presentedby dipl. phys. ETH

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1 Diss. ETH No MAGNETISM IN 30 PICOSECONDSAND MORE A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZÜRICH for the degree of Doctor of Natural Sciences presentedby Andreas Vaterlaus dipl. phys. ETH born September 12,1962 Citizen of Zürich (ZH) accepted on the recommendation of Prof. Dr. H.C. Siegmann,examiner PD. Dr. F. Meier, co examiner Prof. Dr. W. Baltensperger, co examiner 1991

2 Lattice TABLE OF CONTENS Abstract Kurzfassung 1 Time resolved Spin Polarized Photoemissionwith Short Laser Pulses 1.1 Introduction 1.2 Upper and Lower Limits for the Spin of Iron 1.3 Determination of the Spin Gadolinium 1.4 Conclusion RelaxationTime Lattice Relaxation Time for 2 Laser Induced Surface Structures 2.1 Introduction 2.2 Experiment and Interpretation 2.3 Discussion 3 Spin Iron Polarizationof the ElectronsThermoemitted from Nickel and 3.1 Introduction 3.2 Experiment 3.3 Theory References Dank Lebenslauf

3 polarized. probe ABSTRACT The spinlattice relaxation time (%s& is the characteristic time needed for the energy transfer between the lattice and the spin System. Due to a lack of appropriate experimental tools Tse has not been determined so far. In the following it will be demonstratedthat Ts can be measured with laser induced spinpolarized photoemission. Upper and lower limits are derived from a Single pulse experiment where one laser pulse serves for the heating of the sample and for photoemission. By performing this experiment with laser pulses of different time durations it is found that Xst of iron is shorter than 20 nanoseconds (ns) but longer than 30 picoseconds (ps). An exact value of ist is derived from a pump and a second time experiment. One laser pulse heats the sample delayed pulse is used to probe the magnetic State. Xsl is found to be (100 ± 80) ps for ferromagneticgadolinium. Melting a surface with short laser pulses induces a variety of different surface structures. Depending on the laser pulse intensity and the number of pulses linear, quadratic, hexagonal and chaotic topographic patterns are observed. These structures are typical for many nonlinear Systems as, for instance, the Rayleigh new and simple access to a nonlinear System. spin Benard convection. The described experimentoffers a Electrons thermoemitted from ferromagnetic nickel and iron are not This is the simple outcome of the experiments described in the third part of the present work. If cesium is deposited onto well prepared Ni or Fe surfaces a photothreshold as low as 1.4 ev is obtained and found to be stable up to a temperature of 450 K. Under these conditions a thermocurrent of about 1 na is predicted by the Richardson equation and observed in the experiment. The spin polarization of these electrons is found to be zero.

4 Gitter Benard KURZFASSUNG Die Spin Gitter Relaxationszeit (is ) ist die charakteristische Zeit, welche für den Energieübertrag vom Gitter auf das Spinsystembenötigt wird. Da bisher keine geeignetemessmethodezur Verfügung stand, konnte Xsl nicht bestimmt werden. In der vorliegenden Arbeit soll gezeigt werden, dass Xst mit laser induzierter, spin polarisierterphotoemission gemessen werden kann. Untere und obere Schranken werden mit Hilfe eines einfachen Ein Puls Experimenteserhalten. Ein und derselbe Laserpuls dient dabei zum Heizen der Probe und zur Emission von Photoelektronen, deren Spinpolarisation gemessen wird. Durch das Durchführen dieses Experimentesmit Laserpulsen unterschiedlicherpulslänge wird gezeigt, dass Xst von Eisen zwischen 30 Picosekunden (ps) und 20 Nanosekunden (ns) liegen muss. Ein "Pump Probe" Experimentliefert einen genauen Wert für x$l. Bei diesem Experiment dient ein Laserpuls zum Heizen und ein zweiter, zeitlich verzögerter Puls zum Messen der Magnetisierung. Für ferromagnetisches Gadolinium wird eine Spin Relaxationszeit von (100 ± 80) ps gefunden. Durch das Schmelzen einer Probenoberfläche mit kurzen Laserpulsen kann eine Vielfalt von Oberflächenstrukturen erzeugt werden. Lineare, quadratische, hexagonale und chaotische Muster werden je und Anzahl der Laserpulse beobachtet. nichtlineare Systeme, wie zum Beispiel die Raleigh nach Intensität Diese Strukturen sind typisch für viele Konvektion. Das beschriebene Experiment stellt ein neues, einfaches System zum Studium nichtlinearer Phänomene dar. Die aus ferromagnetischem Nickel und Eisen thermoemittierten Elektronen weisen keine Spinpolarisationauf. Dies ist das einfache Resultat der im dritten Teil dieser Arbeit beschriebenen Experimente. Wird Cäsium auf

5 gut präparierte Ni und Fe Oberflächen aufgedampft, so können Proben hergestellt werden, bei welchen die Austrittsarbeit auch bei Temperaturen bis zu 450 K stabil bei 1.4 ev bleibt. Unter diesen Bedingungen werden Thermoemissionsströme bis zu 1 na erwartet und auch beobachtet. Es wird gefunden, dass diese Elektronen keine Spinpolarisationzeigen.

6 1. TIME RESOLVED,SPIN SHORTLASERPULSES POLARIZED PHOTOEMISSIONWITH LI Introduction establish its After a fast change of the lattice temperature a magnet needs time to new equilibrium magnetization. characteristictime, the spinlattice It will be demonstrated that this relaxation time (xs0, can be determinedwith laserinduced spinpolarized photoemission. Apart from its evident fundamental interest the knowledgeof XsC is also of technologicalimportance: it determines, for instance, the maximum speed attainable in Curie point writing, the most widely used technique in magnetooptical recording. Only few experiments are able to probe the spinorbit coupling and its time scale. Kerr effect measurements with short laser pulses gave an upper limit of 40 ns and a lower limit of 20 ps for Xgeof nickel [1]. In ferromagnetic resonance (FMR) the longitudinalrelaxation time Ti contributes Parameters to the line width of the FMR signal. among other Ti is determined by the time the System needs to restore the direction of the magnetization after a small misorientation. The spinlatticerelaxation time Xs& in contrast, is the time the magnetization needs to assume a new equilibrium magnitude after an abrupt change of the lattice temperature. A fast change of the lattice temperature can be induced with short and intense laser pulses. When hitting a sample surface they first induce a hot electron within a few picoseconds [24]. gas which then thermalizes with the lattice Photoemission with spin analysis is described in Ref.5. The spin polarizationp of the photoelectrons is where NT (Ni) is defined as P (NT = NI) / (NT + Ni) the number of electrons with spin magnetic moment parallel

7 (antiparallel) to the surface normal of the sample. P is proportional to the magnetization along the surface normal of the sample. Along this direction an external field up to 3 Tesla can be applied. The temperature of the sample is variable from 30 K up to several hundred K. The experiment is made timeresolved by using short laser pulses as light source for the photoemission [6,7]. Then the magnetization is probed in a time interval determined by the duration of the laser pulse. In the present experimentthe polarization of the total photoyield is measured, i.e. without energy analysis of the electrons. It has been shown in Ref. 6 that space Charge does not affect the total This is below. the polarization of photoyield because of conservation of the total angular momentum. a basic requirementfor the Performanceof the experiments described Upper and lower limits of x^are derived from the set of measurements described in section 1.2. One and the same laser pulse is used to heat the sample and to emit photoelectrons. The polarizationof the photoelectrons measured as function of the laser pulse intensity decreases if the Curie temperature is reached in the laser focus and if the spinlattice relaxation time is short compared to the laser pulse duration. During a laser pulse with a duration much shorter than Xs the magnetization has no time to change its magnitudeeven if the pulse intensity is sufficient to melt the sample. found that Xs of iron lies between 30 ps and 20 ns. in section 1.3. It is A pump probe experiment which gives an exact value of Xs is described A laser pulse with a photon energy below the photothreshold (hv < <(») is used to heat the sample and a second time delayed pulse with hv > $ probes the magnetic State. For ferromagnetic gadoliniumxs is found to be (100 ±80)ps.