Mössbauer spectroscopy is based in the Mössbauer effect, that is, in the emission and resonant absorption of gamma rays by nucleii withouth energy loss due to nuclear recoil. The nuclear resonant absorption has been observed in more than one hundred nuclear transitions of different isotopes of various elements. Mössbauer spectroscopy is only applied to solids or frozen solutions independently of their crystalline or amorphous character. From those all transitions where the Mössbauer effect has been observed the most popular, by far, is the 14.4 keV transition of ⁵⁷Fe (to which most work is devoted). Other popular isotopes (but less used) are ¹¹⁹Sn, ¹⁵¹Eu and ¹²¹Sb.

The characteristics of the 14.4 keV transition of ⁵⁷Fe and its relative isotopic abundance (2%) make possible to work at reasonable experimental conditions: room presure and temperature using reasonable amounts of sample (usually a few miligrams). The possibility of performing Mössbauer spectroscopy with ⁵⁷Fe is very fortunate since iron is an element of the most scientific and technological importance. It is involved in magnetism, catalysis, corrosion, biology, mineralogy, metallurgy and many other interesting and important fields.

By using Mössbauer spectroscopy one can quantify the magnitude of the hyperfine interactions. From the quantification of these interactions, which depend on the environment in which the Mössbauer atom is located, chemical, structural and magnetic information can be obtained. For example, the oxidation state, the cordination type or the magnitude of the hyperfine magnetic field, if there is any kind of magnetic ordering, can be easily determined. By recording spectra at different temperatures, information about magnetic ordering temperatures can be inferred and by the application of external magnetic fields the type of magnetic ordering can be deduced.

Each iron species is characterized by three different hyperfine parameters. In complex samples as multiphasic samples or compounds with various iron sites, Mössbauer spectroscopy can be used to identify each phase ("fingerprint" method) or the different iron sites. Our work is mainly dedicated to ⁵⁷Fe Mössbauer spectroscopy although we also have sources for ¹¹⁹Sn Mössbauer spectroscopy. The spectrometers are located in a separate room in our laboratory area.

Of the three types of Mössbauer spectroscopy (transmission, ICEMS, ILEEMS), in Spain there are about 10 groups that have transmission equipment, less than three with ICEMS (i.e., for thin films) acquisition (including us) and one for ILEEMS.

Transmission spectrometer

Experimentally, the most common mode of operation in Mössbauer spectroscopy is the transmission mode where the gamma quanta rays emmitted from the sample and passing through a thin absorber arrive to an approppriate detector. This mode provides mainly bulk information from the sample, which must be thin enough for the gamma rays to go through the sample. We have an spectrometer devoted to transmission spectroscopy. In this one we often use powder samples, or micrometer-thick foils. The samples can be cooled down with a He-closed cycle cryorefrigerator for the sample which allows us to record spectra at different temperatures between 15 K and 298 K. This is often crucial to detect different iron compounds, as we do for our work on complex oxides.

Integral Conversion Electron Mössbauer Spectrometer (ICEMS)

For some nucleus, such as it occurs for ⁵⁷Fe, the deexcitation after the nuclear resonant absorption is more likely to occur via an internal conversion process, where electrons are emmitted from the sample. Since the mean free path of the electrons in a solid is usually short (depending on their energy) if these electrons are detected, Mössbauer spectroscopy can be turned surface sensitivity. This mode of operation is called Integral Conversion Electron Spectroscopy (ICEMS). Because of the energy of the electrons involved in the process, information from the uppermost 300 nm of a sample can be obtained, although this information is mainly weighted towards the most external 50 nm. By careful preparation of the samples, that usually involves their enrichment with ⁵⁷Fe, the method can be sensitive enough as to detect a fraction of a monolayer.

Our second spectrometer is devoted to ICEMS by means of a Parallel Plate Avalanche Counter (although we can also measure in transmission mode in required). This allows us to routinely measure films in the 10-300 nm thickness range, perfect for Pulse Laser Deposition films of single crystal oxides.

Integral Low Energy Electron Mössbauer Spectrometer (ILEEMS)

A final variant of Mössbauer spectroscopy using electrons is based on the detection of electrons of very low energy (Integral Low Energy Electron Spectroscopy, ILEEMS). By the approppriate application of a positive bias voltage at the cone entrance of a channeltron these low energy electrons can be accelerated and counted more efficiently making the technique very surface sensitive. Our third spectrometer belongs to this class, with both the sample and channeltron in ultra-high vacuum. This allows us to measure samples that are not conducting (unlike CEMS). We have another spectrometer being tested for its connection to our multipurpose growth and characterization chamber.

Fe-57 transmission, CEMS and LEEMS spectra recorded from iron-doped niobium titanium phosphorous oxide (Fe0.33NbTiP3O12). The Fe3+ doublet (dashed in the figure) is enhanced in the spectra recorded in the electron detection mode, particularly in the LEEMS case, indicating the surface sensitivity of the technique.