electronic structure of novel magnetic systems: insights from...
TRANSCRIPT
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Indra Dasgupta
Department of Solid State Physics & Center for Advanced Materials
Indian Association for the Cultivation of Science Kolkata, India
E-mail: [email protected]
Electronic Structure of Novel Magnetic Systems: Insights from Spin Polarized DFT
Calculations
DST Summer School MASTANI, July 2014
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Plan of the Talk
Introduction Stoner Model of Ferromagnetism Ø Electronic structure of Half-metallic ferromagnets
Exchange Mechanism Ø Half Heusler based Diluted Magnetic Semiconductors Ø Magnetism in Type-1 Clathrates
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∑∑< −
++Δ−=ji ji
iii rr
erVH||
)](21[
22
kljijkl
iijkljijiiji
ii ccccvcctnH +++
≠∑∑∑ ++=21
ε
Find some complete one-particle basis:
Hamiltonian
Kinetic Energy Electron-electron int.
The tendency toward magnetism is determined by a competition between exchange and kinetic energy effects
↓ ↓
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Broad energy bands electrons are highly itinerant –itinerant magnetismà SDFT
Narrow energy bands tendency toward localization à “see each other”
Magnetism
jiijHeis SSJH ∑=
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First Principles Electronic Structure Calculations: Foundations
Ø Density functional theory àKohn-Sham + Local density approximation (LDA) → Reduction to an effective non-interacting system.
HΨ = EΨ ∑∑< −
++Δ−=ji ji
iii rr
erVH||
)](21[
22
KOHN-SHAM APPROACH
)()();();();(21 ][ 2 rrrnVrnVrnV iiiGSxcGSextGSH
!!!!!Φ=Φ+++∇− εKS-equationà
Noninteracting part Interacting part
ß Veff à LSDA
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Ø Quantitatively not accurate but captures the essential physics. Ø Provide a framework to interpret results obtained from spin DFT.
Electron density
Magnetization density
“Usually” is a small parameter compared to density , expand in terms of
Exchange-correlation potential for non-spin-polarized electrons.
The average value of is a positive so that more attractive potential acts on the majority electrons compared to minority electrons.
Stoner Model: The potential difference is now independent of .
the exchange integral Stoner parameter and
Stoner Model for Ferromagnetism
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Stoner Model for Ferromagnetism
A constant change of the potential does not change the Kohn-Sham eigenfunctions, but eigenvalues shift by a constant amount.
Exchange splitting:
DOS for a non magnetic system
No. of electrons,
The moment can be determined by the equations M = F(M)
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Finite magnetization always exists, if
The sufficient condition for ferromagnetism
The same result can be obtained starting from Hubbard model using mean field approximation.
Graphical Soln:
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Stoner Criteria and Trends in Magnetism
q I is an intra-atomic, element specific and independent of local environment I3d > I4d > I5d
q n0(EF) à co-ordination number and hopping matrix elements. n0(EF) ~ nd
0(EF) ~ 1/Wd à W3d < W4d < W5d
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Metal n0(EF) [Ry-1] I[Ry] In0(EF)
Na 6.2 0.067 0.42 Al 5.6 0.045 0.25 Cr 9.5 0.028 0.27 Mn 21 0.030 0.63
Fe 42 0.034 1.43 Co 27 0.036 0.97 Ni 55 0.037 2.04 Cu 3.9 0.027 0.11
Pd 31 0.025 0.78
Stoner Model and Ferromagnetism
Local magnetic moments of isolated 3d atomsà Hunds’s Rule
Stoner Condition à Bulk Magnetism
Ref: R Zeller, Computational Nanoscience: Do it yourself 31, 419, 2006
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q Structural properties are well described so the overall charge density is well presented within LSDA, while the energy scale associated with the magnetic instability is not adequately treated in LSDA.
q In LSDA the localization is not controlled by the Hubbard U, but by a quantity which represents the Hund’s rule exchange, the Stoner I. This I is an order of magnitude smaller than U (I ~ 1eV, U ~ 10eV)
q This is due to the homogeneous electron gas picture inherent in LDA, where the spin dependence has its origin in Hunds rule exchange , while in Mott insulators the Hubbard U is the driving force.
q CURE: Hubbard U instead of Stoner I q LDA+U method: Anisimov, Zannen and Andersen,
PRB 44, 943 (1991)
Strongly Correlated Systems and LSDA
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Electronic structure and magnetism in materials for spintronic
application
(a) Half Metallic Magnets (b) Diluted Magnetic Semiconductors
Ranjit Nanda Department of Physics IIT Madras Chennai
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Spin-based Electronics i.e. Exploiting electron’s spin (over and above its charge ) to carry information → new generation of devices with new functionality.
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Existing half-metallic magnets
• Heusler and half-Heusler compounds (e.g. NiMnSb)
Direct observation of half-metalicity in Co2MnSi , Nature Comm. 2014
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Half-Heusler Compounds (XMZ) The unit cell is fcc lattice with three atom as basis.
Ø The Zinc Blende structure adopted by semiconductors like GaAs, ZnSe, InAs. X = Ga, M = Empty, Z = As. Ø Heusler Alloys ⇒ Compatible with semiconductor technology.
← M → ← X → Z
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LDA-LMTO Electronic structure Results: Spin integrated DOS of half-Heusler compounds
• Key Results: A gap close to the Fermi level. VEC = 18 Compounds are semiconductors. VEC > 18 Compounds may be half-metallic ferromagnets. Is gap a generic feature in half-metallic magnets ?
Ref. Electronic structure and magnetism in half-Heusler compounds B.R.K. Nanda and I. Dasgupta J. Phys.: Condensed Matter 15, 7307-7323 (2003)
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Spin integrated DOS of other half-metallic magnets
Fe3O4
CrO2
Gap close to the Fermi level in the spin-integrated state is a generic feature for all existing half-metallic magnets.
Sr2FeMoO6
Zhang and Satpathy, PRB 44 13319 (1991)
Lewis et.al. PRB 55 10253 (1997)
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VEC > 18 and Spin-Polarization
Ø Ferromagnetism: Competition between BETWEEN ⇒ (i) Kinetic energy, (ii) Coulomb repulsion Ø Gap in the DOS is important for half-metallic ferromagnetism.
ENERGY
Ref. Electronic structure of half-metallic magnets B.R.K. Nanda and I. Dasgupta Comp. Mat. Science, 2006
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Doped Semiconducting half-Heusler compounds
• Why doped half-Heuslers ?
Ø Half-Heusler compounds with 18 valence electrons are semiconductors. (e.g. FeVSb,CoTiSb,NiTiSn). ⇒ Doping of Mn/Cr at V/Ti sites will make the number of valence electrons more than 18. Ø Structural similarity between the half-Heusler compounds and semiconductors.
Are doped half-Heusler compounds magnetic ?
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Diluted Magnetic Semiconductors
DMS: Semiconductors doped with TM impurities shows intrinsic ferromagnetism eg. ferromagnetism in Mn-doped GaAs (Ohno 1998)
Semiconducting Material ↔Ferromagnetic Material
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II-VI DMS (MnZnSe)
No carriers, no FM
Carrier-mediated ferromagnetism
III-V DMS (MnGaAs)
Mn = local moments +holes Mn3+ →Mn2+ + holes →act as source of localized spins 5/2 +effective mass acceptors
)().(3 rsrrJSdH ∫=
S(r) à Mn spin, s(r) à hole spin
Mn d-band
Valence band Conduction band
EF
GaAs
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RKKY Model
rkyy
yyyyF
yFJConstrJ
F
pd
2 ,cossin)(
)()(
4
2
=−
=
×=
The effective exchange interaction between the valence band hole and the spin Jpd results in a coupling between TM spins at a separation r given by
So materials with larger Jpd should have higher Tc
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GaN
EF
Valence band Conduction band
Mn d-band
Schematics Mn doped GaN
Impurity band forms in the gap, ferromagnetism is mediated by Double Exchange (Mn 3+) (d 4)
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Double Exchange
Ø If the impurity band is partially occupied and the neighboring site has parallel spin (FM arrangement) then by the allowed hopping to the ne- ighboring site it lowers the kinetic energy and FM is stable.
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Ø XYZ (VEC=18) ⇒ semicondu- cting. Doped system ⇒ X(Y,Mn)Sb (VEC>18) half-metallic
Ref. Electronic Structure and magnetism in doped semiconducting half-Heusler compounds, B.R.K. Nanda and I. Dasgupta J. Phys: Condensed Matter 17 5037 (2005)
Mn doped NiTiSn
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Mn impurity band at EF
Mn Doped NiTiSn Disordered System (KKR-CPA Calculations)
(Ni-d)-(Ti-d) hybridization gap
Mn : 4+, d3 M=3 µB/cell
(MnxTi1-x)NiSn
€
H = − Jiji≠ j∑ !
e i .! e j
€
Jij =14π
dEEF
∫ Im{TrL (Δ iT↑ijΔ jT↓
ji)}
€
Δ i = ti↑−1 − ti↓
−1
Classical Heisenberg Hamiltonian
From Microscopic to Atomistic
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Exchange interaction with disorder (CPA)
Short-ranged interaction Exponential damping due to gap Magnetic percolation
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TC ~ x1/2
€
TCMF =
2x3kB
J0 jj≠0∑ x : Mn conc.
Double exchange : FM is favorable TB model :-
€
<W 2 >conf = <|H0 jj≠0∑ |2>= x | t0 j
j≠0∑ |2
Short-ranged interaction Exponential damping due to gap Magnetic percolation
Mean field estimations
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Monte- Carlo Simulations
Ferromagnetism in Mn doped half-Heusler NiTiSn: Theory and experiment, B. Sanyal et. al. Appl. Phys. Lett. 89, 212502 (2006)
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K. G. Suresh et al.
Experimental Signature of ferromagnetism: Temperature dependence of magnetization of NiTixMn1-xSn
0 50 100 150 200 250 3000.10.20.30.40.50.60.70.80.91.0
M(e
mu/
g)T (K)
NiTi0.75Mn0.25SnH=200 Oe
FC
ZFC
0 50 100 150 200 250 300 3500.0
0.2
0.4
0.6
0.8
1.0
M (e
mu/
g)
T (K)
NiTi0.9Mn0.1SnH=200 Oe
M vs T curve has a typical mean field shape →ferromagnetism is driven by itinerant carriers.
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-60000 -40000 -20000 0 20000 40000 60000
-8
-6
-4
-2
0
2
4
6
8NiTi0.75Mn0.25SnM-H (5K)
Mag
net
izat
ion
(em
u/g
)
Magnetic Field (Oe)
max moment=1.2µB /Mn
M-H plots of NiTi0.75Mn0.25Sn and NiTi0.9Mn0.1Sn K. G. Suresh et al.
-60000-40000-20000 0 20000 40000 60000
-4
-2
0
2
4
M (e
mu/
g)H(Oe)
NiTi0.9Mn0.1SnT=5 K
Max.Moment =
1.7µB per Mn
Experimental Signature of Ferromagnetism
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iopscience.org/jpcm
ISSN 0953-8984
Journal of PhysicsCondensed MatterJournal of Physics
w
Volume 24 Number 50 19 December 2012
Featured in this issueLiquids, Soft Matter and Biological Physics
Topical reviewKinetics of protein unfolding at interfaces Yohko F Yano
Mn doped Ge Clathrates: A Novel DMS System?
Nirmal Ganguli Univ. of Twente, The Netherlands
505501
DST
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Germanium Clathrates • Clathrate (Ge46) ≡ cage like
structure
• Ge46 clathrate can be stabilized by incorporation of Ba
Ø Good thermoelectric material
Ø Useful for hydrogen storage
Ø Exhibits superconductivity
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Magnetism in Clathrates
Ba8Mn2Ge44 clathrate shows FM with 0.8 µB/Mn moment, Tc = 10K
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0
30
60
0
30
0
5
10
DO
S (
stat
es/e
V c
ell)
0
20
-12 -10 -8 -6 -4 -2 0 2E - E
F (eV)
0
1
(a)
(b)
(c)
(d)
(e)
sp gap Ge46 1.19eV
Ba8Ge46
Ge-s
Ge-p
Ba-s
Electronic Structure of Ge46 Clathrates
Doping Mn at 6c sites à Magnetic ?
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Mn doped Ge46 clathrate • Ferromagnetic interaction between Mn atoms for
both distances (a/2 and √6a/4), except vary high value of U
-25
0
25
-7
0
7
DO
S (
stat
es/e
V c
ell)
-3 -2 -1 0 1 2E - E
F (eV)
-10
0
10
(a): Total
(b): Mn-d
(c): Ge-p
spin
spinspin
spinspin
spin
Hybridization induced –ve exchange splitting
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Charge density
0.00
0.08
0.15
0.23
0.31
0.39
The charge density corresponding to valence band hole states
Exchange Interactions
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Mn doped Ba8Ge46
q DOS is metallic q Ex/Ef << 1 à RKKY Limit q Exà Exchange splitting of the host band, Ef à Fermi level
Exchange Interactions
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Mn doped Ba8Ge46
J(r) = Constant × [sin(2kFr) – 2kFr cos(2kFr)]/r4
Tc = 6.3K
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Clathrates: summary • Ge46 clathrate has higher band gap than Ge in diamond
structure • Hybridization induced negative exchange interaction
promotes ferromagnetism for Mn doped Ge46 clathrates • Ba atoms at the center of the cages provide conduction
electrons • In the presence of conduction electrons, RKKY-like
interaction is operative in Mn doped Ba8Ge46 clathrates • Experimental observations (low Curie temperature and low
saturation magnetic moment) can be explained
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