Magnetic anisotropy in (III,Mn)V


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Magnetic anisotropy in (III,Mn)V

  1. 1. Magnetic anisotropy in (III,Mn)V semiconductors: an FMR study Konrad Dziatkowski Division of Solid State Physics Department of Physics Department of Physics University of Texas at Austin University of Warsaw
  2. 2. Advisors •Prof. Andrzej Twardowski (U. of Warsaw) •Prof. Jacek K. Furdyna (U. of Notre Dame) •Prof. Bernard Clerjaud (U. Pierre et Marie Curie, Paris) Coworkers •Dr. Marta Palczewska, Mariusz Pawłowski (Institute of Electronic Materials Technology, Warsaw) •Dr. Tomasz Słupiński (U. of Warsaw) •Dr. Xinyu Liu, Zhiguo Ge, Weng Lee-Lim, Ben Fehrman (U. of Notre Dame) •Prof. Adam Barcz (Institute of Electron Technology, Warsaw) •Dr. Rafał Jakiela (Institute of Physics PAS, Warsaw) Supporting institutions •U.S. Department of State •Foundation for Polish Science •(polish) Ministry of Science and Higher Education
  3. 3. University of Warsaw •established in 1816; then: about 800 of students and 40-50 of faculty •now: about 56,000 of students and 3,000 of faculty
  4. 4. Department of Physics UW •physics was present at UW from the very beginning in 1816; established as an independent department in 1969 •now: about 700 of students and about 200 of faculty •physics of particles, nuclear physics, optics, solid state physics, biophysics, medical physics, relativity and gravitation, geophysics, astronomy, ...
  5. 5. Division of Solid State Physics •about 12 of students and about 25 of faculty •physics of semiconductors (III-V, II-VI, oxides, DMS), superconductors, graphene, polymers, ... •(magneto)optics in FIR-NIR-VIS-UV, (magneto)transport, magnetometry, hydrostatic and uniaxial pressures, electroreflectance, ... •MOCVD, Czochralski, AMMONO (for nitrides), photolitography, Kelvin Probe Microscopy; characterization by Hall, DLTS, SQUID, electrochemical CV profiling, ...
  6. 6. Spintronics (spin-based electronics) •a field of electronics involving nanoscale devices in which the information is carried/stored with use of the spin of an electron rather than its electric charge antiferromagnet (AFM) pinned ferromagnet (FM) nonmagnetic spacer (NM) ferromagnet (FM) SVD - spin valve device MTJ - magnetic tunnel junction •magnitude of passed current is modulated by the relative orientation of the magnetization in ferromagnetic films P. A. Gruenberg, Rev. Mod. Phys. 80, 1531 (2008) A. Fert, Rev. Mod. Phys. 80, 1517 (2008)
  7. 7. Dilute magnetic semiconductors •an alloy of parent, nonmagnetic semiconductor (e.g. GaAs, CdTe) with the atoms of magnetic elements (e.g. Mn, Co) host semiconductor magnetic ions •host materials III-V: GaAs, GaP, GaN, AlAs, AlN, InAs, InN, InP, ... II-VI: CdTe, CdSe, CdS, ZnTe, ZnS, PbTe, ... IV: Ge, Si •magnetic dopants: Mn, Cr, Fe, Co, ...
  8. 8. Ferromagnets among (III,Mn)V alloy Tc Ref. (Ga,Mn)As 173 [1] (In,Mn)As 90 [2] (In,Ga,Mn)As 110 [3] (Ga,Mn)N 8 [4] [1] K. Y. Wang et al., AIP Conf. Proc. 772, 333 (2005) [2] T. Schallenberg and H. Munekata, Appl. Phys. Lett. 89, 042507 (2006) [3] T. Słupiński et al., Appl. Phys. Lett. 80, 1592 (2002) [4] E. Sarigiannidou et al., Phys. Rev. B 74, 041306 (2006) A. H. MacDonald et al., Nature Mater. 4, 195 (2005)
  9. 9. Ferromagnetism of (Ga,Mn)As •exchange interaction between band holes and manganese ions itinerant band holes Jpd localized electrons from of p shell d shell of Mn ions T. Jungwirth et al., Rev. Mod. Phys. 78, 809 (2006) •Jpd < 0 - antiferromagnetic coupling |Jpd| ~ 0.9 - 3.3 ... - 14 eV •effective RKKY-like interaction between manganese ions JMn-Mn ~ Jpd · [sin(2kF r) - 2kF r·cos(2kF r)] / (2kF r)4 2 F. Matsukura et al., Phys. Rev. B 57, R2037 (1998) •ferromagnetic Mn-Mn coupling
  10. 10. Growth of ferromagnetic (Ga,Mn)As •low temperature molecular beam epitaxy formation of MnAs substrate temperature (oC) 300 metallic (Ga,Mn)As 200 insulator roughening 100 policrystalline 0 0.02 0.04 0.06 0.08 concentration of Mn in Ga1-xMnxAs after: H. Ohno, J. Magn. Magn. Mater. 200, 110 (1999) •ion implantation + pulsed layer melting M. A. Scarpulla et al., Appl. Phys. Lett. 82, 1251 (2003)
  11. 11. Growth of ferromagnetic (Ga,Mn)As •substitutional manganese 5 >> electronic configuration: 3d reduction of: >> acceptor >> concentration of holes >> magnetic moment per ion: 5mB >> magnetization >> Curie temperature •interstitial manganese >> double donor (electric compensation) >> antiferromagnetic coupling with substitutional manganese (magnetic compensation) •antistructural defect AsGa >> native for LT-MBE
  12. 12. Growth of ferromagnetic (Ga,Mn)As •TC above 300 K ? S. Mack et al., Appl. Phys. Lett. 92, 192502 (2008) Y. J. Cho et al., J. Appl. Phys. 103, 07D132 (2008)
  13. 13. Motivation •lack of unambiguous description (qualitative and quantitative) for the collective excitations of spin system (ferromagnetic resonance) in dilute ferromagnetic semiconductors •recognition of complex magnetic anisotropy of solitude films and multilayered structure based on (III,Mn)V, identification of various components of the anisotropy and establishing the possibilities for its modifications •understanding of relations joining magnetic anisotropy with interlayer exchange coupling in (III,Mn)V-based SVDs or MTJs antiferromagnet (AFM) pinned ferromagnet (FM) nonmagnetic spacer (NM) ferromagnet (FM) SVD - spin valve device MTJ - magnetic tunnel junction
  14. 14. Materials •solitude films of (Ga,Mn)As and (In,Ga,Mn)As AFM •double layers MnO / (Ga,Mn)As FM •(Ga,Mn)As / GaAs / (Ga,Mn)As structures NM FM growers: T. Słupiński (UW), X. Liu and Z. Ge (U. of Notre Dame) Experimental technique •electron spin resonance (ESR) spectrometer with microwave klystron of X-band (~ 9 GHz) •continuous-flow helium cryostat, T = 4 - 300 K •electromagnet up to 1.4 T
  15. 15. Ferromagnetic resonance (FMR) long range ferromagnetic order + resonance transitions between Zeeman-splitted spin states •ground state: saturation •resonance: collective excitation energy of the entire spin system hw saturation •uniform mode of FMR: coherent magnetic field behavior (”precession”) of all spins w = g×mB×Hrez / h •non-uniform modes (spin waves): P non-trivial spatial dependence of the dP/dH local phase of excitation magnetic field
  16. 16. Anisotropy of FMR (arb. units) anisotropy of FMR ferromagnetic resonance ß ß ß dirt in cryostat anisotropic magnetic properties Hres magnetization [001] magnetic field, easy axis q polar angle H minimum of the resonance field j [100] [010] azimuthal angle
  17. 17. Polar anisotropy [001] (degrees) (degrees) polar q angle H polar angle, (Ga,Mn)As polar angle, 200 nm bufor GaAs 220 nm GaAs (001) magnetic field, SL-A1 magnetic field, magnetic field markers spin wave resonance FMR line K. Dziatkowski et al., Phys. Rev. B 70, 115202 (2004) •both for (Ga,Mn)As/GaAs and (In,Ga,Mn)As/(In,Ga)As/InP the magnetization easy axis is confined in the growth plane
  18. 18. Magnetic interactions vs anisotropy dipol-dipol interaction Þ shape anisotropy in epitaxial (III,Mn)V the crystalline anisotropy dominates over exchange interaction Þ crystalline anisotropy shape anisotropy compressive tensile strain strain X. Liu et al., Phys. Rev. B 67, 205204 (2003) W. L. Lim et al., Phys. Rev. B 74, 045303 (2006) K. Dziatkowski et al., Phys. Rev. B 70, 115202 (2004)
  19. 19. Model of FMR •Laudau-Lifschitz equation: dM / dt = g M ´ H •small deviations of M from equilibrium •harmonic solutions (wres / g)2 = ( Fqq × Fjj - Fqj ) / (M2sin2q) 2 q, j - polar and azimuthal angle of M at equilibrium F = FZeeman + Fshape + Fcrystal •Hres(jH), qH = p/2 Þ parametrized by •Hres(qH), jH = p/4 H4II, H4^, H2II, H2^, geff •Hres(qH), jH = -p/4 J. Smit and H. G. Beljers, Phillips Res. Rep. 10, 113 (1955) M. Farle, Rep. Prog. Phys. 61, 755 (1998)
  20. 20. Polar anisotropy fitting of the model to the experimental data (degrees) (degrees) polar angle, polar angle, magnetic field, magnetic field, magnetic field markers spin wave resonance FMR line polar angle, (degrees)
  21. 21. Azimuthal anisotropy model (arb. units) resonance field, resonance field, resonance field, azimuthal angle, (degrees) azimuthal angle, (degrees) azimuthal angle, (degrees) (In,Ga,Mn)As FCT lattice Þ biaxial anisotropy H 50 nm Þ four-fold symmetry (90o rotation) [100] j (In,Ga)As 100 nm ? ? ? Þ uniaxial anisotropy azimuthal InP (001) Þ two-fold symmetry (180o rotation) angle SL-B2 U. Welp et al., specific reconstruction of (001) GaAs Appl. Phys. Lett. 85, 260 (2004) surface promoting correlated arrangements M. Sawicki et al., of manganese ions along [110] or [1-10] Phys. Rev. B 71, 121302 (2005)
  22. 22. Reorientation of the easy axis of magnetization temperature-induced reorientation of the easy axis of magnetization resonance field, ? different dependence of biaxial azimuthal angle, (degrees) and uniaxial anisotropy constants on magnetization 4.5 Resonance Magnetic Field (kOe) [001] K.-Y. Wang et al., Phys. Rev. Lett. 95, 217204 (2005) [110] two magnetic phases with different 3.0 g=2 anisotropies and different critical [110] temperatures [100] TC K. Hamaya et al., Phys. Rev.Lett. 94, 147203 (2005) 1.5 0 20 40 60 80 Temperature (K)
  23. 23. Other magnetic anisotropies in (III,Mn)V resonance field, polar angle, (degrees) vicinal angle, vicinal GaAs substrate [001] ß anisotropy dependent on the orientation Hrf Hdc of (dynamic) microwave magnetic field speculation !! ß nonlinear response of (Ga,Mn)As [110] on rf magnetic excitation [110]
  24. 24. Exchange bias - discovery and basic idea W. H. Meiklejohn and C. P. Bean, •due to interfacial exchange interaction an Phys. Rev. 102, 1413 (1956) antiferromagnet - which is unaffected by the magnetic field reversal - acts on a ferromagnet as a source of gain or loss of magnetic energy •under field cooling (FC) conditions: the hysteresis loop is shifted out of H = 0 position due to exchange coupling between antiferromagnetic CoO and ferromagnetic Co J. Nogues and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999)
  25. 25. Exchange biasing of (Ga,Mn)As with MnO 40 400 (a) (b) HC HC & HEB (Oe) M (emu/cm ) 3 20 300 HEB 0 200 15 nm -20 30 nm 100 60 nm -40 0 -1.0 -0.5 0.0 0.5 1.0 0 20 40 60 80 H (kOe) dFM (nm) K. Dziatkowski et al., Appl. Phys. Lett. 88, 142513 (2006) •proximity phenomenon •exchange bias in MnO/(Ga,Mn)As reveals no training effect for the alternating magnetic fields up to 9 kOe
  26. 26. Unidirectional anisotropy MnO 15 nm FMR line (Ga,Mn)As (arb. units) 15 - 60 nm dirt in bufor GaAs cryostat ESR line 164 nm GaAs (001) FMR signal, unidirectional anisotropy, magnetic field, interfacial exchange interaction MnO«(Ga,Mn)As ß thickness of (Ga,Mn)As, unidirectional anisotropy, breaking O of 180 -rotation symmetry K. Dziatkowski et al., Acta Phys. Polon. A 110, 319 (2006)
  27. 27. Unidirectional anisotropy anisotropy field, resonance field, unidirectional temperature, temperature, •nonmonotonic temperature dependence of unidirectional anisotropy field Ü interplay between bi- and uniaxial magnetic anisotropies •interfacial exchange coupling in MnO/(Ga,Mn)As relatively robust with respect to temperature
  28. 28. Fully coupled (Ga,Mn)As / GaAs / (Ga,Mn)As (Ga,Mn)As 9 nm GaAs 3 - 12 nm (arb. units) (Ga,Mn)As 14 nm bufor GaAs 137 nm GaAs (001) FMR signal, •for 3nm-thin GaAs spacer the observed anisotropy resembles that observed for a single FM layer magnetic field, •diffusion of manganse from (Ga,Mn)As K. Dziatkowski et al., Acta Phys. Polon. A 112, 227 (2007) into GaAs and the effective thinning of GaAs spacer is a likely cause
  29. 29. Fully coupled (Ga,Mn)As / GaAs / (Ga,Mn)As (Ga,Mn)As 9 nm GaAs 3 - 12 nm resonance field, resonance field, (Ga,Mn)As 14 nm bufor GaAs 137 nm GaAs (001) polar angle, (degrees) polar angle, (degrees) uniform mode of FMR spin wave mode •other authors point out the possible redistribution of the hole wave function resulting in the electronic coupling of two (Ga,Mn)As layers Z. Ge et al., Appl. Phys. Lett. 91, 152109 (2007)
  30. 30. Acoustic and optic modes of FMR acoustic Hrf optic mode mode H (arb. units) dirt in •for GaAs spacer of ³ 6nm thickness: two FMR cryostat FMR signal, lines corresponding to different collective (magnetic) excitations of the entire sample FMR lines magnetic field,
  31. 31. Interlayer exchange coupling vs anisotropy |JIEC| ~ 10 erg/cm optic mode –4 2 (weak) resonance field, separation of modes, acoustic mode (strong) acoustic mode (strong) optic mode (weak) GaAs spacer thickness, GaAs spacer thickness, •separation and ordering of two FMR lines depend on the quantitative balance of magnetocrystalline anisotropy energy and interlayer exchange coupling
  32. 32. Summary •unambiguous qualitative and quantitative description for the anisotropic FMR in (III,Mn)V semiconductors •peculiarities of magnetic anisotropy in solitude layers and heterostructures made of (III,Mn)V, identification of various components of magnetic anisotropy (biaxial, uniaxial, demagnetization, step-induced, dynamic, unidirectional, ...) •dominating role of crystalline anisotropy •reorientation of the magnetization easy axis promoted by an interplay between bi- and uniaxial (magneto)crystalline anisotropies •robust proximity effects - exchange bias and unidirectional anisotropy - in the exchange coupled MnO/(Ga,Mn)As system •influence of bi- and uniaxial anisotropies competition on unidirectional anisotropy •full coupling or acoustic/optic modes of FMR in (Ga,Mn)As/GaAs/(Ga,Mn)As •quantitative relation joining magnetic anisotropy with interlayer exchange coupling in (Ga,Mn)As-based FM/NM/FM trilayers