New magnetic materials for Spintronics Juana Moreno Department of Physics and Astronomy Center for Computation and Technology Louisiana State University 1

Computational Materials Science Petascale computing & the development of new formalism, algorithms and codes will allow the accurate modeling of materials. Huge opportunities for young computational materials scientists to make breakthroughs in the study of complex materials for future high-tech devices.

Exponential growth in computing power: http://i.timeinc.net/time/daily/2011/1102/singularity_graphic.jpg

Analyzer -www.britannica.com Bush Differential Analyzer -www.britannica.com ENIAC - U.S. Army Photo (D. P. Landau, UGA) CRAY 1 - www.ucar.edu CRAY X1 – ORNL/CCS

A Truly interdisciplinary approach Biological Sciences Physical Sciences Medical Science Engineering Social Sciences Computational methods Environmental Sciences Arts & Humanities Economics http://www.ncsa.edu/ Engineering Animation, Inc.

What is spintronics? And why? Spin-unpolarized current: Electrons move with random spin orientation Spin-polarized current: Electrons move with same spin orientation

We need faster, smaller, more efficient chips By 2020 a transistor in a chip may reach the size of a few atoms. Electronics based on a new paradigm is needed!

Devices based on “static” spins Giant magneto-resistance hard-disks GMR effect (1988) IBM hard disk (1997) [Prinz, Science 1998]

The advantage of using spin…. Silicon technology (no equivalent change) 0.1 penny/MB! GMR introduction (advantage of using spin in devices)

Devices based on spin-polarized currents Spin Field Effect Transistor Datta-Das (1990) Spin precession due to spin-orbit interaction with spin-orbit splitting controlled by gate potential p p - - type type n n - - type type Spin LED H split the spin levels circularly polarized light. + - spin injector hν Very small spin injections!

Metal-semiconductor interface Difficulties to build spintronic devices? Metal devices don’t amplify the current. Mixed devices have problems with spin injection. Large scattering at the interface between ferromagnetic metal-semiconductor due to conductivity mismatch. Metal-semiconductor interface Solution: a magnetic semiconductor as spin-injector Easily integrated with current semiconductor technology Multiple functionality Amplification Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Spin transistor (Datta-Das, 1990) Problem at interface with metal-semiconductor junction! Non-magnetic Semiconductor Spin coherence may be preserved Spin coherence destroyed due to conductivity mismatch Ferromagnetic Metal Metal-semiconductor junction Semiconductor-semiconductor junction Ferromagnetic Semiconductor

What is spintronics? And why? Ferromagnetic semiconductors Outline What is spintronics? And why? Ferromagnetic semiconductors Mn

Search for a ferromagnetic semiconductor 1960: Cr spinels 60’s-70’s: Eu-based chalcogenides: EuO, EuS, Eu1-xGdxO Tc up to 137 K (difficult to integrate) 80’s: II-VI alloys: CdMSe, CdMTe, ZnMTe, ZnMSe, … M= V,Cr,Mn,Fe,Co… (low Tc, difficult to dope) 90’s: III-V grown with Molecular Beam Epitaxy (MBE): InMnAs (1992) GaMnAs (1996) Tc > 170 K 00’s: GaMnP,GaMnN, GeMn, GaMnSb Oxides: ZnMnO Organics

Ga1–xMnxAs Tc Room Temperature ( ~ 300K) Magnetization [a.u.] Saturation magnetization (T = 0) Mn2+ (S=5/2) Ga0.92Mn0.08As K. Y. Wang et al., (2005) S. J. Potashnik et al., Phys. Rev. B (2002)

antiferromagnetic correlations In the absence of free carriers: short-range super-exchange Holes mediate ferromagnetic order: antiferromagnetic correlations carrier dispersion Mn spin carrier spin Mn Mn Zener or double-exchange Hamiltonian 15

Optimizing Tc for a ferromagnetic semiconductor Crucial questions: How to increase Tc above room temperature ? Why the saturation magnetization is reduced ? How to understand magnetic anisotropy ? Is there an impurity band? 16

Weak-coupling mean-field theory                                                                Weak-coupling mean-field theory [Dietl et al., Science (2000)] Dietl’s theory is too simplistic to analyze material properties. Room temperature

Theoretical difficulties in (GaMn)As Strongly correlated system Strongly disordered system Non-local effects Large spin-orbit coupling How to model: How multiple carrier bands modify the double-exchange mechanism? How to include short range correlations in a dilute magnetic system? How to incorporate realistic modeling of the host material?  Solution: Dynamical mean field & dynamical cluster approximation

Dynamical Mean-Field Approximation (DMFA) Replace the system by a single impurity in a bath of holes 2. Include local fluctuations of the embedded impurity using a frequency dependent self-energy 3. The self-energy connects the impurity Green function with the mean-field bath mean-field function impurity self-energy Green function 19

Dynamical Cluster Approximation (DCA) DCA provides systematic non-local corrections to the DMFA Finite size cluster self-consistently embedded in a mean-field. Non-local correlations are short-ranged DMFA B. J. van Zeghbroeck (1997) DCA Maier, Jarrell, Pruschke & Hettler, Rev. Mod. Phys. (2005)

Tc for carriers with angular momentum j Tc (j) reaches a maximum for j = 3/2! max Model for GaAs must include two hole bands (j = 3/2) with light and heavy masses. Tc is suppressed due to magnetic frustration. J. M, Fishman & Jarrell, Phys. Rev. Lett. 96, 237204

Magnetic anisotropy in (GaMn)As Tensile strain  Out-of-plane anisotropy  Compressive strain  In-plane anisotropy (GaMn)As substrate (spin reorientation) (GaMn)As substrate [100] z θ M x Φ y [110] [110]

Model for (GaAs)Mn Ga1-xMnxAs non-magnetic ion (Ga) magnetic ion (Mn) Carrier spin Localized spin (Mn) Mn site Spin-orbit coupling & strain effects Ga1-xMnxAs

exact solution at Nc = ∞ Nc = 1 (DMFA) Nc = 14 Nc = 36

Results: Magnetization and Tc Mn concentration : x = 5 % hole concentration : p = 2.5% Nc : Cluster size Mn concentration : x = 5 % Experiments

Results: Magnetic anisotropy z Compressive strain (─0.2%) substrate (GaMn)As θ M x Φ y [110] Tensile strain (0.2%) substrate (GaMn)As

Results: Magnetic anisotropy [100] [110] Compressive strain substrate (GaMn)As Calculation (DCA) z θ M x Φ y Experiments [110]

Outline What is spintronics? And why? Ferromagnetic semiconductors Organic magnets

Organic-based Materials in Spintronics Advantages: smoother interfaces longer spin coherence lengths more flexible metallicity range scaffolding to hold metal centers cheaper bottom-up fabrication low-weight mechanically flexible, .... Disadvantages: low conductivity difficult to grow organic + metal low-weight, ….

Magnetic Organic Semiconductors: Magnetic Organic Semiconductors: Porphyrins & Metalloporphyrins Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005 Porphyrins & Metalloporphyrins Magnetic Organic Semiconductors: Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005 Porphyrins & Metalloporphyrins Magnetic Organic Semiconductors: Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005 Porphyrins & Metalloporphyrins Magnetic Organic Semiconductors: Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005 Porphyrins & Metalloporphyrins Magnetic Organic Semiconductors: Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005 Goldberg, Crystal growth & Design, 2004, 2006 Porphyrins & Metalloporphyrins Magnetic Organic Semiconductors: Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005 Porphyrins & Metalloporphyrins Magnetic Organic Semiconductors: Q. Huo, Langmuir 2004, J. Porphyrins and Phthalocyanines 2005

Majidi, Moreno, Schwalm, Fishman (2007) Metalloporphyrin modeling t1 t1 Co Ni t2 Majidi, Moreno, Schwalm, Fishman (2007)

Conclusions: Huge opportunities in computational materials science

Thanks to: Peter Reis, Ryky Nelson, Karlis Mikelsons (Georgetown), Majid Nili (PennState), Karan Aryanpour (Univ. Arizona) Unjong Yu (GIST, South Korea), Aziz Majidi (Jakarta, Indonesia), Brian Moritz (Standford/SLAC) Alex Brandt (Rackspace), Mason Swanson (Ohio State), Jitu Das (Carnegie Mellon), Jonathan Gluck (Swarthmore), Brittany Shannon (Monmouth) Randy Fishman (ORNL), Mark Jarrell, Dana Browne (LSU) Gonzalo Alvarez, Paul Kent, Thomas Maier, Fernando Reboredo (ORNL)