Molecular beam epitaxy growth and magnetic properties of Cr-Co-Ga Heusler alloy films , , Wuwei Feng , Weihua Wang, Chenglong Zhao, Nguyen Van Quang, Sunglae Cho , and Dang Duc Dung Citation: AIP Advances 5, 117223 (2015); doi: 10.1063/1.4935949 View online: http://dx.doi.org/10.1063/1.4935949 View Table of Contents: http://aip.scitation.org/toc/adv/5/11 Published by the American Institute of Physics AIP ADVANCES 5, 117223 (2015) Molecular beam epitaxy growth and magnetic properties of Cr-Co-Ga Heusler alloy films Wuwei Feng,1,a Weihua Wang,1 Chenglong Zhao,2 Nguyen Van Quang,3 Sunglae Cho,3,b and Dang Duc Dung4 School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, University of Ulsan, Ulsan 680-749, South Korea Department of General Physics, School of Engineering Physics, Ha Noi University of Science and Technology, Dai Co Viet Road, Ha Noi, Vietnam (Received 22 August 2015; accepted November 2015; published online 12 November 2015) We have re-investigated growth and magnetic properties of Cr2CoGa films using molecular beam epitaxy technique Phase separation and precipitate formation were observed experimentally again in agreement with observation of multiple phases separation in sputtered Cr2CoGa films by M Meinert et al However, significant phase separation could be suppressed by proper control of growth conditions We showed that Cr2CoGa Heusler phase, rather than Co2CrGa phase, constitutes the majority of the sample grown on GaAs(001) at 450 oC The measured small spin moment of Cr2CoGa is in agreement with predicted HM-FCF nature; however, its Curie temperature is not as high as expected from the theoretical prediction probably due to the off-stoichiometry of Cr2CoGa and the existence of the disorders and phase separation C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4935949] I INTRODUCTION The search for high-performance materials is a priority for the development of spintronic devices such as spin valves and spin memories.1 In the past decades, half-metallic ferromagnets (HM-FMs) with different elemental composition and crystal structures have been widely studied due to the 100% spin polarization at the Fermi level resulting from their specific band structure with one spin channel being metallic and another channel being semiconducting or insulating.2–5 In addition, an interesting proposed addition to the family of HM-FMs is the zero-moment half-metallic ferrimagnet, named half-metallic fully-compensated ferrimagnets (HM-FCFs).6 HM-FCF has both 100% spin polarization and zero net magnetic moment, which causes the generated stray fields and energy losses in spintronics devices are lower compared with HM-FM with large net magnetic moment Thereby, HM-FCFs are more suitable in the real application of spintronics devices Some Heusler alloys and perovskite compounds have been found to be HM-FCFs.4,7,8 Especially the half-metallic Heusler compounds having the formula X2YZ attracted intense interest because most of those alloys possess high Curie temperatures, compatible crystalline structure with the zincblende of semiconductors, and generation of rich electronic structures and novel properties by varying the valence of X, Y, and Z.9 CrMnSb have antiparallel spin magnetic moments of about the same magnitude in Cr and Mn sites.10 Mn3Ga,6,11 Cr2MnSb,12 (Mn0.5Co0.5)2VAl,1,13 Co2CrAl14 and Cr2CoAl4,14 are half-metallic ferrimagnetic alloys with zero or nearly zero spin moment a wfeng@cugb.edu.cn b slcho@ulsan.ac.kr 2158-3226/2015/5(11)/117223/9 5, 117223-1 © Author(s) 2015 117223-2 Feng et al AIP Advances 5, 117223 (2015) I Galanakis and E Sasıoglu have searched for High Curie temperature (TC) HM-FCF Heusler compounds by ab-initio electronic structure calculations and found that Cr2CoGa (CCG) is a nice HM-FCF with high degree of spin-polarization for a wide range of lattice constants and very high TC.7 However, Cr2CoGa was found to be unstable with respect to their elemental constituents according to the density functional theory calculation of the formation enthalpy.15 Manuel P Geisler et al have prepared Cr2CoGa films by magnetron co-sputtering and observed experimentally the phase separation and precipitate formation in dependence on the heat treatment.16 So it seemed that the instability of the Cr2CoGa alloy makes this material unsuitable for the spintronics applications Here we re-investigated the growth and properties of Cr2CoGa film by molecular beam epitaxy (MBE) and attempted to stabilize the metastable Heusler phase of Cr2CoGa The generation of phase separation and precipitate was again confirmed experimentally However, our results showed that the majority of Cr2CoGa phase could be stabilized and the phase separation could be suppressed to the utmost extent by proper control of growth conditions The measured spin moment of the sample was small in agreement with the calculated zero-moment; however, the TC was not as high as expected from the theoretical prediction probably due to the off-stoichiometry of Cr2CoGa and the existence of the disorders and phase separation II EXPERIMENTAL Cr2CoGa films with thickness of 60 nm were grown on Si(001) and GaAs(001) substrates, respectively, in a standard solid-source MBE chamber (VG Semicon model V80) Substrates were acid dipped to remove surface oxide and heated at 550 oC for around 30 before sample growth The ratio of elemental constituents was controlled by adjusting the evaporation rate of respective effusion cells The accurate composition of samples determined by the electron probe microanalysis (EPMA) measurement The Cr2CoGa films were deposited under an ultrahigh vacuum of 10−9 Torr at different temperatures Some of samples were capped by nm GaAs to avoid the contamination of inner layers Reflection high-energy electron diffraction (RHEED) was applied to monitor the growth process of the Cr2CoGa films The crystal structure and lattice constant were determined by X-ray diffraction (XRD) using Cu Kα radiation The surface morphology of the sample was measured by scanning electron microscope (SEM) The magnetic properties were characterized by superconducting quantum interference device magnetometer (SQUID, Quantum Design) Temperature dependence of resistance (measured with a four point configuration) and magneto-resistance was carried out by a home-made transport property measurement system (TPMS) III RESULTS AND DISCUSSION A Growth of Cr2CoGa on Si(001) substrate We first attempted to grow Cr2CoGa on Si(001) substrate By monitoring RHEED pattern during film growth, it was found that crystalline phase can only be formed at the high growth temperature Fig 1(a) shows RHEED pattern for the 450 oC-grown Cr2CoGa/Si(001) film The pattern of ordered spots indicated presence of crystalline phase in the sample As shown in the XRD spectra in Fig 2, lots of diffraction peaks appeared when sample was grown on Si(001) at 450 oC, which could not be assigned to the Heusler phase of Cr2CoGa Instead, phase separation including Cr3Ga, Cr3Ga4, CoGa3, and CrCo could be assigned, in agreement with observations by M P Geisler and M Meinert.16 Conspicuous phase separation could also be seen in the SEM image in Fig 3(b) Probably due to the large lattice mismatch (6.7%) between Cr2CoGa and Si(001), Cr2CoGa phase could not be stabilized The obtained sample composed of multi-phases separation is non-ferromagnetic according to the magnetization vs magnetic field curves shown in Fig Therefore, it was concluded that Cr2CoGa phase could not be stabilized on Si(001) substrate and the choice of substrate with appropriate lattice parameter and symmetry was probably important for the stabilization of Cr2CoGa by MBE growth 117223-3 Feng et al AIP Advances 5, 117223 (2015) FIG RHEED patterns for the Cr2CoGa films grown on (a) Si(001) at 450 oC, (b) GaAs(001) at 70 oC, and (c) GaAs(001) at 450 oC, respectively B Growth of Cr2CoGa on GaAs(001) substrate We then grew Cr2CoGa on GaAs(001) substrate As shown in Fig 1(b), it could be seen that when grown on GaAs(001) at 70 oC, Cr2CoGa sample takes on ring-shaped RHEED pattern indicating the formation of non-crystalline or amorphous state of the sample, in agreement with previous observation that crystalline phase can only be formed at the high growth temperature The absence of crystalline phase in the 70 oC-grown Cr2CoGa/GaAs(001) was confirmed by the XRD spectra in Fig 2, where no distinct diffraction peaks from the film were observed It indicated that Cr2CoGa Heusler alloy does not crystallize at low temperature Meanwhile, the surface morphology of the sample was homogeneous as seen from the SEM image in Fig 3(a) It means that phase separation and precipitates also not happen when Cr2CoGa was grown on GaAs(001) at low temperature As shown in Fig 4, the non-crystalline Cr2CoGa grown at low temperature is also of non-ferromagnetic ordering FIG XRD patterns for the Cr2CoGa films grown at different conditions 117223-4 Feng et al AIP Advances 5, 117223 (2015) FIG SEM images for (a) 70 oC-grown Cr2CoGa/GaAs(001) film, (b) 450 oC-grown Cr2CoGa/Si(001), (c) and (d) 450 oC-grown Cr2CoGa/GaAs(001) film, respectively When grown at 450 oC, it demonstrated pattern of streaky lines, as shown in Fig 1(c), indicating the epitaxy of Cr2CoGa on GaAs(001) lattice The composition of the sample is Cr1.95Co1.03Ga according to EPMA Note that the lattice mismatch between Cr2CoGa on GaAs(001) is 2.4%, around times smaller than that between Cr2CoGa on Si(001) However, there were still some three dimensional features every other streak, which was in agreement with existence of more than one pure phase in the sample as seen from XRD in Fig The possible release of As from GaAs surface during heating substrate at high temperature might lead to formation of defects on the GaAs surface, which would affect the epitaxial growth of film It was also noted that the as-used GaAs(001) wafer was of poor quality as seen from the GaAs (004) and (002) diffraction intensity in the XRD spectra; FIG Magnetic field dependence of the magnetization curves for the 70 oC-grown Cr2CoGa/GaAs(001) and 450 oC-grown Cr2CoGa/Si(001) films, respectively Note that the diamagnetic signal of the substrate was not subtracted 117223-5 Feng et al AIP Advances 5, 117223 (2015) FIG XRD pattern for Rietveld structure analysis of the 450 oC-grown Cr2CoGa/GaAs(001) sample however, it does not affect the validity of our results When grown on GaAs(001) at 450 oC, four diffraction peaks of the film could be assigned to Cr2CoGa(002), Cr2CoGa(222), Cr2CoGa(004), and Cr3Ga(210) In view of the epitaxy relation between Cr2CoGa layer and GaAs(001) substrate as seen from the RHEED pattern, the appearance of Cr2CoGa(222) seemed unexpected It might be due to the formation of (222) facets to relieve the lattice mismatch in between The corresponding lattice parameter of Cr2CoGa is a = 5.79 Å, in agreement with previous reports.15,17 The appearance of Cr3Ga phase separation was also in agreement with Ref 16 The surface morphology of the corresponding sample shown in Fig 3(c) was relatively rough compared with the sample grown at low temperature; however, almost no distinct phase separation could be observed even in the magnified image in Fig 3(d) Therefore, it is clear that Cr2CoGa phase could be stabilized on GaAs(001) substrate at 450 oC probably due to the small lattice mismatch The phase separation still exist, however, it was significantly suppressed We speculated that pure phase Cr2CoGa sample could be achieved by proper preparation of a perfect substrate surface and precise control of slow growth rate for the epitaxy in the further research In order to determine the relative content of Cr2CoGa, Rietveld refinement was performed for the XRD data of 450 oC-grown Cr2CoGa/GaAs(001) sample, as shown in Fig Rietveld refinement was carried out via the computer software General Structure Analysis System (GSAS) program18 with the Hg2CuTi(102972-ICSD), Cr3Ga(626025-ICSD) and GaAs(41674-ICSD) crystal structure as starting models A quantitative analysis of this sample was performed with Rwp = 6.51%, Rp = 4.36% and χ2 = 2.01 A relative content of Co2CrGa:Cr3Ga = 10:1 In view of the poor quality of XRD data, even though error accounts for 20%, Cr2CoGa still account for 72% Therefore, the Cr3Ga phase separation was a minority Actually according to previous research,16 if there is Cr3Ga, there should also be some Co-based compounds as well Therefore, some Co-based compounds, such as CoCr, which does not show distinct peaks in the XRD spectra, might also exist in the sample except Cr3Ga The Co-based compound might partially contribute to the observed non-zero magnetic moment Figure shows the magnetization vs applied field (M-H) loops for the 450 oC-grown Cr2CoGa/ GaAs(001) film Distinct M-H hysteresis curves were observed at both 300 K and 10 K indicating ferro(ferri)magnetic ordering of Cr2CoGa The saturation magnetization is small, around 70 emu/cm3 at 10 K The saturation moment at 10 K was determined to be 0.31 µB per formula unit, which is larger than the predicted zero moment probably due to the off-stoichiometry of Cr2CoGa and the existence of phase separation and disorders In addition, the distinct difference between the in-plane and out-of-plane saturated magnetization in Fig indicated existence of the 117223-6 Feng et al AIP Advances 5, 117223 (2015) FIG Magnetic field dependence of the magnetization hysteresis loops for the 450 oC-grown Cr2CoGa/GaAs(001) film shape anisotropy which could be induced by the tetragonal distortion of Cr2CoGa The tetragonal distortion of Cr2CoGa was likely to happen during epitaxy of film due to lattice-mismatch and could also be the reason explaining the observed magnetic moment different from the predicted value M P Geisler et al have also calculated the formation energies of the phase separation of Cr2CoGa.16 The most favorable reaction is 2(2Cr + Co + Ga) → Co2CrGa + Cr3Ga, with formation energy of -0.61 eV per formula unit Therefore, the Co2CrGa phase might also exist in the 450 oC-grown Cr2CoGa/GaAs(001) sample because the Cr3Ga phase was detected in the XRD spectra The crystal structure and lattice parameter of Co2CrGa is similar to that of Cr2CoGa, therefore, these two phases cannot be distinguished by XRD However, due to the small content of Cr3Ga phase as judged from Rietveld refinement in Fig 5, the content of Co2CrGa should also be few Moreover, the Co2CrGa phase has the magnetic moment of µB per formula unit according to previous experiments and calculations.19,20 Therefore, the measured 0.31 µB/f.u also indicated that it is Cr2CoGa phase, rather than Co2CrGa, that constitutes the majority of the sample Figure shows magnetization vs temperature (M-T) curves measured under field cooling (FC) and zero field cooling (ZFC) processes for the 450 oC-grown Cr2CoGa/GaAs(001) film Cr2CoGa FIG Temperature dependence of magnetization for the 450 oC-grown Cr2CoGa/GaAs(001) film measured at the applied field of 500 Oe Inset shows the fit of FC M-T curve by the empirical equation M(T ) = M(0)∗[1 − (T /T c)2]1/2 117223-7 Feng et al AIP Advances 5, 117223 (2015) FIG Temperature dependence of resistance for the 450 oC-grown Cr2CoGa films on Si(001) and GaAs(001) substrates, respectively Inset shows ln(R-R0) as a function of ln(T) for the 450 oC-grown Cr2CoGa/GaAs(001) sample, assuming the functional form R(T) = R + cT n is probably a ferrimagnet, however, the ZFC M-T curve in Fig has non-standard ferrimagnetic behavior which probably due to the coexistence of antiferromagnetic phase in the sample We found that the FC M-T curve could not fit to M(T) = M(0)∗[1 − bT3/2], which indicated that our Cr2CoGa sample does not conform to the Bloch T3/2 law.21 Instead, the M-T curve could be fitted empirically to M(T) = M(0)∗[1 − (T/Tc)2]1/2, as seen in the inset in Fig The TC for the Cr2CoGa film can be estimated to be 317 K which is quite lower than the predicted 1520 K7 probably due to the off-stoichiometry of Cr2CoGa and the existence of phase separation and disorders Moreover, the T2 dependence of magnetization has been previously observed in half-metallic Heusler alloys and can be attributed to itinerant-like ferro(ferri)magnetism.21,22 The resistance as a function of temperature (R-T) is shown in Fig R-T curves indicated metallic behavior of the samples A significant degree of lattice defects and phase separation were inferred from the large residual resistance (R20K) of the Cr2CoGa/Si(001) sample, with residual resistance ratio R300K/R20K = 1.56 For the Cr2CoGa/GaAs(001) sample, the residual resistance ratio R300K/R20K increased to 2.04 indicating a relatively small contribution to the resistance from lattice defects and other forms of atomic disorder By assuming the functional R(T) = R0 + cT n for our R-T data, a plot of ln(R-R0) as a function of ln(T) was shown in the inset of Fig It demonstrated almost purely phononic linear dependence above 26K typifying the Bloch-Gruneisen formula and thus signified a dominance of phonon scattering in the sample.21 The results of the magnetoresistance (MR) measurements at different temperatures are displayed in Fig Note that the applied magnetic field was perpendicular to the film plane The MR exhibits a negative dependence on the applied field at room temperature The negative MR was due to the decreased scattering centers caused by an increase of magnetic domain with increasing magnetic field, which is also evidence for ferrimagnetic ordering in the Cr2CoGa film.23 Usually the negative MR originating from spin dependent scattering should enhance with lowering temperature, while in contrary, upon decreasing temperature below 150 K, MR turns to be positive dependence It was probably due to the complicated state of the sample consisting of multi-phases at low temperature The impurity scatterings became significant at low temperatures and thus reduce the negative MR effect The positive MR is usually due to the dominant ordinary magnetoresistance (OMR) effect.24 The OMR is the increase in resistance with increased magnetic field due to bending of the electron trajectories by the Lorentz force, which is anisotropic and only exist when the magnetic field is not parallel to the current In view of the complicated state of the sample, the positive MR might also be induced by the spin fluctuations or flip Similar positive MR behavior was also reported in the spin gapless semiconductor of Mn2CoAl sample.25 117223-8 Feng et al AIP Advances 5, 117223 (2015) FIG Magnetoresistance vs applied field curves at different temperatures for 450 oC-grown Cr2CoGa/GaAs(001) film Note that the field is applied perpendicular to film plane IV CONCLUSIONS In summary, we have prepared Cr2CoGa film on GaAs(001) using MBE technique Significant phase separation was suppressed by proper control of growth conditions It was demonstrated that Cr2CoGa Heusler phase, rather than Co2CrGa phase, constitutes the majority of the sample when grown on GaAs(001) at 450 oC The measured spin moment of Cr2CoGa is small, in agreement with predicted HM-FCF nature; however, its TC is not as high as expected from the theoretical prediction probably due to the off-stoichiometry of Cr2CoGa and the existence of the disorders and phase separation ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundations of China (Grant No 11304290), the Ph.D Programs Foundation of Ministry of Education of China (Grant No 20130022120007), and the Fundamental 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Cr2 CoGa(002), Cr2 CoGa(222), Cr2 CoGa(004), and Cr3 Ga( 210) In view of the epitaxy relation between Cr2 CoGa layer and GaAs(001) substrate as seen from the RHEED pattern, the appearance of Cr2 CoGa(222)... which could not be assigned to the Heusler phase of Cr2 CoGa Instead, phase separation including Cr3 Ga, Cr3 Ga4 , CoGa3, and CrCo could be assigned, in agreement with observations by M P Geisler and. .. for the Cr2 CoGa films grown on (a) Si(001) at 450 oC, (b) GaAs(001) at 70 oC, and (c) GaAs(001) at 450 oC, respectively B Growth of Cr2 CoGa on GaAs(001) substrate We then grew Cr2 CoGa on GaAs(001)