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The role of the Mn, Fe and La dopants on magn etic prope rties and electron ic structures of strontium titanate quinar y compo unds

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In this article, magnetic properties and electronic structures of the quinary compounds (Sr1-yLay)(Ti0.90D0.10)O3 are investigated using the Korringa-Kohn-Rostoker (KKR)-Green''s function method, where D indicates the TM atoms (Mn, Fe) and y represents the fractional concentration of La atom. We focus on the substitution of the two cation sites (Sr and Ti) of STO with metallic ions.

Journal of Science: Advanced Materials and Devices (2018) 342e347 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article The role of the Mn, Fe and La dopants on magnetic properties and electronic structures of strontium titanate quinary compounds M.J Sadique a, M Shahjahan a, b, * a b Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea a r t i c l e i n f o a b s t r a c t Article history: Received 23 May 2018 Received in revised form August 2018 Accepted August 2018 Available online 28 August 2018 Magnetic properties and electronic structures of the quinary compounds (Sr1ÀyLay)(Ti0.90D0.10)O3 have been calculated using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method, where y represents La concentration (y ¼ 0e0.15) and D indicates the magnetic cations (Mn, Fe) The stability of the magnetic states changes in the opposite manner for the (La, Mn) and (La, Fe) pairs doped into the strontium titanate The Sr(Ti0.90Fe0.10)O3 (y ¼ 0) shows stable ferromagnetic (FM) state with a high Curie temperature (TC ) and a large magnetic moment (MM), whereas the spin glass (SG) state is found for Sr(Ti0.90Mn0.10)O3 (y ¼ 0) A variation of the magnetic properties of (Sr1-yLay)(Ti0.90Fe0.10)O3 (SLTFO) and (Sr1-yLay)(Ti0.90Mn0.10)O3 (SLTMO) is found for different La concentrations The TC and MM for SLTFO decrease with increasing La concentration Interestingly, SLTMO compounds exhibit a phase transition from SG to FM states upon introducing La at the Sr site The density of states (DOS) for La concentrations (y ¼ 0.01, 0.05, 0.10, and 0.15) in SLTFO and SLTMO are calculated to understand the mechanism of the FM stability The bands shaping and shifting are found in the DOS of SLTFO and SLTMO around the Fermi level The band manipulations are explained in terms of the orbital hybridization and exchange mechanisms © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Quinary compounds KKR-CPA Curie temperature Density of states Strontium titanate Band shifting Introduction Spintronics is an attracting field of current research, where the spin degrees of freedom are exploited along with the electronic charge for microelectronic applications [1] Spintronic devices carry information encoded in their spin states They utilize semiconductors with the ferromagnetic (FM) property, which are familiar as dilute magnetic semiconductors (DMS) [2] A promising DMS can be formed by doping suitable transition metals (TM) into a hosting semiconductor material The induced magnetic properties and the interaction between the electrons in the valence and the conduction bands affect the positions of the energy band and the band-gap in DMS [3] The growth technology of DMS is quite sophisticated and the magnetic properties mainly depend on the impurity concentrations of the properly grown multicomponent compounds * Corresponding author Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh E-mail address: mjahan@du.ac.bd (M Shahjahan) Peer review under responsibility of Vietnam National University, Hanoi Currently, oxide perovskites have drawn much attention preferably for their fascinating properties of multifunctional applications Among the oxides, strontium titanate SrTiO3 (STO) is an excellent substrate for epitaxial growth of thin films At room temperature (RT), STO is a non-magnetic semiconductor with a model ABO3 type perovskite structure, here A and B indicate the Sr and Ti cation sites, respectively The STO semiconductor is changed into a magnetic material by introducing TM atoms at either of the cation sites Numerical design of the STO-based FM semiconductors is challenging with the suitable control doping of magnetic ions Recently, Kim et al have studied the magnetic and magnetooptical properties and found a high Faraday rotation with low optical loss for 40% Fe doping at the Ti site of STO films [4] Egilmez group has measured the magnetic properties of LaySr1-yTi0.9 Fe0.1O3-d films for compositions y ¼ 0, 0.2, 0.5, and 0.7, which exhibit RT ferromagnetism with magnetic moments ranging from 0.7mB =Fe to 0.2 mB =Fe [5] The variant pattern of magnetic moments per Fe atom and TC agree well with their measured values Zhang's team has reported the magnetic and electrical properties of (Mn, La) Co-doped STO thin films and observed FM behavior at RT for magnetic coupling between the induced electrons and the Mn 3d spins [6] Modak and Ghosh have explained the role of La and Ru https://doi.org/10.1016/j.jsamd.2018.08.001 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices (2018) 342e347 codoped at STO for enhanced hydrogen evolution using the band structure calculations [7] Again, Jiang-Ni group has investigated the electronic structures and optical properties of La-doped STO, where they found broadened optical band gaps for the La substitution [8] Besides, Guo group has argued that the band structure of Nb doping at the Ti site of STO exhibits metallic behavior for 12.5% Nb concentration [9] An extensive spectroscopic analysis has been carried out by Higuchi et al on the electronic structures of STO with 2% Sc or Nb doping at the Ti site, where the band gap was found to increase by increasing the dopant concentrations [10] Furthermore, Matsushima group has analyzed the electronic structures of pure and La-doped STO using X-ray photoemission spectroscopy and reported the improvement of electrical conductivity by La doping [11] Yahyaoui and Diep have investigated the magnetic properties of (La0.56Ce0.14)Sr0.30MnO3 using Monte Carlo simulation and found a very sharp FM-paramagnetic transition at 357 K [12] In addition, Berri group has performed the first-principle calculations of the structural, electronic and magnetic properties of CeMnO3 and found half-metallic FM ground state in the GGA ỵ U treatment [13] In this article, magnetic properties and electronic structures of the quinary compounds (Sr1-yLay)(Ti0.90D0.10)O3 are investigated using the Korringa-Kohn-Rostoker (KKR)-Green's function method, where D indicates the TM atoms (Mn, Fe) and y represents the fractional concentration of La atom We focus on the substitution of the two cation sites (Sr and Ti) of STO with metallic ions The Curie temperature (TC ) is estimated using the mean field approximation (MFA) In our previous paper [14], Mn doped STO was found instable in the FM state We have been motivated to search for its stability in the FM phase by tuning concentrations or by some suitable double doping approaches Therefore, substitution of La at the Sr site and Mn at the Ti site as (Sr1-yLay)(Ti0.90Mn0.10)O3 (SLTMO) shows a phase transition from the spin glass (SG) to the FM states, where La concentrations are taken in the range y ¼ 0.01e0.15 By contrast, 10% Fe substitution at the Ti site of STO exhibits a stable FM state, whereas inclusion of La at the Sr site and Fe at the Ti site as (Sr1-yLay)(Ti0.90Fe0.10)O3 (SLTFO) shows a variation of the magnetic moments and TC with the La concentrations The net magnetic moment (NM) and TC decrease with the increase of La concentrations in SLTFO To understand the effect of La and the magnetic ions in the STO-based DMS, the density of states (DOS) are calculated for different La concentrations Total and component DOS exhibit the induced magnetic properties of SLTFO and SLTMO by varying La concentrations The shaping and shifting of the energy bands occur due to the orbital hybridization in SLTFO and SLTMO The magnetic properties of the quinary compounds are calculated and the role of dopants on magnetic states is discussed The article is organized in the following way: in section 2, the computational details are described briefly The magnetic phase 343 stability, NM, spin magnetic moment (SM), TC , total and component DOS of perovskite STO type DMS are presented and the underlying mechanisms are explained in section Graphical presentations of the calculated results are also shown and discussed in this section The results are briefly summarized in section Computational details Electronic structures and magnetic properties of the quinary compounds (Sr1-yLay)(Ti0.90D0.10)O3 (D ¼ Mn, Fe) have been calculated using the KKR-coherent potential approximation (CPA) method [15e19] In the CPA scheme, one can simulate arbitrary the concentrations of impurities at any constituent sites of the host semiconductor [20] The CPA is the procedure of averaging over random impurity configurations in terms of the coherent t-matrix of the muffin-tin potential, which describes the average atom The generalized gradient approximation (GGA) is used to estimate the exchange correlation (XC) energy functional of the inhomogenous systems [21,22] In the GGA, the XC energy functional is approximated as a function of the electron density and the gradient of the electron density of inhomogenous systems, where the electron density changes rapidly The muffin-tin (MT) potential approximation is used to describe the shape of the potential, which assumes that the potential is spherically symmetric inside the atomic spheres and constant in the interstitial areas of many body systems The MT potentials are restricted by the condition that they may not overlap each other, but there is no restriction on the depth of the potential The scalar relativistic approximation was used to account the relativistic effects of the calculation At RT, pure STO crystallizes into the cubic perovskite structure of the space group Pm-3m (group number 221) [23] The experimental indirect band gap of STO is 3.25 eV and the direct one is 3.75 eV [24] We use the lattice constant a ¼ 0.3905 nm for the numerical calculations [25] The unit cell of the perovskite STO with the schematic substitution of La and D atoms are shown in Fig 1(a) Calculated total and partial DOS of Sr d, Ti d, and O p states of the basic STO semiconductor are shown in Fig 1(b) The band gap, as an evidence of a semiconductor, was found around the Fermi level, which is set at zero Ry energy Although, the basic material is a non magnetic semiconductor, we have performed spin polarized calculations and plotted DOS exhibiting up spin and down spin states as identical (see Fig 1(b)) The local lattice distortion was ignored in the dilute limit of impurity concentrations (y ¼ 0.01e0.15) The experimental lattice constant of the pure STO was used to calculate the electronic structures and magnetic properties of the quinary compounds In the calculation, the assigned angular momenta to identify the spherical harmonics were considered up to l ¼ for the electronic wave functions The Brillouin zone integration was carried out with 286 k sampling Fig (a) Unit cell of a perovskite SrTiO3 structure with dopant atoms, (b) total and component DOS of SrTiO3 Vertical dashed line indicates the Fermi energy level 344 M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices (2018) 342e347 points of the first Brillouin zone The calculations were implemented by KKR-CPA program package “Machikaneyama” developed by Akai [26] Results and discussion La conc In the perspective of electron-spin polarization, the effect of dopants on magnetic properties and electronic structures of (Sr1-yLay)(Ti0.90D0.10)O3 (D ¼ Mn, Fe) was investigated for a range of La concentrations y ¼ 0e0.15 We calculated the fascinating properties of the quinary DMS such as, NM per unit cell, SM per magnetic ions, DOS and the Fermi energies of the compounds The compound (Sr1-yLay)ðTi0:90 D[ 0:10 Þ O3 indicates the FM configuration, where up arrow denotes the spin in a particular orientation with nonzero net magnetization On the Y other hand, the compound (Sr1-yLay) ðTi0:90 D[ 0:05 D0:05 ÞO3 seems a good choice to consider as SG configuration, where the bidirectional arrows denote the random orientation of spins with zero net magnetization There is a controversy of using the term SG state in the literature In this article, the SG phase is simply meant a simulating paramagnetic phase, where the net moment is zero by cancelling of local moments in the spin disordered system In order to design the DMS materials, total energies (TE) per unit cell for FM and SG states were calculated and the stability of magnetic state was fixed out from the lower energy state Then, the energy deference (DE) between FM and SG states was calculated as DE ¼ TESGeTEFM Subsequently, TC is estimated using DE in the DE was used to estiframework of MFA The expression, kB TC ¼ 3z mate TC in the present calculation, where kB is the Boltzmann constant and z ẳ (y ỵ 0.10) is the total concentration of dopants [27] The NM per unit cell, SM per atom, DE in mRy, and estimated TC in Kelvin are tabulated in Table and Table for SLTMO and SLTFO, respectively The positive DE indicates the stable FM state, whereas negative DE denotes the relatively lower energy case of the SG state The SLTMO (y ¼ 0) exhibits lower energy situation in the SG state, whereas the SLTFO (y ¼ 0) shows a stable FM state with TC higher than RT Interestingly, when the Sr site of SLTMO was doped with La, a change in phase was found from SG to FM states The SM per Mn atom is ascending with the increasing of La concentrations, whereas NM per unit cell gradually rises to a top value and then slowly decreases in SLTMO compounds On the other hand, in SLTFO compounds, NM per cell spans from 0.384 ðmB =cellÞ to 0.056 ðmB =cellÞ for the specific concentrations y ¼ 0e0.15, as shown in Table At y ¼ 0, the largest numerical value of NM, SM, and TC was found The calculated magnetic properties over the two cases oppositely rise or lower (fall) with La concentrations The (La, Mn) based systems (see Table 1) might have a disadvantage for the conventional electronics, because of their low operational TC Very recently, Yahyaoui et al have reported the magnetic properties due to the Ti substitution with Mn in the compounds La0.7Sr0.3Mn1Table Net magnetic moment (NM) per unit cell, spin magnetic moment (SM) of Mn atom and energy difference (DE) between the FM and SG states with Curie temperature (TC ) of (Sr1-yLay)(Ti0.90Mn0.10)O3 compounds with a range of La concentrations y ¼ 0e0.15 La conc y y y y y y y y ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ 0.01 0.03 0.05 0.07 0.10 0.13 0.15 Table Net magnetic moment (NM) per unit cell, spin magnetic moment (SM) of Fe atom and energy difference (DE) between the FM and SG states with Curie temperature (TC ) of (Sr1-yLay)(Ti0.90Fe0.10)O3 compounds with a range of La concentrations y ẳ 0e0.15 NM mB =cellị SM mB =Mnị DE (mRy) T calc: (K) C 0.302 0.310 0.325 0.338 0.345 0.332 0.325 0.323 2.786 2.834 2.915 2.982 3.029 3.031 3.033 3.036 À0.029 0.036 0.156 0.261 0.349 0.409 0.435 0.441 e 34 126 183 216 215 199 186 y y y y y y y y ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ 0.01 0.03 0.05 0.07 0.10 0.13 0.15 NM ðmB =cellÞ SM ðmB =FeÞ DE (mRy) T calc: (K) C 0.384 0.394 0.166 0.147 0.129 0.102 0.075 0.056 3.234 3.307 1.642 1.479 1.327 1.112 0.907 0.769 0.363 0.337 0.286 0.285 0.279 0.256 0.213 0.173 382 323 232 200 173 135 98 73 xTixO3 (x ¼ 0.1, 0.2, 0.25) and analyzed the effect of nearestneighbor interactions in the FM ordering [28] The calculated magnetic moments concur well with their reported values The La dependence of TC and NM per unit cell in the quinary compounds SLTMO and SLTFO are shown in Fig 2(aed), respectively The tiny solid squares indicate the data points and the connecting line indicates the overall trend of the TC and moments In SLTMO compounds, TC and NM are rising at low concentrations, peaked at around 7% and onwards decreasing with the La concentrations, as shown in Fig 2(a, c), respectively On the contrary, Fig 2(b, d) indicates that in the SLTFO compounds, TC and NM gradually decrease and it seems that the FM behavior will be ceased at some higher La concentrations In the Mn doped cases, NM and TC slowly decrease due to the weakening of the double-exchange mechanism Beyond 7% La concentration, super exchange mechanism dominates over double exchange mechanism Additional electrons are introduced to the system by substituting Sr with La Therefore, at concentrations of over 7% La, NM and TC slowly decrease through the disappearing of ferromagnetism in (La, Mn) doped STO due to the dominating super-exchange mechanism [29] In order to elucidate the origin of magnetism, electronic structures of the quinary compounds SLTMO and SLTFO were calculated The physical origin is the mixed valence magnetism between the orbitals as well as the p-d orbital hybridization, which dominate the main electronic properties of the system As a result, the system becomes stable in the FM state and hence produces magnetic moments The La doping enhances the spin band coupling and shifting towards the lower energy state and therefore stabilizes the FM state The total DOS per unit cell and local DOS of Mn and La d states in the SLTMO compounds are shown in Fig 3(aed) The variation of DOS is found for 10% Mn and different La concentrations of y ¼ 0.01, 0.05, 0.10, and 0.15 The DOS data show impurity bands around the Fermi level in both spin directions The partial spin polarization at the Fermi level indicates the FM situation and produces NM The spin band changes significantly around the Fermi level for the varying concentrations of La The SG state previously shown of SLTMO (y ¼ 0) changes into the stable FM state even for a low doping concentration of La at the Sr site [14] Noticeable band shifting is seen in the DOS of the majority and minority spin states The La3ỵ ions fully act as electron donors in the compound system [8] Clearly, the spin polarization at the Fermi level is increasing with the rising of the electron density due to the La substitution The domination of La 5d states induces delocalized electrons near the Fermi level Therefore, the delocalized electrons can be exited to the conduction band (CB) as free carriers for electrical conduction by the thermal ionization or carrier diffusion [8] The total and component DOS of SLTFO for various La concentrations y ¼ 0.01, 0.05, 0.10, and 0.15 are shown in Fig 4(aed), respectively The spin polarized DOS are found at the Fermi level for the compounds The plotted DOS explore a band shifting around M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices (2018) 342e347 345 Fig Curie temperature TC in Kelvin (K) of (a) Mn, (b) Fe doped STO and net magnetic moment (NM) in Bohr magneton of (c) Mn, and (d) Fe doped STO as a function of the La concentrations the Fermi level as well as the La 5d states modify the electronic structures of the SLTFO compounds The Fe 3d states (sharp peak) are responsible for the ferromagnetism in the SLTFO compounds, whereas the La impurity attempts to slowly diminish the FM stability [29] The DOS of the Fe 3d states are shifted oppositely in both spin directions for the band coupling with the La 5d states However, the introduction of n-type carriers (La atom) into the SLTFO shifts the Fermi level towards the conduction band CB, which is a good agreement with previously reported results [8,30] Therefore, the rigid band shifting at the Fermi level is ensured by the electron doping At the same time, the SLTFO compounds show half metallic behavior for 5%, 10%, and 15% La impurity concentrations The half metallicity in the SLTFO type of DMS can be manifested significantly by n-type carrier doping at Sr site Fig Total density of states per unit cell (red solid line) of (Sr1-yLay)(Ti0.90Mn0.10)O3 compounds for different La concentrations at (a) y ¼ 0.01, (b) y ¼ 0.05, (c) y ¼ 0.10, and (d) y ¼ 0.15 Blue dashed line and black dotted line indicate the partial DOS of the 3d states of Mn and the 5d states of La atoms, respectively Vertical dashed line indicates the Fermi energy level 346 M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices (2018) 342e347 Fig Total density of states per unit cell (red solid line) of (Sr1-yLay)(Ti0.90Fe0.10)O3 compounds for different La concentrations at (a) y ¼ 0.01, (b) y ¼ 0.05, (c) y ¼ 0.10, and (d) y ¼ 0.15 Blue dashed line and black dotted line indicate the partial DOS of the 3d states of Fe and the 5d states of La atoms, respectively Vertical dashed line indicates the Fermi energy level Conclusion The electronic structures and magnetic properties of the proposed quinary compounds SLTMO and SLTFO type DMS are reported by ab-initio calculations Estimated TC was obtained by tuning the controlled doping at the Sr and Ti sites The significance of tunable La doping has been found to play a reverse role of La in the SLTMO and SLTFO compounds The SLTMO (y ¼ 0) exhibits an SG state, whereas the FM stability is found in SLTMO compounds with La concentrations y ¼ 0.01e0.15 The low TC of 216 K and a high NM of 0.345 mB =cell were found for 7% y concentration in SLTMO On the other hand, high TC of 382 K and NM of 0.384 mB =cell have been found in SLTFO (y ¼ 0) with slowly decreasing trend of TC and NM at higher y concentrations The magnetic properties are induced by incorporating Mn and Fe at the Ti site and are manipulated by the La doping The plotted DOS data reveal the band shifting in both the compounds around the Fermi level The carrier induced FM behavior is hindered due to excess amount of La doping Moreover, FM half metallicity has been found for 5%, 10%, and 15% La concentrations at the Sr site of SLTFO The fully spin polarized results offer the possibility to magnetically store information for spintronic applications, where spins are the key controlling factor Acknowledgments The authors are thankful to the authority of the center for advanced research in sciences (CARS), University of Dhaka, Dhaka 1000 for providing advanced computing facilities to perform the numerical computation References r, [1] S.A Wolf, D.D Awschalom, R.A Buhrman, J.M Daughton, S von Molna M.L Roukes, A.Y Chtchelkanova, D.M Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488e1495 [2] H Ohno, Making nonmagnetic semiconductors ferromagnetic, Science 281 (1998) 951e956 [3] H.-F Lin, W.-M Lau, J Zhao, Magnetism in the p-type monolayer IIeVI semiconductors SrS and SrSe, Sci Rep (2017) 45869e45878 [4] H.-S Kim, L Bi, G.F Dionne, C.A Ross, Magnetic and magneto-optical properties of Fe-doped SrTiO3 films, Appl Phys Lett 93 (2008) 0925061e0925063 [5] M Egilmez, G.W Leung, A.M.H.R Hakimi, M.G Blamire, Origin of 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structures and magnetic properties of the quinary compounds In the calculation, the assigned angular momenta to identify the spherical harmonics were considered... by varying La concentrations The shaping and shifting of the energy bands occur due to the orbital hybridization in SLTFO and SLTMO The magnetic properties of the quinary compounds are calculated... variation of the magnetic moments and TC with the La concentrations The net magnetic moment (NM) and TC decrease with the increase of La concentrations in SLTFO To understand the effect of La and the

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    The role of the Mn, Fe and La dopants on magnetic properties and electronic structures of strontium titanate quinary compounds

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