LABORTORY FOR DIFFRACTION, MAGNETOMETRY AND MAGNETIC RESONANCES
Learning outcomes of the course unit
The main goals of this course, either specific to the experimental techniques and physics subjects dealt with, or belonging to a broader scope, are detailed as follows. We stress their correspondence to several Dublin Descriptors (DD).
Specific targets are
1) learning fundamentals of some important experimental techniques (DD: knowledge and understanding);
2) acquiring practical skills about important supporting technologies for experimental Physics (for instance: cryogenics, vacuum techniques, electric signal conditioning);
3) getting a deeper understanding of a few selected subjects already treated in introductory solid state physics courses, by combining theory with an experimental point of view (DD: applying knowledge and understanding); possibly, extending the body of knowledge from those courses by the student's individual study of
materials provided by the teacher, though under the supervision of the latter (DD: learning skills).
Beyond its specific contents, the main purpose of this course may be summarized in helping the student to develop a correct experimental attitude. The latter involves
1) conceiving and setting up experiments independently;
2) understanding the operation principles of the equipment;
3) developing criticism towards experimental results (DD: making judgements), namely, taking into account their range of confidence, minimizing the sources of systematic errors, and recognizing the possible effect of instrument malfunctions;
4) evaluating and presenting the experimental data with appropriate methods and software tools (DD: applying knowledge and understanding);
5) following good laboratory practice.
None beyond the body of knowledge and skills already supplied by the first-level graduation course.
Course contents summary
The essential part of the course consists in performing two long experiments out of three available, each one involving a spectroscopy of great relevance for experimental condensed matter physics: X-ray diffraction (XRD), solid-state nuclear magnetic and/or quadrupole resonance (NMR/NQR), electron paramagnetic resonance. XRD and NMR experiments are carried out in respective research laboratories of the Physics and Earth Science Department of this university, while a continuous-wave EPR spectrometer belonging to the teaching laboratory is employed for electron spin resonance. The application of these techniques is focused on studying specific physical phenomena: the temperature-dependent spontaneous magnetization in a ferro- or ferrimagnet (NMR), structural transitions in solids (XRD and/or NQR), hyperfine interactions (EPR). Laboratory sessions are preceded by a few introductory classroom lectures, whereby students are provided with the necessary background of physical knowledge, and the operation principles of the experimental equipments are explained in some detail.
Experiments will presumably deal with the following subjects.
1) NMR. Study of the magnetic order parameter in ferromagnetic (FM) or ferrimagnatic (FIM) compounds, by means of the nuclear probes available in the material. Candidate compounds are, for instance, lanthanum Sr-doped manganites La(1-x)Sr(x)MnO3 (FM), where the 55Mn and 139La resonances may be exploited; and Mn-doped magnetite Fe(3-x)Mn(x)O4 (FIM), to be studied by means of 55Mn and 57Fe. These materials are particularly suited to be studied between 77 e 300K, owing to their their very intense (55Mn) or intense enough (139La, 57Fe) NMR signals thanks to the so-called enhancement mechanism, and to Curie temperature well above room temperature. After preparing the sample, the tank circuit, and setting up the cryogenic apparatus, students will record NMR spectra in the spontaneous hyperfine field at several temperatures, analyze them an fit mean resonance frequencies as a function of temperature, which are proportional to the local magnetic moment. Students will be required to compare the results from different nuclear probes and to discuss possible discrepancies.
2) EPR. Study of the anisotropic g-factor of Cr3+ or Cu2+ ions in single crystals, by recording absorption spectra at different crystal orientations with respect to the applied magnetic field. Resolution of the hyperfine structure and determination of the hyperfine coupling constants e.g. in Mn2+ salts. Study of the influence of dilution and/or the exchange interaction on the linewidth.
3) XRD. Acquisition of powder spectra at room temperature by means of a Bruker diffractometer, refinement of space group and lattice parameters by means of dedicated computer programs (e.g. Fullprof). Study of a structural transition taking place just above or just below room temperature, by recording and refining diffractograms as a function of temperature.
Charles Kittel, "Introduction to Solid State Physics", John Wiley & Sons
D. Preston E. Dietz, "The art of experimental physics", John Wiley & Sons
Research and review papers from international scientific journals.
The course is opened by a few introductory classroom lectures aimed at providing the students with the essential theoretical and technical-instrumental background. Student groups (2-3 members each) are then led to run a couple of experiments out of three ones proposed, each one presumably taking 7-8 laboratory sessions (4 hours each) on average. Owing to the employment of "big instruments" (NMR and EPR spectrometers, XRD diffractometer), experiments are initiated under the close supervision of the teacher. During each experiment, however, students are encouraged to develop increasing autonomy in setting up and running measurements, in using the equipment and evaluating the experimental results.
Assessment methods and criteria
Learning verification takes first place in the laboratory sessions, thanks to the continuous presence of the teacher. After each experiments and before the beginning the next one, students are then supposed to write a concise and thorough experimental report. The latter must provide a clear description of the experiment purpose, the most relevant physics involved, the instrumentation employed, the analysis of the experimental data, and the conclusions. The final examination consists in a colloquium on the experimental reports.
Compatibly with the schedules of the hosting laboratories, students will be generally granted the possibility of setting up and running experiments beyond the official timetable, in order to meet specific needs of either students (e.g. working students) or of the experiment itself (e.g. long measurements to be run overnight).