Structural and magnetic properties of V2O3-based magnetic heterostructures

Abstract

Vanadium sesquioxide, typified by its first-order metal-insulator transi- tion, structural phase transition, and magnetic changes, transition from a high-temperature rhombohedral metallic and paramagnetic state to a low- temperature insulating monoclinic antiferromagnet. The three transitions occur simultaneously, presenting a complex, intertwined phenomenon. In- vestigating the phase coexistence region in V2O3 is crucial, as its transition is intrinsically inhomogeneous, offering a unique scenario where two phases exist simultaneously within a narrow temperature range. This characteris- tic, coupled with the ability to manipulate it through various parameters, not only underscores V2O3’s distinctiveness but also amplifies its appeal from an applications standpoint, signaling a rich avenue for further research into this material’s potential. The complex nature of V2O3, characterized by simultaneous transitions, offers a unique opportunity to explore the properties of bilayers and observe how overlying layers are affected by the transitions through the interface. For instance, it allows for the creation of a system with a ferromagnet/paramagnet interface at room temper- ature and a ferromagnet/antiferromagnet interface at low temperatures. Particularly at low temperatures, interfacing a ferromagnetic layer with V2O3 introduces the potential for an exchange bias effect, which arises from the ferromagnetic/antiferromagnetic interface interaction. Exchange bias systems are extensively researched for their capacity to increase ferromag- netic coercivity and stabilize magnetization from thermal fluctuations, thus providing a method to manipulate magnetic properties. V2O3 demonstrates that alterations in crystal structure due to structural phase transitions can be leveraged to influence magnetic characteristics, thereby introducing an extra dimension of control via its magnetic transition phenomena. This thesis delves into the study of phase transitions in V2O3 and their inherent characteristics, as well as the subsequent impact on the magnetic properties of overlying magnetic layers. Specifically, we investigate the coercivity response to the phase transitions in V2O3 and emergence of exchange bias within a bilayer system incorporating a nickel (Ni) or a permalloy (Py) layer. The first part of the project was dedicated to optimizing the V2O3 layer on various substrates, including silicon and sapphire with different crystallographic orientations, aiming to achieve the highest crystallinity and effective epitaxy of V2O3. This optimization was crucial for observing the most pronounced metal-insulator transition and structural phase transition. To maintain a focused and concise discussion, these optimization details are not included in this thesis; however, they are documented and presented in the publications referenced herein that are not discussed in this thesis (Paper V and Paper VI). The second part of the project was devoted to investigating the coupling effects between phase transitions in V2O3 and magnetism of the Ni layer. Following the first part, a decision was made to grow a magnetic heterostructure on a sapphire substrate, specifically on two distinct crystallographic directions. This resulted in differing crystallographic and microstructural properties of V2O3, which in turn impacted the magnetic layer in various ways, as detailed in Paper I and Paper III. Concurrently, a different study focused on a magnetic layer composed of Py (Ni80Fe20), presented in Paper II. However, studying magnetic Py films on V2O3 presented challenges—discussed later in this thesis—and thus far, no significant magnetic response or effect of the SPT has been observed in the Py films. The bilayer structure, composed of Ni and V2O3, grown on sapphire r-plane, exhibited the most sensitive response to the phase transition, manifested in enhanced coercivity and tunable exchange bias. Consequently, a comprehensive study of the structural phase transition of V2O3 grown on r-plane sapphire was conducted, the findings of which are detailed in Paper III. Subsequently, the research emphasis shifted to a more detailed analysis of exchange bias, which is discussed in Paper IV, thus highlighting another facet of this thesis with equal foundational and practical significance.

Vanadínoxíð (V2O3) er hálfmálsoxíð sem fer í gegnum fasabreytingu þar sem kristalgerð þess, seguleiginleikar og rafeiginleikar breytast með hitastigi. Við stofuhita hefur það tígulflötungskristalgerð, er málmleiðandi og hefur meðseglandi seguleiginleika en ef það er kælt niður fyrir 150 K breytist það í einangrara með einhalla kristalgerð og andjárnseglandi segulgerð. Allar þessar þrjár fasabreytingar gerast samtímis og rannsóknir á þeim veita því innsýn í flókið samspil þeirra, sérstaklega þegar bæði háhita og lághita fasarnir eru til staðar samtímis. Með því að tvinna saman vanadíumoxíð við önnur efni í lagskiptum efnum má rannsaka hvernig fasabreytingarnar hafa áhrif á eiginleika nærliggjandi efna yfir samskeyti þeirra. Þannig má til dæmis byggja samskeyti efna sem eru járnseglandi/meðseglandi við stofuhita en járnseglandi/andjárnseglandi við lág hitastig. Sérstaklega er áhugavert að skoða slík samskeyti við lág hitastig því andjárnseglandi ástand vanadínoxíðs getur valdið skiptavíxlverkun á samskeytunum sem veldur hliðrun í seguleiginleikum. Vanadíumoxíð í lagskiptum efnum sýna því vel hvernig breytingar í kristalgerð nýtast til að hafa áhrif segulgerð og jafngilda því nýrri aðferð til að stjórna seguleiginleikum efna. Í þessari rannsókn er fasabreyting vanadínoxíðs skoðuð og áhrif hennar á seguleiginleika nærliggjandi efna. Í fyrri hluta verkefnisins var unnið að ræktun vanadínoxíð kristalla á ólík undirlög með það að markmiði að hámarka kristalgæði efnisins en kristalgerð vanadínoxíðs ræður miklu um eiginleika fasabreytingarinnar í efninu. Ritgerðin sjálf fjallar ekki sérstaklega um þennan hluta verkefnisins en vísindagreinar sem tengjast þessum rannsóknarhluta verkefnisins er hafðar með til hliðsjónar (greinar V og VI). Í seinni hluta verkefnisins var unnið að rannsóknum á hvernig fasabreytingar í vanadínoxíðihafa áhrif á seguleiginleika segulefna í tvílögum (greinar I, II, III og IV). Unnið var með vanadínoxíð húðir ræktaðar á safír undirlög og skoðað hvernig kristalgerð, fasabreytingar og kristalbygging vanadínoxíðs hefur áhrif á breytingar á eiginleikum segulhúða. Tvílög af vanadínoxíði og Ni segulhúðum sýndu sterkustu áhrif fasabreytingarinnar á seguleiginleikana sérstaklega hvað varðar segulheldnisvið og skiptavíxlverkun.