Lanthanum Magnesium Nitrate Research Articles

Polarised neutron scattering from dynamic polarised nuclei 1972–2022

With the inauguration of the small-angle instrument D11 of the Institute Laue–Langevin (ILL) in September 1972 neutron scattering revolutionized methods of contrast variation. Very soon D11 was oversubscribed by proposals relying on isotopic substitution of hydrogen isotopes. At the same time in Oxford first experiments of polarised neutron diffraction from dynamic polarised protons in lanthanum magnesium nitrate crystals demonstrated the great utility of this approach. In the early eighties a new type of polarised target material led to a boom of contrast variation by nuclear polarisation. The new samples of frozen solutions of macromolecules lent themselves to small-angle scattering. Often in collaboration with research centres of High Energy Physics various groups in Europe and Japan started experiments of polarized neutron scattering from dynamic polarised protons. Techniques of NMR and EPR considerably enlarged the spectrum of nuclear contrast variation. This is shown with time-resolved polarised neutron scattering from dynamic polarized proton spins of a free radical and of tyrosyl doped catalase using D22 at the ILL.Graphical abstract

High-Resolution 17O NMR Spectroscopy of Structural Water.

The importance of studying site-specific interactions of structurally similar water molecules in complex systems is well known. We demonstrate the ability to resolve four distinct bound water environments within the crystal structure of lanthanum magnesium nitrate hydrate via 17O solid state nuclear magnetic resonance (NMR) spectroscopy. Using high-resolution multidimensional experiments at high magnetic fields (18.8-35.2 T), each individual water environment was resolved. The quadrupole coupling constants and asymmetry parameters of the 17O of each water were determined to be between 6.6 and 7.1 MHz, 0.83 and 0.90, respectively. The resolution of the four unique, yet similar, structural waters within a hydrated crystal via 17O NMR spectroscopy demonstrates the ability to decipher the unique electronic environment of structural water within a single hydrated crystal structure.

Polarized neutron Laue diffraction on a crystal containing dynamically polarized proton spins

A polarized neutron Laue diffraction experiment on a single crystal of neodymium-doped lanthanum magnesium nitrate hydrate containing polarized proton spins is reported. By using dynamic nuclear polarization to polarize the proton spins, it is demonstrated that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.

Studies of the EPR Parameters and Local Structure for the Cu<sup>2+</sup> Center in Lanthanum Magnesium Nitrate

The electron paramagnetic resonance parameters (g-factors and the hyperfine structure constants) for the Cu2+ center in lanthanum magnesium nitrate (LMN) are theoretically studied from the conventional perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedra. The studied complex is found to exhibit the slight tetragonal elongation (characterized by the relative elongation ratio ρ  4%) due to the Jahn-Teller effect, which may entirely conceal the original trigonal distortion of the host Mg2+ site in LMN. The conventional formulas containing only the metal orbital and spin-orbit coupling contributions are proved to be valid for the Cu2+ center in view of the weak covalency and ligand spin-orbit coupling interactions. This defect is also compared with the similar Cu2+ center of the Jahn-Teller nature on the octahedral interstitial site in the CdSe nanocrystals.

Polarized targets in biology

Abstract Polarized nuclear spin targets are playing an important role in high energy physics experiments. In many cases they are hydrogenous solids, the nuclear spins of which are almost completely aligned with respect to an external magnetic field. In the seventies, polarized targets started exciting the interest of neutron physicists, e.g. at the Institut Laue-Langevin (ILL), Grenoble, the reason for this being the sensitivity of the amplitude of the interaction of neutron spins with proton spins to their mutual spin orientation. Polarized neutrons are an ideal probe of polarized nuclear spin densities in solids. In a first experiment at the ILL, Hayter, Jenkin and White (1974) obtained a proton density distribution in the unit cell of a lanthanum magnesium nitrate crystal, which because of its high water content was assumed to reflect some properties of biological material.

Deuterized cerium lanthanum magnesium nitrate as a magnetic thermometer

The substitution of deuterium for hydrogen in the water of crystallization of lanthanum-diluted cerium magnesium nitrate improves some of its thermometric characteristics. This paramagnetic salt makes it possible to measure lower temperatures, and the difference between the magnetic and absolute temperature is smaller. The time constant for attaining thermal equilibrium is probably shorter than in ordinary cerium lanthanum magnesium nitrate.

Theory of ESR dipolar splitting of a spin pair in the presence of avalanche phonons

The ESR spectrum of a pair of paramagnetic spins $S=\frac{1}{2}$, interacting via dipolar magnetic forces, is calculated in the presence of an incoherent phonon avalanche, which is coupled to the spin pair by linear spin-phonon interaction. The problem is approximately solved by a Green's-function approach. Two cases are discussed, the first for an infinitely narrow phonon spectrum, and the second for finite bandwidth of the avalanche phonons. When the ESR frequency falls out of the avalanche band, the spectrum of the pair is shown to consist of two lines shifted from the no-avalanche resonant field and more closely spaced than in the no-avalanche situation. When the ESR frequency is within the avalanche band, a sharp decrease of the ESR intensity is expected. These results are interpreted in terms of ac Stark shift and of screening of dipolar interaction by the avalanche phonons. Their relationships to electromagnetic power narrowing of ESR lines and to the "magic angle" effect in NMR are discussed. Finally the theoretical results are examined in connection with phonon-avalanche experiments on ${\mathrm{Ce}}^{3+}$-doped lanthanum magnesium nitrate.

The crystal magnetic field in a proton-polarized lanthanum magnesium nitrate single crystal

Knowledge of the crystal magnetic fieldHc is necessary for proton polarization measurements in polarized targets by the internal field method [1, 2]. Because of the importance of lanthanum magnesium nitrate (LMN) single crystals for low-energy polarized proton targets [3],Hc has been calculated for LMN on the basis of new x-ray diffraction data. We find thatHc=ApP2(cosθ), wherep is the proton polarization andP2 the second Legendre polynomial depending onθ, the angle between crystalc-axis and external magnetic field, andA is a constant characteristic for LMN. Our result\(A = - \left( {0.242 \pm \begin{array}{*{20}c} {0.028} \\ {0.090} \\ \end{array} } \right) Oe\) leads to a field only about half as large as expected according to previous assumptions. The difference is due to the markedly different proton positions, revealed by the x-ray structure analysis.

Polarized-Neutron Diffraction from Spin-Polarized Protons: A Tool in Structure Determination?

We have observed the spin-dependent diffraction of slow neutrons by differentially polarized nuclei in single crystals of lanthanum magnesium nitrate. The effects are accompanied by enhanced Bragg diffraction and reduced incoherent scattering at high proton polarizations. The diffraction pattern is very sensitive to selective depolarization of nuclei, and proton resonances have been observed through changes in the diffracted intensities when the sample is irradiated at the Larmor frequencies.

Dynamic nuclear polarization in samarium-doped lanthanum magnesium nitrate

We report theoretical and experimental investigations of the dynamic nuclear polarization (DNP) and nuclear spin-lattice relaxation of protons in single crystals of lanthanum magnesium nitrate (LMN) weakly doped (\ensuremath{\cong}0.1-1.1-mole%) with samarium ions (LMN:Sm) at 9.1 GHz and liquid-helium temperatures. Theoretical expressions for DNP and nuclear relaxation are given for a homogeneously broadened ESR line in the high-temperature limit. DNP effects due to the electron dipole-dipole reservoir (EDDR) are predicted for the case of well-resolved satellites. Our DNP experiments on LMN:Sm are in general agreement with the conventional theory, but no evidence for direct EDDR effects on DNP with well-resolved satellites is seen, despite suggestive nuclear-relaxation data. Various explanations for this disagreement are proposed. We suggest that these EDDR effects may be seen in crystals of \ensuremath{\cong}3-5-mole% LMN:Sm or in equivalent systems.

The heat capacity of cerous magnesium nitrate and some related materials between 0.3 and 4 K

Cerous magnesium nitrate (CMN) is the preeminent electronic paramagnet in use in cryogenic physics for magnetic thermometry and adiabatic cooling. In demagnetization experiments designed to establish the thermodynamic temperature relations for CMN, an inexplicable heat capacity anomaly was found to occur above 20 mK and is shown here to persist to temperatures near 1 K. The anomaly is small but its presence interferes with and may cause errors in the analysis of thermometric data. We have measured the heat capacity of CMN, lanthanum magnesium nitrate (LMN), cerous nitrate hexahydrate, and a saturated aqueous solution of CMN (CMN liquor) in the temperature range 0.3–4 K in an attempt to find the source of the anomaly. The LMN heat capacity shows no anomaly and is used to approximate the lattice heat capacity of CMN. At low temperatures the CMN heat capacity, exclusive of the lattice contribution, is some 2 1/2 times larger than the magnetic heat capacity predicted by other investigations. At high temperatures an exponentially increasing heat capacity due to the first excited electronic level is observed and indicates a splitting which is in accurate agreement with the spectroscopic value. There is evidence that the lattice heat capacity in CMN is about 1% smaller than in LMN, which is probably the result of the crystal-field interaction with the electronic states of the cerous ions. The lattice terms and theT−2term of the magnetic heat capacity for cerous nitrate have been determined, the latter being 25 times larger than the predictedT−2term in CMN. The CMN liquor measurements indicate that this sample had probably become a glass on cooling. The lattice heat capacity is considerably larger than could be predicted from the separate components and there is no indication of the exponential term which would be observable if appreciable crystalline CMN were present. These measurements help to define the nature of the anomalous heat capacity and remove from consideration some possible explanations, but they do not reveal the cause of the anomaly.

The thermal-expansion coefficients of La 2Mg 3(NO 3) 12·24H 2O and Ce 2Mg 3(NO 3) 12·24H 2O between 10 K and 300 K

The thermal-expansion coefficient has been measured along the trigonal axis of the crystal (α [Verbar]) and in a direction perpendicular to this axis (α⊥) by means of a Fizeau interferometer. The thermal-expansion coefficients of lanthanum magnesium nitrate hydrate (LMN) and cerous magnesium nitrate hydrate (CMN) appear to be identical within the accuracy of the measurements There is a large anisotropy: at room temperature α [Verbar]α⊥ ≈ 7, whereas at 10 K the sign of α [Verbar] is negative, α⊥ remaining positive. The expansion coefficients and the relative expansions at several temperatures between 10 K and 300 K have been calculated from the experimental data, as have also the lengths of the c and a axes, using data obtained by Zalkin et al. at room temperature. The quality of the crystals is shown.

Nuclear magnetic resonance on oriented nuclei in a dilute paramagnetic crystal;57Co, 58Co and 60Co in La2Mg3(NO3)12.24H2O

The method of nuclear magnetic resonance on radioactive nuclei, oriented in a dilute paramagnetic crystal, was applied to three Co isotopes, incorporated in single crystals of lanthanum magnesium nitrate (LMN). Nuclear orientation was achieved by means of cross relaxation with Ce ions, partly replacing the La ions. Resonance transitions between successive hfs levels were detected by observing changes in the directional anisotropy of the emitted gamma rays.Resonance transitions could be observed only for Co ions at site II of the LMN crystal. From the observed changes in the gamma anisotropy it was concluded that 58% of the Co ions occupy site II, which is somewhat lower than 2/3, the relative abundance of this site.The width of the resonance lines was ascribed to a spread in the hfs parameters A and B. The contribution of homogeneous broadening to the total linewidth could not be observed.From the measured resonance frequencies accurate values of the hfs parameters could be derived. By comparing these with the hfs parameters of stable 59Co in the same salt, accurate values for the magnetic moments of 57Co, 58Co and 60Co were obtained: μ(57Co) = +4.722 ± 0.017n.m., μ(58Co) = +4.035 ± 0.008n.m. and μ(60Co) = +3.790 ± 0.008n.m. The quadrupole moments of the three Co isotopes could be determined for the first time: Q(57Co) = +0.49 ± 0.09b, Q(58Co) = +0.21 ± 0.03b and Q(60Co) = +0.42 ± 0.05b.In the case of 57Co, a considerable attenuation of the gamma anisotropy was observed, which must be attributed to reorientation during the 8.7 ns lifetime of the excited state of 57Fe at 136 keV. From the anisotropy measurements the mixing ratio δ(E2/M1) of the 122 keV transition was obtained.

Dynamic coupling between two paramagnetic spin species induced by resonant phonons

Results of paramagnetic relaxation studies conducted at ≈11 GHz and liquid helium temperature, involving lanthanum magnesium nitrate doped with Ce 3+ and Pr 3+ in spatially separated regions, are presented. The Ce transition alone was excited by fast passage, and the Pr paramagnetic resonance signal was observed. The spectral separation of the center frequencies of the two transitions, and the time during which the separation was maintained, could be independently adjusted. In this way both the temporal and spectral properties of the phenomenon could be studied. The Pr transition was found to exhibit in time a delayed onset of rapid heating followed by a gradual return to thermal equilibrium. The Pr line shape exhibited heating in those portions of the line localized in frequency nearest to the center of the Ce line. The spectral and temporal behavior of the Pr signal were both sensitive to the degree of Ce excitation as well as the relative spectral separation between the two lines. The observed effects are attributed to heating of Pr spin packets induced by hot photons created in the course of the paramagnetic relaxation of the Ce transition. Two simple analytical descriptions of the Ce-Pr interaction are presented.

Nuclear relaxation and NMR saturation in a crystal with a magnetic impurity

AbstractA theoretical treatment of nuclear relaxation in non‐metallic diamagnetic crystals with a magnetic impurity is carried out for the case when the dipole—dipole reservoir of magnetic ions plays an important role. The theoretical results are compared with the data of the Leiden group on relaxation of protons in lanthanum magnesium nitrate with neodymium impurities. An elementary theory of NMR saturation is given taking into account saturation transfer from the nuclear Zeeman reservoir to the dipole‐dipole reservoir of magnetic ions. A special analysis is made of the possible role in nuclear relaxation of an inhomogeneous broadening of the EPR line, of a phonon bottleneck in the spin‐lattice relaxation of magnetic ions, and of exchange pairs.

Concentration-Dependent Orbach Relaxation Rates in Nd-Doped Lanthanum Magnesium Nitrate

Electron-spin-resonance linewidth data, taken between 6 and 23 K, are compared with pulse-saturation-recovery times, measured between 2.5 and 3.5K, for three samples of Nd-doped lanthanum magnesium nitrate. Both types of measurements indicate a dominant Orbach relaxation process. The relationship between the two types of data in the presence of a phonon-limited Orbach relaxation process is discussed. The fact that the linewidth data were uniform among the different samples, whereas the pulse-saturation-recovery data were concentration dependent, is interpreted as evidence that the Orbach relaxation process in this salt is phonon limited.

Determination ofdE/dx for 3–17 MeV alpha particles from energy loss measurements in lanthanum magnesium nitrate single crystals

dE/dx for 3–17 MeV alpha particles in lanthanum magnesium nitrate single crystals parallel to the crystalc-axis has been determined experimentally. We find that the Livingston-Bethe-Bloch theory essentially holds in this range of energies, but that values fordE/dx calculated simply by addition of the energy losses caused by the different constituent atoms of the compound are in error up to 20%.

Thermal Conductivity of Double-Nitrate Crystals below 1°K

The thermal conductivities of high-purity cerium magnesium nitrate and lanthanum magnesium nitrate single crystals have been measured in zero or small magnetic fields. The phonon mean free path is found to be limited by processes intrinsic to the crystal. This mean free path can be longer than nominally allowed by the dimensions of the specimen. Measurements were also made on samples doped with other rare-earth ions. The resonant absorption of phonons by the non-Kramers ions Pr and Ho is observed and is shown to result from electronic transitions between hyperfine energy levels.

Dynamic Proton Polarization in AlK Alum and AlNH4Alum

Over 50% proton polarization has been attained in three sulfate alum crystals containing dilute ${\mathrm{Cr}}^{3+}$ ions, at 1 K and \ensuremath{\sim}19.5 kG, with \ensuremath{\sim}\textonequarter{}-W microwave power. We measured polarization growth-rate curves, polarization as a function of magnetic field, proton line shapes and widths, and proton spin-lattice relaxation times. The theory of dynamic polarization for well-separated allowed and forbidden transitions applies to the alums. To obtain high proton polarization, the alums require about three times as much microwave power as lanthanum magnesium nitrate (LMN). A pure AlK alum had a very long proton relaxation time (1.3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ sec at 0.96 K), making it a promising crystal for "storing" polarization. AlN${\mathrm{H}}_{4}$ alum should make useful polarized targets for low-energy scattering experiments, because it contains 1.97 times as much hydrogen by weight as LMN.

Nuclear magnetic resonance on oriented nuclei in a paramagnetic crystal, 52Mn and 54Mn in La 2Mg 3(NO 3) 12.24H 2O

Transitions between hyperfine-structure levels of oriented 52Mn and 54Mn ions included in lanthanum magnesium nitrate were induced by radiofrequency irradiation. The Mn ions were oriented by cross relaxation with Ce ions, partly replacing the La ions (Ce: La=1: 10 3). Changes in the directional anisotropy of the gamma rays emitted by the nuclear spin system were utilized to detect the rf resonance transitions. Different hfs transitions could be induced separately. Complete saturation of each transition was accomplished by slowly sweeping the frequency of the rf field across the resonance, the rf field amplitude being as low as 10 -4Oe. A theoretical description of the process is given in which the enhancement of the rf amplitude at the position of the nucleus through the intermediary of the electronic spin plays a major role. The agreement with the experimental results is satisfactory. The observed resonance lines were very narrow; their width was mainly determined by the inhomogeneity of the external magnetic field. Also a homogenous linewidth was shown to be present; it is argued that it originates from the protons surrounding the Mn ions. By taking data at various field strengths and directions very accurate values of the hfs parameters were obtained. By comparing these with the data on the stable isotope 55Mn the magnetic moments of 52Mn and 54Mn were derived with a precision determined only by the error in the value of the magnetic moment of 55Mn. The quadrupole moments of both radioactive isotopes were determined for the first time.