2017-5-31, KITS, Beijing 　Numerical study of electron correlation effects in spintronic materials Bo Gu (顾波) Advanced Science Research Center (ASRC) Japan Atomic Energy Agency (JAEA) 1 1 1

Self-introduction Dec. 1977 born in Hubei, China Education: 1996.9 – 2000.7 Wuhan University, Bachelor 2000.9 - 2003.7 Peking University, M.S. （Supervisor: 苏肇冰, Co-supervisors: 向涛, 王孝群，覃绍京） 2004.2 - 2007.1 Graduate University of Chinese Academy of Sciences, Ph.D. （ Supervisor: 苏刚） Employment: 2007.5 – 2010.3 Tohoku University, Japan, Post-doc (Supervisor: Sadamichi Maekawa) 2010.4 – 2012.3 Japan Atomic Energy Agency, Post-doc 2012.4 – 2016.6 Japan Atomic Energy Agency, Scientist (Permanent Staff) 2016.7 – now Japan Atomic Energy Agency, Senior Scientist (Permanent Staff)

Outline 1. Introduction on diluted magnetic semiconductor (DMS) 2. DMS with wide band gap 3. DMS with narrow band gap I am sorry to skip the part of spin Hall effect due to time limit. I am happy to answer questions on spin Hall effect.

Spintronics Conventional electronic devices charge (e) electron spin (μB) Spintronic devices Advantage: Non-volatility (data are retained with power off) Low electric power consumption etc…

Diluted magnetic semiconductors (DMS) e.g. GaAs e.g. (Ga,Mn)As Nonmagnetic semiconductors (charge) Diluted magnetic semiconductors (charge + spin) Classic DMS (Ga,Mn)As: Curie Tc ~ 200 K. (2)　p-type (hole) (Ga3+  Mn2+) Challenges: High Tc > Room Temperature p-type (hole) & n-type (electron) (e.g. spin p-n junction)

A picture of ferromagnetism (FM) in DMS Bulut PRB 76, 045220 (2007); Tomoda Physica B 404, 1159 (2009) Host band gap △g = 2 eV IBS: impurity bound state (split-off state) ~ 0.1 eV Symmetric model at μ = △g/2 μ Quantum Monte Carlo (QMC) method Chemical potential μ = - 6.0 eV (metallic)  RKKY-type (Ruderman-Kittel-Kasuya-Yoshida) impurity-host impurity-impurity β=1/T impurity-impurity distance impurity-host distance

Chemical potential μ = 0.1 eV (semiconductor)  long-range FM IBS: impurity bound state μ impurity-host impurity-impurity Ferromagnetic β=1/T Antiferromagnetic impurity-impurity distance impurity-host distance Impurity Carrier-mediated ferromagnetism ! Host IBS

IBS: impurity bound state ~ 0.1 eV Impurity-Impurity β=1/T μ ~ Condition for FM IBS Bulut PRB 76, 045220 (2007) Chemical potential μ (eV) Hartree-Fock approximation Spatial distribution of spin of host Impurity IBS Host effective mass of host energy level of IBS M. Ichimura et al, Proceedings of ISQM-Tokyo 2005, p.183-186; cond-mat/0701736.

A picture of p-type ferromagnetism (FM) in DMS materials FM in p-type DMS VB CB μ p-type μ n-type μ Strong p-d mixing in VB  IBS near VB Weak s-d mixing in CB  no IBS near CB p-type: μ ~  FM n-type: μ >>  No FM IBS: impurity bound state (split-off state) The picture for (Ga,Mn)As, (Zn,Mn)O, Mg(O,N), Li(Zn,Mn)P wide band gap

DMS: Semiconductor host + Magnetic impurity Carriers (electrons, holes) Localized moment VB: p-orbital; CB: s-orbital Band structure Ferromagnetism Electron correlations Density functional theory (DFT) Quantum Monte Carlo (QMC) Advantage of QMC 1. Treat electrons correlations correctly. 2. Not rely on separation between spin and charge fluctuations.

Anderson impurity model: Our method for DMS Anderson impurity model: Host band Mixing Impurity level  Density functional theory (DFT) Coulomb correlations of impurity  Quantum Monte Carlo (QMC) with Hirsch-Fye algorithm DFT+QMC: Electron Correlations; Spin and Charge Fluctuations Chemical potential μ: a free parameter, model p- or n-type carriers. Occupation number of impurity Magnetic correlations between impurities

Outline 1. Introduction on diluted magnetic semiconductor (DMS) 2. DMS with wide band gap 3. DMS with narrow band gap

Case 1: (Zn,Mn)O & Impurity bound state (IBS) zincblend: ~ 0.1 eV wurtzite: ~ 0.2 eV wurtzite (Shallow IBS) (Local moment)2 zincblende rocksalt rocksalt: ~ 1.6 eV IBS IBS (Deep IBS) Band gap △g = 3.45 eV Chemical potential μ (eV) Gu, Bulut, Maeawa, J. Appl. Phys. 104, 103906 (2008). μ

Ferromagnetic (FM) correlation in (Zn,Mn)O ~0.2 eV FM in p-type DMS VB CB ~0.1 eV p-type μ μ ~ Condition for FM ~1.6 eV No FM (Deep IBS) Hartree-Fock result: Mn-Mn distance (alattice) Gu, Bulut, Maeawa, J. Appl. Phys. 104, 103906 (2008).

B. Debate on experiments A. Many experiments declare high-temperature ferromagnetism in p-type (Zn,Mn)O M. Ivill, et al., J. Appl. Phys. 97, 053904 (2005) K. R. Kittilstved, et al., Nat. Mater. 5, 291 (2006) J. R. Neal, et al., PRL 96, 197208 (2006) B. Debate on experiments Unexpected ferromagnetic materials (cluster etc.) Nature of the experimentally observed ferromagnetic signals Intrinsic (Carrier-mediated) ferromagnetism K. Ando, Science 312, 1883 (2006) Our message on (Zn,Mn)O It is possible to have intrinsic ferromagnetism in (Zn,Mn)O. We predict that zincblende structure (stable in thin film) is better than wurtzite structure (most common phase) in terms of FM. Gu, Bulut, Maeawa, J. Appl. Phys. 104, 103906 (2008).

Case 2: New generation 111-type DMS (Ga3+, Mn2+)As 111-type Li1+x(Zn2+, Mn2+)As Li1+x(Zn2+, Mn2+)P Hole & Spin Hole/Electron Spin Independently dope charge and spin ! Experiments on 111-type DMS Li(Zn,Mn)As Tc ~ 50 K, P-type, Band gap = 1.61 eV Z. Deng et al, Nat. Commun. 2, 422 (2011) Li(Zn,Mn)P Tc ~ 36 K, P-type, Band gap ~ 2 eV Z. Deng et al, PRB 88, 081203(R) (2013).

Ferromagnetic (FM) correlations in Li(Zn,Mn)P Impurity bound state (IBS). Impurity-Impurity FM coupling Occupation number (Mn) ~ 0 eV IBS Chemical potential μ (eV) Mn-Mn distance (alattice) Reasonable p-type μ FM in p-type DMS VB CB μ p-type μ

How to obtain n-type DMS ? DMS with wide band gap FM in p-type DMS IBS: impurity bound state VB CB μ p-type μ n-type μ Our idea: DMS with narrow band gap FM in p & n-type DMS VB CB μ p-type μ n-type μ

Outline 1. Introduction on diluted magnetic semiconductor (DMS) 2. DMS with wide band gap 3. DMS with narrow band gap

FM in n-type (electron) Good materials to check our idea: 　 122-type DMS: Mn-doped BaZn2As2 FM in p-type (hole) FM in n-type (electron) (Ba2+,K+)(Zn,Mn)2As2　　　　　Tc ~ 230 K K. Zhao et al, Nat. Commun. (2013) ; Chin. Sci. Bull. (2014). Ba(Zn,Mn,Co)2As2 Tc ~ 80 K H. Man et al, arXiv.1403.4019 (2014) BaZn2As2 : Gap = 0.2 eV  Mn-doped BaZn2As2 and BaZn2Sb2 ! BaZn2As2　(I4/mmm) Gap = 0.2 eV BaZn2Sb2　(Pnma) Gap = 0.2 eV

Density of state (Mn-3d) Case 1: Ba(Zn,Mn)2As2 VB CB Density of state (Mn-3d) IBS IBS IBS Chemical potential μ (eV) ARPES: H. Suzuki et al, PRB 92, 235120 (2015). p-type μ n-type μ

Ferromagnetic (FM) correlation in Ba(Zn,Mn)2As2 P-type Long-range FM (larger <M1M2>) Tc (exp) ~ 230 K N-type Long-range FM (smaller <M1M2>) Tc (exp) ~ 80 K Mn-Mn distance (Å) Gu and Maekawa, PRB 94, 155202 (2016)

A simple estimation of exchange coupling J12 by <M1M2> Ferromagnetic Antiferromagnetic Gu, Ziman, Maekawa, PRB 79, 024407 (2009)

Case 2: Ferromagnetic (FM) correlation in Ba(Zn,Mn)2Sb2 P-type Long-range FM Mn-Mn distance (Å) p-type FM ＜M1M2＞ (2nd n.n.) FM range Tc Mn in BaZn2As2 ~ 0.08 ~ 6 Å 230 K (Zhao, 2014) Mn in BaZn2Sb2 ~ 0.14 ~ 10 Å > 230 K (expected) Gu and Maekawa, PRB 94, 155202 (2016)

Case 3: Cr vs. Mn impurities in BaZn2As2 VB CB Impurity bound state (IBS) Cr: bottom of CB Mn: top of VB Density of state (3d) IBS(Mn) IBS(Cr) N-type FM: Cr : more promising Chemical potential μ (eV) Impurity level : Ed (Mn2+: d5) < Ed (Cr2+: d4) < EF  IBS(Mn) < IBS (Cr) AIP Advances 7, 055805 (2017).

Summary of diluted magnetic semiconductor (DMS) FM in p-type DMS Found ferromagnetic (FM) correlations in some DMSs with wide band gap. Clarify the controversial experiments. VB CB (Zn,Mn)O, Mg(O,N) Contributed to a new promising direction IBS: impurity bound state Li(Zn,Mn)As, Li(Zn,Mn)P p-type μ n-type μ Proposed a way to realize p- and n-type DMS: Narrow band gap. FM in p & n-type DMS Consistent with exp. in Ba(Zn,Mn)2As2 VB CB Predict p- and n-type FM in Ba(Zn,Cr)2As2 Predict high Tc (> 230K) in Ba(Zn,Mn)2Sb2 p-type μ n-type μ

Outlook & Future Conduction electrons Localized electrons Electron correlations Band structure Density functional theory (DFT) Quantum Monte Carlo (QMC) DFT + QMC method: Useful in a wide range of materials !

Acknowledgments (Diluted Magnetic Semiconductor) Theories: S. Maekawa (ASRC, JAEA) N. Bulut (Izmir Inst. Tec.): QMC, (Zn,Mn)O T. Ziman (Inst. Laue Langevin): Mg(O,N) J. Ohe (Toho U): (Ga,Mn)As Experiments: F. L. Ning（宁凡龙）(Zhejiang U)（浙大）: n-type Ba(Zn,Mn)2As2 C. Q. Jin （靳常青）(IOP, CAS)（物理所）: Li(Zn,Mn)As, Ba(Zn,Mn)2As2 A. Fujimori (U Tokyo): ARPES Y. J. Uemura (Columbia U): muSR, organization

Thank you very much !