IMFM, Physics Dept.

Welcome to the Department of Physics at IMFM -

Laboratory for NQR and weak magnetic fields

Our research is directed towards studies of magnetism and it spans from magnetism in condensed matter physics to magnetism in living state - biomagnetism. Cryophysics and cryotechnology are also parts of our interest. We are active in basic research as well as in applied research.

Research of magnetic properties in condensed matter physics

The research of magnetic properties in condensed matter physics in its broadest sense is an important part of research in physics. It is one of the earlier methods of research in solids. The results of basic research have reached here exceptionally high number of applications and this tendency is present still today. The new nanomaterials are often characterized with interesting magnetic properties and a large variety of applications are expected, spanning from humane health care to defense purposes. Expansions in magnetochemistry and magnetic research in biology is expected in Ljubljana.

Several institutions in Slovenia are active in the above mentioned research. The traditional research in nuclear magnetism (NMR, NQR, EPR) in Ljubjana was complemented by a home built SQUID magnetometer about 10 years ago. Its measuring capacity soon appeared to be too small and we recently purchased a new Magnetic Properties Measuring System MPMS-XL-5 (Quantum Design, San Diego, USA) with improved sensitivity and user friendly automated measuring software. It contains facilities for the dc and ac magnetization measurements as a function of temperature and applied magnetic field; zero field and sample rotation option is available. This way a high quality magnetic measuring system is added and we can easily cover the growing magnetic research needs.

In order to achieve better efficiency of this powerful equipment we established an association of several research institutions, called the Center for Magnetic measurements - CMag. Present members are;

Institute of Mathematics, Physics and Mechanics
"Jožef Stefan" Institute
National Institute of Chemistry Slovenia
Faculty of Mathematics and Physics, University of Ljubljana
Faculty of Chemistry and Chemical Technology, University of Ljubljana

CMag is open for new national and international partners according to the rules of the center.

A heat capacity measuring system, working in the low temperature region (4K-30K) was recently added.

NQR research

Nuclear Quadrupole Resonance (NQR) with its ability of identification specific molecules in measured sample is potentially powerful method in solid state physics, chemistry and pharmacy. NQR data can be used to get additional information on molecular and crystal structure, on lattice dynamics, on chemical bond character, for nondestructive characterization of some solid chemical compounds, etc. The NQR spectroscopy is a good supplement to NMR and EPR spectroscopy. Sharp, orientation independent NQR lines can be obtained using either powder samples or single crystals. In NQR it is the interaction of nuclear quadrupole moment with the electric field gradient at the site of the nucleus (the intrinsic crystal field caused by the intra- and interatomic electric fields) which determines the energy splitting of nuclear quantum states. The rf magnetic field is the only external field, necessary to excite the quadrupole transitions. Additional external (quasi)static magnetic or electric field may be applied only to get more information. Drawback of the pure NQR method is that relatively large number of chemically equivalent quadrupole nuclei is needed (approx. 10¹⁹) to obtain sufficient S/N ratio.

Some subjects of our interest are:

the detection of some illicit materials such as drugs and explosives via the ¹⁴N NQR,
the phase transitions in molecular crystals with halogen atoms (nitrosyl halogenides NOX (X=Cl, Br), methyl mercury halogenides CH₃HgX (X=Cl,Br,I), studied by halogen NQR,
arsenic NQR in M(AsF6)_n complexes, containing paramagnetic ions M,
different less ordered structures (incommensurate structures, nonstoichiometric samples, high temperature superconductors, drying process in stanous chloride hydrates, etc.)

Measurements of biomagnetic signals

magnetocardiograms
other biomagnetic fields. e.g. magnetic field originated from intracellular current in a single cell of green algae Chara corallina. Injury induced ionic currents and the corresponding magnetic fields in Vicia faba plants.
Measurements of biomagnetic fields are usually followed by calculations (modeling) of the current sources causing the measured magnetic field. These calculations are known as the so called inverse problem.
Model studies in cardiac electrophysiologic research is here the main activity. Both electrocardiography (ECG) and magnetocardiography (MCG) are included. The very detailed heart model allows us to simulate many of actual pathological events in human heart and this way contribute to improvement of some heart diagnosis.

Main experimental instrumentation:

Quatum Designa MPMS-XL5 magnetometer with 5 T superconducting magnet, temperature range from 1.9 K up to 400 K, ac-susceptibility and ultra-low field option, with sample rotator. ...more in CMag ...,
Two commercial NQR spectrometers, a superregenerative type 2 - 300 MHz and a pulse type 1 - 100 MHz, are used to investigate the NQR spectra in crystalline materials and the dependence of NQR spectra on outer parameters (temperature, electric and magnetic fields, etc.). Polarization enhanced (level crossing) pulsed NQR spectrometer (0.1 - 10 MHz) for ¹⁴N NQR measurements. Alternative ways of resonance detection (using SQUID) are studied as well.
SQUID based magnetometric system has been constructed and is used to measure electronic magnetization in the temperature range 4-30 K and in fields up to 150 Gauss.
Single channel SQUID magnetogradiometer for biomagnetic measurements in our remote magnetically quiet laboratory (Golovec) is sometimes used for preliminary test measurements. Majority of our biomagnetic measurements are done in our partner institute laboratories (PTB, Institut Berlin) in Berlin Charlottenburg and Berlin Steglitz.

Cryophysics

Cryophysics has been an accompanying part of our research activities practically from the beginning. However, with the applications of SQUID sensor it became more important one and now it can be divided in two parts:

Research end development of small refrigerators (up to 2 W cooling power at 77 K) of Stirling type as well as an extension towards pulse tube type of small refrigerators. The second one is especially interesting because of its potential possibility to replace the Stirling type of refrigerator in all applications where the reduced mechanical vibrations are necessary.
Research of electron gas cooling resulting during tunneling of electrons trough junction SIN, SINIS. Here stands S for superconductor, I for insulator and N for normal metal. Under the favorite conditions the electron gas can be coupled to the phonons and the lattice can be slightly cooled for few mK. That takes place at the lattice temperature around 100 mK.

Applied physics

Various contributions to applied research came from our group in the last 20 years, like:

Research and manufacturing of the prototype of the m-computer controlled apparatus for the determination of colour components. Work done for Saturnus R.&D. Laboratory, 1978/79. J. Luznik, A. Kogovsek, J. Pirnat, E. Srebotnjak and Z. Trontelj.
Research and manufacturing of the prototype of the apparatus for the measurements of the amount of adsorbed gases on glass surfaces. Work done for the Sava R.&D. Laboratory, 1981. J. Luznik and Z. Trontelj.
Development and manufacturing of the prototype of the m-computer controlled apparatus for the measurements of the heat conductance of thin isolation materials. Work done for the Sava R.&D. Laboratory, 1981, 1982. J. Luznik, A. Kogovsek, J. Pirnat, Z. Trontelj.
Research and development of the prototype of the linear resonance motor. Work done for the Institute Zoran Rant Skofja Loka 1984, 1985. J. Luznik and Z. Trontelj.
Research and development of the prototype of the apparatus for the detection of local obstacles in the tube transportation system. Work done for the Sava R.&D. Laboratory, 1985. J. Luznik and Z. Trontelj.
Research and development of the prototype of Stirling type of minirefrigerator. Work in cooperation with the Institute Z. Rant, Skofja Loka. 1987-89, 1993-99. J. Luznik, A. Jeromen, Z. Trontelj and several members of the Institute Z. Rant.

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