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Room temperature ferromagnetism in pristine MgO 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.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
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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.

Place, publisher, year, edition, pages
2010. Vol. 96, no 23
Keyword [en]
ENERGY
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-13939DOI: 10.1063/1.3447376ISI: 000278695900048Scopus ID: 2-s2.0-77953525394OAI: oai:DiVA.org:kth-13939DiVA: diva2:328494
Note

QC20100705

Available from: 2010-07-05 Created: 2010-07-05 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Functional Metal Oxide Materials Deposited by Inkjet Printing Technique
Open this publication in new window or tab >>Functional Metal Oxide Materials Deposited by Inkjet Printing Technique
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents a comprehensive study of the intrinsic room temperature ferromagnetism in 85 to 100nm thin films produced by in situ deposition of industrially important pristine MgO and ZnO by inkjet printing. It is suggested that the observed long range magnetic order, the so-called do magnetism in these oxides arises from cation vacancies. Wide range scanned XPS studies rule out any other source but of intrinsic nature in this first report on inkjetted oxide films. (Accepted for publication in APL 2010; and JPC Letters 2010)

The functional properties of the above oxide films have been exploited by inkjet patterning devices based on UV-sensing, photoconductivity and field-effect transistors with attractive features. Development of devices and prototypes is an area of growing importance among future technologies. Complementary to photolithography, inkjet printing is increasingly considered a cost-effective and flexible microfabrication method to structure functional materials. The ease of mass fabrication and the inherent flexibility of inkjet technology make it a suitable method for the manufacture of microsystems and components. The results presented in this dissertation summarize recent achievements in this relatively new technology for the development of miniaturized devices:

1)      RTFM in pure and Mn-doped ZnO thin films: (IEEE Trans. Magnetics 2010) Tailored RTFM in Mn-doped ZnO thin films synthesized by inkjet printing has been realized. Highly c-axis oriented 80 to 400 nm thin films are obtained with a magnetic moment as large as 2.1 μB/Mn2+. X-ray absorption and emission spectra measurements at the O K edge and the Mn L edge suggest strong p-d hybridization between the Mn2+ and O2-. Furthermore, Mn L edge XAS indicates the emergence of Mn3+/Mn4+ mixed valence states for films annealed above 500 °C that leads to suppression of ferromagnetic ordering.

2)      RTFM in MgO thin films (JPC Letters 2010) Solution processed homogeneous (200) oriented MgO ~85 nm thin films show room temperature ferromagnetism with a saturation magnetization MS as high as ~0.63 emu/g. X-ray photoelectron spectroscopy investigations show the absence of any contamination while the Mg 2p, and O 1s spectra indicate the role of defect structure at the Mg site. By controlling the pH values of the precursors the concentration of the defects can be varied and hence to tune the magnetization.

3)      Photoconductivity of pure and Al-doped ZnO thin films: (2009 MRS proceedings). Pure and Al-doped ZnO, 120-300 nm thin films are found to exhibit a transmittance above 85-90% in the visible wavelength range. The electrical conductivity of Al-doped ZnO thin films is found to be larger by two orders of magnitude than that for pure ZnO films while the photoconductivity is found to increase by about three orders of magnitude under UV irradiation.

4)      The photosensitivity of CaS-composited ZnO thin films: (JPC Letters 2010) We have engineered a 3 orders of magnitude enhancement of the ultraviolet photoresponse of ZnO thin films, fabricated by inkjet printing, and then capped with CdS nanoparticles by dip coating. As a consequence, the decay time of the photoresponse is reduced to about 4 ms. Capping with CdS not only suppresses the detrimental passivation layer of ZnO thin films, but also generates an interfacial carrier transport layer to reduce the probability of carrier recombination.

5)      Indium-doped Zinc oxide field effect transistors: (Materials Letters 2010) In darkness the In-doped zinc oxide field effect transistor (IZO-FET) exhibits a saturation current level of about 10 μA, an incremental mobility as high as 8 cm2 V-1 s-1, and a current on/off ratio of 104 ~105. When illuminated by 363 nm, 1.7 mW/cm2 UV light, the IZO-FET displays a photocurrent of 2 mA, and a darkness current of ~20 nA at an optimized gate voltage of -2V. The device is effectively turned on in about 5 ms and off in 10 ms.



Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xii, 63 p.
Keyword
Inkjet printing, photoconductivity, ZnO, Al-ZnO, composite, electron transfer, CdS, field effect transistor, photosensitive, RT do-ferromagnetism, Mn-doped ZnO, MgO
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13191 (URN)978-91-7415-667-6 (ISBN)
Public defence
2010-06-11, Salongen KTHB, Osquars Backe 31, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100706Available from: 2010-06-02 Created: 2010-06-02 Last updated: 2010-07-06Bibliographically approved
2. 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
3. Atomistic modelling of functional solid oxides for industrial applications: Density Functional Theory, hybrid functional and GW-based studies
Open this publication in new window or tab >>Atomistic modelling of functional solid oxides for industrial applications: Density Functional Theory, hybrid functional and GW-based studies
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this Thesis a set of functional solid oxides for industrial applications have been addressed by first principles and thermodynamical modelling. More specificially, measurable quantities such as Gibbs free energy, geometry and electronic structure have been calculated and compared when possible with experimental data. These are crystalline and amorphous aluminum oxide (Al2O3), Zirconia (ZrO2), magnesium oxide (MgO), indiumoxide (In2O3) and Kaolinite clay (Al2Si2O5(OH)4).

The reader is provided a computation tool box, which contains a set of methods to calculate properties of oxides that are measurable in an experiment. There are three goals which we would like to reach when trying to calculate experimental quantities. The first is verification. Without verification of the theory we are utilizing, we cannot reach the second goal -prediction. Ultimately, this may be (and to some extent already is) the future of first principles methods, since their basis lies within the fundamental quantum mechanics and since they require no experimental input apart from what is known from the periodic table. Examples of the techniques which may provide verification are X-Ray Diffraction (XRD), X-ray Absorption and Emission Spectroscopy (XAS and XES), Electron Energy Loss Spectroscopy and Photo-Emission Spectroscopy (PES). These techniques involve a number of complex phenomena which puts high demands on the chosen computational method/s. Together, theory and experiment may enhance the understanding of materials properties compared to the standalone methods. This is the final goal which we are trying to reach -understanding. When used correctly, first principles theory may play the role of a highly resolved analysis method, which provides details of structural and electronic properties on an atomiclevel. One example is the use of first principles to resolve spectra of multicomponentsamples. Another is the analysis of low concentrations of defects. Thorough analysis of the nanoscale properties of products might not be possible in industry due to time and cost limitations. This leads to limited control of for example low concentrations of defects, which may still impact the final performance of the product. On example within cutting tool industry is the impact of defect contents on the melting point and stability of protective coatings. Such defects could be hardening elements such as Si, Mn, S, Ca which diffuse from a steel workpiece into the protective coating during high temperature machining. Other problems are the solving of Fe from the workpiece into the coating and reactions between iron oxide, formed as the workpiece surface is oxidized, and the protective coating.

The second part of the computational toolbox which is provided to the reader is the simulation of solid oxide synthesis. Here, a formation energy formalism, most often applied to materials intended in electronics devices is applied. The simulation of Chemical Vapour Deposition (CVD) and Physical Vapor Deposition (PVD) requires good knowledge of the experimental conditions, which can then be applied in the theoretical simulations. Effects of temperature, chemical and electron potential, modelled concentration and choice of theoretical method on the heat of formation of different solid oxides with and without dopants are addressed in this work. A considerable part of this Thesis is based upon first principles calculations, more specifically, Density Functional Theory (DFT) After Kohn and Pople received the Nobel Prize in chemistry in 1998, the use of DFT for computational modelling has increased strikingly (see Fig. 1). The use of other first principles methods such as hybrid functionals and the GW approach (see abbreviations for short explanations and chapter 4.5 and 5.3.) have also become increasingly popular, due to the improved computational resources. These methods are also employed in this Thesis.

Place, publisher, year, edition, pages
Stockholm: KTH, 2011
Keyword
density functional theory, oxides, GW
National Category
Condensed Matter Physics Condensed Matter Physics Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-29257 (URN)978-91-7415-868-7 (ISBN)
Public defence
2011-02-18, F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110201Available from: 2011-02-01 Created: 2011-01-28 Last updated: 2012-03-28Bibliographically approved
4. Multifunctional magnetic materials prepared by Pulsed Laser Deposition
Open this publication in new window or tab >>Multifunctional magnetic materials prepared by Pulsed Laser Deposition
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

     Pulsed LASER deposition (PLD) is widely recognized as excellent deposition technique owing to stoichiometric transfer of target material, easy preparation and high quality. Thin films from few nanometers to micrometer regime can be fabricated with equal ease. Although a batch process is not suitable for mass scale industrial production, PLD is a versatile technique, efficient and convenient for high quality basic research.  This thesis illustrates the use of PLD technique to study the emerging trends in tailoring multifunctional magnetic thin films both from basic nanoscience and device development point of view.

     After a comprehensive review of magnetism in chapter 1: entitled ‘A journey through classical to modern trends in magnetism, and multifunctional thin film devices’ followed by a reasonably thorough discussion on Pulsed Laser thin film technique in Chapter 2, we present:

  1. Studies of tailoring composite high energy product permanent magnetic FePtB based thin films for applications in NEMS /MEMS, (Chapter 3).
  2. Study of search for new multiferroic materials by investigating the properties of Chromites. Crystalline Chromites are antiferromagnetic below 150oC.  However depositing thin films by PLD of the crystalline 95.5% dense targets produced by Surface Plasma Sintering, we discovered that the resulting films were amorphous and ferromagnetic beyond room temperature. Moreover advanced spectroscopic techniques revealed that the amorphized state is metallic with Cr in a mixed valence state.   An understanding of the underlying physics of the observed phenomenon has been carried out based on first principles calculations.  These results are now being considered for publication in a high profile journal.  Extensive studies on the films showing that these materials are ferromagnetic, but not ferroelectric are discussed in chapter 4. A preliminary presentation of these studies was pier reviewed and published in MRS symposium proceedings.
  3. Fabrication of Room temperature, Transparent, high moment soft ferromagnetic amorphous Bulk metallic glass based FeBNbY thin films by PLD, suitable for Nanolithography in NEMS/MEMS device development .  (Chapter 5)

From a basic study point of view on new trends on magnetism we present:

4. The use of PLD technique to demonstrate room temperature ferromagnetism in undoped MgO, and V-doped MgO thin films.  Both of these oxides which do not contain any intrinsically magnetic elements and are diamagnetic in their bulk form belong to a new class of magnetic films, the so called d0magnets signifying that robust above room temperature ferromagnetism arising from defects and controlled carriers and no occupied d-states can be tailored in semiconductors and insulators.  These, mostly ZnO and MgO based thin films which may be classified as Dilute Magnetic Semiconductors, DMS, and Dilute Magnetic Insulators, DMI, are now the materials of active interest in future Electronics involving components which exploit both charge and spin of electrons in the arena of SPINTRONICS.

Extensive characterization of magnetic, electrical, optical properties and microscopic structure has ensured development of high quality magnetic materials for future applications. Further research on these promising materials is expected to yield new generation spintronic devices for better performance in terms of efficiency, energy consumption and miniaturization of sizes.

Place, publisher, year, edition, pages
Stokholm: KTH Royal Institute of Technology, 2012. xxii, 140 p.
Keyword
Magnetic materials, thin films, pulsed laser depsoition, spintronics, chromites, MgO, NEMS, MEMS
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-94852 (URN)978-91-7501-386-2 (ISBN)
Public defence
2012-05-29, F3, Lindstedtsvagen 26 KTH, Stockholm, 14:30 (English)
Opponent
Supervisors
Note
QC 20120511Available from: 2012-05-11 Created: 2012-05-11 Last updated: 2012-05-11Bibliographically approved

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