Although discovered about three decades ago, the peculiar properties of synthetic antiferromagnetic (SAF) thin films consisting of two ferromagnetic layers separated by a non-magnetic metal spacer have recently revived a renewed interest as potential candidates for a number of innovative and advanced applications including spintronics and biotechnology. SAFs are key component in spintronic devices and a significant attention has been recently paid on the preparation of such devises on flexible substrates, which provide wide advantages over their conventional rigid-substrate counterparts, such as the ability to bend and adjust the shape of a device, a light-weight and low costs. While the progress and development of systems with longitudinal magnetic anisotropy on non-planar substrates has been remarkable over the last few years, flexible magneto-resistive heterostructures with perpendicular magnetic anisotropy (PMA) are rather unexplored despite they allow for additional functionality and improved performance. On the other hand, for diagnostic and therapeutic applications, perpendicular magnetized SAF microdisks prepared by top-down lithographic approaches have been recently proposed as a valid alternative to the most investigated superparamagnetic particles synthetized by chemical routes as they fulfill all the key criteria required for biomedical applications while allowing a significant degree of control and tunability of the magnetic properties. Within this context, this thesis aims at developing and studying magneto-resistive spintronic devices on flexible substrates and microdiscs for biomedical applications based on SAF thin film stacks with PMA. The focus was on Co/Pd- and Co/Ni-based systems due to their strong PMA (~106 J/m3) and the possibility to finely tune their magnetic properties by varying the thickness of the individual layers and the number of repetitions N of the Co/Pd(Ni) bilayer. In particular, flexible Co/Pd(Ni)-based giant magnetoresistance spin-valve thin film stacks consisting of a [Co/Pd(Ni)]N free layer and a fully compensated [Co/Pd(Ni)]N/Ru/[Co/Pd(Ni)]N synthetic antiferromagnet reference electrode separated by a Cu spacer, were prepared by direct deposition on flexible substrates and by exploiting both wet and dry-etching transfer-and-bonding approaches. Measurements under bending conditions were also performed to investigate the robustness of the flexible spin-valves and the possibility for their integration on curved surfaces. The optimized SAF stacks were also used for the preparation of thin fil stacks consisting of multiple repeats of single [Co/Pd]N/Ru/[Co/Pd]N SAF units with perpendicular magnetic anisotropy with the aim to fabricate free-standing SAF microdisks by using lithographic processes.
Antiferromagnetici sintetici (SAF) costituiti da due strati ferromagnetici separati da un sottile strato metallico non magnetico hanno recentemente suscitato un rinnovato interesse come potenziali candidati per una serie di applicazioni innovative e avanzate nell’ambito della spintronica e della biotecnologia. I SAF sono componenti chiave nei dispositivi spintronici e una significativa attenzione è stata recentemente prestata alla preparazione di tali sistemi su substrati flessibili in virtù dei significativi vantaggi che offrono rispetto a dispositivi fabbricati su substrati rigidi convenzionali, come la capacità di piegare e regolare la forma del substrato, un minor peso e costi contenuti. Sebbene il progresso e lo sviluppo di sistemi spintronici con anisotropia magnetica longitudinale su substrati non planari sia stato notevole nel corso degli ultimi anni, eterostrutture magneto-resistive flessibili con anisotropia magnetica perpendicolare (PMA) sono piuttosto inesplorate nonostante consentano funzionalità aggiuntive e prestazioni migliorate. D'altra parte, per applicazioni diagnostiche e terapeutiche, microdischi SAF con magnetizzazione perpendicolare preparati con approcci litografici top-down sono stati recentemente proposti come valida alternativa alle più studiate particelle superparamagnetiche sintetizzate per via chimica in quanto soddisfano tutti i criteri chiave richiesti per applicazioni biomedicali, consentendo al contempo un significativo grado di controllo e modulazione delle proprietà magnetiche. In questo contesto, la tesi si propone di sviluppare e studiare dispositivi magneto-resistivi su substrati flessibili e microdischi per applicazioni biomedicali basati su SAF a film sottile con PMA. L'attenzione si è concentrata su sistemi basati su Co/Pd e Co/Ni in virtù dell’elevata anisotropia magnetica (~106 J/m3) e della possibilità di regolare in maniera fine le proprietà magnetiche variando lo spessore dei singoli strati e il numero di ripetizioni N del doppio strato di Co/Pd(Ni). In particolare, multistrato flessibili con struttura spin-valve e magnetoresistenza gigante, costituiti da un free layer di [Co/Pd(Ni)]N e da un reference layer SAF con struttura [Co/Pd(Ni)]N/Ru/[Co/Pd(Ni)]N separati da uno strato di Cu, sono stati preparati sia mediante deposizione diretta su substrati flessibili che attraverso l’utilizzo di strategie transfer-and-bonding di tipo wet e dry. Sono state inoltre eseguite misurazioni in condizioni di flessione per indagare la robustezza delle spin-valve flessibili e la possibilità della loro integrazione su superfici curve. Film SAF ottimizzati sono stati infine impiegati per la preparazione di multistrato a film sottile costituiti da ripetizioni multiple di singole unità SAF con struttura [Co/Pd]N/Ru/[Co/Pd]N e anisotropia magnetica perpendicolare allo scopo di fabbricare microdischi SAF free-standing mediante processi litografici.
Perpendicularly magnetized synthetic antiferromagnets for flexible spintronic and biomedical applications / Hassan, Mariam. - (2022 Sep 07).
Perpendicularly magnetized synthetic antiferromagnets for flexible spintronic and biomedical applications
HASSAN, MARIAM
2022-09-07
Abstract
Although discovered about three decades ago, the peculiar properties of synthetic antiferromagnetic (SAF) thin films consisting of two ferromagnetic layers separated by a non-magnetic metal spacer have recently revived a renewed interest as potential candidates for a number of innovative and advanced applications including spintronics and biotechnology. SAFs are key component in spintronic devices and a significant attention has been recently paid on the preparation of such devises on flexible substrates, which provide wide advantages over their conventional rigid-substrate counterparts, such as the ability to bend and adjust the shape of a device, a light-weight and low costs. While the progress and development of systems with longitudinal magnetic anisotropy on non-planar substrates has been remarkable over the last few years, flexible magneto-resistive heterostructures with perpendicular magnetic anisotropy (PMA) are rather unexplored despite they allow for additional functionality and improved performance. On the other hand, for diagnostic and therapeutic applications, perpendicular magnetized SAF microdisks prepared by top-down lithographic approaches have been recently proposed as a valid alternative to the most investigated superparamagnetic particles synthetized by chemical routes as they fulfill all the key criteria required for biomedical applications while allowing a significant degree of control and tunability of the magnetic properties. Within this context, this thesis aims at developing and studying magneto-resistive spintronic devices on flexible substrates and microdiscs for biomedical applications based on SAF thin film stacks with PMA. The focus was on Co/Pd- and Co/Ni-based systems due to their strong PMA (~106 J/m3) and the possibility to finely tune their magnetic properties by varying the thickness of the individual layers and the number of repetitions N of the Co/Pd(Ni) bilayer. In particular, flexible Co/Pd(Ni)-based giant magnetoresistance spin-valve thin film stacks consisting of a [Co/Pd(Ni)]N free layer and a fully compensated [Co/Pd(Ni)]N/Ru/[Co/Pd(Ni)]N synthetic antiferromagnet reference electrode separated by a Cu spacer, were prepared by direct deposition on flexible substrates and by exploiting both wet and dry-etching transfer-and-bonding approaches. Measurements under bending conditions were also performed to investigate the robustness of the flexible spin-valves and the possibility for their integration on curved surfaces. The optimized SAF stacks were also used for the preparation of thin fil stacks consisting of multiple repeats of single [Co/Pd]N/Ru/[Co/Pd]N SAF units with perpendicular magnetic anisotropy with the aim to fabricate free-standing SAF microdisks by using lithographic processes.File | Dimensione | Formato | |
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