Tuning magnetic anisotropy, Kondo screening and Dzyaloshinskii-Moriya interaction in pairs of Fe adatoms Department of Physics, Hamburg University, Hamburg, Germany SPICE Workshop, Mainz
Outline Tune magnetic properties of single Fe Hund s impurities by Hydrogenation A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Trace exchange interactions in FeH 2 -Fe pairs A. A. Khajetoorians, M. Steinbrecher et al., submitted (2015) Ho 4f adatoms as single quantum bits? M. Steinbrecher et al., Nat. Comm., submitted (2015) Outline 2
charge fluctuations What is a Hund s impurity? L. Huang et al., arxiv:1402.3181v1; A. Georges et al., Annu. Rev. Condens. Matter Phys. 4, 137 (2013) Different energetic contributions create various electronic states five orbital Anderson model description Hund s rules: How to arrange e - in orbitals Competition: CF+SO coupling, Hund s exchange, Kondo screening Hybridization V dk : U coulomb small charge fluctuations A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 3
What is a Hund s impurity? L. Huang et al., arxiv:1402.3181v1; A. Georges et al., Annu. Rev. Condens. Matter Phys. 4, 137 (2013) 1. Mixed valency: weak hybridization no (half-) integer spin J 5/2 2. Non-zero, well-defined magnetic moment: stabilized by intra-atomic Hund s exchange strong hybridization (U coulomb < V dk < J Hund ) Hund s impurity explains physics in TMOs, Fe pnictides, etc. Tune magnetic properties of Fe Hund s impurities 4
Fe adatoms on Pt(111) - Hydrogenation A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 5
Fe adatoms on Pt(111) - Hydrogenation Cold deposition ( 4 K) of single Fe atoms ( 1/100 ML) on Pt(111) 2 adsorption sites: Fe fcc, Fe hcp A. A. Khajetoorians et al., Phys. Rev. Lett. 111, 157204 (2013) Hydrogenation (2 species per site): Fe fcc H, Fe hcp H, Fe fcc H 2, Fe hcp H 2 fcc Fe (100 pm) FeH (170 pm) Tip induced 500 mv pulse similar: F. D. Natterer et al., Surf. Sci. 615, 80-87 (2013) F. Donati et al., Phys. Rev. Lett. 111, 236801 (2013) D. Serrate et al., J. Phys. Chem. C 118, 5827 (2014) Q. Dubout et al., Phys. Rev. Lett. 114, 106807 (2015) 5 nm hcp Tune magnetic properties of Fe Hund s impurities FeH 2 (100 (180 pm) pm, 1%) Remove H x with STM tip V = -100mV, I = 1nA A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) 6
Ab initio calculation: Fe magnetic moments fully relativistic KKR, including 62 Pt atoms, spin and orbital momenta (S. Lounis et al., FZ Jülich) A. A. Khajetoorians et al., Phys. Rev. Lett. 111, 157204 (2013) fcc hcp m S (in µ B ) 4.42 4.57 m L (in µ B ) 0.23 0.22 m tot (in µ B ) 4.65 4.79 J = m total gµ B 5 2 H = g J μ B B J + DJ z 2 approximate value used for effective spin hamiltonian crystal field theory for C 3v symmetry e.g.: T. Schuh et al., PRB 84, 104401 (2011) FeH n -complexes: (A. Lichtenstein et al.) magnetic moment nearly unchanged Tune magnetic properties of Fe Hund s impurities 7
Quantum Monte Carlo calculations Using ab inition calculated hybridization functions (A. Lichtenstein et al., Uni HH) l 1. Metallic behaviour of 3d occupancy 2. Finite scattering rate at lowest Matsubara frequency nonzero magn. moment clean Fe Fe-H, Fe-H 2 all 6 complexes are Hund s impurities! A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 8
Effect of Hydrogenation Perform Inelastic Scanning Tunneling Spectroscopy (ISTS) @ 0.3 K Hydrogen affects magnetic properties Q. Dubout et al., Phys. Rev. Lett. 114, 106807 (2015) Kondo state for Fe hcp H 2 fcc hcp FeH 2 FeH Fe 0.75 mev 0.19 mev A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 9
Anisotropy D more negative stronger easy axis Effect of Hydrogenation - fcc fcc: easy axis out-of-plane J z = ± 5/2 ground state A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 10
Anisotropy D decreases Effect of Hydrogenation - hcp hcp: easy-plane J z = ± 1/2 ground state A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 11
Multi-orbital Kondo effect Temperature & B dependent measurements characterize Kondo Wilson s definition of T K : H. Prüser et al., Nature Physics 7, 1745 (2011) T K = 0.27Γ k B 2.8 K (T exp = 0.3 K) k B T K 230 µev > Δ 1 2 3 2 100 µev Δ: Peak splitting spin ½ Various deviations from spin ½ Kondo theories multi orbital Kondo effect Test system to develop advanced theories Use sensitive Kondo resonance as a sensor! A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Tune magnetic properties of Fe Hund s impurities 12
Tune magnetic properties of single Fe Hund s impurities by Hydrogenation A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Trace exchange interactions in FeH 2 -Fe pairs A. A. Khajetoorians, M. Steinbrecher et al., submitted (2015) Use the very sensitive Kondo resonance as a sensor! Ho 4f adatoms as single quantum bits? M. Steinbrecher et al., Nat. Comm., submitted (2015) Outline 13
0 height (pm) 220 Effect of magnetic coupling to spectral signature Place an Fe atom next to a Kondo impurity Kondo peak shows splitting Excitation shows shift Offset: artificial Fe hcp H 2 Fe hcp 8.31 Å 1 nm Splitting 500 µev V stab = 6 mv // I stab = 3 na // V mod = 40 µa Trace exchange interactions in FeH2-Fe pairs 14
Distance dependent Kondo splitting V stab = 6 mv // I stab = 3 na // V mod = 40 µa Trace exchange interactions in FeH2-Fe pairs 15
Substrate mediated contributions to Kondo splitting Simulate experimental results (in B-field) with local spin Hamiltonian H = H Zeeman + H Ani + H J + H D = g 1 μ B B S 1 + g 2 μ B B S 2 + K 1 S 2 2 1,z + K 2 S 2,z JS 1 S 2 + D (S 1 S 2 ) 1. Anisotropy (lowest order, K) 2. Isotropic exchange coupling (J) 3. Dzyaloshinskii Moriya interaction (D) only D important! Figure inspired by: A. Fert, V. Cros and J. Sampaio, Nat. Nanotechn. 8, 152 (2013) Use 3rd order perturbation theory to model coupling to substrate/tip e- including anisotropy and RKKY exchange terms (M. Ternes) M. Ternes, New J. of Phys., 17, 063016 (2015) Approximation: use g 1, g 2, K 1, K 2 from fitting spectra of uncoupled Fe hcp H 2 /Fe hcp change J and D to fit pair spectra Trace exchange interactions in FeH2-Fe pairs 16
Model experimental data, d = 8.31 Å D = 0.0 mev Dzyaloshinskii-Moria interaction needed for description ± 5/2, ± 3/2, ± 1/2 Trace exchange interactions in FeH2-Fe pairs 17
normalized Distance dependency of J and DMI ISTS experiments in B-field + simulations in model ab-initio calculation using KKR Greens function approach by M. Bouhassoune, S. Lounis (hcp-hcp pairs only) D = D 2 + D 2 + D z 2 D plane = D 2 + D 2 Trace exchange interactions in FeH2-Fe pairs 18
Distance dependent chirality positive chirality D z not always negligible sign(d ) changes chirality (sense of rotation) D z negative chirality D Fehcp Fe hcp H 2 D Spin expectation values show different rotation of spins chirality Trace exchange interactions in FeH2-Fe pairs 19
Tune magnetic properties of single Fe Hund s impurities by Hydrogenation A. A. Khajetoorians,, M. Steinbrecher et al., Nat. Nanotech., accepted (2015) Trace exchange interactions in FeH 2 -Fe pairs A. A. Khajetoorians, M. Steinbrecher et al., submitted (2015) Ho 4f adatoms as single quantum bits? M. Steinbrecher et al., Nat. Comm., submitted (2015) Outline 20
Localized 4f orbital decouples spin? Ultimate goal: Single spins for data storage and processing devices! Use rare earth atoms with strongly localized 4f states A. J. Freeman and R. E. Watson, Phys. Rev. 127, 2058 (1965) A. A. Khajetoorians, et al., Science 339, 552 (2013) Ho/Pt(111), STM stable magn. state of single atom Ho/Pt(111), XMCD magnetic, different ground state T. Miyamachi et al., Nature 503, 242 (2013) F. Donati et al., PRL 113, 237201 (2014) Ho 4f adatoms as single quantum bits? 21
Spin-excitation: expectations 2 different magnetic ground states reported: T. Miyamachi et al.: J z = 8 DFT F. Donati et al.: J z = 6 XMCD + Multiplet calculations Spin-excitations: - fcc: 8 mev - hcp: 5 mev Zero-Field Splitting 3.9 mev Symmetry protection of ground state T. Miyamachi et al., Nature 503, 242-246 (2013) F. Donati et al., PRL 113, 237201 (2014) Ho 4f adatoms as single quantum bits? 22
ISTS of Fe and Ho ISTS on arranged Ho & Fe atoms with the same tip Fe system well known Ho fcc Fe fcc Fe hcp step energy 0.75 mev 0.19 mev step intensity 8 % 12 % Fe hcp Fe fcc Ho hcp A. A. Khajetoorians et al., PRL 111, 157204 (2013) I stab = 5 na // V stab = 10 mv // V mod = 40 µv // f mod = 4.142 khz Does it interact magnetically? Ho 4f adatoms as single quantum bits? 23
RKKY coupled pairs of Ho & Fe atoms Manipulate pairs of Ho & Fe atoms with various distances 4.24 Å 5.55 Å 13.9 Å Ho only affects Fe in direct exchange regime Only excitation intensity is changed, not its energy Ho 4f adatoms as single quantum bits? 24
SP-STM measurements Try to find switching of magnetic ground state on Ho atoms Build Fe 3 -cluster as a reference Get spin-polarized tip Fe 5 /Cu(111): A. A. Khajetoorians et al., Science 339, 55 (2013) U = 5 mv, I = 1 na, B = 0.2 T Fe 3 200 pm Ho 1 nm 180 pm 1 nm C. Hübner et al., PR B 90, 155134 (2014) No magnetization switching observed on Ho atom (with various parameters) paramagentic behaviour, or too weak spin-polarization Ho 4f adatoms as single quantum bits? 25
Summary Fe atoms on Pt(111) are Hund s impurities H-doping changes anisotropy and leads to complex multi-orbital Kondo effect Heisenberg and Dzyaloshinskii-Moriya interactions tuned in FeH 2 -Fe pairs DMI induces non-collinear ground states (chirality) and drives a system faster into classical behaviour Single Ho atoms show no magnetic signs in STM No single atom quantum bit! Summary 26
Acknowledgements Experiments: Uni Hamburg (*former group member) Model: MPI Stuttgart Dr. A. Sonntag Jan Hermenau Dr. T. Schlenk* Prof. A. A. Khajetoorians* Dr. J. Wiebe Prof. R. Wiesendanger Dr. M. Ternes Theory: Uni Hamburg Uni Bremen FZ Jülich M. Valentyuk Prof. Dr. A. I. Lichtenstein Prof. Dr. T. O. Wehling Dr. M. Bouhassoune Dr. M. dos Santos Dias Dr. S. Lounis Acknowledgements 27