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Supermagnetism in Magnetic Nanoparticle Systems

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Supermagnetism in Magnetic Nanoparticle Systems Supermagnetism in magnetic nanoparticle systems (Supermagnetismus in magnetischen Nanoteilchensystemen) Vom Fachbereich Physik der Universität Duisburg-Essen (Campus Duisburg) zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation von Subhankar Bedanta aus Jignipur, Cuttack, Indien Referent : Prof. Dr. Wolfgang Kleemann Korreferent : Prof. Dr. Michael Farle Tag der mündlichen Prüfung : 11. Dezember 2006 Dedicated to my parents I Abstract Nanoscale magnetic materials are of interest for applications in ferrofluids, high-density magnetic storage, high-frequency electronics, high performance permanent magnets, and, magnetic refrigerants. Magnetic single-domain nanoparticles (“superspins) are very interesting not only for potential applications, e.g. high density storage devices, but also for fundamental research in magnetism. In an ensemble of nanoparticles in which the inter- particle magnetic interactions are sufficiently small, the system shows superparamagnetic (SPM) behavior as described by the Néel-Brown model. On the contrary, when inter- particle interactions are non-negligible, the system eventually shows collective behavior, which overcomes the individual anisotropy properties of the particles. In order to address the effect of interactions, we have investigated two different magnetic nanoparticle systems. The first part of this thesis focuses on the magnetic properties of ensembles of magnetic single-domain nanoparticles in an insulating matrix. The samples have a granular multilayer structure prepared as discontinuous metal-insulator multilayers (DMIM) [Co80Fe20 (tn)/Al2O3 (3nm)]m where the nominal thickness of CoFe is varied in the range 0.5 £ tn £ 1.8 nm, and the number of bilayers m is varied between 1- 10. The DMIMs represent a model system to study the effect of inter-particle interactions by varying the nominal thickness which corresponds to the magnetic particle concentration. The structural properties are investigated by transmission electron microscopy, small angle X-ray reflectivity and electric conductivity measurements. It is found that CoFe forms well- separated and quasi-spherical nanoparticles in the Al2O3 matrix, and the samples exhibit a regular multilayer structure. The magnetic properties are investigated by means of dc magnetization, ac susceptibility, polarized neutron reflectometry (PNR), magneto-optic Kerr effect and ferromagnetic resonance. The DMIM system with the lowest tn = 0.5 nm, in which the inter-particle interaction is almost negligible, single particle blocking has been observed. When increasing the nominal thickness to tn = 0.7 nm and, hence, increasing the inter-particle interaction, the system shows spin glasslike cooperative freezing of magnetic moments at low temperatures. Superspin glass properties have been evidenced by static and dynamic criticality studies such as memory and rejuvenation. With further increase of nominal thickness and hence stronger interaction, the system shows a superferromagnetic (SFM) state, e.g., at tn = 1.3 nm. A SFM domain state has been evidenced by Cole-Cole II analysis of the ac susceptibility and polarized neutron reflectivity measurements. Finally, the SFM domains have been imaged by synchrotron based photoemission electron microscopy (PEEM) and magneto-optic Kerr microscopy. Stripe domains stretched along the easy in-plane axis, but exhibiting irregular walls and hole-like internal structures (“domains in domains”) are revealed. They shrink and expand, respectively, preferentially by sideways motion of the long domain walls as expected in a longitudinal field. The SFM domain state is explained by dipolar interaction and tunneling exchange between the large particles mediated by ultrasmall atomically small magnetic clusters. These have been evidenced by their sizable paramagnetic contributions, first in systems referring to tn = 0.5 nm and 0.7 nm, but later on also at SFM coverages, tn = 1.3 nm and at higher coverages. These ultrasmall particles (atoms?) are undetectable in transmission electron microscopy. At tn = 1.4 nm, physical percolation occurs and a conventional three-dimensional (3D) ferromagnetic phase with Ohmic conduction is encountered. Polarized neutron reflectivity and magnetometry studies have been performed on the DMIM sample with tn = 1.6 nm which exhibits dominant dipolar coupling between the ferromagnetic layers. Our PNR measurements at the coercive field reveal a novel and unexpected magnetization state of the sample exhibiting a modulated magnetization depth profile from CoFe layer to layer with a period of five bilayers along the multilayer stack. With the help of micromagnetic simulations we demonstrate that competition between long and short-ranged dipolar interactions apparently gives rise to this unusual phenomenon. In the second part of the thesis the structural and magnetic properties of FeCo nanoparticles in liquid hexane will be analyzed for two different concentrations of the ferrofluids. Inter-particle SFM ordering between FeCo nanoparticles are evidenced by magnetization measurements and ac susceptibility measurements. Mössbauer spectroscopy measurements are shown to evidence collective inter-particle correlations between the nanoparticles. III Kurzfassung Magnetische Materialien auf der Nanoskala sind von hohem Interesse in zahlreichen Anwendungen, wie z.B. Ferrofluiden, Speichermedien, Hochfrequenzelektronik, Permanentmagneten und magnetischen Kühlmitteln. So sind insbesondere magnetisch eindomänige Nanopartikel ("superspins") nicht nur für Anwendungen, wie z.B. in der Speichertechnologie interessant, sondern auch für das Grundlagenverständnis im Magnetismus. In einem Ensemble von Nanopartikeln mit genügend kleiner magnetischer Wechselwirkung zwischen den Partikeln, zeigt das System superparamagnetisches (SPM) Verhalten, welches durch das Néel-Brown- Modell beschrieben werden kann. Umgekehrt, wenn die Inter-Partikel-Wechselwirkungen nicht vernachlässigbar sind, zeigt es kollektives Verhalten, welches dabei die individuellen Anisotropieeigenschaften der Partikel überwindet. Um diesem Effekt der Wechselwirkungen nachzugehen, haben wir zwei unterschiedliche Nanopartikelsysteme untersucht. Der erste Teil dieser Arbeit behandelt die Eigenschaften von Ensembles von magnetisch eindomänigen Nanopartikeln in einer isolierenden Matrix. Die Proben haben eine granulare Multilagenstruktur, die als diskontinuierliche Metall-Isolator- Vielfachschichten (DMIMs) der Form [Co80Fe20 (tn)/Al2O3(3nm)]m hergestellt werden. Die nominelle Dicke der CoFe-Schicht liegt dabei im Bereich 0.5 £ tn £ 1.8 nm und die Anzahl der Bilagen im Bereich 1 £ m £ 10. Diese DMIMs stellen ein hervorragendes Modell-System zum Untersuchen des Effekts der Inter-Partikel-Wechselwirkungen dar. Die nominelle Dicke entspricht hierbei der Partikelkonzentration. Die strukturellen Eigenschaften wurden mit Hilfe von Transmissionselektronenmikoskopie (TEM), Kleinwinkel-Röntgen-Streuung und elektrischen Transportmessungen studiert. So findet man, dass das CoFe getrennte und nahezu sphärische Nanopartikel in der Al2O3-Matrix bildet, und das ganze System eine exzellente Multilagenstruktur aufweist. Die magnetischen Eigenschaften wurden mittels DC-Magnetisierung, AC-Suszeptibilität, DC- Relaxation, magneto-optischem Kerr-Effekt (MOKE) und ferromagnetischer Resonanz untersucht. Im DMIM-System mit der niedrigsten nominelle Dicke, tn = 0.5 nm, und somit kleinster Inter-Partikel-Wechselwirkung wurde individuelles Blocking (SPM-Verhalten) gefunden. Bei einem größeren Wert von tn = 0.7 nm, und somit stärkeren Wechselwirkungen, zeigt das System spinglas-artiges kooperatives Einfrieren der magnetischen Partikelmomente bei niedrigen Temperaturen. Diese 'Superspinglas'- IV Eigenschaften wurden nachgewiesen durch statische und dynamische Untersuchungen, wie z.B. den Memory- und Rejuvenation-Effekt. Bei weiterer Vergrößerung der nominellen Dicke und somit stärkeren Wechselwirkungen zeigt das Ensemble einen superferromagnetischen (SFM) Zustand. Dieser SFM-Domänen-Zustand wurde nachgewiesen durch eine Cole-Cole-Plot-Analyze der AC-Suszeptibilität und durch polarisierte Neutronenreflektometrie (PNR). Es ist sogar gelungen diese SFM-Domänen direkt durch Photoelektronen-Emissionsmikroskopie (PEEM) an einem Synchrotron und MOKE-Mikroskopie darzustellen. Sichtbar sind Streifendomänen entlang der leichten planaren Achse, jedoch mit unregelmäßigen Wänden und loch-artigen Strukturen ("Domänen in Domänen") Wie erwartet wachsen bzw. schrumpfen die Domänen vorzugsweise durch seitliche Bewegung der langen Wände in einem longitudinalen Feld. Der SFM-Domänenzustand kann erklärt werden durch Dipol- und Tunnelaustausch- Wechselwirkung der Partikel sowie Wechselwirkungen über atomare magnetische Cluster. Diese extrem kleinen Cluster wurden durch deren paramagnetischen Beitrag zunächst in Systemen mit tn = 0.5 nm und 0.7 nm nachgewiesen, dann aber auch in SFM-Systemen mit tn = 1.3 nm. In beiden Fällen sind sie nicht durch TEM nachweisbar. Bei tn = 1.4 nm findet strukturelle Perkolation der Partikel statt und es wird eine gewöhnliche drei-dimensionale (3D) ferromagnetische Phase mit Ohm'schen Widerstand gefunden. PNR und Magnetisierungs-Messungen an der DMIM-Probe mit tn = 1.6 nm zeigen dominante dipolare Kopplung der ferromagnetischen Lagen. So zeigen die PNR- Daten nahe der Koerzitivfeldstärke einen neuartigen und unerwarteten Zustand, bei dem ein moduliertes Magnetisierungs-Profil im Multilagenstapel vorzufinden ist. Mit Hilfe von mikromagnetischen
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