Net magnetic moment (NM) per unit cell, spin magnetic moment (SM) of Mn atom and energy difference ( D E) between the FM and SG states with Curie temperature (T C ) of (Sr 1-y La y )(Ti [r]
Trang 1Original Article
The role of the Mn, Fe and La dopants on magnetic properties and
electronic structures of strontium titanate quinary compounds
M.J Sadiquea, M Shahjahana,b,*
a Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh
b 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
Article history:
Received 23 May 2018
Received in revised form
4 August 2018
Accepted 5 August 2018
Available online 28 August 2018
Keywords:
Quinary compounds
KKR-CPA
Curie temperature
Density of states
Strontium titanate
Band shifting
a b s t r a c t
Magnetic properties and electronic structures of the quinary compounds (Sr1yLay)(Ti0.90D0.10)O3have been calculated using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method, where y represents La concentration (y¼ 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(y¼ 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(y¼ 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 TCand 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 (y¼ 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/)
1 Introduction
Spintronics is an attractingfield 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
semi-conductors 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
so-phisticated and the magnetic properties mainly depend on the
impurity concentrations of the properly grown multicomponent
compounds
Currently, oxide perovskites have drawn much attention pref-erably for their fascinating properties of multifunctional applica-tions 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 ABO3type 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 magneto-optical properties and found a high Faraday rotation with low optical loss for 40% Fe doping at the Ti site of STOfilms[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 0.2mB=Fe[5] The variant pattern of magnetic moments per Fe atom and TCagree well with their measured values Zhang's team has reported the magnetic and electrical properties of (Mn, La) Co-doped STO thinfilms 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
* 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.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
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
Journal of Science: Advanced Materials and Devices 3 (2018) 342e347
Trang 2codoped 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
substi-tution[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]
Further-more, 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)Sr0.30MnO3using Monte Carlo simulation
and found a very sharp FM-paramagnetic transition at 357 K[12] In
addition, Berri group has performed thefirst-principle calculations
of the structural, electronic and magnetic properties of CeMnO3and
found half-metallic FM ground state in the GGAþ 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
repre-sents 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
meanfield 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 y¼ 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 TCwith
the La concentrations The net magnetic moment (NM) and TC
decrease with the increase of La concentrations in SLTFO To
un-derstand 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
concen-trations 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 section2, the
computational details are described briefly The magnetic phase
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 section3 Graphical presentations of the calculated results are also shown and discussed in this section The results are briefly summarized in section4
2 Computational details Electronic structures and magnetic properties of the quinary compounds (Sr1-yLay)(Ti0.90D0.10)O3(D¼ Mn, Fe) have been calcu-lated 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 approxi-mated 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 approxi-mation is used to describe the shape of the potential, which as-sumes 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 experi-mental indirect band gap of STO is 3.25 eV and the direct one is 3.75 eV[24] We use the lattice constant a¼ 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 inFig 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 concentra-tions (y¼ 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 l¼ 2 for the electronic wave functions The Brillouin zone integration was carried out with 286 k sampling
M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices 3 (2018) 342e347 343
Trang 3points of the first Brillouin zone The calculations were
imple-mented by KKR-CPA program package“Machikaneyama”
devel-oped by Akai[26]
3 Results and discussion
In the perspective of electron-spin polarization, the effect of
dopants on magnetic properties and electronic structures of
(Sr1-yLay)(Ti0.90D0.10)O3(D¼ Mn, Fe) was investigated for a range of
La concentrations y¼ 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)ðTi0:90D[0:10Þ
O3indicates the FM configuration, where up arrow denotes the spin
in a particular orientation with nonzero net magnetization On the
other hand, the compound (Sr1-yLay)ðTi0:90D[0:05DY0:05ÞO3seems 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
simu-lating 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
DE¼ TESGeTEFM Subsequently, TC is estimated using DE in the
framework of MFA The expression,kBTC¼ 2
3zDE was used to esti-mate TC in the present calculation, wherekB is the Boltzmann
constant and z¼ (y þ 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 inTable 1andTable 2for SLTMO and SLTFO,
respectively The positiveDE indicates the stable FM state, whereas
negativeDE denotes the relatively lower energy case of the SG state
The SLTMO (y¼ 0) exhibits lower energy situation in the SG state,
whereas the SLTFO (y¼ 0) shows a stable FM state with TChigher
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 0.384ðmB=cellÞ to 0.056
ðmB=cellÞ for the specific concentrations y ¼ 0e0.15, as shown in
Table 2 At y¼ 0, the largest numerical value of NM, SM, and TCwas
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 La0.7Sr0.3Mn
1-xTixO3 (x ¼ 0.1, 0.2, 0.25) and analyzed the effect of nearest-neighbor interactions in the FM ordering [28] The calculated magnetic moments concur well with their reported values The La dependence of TCand NM per unit cell in the quinary compounds SLTMO and SLTFO are shown inFig 2(aed), respec-tively The tiny solid squares indicate the data points and the con-necting line indicates the overall trend of the TCand moments In SLTMO compounds, TCand NM are rising at low concentrations, peaked at around 7% and onwards decreasing with the La con-centrations, as shown inFig 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
TCslowly decrease due to the weakening of the double-exchange mechanism Beyond 7% La concentration, super exchange mecha-nism dominates over double exchange mechamecha-nism 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 struc-tures 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 inFig 3(aed) The variation of DOS is found for 10% Mn and different La concentra-tions of y¼ 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 (y¼ 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 La3þions 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 concen-trations y¼ 0.01, 0.05, 0.10, and 0.15 are shown inFig 4(aed), respectively The spin polarized DOS are found at the Fermi level for the compounds The plotted DOS explore a band shifting around
Table 1
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
(T C ) of (Sr 1-y La y )(Ti 0.90 Mn 0.10 )O 3 compounds with a range of La concentrations
y ¼ 0e0.15.
La conc NM ðmB =cellÞ SM ðmB =MnÞ DE (mRy) Tcalc:C (K)
y ¼ 0 0.302 2.786 0.029 e
y ¼ 0.01 0.310 2.834 0.036 34
y ¼ 0.03 0.325 2.915 0.156 126
y ¼ 0.05 0.338 2.982 0.261 183
y ¼ 0.07 0.345 3.029 0.349 216
y ¼ 0.10 0.332 3.031 0.409 215
y ¼ 0.13 0.325 3.033 0.435 199
y ¼ 0.15 0.323 3.036 0.441 186
Table 2 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 (T C ) of (Sr 1-y La y )(Ti 0.90 Fe 0.10 )O 3 compounds with a range of La concentrations
y ¼ 0e0.15.
La conc NM ðmB =cellÞ SM ðmB =FeÞ DE (mRy) Tcalc:C (K)
y ¼ 0 0.384 3.234 0.363 382
y ¼ 0.01 0.394 3.307 0.337 323
y ¼ 0.03 0.166 1.642 0.286 232
y ¼ 0.05 0.147 1.479 0.285 200
y ¼ 0.07 0.129 1.327 0.279 173
y ¼ 0.10 0.102 1.112 0.256 135
y ¼ 0.13 0.075 0.907 0.213 98
y ¼ 0.15 0.056 0.769 0.173 73 M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices 3 (2018) 342e347
344
Trang 4the 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
sta-bility[29] The DOS of the Fe 3d states are shifted oppositely in both
spin directions for the band coupling with the La 5d states
How-ever, 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 signi fi-cantly by n-type carrier doping at Sr site
Fig 2 Curie temperature T C 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.
Fig 3 Total density of states per unit cell (red solid line) of (Sr 1-y La y )(Ti 0.90 Mn 0.10 )O 3 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
M.J Sadique, M Shahjahan / Journal of Science: Advanced Materials and Devices 3 (2018) 342e347 345
Trang 54 Conclusion
The electronic structures and magnetic properties of the
pro-posed quinary compounds SLTMO and SLTFO type DMS are
re-ported 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 (y¼ 0) exhibits an
SG state, whereas the FM stability is found in SLTMO compounds
with La concentrations y¼ 0.01e0.15 The low TCof 216 K and a
high NM of 0.345 mB=cell were found for 7% y concentration in
SLTMO On the other hand, high TCof 382 K and NM of 0.384mB=cell
have been found in SLTFO (y¼ 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
manip-ulated 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
infor-mation for spintronic applications, where spins are the key
con-trolling 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
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