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Transition from ferromagnetism to diamagnetism in undoped ZnO thin films
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.ORCID iD: 0000-0003-2170-0076
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
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2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 3Article in journal (Refereed) Published
Abstract [en]

We report a systematic study of the film thickness dependence (0.1-1 mu m) of room-temperature ferromagnetism in pure magnetron-sputtered ZnO thin films wherein a sequential transition from ferromagnetism to paramagnetism and diamagnetism as a function of film thickness is observed. The highest saturation magnetization (M-S) value observed is 0.62 emu/g (0.018 mu(B)/unit cell) for a similar to 480 nm film. On doping the ZnO film with 1 at. % Mn enhances the M-S value by 26%. The ferromagnetic order in ZnO matrix is believed to be defect induced. In addition, on doping with Mn hybridization between the 2p states of O and the 3d states of Mn occurs.

Place, publisher, year, edition, pages
2009. Vol. 95, no 3
Keyword [en]
diamagnetic materials, ferromagnetic materials, ferromagnetic-paramagnetic transitions, II-VI semiconductors, magnetic semiconductors, magnetic thin films, manganese, paramagnetic materials, semiconductor doping, semiconductor thin films, spontaneous magnetisation, sputter deposition, zinc compounds
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-14173DOI: 10.1063/1.3180708ISI: 000268405300055Scopus ID: 2-s2.0-67749124267OAI: oai:DiVA.org:kth-14173DiVA: diva2:331398
Note

QC 20100722

Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Defect Induced Room-Temperature Ferromagnetism in ZnO and MgO Thin FIlms and Device Development
Open this publication in new window or tab >>Defect Induced Room-Temperature Ferromagnetism in ZnO and MgO Thin FIlms and Device Development
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: KTH, 2009. 50 p.
Keyword
Room-temperature ferromagnetism, intrinsic defects, un-balanced/balanced magnetron sputtering, magnetic anisotropy and photoconductivity
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11330 (URN)978-91-7415-456-6 (ISBN)
Public defence
2009-11-06, FB42 AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100722Available from: 2009-10-27 Created: 2009-10-26 Last updated: 2010-07-22Bibliographically approved

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