Nonthermal ultrafast optical control of the magnetization in garnet films

Hansteen, Fredrik; Kimel, A.V.; Kirilyuk, Andrei; Rasing, T.H.M.

doi:10.1103/physrevb.73.014421

Cited by 171 publications

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“…As with the thermomagnetic methods, the source of angular momentum in the IFE has not been identified despite much theoretical and experimental work. [11][12][13][19][20][21] I show below that the angular momentum is provided by the photons. No other angular momentum reservoir is needed.…”

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confidence: 99%

“…The inverse Faraday effect ͑IFE͒ offers the possibility of nonthermally controlling the magnetization and avoiding this problem. 11,12 The inverse Faraday effect is the generation of an effective magnetic field using nonresonant circularly polarized light. [19][20][21] It is the "inverse" of the well-known Faraday effect-the rotation of linearly polarized light propagating through a medium in a magnetic field 22,23 -because it is derived from the same free energy.…”

mentioning

confidence: 99%

“…However, due to the difficulty of creating ultrashort magnetic pulses and the recent discovery that magnetic switching by strong magnetic fields can be unpredictable, 1 alternative techniques of controlling magnetization are under intense investigation. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Optical methods 2-13 are particularly promising due to the availability of ultrashort laser pulses. However, the fundamental mechanisms of optically induced demagnetization and magnetic switching are not fully understood.…”

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confidence: 99%

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Conservation of angular momentum and the inverse Faraday effect

Woodford

2009

Phys. Rev. B

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Precession of magnetization via the inverse Faraday effect is investigated with a view of determining the fundamental limit on the precession speed. Such a limit could have important consequences for ultrafast magnetic switching. The angular momentum required for precession is shown to be supplied by the light. This indicates that there is no fundamental obstruction to magnetization reversal on the time scale of a laser pulse provided that a material with a sufficiently strong magneto-optical response can be found. DOI: 10.1103/PhysRevB.79.212412 PACS number͑s͒: 75.60.Jk, 75.40.Gb, 78.20.Ls, 85.70.Li The ability to control magnetization on a subpicosecond time scale is growing in importance as the speed of electronic devices increases. The current generation of technology employs magnetic fields to induce magnetization dynamics. However, due to the difficulty of creating ultrashort magnetic pulses and the recent discovery that magnetic switching by strong magnetic fields can be unpredictable, 1 alternative techniques of controlling magnetization are under intense investigation. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Optical methods 2-13 are particularly promising due to the availability of ultrashort laser pulses. However, the fundamental mechanisms of optically induced demagnetization and magnetic switching are not fully understood. In particular, the question of which reservoir supplies the angular momentum needed for demagnetization remains controversial. This reservoir plays a decisive role in determining the maximum demagnetization speed so resolving this issue is important for technological applications.Most experiments on optical demagnetization and magneto-optical switching employ thermal methods. An optical pulse is absorbed, the electrons are driven far from equilibrium, and the sample is almost completely demagnetized within a few hundred femtoseconds. [3][4][5][6][7][8][9][10] If this is performed in the presence of a magnetic field, the magnetization can be reversed. [2][3][4][5] Some estimates show that thermal demagnetization occurs too rapidly for phonon processes to be relevant and it has been suggested that angular momentum is transferred between the spin and orbital components of the electrons. 9 On the other hand, it is possible that the nonequilibrium electrons experience a spin-phonon interaction that is much stronger than usual. In this case, the phonons could provide the angular momentum. 8,10 Transfer of angular momentum by the absorption of photons has also been considered. 9,13,18 Theoretical arguments based on the number of photons absorbed 9 and experiments using circularly polarized light with nickel 18 indicate that the photon angular momentum is irrelevant, although experiments with GdFeCo yielded the opposite conclusion. 13 Thermomagnetic switching is associated with an increase in temperature, so devices employing these methods will suffer a significant cooling time before new information can be written to them. The inverse Faraday effect ͑IFE͒ offers the p...

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Conservation of angular momentum and the inverse Faraday effect

Woodford

2009

Phys. Rev. B

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“…In addition, the magnetization dynamics were found to be consistent with a weakly perturbed equilibrium state: the IFE does not cause significant departures from equilibrium and is a truly nonthermal effect. Full reversal of the magnetization could not be attained with these pulse strengths and durations, but control of the magnetization was demonstrated [66]. Further results from recent IFE experiments can be found in the review [98].…”

Section: The Ultrafast Inverse Faraday Effectmentioning

confidence: 99%

“…As with other optical methods of magnetic switching, the source of angular momentum in the IFE has not been identified 1 , despite much theoretical and experimental work [66,99,150,152,179,187]. However, the nonthermal nature of the IFE simplifies the discussion, since several of the reservoirs discussed in Chapter 1 cannot play a role.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast Magnetization Dynamics

Woodford¹,

Blügel²

Magnetism

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This thesis addresses ultrafast magnetization dynamics from a theoretical perspective. The manipulation of magnetization using the inverse Faraday effect has been studied, as well as magnetic relaxation processes in quantum dots.The inverse Faraday effect -the generation of a magnetic field by nonresonant, circularly polarized light -offers the possibility to control and reverse magnetization on a timescale of a few hundred femtoseconds. This is important both for the technological advantages and for the deeper fundamental understanding of magnetization dynamics that can be gained. However, several aspects of the inverse Faraday effect have remained poorly understood. The question of whether light can manipulate magnetization alone or whether an additional angular momentum reservoir is needed, in particular, remains unanswered.This question is answered here: the light beam that causes the inverse Faraday effect provides the angular momentum required for the magnetization to precess. No other reservoir is needed. This implies that manipulation of the magnetization occurs on the timescale of a laser pulse, which can be made extremely short. Even magnetization reversal on this timescale could be possible, provided a material with a sufficiently strong magnetooptical response can be found. This is a technical challenge, not a fundamental obstacle.The Faraday effect in the presence of optical birefringence has also been analyzed. This effect has been used for imaging magnetization dynamics in transparent media on an ultrafast timescale, but transparent magnetic materials usually have a complex crystal structure and complicated optical properties, which render the relationship between Faraday rotation and magnetization unclear. We have shown that the Faraday effect can be used to measure the instantaneous magnetization, even in the presence of birefringence, provided certain experimental conditions are met. Suggestions concerning these experimental conditions are made.The relaxation of magnetization, particularly the relaxation of a spin in a quantum dot, has been studied. This problem is relevant to the fields of quantum computing and highly-multiplexed optical memory, both of which are of great current interest. We have investigated the interaction of the spin with a metallic electrode and calculated the dephasing and dissipation rates. We found that under current experimental conditions, this relaxation pathway is negligible compared to spin-phonon scattering, but as systems are miniaturized, interactions with electrodes become more important.The methods developed to study the relaxation of a spin were also applied to the relaxation of a charge in a double quantum dot, another important problem in quantum computing. Again, we found that the interaction with a gate electrode is generally much weaker than the interaction with phonons but may be important for smaller systems. Kurzfassung

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Magneto‐Optical Characterization of Magnetic Thin Films, Surfaces, and Interfaces at Small Length and Short Timescales

Kirilyuk

Kimel

Savoini

et al. 2012

Characterization of Materials

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The understanding of magnetism at its fundamental length and timescales related to the exchange interaction, that is, at nanometer (nm) length and femtosecond (fs) timescales, is becoming essential as future magnetic recording aims at Tbit densities switched at THz rates, which is exactly this regime. Magnetization‐sensitive second harmonic generation (MSHG) is a nonlinear optical technique that, due to the dipole selection rules, is specifically sensitive to surfaces and interfaces of centrosymmetric media. This surface/interface sensitivity of MSHG in combination with very large magneto‐optical effects has lead to a fast development of this technique over the past decade, demonstrating atomic scale resolution in the direction perpendicular to the interfaces. To increase the lateral resolution of magneto‐optics beyond the diffraction limit, scanning near‐field optical microscopy techniques are more appropriate. The development of novel probes that preserve the polarization has led to the possibility of magneto‐optical characterization of magnetic nanostructures with spatial resolution beyond 100 nm. The combination of these and other magneto‐optical techniques with pump–probe approaches finally allows one to include time resolution down to the femtosecond range in the magneto‐optical characterization of magnetic structures. The main attention of the present chapter is on the application of the MSHG, SNOM, and pump–probe techniques to magnetic thin films and surfaces, with a focus on the achieved progress in understanding of magnetic phenomena at the smallest length and shortest timescales. On the one hand, an extreme sensitivity of MSHG to the electronic and magnetic structure of clean surfaces has been successfully demonstrated. On the other hand, the penetration depth of light has allowed to use this sensitivity to study buried interfaces in multilayer systems. Various phenomena, such as surface states on magnetic metals, enhanced magnetic moments of low‐coordinated atoms, and quantum well states, have been studied. Complimentary to these developments, we show how SNOM gives in‐plane information about submicron magnetic domain structures. Finally, we demonstrate how magneto‐optical pump–probe techniques can be used to study the dynamics of magnetic phenomena with a time resolution down to femtoseconds.

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Nonthermal ultrafast optical control of the magnetization in garnet films

Cited by 171 publications

References 65 publications

Conservation of angular momentum and the inverse Faraday effect

Conservation of angular momentum and the inverse Faraday effect

Ultrafast Magnetization Dynamics

Magneto‐Optical Characterization of Magnetic Thin Films, Surfaces, and Interfaces at Small Length and Short Timescales

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