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Coexistence of ultraviolet photo-response and room-temperature ferromagnetism in polycrystalline ZnO thin films
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
Vise andre og tillknytning
2010 (engelsk)Inngår i: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, ISSN 0167-577X, Vol. 64, nr 11, s. 1291-1294Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The coexistence of ultraviolet (UV) photoconductivity (PC) and room-temperature ferromagnetism (RTFM) is observed in polycrystalline ZnO thin films deposited by unbalanced magnetron sputtering under high oxygen pressure. A significant increase in PC (similar to 870% to 40000%) is observed with increasing film thickness and the consequent structural disorder and film porosity. In contrast, the saturation magnetization (M(S)) at room temperature is found to decrease from 1.02 emu/g to 0.53 emu/g with increasing film thickness from 50 to 150 nm.

sted, utgiver, år, opplag, sider
2010. Vol. 64, nr 11, s. 1291-1294
Emneord [en]
Photoconductivity, Room-temperature ferromagnetism, Unbalanced magnetron sputtering
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-14177DOI: 10.1016/j.matlet.2010.03.011ISI: 000278148900014Scopus ID: 2-s2.0-77950516143OAI: oai:DiVA.org:kth-14177DiVA, id: diva2:331441
Forskningsfinansiär
Swedish Foundation for Strategic Research VINNOVASwedish Research CouncilCarl Tryggers foundation
Merknad

QC 20100722

Tilgjengelig fra: 2010-07-22 Laget: 2010-07-22 Sist oppdatert: 2017-12-12bibliografisk kontrollert
Inngår i avhandling
1. Defect Induced Room-Temperature Ferromagnetism in ZnO and MgO Thin FIlms and Device Development
Åpne denne publikasjonen i ny fane eller vindu >>Defect Induced Room-Temperature Ferromagnetism in ZnO and MgO Thin FIlms and Device Development
2009 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

This thesis presents the discovery of defect induced room-temperature ferromagnetism in industrially important ZnO and MgO thin films, and establishes from a systematic study, in both ZnO and MgO films, the unique phenomenon of the sequences of transitions from ferromagnetism to para-, and eventually the well known diamagnetism of the bulk as a function of film thickness.

Highly oriented and high quality dense thin films of ZnO and MgO have been deposited by reactive (balanced) magnetron sputtering under different ambience conditions and deposition temperatures. The ZnO thin films were deposited from a Zn metal target whereas the MgO thin films were deposited from an MgO ceramic target. Their magnetic properties have been studied as a function of both film thickness and variation in oxygen deposition pressure (for a given thickness) using a SQUID magnetometer. The ferromagnetic ordering in these materials is shown to arise from lattice defects situated at the cation sites. We discuss in detail the observed variation in their saturation magnetization, MS, as a function of the various deposition conditions and film characteristics (i.e. film thickness), and relate these to the nature and role of the intrinsic defects in giving rise to the observed magnetism. The in-plane saturation magnetization obtained in these films is at least two orders of magnitude larger as compared to what is measured in nanoparticles of similar dimensions. Furthermore it is shown that the magnetic properties in these thin films is directional dependent and that along the diagonal of the wurtzite structure at 45 degrees to the c-axis the MS values are about 60% larger. This we correlate with a calculation based on the structure which shows that the cation- cation distances along the diagonal is the shortest by similar magnitude. A Zn57O57 super-cell has been modelled using the Inorganic Crystal Structure Database (ICSD Diamond 3.0), from which we have calculated the shortest distance between two adjacent cation sites (i.e. potential cation vacancy sites) along the c-axis as well as perpendicular and along the diagonal (i.e. 45°) to the c-axis (along which the films have grown). Such possibilities to tailor defect induced ferromagnetism resulting in saturation magnetization of ≈ 5 emu/g, is indeed highly important information in understanding and designing thin film devices. In order to further tailor the physical property of polycrystalline ZnO thin films, un-balanced magnetron sputtering was used to obtain porous microstructured ZnO thin films to induce significant UV photoconductivity and demonstrate plausible device application.

The above studies have been made possible using extensive characterization of the high quality films, in the thickness range from a few nanometers to almost a micron, using XRD for structure, Dual beam HRSEM/FIB and AFM for accurate film cross-sectioning and surface morphology, EDXS for elemental analysis and electrical/photo- conductivity measurements over a wide range of incident radiation from UV to visible.

The overall conclusion is that the room-temperature ferromagnetic ordering in the ZnO and MgO thin films originates from cation vacancies which couple ferromagnetically and establish long range magnetic order.

sted, utgiver, år, opplag, sider
Stockholm: KTH, 2009. s. 50
Emneord
Room-temperature ferromagnetism, intrinsic defects, un-balanced/balanced magnetron sputtering, magnetic anisotropy and photoconductivity
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-11330 (URN)978-91-7415-456-6 (ISBN)
Disputas
2009-11-06, FB42 AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00 (engelsk)
Opponent
Veileder
Merknad
QC 20100722Tilgjengelig fra: 2009-10-27 Laget: 2009-10-26 Sist oppdatert: 2010-07-22bibliografisk kontrollert

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