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Ultrafast all-optical control of the magnetization in magnetic dielectrics
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Image of FIG. 1.
FIG. 1.

Garnet sample characteristics: (a) Measured Faraday rotation at as a function of temperature with a saturating applied field normal to the film plane. (b) Hysteresis loop at measured with normal to the film. (c) Hysteresis loop measured at a small angle of incidence with in the sample plane, indicating the presence of anisotropy fields of about .51

Image of FIG. 2.
FIG. 2.

(a) Crystallographic and spin structure of showing different phases as well as the reorientation of the ferromagnetic and antiferromagnetic vectors. (b) Temperature dependence of the birefringence along the three principal crystallographic directions. The birefringence anomalies occur at the orientational-transition temperatures and .63

Image of FIG. 3.
FIG. 3.

The energy level scheme and the absorption spectra of at room temperature (solid line) and at (dashed line). The spectral lines from 1 to 4 correspond to the transitions from the ground state to the excited state split by the spin-orbit coupling.44

Image of FIG. 4.
FIG. 4.

Experimental geometry. Pump and probe pulses were incident on the garnet film at near normal incidence. The magnetization of the sample forms an angle with the sample normal [001] and an angle with the crystallographic [100] axis of the film. For linearly polarized pump pulses the angle of the electric field component of the light with respect to the sample axis is denoted .

Image of FIG. 5.
FIG. 5.

(a) The transmission as a function of time delay, (b) the intensity dependence of the ultrafast Faraday rotation (symbols); linear fit with a slope of (line).44

Image of FIG. 6.
FIG. 6.

The long-term transient Faraday rotation measured as a function of temperature. The antiferromagnetic order is destroyed at a time delay of for .44

Image of FIG. 7.
FIG. 7.

The Faraday rotation without pump (solid squares) and at negative (solid circles) and zero (open circles) time delay as a function of the bias temperature with the fit to Eq. (7) (solid and dotted lines, respectively). The difference between the intrinsic magnetooptical signal and that at is shown by diamonds together with the calculation based on the fitted parameters (dashed line). The inset shows the transient component of the magnon temperature as a function of the time delay. The solid line is the fit according to Eq. (8).44

Image of FIG. 8.
FIG. 8.

Precession following excitation with circularly polarized light. The two helicities and give rise to precessions with opposite phase and different amplitude. During the presence of the laser pulse the magnetization precesses in the dominating axial magnetic field created by the circularly pump pulse. Subsequent precession takes place in the effective magnetic field (Refs. 50 and 51).

Image of FIG. 9.
FIG. 9.

Precession frequency as function of the externally applied magnetic field, measured with polarized excitation. Circles represent measurements and the solid line is a best fit using and .51

Image of FIG. 10.
FIG. 10.

Illustration of the stimulated Raman-like coherent scattering mechanism believed to be responsible for the ultrafast optically generated magnetic field. Two frequency components of electromagnetic radiation from the spectrally broad laser pulse take part in the process. The frequency causes a transition to a virtual state with strong spin-orbit coupling. Radiation at the frequency stimulates relaxation back to the ground state with the creation of a magnon.

Image of FIG. 11.
FIG. 11.

Magnetic excitations in probed by the magnetooptical Faraday effect. Two processes can be distinguished: 1) instantaneous changes of the Faraday effect due to the photoexcitation of Fe ions and relaxation back to the high-spin ground state ; 2) oscillations of the Fe spins around their equilibrium direction with an approximately period. The circularly polarized pumps of opposite helicities excite oscillations of opposite phase. The inset shows the geometry of the experiment. Vectors and represent the effective magnetic fields induced by right- and left-circularly polarized pumps, and , respectively.10

Image of FIG. 12.
FIG. 12.

Excitation of the spin oscillations in measured at different temperatures in the range between 18 and . In order to exclude effects not relevant to magnetic excitations, the difference between the signals for right- and left-circularly polarized pump pulses is plotted. Each new curve is shifted from the previous one along the vertical axis by 0.06°. The inset shows the amplitude of the spin oscillations as a function of pump fluence.10

Image of FIG. 13.
FIG. 13.

Temperature dependence of the frequencies of the observed spin oscillations. Filled and open circles show the frequencies of the excited oscillations for laser pulses propagating along the and axis, respectively. Lines show the frequency of the quasi-antiferromagnetic (quasi-AFM) and the quasi-ferromagnetic (quasi-FM) resonance modes from Refs. 67, 83, and 84. Top right inset shows the temperature dependence of the oscillation amplitudes. Top left and bottom right insets are, respectively, schematic representations of the quasi-FM and quasi-AFM modes of the spin resonance. Vectors show the directions of the instantaneous magnetic field that is equivalent to the photoexcitation.10

Image of FIG. 14.
FIG. 14.

Excitation and relaxation of the AFM moment measured via changes in the magnetic birefringence. On the figure one can distinguish three processes: 1) electron-phonon thermalization with relaxation time; 2) rotation of the AFM vector with response time; 3) oscillations of the AFM vector around its equilibrium direction with an approximate period.9

Image of FIG. 15.
FIG. 15.

Schematic illustration of the spin relaxation in an antiferromagnet as compared to that in a ferromagnet: in contrast to the spiral FM precession, the AFM vector moves in a plane.9

Image of FIG. 16.
FIG. 16.

Temperature dependences of the amplitudes and frequencies of the observed oscillations, as well as the amplitude of the spin reorientation. Thin line shows the frequency change at the reorientational transition from Ref. 86. Inset shows the oscillations of spins in the plane. Nonzero reorientation amplitude at corresponds to an instantaneous local laser-induced heating of more than .9

Image of FIG. 17.
FIG. 17.

Coherent precession of the magnetization triggered by linearly polarized laser pulses. (a) Time dependence of the precession for different planes of pump polarization , with an applied field of in the plane of the sample. Circles represent measurements and solid lines simulations based on the Landau-Lifshitz equation. (b) Precessional amplitude as a function of the plane of polarization of the pump. Round and square symbols represent amplitudes extracted from measurements at . The solid line is a best fit. (c) Pump-induced change of the sample transmissivity .51

Image of FIG. 18.
FIG. 18.

Dependence of the precessional amplitude on the applied in-plane magnetic field . Round and square symbols represent amplitudes extracted from measurements at .

Image of FIG. 19.
FIG. 19.

Graphical illustration of the process of photoinduced magnetic anisotropy caused by linearly polarized laser excitation and the subsequent precessional dynamics.

Image of FIG. 20.
FIG. 20.

(a) Precession of the magnetization following excitation with linearly polarized light for different values of the magnetic field applied at an angle of about 45° with the sample normal. (b) The excitation shown on a finer time scale.51

Image of FIG. 21.
FIG. 21.

Dependence of the precession amplitude on the excitation pulse energy.51

Image of FIG. 22.
FIG. 22.

Illustration of the photoexcitation of electrons between iron ions in different crystallographic sites. A laser pulse induces electron transfer from a ion in the octahedral site (denoted by [a]) to a ion in the tetrahedral site (denoted by (d)). The dodecahedral site with the divalent lead impurity is denoted by {c}.

Image of FIG. 23.
FIG. 23.

Double pump experiment with circularly polarized laser pulses of opposite helicity and pulse power. The upper panel shows the pump-induced change of the sample transmissivity due to the photoexcitation of impurities. The lower panel shows how amplification and complete stopping of the magnetization precession can be achieved depending on the phase of the precession when the second laser pulse arrives. The time delay between the two pump pulses is fixed at approximately , and the precession frequency is controlled by varying the external field.51

Image of FIG. 24.
FIG. 24.

Illustration of the double-pump experiment for circularly polarized pump pulses of opposite helicity arriving at (a) an odd number of half precessional periods and (b) an integer number of full precessional periods. The magnetization is either rotated further away from the effective field direction causing subsequent precession to take place with almost twice the original amplitude, or the magnetization is rotated back into the effective field direction and no further precession takes place.

Image of FIG. 25.
FIG. 25.

Direct optical control of the magnetization dynamics in the dysprosium orthoferrite sample by two circularly polarized pump pulses of the same helicity. Depending on the time of arrival of the second pump pulse, the precession can be amplified or stopped completely.

Image of FIG. 26.
FIG. 26.

A double-pump experiment with two orthogonal linearly polarized pump pulses separated in time by approximately . Timing with respect to the spin precession is done by varying the in-plane applied magnetic field and thereby the precession frequency. The bottom panel shows the photoinduced change of sample transmissivity. Partial quenching (top panel) and amplification (middle panel) of the precession was achieved.51

Image of FIG. 27.
FIG. 27.

Precession of the magnetization triggered by left- and right-circularly polarized laser pulses at different values of the in-plane applied magnetic field. For the helicity, at an applied field of , no precession is observed due to a perfect balance of the two photomagnetic effects and .

Image of FIG. 28.
FIG. 28.

Illustration of the switching process. Initially at the magnetization is along . During the existence of the laser pulse photoinduced modification of the anisotropy fields leads to a new long-lived equilibrium along . Simultaneously, the strong optomagnetically generated field causes the magnetization to precess into the new state. After the optical pulse is gone and the approximately 0.6° switching of is complete.51


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Ultrafast all-optical control of the magnetization in magnetic dielectrics