Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-Pérot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillatordriven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost. IntroductionThe terahertz (THz) window, loosely defined as the frequency range from 0.3 to 30 THz in the electromagnetic spectrum, is located between the realms of electronics and optics 1,2 . As this region coincides with many fundamental resonances of materials, THz radiation enables very selective spectroscopic insights into all phases of matter with high temporal 3,4 and spatial 5,6,7,8 resolution. Consequently, numerous applications in basic research 3,4 , imaging 5 and quality control 8 have emerged.To fully exploit the potential of THz radiation, energy-efficient and low-cost sources of ultrashort THz pulses are required. Most broadband table-top emitters are driven by femtosecond laser pulses that generate the required THz charge current by appropriately mixing the various optical frequencies 9,10 . Sources made from solids usually consist of semiconducting or insulating structures with naturally or artificially broken inversion symmetry. When the incident photon energy is below the semiconductor band gap, optical rectification causes a charge displacement that follows the intensity envelope of the incident pump pulse 9,10,11,12,13,14,15,16,17 . For above-band-gap excitation, the response is dominated by a photocurrent 18,19,20,21,22,23,24 with a temporally step-like onset and, thus, generally smaller bandwidth than optical rectification 9 . Apart from rare exceptions 14 , however, most semiconductors used are polar 1,2,12,13,15,16,17,21,22 and strongly attenuate THz radiation around optical phonon resonances, thereby preventing emission in the so-called Reststrahlen band located between ~1 and 15 THz.The so far most promising sources covering the full THz window are photocurrents in transient gas plasmas 9,10,25,26,27,28,29 . The downside of this appealing approach is that the underlying ionization process usually requires amplified laser pulses with high threshold energies on the order of 0....

We propose a semi-classical model for femtosecond-laser induced demagnetization due to spinpolarized excited electron diffusion in the super-diffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.Excitation with femtosecond laser pulses is known for more than a decade to cause an ultrafast quenching of the magnetization in metallic ferromagnets [1]. The achieved demagnetization times are typically 100-300 fs for ferromagnets such as Ni [1,2]. Hence, laser-induced demagnetization opens up new, interesting routes for magnetic recording with hitherto unprecedented speeds [3]. However, in spite of the technological importance the mechanism underlying the femtosecond demagnetization remains highly controversial. A common belief is that there should exist an ultrafast channel for the dissipation of spin angular momentum [4][5][6][7][8]. Several such mechanisms through which an excited electron can undergo a spinflip in a ferromagnetic metal are currently being debated. The main proposed mechanisms for a fast spin-flip process are a Stoner excitation, an inelastic magnon scattering, an Elliott-Yafet-type of phonon scattering [4,5], spin-flip Coulomb scattering [6], laser-induced spin-flips [7], or relativistic quantum electrodynamic processes [8]. An effect that, until recently [9], has been regarded to play only a marginal role is the spin-polarized transport of laser-excited hot electrons.In this Letter we show that spin-dependent transport of laser-excited electrons provides a considerable contribution to the ultrafast demagnetization process and can even completely explain it. We demonstrate this by developing a transport equation for the super-diffusive flow of spin-polarized electrons. A few approaches to describe the electron motion have been attempted previously [10,11]. In our theory, however, we take into account the whole process of multiple, spin-conserving electron scattering events and electron cascades created by inelastic electron scattering. Also the presence of different metallic films in the probed material is treated. We solve the developed theory numerically for ferromagnetic Ni, for which the femtosecond demagnetization is well documented [1,2,12], and show that a large demagnetization in a few hundred femtoseconds is generated.The typical geometry for a femtosecond laser experiment is depicted in Fig. 1. The intense laser beam creates excited hot electrons in the ferromagnetic film, which will start to move in a random direction. Our goal is to compute the time-dependent magnetization resulting from the super-diffusive motion in the laser spot. Due to the fact that the electronic mean-free-path (up to a few tens of nm) is much smaller tha...

In spin-based electronics, information is encoded by the spin state of electron bunches 1,2,3,4 . Processing this information requires the controlled transport of spin angular momentum through a solid 5,6 , preferably at frequencies reaching the so far unexplored terahertz (THz) regime 7,8,9 . Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin-current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is employed to drive spins 10,11,12 from a ferromagnetic Fe thin film into a nonmagnetic cap layer that has either low (Ru) or high (Au) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter 13 based on the inverse spin Hall effect 14,15 that converts the spin flow into a THz electromagnetic pulse. We find that the Ru cap layer yields a considerably longer spin-current pulse because electrons are injected in Ru d states that have a much smaller mobility than Au sp states 16 . Thus, spin current pulses and the resulting THz transients can be shaped by tailoring magnetic heterostructures, which opens the door for engineering high-speed spintronic devices as well as broadband THz emitters 7,8,9 , in particular covering the elusive range from 5 to 10THz.Contemporary electronics is based on the electron charge as information carrier whose presence or absence encodes the value of a bit. Much more efficient devices for low-power data storage and processing could be realized if the spin degree of freedom were used in addition 1,2,3,4 . The spintronics approach requires the generation and control of spin currents, that is, the transport of spin angular momentum through space 5,6 . Spintronic operations should be performed at a pace exceeding that of today's computers, which ultimately requires the generation of spin current pulses with terahertz (1 THz = 10 12 Hz) bandwidths 7,8 as well as the possibility to manipulate them in novel structures 17,18 . To date, femtosecond spin-current pulses have been successfully launched by optically exciting electrons in semiconductors 10 or ferromagnetic metals 11,12 . However, to enable ultrafast basic operations on these transients (such as buffering or delaying), their shape and propagation have to be controlled on subpicosecond time scales.Here, we employ magnetic heterostructures containing an optimally chosen nonmagnetic metallic layer whose electron mobility allows us either to trap or to transmit electrons and, thus, to engineer ultrafast spin pulses. The spin flow is probed in a contactless manner using the inverse spin Hall effect 14,15 (ISHE) that converts the spin current into a detectable THz electromagnetic pulse 13 . Our findings open up a route to device-oriented femtosecond spintronics as well as novel broadband emitters of THz radiation 7,8,9 .Our idea is illustrated in Fig. 1a, which shows a schematic of a ferromagnetic Fe film capped by a thin layer of Ru or Au. Absorption of a femtosecond laser pulse (photon energy 1...

To realize molecular spintronic devices, it is important to externally control the magnetization of a molecular magnet. One class of materials particularly promising as building blocks for molecular electronic devices is the paramagnetic porphyrin molecule in contact with a metallic substrate. Here, we study the structural orientation and the magnetic coupling of in-situ-sublimated Fe porphyrin molecules on ferromagnetic Ni and Co films on Cu(100). Our studies involve X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments. In a combined experimental and computational study we demonstrate that owing to an indirect, superexchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferromagnetically. The Fe magnetic moment can be rotated along directions in plane as well as out of plane by a magnetization reversal of the substrate, thereby opening up an avenue for spin-dependent molecular electronics.

This Colloquium reviews the 25 year quest for understanding the continuous (2 nd ) order, meanfield-like phase transition occurring at 17.5 K in URu 2 Si 2 . Since ca. ten years the term hidden order (HO) has been coined and utilized to describe the unknown ordered state, whose origin cannot be disclosed by conventional solid-state probes, such as x-rays, neutrons, or muons. HO is able to support superconductivity at lower temperatures (Tc ≈ 1.5 K) and when magnetism is developed with increasing pressure both the HO and the superconductivity are destroyed. Other ways of probing the HO are via Rh-doping and very large magnetic fields. During the last few years a variety of advanced techniques have been tested to probe the HO state and their attempts will be summarized. A digest of recent theoretical developments is also included. It is the objective of this survey to shed additional light on the HO state and its associated phases in other materials. VII. Present State of HO 12Acknowledgments 16 References 17Figures 21 I. HISTORICAL BACKGROUNDUranium is a most intriguing element, not only in itself but also as a basis for forming a variety of compounds and alloys with unconventional or puzzling physics properties (for recent reviews, see Sechovský and Havela (1998), Santini et al. (1999), Stewart (2006)). Natural or depleted uranium, i.e. containing 99.5 percent 238 U has a * Electronic address: mydosh@physics.leidenuniv.nl † Electronic address: peter.oppeneer@physics.uu.se mild alpha radioactivity of 25 kBq/g, which allows Ubased samples to be fabricated and studied in university laboratories with a minimum of safety precautions. Following initial discoveries of unexpected superconductivity and heavy-fermion behavior in uranium-based compounds as UBe 13 (Ott et al., 1983) and UPt 3 (Stewart et al., 1984) it has become popular to synthesize uranium compounds and to cool these in search for exotic ground states. Over the past 50 or so years many conducting and insulating systems were synthesized, analyzed and structurally characterized (Sechovský and Havela, 1998;Stewart, 2001Stewart, , 2006. The usual classification of the metallic samples at low temperature is superconducting and/or magnetic or with some of the modern compounds designated as "exotic" (Pfleiderer, 2009).Why are uranium-based materials so interesting? The observed variety of unusual behaviors derive directly from the U open 5f shell. Several defining electronic structure quantities of the U f electrons are all on the same energy scale: the exchange interaction, 5f bandwidth, the spin-orbit interaction, and intra-atomic f − f Coulomb interaction. As a consequence, i) elemental uranium displays intermediate behavior between the transition metals and the rare-earths in their characteristic bandwidths, yet it generates the largest spin-orbit coupling. ii) U lies directly on the border between localized and itinerant (or overlapping) 5f wavefunctions. iii) The WignerSeitz radii R WS of comparative elements places U near the minimum between metallic and...