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Defect Induced Room-Temperature Ferromagnetism in ZnO and MgO Thin FIlms and Device Development
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
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 [en]
Room-temperature ferromagnetism, intrinsic defects, un-balanced/balanced magnetron sputtering, magnetic anisotropy and photoconductivity
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-11330ISBN: 978-91-7415-456-6 (print)OAI: oai:DiVA.org:kth-11330DiVA: diva2:274073
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
List of papers
1. Transition from ferromagnetism to diamagnetism in undoped ZnO thin films
Open this publication in new window or tab >>Transition from ferromagnetism to diamagnetism in undoped ZnO thin films
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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.

Keyword
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:nbn:se:kth:diva-14173 (URN)10.1063/1.3180708 (DOI)000268405300055 ()2-s2.0-67749124267 (Scopus ID)
Note

QC 20100722

Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2017-12-12Bibliographically approved
2. On the defect induced ferromagnetic ordering above room-temperature in undoped and Mn doped ZnO thin films
Open this publication in new window or tab >>On the defect induced ferromagnetic ordering above room-temperature in undoped and Mn doped ZnO thin films
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2009 (English)In: NOVEL MATERIALS AND DEVICES FOR SPINTRONICS / [ed] Sanvito S, Heinonen O, Dediu VA, Rizzo N, Warrendale, PA: MATERIALS RESEARCH SOCIETY , 2009, Vol. 1183, 3-8 p.Conference paper, Published paper (Refereed)
Abstract [en]

Evidence for long range ferromagnetic order above room-temperature, RTFM, in pristine ZnO, In2O3, TiO2 nanoparticles and thin films, containing no nominal magnetic elements have been reported recently. This could question the origin of RTFM in doped dilute alloys if for example the ZnO matrix itself develops a defect induced magnetic order with a significant moment per unit cell. In this presentation we report a systematic study of the film thickness dependence of RTFM in pure ZnO deposited by DC Magnetron Sputtering. We observe a maximum in the saturation magnetization, Ms, value of 0.62 emu/g (0,018 ÎŒB/unit cell), for a -480 nm film deposited in an oxygen ambience of appropriate pressure. Above a thickness of around 1 ÎŒm the films are diamagnetic as expected. We thus see a sequential transition from ferromagnetism to para- and eventual diamagnetism as a function of film thickness in ZnO. We also find that in such a ZnO matrix with a maximum intrinsic defect induced moment, on doping with Mn the maximum enhanced Ms value of 0.78 emu/g is obtained for 1 at. % Mn doping. With this approach of appropriate doping in a defect tailored matrix, we routinely obtain RTFM in both undoped and Mn- doped ZnO thin films.

Place, publisher, year, edition, pages
Warrendale, PA: MATERIALS RESEARCH SOCIETY, 2009
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172 ; 1183
Keyword
Dc magnetron sputtering, Dilute alloys, Ferromagnetic orderings, Induced magnetic order, Intrinsic defects, Long range ferromagnetic order, Magnetic elements, matrix, Mn-doped ZnO, Mn-doping, Per unit, Pure ZnO, Room temperature, Significant moments, Systematic study, TiO, Unit cells, ZnO, ZnO matrix, Defects, Diamagnetism, Doping (additives), Ferromagnetic materials, Ferromagnetism, Magnetic films, Manganese, Manganese oxide, Metallic films, Nanomagnetics, Optical films, Oxygen, Saturation magnetization, Thin films, Zinc oxide
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-14174 (URN)000284863300001 ()2-s2.0-77950964230 (Scopus ID)
Conference
Symposium on Novel Materials and Devices for Spintronics San Francisco, CA, APR 14-17, 2009
Note
QC 20100722Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2011-01-04Bibliographically approved
3. Room temperature ferromagnetism in pristine MgO thin films
Open this publication in new window or tab >>Room temperature ferromagnetism in pristine MgO thin films
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 23Article in journal (Refereed) Published
Abstract [en]

Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/cm(3) is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.

Keyword
ENERGY
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13939 (URN)10.1063/1.3447376 (DOI)000278695900048 ()2-s2.0-77953525394 (Scopus ID)
Note

QC20100705

Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2017-12-12Bibliographically approved
4. Experimental evidence for ferromagnetism at room-temperature in MgO thin films
Open this publication in new window or tab >>Experimental evidence for ferromagnetism at room-temperature in MgO thin films
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2010 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 34, 345004- p.Article in journal (Refereed) Published
Abstract [en]

Ferromagnetic ordering at room temperature (RTFM) in MgO thin films deposited by RF magnetron sputtering under various atmospheric conditions and temperatures is reported. A saturation magnetization (MS) value as high as 1.58 emu g(-1) is (0.046 mu B/unit cell) observed for a 170 nm film deposited at RT under an oxygen pressure of 1.3 x 10(-4) mbar. In contrast, films deposited at elevated temperature (under an identical oxygen pressure), or at higher oxygen pressures, as well as under a nitrogen atmosphere at RT show significantly suppressed magnetization. The ferromagnetic order in the MgO matrix is believed to be defect induced

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-14175 (URN)10.1088/0953-8984/22/34/345004 (DOI)000280847200006 ()2-s2.0-77957135524 (Scopus ID)
Note

QC 20100722. Updated from submitted to published 20120327

Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2017-12-12Bibliographically approved
5. Defect induced magnetic anisotropy in undopted ZnO thin films
Open this publication in new window or tab >>Defect induced magnetic anisotropy in undopted ZnO thin films
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(English)Article in journal (Other academic) Submitted
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-14176 (URN)
Note
QS 20120327Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2012-03-27Bibliographically approved
6. Coexistence of ultraviolet photo-response and room-temperature ferromagnetism in polycrystalline ZnO thin films
Open this publication in new window or tab >>Coexistence of ultraviolet photo-response and room-temperature ferromagnetism in polycrystalline ZnO thin films
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2010 (English)In: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, ISSN 0167-577X, Vol. 64, no 11, 1291-1294 p.Article in journal (Refereed) 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.

Keyword
Photoconductivity, Room-temperature ferromagnetism, Unbalanced magnetron sputtering
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-14177 (URN)10.1016/j.matlet.2010.03.011 (DOI)000278148900014 ()2-s2.0-77950516143 (Scopus ID)
Funder
Swedish Foundation for Strategic Research VINNOVASwedish Research CouncilCarl Tryggers foundation
Note

QC 20100722

Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2017-12-12Bibliographically approved
7. Effects of hole doping in the ferromagnetic semiconductor Mn-doped ZnO thin film studied by x-ray magnetic circular dichoism
Open this publication in new window or tab >>Effects of hole doping in the ferromagnetic semiconductor Mn-doped ZnO thin film studied by x-ray magnetic circular dichoism
2009 (English)In: "Solid state devices and materials(SSDM2009)" held in Japan(7-9 october 2009), 2009Conference paper, Published paper (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-14178 (URN)
Note
QC 20100722Available from: 2010-07-22 Created: 2010-07-22 Last updated: 2010-07-22Bibliographically approved

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