versatile materials since their highly adaptive crystal structure, rich magnetic ordering, strong magnetic anisotropy and variable magnetic moment per formula unit can be adjusted by changing growth and annealing temperatures, 15,16 substrates 12,17 and continuously varying the stoichiometry 1,4,7,11,13,18–21 . Mnx Ga alloys with 2 ≤ x < 3 have tetragonal D022 ferrimagnetic phase and hexagonal D019 antiferromagnetic phase for x = 3. For x < 2, tetragonal L10 ferromagnetic phase is commonly observed 11,22–26 . However, nanostructured MnxGa (x ≤ 2) alloys with the majority D022 phase have also been reported 18,27,28 . In this work, we investigate the structural and magnetic characteristics of Mnx Ga (x ∼ 3



2, 1.97, 1.76, and 1.64) thin films as well as the influence of the thermal annealing in these properties.

Experimental details The Mnx Ga films were grown by molecular beam epitaxy (MBE) on the native SiO2 layer of about 5-nm-thick on commercial Si(100) wafers, that are suitable for producing highcoercivity films 17,29–31 , using a customized ultra-high vacuum MBE system equipped with Ga and Mn effusion cells. The growth procedure on SiO2 /Si substrates was performed at room temperature and the growth chamber pressure was 1 x 10-8 Torr. The Mn and Ga cells were opened simultaneously with a suitable Mn/Ga flux ratio to obtain the Mn:Ga films with a stoichiometric ratio of 2:1. The growth rate was about 0.2 nm/min and the nominal thickness of the films is 10 nm. After growth, some samples were thermally annealed at Ta = 200, 300 and 400 °C for 30 minutes, as well as at Ta = 300 °C for 15 min for comparison purposes. The annealing procedures were done in situ immediately after closing the shutters of the Mn and Ga cells under high vacuum condition ( 10-8 Torr) of the growth chamber. The surface stoichiometry of the films was monitored in situ by X-ray photoemission spectroscopy (XPS) performed using a VG Microtech ESCA3000 spectrometer using a conventional Mg-Kα X-ray source and a 250 mm hemispherical energy analyzer with an overall resolution of 0.8 eV at a 45° emission angle. X-ray diffraction (XRD) experiments were performed with a Shimadzu diffractometer using Cu-Kα radiation. The surface morphology of the films was probed by atomic force microscopy (AFM) experiments using a Shimadzu SPM-9500J8 microscope. Magnetic measurements were performed using a Physical Properties Measurements Systems (PPMS) model Evercool II fabricated by Quantum Design.

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Results and discussion XPS spectra for Mn 2p and Ga 2p core levels are shown in figures 1(a) and 1(b), where can be observed a spin-orbit splitting of 11 eV for Mn 2p1/2 – 2p3/2 and 27 eV for Ga 2p3/2 – 2p5/2 , results consistent with previously published values 32 . The stoichiometry of the samples were derived from XPS data by evaluating the ratio between the areas of the Mn-2p and Ga-2p photopeaks, using standard procedure with Shirley background subtraction and the areas were normalized using their respective atomic sensitivity factors. The maximum error determining the stoichiometry is about 0.2 %. The Mn:Ga stoichiometries found are 67:33 (S0; x = 2: sample as-deposited), 65:35 (S1; x = 1.97: 200°C for 30 min.), 54:46 (S2; x = 1.64: 300°C for 30 min.) and 58:42 (S3; x = 1.76: 300°C for 15 min.). These results show significant changes in the stoichiometry of the films as a function of the annealing temperature and time. Although the XPS technique is very sensitive to the surface stoichiometry, eventual microscopic inhomogeneities may result from the re-crystallization and atom migrations during annealing. However, intermediate phases are not detected by X-ray diffraction. Therefore, we consider in our analysis that Mn-rich and poor regions are homogeneously distributed for the samples S0 to S3. Such an assumption is important since the observed magnetic data can be explained by intrinsic magnetic anisotropy or extrinsic contributions (see DFT Calculations section). The sample annealed at 400 °C (S4) shows surface stoichiometry consisting of 15 at. % of Mn, 46 at. % of Ga and 39 at. % of O, indicating a migration of Mn atoms towards Si substrate followed by reaction with SiO2 and release of O towards the surface. The spectral deconvolution analyses of the Mn 2p3/2 and Ga 2p3/2 photopeaks for the sample annealed at 300°C for 15 minutes are shown in figures 1(c) and 1(d). Similar data analysis was performed for all samples. For the Mn 2p3/2 , the main contributions are due to metallic Mn (Mn0 at 638.5 eV) and two others contributions due to Mn+2 that are centered at 640.0 eV (Mn type II) and 642.0 eV (Mn type I). The Mn+2 type I and II have octahedral and tetrahedral coordination, respectively, leading to distinct binding energies. Although 5



Mn type I is expected to appear in Mn3 Ga compounds 8 , local stoichiometric fluctuations in Mn2 Ga can happen by the occupation of Mn atoms in the octahedral sites. The Ga 2p3/2 photopeaks reveal three contributions denoted as metallic Ga0 centered at 1114 eV, the main spectral component from alloying for both samples that is centered at 1116 eV, that can be assigned to the Ga+1 oxidation state, and a contribution centered at 1117.4 eV, which is assigned to Ga+3 valence state. As-grown sample and the sample which was annealed at 200 °C present the same trends in XPS spectra (not shown). The Mn0 contribution arises from the fact that Mn-Mn distances in the binary Mn-Ga 4 and in the pure Mn 33 are about the same, what is interesting for magnetic properties. Rode and coworkers reported in their studies that the non-collinear magnetic ground state of the D022 Mn3 Ga is reminiscent of the magnetic ground state of α-Mn and thus D022 -Mn2 Ga should behave similarly 34 . The XRD results are shown in figure 2(a) and reveal that the D022 -Mn2 Ga is the main phase stabilized with a preferential alignment of the (112) crystal planes with respect to the SiO2 /Si substrates. The 2(b) shows the rocking curve on the (112) Bragg reflection for sample S3, corroborating that the sample is textured consisting of slightly misoriented mosaic blocks with atomic distance d(112) = 2.180 Å, which is compatible with relaxed Mn2 GaD022 . The full width at half maximum of the Mn2 Ga(112) diffraction peaks decreases with the increase of the annealing temperature (Ta ) indicating the improvement of the crystalline quality of the films. The samples annealed at 300 °C for 30 min indicate the formation of a secondary phase Mn8 Ga5 with a predominant crystalline texture consisting of (100) planes stacking along the normal of the substrates. The inset in figure 2(a) shows that the (112) atomic planes distances tend to increase by increasing x, discarding the possibility of D022 → L10 phase transitions, as the L10 phase behaves in the opposite way with d111 decreasing with increasing x 19 . Annealing at 400 °C reveals a strong chemical reaction between films and substrate with formation of manganese silicides and oxides, as it can be seen in figure 2(b). Next, we will focus only on the non-reacted films. The morphology was investigated by AFM, revealing a uniform surface with submicro6


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metric grains and hillocks as shown in figure 3. By increasing the temperature, the Mn and Ga surface diffusion increases and grains acquire mobility to move randomly on the film surface increasing the coalescence. As a result, there was an increase in the probability to improve the mixture of Ga and Mn, which are mutually miscible, leading to the formation of bigger size grains. However, we observed the formation of grains with a diameter of about 20 nm for samples S0, S1 and S3. Sample S2 presented bigger grains with diameters between 100 nm and 270 nm. Sample S3 presents a more homogeneous surface with flat zones covered by more fine grains. This corroborates with the assumption of better crystalline quality of S3 sample, comparatively to the other samples, which is indicated by narrower peaks in XRD pattern shown in figure 2. Magnetic data obtained from the magnetic hysteresis loops are given in Table I. The magnetic behavior is strongly dependent on thermal annealing, which can induce significant changes in the values of saturation magnetization (Ms), remanence (Mr) and coercive fields (Hc). Samples S0 and S1 show soft magnetic material behavior with lower Ms comparatively to sample S3, whose magnetic behavior is in agreement with those high-quality samples reported in the literature 6,9,10 . Sample S2 which has a tetragonal D022 crystal structure also show low Ms, but the coexistent Mn8 Ga5 phase is paramagnetic at room temperature. In these cases, the specific order of occupation of the atomic sites imposes a strong influence on the magnetic properties 12 . Figure 4(a) shows the in-plane (IP) magnetic hysteresis loop measured at room temperature for all samples, whereas figure 4(b) shows the IP hysteretic behavior for sample S3 measured with the magnetic field applied along two crystallographic axes of the substrate, indicating that film plane is an isotropic magnetic plane. In figure 4(c) is shown Ms dependence on the Mn content in the films. If sample S2, which contain two alloy phases, is disregarded, Ms decrease with increasing Mn content in the films. This behavior is in agreement with the literature 10,35 , and it is consistent with the assumption that extra Mn atoms occupying Ga sites favor an antiferromagnetic coupling with the remaining Mn atoms 35 . 7



This, together with our results from XRD, corroborates that there are no D022 -L10 phase transitions as a function of thermal annealing. According to the XPS analyses, the ratio between Mn type I and Mn type II remains almost constant in the off-stoichiometric samples, even with the increase in the thermal annealing temperature. This can be understood by assuming that Ms value assigned to Mn type II in ordered atomic sites have a dominant effect in the samples and, therefore, Ms increases with thermal annealing up to 300 °C for 15 minutes. High coercivity is observed in figure 4 for samples S2 and S3. The in-plane hard magnetic behavior was previously observed by Zha et al. 29 and by Nummy et al. 31 in Mn-Ga films with similar stoichiometry grown on SiO2 /Si substrates. They reported that the high coercive fields observed are due to nanomorphology and high magnetocrystalline anisotropy of the films. In agreement with this work 29 , the AFM images shown in figure 3 suggest that the origin of the high Hc in our samples can also be attributed to the nanostructured morphology of the films. However, differently from the polycrystalline samples reported by Zha et al. 29 , our out-of-plane (OP) magnetic measurements show perpendicular anisotropy with peculiar step-like features around zero-field in the hysteresis loops, as shown in figure 4(d). In another work reported by Zha et al. 30 is observed a similar behavior in a highly (112)-textured sample. Therefore, these magnetization steps appear to be a common feature of (112)-textured Mn2 Ga films. It is interesting to note that the S3 sample has a unique phase and better structural quality than S2, according to X-ray diffraction. Besides, AFM measurements show that S3 presents a more homogeneous surface morphology with flat zones, whereas S2 presents a surface morphology with much larger granularity. These effects of the annealing time can induce the observed distinct IP- and OP-Ms in agreement with Ref. 20 Qualitatively, there is no significant difference between the anisotropic magnetic behavior observed for S2 and S3 at room temperature. Notice that the secondary phase in S2 is paramagnetic at room temperature and with a low magnetic moment. Therefore, the IP and OP hysteresis shapes result qualitatively the same for S2 and S3, as expected. 8


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It is also worth noticing the high magnetization of the alloy films corresponding to S2 and S3 samples (see Table I). As already pointed out by Kurt et al 6 , these compounds exhibit quite high values of both remanence and energy product of BHmax as also observed for the samples S2 and S3, which can be considered as an isotropic in-plane hard magnet.

DFT calculations In order to better understand the magnetic anisotropy observed in these Mn-Ga thin films, ab initio calculations were performed to determine the magnetic moment distributions (MMD) in the experimental conditions of our samples. Density functional theory (DFT) calculations were performed for ordered Mn2 Ga with D022 structure (space group: I4/mmm) and with lattice parameters 22,29 a = 3,905 Å and c = 7,172 Å, which are consistent with our films where atomic distance d(112) = 2,180 Å was determined by XRD experiments. DFT calculations were performed using the all-electron full-potential linearized augmented-plane-wave (FPLAPW) method, as implemented in the ELK code 36 . We used GGA exchange-correlation functional within the PBEsol approximation 37 for the non-collinear spin-polarized calculations. A grid of 13 x 13 x 7 k points in the Brillouin zone was used for the integration in reciprocal space. The total energy and the Kohn-Sham potential convergences were better than 10-6 Ha and 10-8 Ha, respectively. Some of the figures shown below were made using the VESTA software 38 . The total density of states (TDOS), as well as the most important partial density of states for each atom, are shown in figure 5. The Mn 3d states are the dominant contribution for TDOS, whereas 4s and 4p hybridized states are the most important contributions for Ga. As expected, the Mn2 Ga-D0ztsub22 is metallic and ferrimagnetic 8 with a spin polarization at Fermi level of 68%, well above the 40 % measured by Kurt et al. 6 for samples with Ms values similar to those reported in this present work. The total magnetic moment of 2.32 µB per formula unit found in our calculations is also higher than the experimental value of

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1.64 µB found for S3 sample. It is important to remember that the S3 sample shows the highest Ms value among our (112)-texture D022 -Mn2 Ga samples. Although our experimental values for magnetic moments are in good agreement with the literature 6,8 , DFT calculations suggest that such values are much lower than those theoretically predicted ones and perhaps it indicates that the samples present some degree of chemical disorder, which is corroborated by XPS analyses 12 . The origin of the observed magnetic anisotropy can be understood with the theoretical results shown in figure 6. The ground state of magnetic moments in D022 Mn2 Ga was found to be non-collinear in the calculations, which is in agreement with measurements and discussions reported by Rode et al. 34 . The MMD projection on the (112) planes of the D022 structure, which are parallel to the surface of the substrate according to the XRD experiments, is shown in figure 6(b). As can be clearly observed, the MMD contours have a rounded shape. The corresponding detail (in figure 6(c)) with magnetic moment isolines shows that at the core of these distributions, where magnetic moment is most highly localized, there is some small anisotropy with a clear deviation from the rounded contour forms. Figure 6(b) also shows the MMD contours in the (110) planes, which are perpendicular to the substrate surface. In these planes, the MMD contours are ellipsoidal even at the core (see figure 6(c)). The major ellipsoid axis of the MMD contour is inclined with respect to the substrate surface as well as to the (112) film planes by about 35 degrees. Since MMD contours indicate the directions that magnetic moments point for lowering the total system energy, then, intrinsically isotropic MMD contours in the (112) planes as well as the ellipsoidal (110) planes explain the observed isotropic IP hysteresis loops and the observed IP-OP magnetic anisotropy of hysteresis loops for S3 sample, shown in figure 4(d). Since the system does not show an intrinsic biaxial magnetic anisotropy, as can be observed in 6, an intrinsic uniaxial anisotropy associated with extrinsic shape anisotropy causes the magnetic observations for IP and OP directions. The global magnetic behavior is perfectly consistent with an isotropic IP easy magnetization in the (112) planes with a 35-degree-off OP easy magnetization axis in the (110) planes. The 10


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demagnetization energy helps to force the magnetization towards the film plane, whereas grains shape possibly determines the coercivity strength, as pointed out by Zha et al. 29 and by Nummy et al. 31 . Distinct Ms values for IP and OP loops were observed for the S2 film, and it was also observed by Gutiérrez-Pérez et al. 20 , in τ -Mn3 Ga samples, as a property dependent on the film thickness up to 50 nm. Giant magnetic moment observed in OP directions were attributed to uncompensated ferrimagnetic moments at the surface of Mn3 Ga nanocrystals, leading to ferromagnetic order on the shell of the nanograins 20 . For the S2 sample, the observed Ms values for IP and OP loops in our 10-nm-thick Mnx Ga (x = 1.64) are comparable to those reported by Gutiérrez-Pérez et al. 20 . More detailed studies are needed to characterize the magnetic behavior of the S2 sample.

Conclusions In summary, we have studied D022 Mnx Ga thin films grown on SiO2 /Si substrates with stoichiometric deviations of 1.64 ≤ x ≤ 2 resulting from thermal annealing at different temperatures. The magnetic behavior is strongly dependent on the annealing time and temperature. This procedure enables a wide control of magnetic properties interesting for applications. Thermally-induced changes in the coercivity associated with the samples nanomorphology as well as the saturation magnetization connected to the degree of chemical order are suitable for rare-earth-free magnets applications. Besides, peculiar out-of-plane magnetic anisotropy with step-like magnetization behavior could be used as a two-three state part of magnetic storage devices. These magnetic properties are related to the extrinsic morphology of the samples and intrinsic magnetic moment distributions of the Mn2 Ga with D022 structure, as demonstrated by first-principle calculations of the electronic structure.

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Acknowledgement The authors thank the financial support from CNPq, CAPES, Fundação Araucária/PRONEX, and SISNANO/Sibratec and computational support from LCPAD-UFPR.

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