Experiments
3-1 Sample preparation
We experimentally demonstrate that it is possible to induce room temperature ferromagnetism in 2p-light-element doped TiO2 prepared simply by conventional solid state reaction method. We divide our experiment into two part, one is to anneal the high purity rutile TiO2 (99.998%) powders twice under different nitrogen pressures at 1273K for 2hours. The other is utilized high purity TiO2 to mix with carbon (99.999%) powders. The mixtures were then heated at 1273K for 6 hours under a reduced pressure of <6x10-4 torr to promote reactions among the mixing constituents. The solidified mixture was then grinded and pressed using a mold to form the target body; the target body was heated twice at 1273K for 12 hours. This step is intended to increase the contact area between carbon and TiO2 powders. Care was taken to prepare sample from directly contacting any ferrous tools or vessels during synthesis and characterization processes. Structural and magnetic characterizations of 2p-doped TiO2 samples were carried out by means of x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID), and electron paramagnetic resonance (EPR).
3-2 Measurements
(I) X-ray diffraction (XRD)
X-ray diffraction is a common technique for studying crystal structures and variations in atomic spacing. It is based on constructive interference of
monochromatic X-rays when passing through a crystalline sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ) (Fig.11). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, counted and processed. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacing allows identification of the mineral because each mineral has a set of unique d-spacing.
Typically, this is achieved by comparisons of the obtained d-spacing information with the standard reference patterns.
Fig. 11 The illustration of XRD process.
(II) X-ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is a popular surface analytic technique. It can provide detailed information about the elemental composition, chemical state and electronic structure of the elements that exist within a material [66-68]. XPS is based on the photoelectric effect, whereby a sample surface emit electrons after being irradiated by a photon source of sufficiently high energy, as shown in Fig.12 . For XPS, a soft x-ray
(50－1500 eV) are used as the exciting photon source. The surface atoms emit electrons (called photoelectron) after direct transfer of energy from the photon to the core-level electron, as illustrated in Fig. 13. The emitted electron is called photoelectron and the emitting process is named photoemission. The photoelectrons are subsequently separated according to their energy via an electron spectrometer.
Fig. 12 Illustration of photoelectric effect
Fig. 13 The X-ray photon transfers its energy to a core-level electron imparting enough energy for the electron to leave the atom
E E E Φ
Because the energy of a particular x-ray wavelength equals a known value, we can determine the electron binding energy of each of the emitted electrons by using an equation that is based on the work of Ernest Rutherford:
where E is the energy of the electron emitted from one electron configuration within the atom, E is the energy of the x-ray photons being used, E is the kinetic energy of the emitting electron as measured by the instrument and Φ is the work function of the spectrometer (not the material).
Core level peaks can provide direct information on the chemical bonding by XPS measurement. The characteristic binding energies of photoelectrons are not only element specific, but also depend on the chemical state of the corresponding atom.
Chemical shifts are recorded as a displacement (typically in the range of 0 to 4 eV) in binding energies of photoelectrons excited from atoms in a compound compared to the energies of the corresponding pure substance. Binding energies increase, e.g., with the oxidation state of a substance, as part of the electronic density is transferred to the oxidizing species, leaving the remaining electronic density unbalanced against the positive nuclear charge.
(III) Superconducting Quantum Interference Device (SQUID)
A superconducting quantum interference device (SQUID) is equipment used to measure extremely weak signals, such as subtle the magnetic field changes in living organisms. SQUID is designed by use of the principal of macroscopic long-range quantum interference and consists of two parallel Josephson junctions made of two superconductors separated by a thin insulating layer. The device may be configured as a magnetometer to detect incredibly small magnetic fields. The great sensitivity of the SQUID devices is associated with measuring changes in magnetic field associated with one flux quantum. One of the discoveries associated with Josephson junctions were the flux is quantized in units. If a constant biasing current is maintained in the SQUID device, the measured voltage oscillates with the changes in phase at the two junctions, which depends upon the change in the magnetic flux. Recording the
oscillations allows us to evaluate the flux change which has occurred. Hence, SQUID as magnetometer provides the information of magnetic field gradient or magnetic field of sample.
(IV) Electron Paramagnetic Resonance (EPR)
Electron Paramagnetic Resonance (EPR) often called Electron Spin Resonance (ESR) is a branch of spectroscopy in which electromagnetic radiation (usually of microwave frequency) is absorbed by molecules, ions or atoms possessing electrons with unpaired spins.
Ever since the discovery of Electron Paramagnetic Resonance by
Zavoiskii, it has been developed into an important branch of spectroscopy and becomes a valuable tool in all branches of science wherever systems containing unpaired electrons are investigated.
Intrinsic spin is a unique quantum property of electron, nuclei, and nuclear and sub nuclear particles. Charged particles with spin or orbital angular momentum will have magnetic moments, a vector quantity denoted as such by the bold character μ. The magnetic moment μ interacts with the static vector magnetic field B to produce an energy U through the following scalar product relationship.
U µ · B
This component of the electron energy is designated as the Zeeman interaction. Static magnetic field generates an energy-level structure. Magnetic fields that vary rapidly with time stimulate transitions between energy-levels or states, giving rise to spectral lines. Energy states are quantitatively called the spin Hamiltonian. From the Hamiltonian, the energy level structure can be derived. The intrinsic spin gives the electron its intrinsic spin magnetic charge will create a current loop. An electron can have angular momentum as it moves not only around its own axis but also in an orbit.
The component µ of electron-spin magnetic moment along the direction of the magnetic field B applied alon tg he direct on z is i
µ g β M ; β |q|
2m
where g is the Zeeman factor. This was originally introduced to correct and increase, by a factor of 2, the relationship between magnetic moment and spin from what would be expected from the relationship between magnetic moment and orbital angular momentum.
For a single unpaired electron, the possible values of Ms are and . Hence
the possible values of µ are g β and the values of U are g β B (Fig. 14).
These are sometimes referred to as the e ectr nl o ic Zeeman energy.
∆U g β B
Fig. 14 Energy-level scheme for the simplest system (e.g., free electron) as a function of applied magnetic field B, showing EPR absorption.
By placing the unpaired electrons in a magnetic field, we have increased the number of energy levels from one to two. In fact, sometimes in the region near the electron there is an atomic nucleus with a magnetic moment. The magnetic moment of
the nucleus is restricted to a few orientations with respect to an external magnetic field. The magnetic energy of the electron is affected by the orientation of the nuclear magnetic moment. It is the reason why two magnetic energy levels for the electron are each further split into a few sublevels. This interaction of the nucleus and the electron is called the hyperfine interaction.
Fig. 15 Hyperfine splitting in the hydrogen atom in a high field.
The applied magnetic field splits the energy levels in to M and states.
Each Ms state splits into two MI states due to interaction with He nucleus (I= ). We observe transitions between levels 4 and 1, 3 and 2. Therefore the “selection rules” for EPR transitions are ∆M 1, ∆MI 0. The proton has a spin of . By the 2I+1 rule, the number of possible orientations is two. The number of levels is produced by hyperfine splitting. That is the reason there will be four levels. The Hamiltonian for the magnetic interaction in a trong ext s ernal fie d s l i
K gµ H S aI S
By using the Hamiltonian we find that the energy in the s te M Mta I is U M MI gµ H M aM MI
Most of the electrons in atoms, molecules, and solids do not give EPR signals. Due to electrons are not unpaired in these substances. This means that for every electron in the Ms= state there is another electron in the same orbital and in the Ms= .
Transition from the state to the state would put two electrons in the same orbital and the same spin state. Unpaired electrons of substance are called paramagnetic substances which include organic free radicals, transition metal ions, metals, crystals with certain defects.
EPR experiments manipulate a Bruker spectrometer at X-band (ν=9 GHz). DPPH was used as a reference for calibration of g factors. The shape and the area of the EPR spectra were analyzed by standard numerical methods.
Chapter 4