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.
