Beyond ferromagnetic spintronics: antiferromagnetic I-Mn-V semiconductors Tomas Jungwirth Institute of Physics in Prague & University of Nottingham

Kvantová relativistická fyzika Spintronics ← relativistic quantum physics

Kvantová relativistická fyzika Spintronics ← relativistic quantum physics

Kvantová relativistická fyzika Spintronics ← relativistic quantum physics

} } Ultra-relativistic particles with spin (neutrino): Spin-orbit coupling Weaker but also present in electrons in solids

Electron has spin & charge → magnetic moment Collective behavior of spins due to Coulomb interaction → magnetism Provides sensitivity to weak external fields & yields strong electrical signals

Electron has spin & charge → magnetic moment Collective behavior of spins due to Coulomb interaction → magnetism Provides sensitivity to weak external fields & yields strong electrical signals

... and memory Electron has spin & charge → magnetic moment Collective behavior of spins due to Coulomb interaction → magnetism Provides sensitivity to weak external fields & yields strong electrical signals

Lord Kelvin 1857 First spintronic devices Poor scalability to small dimensions & small MR (subtle spin-orbit origin) Current spintrnic devices Interface effect → nanoscale in nature & large MR (robust ferromagnetic origin) Fert, Grünberg et al Bulk AMRTMR (GMR) Spintronic magnetoresistance effects in metals HDD read-head sensors Magnetic RAM

Towards semiconductor spintronics FM semiconductors Ohno et al. Science’98, Dietl et al PRB’00, Jungwirth, MacDonald et al PRB’99 Archetypical material (Ga,Mn)As: favorable FM and spin-orbit coupled bands & semiconductor nano-fabrication → revived interest in spin-orbit phenomena like AMR in nanostructures

Huge (~1000%) AMR-type effects in (Ga,Mn)As nanostructures Wunderlich, Irvine, Jungwirth et al. PRL’06, Schlapps, Weiss et al. PRB’09 Electrical control of spintronics B (T) → rotating m → V G1 V G2 Positive & negative MR Spintronic control of electronics m1m1 → m2m2 → p-type & n-type transistor (m)(m) →

(Ga,Mn)As...FM at huge dopings > 1% (> cm -3 ) → more of a low-density metallic alloy T c below room-T (  190K) Novák, Jungwirth et al. PRL ’08 Limitations of ferromagnetic semiconductor (Ga,Mn)As TcTc Well behaved Itinerant ferromagnet but...

Shick, Jungwirth et al. ‘06 Wunderlich, Jungwirth, Shick et al. ’06 Bernand-Mantel, Fert et al. ‘09 Theory predictions Confirmed by experiments Gao, Tsumbal, Parkin et al. ’07 Park,Wunderlich, Jungwirth et al. 08 AMR-type effects predicted and observed in high-T c FM metal nanostructures cobalt Pt AlO x Pt/Co

spontaneous moment spin-orbit coupling FMAFM Shick, Wunderlich, Jungwirth, et al., PRB‘10 Magnetic and magneto-transport anisotropy effects present in AFMs with spin-orbit equally well as in FMs Maximizing the anisotropy phenomena in metals → spintronics in the AFMs AFM metal MnIr

Much easier to realize strong AFM-SC than FM-SC Can AFMs resolve the problem of high-T SEMICONDUCTOR spintronics? Jungwirth, Novak, et al., preprint ‘10 E Fermi E gap E exchange E exchange competing with E gap in FM-SCs No E exchange competing with E gap in AFM-SCs Strong FM exchange spitting turns the system into metal

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs Si 2 group-IV Si per elementary cell → 8 (sp) valence electrons

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. IV: no magnetic SC analogue Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII Si 1 proton transfer IVIII-V IV: no magnetic SC analogue Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. Magnetic SCs derived from common 8-valence non-magnetic SCs III-V: FeAs – SC, AFM T N =77K GdN – SC, FM T c =72K (Ga,Mn)As – low-density metal, FM T c <190K Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII IV: no magnetic SC analogue Lower moment Fe (Gd) less favorable than high moment Mn → II-VI intrinsic magnetic SCs

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. Magnetic SCs derived from common 8-valence non-magnetic SCs Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII IV: no magnetic SC analogue III-V: FeAs – SC, AFM T N =77K GdN – SC, FM T c =72K (Ga,Mn)As – low-density metal, FM T c <190K

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. II-VI: MnO, MnS, MnSe, MnTe - SC, AFM T N ~ K EuO, EuS – SC, FM T c <70K EuSe, EuTe - SC, AFM T N <10K All III-V and II-VI magnetic SCs have low transition-T Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII IV: no magnetic SC analogue Larger more ionic bonds weaken magnetic interactions in II-V‘s Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs Can we make high moment (Mn) and smaller lattice (pnictides) intrinsic SC? III-V: FeAs – SC, AFM T N =77K GdN – SC, FM T c =72K (Ga,Mn)As – low-density metal, FM T c <190K

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. I (AM) Li, Na,.. (TM) Cu, Ag,.. Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. I (AM) Li, Na,.. (TM) Cu, Ag,.. I-II-V: LiMnAs, NaMnAs, LiMnP, LiMnSb... - AFM T N >> room T Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII Bronger et al., Z. among. allg. Chem. ’86 Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. I (AM) Li, Na,.. (TM) Cu, Ag,.. Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII III-VI-II-V Twin SCs I-Mn-V Bronger et al., Z. among. allg. Chem. ’86 Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs I-II-V: LiMnAs, NaMnAs, LiMnP, LiMnSb... - AFM T N >> room T

II Zn, Cd,.. III Al, Ga,.. IV Si, Ge,.. V (pnictides) N, P, As,.. VI (chalcogenides) O, S, Se, Te,.. I (AM) Li, Na,.. (TM) Cu, Ag,.. Mn (d 5 s 2 ) Fe Eu (f 7 s 2 )Gd IIIII I-Mn-V No report on electronic structure of AFM I-Mn-V: Are they SCs? No report on MBE growth of group-I compounds: Can they be grown as single-crystal epilayers? Bronger et al., Z. among. allg. Chem. ’86 Magnetic (FM & AFM) SCs derived from common 8-valence non-magnetic SCs I-II-V: LiMnAs, NaMnAs, LiMnP, LiMnSb... - AFM T N >> room T

InAs LiMnAs MBE growth of I-Mn-V: LiMnAs on nearly lattice matched InAs 4.27A 4.28A

[110][-110] LiMnAs MnAs growth drection log(intensity) x (  m) profile (nm) wavelength (nm) LiMnAs InAs cap substrate LiMnAs In situ RHEEDIn situ optical reflectivity Ex situ profile Sharp 2D cubic single-crystal growth... poor growth of control umatched MnAs Fabry-Perot oscillations → semiconductor

log(intensity) X-ray diffraction All LiMnAs crystal peaks observed Fully tensile strained on InAs (0.2% increase of LiMnAs volume) InAs LiMnAs 4.27A 4.28A

Expected 45 o rotation of LiMnAs with respect to the InAs substrate InAs LiMnAs [110] LiMnAs InAs[100] X-ray diffraction

M (10 4 emu) H (T) MnAs Mn S=5/2 LiMnAs energy (meV) I T /I 0 InAs Li:InAs LiMnAs MnAs temperature (K) M rem (10 4 emu) LiMnAs MnAs Ex situ optical transmission Squid magnetization Transparent at least up to InAs band-gap Consistent with in situ Febry-Perot oscillations and compare with non-transparent metal MnAs Magnetization consistent with compensated AFM moments in LiMnAs upto studied 400K Compare with FM MnAs with same amount of Mn

Ab initio theory Stoichiometric I-Mn-V are strong AFMs & intrinsic semiconductors

Magnetic and correlated Mn d-states mixed near band gap → low √  (refractive index), strong and gatable magnetic anisotropy effects LDA

AFM semiconductors for spintronics AFM 1. Electrically gatable magnetic and magneto-transport anisotropy effects Feasible to rotate magnetic easy-axis electrically in high-doped (Ga,Mn)As → should be much more accessible in intrinsic SCs I-Mn-V FM

AFM semiconductors for spintronics 2. Exchange-biasing AFM with embeded conventional semiconductor devices Fixed by exchange- biasing AFM Transistor directly in the AFM layer Opto-electronics directly in the AFM layer Discrete spintronic and transistor elements in current MRAM

FM SCs (GaMnAs) favorable model spintronic systems but low transition T AFM I-Mn-V compounds: - Simplest magnetic counterparts to conventional SCs with high transition T - We showed that they are semiconductors and that the group-I alkali metal compounds can be grown by MBE as high quality single-crystal epilayers - Admixture of magnetic d-states yields unconventional SC properties and theory predicts very strong and gatable spintronic responses Conclusions Prospect for high-T semiconductor spintronics but first sytematic materials research needs to be completed

University of Nottingham Tom Foxon, Richard Campion, Bryan Gallagher, et al. Hitachi & Cavendish Laboratories at Cambridge Jorg Wunderlich, Andrew Irvine et al. Institute of Physics ASCR, Prague Vít Novák, Miroslav Cukr, Jan Mašek, Alexander Shick, František Máca,Petr Kužel, et al. Charles University, Prague Xavi Marti, Petra Horodyská, Václav Holý, Petr Němec, et al. Texas A&M and University of Texas Jairo Sinova, Allan MacDonald, et al.