Journal of Magnetism and Magnetic Materials 331 (2013) 1–6
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Variation of half metallicity and magnetism of Cd1 xCrxZ (Z ¼S, Se and Te)
DMS compounds on reducing dilute limit
Hardev S. Saini a, Mukhtiyar Singh a, Ali H. Reshak b,c, Manish K. Kashyap a,n
a
Department of Physics, Kurukshetra University, Kurukshetra 136119, Haryana, India
School of Complex systems, FFPW, CENAKVA, University of South Bohemia in CB, Nove Hrady 37333, Czech Republic
c
School of Material Engineering, Malaysia University of Perlis, P.O. Box 77, d/a Pejabat Pos Besar, 01007 Kangar, Perlis, Malaysia
b
a r t i c l e i n f o
abstract
Article history:
Received 22 August 2012
Received in revised form
16 October 2012
Available online 16 November 2012
The electronic and magnetic properties of Cr-doped Cd-Chalcogenides, Cd1 xCrxZ (Z ¼S, Se and Te) for
dopant concentration, x ¼0.25 and 0.125 are presented in order to search new Dilute Magnetic
Semiconductor (DMS) compounds suitable for spintronic applications. The calculations have been
performed using full potential Linear Augmented Plane Wave (FPLAPW) method within generalized
gradient approximation (GGA) as exchange–correlation (XC) potential. The calculated results show that
the doping of Cr atom induces ferromagnetism in these compounds. Moreover, all DMS compounds
retain half metallicity at both dopant concentrations with 100% spin polarization at Fermi level (EF). The
total magnetic moments of these compounds are mainly due to Cr-d states present at EF where as there
exist small induced magnetic moments on other non-magnetic atoms as well.
& 2012 Elsevier B.V. All rights reserved.
Keywords:
Band structure
DFT
FPLAPW method
Spintronics
DMS
1. Introduction
Dilute magnetic semiconductors (DMS) are the conventional
semiconductors in which appropriate fraction of atoms is substituted
by the elements which are capable to add localized magnetic
moments. Due to this substitution, these materials not only retain
the semiconducting properties but also acquire the magnetism.
A strong spin dependent coupling exists between the electronic states
of parent semiconductor and dopant atoms which results metallic
behavior for one spin channel and semiconducting for the second in
the resultant DMS compound. In other words, DMS compounds show
100% spin polarization at Fermi level (EF) with suitable magnetism so
that these are characterized as half metallic (HM) ferromagnets [1,2].
Moreover, these compounds are expected to have large Curie
temperature (Tc). All of these properties make DMS compounds as
the functional materials for spintronic applications.
To find half metallicity or spin polarization experimentally, is a
great challenge for different types of compounds. The spin-resolved
photoemission measurements of a ferromagnetic Manganese
perovskite, La0.7Sr0.3MnO3 by Park et al. [3] directly manifested the
half-metallic nature well below Tc with a band gap in minority spin.
First experimental observation of half-metallic gap in Co2MnSi
(CMS) Heusler alloy by tunneling conductance measurements for
n
Corresponding author. Tel.: þ91 1744 238410x2482, 2130;
fax: þ 91 1744 238277.
E-mail addresses: [email protected], [email protected] (M.K. Kashyap).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmmm.2012.10.044
CMS/Al–O/CoFe and CMS/Al–O/CMS- Magnetic Tunnel Junctions
(MTJs) was made by Sakuraba et al. [4]. Most MTJs that have been
studied commonly are based on 3d-ferromagnetic metal electrodes.
Semiconductor-based MTJs (SMTJs) composed of two DMS electrodes and a semiconductor barrier film assembled as a DMS/semiconductor/DMS junction, are expected to become key components
in multifunctional spintronic devices that will combine memory and
logic functions [5]. However, the spin-dependent transport properties of SMTJs are not yet well understood. The applications of SMTJs
have been difficult as the tunnel resistance shows a sizeable
variation due to the uncertainty in the magnetism of the DMS
electrodes [6,7].
In 1970s, the extensive studies on DMS compounds were
started particularly when appropriately purified Mn was
employed to grow bulk Mn-based II–VI alloys [8]. Later on, the
rapid progress on the synthesis of DMS compounds such as
transition metal (TM) doped II–VI, IV–VI and III–V semiconductors was started to a large extent by the Molecular Beam Epitaxy
(MBE) and Laser Ablation method [2,9] in 1990s. Recently,
Cr-doped II–VI type DMS compounds have been studied extensively in order to find out the improved ferromagnetic properties
and to explore the mechanism behind the half metallic ferromagnetism (HMF) [10–14]. This kind of study is very useful to design
new spintronics devices like spin valves, spin light emitting
diodes, magnetic sensors, logic devices and ultra-fast optical
switches.
Cr-doped Cd-chalcogenides are the important II–VI semiconductor
based DMS compounds on which considerable efforts have been
2
H.S. Saini et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 1–6
employed to find out the HMF, spin polarization and high Curie
temperature (Tc). Twardowski et al. [15] and Herbich et al. [16]
cultivated the Cd1 xCrxS crystal with very low concentration of x by
the modified high pressure Bridgman technique and predicted the
s–d and p–d exchange constants. These predictions showed that the
ferromagnetism significantly depends on the p–d interaction. Cr- and
Co-doped bulk CdSe crystals were synthesized by a high temperature
solution growth technique using Se as solvent by Adetunji et al. [17].
They reported that the resistivity of the doped crystal increases/
decreases on Cr/Co doping. Kasiyan et al. [18] synthesized high
perfection Cr2þ -doped CdSe and CdS0.67Se0.33 single crystals using
the seeded physical vapor-phase free growth technique and observed
the transport properties, including the Hall effect and electrical
conductivity. Cr-doped CdTe bulk crystals with doping concentration,
x¼0.04 were grown by Ko and Blamire [19] using vertical solidification method and found ferromagnetism in resultant DMS compounds
with Tc ¼ 395 K. Stefaniuk et al. [20] prepared Cd1 xCrxTe (x¼0.05)
solid solution in polycrystalline form by liquid state synthesis of
stoichiometric powered quantities, Cd and Cr2Te3. This solution
showed ferromagnetic properties at room temperature induced by
sp–d interaction.
On the theoretical front, the stability of ferromagnetic state in
Cr-doped II–VI type DMSs was established by Blinowski et al. [21]
using the tight binding approximation. Liu and Liu [22] performed the
first principle calculation using full potential approach to investigate
the ferromagnetism in some TM doped II–VI compounds. They
observed that out of the studied compounds, CdTe and ZnTe on
doping with 3d TMs (i.e. Cr and V) for concentrations, x¼0.25 and
0.75, show robust half metallicity. The ab-initio calculations of
Cd1 xTMxSe (TM¼Cr, V and Mn) with x¼ 0.25, 0.50 and 0.75 were
explored by Zhang et al. [23]. They evaluated the HMF in V- and Crdoped CdSe compounds due to the hybridization of TM-d states with
anion p-states. Nazir et al. [13] studied the effect of contracting the
unit cell on half metallicity of Zn1 xCrxS and Cd1 xCrxS (x¼ 0.25)
compounds and governed the existence of half metallicity up to 8%
and 6% compression, respectively. They also estimated that the p–d
hybridization between TM-d states and anions-p states was responsible in reducing the magnetic moment of impurity atoms (Cr) from
its free space charge value. Recently, Noor et al. [14] systematically
analyzed the structural, electronic and magnetic properties of Crdoped CdTe for various Cr concentrations (x¼0.25, 0.50, 0.75 and 1.0)
using FPLAPW method within GGA as XC functional. They envisaged
the large HM gap in minority spin channel and studied the robustness
of half-metallicity with respect to the variation of lattice constants.
On reviewing the literature, it has been observed that the II–VI
DMS compounds have been studied theoretically up to a dopant
concentration, x¼0.25 only. But for practical purpose, the dilute limit
of TMs in II–VI semiconductors is actually smaller [2,24,25]. Therefore, an emphasis is to be conferred on the study with concentration
xo0.25 for exhaustive comparison with experiments. Keeping in
mind the dilute limit of TMs (10–25%) in these compounds, we have
calculated the electronic and magnetic properties of Cr-doped
Cd-chalcogenides for two Cr-concentrations, x¼0.25 and 0.125.
Moreover, the other aim of present study is to check the retainness
of half metallicity at lower dopant concentration.
The paper is organized as follows: brief description of computational details is outlined in Section 2. The analysis of electronic
and magnetic properties along with band structures of studied
compounds is described in Section 3. In Section 4, the results are
summarized and conclusions are presented.
2. Theoretical approach
The concept of HM ferromagnetism was first presented by de
Groot et al. [26] in semi Heusler alloy, NiMnSb in terms of spin
polarization. These types of materials exhibiting a gap in one spin
channel are supposed to have 100% spin polarization (P) at EF. The
spin polarization of a system at EF can be defined as
P¼
Nm ðEF ÞNk ðEF Þ
N m ðEF Þ þ Nk ðEF Þ
ð1Þ
where Nm ðEF Þ and N k ðEF Þ are the majority and minority spin
densities at EF, respectively. Thus, for 100% spin polarization
(P¼ 1), a material should have either N m ðEF ÞorNk ðEF Þ equal to
zero. In this way, the material can have fully spin polarized
electrons which are able to generate spin polarized current.
In the present work, the calculations were carried out using
the FPLAPW method based on density functional theory (DFT)
[27] as implemented in WIEN2k code [28]. The XC potential has
been constructed using GGA within the parameterization of
Perdew–Burke–Ernzerhof (PBE) [29]. In FPLAPW calculations,
the core states are treated fully relativistically where as for the
valence states, a semirelativistic calculation is performed. The
plane wave cut off parameters were decided by RMTkmax ¼7
(where kmax is the largest wave vector of the basis set) and
Gmax ¼12 a.u. 1 for Fourier expansion of potential in the interstitial region. The calculations were based on the supercell
approach where one Cd atom at (000) in the supercell cell of
CdZ (Z¼S, Se and Te) is replaced by Cr atom. The supercell is cubic
(i.e. 1 1 1) with eight atoms and tetragonal (i.e. 2 1 1) with
16 atoms for x¼0.25 and 0.125, respectively. The radius of MT
spheres (RMT) for Cd, Cr, S, Se and Te atom were chosen to be 2.3,
2.0, 2.0, 2.3 and 2.4 a.u., respectively, ensuring nearly touching
spheres and minimizing the interstitial space. The energy convergence criterion was set to 10 4 Ry and the charge convergence
was also monitored along with it. The k-space integration has
been carried out using the modified tetrahedron method [30]
with 63 and 40 k-points in the irreducible Brillouin zone (IBZ) for
doping of 25% and 12.5%, respectively.
3. Result and discussion
The stability of FM configuration for Cr-doped II–VI DMS
compounds as predicted by Blinowski et al. [21] has provided
us a strong base to perform the structural optimization in this
configuration. In Fig. 1, the structural optimization performed in
the neighborhood of lattice parameters of parent CdZ (Z ¼S, Se
and Te) semiconductor [2] for Cd1 xCrxZ (Z¼S, Se and Te)
compounds with x ¼0.25 and 0.125 is depicted and equilibrium
values of optimized lattice parameters are listed in Table 1.
The optimized parameters were used for further calculations of
electronic and magnetic properties of studied DMS compounds. A
small increment in lattice constant is observed on reducing the
dopant concentration for all cases. The optimized lattice parameters for x¼ 0.25 are in accordance with other theoretical
predictions [13,14,23] where as for x¼0.125, the same are
reported first time. In Fig. 2, the total density of states (TDOS)
of studied compounds is presented. The calculated DOS of all
compounds are generic in nature and the splitting of minority
DOS in vicinity of EF is clearly visible which endows the 100% spin
polarization with a band gap for this spin channel. Thus, these
compounds are able to generate fully spin polarized current and
are responsible for maximizing the efficiency of spintronic
devices. The value of minority band gap as summarized in
Table 1 decreases on reducing the doping concentration (x) from
0.25 to 0.125. This behavior is attributed to the tendency of Cr-d
states to be more localized for lower concentration. The localized
Cr-d states hybridize weakly at x ¼0.125 with Z-p states as
compared to that at x¼0.25 and result in smaller separation of
bonding and antibonding states. The minority band gap also
H.S. Saini et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 1–6
3
Fig. 2. Comparison of calculated total DOS of Cd1 xCrxZ (Z ¼ S, Se and Te)
compounds with x ¼0.25 and 0.125.
Fig. 1. Total energy versus lattice parameters of Cd1 xCrxZ (Z ¼S, Se and Te) DMS
compounds with x¼ 0.25 and 0.125. The solid lines show a polynomial fits for
determining the optimized lattice constants. Eequi corresponds to equilibrium
energy at optimized lattice constant.
Table 1
Optimized lattice parameters, the calculated DOS for majority and minority spin
(i.e. N m (EF) and N k (EF) in states/eV) and spin polarization (‘P’) at EF. The calculated
band gaps of Cd1 xCrxZ (Z ¼ S, Se and Te) with doping concentration, x¼ 0.25 and
0.125 with EF fixed at 0 eV are also presented. The change in band gap on reducing
dilute limit within same compound (DEconc.) is also reported.
Cd1 xCrxZ
a (Å) DE (eV) DEconc. (eV) N m (EF) N k (EF) ‘P’
Cd0.75Cr0.25S
5.84
5.72
5.89
–
6.11
6.15
6.16
–
6.54
6.43
6.58
–
This work
Othersa
Cd0.875Cr0.125S This work
Others
Cd0.75Cr0.25Se
This work
Othersb
Cd0.875Cr0.125Se This work
Others
Cd0.75Cr0.25rTe This work
Othersc
Cd0.875Cr0.125Te This work
Others
a
b
c
2.11
2.03
1.59
–
1.60
0.39
1.11
–
1.58
1.56
1.10
–
–
–
0.52
–
–
–
0.49
–
–
–
0.48
–
0.59
–
0.75
–
0.61
–
0.62
–
0.52
–
0.47
–
0
–
0
–
0
–
0
–
0
–
0
–
1
–
1
–
1
–
1
–
1
–
1
–
Ref. [13].
Ref. [23].
Ref. [14].
decreases on changing chalcogen atom (S) with heavier one in
compound Cd1 xCrxS at both dopant concentrations. The maximum value of band gap among present compounds is 1.21 eV for
Cd0.75Cr0.25S and lowest (1.10 eV) for Cd0.875Cr0.125Te. The
observed trend can be explained by weaker hybridization
between Cr-d and Te-p states due to larger size of p-orbital. It is
interesting to note that the decrease in minority band gap on
reducing dopant concentration (DEconc ¼ DE0.125 DE0.25) in
Table 1 is almost constant for all the compounds. This means
that the interaction between dopant Cr-d states and isovalent Z-p
states reduces by the same amount at lower doping.
Due to the similar environment, each partial contribution
towards total DOS is almost similar in the studied compounds.
Therefore, we have explored the spin dependent partial DOS (PDOS)
of Cr-doped CdS compound only as a representative for both
concentrations (i.e. x¼ 0.25 and 0.125) as shown in Fig. 3. The
detailed investigation of this compound reveals that the DOS in
valence band (VB) in energy range, 5.2 eV to 1.2 eV is mainly
contributed by the S-p states for both spin channels. The EF is
occupied by the majority Cr-d states with a small admixture of S-p
states for both concentrations. But on reducing Cr-concentration
from 0.25 to 0.125, the splitting of majority d-states takes place and
their effective contribution at EF increases. The minority Cr-d states
remain localized in the conduction band only leaving a band gap in
this spin channel for the resultant compound. In conduction band
(CB), there is a presence of crowded minority DOS only. As the Cr
atom has five majority electrons in d-orbital, therefore the unfilled
states are available mainly for minority electrons. Moreover, the sand p- states of host Cd and S atoms also have smaller contributions
towards TDOS in CB which increases on reducing the doping
concentration.
The Cr-d states play a crucial role to decide the ferromagnetism
induced in the resultant DMS compound. When a Cr atom is
substituted in place of one Cd atom, it creates five majority spin
d-states in the semiconducting gap of CdZ (Z¼S, Se and Te).
According to the crystal field theory [31], the five fold degenerate
atomic levels of Cr-d states split into three fold degenerate (t2g) and
two fold degenerate (eg) symmetry states due to the tetrahedral
environment of Z- atoms. The separate contribution of these two
symmetry states towards TDOS are presented in Fig. 4 for Cd1 xCrxS
(x¼0.25 and 0.125). It is clear that Cr-eg states are more energetic
than corresponding t2g states in VB due to large coulomb interaction.
4
H.S. Saini et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 1–6
Fig. 3. Calculated total and partial DOS of Cd1 xCrxS (x ¼0.25 and 0.125) compounds.
Table 2
Total and atom resolved magnetic moments of Cd1 xCrxZ (Z¼ S, Se and Te) DMS
compounds, where Mint represents the magnetic moments at interstitial site.
Cd1 xCrxZ
MCd (mB) MCr (mB) MZ (mB)
Mint (mB) Mtot
(mB)
Cd0.75Cr0.25S
0.017
0.017
0.026
–
0.016
0.010
0.026
–
0.014
0.013
0.021
–
0.414
0.453
0.410
–
0.426
–
0.418
–
0.419
–
0.422
–
This work
Othersa
Cd0.875Cr0.125S This work
Others
Cd0.75Cr0.25Se
This work
Othersb
Cd0.875Cr0.125Se This work
Others
Cd0.75Cr0.25rTe This work
Othersc
Cd0.875Cr0.125Te This work
Others
a
b
c
Fig. 4. Calculated DOS of Cr-d states and d-plit components (eg and t2g) for
Cd1 xCrxS (x ¼0.25 and 0.125) compounds.
The eg and t2g states for majority spin are localized mainly in VB
such that a fraction of t2g states crosses EF. On the other hand, these
states are concentrated wholly in CB for minority spin within both
dopant concentrations. On reducing the dilute limit, the broadening
of eg and t2g states decrease in the VB for minority spin channel. A
strong p–d exchange interaction in Cr-d and Z-p is responsible for
separation of these two symmetry states. This p–d hybridization in
these compounds leads to the double exchange interaction. Consequently, this double exchange mechanism attributes the different
electrons densities for majority and minority electrons and thus is
responsible for emergence of ferromagnetism in this system.
The total magnetic moment per unit cell (Mtot) and atom
resolved spin magnetic moments calculated at optimized lattice
parameters for present compounds are listed in Table 1.
The substitution of one Cd atom with Cr atom implies that two
3.670
3.603
3.670
–
3.718
3.550
3.729
–
3.777
3.745
3.782
–
0.034
0.027
0.0.033
–
0.047
0.050
0.048
–
0.059
0.013
0.056
–
4.001
4.000
4.008
–
4.009
4.000
4.005
–
3.999
4.000
3.994
–
Ref. [13].
Ref. [23].
Ref. [14].
Cr-d electrons contributes to the anion (Z) to form a dangling
bond and the remaining d-electrons stay on the dopant (Cr)
position which become responsible for magnetic state of resultant
DMS compounds. The observed magnetic moments of these
compounds are very close to the integer value (4.0mB) confirming
their HM characteristics. Almost same value of magnetic moment
for all compounds is a characteristic of similar valence configuration of constituent atoms. The large exchange splitting of Cr-d
states leads to the localized spin magnetic moment at Cr site. It is
also observed that in all compounds, the local magnetic moment
of Cr has reduced from its elemental value (4.0 mB) due to the
strong hybridization between Cr-d and Z-p states. The induced
spin magnetic moment on Cd/Z atom is negligibly small and
aligns parallel/antiparallel to Cr atom. Thus, Cd/Z atom interacts
ferromagnetically/antiferromagnetically with Cr atom. The negative p–d interaction between Cr-d and Z-p states lowers the total
energy and stabilizes the magnetic state of these compounds
Table 2.
H.S. Saini et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 1–6
5
Fig. 5. Spin resolved band structure and total DOS of Cd1 xCrxS (x ¼0.125). The horizontal line at E ¼0 eV marks the Fermi level (EF).
The spin resolved band structure of Cd1 xCrxS (x ¼0.125) as a
reference compound is presented in Fig.5. The band gap is direct
along G–G direction along the high symmetry direction of the first
Brillouin zone in all these compounds. The S-p states are extended
over entire VB. In both spin channels of Cd0.875Cr0.125S, a low lying
band at 4.7 eV to 3.7 eV arises due to Z-p states with a
small admixture of Cd-s states. A series of bands, ranging from
3.7 eV to 1.2 eV is mainly due to the S-p states. The next
two bands near and at EF are contributed significantly by Cr-d
states with a small contribution of S-p states which governs the
metallic nature of this compound for majority spin. In minority
spin channel, there is no band present at EF and above EF the next
empty band at bottom of CB is due to triple-degenerated (t2g) of
Cr-d states, followed by the double-degenerated band (eg).
Acknowledgment
The computation in this work was performed on Kalki server
of Inter University Accelerator centre (IUAC), New Delhi, INDIA.
One of the authors (M. K. Kashyap) gratefully acknowledges the
support from University Grant Commission, New Delhi, INDIA for
providing the computational and software facilities under the
scheme SAP-I for faculty members. For the author—A.H. Reshak,
the work was supported from the institutional research concept
of the project CENAKVA (no. CZ.1.05/2.1.00/01.0024), the Grant
no. 152/2010/Z of the Grant Agency of the University of South
Bohemia and the School of Materials Engineering, University
Malaysia Perlis (UniMAP), Perlis, Malaysia.
References
4. Summary and conclusions
A full potential treatment of electronic and magnetic properties of Cr-doped Cd-chalcogenides, Cd1 xCrxZ (Z¼S, Se and Te) at
dopant concentration, x ¼0.25 and 0.125 is presented. The half
metallicity remains intact on reducing the dilute limit for all the
compounds with a band gap in minority spin channel. This
property makes these compounds suitable for practical spintronic
devices. The total magnetic moments for all compounds come out
to be an integer value confirming the HM characteristics of these.
The magnetism comes essentially from the d-states of impurity
atom (Cr). The minority band gap decreases on reducing the
dilute limit for all compounds. Moreover, it also drops off on
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HM character for studied DMS compounds will motivate
researchers to study spin-dependent transport properties in
semiconductor-based MTJs and to remove elusive nature of half
metallicity in these, experimentally.
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