Ultrafast Optical Manipulation of Magnetic Order
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REVIEWS OF MODERN PHYSICS, VOLUME 82, JULY–SEPTEMBER 2010 Ultrafast optical manipulation of magnetic order Andrei Kirilyuk,* Alexey V. Kimel, and Theo Rasing Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands ͑Published 22 September 2010͒ The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of ͑nearly͒ free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism. DOI: 10.1103/RevModPhys.82.2731 PACS number͑s͒: 75.78.Jp, 85.75.Ϫd, 78.47.jh, 75.60.Ϫd CONTENTS a. Time-resolved electron photoemission experiments 2744 b. Time-resolved x-ray experiments 2745 I. Introduction 2732 5. Microscopic models of ultrafast A. Issues and challenges in magnetization dynamics 2732 demagnetization 2746 B. The problem of ultrafast laser control of magnetic B. Demagnetization of magnetic semiconductors 2747 order 2733 C. Demagnetization of magnetic dielectrics 2748 II. Theoretical Considerations 2734 D. Demagnetization of magnetic half metals 2749 A. Dynamics of magnetic moments: E. Laser-induced coherent magnetic precession 2750 Landau-Lifshitz-Gilbert equation 2734 1. Precession in exchange-biased bilayers 2750 B. Finite temperature: Landau-Lifshitz-Bloch equation 2735 2. Precession in nanostructures 2751 C. Interaction of photons and spins 2735 3. Precession in ͑III,Mn͒As ferromagnetic III. Experimental Techniques 2737 semiconductors 2752 A. Pump-and-probe method 2737 4. Precession in ferrimagnetic materials 2753 B. Optical probe 2737 F. Laser-induced phase transitions between two C. Ultraviolet probe and spin-polarized electrons 2738 magnetic states 2755 D. Far-infrared probe 2739 1. Spin reorientation in TmFeO 2755 E. X-ray probe 2739 3 2. Spin reorientation in ͑Ga,Mn͒As 2756 IV. Thermal Effects of Laser Excitation 2739 3. AM-to-FM phase transition in FeRh 2756 A. Ultrafast demagnetization of metallic ferromagnets 2739 G. Magnetization reversal 2757 1. Experimental observation of ultrafast V. Nonthermal Photomagnetic Effects 2758 demagnetization 2740 A. Photomagnetic modification of magnetocrystalline 2. Phenomenological three-temperature model 2742 anisotropy 2758 3. Spin-flip and angular momentum transfer in 1. Laser-induced precession in magnetic garnet metals 2743 films 2758 4. Interaction among charge, lattice, and spin a. Experimental observations 2758 subsystems 2744 b. Phenomenological model 2759 c. Microscopic mechanism 2760 2. Laser-induced magnetic anisotropy in *[email protected] antiferromagnetic NiO 2761 0034-6861/2010/82͑3͒/2731͑54͒ 2731 ©2010 The American Physical Society 2732 Kirilyuk, Kimel, and Rasing: Ultrafast optical manipulation of magnetic order 3. Photomagnetic excitation of spin precession in ͑Ga,Mn͒As 2761 B. Light-enhanced magnetization in ͑III,Mn͒As semiconductors 2762 1ns VI. Nonthermal Optomagnetic Effects 2763 Magnetic A. Inverse magneto-optical excitation of magnetization 100 ps Spin field precession dynamics: Theory 2763 10 ps ~1ps-1ns Laser 1. Formal theory of inverse optomagnetic L S 1ps effects 2763 Spin-orbit (LS) interaction~ 0.1 ps-1 ps 2. Example I: Cubic ferromagnet 2765 100 fs 3. Example II: Two-sublattice antiferromagnet 2766 Exchange 10 fs ~ 0.01 ps – 0.1 ps B. Excitation of precessional magnetization dynamics interaction 1fs with circularly polarized light 2766 1. Optical excitation of magnetic precession in FIG. 1. ͑Color online͒ Time scales in magnetism as compared garnets 2766 to magnetic field and laser pulses. The short duration of the 2. Optical excitation of antiferromagnetic laser pulses makes them an attractive alternative to manipulate resonance in DyFeO3 2767 the magnetization. 3. Optical excitation of precession in GdFeCo 2768 C. All-optical control and switching 2769 1. Double-pump coherent control of magnetic corresponds to the ͑equilibrium͒ exchange interaction, precession 2769 responsible for the existence of magnetic order, while 2. Femtosecond switching in magnetic garnets 2771 being much faster than the time scale of spin-orbit inter- 3. Inertia-driven switching in antiferromagnets 2771 action ͑1–10 ps͒ or magnetic precession ͑100–1000 ps͒; 4. All-optical magnetization reversal 2772 see Fig. 1. Because the latter is considered to set the 5. Reversal mechanism via a nonequilibrium limiting time scale for magnetization reversal, the option state 2773 of femtosecond optical excitation immediately leads to D. Excitation of the magnetization dynamics with the question whether it would be possible to reverse linearly polarized light 2774 magnetization faster than within half a precessional pe- 1. Detection of the FMR mode via magnetic riod. As magnetism is intimately connected to angular linear birefringence in FeBO3 2775 momentum, this question can be rephrased in terms of 2. Excitation of coherent magnons by linearly the more fundamental issues of conservation and trans- and circularly polarized pump pulses 2776 fer of angular momentum: How fast and between which 3. ISRS as the mechanism of coherent reservoirs can angular momentum be exchanged and is magnon excitation 2776 this even possible on time scales shorter than that of the 4. Effective light-induced field approach 2778 spin-orbit interaction? VII. Conclusions and Outlook 2778 While such questions are not relevant at longer times Acknowledgments 2779 and for equilibrium states, they become increasingly im- References 2779 portant as times become shorter and, one by one, the various reservoirs of a magnetic system, such as the I. INTRODUCTION magnetically ordered spins, the electron system, and the lattice, become dynamically isolated. The field of ul- A. Issues and challenges in magnetization dynamics trafast magnetization dynamics is therefore concerned with the investigation of the changes in a magnetic sys- The time scale for magnetization dynamics is ex- tem as energy and angular momentum are exchanged tremely long and varies from the billions of years con- between the thermodynamic reservoirs of the system nected to geological events such as the reversal of the ͑Stöhr and Siegmann, 2006͒. magnetic poles down to the femtosecond regime con- Although deeply fundamental in nature, such studies nected with the exchange interaction between spins. are also highly relevant for technological applications. From a more practical point of view, the demands for Indeed, whereas electronic industry is successfully enter- the ever-increasing speed of storage of information in ing the nanoworld following Moore’s law, the speed magnetic media plus the intrinsic limitations that are of manipulating and storing data lags behind, creating connected with the generation of magnetic field pulses a so-called ultrafast technology gap. This is also evident by current have triggered intense searches for ways to in modern PCs that already have a clock speed of a control magnetization by means other than magnetic few gigahertz while the storage on a magnetic hard disk fields. Since the demonstration of subpicosecond demag- requires a few nanoseconds. A similar problem is expe- netization by a 60 fs laser pulse by Beaurepaire et al. rienced by the emerging field of spintronics as in, for ͑1996͒, manipulating and controlling magnetization example, magnetic random access memory devices. with ultrashort laser pulses has become a challenge. Therefore, the study of the fundamental and practical Femtosecond laser pulses offer the intriguing possi- limits on the speed of manipulation of the magnetiza- bility to probe a magnetic system on a time scale that tion direction is obviously also of great importance for Rev. Mod. Phys., Vol. 82, No. 3, July–September 2010 Kirilyuk, Kimel, and Rasing: Ultrafast optical manipulation of magnetic order 2733 magnetic recording and information processing