Paramagnetic Cr Research Articles

Dynamic orientation of nuclei at low temperatures

A study has been made of the mechanisms of dynamic polarization of nuclei in organic materials at temperatures below 1 K and in magnetic fields up to 50 kG. After a summary of the mechanisms of dynamic polarization, the experimental results on 1,2-propanediol, doped with paramagnetic Cr V complexes, are compared quantitatively with a spin temperature model of Borghini, in which it is assumed that the nuclear polarization is obtained via thermal contact with a dynamically cooled electron spin-spin interaction reservoir. The strong influence of this latter reservoir was apparent from the transient behavior of different nuclear spin systems contained in the same sample. The abovementioned spin temperature model has the unique property of not using the simplifying “high-temperature approximation” and it was the first time that this model has been found to be in reasonable agreement with experimental data in the 100 mK region. Furthermore, dynamic polarization experiments with the free radical BDPA are reviewed. These measurements show clearly the contributions of different mechanisms of dynamic polarization. The results are found to be in agreement with formulas derived from the spin temperature theory after introduction of some simplifying assumptions in order to be able to deal with low temperatures.

High proton polarization in 1,2-propanediol at 3He temperatures

A study is made of the dynamic proton polarization in 1,2-propanediol doped with paramagnetic Cr V complexes. A method is described by which samples with a high concentration of Cr V complexes can be prepared. Samples of 1,2-propanediol prepared in this way yielded a maximum proton polarization of 92% at a temperature of 0.48 K in a 2.5 T magnetic field. The polarization build-up time was found to be 2 to 3 min in a 7 cm 3 sample, with about 25 mW microwave power in the cavity. The microwave power needed to maintain the polarization in such a sample was about 4 mW.

ENDOR investigations of Cr+ centres in NaF, NaCl, and NaBr

AbstractIsolated paramagnetic Cr+ centres in sodium halide crystals are studied by EPR and ENDOR. They show cubic symmetry. Superhyperfine and quadrupole interactions with many neighbouring ions (up to 〈300〉 in NaF) could be determined. The experimental results are interpreted in terms of two models for the wavefunction of the centre: overlap and covalency. A new way for the experimental determination of the coefficients for σ‐ and π‐covalency, using the isotropic superhyperfine interactions of neighbours 〈110〉 and 〈200〉, is proposed. The small anisotropic superhyperfine interaction of the nearest neighbours, which becomes even negative for Br− in NaBr, cannot be understood along these lines.

Proton NMR Investigation of Hindered Methyl Rotation in Paramagnetic Cr(II) Complexes

It is shown that in a paramagnetic complex, methyl rotation can act as an effective correlation time for spin relaxation through the Fermi contact interaction. For unhindered methyl groups, this mechanism can lead to methyl line narrowing. In the limit of slow methyl rotation resulting from steric hinderance, methyl rotation is capable of modulating the average hyperfine interaction (contact shift), which leads to methyl line broadening similar to that observed from chemical exchange effects. The proton linewidths of a series of tris-Cr(II) chelates with methyl-substituted ortho-phenanthrolines have been investigated. The linewidths of the aromatic protons in all complexes are determined by primarily the dipolar interaction. The linewidths of methyl groups which are expected to be sterically hindered are considerably broader than accountable by either dipolar or hyperfine exchange relaxation. We conclude that the anomalous methyl linewidths arise from modulation of the average hyperfine interaction by methyl rotation. The temperature dependence of the observed linewidths yields the barrier to methyl rotation.

Calculation of Paramagnetic Susceptibilities for B.C.C. Transition Metals and Their Alloys

The approximate energy bands for Cr, Mo and W metals are built up from the energy bands for the paramagnetic Cr metal, by taking into account the spin-orbit interaction. The effect of the spin-orbitinteraction to the paramagnetic susceptibilities for 3 d , 4 d and 5 d b.c.c. transition metals and their alloys are investigated. The spin paramagnetic susceptibilities χ s and the orbital paramagnetic susceptibilities χ orb for 3 d metals and their alloys are the largest, followed by those for 4 d and 5 d metals and their alloys in that order, and the mixed contribution due to the spin and orbital angular momentums, χ so , is negligibly small. The calculated paramagnetic susceptibilities agree with the results of the experiments, by adopting the reasonable values of a molecular field coefficient. The method to analyze the paramagnetic susceptibilities for the alloys composed of 3 d and 4 d , 4 d and 5 d or 3 d and 5 d transition metals is suggested and the experimental results for V–Nb and Nb–Ta alloys are an...

De Haas-van Alphen Effect in Antiferromagnetic Chromium

de Haas-van Alphen torque oscillations have been studied in single-domain crystals of Cr in magnetic fields to 33 kG and have been analyzed by computer calculation of their Fourier transform. Extensive data in all major symmetry planes and several nonsymmetry planes confirm the tetragonal symmetry of the Fermi surface in the antiferromagnetic state. The frequency resolution is sufficient to isolate several families of data, a number of which can be represented well by simple geometrical Fermi-surface models. Band calculations for paramagnetic Cr indicate a Fermi surface which contains a closed hole sheet at the point $N$ of the Brillouin zone and a closed electron sheet located along $\ensuremath{\Gamma}H$. For simplicity, it is assumed that the electron and hole octahedra at $\ensuremath{\Gamma}$ and $H$, respectively, are destroyed by the interaction which stabilizes the antiferromagnetic state. The surviving Fermi-surface sheets at $N$ and along $\ensuremath{\Gamma}H$ are truncated by energy gaps due to the antiferromagnetic Q vector, as proposed by Lomer. Several of the resulting surfaces resemble those obtained from the geometrical analysis of the data. By adjusting the dimensions and shape of the hole surfaces at $N$, it is possible to interpret three families of data with an accuracy of \ensuremath{\sim}10% in area. Applying a similar procedure to the electron surface along $\ensuremath{\Gamma}H$ yields \ensuremath{\sim}20% agreement with a fourth family of data. The many remaining frequencies receive either a tentative assignment or none at all.