Magnetic and spin dependent transport properties of SrC/NaCl(CaS)/SrC (001)
tunnel junctions
P. Vlaic1,2, E. Burzo1
1Faculty
of Physics, Babes Bolyai University, 400084 Cluj-Napoca, Romania
of Medicine and Pharmacy ’’Iuliu Hatieganu’’, Physics and Biophysics Department, RO 400023 Cluj-Napoca,
Romania
I. SrC/NaCl(CaS)/CaC (001) Multilayer Structure
structure is considered Pd/SrC
(001) and Pd/SrC (001) interface:
¾IC1 Pd/SrC (001) interface: Pd
atoms located atop Sr and C sites
(Fig. 2)
-0.5
20
SrC
Pd electrode
0
SrC
electrode
NaCl spacer
5
z (a
NaCl
10
15
/2 lattice spacing)
Pd
20
NaCl
2.5
Pd
2Pd/5CaC/9CaS/5CaC/3Pd
2
B
0
-1
2Pd/5CaC/9CaS/5CaC/3Pd
-2
Pd/SrC (001)
IC interface
-3
1
0
5
z (a
Pd
Es
Sr
C
Ca
S
10
15
/2 lattice spacing)
1
-0.5
20
SrC
Pd electrode
0
CaS spacer
5
z (a
CaS
10
0
10
20
Na(I)
30
Cl(I)
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
0
0
0
10
Sr(I+2)
20
C(I+2)
40
-1
0.2
40
2Pd/5SrC/9CaS/5SrC/3Pd
30
Pd/SrC (001) IC
E
1
F
20
10
0
10
20
Na(I+1)
30
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
F
20
40
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
E
1
0
0
10
Sr(I)
20
C(I)
-0.8
-0.6 -0.4 -0.2
Energy (Ry)
40
10
0
10
Cl(I+1)
30
Cl(I+2)
0
40
-1
30
20
SrC
electrode
10
15
/2 lattice spacing)
Pd
20
CaS
5
0
s-electr.
p-electr.
d-electr.
5
10
V. Exchange coupling in 2Pd/5SrC/mNaCl(CaS)/5SrC/3Pd
(001) heterostructures
1
Sr(I)
-1
0
10
20
Ca(I)
30
S(I)
-1
F
0
10
0.2
F
s-electr.
p-electr.
d-electr.
5
E
-0.8 -0.6 -0.4 -0.2
E
1
C(I)
0
-1
0
s-electr.
p-electr.
d-electr.
2
4
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
4 2Pd/5SrC/9NaCl/5SrC/3Pd
Pd/SrC (001) IC
E
1
F
2
Na(I)
0
Energy (Ry)
0.2
-1
4 2Pd/5SrC/9NaCl/5SrC/3Pd
Pd/SrC (001) IC
E
1
F
2
Cl(I)
0
s-electr.
p-electr.
d-electr.
2
4
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
0
0.2
-1
10
Sr(I+1)
20
30
C(I+1)
20
-0.8
-0.6 -0.4 -0.2
Energy (Ry)
0
1
F
0
10
Sr(I+2)
20
C(I+2)
30
40
-0.8
-0.6 -0.4 -0.2
Energy (Ry)
0
-1
0.2
10
0
10
20
Ca(I+1)
30
S(I+1)
-0.8
40
2Pd/5SrC/9CaS/5SrC/3Pd
30
Pd/SrC (001) IC
E
1
F
20
40
-1
0.2
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
E
10
30
20
-0.6 -0.4 -0.2
Energy (Ry)
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
E
1
0
0.2
0
0.2
F
10
0
10
20
Ca(I+2)
30
S(I+2)
40
-0.8
-0.6 -0.4 -0.2
Energy (Ry)
0
0.2
-1
-0.8
-0.6 -0.4 -0.2
Energy (Ry)
Fig. 7 Layer and atom resolved DOS for 2Pd/5SrC/9CaS/5SrC/3Pd (001) heterostructure
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
0
0.2
Fig.6 l-resolved DOS for Sr,
Sr, C, Na and Cl atoms at SrC/NaCl (001) inerfaces in 2Pd/5SrC/9NaCl/5SrC/3Pd
(001) heterostructure
10
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
1
5
Sr(I)
0
s-electr.
p-electr.
d-electr.
5
10
-1
-0.8
-0.6
-0.4
E
-0.2
5
10
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
1
E
F
C(I)
s-electr.
p-electr.
d-electr.
F
0
0.2
Energy (Ry)
2Pd/5SrC/9CaS/5SrC/3Pd
0
5
10
-1
-0.8
-0.6 -0.4 -0.2
Energy (Ry)
10
0
0.2
5
Pd/SrC (001) IC
2Pd/5SrC/9CaS/5SrC/3Pd
1
Ca(I)
0
s-electr.
p-electr.
d-electr.
5
10
-1
-0.8
E
-0.6 -0.4 -0.2
Energy (Ry)
5
Pd/SrC (001) IC
E
1
S(I)
F
0
s-electr.
p-electr.
d-electr.
5
F
0
0.2
10
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
Energy (Ry)
Fig.8 l-resolved DOS for Sr,
Sr, C, Ca and S atoms at SrC/CaS (001) inerfaces in 2Pd/5SrC/9CaS/5SrC/3Pd
(001) heterostructure
VI. Spin dependent transport properties
10
3
AFM spin up down
AFM spin down up
FM spin up
FM spin down
10
10
-1
10
-3
10
-5
10
-7
10
-9
2
conductance (e /h)
0.05
Pd/SrC (001) IC
5
0
-1
F
10
2Pd/5SrC/9NaCl/5SrC/3Pd
2Pd/5SrC/9NaCl/5SrC/3Pd
Pd/SrC (001) IC
F
10
40
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
E
1
0.2
10
0.2
10
0
0
40
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
1
20
40
-1
2Pd/5SrC/9CaS/5SrC/3Pd
30
Pd/SrC (001) IC
E
1
F
20
40
2Pd/5SrC/9CaS/5SrC/3Pd
Pd/SrC (001) IC
E
30
30
40
0.2
Na(I+2)
F
10
30
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
20
0.2
30
DOS (states/Ry spin)
1
30
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
40
-1
0.2
40
2Pd/5SrC/9NaCl/5SrC/3Pd
Pd/SrC (001) IC
E
10
DOS (states/Ry spin)
C(I+1)
20
DOS (states/Ry spin)
Sr(I+1)
20
30
DOS (states/Ry spin)
10
40
10
Fig. 4 Layer resolved charge transfers and magnetic moments for
2Pd/5SrC/9CaS/5SrC/3Pd (001) heterostructure
0.1
0
40
-1
0.2
F
Fig. 5 Layer and atom resolved DOS for 2Pd/5SrC/9NaCl/5SrC/3Pd (001) heterostructure
Pd
Es
Sr
C
Ca
S
0.5
0
2Pd/5SrC/9CaS/5SrC/3Pd
30
Pd/SrC (001) IC
E
1
F
20
40
-1
1
1.5
0
Pd/SrC (001)
IC interface
DOS (states/Ry spin)
CaS spacer
SrC
electrode
magnetic moment (μ /atom)
charge transfer (e)
SrC
electrode
1
30
-0.8 -0.6 -0.4 -0.2
Energy (Ry)
40
Fig. 3 Layer resolved charge transfers and magnetic moments for
2Pd/5SrC/9NaCl/5SrC/3Pd (001) heterostructure
1 Pd
C(I)
40
2Pd/5SrC/9NaCl/5SrC/3Pd
Pd/SrC (001) IC
E
10
DOS (states/Ry spin)
z (a
10
15
/2 lattice spacing)
Sr(I)
20
20
DOS (states/Ry spin)
5
10
30
DOS (states/Ry spin)
1
0
F
0
40
-1
0
Pd[110] || SrC [100]
DOS (states/Ry spin)
-3
1
30
0.5
• Epitaxial relations:
SrC [100] || [100] NaCl(CaS)
NaCl(CaS)
40
2Pd/5SrC/9NaCl/5SrC/3Pd
Pd/SrC (001) IC
E
10
• The conductances through Pd/SrC/NaCl(CaS)/SrC/Pd
Pd/SrC/NaCl(CaS)/SrC/Pd (001) tunnel
junctions are evaluated in the currentcurrent-perpendicularperpendicular-toto-plane (CPP)
geometry by means of the linear response of Kubo approach
implemented within TBTB-LMTO formalism and including vertex corrections
[4, 5].
•A 16x16 in plane k||-point grid is used for electronic structure
calculations and a 60x60 k||-point grid is used for spin dependent
transport calculations.
Fig. 2 Pd/SrC (001) interface
configuration
aSrC
= aPd
2
DOS (states/Ry spin)
Pd/SrC (001)
IC interface
1
aSrC=aNaCl(CaS)
NaCl(CaS)
DOS (states/Ry spin)
Pd
Es
Sr
C
Na
Cl
2Pd/5CaC/9NaCl/5CaC/3Pd
-2
Pd
Es
Sr
C
Na
Cl
1.5
20
Spin dependent transport properties:
IC1
fccfcc-type
str.
str.
DOS (states/Ry spin)
-1
1
30
DOS (states/Ry spin)
B
0
C Na(Ca) Cl(S)
DOS (states/Ry spin)
2
Sr
40
Pd/SrC (001)
IC interface
2Pd/5CaC/9NaCl/5CaC/3Pd
Pd
SrC
Fig. 1 Structure of
Pd/SrC/NaCl(CaS)/SrC/Pd
Pd/SrC/NaCl(CaS)/SrC/Pd (001)
heterojunctions
DOS (states/Ry spin)
Pd
NaCl (CaS)
B1B1-type structure
Pd
DOS (states/Ry spin)
SrC
electrode
magnetic moment (μ /atom)
charge transfer (e)
NaCl spacer
SrC
• Performed by means of a first principle Green’
Green’s function technique for
surfaces and interfaces implemented within the tighttight-binding linear
muffinmuffin-tin orbital method in the atomic sphere approximation (TB(TB-LMTOLMTOASA) [2].
• The local spin density approximation (LSDA) was used for exchange
exchange
correlation potential within VoskoVosko-WilkWilk-Nusair parameterisation [3].
DOS (states/Ry spin)
Pd
fccfcc-type
str.
str.
2.5
SrC
electrode
Electronic Structure Calculations:
/(semi-infinite) Pd (001)/2Pd (001)/mSrC (001)/nNaCl(CaS) (001)/mSrC
(001)/3Pd (001)/Pd (001) (semi-infinite)/
One interface geometrical
III. Ground state electronic and magnetic properties
1 Pd
II. Computational Details
Studied system:
DOS (states/Ry spin)
Half metallic ferromagnetic (HMF) materials with metallic properties
properties in only one spin
direction and therefore having full spin polarization at the Fermi
Fermi level are seen the most promising
candidates for high performance spintronic device applications. Electronic structure calculations
performed for SrC compound by using tighttight-binding linear muffinmuffin-tin orbital (TB(TB-LMTO) method
show that metastable rocksaltrocksalt-type phase with an equilibrium lattice parameter aSrC=5.55 Å has
half metallic characteristics with a total magnetic moment of 2 μB/f.u.,
/f.u., in agreement with previous
predictions [1]. Therefore SrC is epitaxially compatible with both B1B1-type direct band gap NaCl
(aNaCl=5.64 Å) and Γ-X indirect band gap CaS (aCaS=5.69 Å) barriers. Thus SrC/NaCl(CaS)/SrC (001)
magnetic tunnel junctions (MTJs
(MTJs)) represent feasible heterostructures for theoretical investigations
as well as for potential technological applications.
Ground state electronic and magnetic properties of SrC/NaCl/SrC (001) and SrC/CaS/SrC
(001) heterostructures are studied by using a first principles Green’
Green’s function technique for
surface and interfaces implemented within TBTB-LMTO formalism. The spin dependent transport
properties in the currentcurrent-perpendicularperpendicular-toto-plane (CPP) geometry are determined by means of the
linear response of Kubo approach implemented within TBTB-LMTO formalism. At SrC/NaCl(CaS)
SrC/NaCl(CaS)
(001) interfaces Sr and C atoms have magnetic moments little reduced compared with the
corresponding bulk values. Small spin polarizations are induced on both Na(Ca)
Na(Ca) and Cl(S)
Cl(S)
interfacial sites. A ferromagnetic (FM) coupling is observed for NaCl based junctions while for CaS
based ones it is antiferromagnetic (AFM). For both SrC/NaCl/SrC (001) and SrC/CaS/SrC (001)
heterostructures the exchange couplings are small and decrease exponentially with
with the barriers
thicknesses. In the FM state of the junctions the highest contributions
contributions to the total conductances
are given by the minority spin electrons. All conductances decrease exponentially with the barrier
thicknesses. The spin dependent transport properties are mostly determined by the electronic
characteristics of interfacial SrC layers as well as by the complex band structures of the insulating
insulating
(semiconducting)
semiconducting) spacers. For CaS based magnetic tunnel junctions, tunnelling
magnetoresistance ratio increases almost exponentially with the barrier thickness.
thickness.
DOS (states/Ry spin)
ABSTRACT
DOS (states/Ry spin)
2University
2Pd/5SrC/nCaS/5SrC/3Pd
-0.1
2Pd/5SrC/nNaCl/5SrC/3Pd
10
-1
10
-2
10
-0.15
0.0001
2
2.5
3
n
3.5
4
3
4
5
2
3
4
5
6
7
8
9
10
n
-3
2
6
n
10
6
10
5
10
4
10
3
10
2
2
10
4
5
7
8
9
2Pd/5SrC/nNaCl/5SrC/3Pd
2Ps/5SrC/nCaS/5SrC/3Pd
AFM spin up down
AFM spin down up
FM spin up
FM spin down
1
3
n
2
Fig. 10 FM and AFM conductances vs.
insulating spacer thickness for
2Pd/5SrC/nNaCl
/5SrC/3Pd (001)
2Pd/5SrC/nNaCl/5SrC/3Pd
heterojunction
2
-0.05
conductance (e /h)
0.001
1
/J exch / (mRy)
0.01
J exch (mRy)
J exch (mRy)
0
10
TMR (%)
2Pd/5SrC/nNaCl/5SrC/3Pd
-2
10
-4
Fig. 12 k‫װ‬-resolved
conductances of FM and AFM
states for
2Pd/5SrC/9NaCl/5SrC/3Pd (001)
heterojunction
10
-6
10
2Pd/5SrC/nCaS/5SrC/3Pd
Fig. 9 The exchange couplings versus barrier thicknesses for
2Pd/5SrC/nNaCl(CaS)/5SrC/3Pd (001) heterostructures
-8
10
2
3
4
5
6
7
n
8
9
10
2
3
4
5
6
n
7
8
9
10
Fig. 11 FM and AFM conductances and TMR values vs. insulating spacer
spacer thickness for
2Pd/5SrC/nCaS
/5SrC/3Pd (001) heterojunctions
2Pd/5SrC/nCaS/5SrC/3Pd
Conclusions
• Electronic structure and magnetic properties of Pd/SrC/NaCl/SrC/Pd
Pd/SrC/NaCl/SrC/Pd (001) and
Pd/SrC/CaS/SrC/Pd
Pd/SrC/CaS/SrC/Pd (001) heterostructures have been studied by using a Green’
Green’s
function technique for surface and interfaces implemented within TBTB-LMTO
formalism. The spin dependent transport properties in currentcurrent-perpendicularperpendicular-totoplane geometry have been determined by means of KuboKubo-Landauer approach
implemented within TBTB-LMTO formalism.
•The main contribution to FM conductances is given by the minorityminority-spin
electrons.
•Very large magnetoresistive effects are predicted for Pd/SrC/CaS/SrC/Pd
Pd/SrC/CaS/SrC/Pd (001)
heterojunctions.
heterojunctions.
• A charge transfer between Pd and SrC interfacial layers is observed.
References
• Magnetic moments of interfacial SrC layers at Pd/SrC
Pd/SrC (001) interfaces are
reduced compared with corresponding bulk values.
[1] Wenxu Zhang, Z. Song, B. Peng,
Peng, and Wanli Zhang, J. Appl.
Appl. Phys. 112 (2012) 43905.
[2] I. Turek,
Turek, V. Drchal,
Drchal, J. Kudrnovský,
Kudrnovský, M. Šob,
ob, and P. Weinberger in ’’Electronic
’’Electronic Structure
of Disordered Alloys, Surfaces and Interfaces’’
Interfaces’’ (Kluwer Academic Publishers, 1997).
[3] S. H. Vosko,
Vosko, L. Wilk,
Wilk, and M. Nusair,
Nusair, Can. J. Phys. 58 (1980) 1200.
[4] K. Carva,
Carva, I. Turek,
Turek, J. Kudrnovský,
Kudrnovský, and O. Bengone,
Bengone, Phys. Rev. B 73 (2006) 144421.
[5] J. Kudrnovský,
Kudrnovský, V. Drchal,
Drchal, C. Blass, P. Weinberger, I. Turek and P. Bruno, Phy.
Phy. Rev. B 62
(2000) 15084.
• Magnetic moments of interfacial SrC layers at SrC/NaCl (001) and SrC/CaS (001)
interfaces decrease little compared with corresponding bulk values.
values.
Fig. 13 k‫װ‬-resolved
conductances of FM and AFM
states for
2Pd/5SrC/9CaS/5SrC/3Pd (001)
heterojunction
• Small exchange couplings with exponential decay are evidenced.
• Away from Pd/SrC
SrC/NaCl(CaS) (001) interfaces, SrC layers tend to
Pd/SrC (001) and SrC/NaCl(CaS)
have bulkbulk-like behaviors with the magnetic moments mostly carried by C anions.
anions.
• Induced gap states are observed on both Na(Ca)
Na(Ca) and Cl(S)
Cl(S) ions at SrC/NaCl(CaS)
SrC/NaCl(CaS)
(001) interfaces.
• The insulating character in NaCl and CaS barriers is rapidly recovered.
Acknowledgment
This work was realized in the framework of project PN-II-ID-PCE-2012-4-0028.