Static Magnetic Field Distribution Research Articles

Proton NMR measurements of the local magnetic field in the paramagnetic metal and antiferromagnetic insulator phases ofλ−(BETS)2FeCl4

Measurements of the $^{1}$H-NMR spectrum of a small ($\sim$ 4 $\mu$g) single crystal of the organic conductor $\lambda$-(BETS)$_{2}$FeCl$_{4}$ are reported with an applied magnetic field $\bf{B}$$_{0}$ = 9 T parallel to the a-axis in the $ac$-plane over a temperature $(T)$ range 2.0 $-$ 180 K. They provide the distribution of the static local magnetic field at the proton sites in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along with the changes that occur at the PM$-$AFI phase transition. The spectra have six main peaks that are significantly broadened and shifted at low $T$. The origin of these features is attributed to the large dipolar field from the 3d Fe$^{3+}$ ion moments (spin $S_{\rm{d}}$ = 5/2). Their amplitude and $T-$dependence are modeled using a modified Brillouin function that includes a mean field approximation for the total exchange interaction ($J_{0}$) between one Fe$^{3+}$ ion and its two nearest neighbors. A good fit is obtained using $J_{0}$ = $-$ 1.7 K. At temperatures below the PM$-$AFI transition temperature $T_{MI}$ = 3.5 K, an extra peak appears on the high frequency side of the spectrum and the details of the spectrum become smeared. Also, the rms linewidth and the frequency shift of the spectral distribution are discontinuous, consistent with the transition being first-order. These measurements verify that the dominant local magnetic field contribution is from the Fe$^{3+}$ ions and indicate that there is a significant change in the static local magnetic field distribution at the proton sites on traversing the PM to AFI phase transition.

High-sensitivity detection of static magnetic field in a SQUID probe microscope

We have developed a high-Tc SQUID probe microscope. A high permeability probe was used as a flux guide toimprove its spatial resolution. The SQUID head with the probe makes it possible tomeasure samples with high spatial resolution in air at room temperature. Toachieve a high spatial resolution, the microscope should have high signal to noise(S/N) ratio. We have investigated a modulation method of the static magneticfield emanating from a sample with the aim of achieving a higherS/N ratio. The sample stage was vibrated perpendicular to the plane of the SQUIDwith a frequency of 120 Hz by being mounted on a piezo stage attachedto a mechanical stage. The oscillation amplitude of the sample was about44 µm when the piezo stage was vibrated with amplitude20 µm. The SQUID output was connected to a lock-in amplifier, and the magnetic field signalwas obtained using lock-in detection. In order to evaluate the performance of thismodulation method, we measured static magnetic field distributions of fine meander linesand fine patterns of a Ni thin film. Observed line scans showed that the improvedS/N ratio of the SQUID microscope led to a higher spatial resolution.

Brief encounter at the molecular level: what muons tell us about molecule-based magnets

Spin-polarized muons can be implanted in various molecular magnetic materials in order to measure static and dynamic magnetic field distributions at a local level. The positively-charged muon is an unstable, radioactive particle which has spin–1/2, a lifetime of 2.2 μ S, about one-ninth of the proton mass and a magnetic moment of approximately 1/200 μ B. Both pulsed and continuous beams of muons can be produced with almost 100% spin polarization and significant intensity at various accelerator facilities. The subsequent decay of the muon into a positron allows the extraction of the muon-spin autocorrelation function which can be related to the magnetic field distribution inside a sample. This experimental technique has found particular application to the problem of hydrogen in semiconductors, as well as the study of the vortex lattice in both high-temperature and organic superconductors. Nevertheless, it has been most widely employed in the field of magnetism. We describe how our experiments using spin-polarized muons have been used to provide information about organic ferromagnets, molecular magnets, spin-chains and single molecule magnets.

Quest for Frustration Driven Distortion inY2Mo2O7

We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin glass Y2(Mo2)O(7) by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muon spin relaxation technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magnetoelastic coupling at T > 0.

Modified distributed electron cyclotron resonance ion source for the production of large diameter ion beams

The work presented in this article concerns the conception and the study of a multiantenna electron cyclotron resonance (ECR) ion source at 2.45 GHz dedicated to the surface treatment of large area materials. In this original device, a microwave plasma is created in a 20 cm in diameter ionization chamber. The 15 cm in diameter extracted ion beam has a current density of about 1 mA/cm2. The applicator is an N-way coaxial power divider in which the N=20 antennas are individually magnetized by internal SmCo bars in order to produce ECR zones in their vicinity. Moreover, the microwave plasma is confined by a multicusp magnetic structure surrounding the ionization chamber. The choice of the source geometry has been guided by the study of some theoretical considerations such as the characteristic diffusion length, the minimum breakdown field, and the penetration depth of the wave into the plasma. Simulations both of the electromagnetic field and the static magnetic field distribution have been carried out in order to validate the final choice of the source geometry. A full characterization of the ECR plasma and of the extracted ion beam was made with different experimental techniques. These results allow us to localize the plasma creation zones inside the ionization chamber. A 120 mA singly charged argon ion beam with a profile homogeneity better than ±5% over 10 cm was obtained 5 cm downstream the extraction system with a 300 W microwave power and a neutral argon pressure of 10−3 mbar in the ionizing chamber.

NMR studies of two unusual f-electron metals

We present the results of nuclear magnetic resonance (NMR) studies down to very low temperatures for two U compounds, UCu3.5Pd1.5 and U0.8Y0.2Pd3, where the conduction electrons experience strong interactions and for which the temperature dependences of the thermal and transport properties do not obey the expectations of a simple Fermi-liquid model. Also the temperature and field dependences of the nuclear magnetization recovery of UCu3.5Pd1.5 in Cu nuclear quadrupole resonance show unusual features that cannot be reconciled with common expectations for a simple metal, but they are well accounted for by the Kondo disorder model. For U0.8Y0.2Pd3, our Y NMR results indicate a distribution of internal static magnetic fields at the Y sites and a small temperature-dependent enhancement of the spin-lattice relaxation rateT 1 − with respect to YPd3. The NMR spectra are consistent with the presence of very small frozen U moments, but the temperature dependence of the spin-lattice relaxation rate indicates a more complicated situation.

Two-dimensional nuclear magnetic resonance study of a hydrated porous medium: An application to white cement

Structure and dynamics of a hydrated white cement were investigated by the T1-weighted lineshape, two-dimensional (2D) exchange, and 2D separation of inhomogeneous and homogeneous lineshapes nuclear magnetic resonance (NMR) techniques. T1 weighting of the proton spectrum eliminates the strong bulk water signal from the pores so that the weaker spectrum of the solid cement matrix becomes observable. The proton spectrum of the solid cement fraction exhibits a characteristic Pake doublet shape and shows a close similarity to the powder spectra of pure calcium hydroxide and the crystalline water of gypsum. The 2D exchange NMR spectrum of white cement demonstrates the existence of slow chemical exchange processes of protons in the solid matrix on a sub-kHz frequency scale. This exchange is orders of magnitude slower than the exchange between the surface and bulk water. The 2D NMR separation of inhomogeneous and homogeneous lineshapes technique demonstrates that the absorption lines of a set white cement are inhomogeneously broadened due to the existence of a distribution of static local magnetic fields. The major contribution to these fields comes from the dipolar fields of the paramagnetic electrons whereas the magnetic susceptibility differences of the liquid and solid fractions add a small part at the solid-liquid boundary where the direction of the external magnetic field is changed.

A computer simulation of the static magnetic field distribution in the human head.

Distortion of the static magnetic field inside the human head is dependent on regional tissue susceptibility variations and geometrical shape. These effects result in resonance line broadening and frequency shifts and consequently, intensity and spatial errors in both magnetic resonance imaging (MRI) and magnetic resonance (MR) spectroscopy. To calculate the field distortion due to the susceptibility's geometry, two dimensional (2D) finite element analysis was applied to simulate the field distribution in a 2D model of the human head, placed in a uniform magnetic field. The model contains air-filled cavities and sinuses, and the remainder is treated as water. The magnetic field deviation was evaluated using gray scale plots and histograms of the magnetic field. The shifts in parts/million and broadening of the histograms correspond to the NMR of the sampled region. The field distribution of the human head was also experimentally mapped using the DANTE tagging sequence. The calculated and experimental field maps are in good agreement. Thus, geometric considerations with uniform susceptibilities are sufficient to explain most of the static magnetic field distribution in the human head.

4833408 NMR chemical shift imaging method with influence of intensity distribution of static magnetic field removed

In an NMR imaging method of distinguishing two chemical shifts of unclear spins and obtaining respective spin distribution images, 180° RF. pulses are applied twice to generate two spin echoes after spins of an object for inspection have been exicted. In the second echo among them, a pulse time-interval is so set that the time difference between the Hahn echo and the gradient echo may provide two chemical shifts with a phase difference of 2nπ. On the basis of information representing the distribution of a static magnetic field included in an image obtained from the second pulse, an image obtained from the first echo is compensated. By using the compensated image, spin distribution images with two chemical shifts distinguished are obtained.

4833408 NMR chemical shift imaging method with influence of intensity distribution of static magnetic field removed : Etsuji Yamamoto, Takashi Onodera, Akishima, Japan assigned to Hitachi Ltd

4833408 NMR chemical shift imaging method with influence of intensity distribution of static magnetic field removed : Etsuji Yamamoto, Takashi Onodera, Akishima, Japan assigned to Hitachi Ltd

Evidence for complex magnetic order in U2Zn17: A μ+ SR studySR study

μ+ SR-measurements in transversally applied magnetic fields of 2000 G and 4000 G on heavy-electron single crystal U2Zn17 are presented. They reveal that at least two types of interstitial sites are occupied by the positive muons. One of these sites (1/3, 2/3, 5/6) could be identified via induced local dipolar fields which aboveTN=9.7 K can exactly be derived from the magnetic susceptibility. The corresponding component of the μ+-signal exhibits a steplike decrease by about 40% atTN which is caused by the onset of a very broad distribution of static internal magnetic fields (ΔB≈1000 G) with zero average. Such a field distribution is in distinct contrast to dipolar-field calculations performed for the simple antiferromagnetic structure deduced from neutron diffraction. The remaining 60% of the muons contributing to this component belowTN are subject to a narrow static field distribution (ΔB≈1 G). The induced dipolar fields at the site (1/3, 2/3, 5/6) are temperature-independent belowTN. A weak dipolar coupling to the U-moments renders similar observations for muons occupying the second type of interstitial impossible.

A new method of measuring static field distribution using modified Fourier NMR imaging

A new method for mapping the static magnetic field of NMR imagers is presented. The method uses a conventional Fourier imaging pulse-sequence with the exception that the time interval between the 90 degrees and 180 degrees pulses does not equal that between the 180 degrees pulse and spin-echo formation. In images obtained from the pulse sequence, the difference between those two time intervals causes phase shifts that depend on the static magnetic field distribution. Thus, the information on the field distribution can be extracted from these phase shifts. This method is quicker and more easily implemented than the method proposed by Maudsley et al. (1979, 1983, 1984).

Half-space radiation by EMATs

A Green's function calculation of the far-field radiation patterns of EMATs is presented. The approach is based on (a) closed form expressions for the eddy current and static magnetic field distributions, established by the EMATs, which react to produce the driving Lorentz forces, and (b) a Green's function derived from the steepest descent approximation to the far-field response of an arbitrary surface point force on a half space. Numerical results are presented, illustrating the radiation patterns of the three common EMAT designs. Included are vertically polarized shear waves as radiated by both meander coil and periodic magnet EMATs and horizontally polarized shear waves as radiated by the latter.