Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

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 nearly 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.

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Ultrafast optical manipulation of magnetic order

Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed Antiferromagnetic spintronics started out with studies on spin transfer, and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics Central to these endeavors are the need for predictive models, relevant disruptive materials and new experimental designs This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials It also details some of the remaining bottlenecks and suggests possible avenues for future research

Antiferromagnetic spintronics

The demand for ever-increasing density of information storage and speed of manipulation has triggered an intense search for ways to control the magnetization of a medium by means other than magnetic fields. Recent experiments on laser-induced demagnetization and spin reorientation use ultrafast lasers as a means to manipulate magnetization, accessing timescales of a picosecond or less. However, in all these cases the observed magnetic excitation is the result of optical absorption followed by a rapid temperature increase. This thermal origin of spin excitation considerably limits potential applications because the repetition frequency is limited by the cooling time. Here we demonstrate that circularly polarized femtosecond laser pulses can be used to non-thermally excite and coherently control the spin dynamics in magnets by way of the inverse Faraday effect. Such a photomagnetic interaction is instantaneous and is limited in time by the pulse width (approximately 200 fs in our experiment). Our finding thus reveals an alternative mechanism of ultrafast coherent spin control, and offers prospects for applications of ultrafast lasers in magnetic devices.

Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses.

Magnetic switching is typically accomplished by using a driving field that stays on until the magnetization is rotated to its final position. An experiment demonstrates that, in antiferromagnets, inertial effects can be harnessed, such that only a short ‘kick’ is required to transfer sufficient momentum to the spin system for it to reorient. It is generally accepted that the fastest way to reorient magnetization is through precessional motion in an external magnetic field1,2,3,4,5,6,7. In ferromagnets, the application of a magnetic field instantaneously sets spins in motion and, in contrast to the inertial motion of massive bodies, the magnetization can climb over a potential barrier only during the action of a magnetic-field pulse. Here we demonstrate a fundamentally different scenario of spin switching in antiferromagnets, where the exchange interaction between the spins leads to an inertial behaviour. Although the spin orientation hardly changes during the action of an optically generated strong magnetic-field pulse of 100 fs duration, this pulse transfers sufficient momentum to the spin system to overcome the potential barrier and reorient into a new metastable state, long after the action of the stimulus. Such an inertia-based mechanism of spin switching should offer new opportunities for ultrafast recording and processing of magnetically stored information.

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Inertia-driven spin switching in antiferromagnets

The optical response of the technologically interesting multiferroic ${\text{BiFeO}}_{3}$ and related complex iron oxides, having high N\'eel or Curie temperature, is studied in the wide spectral range from 0.6 up to 5.8 eV by means of spectroscopic ellipsometry. The investigated iron oxides have different crystal symmetry with ${\text{FeO}}_{6}$ octahedral and ${\text{FeO}}_{4}$ tetrahedral centers distorted to a certain degree. One of the two groups of materials includes ${\text{BiFeO}}_{3}$, ${\text{ErFeO}}_{3}$, ${\text{Y}}_{.95}{\text{Bi}}_{.05}{\text{FeO}}_{3}$, $\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3}$, ${\text{Fe}}_{2\ensuremath{-}x}{\text{Ga}}_{x}{\text{O}}_{3}$, and ${\text{Fe}}_{3}{\text{BO}}_{6}$ in which iron ${\text{Fe}}^{3+}$ ions occupy only octahedral centrosymmetric or noncentrosymmetric positions and distortions range of 1--20 %. The second group includes ${\text{LiFe}}_{5}{\text{O}}_{8}$, ${\text{BaFe}}_{12}{\text{O}}_{19}$, ${\text{Sm}}_{3}{\text{Fe}}_{5}{\text{O}}_{12}$, and ${\text{Ca}}_{2}{\text{Fe}}_{2}{\text{O}}_{5}$ in which ${\text{Fe}}^{3+}$ ions occupy both octahedral and tetrahedral positions with a rising tetra/ortho ratio. We show that in the spectral range up to $\ensuremath{\sim}3.7\text{ }\text{eV}$, the optical response is dominated by $p\text{\ensuremath{-}}d$ charge transfer (CT) transitions, while at $Eg3.7\text{ }\text{eV}$ both $p\text{\ensuremath{-}}d$ and $d\text{\ensuremath{-}}d$ CT transitions are revealed. At variance with several previous investigations, we present a correct and unified assignment of different dipole-allowed and dipole-forbidden CT transitions. All the ferrites investigated are CT insulators with the band gap determined by a dipole-forbidden $p\text{\ensuremath{-}}d$ CT transition ${t}_{1g}\ensuremath{\rightarrow}{t}_{2g}$, forming a $\ensuremath{\sim}2.5\text{ }\text{eV}$ band on the tail of a strong 3.0 eV band assigned to dipole-allowed $p\text{\ensuremath{-}}d$ CT transitions ${t}_{2u}(\ensuremath{\pi})\ensuremath{\rightarrow}{t}_{2g}$ in octahedral ${\text{FeO}}_{6}$ centers. A noticeable enhancement of the optical response in ${\text{BiFeO}}_{3}$ at $\ensuremath{\sim}4\text{ }\text{eV}$ as compared with other related iron oxides is attributed to CT transitions within the Bi-O bonds. We report an observation of unexpected midinfrared CT bands in calcium ferrite ${\text{Ca}}_{2}{\text{Fe}}_{2}{\text{O}}_{5}$ and an enhanced structureless spectral weight in a wide range below the main CT bands in ${\text{BiFeO}}_{3}$ with a remarkable smearing of the fundamental absorption edge. All these anomalies are assigned to CT instabilities accompanied by a self-trapping of $p\text{\ensuremath{-}}d$ CT excitons and nucleation of electron-hole droplets. The optical detection of this CT instability agrees with the observation of a metal-insulator transition in bismuth ferrite.

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Charge transfer transitions in multiferroic BiFeO3 and related ferrite insulators

Using 100-fs optical laser pulses, we have been able to excite and probe spin dynamics in the rare-earth orthoferrite ${\mathrm{ErFeO}}_{3}$. The investigation was performed in a broad temperature range with the focus on the vicinities of the compensation point ${T}_{\mathrm{comp}}\ensuremath{\approx}47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the spin reorientation transition region in the interval $86\phantom{\rule{0.16em}{0ex}}\mathrm{K}\ensuremath{\lesssim}T\ensuremath{\lesssim}99\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Spin precession excited by the laser pulse was present in a large part of the investigated temperature range, but was especially strong near the spin reorientation region. In this region the laser pulse also caused an ultrafast spin reorientation. By changing the laser pulse fluence, we could vary both the reorientation amplitude and the reorientation speed. We show that the laser-induced spin dynamics in ${\mathrm{ErFeO}}_{3}$ is caused in part by heating and in part by the inverse Faraday effect. Comparing to the results of similar experiments in other rare-earth orthoferrites, we found the speed of the laser-induced spin reorientation to be significantly lower. We attribute this finding to the weaker electron-phonon coupling of the ${\mathrm{Er}}^{3+}$ 4f electrons with the lattice.

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Laser-induced ultrafast spin dynamics in ErFeO 3

Pump-pulse induced magnetization reversal of amorphous ${\mathrm{Gd}}_{23.1}{\mathrm{Fe}}_{71.9}{\mathrm{Co}}_{5.0}$ showed a subpicosecond magnetization collapse followed by a slower reversal. The reversal dynamics is well described by the Bloch equation via a reversal time that does not depend on temperature, but strongly decreases with increasing pump fluence. A comparison to data obtained in external saturation field and in remanence opened the way to separate the contributions due to temperature induced effects within single domains from those related to field induced domain formation.

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Fast magnetization reversal of GdFeCo induced by femtosecond laser pulses

fcc Co on Cu(001) and multilayers of Cu/Co/Cu(001) are studied by magnetization induced SHG (MSHG) in combination with linear Kerr measurements, for thicknesses between 1 and 20 monolayers (ML). Interface sensitivity of MSHG is demonstrated by its Co thickness dependency. Aside from a weak modulation, the MSHG signal from Co/Cu is nearly constant for thicknesses between 6 and 20 ML. Very strong oscillations in the MSHG signal are found for the Cu/Co/Cu system as a function of the thickness of the Cu coverlayer, which are possibly related to quantum well states in these thin Cu films.

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T.H.M. Rasing

Papers

Inertia-driven spin switching in antiferromagnets

Charge transfer transitions in multiferroic BiFeO3 and related ferrite insulators

Laser-induced ultrafast spin dynamics in ErFeO 3

Fast magnetization reversal of GdFeCo induced by femtosecond laser pulses

Interface magnetism and possible quantum well oscillations in ultrathin Co/Cu films observed by magnetization induced second harmonic generation.