Zvonko Trontelj

Electron paramagnetic resonance of magnetoelectric Pb(Fe1∕2Nb1∕2)O3

To check on the nature of the weak magnetic order in polycrystalline magnetoelectric Pb(Fe1∕2Nb1∕2)O3 the X-band, Q-band, and far infrared electron paramagnetic resonance (EPR) spectra have been measured between 4 and 600K and compared with magnetic susceptibility and magnetization data. The asymmetric line shapes can be simulated at higher temperature by thermally fluctuating superparamagnetic nanoclusters. The pronounced temperature dependence of the position of the spectra demonstrates the presence of an internal magnetic field which is small but nonzero even at room temperature, i.e., far above the antiferromagnetic transition. The electronic spin-spin exchange has been found to be in the terahertz range. The magnetization data reveal a weak ferromagnetism even above 300K and a break in the temperature dependence of susceptibility at the paramagnetic to ferromagnetic transition.

Ferromagnetism in a cobaltocene-doped fullerene derivative below 19 K due to unpaired spins only on fullerene molecules

Abstract We report the discovery and physical properties of a new organic ferromagnet, 3-aminophenyl-methano-fullerene[60]-cobaltocene, with a Curie temperature T C of 19 K. Near- and mid-infrared spectroscopy together with electron spin resonance experiments show that spins reside entirely on the fullerene units, from which we deduce that the ferromagnetism is a consequence solely of exchange interactions between distributed p-electrons on the 3-aminophenyl-methano-fullerene[60] molecules. The ferromagnetic state of the compound is characterized by a very rapid transition to ferromagnetic order below T C , a low saturation field of H s =40 Oe and no measurable hysteresis.

Multi-channel atomic magnetometer for magnetoencephalography: A configuration study

Atomic magnetometers are emerging as an alternative to SQUID magnetometers for detection of biological magnetic fields. They have been used to measure both the magnetocardiography (MCG) and magnetoencephalography (MEG) signals. One of the virtues of the atomic magnetometers is their ability to operate as a multi-channel detector while using many common elements. Here we study two configurations of such a multi-channel atomic magnetometer optimized for MEG detection. We describe measurements of auditory evoked fields (AEF) from a human brain as well as localization of dipolar phantoms and auditory evoked fields. A clear N100m peak in AEF was observed with a signal-to-noise ratio of higher than 10 after averaging of 250 stimuli. Currently the intrinsic magnetic noise level is 4fTHz(-1/2) at 10Hz. We compare the performance of the two systems in regards to current source localization and discuss future development of atomic MEG systems.

Weak ferromagnetism and ferroelectricity in K3Fe5F15

Here, we report on the observation of a weak ferromagnetic transition at TN=122K in a K3Fe5F15 system which is ferroelectric and ferroelastic below Tc=490K. The magnetization and the susceptibility continuously increase with decreasing T down to 2K. The Mossbauer spectra show a spontaneous magnetic ordering and at least three sites corresponding to Fe2+ and Fe3+. The ratio between Fe2+ and Fe3+ is 60:40. At 6K, there are two magnetically ordered sextets with internal fields of 585 and 263kOe. The “slim” hysteresis loops observed are as well characteristic of weak ferromagnetism. At 122K and at low frequencies, the system shows dielectric anomaly characteristic of magnetoelectric behavior.

Polarization enhanced 14N NQR detection with a nonhomogeneous magnetic field

Abstract The use of proton polarization enhancement of low frequency 14 N NQR signals was tested. An improvement of NQR signals was observed in samples of (N 3 (NO 2 ) 3 (CH 2 ) 3 , (CH 2 ) 6 N 4 and (NO 2 )C 6 H 4 COOH). A very nonhomogeneous magnetic field obtained with a permanent magnet was used for proton polarization.

nuclear magnetic resonance and electron spin resonance of amorphous hydrogenated carbon

The temperature dependences of the and magnetization recovery and spin-lattice relaxation times as well as the NMR spectra have been studied between 300 K and 4 K. The observed short and nearly temperature independent proton and spin-lattice relaxation times demonstrate that the dominant spin-lattice relaxation mechanism is spin diffusion to paramagnetic impurities. The fact that the magnetization recovery curves clearly deviate from the single-exponential form expected for the case of spin diffusion and randomly distributed paramagnetic centres demonstrates that the paramagnetic centres aggregate in clusters. Superparamagnetic freezing effects expected for such an inhomogeneous distribution are indeed seen below 50 K in the temperature dependence of the electron spin-resonance (ESR) intensity, the NMR spectra and the SQUID magnetization measurements which show a distinct magnetic hysteresis loop.

X-ray powder diffraction line broadening analysis and magnetism of interacting ferrite nanoparticles obtained from acetylacetonato complexes

A study of the microstructures and magnetic properties of nanosize Zn ferrite (ZnFe2O4), Mn ferrite (MnFe2O4), and the cation deficit Zn–Mn ferrites Zn0.70Mn0.23Fe1.89O4 (S1), Zn0.41Mn0.50Fe1.84O4 (S2) and Zn0.18Mn0.67Fe1.85O4 (S3) was performed. The crystallite size for all samples was determined by x-ray powder diffraction (XRPD) analysis using four different methods, and was close to the particle size found from transmission electron microphotography. Among different methods of XRPD line broadening analysis it seems that the cubic harmonic function method is more precise and reliable than the Warren–Averbach and simplified integral breadth methods. M(T) and M(H) magnetization curves at different fields/temperatures indicate superparamagnetic behaviour of the samples. Asymmetric hysteresis loops and differences in coercive fields, HC−(FC) − HC−(ZFC), are discussed by both the core/shell model of nanoparticles and spin canting. The magnetic measurements with a maximum in the FC magnetization branches, the difference in M/MS versus H/T curves above Tmax (temperature of maximum in ZFC magnetization), the nonlinearity in HC versus T1/2, the remanence/saturation ratio value, MR/MS and observation of the Almeida–Thouless line for low-field magnetization data (Tmax versus H2/3) indicate that the samples consist of an interacting ferrite nanoparticle ensemble.

Magnetic properties of multiferroic K3Cr2Fe3F15

The local electronic and structural as well as the macroscopic magnetic properties of K3Cr2Fe3F15 have been studied between room temperature and 4 K. The system has been found to be isostructural with ferroelectric and weakly ferrimagnetic K3Fe5F15 above the ferroelectric transition temperature Tc. The X-band and 216 GHz Cr3+ electron paramagnetic resonance (EPR) spectra as well as the magnetic susceptibility and Mossbauer data show the existence of two magnetic relaxor type transitions around 37 and 17 K. The K39 magic angle sample spinning NMR, EPR, and the Mossbauer data further demonstrate the existence of two nonequivalent Fe, Cr, and K sites in the unit cell as well as the presence of rapid exchange at higher temperatures. The observation of the Fe2+ EPR and Mossbauer spectra shows that the Fe2+ ion is in a high spin state.

Electron paramagnetic resonance and Mössbauer study of antiferromagnetic K3Cu3Fe2F15

The electron paramagnetic resonance (EPR) spectra of K3Cu3Fe2F15 consist above 85 K of a superposition of two Lorentzians with different intensities, T-dependences, linewidths, and g-factors. The broad line shows an antiferromagnetic (AFM) behavior and the narrow one shows a paramagneticlike behavior. The Mossbauer spectra consist at room temperature of two quadrupolar doublets and below 77 K of two magnetic sextets and one quadrupolar doublet. Below 85 K the broad line EPR disappears and the intensity of the narrow line increases nearly exponentially with decreasing temperature. The broad EPR line originates from regions with local AFM order, whereas the narrow line comes from regions without AFM order, e.g., grain boundaries.

Improved N14 nuclear quadrupole resonance detection of trinitrotoluene using polarization transfer from protons to N14 nuclei

Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes N14 nuclear quadrupole resonance (NQR) in trinitrotoluene fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for all three N14 NQR lines (the case with η≠0) were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. N14 NQR signals in a 15g trinitrotoluene sample of predominantly monoclinic modification were measured in 15s in different polarization magnetic fields. The conditions for optimal proton-nitrogen level crossing were determined.