Probing and controlling spin dynamics with THz pulses Spin Charge THz pulse Tobias Kampfrath Freie Universität Berlin and FHI/Max Planck Society
PhD students: organize your own symposium … at the 2019 Spring Meeting of the German Physical Society in Regensburg! ƒ Your chance to implement a symposium you always wanted to attend ƒ To get in personal contact with leading scientists at an early stage of your career How? ƒ Pick 1-5 colleagues as organization committee and fill out the online application http://www.dpg-physik.de/dpg/gliederung/junge/profil/ateam/wissenschaftlich/tagungen/2019/phd-symposien/announcement.html ƒ Timely topic related to magnetism ƒ Invite speakers, compile the program, organize the day ƒ Deadline: October 15
Three elementary spin operations 1. Turn spins around 2. Transport spins 3. Detect spin dynamics Goal: reach speed of other information carriers, i.e. THz bandwidth Light in fibers: >10 Tbit/s Electrons in a FET: ~1 THz cut-off How to manipulate magnetic order ultrafast? Two approaches
Spintronics and femtomagnetism Spintronics: voltages in circuits Femtomagnetism: fs light fields ~ ~ ƒ Freq. ~400 THz ⇑ Need rectification ƒ Bandwidth <10 GHz ƒ Force/torque × applied field ƒ Force/torque × light intensity See e.g. Magnetism Roadmap (2017) Kirilyuk, Kimel, Rasing, Rev. Mod. Phys. (2010) DC 0.01 THz 100 THz 1000 THz Terahertz gap 1…30 THz THz fields + magnetism = useful? 4…120 meV
Why THz magnetism? 1) Reveal speed and initial elementary steps of spintronic effects E.g. spin-Hall, spin-Seebeck and GMR 2) New physics, new methods 3) Reward for THz technology as THz coincides with many e.g. THz sources and modulators fundamental modes for spectroscopy and imaging + + - - Magnons Phonons - - Bound electron states: ƒ Cooper pairs Hillenbrand et al. , Nano Lett. (2008) ƒ Excitons 1 THz ≙ 4 meV Intraband transport How to get THz pulses?
Intense THz pulses by optical rectification Rectified part of electron displacement � � � ∝ � � � �� � � � - ~400 THz ~0.5…50 THz Nonlinear- optical Femtosecond THz pulse crystal pulse Nonlinear electron displacement Linear electron displacement How to detect � � the THz pulse? � � ∝ � �� ∝ 2 nd harmonic + { envelope { 2 Reviews: Hoffmann, Fülöp, J. Phys. D (2011); Reimann, Rep. Prog. Phys. (2007)
THz detection: electro-optic sampling Delay � Sampling THz pulse field � ��� Nonlinear- optical Wu, Zhang, crystal APL (1995) Electrooptic effect : Change in refractive index ∝ � ��� ⇑ Crystal becomes birefringent Scan ellipticity of sampling pulse vs � ⇑ Get THz electric field � ��� ( � ) A typical THz pulse…
Example of an ultrashort THz pulse 0.2 ZnTe ƒ Duration down to 50 fs ƒ Tunable center frequency 0.5…50 THz, i.e. 2…200 meV ƒ Peak fields up to ~30 MV/cm (~10 T) ƒ But: gaps between 5 and 15 THz ƒ Detection of full transient field , threshold down to 1 V/m How to control magnetic order by THz fields? Consider equation of motion of spins
How can one control spin dynamics? Total effective Equation of � � � � = −�� � � � × � ���� field acting motion of spin � on spin � � � ���� = � ��� + � ����� + � � In Heisenberg- � × � � + � � �� � � type magnet ��� Zeeman coupling to SO coupling to Exchange external field + other spins total electric field coupling � ���� is the handle to (ultrafast) control over magnetic order ƒ Directly by external fields � ��� , � ��� ( ↑ Kim) ƒ Indirectly by modulation of coupling parameters (e.g. � �� ) using light, currents, strain, heat, … ( ↑ Kirilyuk, Kalashnikova) Start simple: Zeeman torque
How to control spins as fast as possible? Most natural stimulus: magnetic field � � � ��� �(� ) THz �(� ) pulse � � ∝ � × � Zeeman torque
How to control spins as fast as possible? Most natural stimulus: magnetic field � � � ��� Most efficient coupling on resonance Larmor frequency ℏω � = �� � |� ��� | � Ferromagnets Antiferromagnets ƒ ω � determined by anisotropy field ƒ Exchange causes additional repulsion ƒ ω � /2 � ≪ 1 THz ƒ ω � /2 � ∼ 1 THz ⇑ Conduct a THz-pump magnetooptic-probe experiment
THz magnetic pump – infrared probe 2� /| � | Detect Faraday rotation Sample: antiferromagnetic NiO ∝ � ����� ⋅ � � ƒ Neel temperature 523 K ƒ Magnon ( � = 0 ) at 1 THz In the lab…
Simplistic THz setup in the lab Pump beam: generates the THz beam THz emitter Si Probe Parabolic mirror beam Sample To detection of Faraday rotation
THz-induced magnon oscillation Incident magnetic pulse
THz-induced magnon oscillation Incident magnetic pulse Faraday response ⇑ Signature of � = 0 magnon at 1 THz Oscillation at 1 THz, decay time ~40 ps Driven by electric or magnetic field component?
The magnon is driven by the magnetic field Observation: Induced magnetization × driving field Is � = �� possible? ƒ NiO is centrosymmetric ⇑ � = 0 Driving force is magnetic ƒ No linear magnetoelectric effect (not electric) field in centrosymmetric NiO Idea: use double pulses to control magnon amplitude
Coherent spin control with THz pulse pairs 1 2 Second pulse after 6 cycles: amplifies magnon 1 2 Second pulse after 6.5 cycles: switches magnon off THz spin control is feasible by the simple Zeeman torque of THz magnetic pulses Kampfrath, Sell, Fiebig, Wolf, Huber et al. , Nature Phot. (2011) Baierl, Kampfrath, Huber et al. , PRL (2016) Interesting application: THz magnon spectroscopy
THz magnon spectroscopy ƒ Dynamics of � ���� � ��� following ƒ Characterization of antiferromagnets optical excitation ƒ Magnons probe � ���� � ��� Nishitani, Hangyo et al. , APL (2010), PRB (2012) Bowlan, Prasankumar et al. , PRB (2016) Kanda, Kuwata-Gonokami et al. , Nature Comm. (2012) Mikhaylovskiy, Kimel et al. , Nature Comm. (2015) ƒ Not easy with non-optical methods ƒ Many more opportunities with stronger THz fields: probe spin couplings
Reveal elementary spin couplings 3 MV/cm Spin-electron coupling 1 T ƒ Mikhaylovskiy et al. , Nature Phot. (2016) ƒ Bonetti, Dürr et al. , PRL (2016) Electron orbits Electron spins Magnon-magnon coupling ƒ Mukai, Hirori, Tanaka et al. , New J Phys (2016) Ionic ƒ Bocklage et al. , lattice PRL (2015) ƒ Lu, Suemoto, Nelson et al. , PRL (2017) Spin-phonon coupling highly unexplored at >1 THz How to probe coupling of spins and phonons?
Probing spin-phonon coupling Use an insulator ⇑ Electron orbital excitations are frozen out Faraday probe: measures magnetic state Optical phonon pump Observation of new coherent coupling channels ƒ Kubacka, Johnson, Staub et al. , Science (2014) ƒ Nova, Cavalleri et al. , Nature Phys. (2016) How fast is spin-lattice equilibration? ⇑ Study model magnet YIG
Spin-lattice equilibration in YIG THz phonon pump Many open questions, e.g.: Time scale and mechanism of spin-phonon equilibration unknown ~1 ps ~250 ps ~1 µs Rezende et al. , Schreier et al. , Xiao et al. , JMMM (2016) PRB (2013) PRB (2010) Relevant for ƒ Magnetization switching ƒ Spin Seebeck effect Sample: ferrimagnet YIG Experiment ƒ Has two spin sublattices (a and d) ƒ Excite Fe-O lattice vibrations ƒ Band gap of 2.8 eV ƒ Probe spin dynamics from ƒ Magnonic model material femtoseconds to microseconds
THz lattice pump–magnetooptical probe B ext Detect Faraday rotation ( ° 100 mT) � = � � � � + � � � � � Krumme et al. , Thin Solid Films (1984) Pump on and off the phonon resonances
Phonon-driven magnetization dynamics Transient Faraday rotation (%) On- Absorption (arb. units) 0 resonant Off-resonant Off- resonant On-resonant -1 Phonons 0 20 25 30 35 -1 0 1 2 3 4 5 6 7 Pump-probe delay (ps) Frequency (THz) Surprisingly fast loss of magnetic order within ~1 ps: ƒ ~10 5 faster than lifetime of YIG’s zone-center magnons (FMR) ƒ Response speed is comparable to laser-excited metals Behavior on longer time scales?
From femtoseconds to milliseconds 0 (arb. untis) Two very different time scales: -1 interpretation? -2 Faraday rotation -3 -4 in ~1 ps ~300 λ s ~100 ns -5 -6 0 2 4 200 400 600 200 400 600 800 Time ( λ s) Time (ps) Time (ns) Ultrafast magnetic- Full equilibration: Heat flow to substrate: order quenching deduced from From simulations, temp.-dependence different substrates
Summary: spin-phonon equilibration in YIG Phonons THz pump � � Energy Δ� �� (� ) � �� TO( Φ ) � � Spin angular momentum a-Fe 3+ O 2- d-Fe 3+ Spins Δ� �� (� ) = �� �� �� Δ�(� ) Maehrlein, Barker, Kampfrath et al. , � ( � ) Science Adv. (2018)
Summary: spin-phonon equilibration in YIG Phonons Identical sublattice demagnetization THz pump � � in ~1 ps ⇑ Constrained state, lives ~10 ns Energy Δ� �� (� ) � �� TO( Φ ) � � Spin angular momentum Global equilibration: ~100 ns Spins Reveals spin-phonon equilibration in YIG: Maehrlein, Barker, ƒ Transfer of energy: in ~1 ps Kampfrath et al. , Science Adv. (2018) ƒ …and angular momentum: ~100 ns
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