Internal Magnetic Field Distribution Research Articles

The internal magnetic field distribution, and single exponential magnetic resonance free induction decay, in rocks

When fluid saturated porous media are subjected to an applied uniform magnetic field, an internal magnetic field, inside the pore space, is induced due to magnetic susceptibility differences between the pore-filling fluid and the solid matrix. The microscopic distribution of the internal magnetic field, and its gradients, was simulated based on the thin-section pore structure of a sedimentary rock. The simulation results were verified experimentally. We show that the ‘decay due to diffusion in internal field’ magnetic resonance technique may be applied to measure the pore size distribution in partially saturated porous media. For the first time, we have observed that the internal magnetic field and its gradients in porous rocks have a Lorentzian distribution, with an average gradient value of zero. The Lorentzian distribution of internal magnetic field arises from the large susceptibility contrast and an intrinsic disordered pore structure in these porous media. We confirm that the single exponential magnetic resonance free induction decay commonly observed in fluid saturated porous media arises from a Lorentzian internal field distribution. A linear relationship between the magnetic resonance linewidth, and the product of the susceptibility difference in the porous media and the applied magnetic field, is observed through simulation and experiment.

Micromagnetic configurations and switching mechanism in Pac-man-shaped submicron Ni80Fe20 magnets

Micromagnetic modeling analysis and magnetic force microscopy studies were performed in order to characterize the magnetization configuration and magnetic switching behavior in two types, PM I and PM II, of submicron Pac-man-shaped Ni80Fe20 magnetic elements. It was found that a slight variation in the shape of the elements has a striking influence on the internal magnetic structures and switching field distribution. In particular, the vortex-formation driven switching is replaced by quasicoherent reversal by removing the central core part at the center of element. The sensitive dependence of remanent magnetic configuration and switching behavior on sample geometry is discussed in terms of the competition between the exchange and demagnetizing energy terms.

Room temperature solid–solid reaction preparation of iron [sbnd]boron alloy nanoparticles and Mössbauer spectra

The amorphous iron boron alloy can be prepared very easily by a room temperature solid–solid reaction of the FeCl 3·6H 2O, CrCl 3·6H 2O and KBH 4 powders. The characterizations show that the product is mainly composed of amorphous iron boron alloy nanoparticles. 57Fe Mossbauer spectra at 84 and 295 K show that there are three kinds of Fe-containing components in the sample. There are the superparamagnetic relaxation in Mössbauer spectra and the continuous distribution of internal magnetic field in the iron boron alloy.

Distribution of internal transverse magnetic fields in aMn12-based single molecule magnet

A distribution of internal transverse magnetic fields has been observed in single molecule magnet (SMM) Mn12-BrAc in the pure magnetic quantum tunneling (MQT) regime. Magnetic relaxation experiments at 0.4 K are used to produce a hole in the distribution of transverse fields whose angle and depth depend on the orientation and amplitude of an applied transverse ``digging field.'' The presence of such transverse magnetic fields can explain the main features of resonant MQT in this material, including the tunneling rates, the form of the relaxation and the absence of tunneling selection rules. We propose a model in which the transverse fields originate from a distribution of tilts of the molecular magnetic easy axes.

Magnetic field distributions in a large single crystal of YBa 2Cu 3O 7− δ with cylindrical non-conducting inclusions

μSR experiments on a large single crystal sample of YBa 2Cu 3O 7− δ with non-conducting Y 2BaCuO 5 inclusions show some interesting characteristics, especially the magnetic field distribution in the inclusion regions. This internal magnetic field distribution has been calculated using the London equations for the superconducting regions. In the model we develop here we assume that the inclusions are cylindrical and infinite in length. The interaction energy between an inclusion and an hexagonal array of vortices is also obtained. The difference between the field value in the inclusions and the value at the saddle point is sensitive, in our model, to the penetration depth. Comparing the calculated to observed field differences provides a new alternative method of determining the penetration depth.

Paramagnetic vortex state in Pr2$minus;xCexCuO4 single crystals

Transverse-field muon spin rotation (TF-μSR) measurements of the internal magnetic field distribution of Pr2−xCexCuO4 single crystals reveal a large increase in the magnitude of the average field in the vortex state under field-cooling conditions. The observed increase in the average internal magnetic field measured by μSR is discussed in the context of the paramagnetic Meissner effect.

μSR study on ferromagnetism of potassium clusters in aluminosilicate zeolite LTA

Potassium clusters stabilized in regular cages of aluminosilicate zeolite LTA are known to show ferromagnetic properties depending on the loading density of guest K atoms. In the present paper, a muon spin rotation/relaxation (μSR) experiment is reported for a sample at a K-loading density of 4.8 atoms per cage. This sample shows ferromagnetic properties below 6.5 K. An increase in the muon spin depolarization rate, a decrease in the initial asymmetry and an appearance of the muon spin precession signal are all observed at low temperatures in the zero field. The internal magnetic field distribution is roughly estimated from the depolarization rate and the precession frequency. Possible magnetic structures are discussed in comparison with the internal field distribution. A recovery of the initial asymmetry is observed for very weak longitudinal field (LF). This behavior is ascribed to the reorientation of spontaneous magnetization along LF. The present result is discussed in relation to the spin-canting model.

Evidence for static magnetism in the vortex cores of ortho-II YBa2Cu3O6.50.

Evidence for static alternating magnetic fields in the vortex cores of underdoped YBa2Cu3O6+x is reported. Muon spin rotation measurements of the internal magnetic field distribution of the vortex state of YBa2Cu3O6.50 in applied fields of H = 1 T and H = 4 T reveal a feature in the high-field tail of the field distribution which is not present in optimally doped YBa2Cu3O6.95 and which fits well to a model with static magnetic fields in the vortex cores. The magnitude of the fields is estimated to be 18(2) G and decreases above T = 10 K. We discuss possible origins of the additional vortex core magnetism within the context of existing theories.

Organic Magnetic Materials Studied by Positive Muons

Positive muons can be implanted into organic and molecular magnets in order to study their internal magnetic field distribution and any associated dynamics. The muon behaves essentially as a “microscopic magnetometer”, sensitive to local magnetic order and magnetic fluctuations. We describe some recent experiments using this technique which were performed on a variety of organic systems, including nitronyl nitroxide magnets and materials with spin-Peierls ground states, MEM(TCNQ)2 and DEM(TCNQ)2, and demonstrate how the technique can give information concerning their ground states.

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.

Study of Composites Consisting of High-TcSuperconductor and Fine Magnetic Particles

The composite systems as a mixture of fine magnetic particles of Fe3O4 and superconducting powder such as Bi1.8Pb0.2Sr2Ca2Cu3O10+x were prepared. The influence of internal magnetic field (generated by magnetic particles) on the superconducting properties of prepared composites were studied as a function of concentration of magnetic particles and their magnetic state. The observed data both of the shielding and the Meissner effect were compared with corresponding linear combination of pure signals of magnetite and superconductor, respectively. The large differences were observed for demagnetized samples. It means that result must be discussed in the frame of the distribution of internal magnetic field created by the magnetic particles. PACS numbers: 74.72.Ηs, 75.50.Tt

熱時効による2相ステンレス鋳鋼のフェライトの組成変化

A cast duplex stainless steel practically used in the intermediate temperature range of 280 to 330°C was investigated in terms of its aging behavior in the temperature range of 350 to 450°C for aging times up to 20000 h. A change in ferrite composition with increasing aging time was detected at room temperature by Mösbauer spectroscopy. The characteristic changes in internal magnetic field distribution and its full width at half maximum revealed that the ferrite phase decomposes by the spinodal process. The change in ferrite composition with time during aging was analyzed and the corresponding equation proposed was that of the sum of two hyperbolic tangent curves. The activation energy for the compositional change during decomposition was about 200 kJ/mol, which is comparable to that for volume diffusion of Cr or Fe in ferrite. An aging parameter p(t,T) calculated from the above activation energy was introduced to estimate the decomposition behavior at temperatures lower than 350°C. The time required for the ferrite phase to reach completion of phase decomposition was estimated to be about 1700000 h at 320°C.

Nuclear magnetic resonance study of the internal magnetic field distribution in water base ionic and surfacted ferrofluids

This work reports a nuclear magnetic resonance (NMR) study of both hydrogen and deuterium on two different kinds of ferrofluids. One of them is a colloidal suspension of surfacted magnetic grains dispersed in a mixture of light and heavy water. The second one is an ionic ferrofluid composed by positively charged magnetic grains of Co–Fe2O4 dispersed in the same solution of water. A model of the local magnetic field distribution in the interstitial (bulk) water is worked out in order to account for the broadening of the NMR spectra observed in both samples.

Influence of Annealing on Electrical Resistance and Internal Magnetic Field Distribution Function of Fe75Cr5B20 Amorphous Alloy

The influence of annealing under various temperatures on the temperature dependences of the electrical resistance R(T) of amorphous Fe75Cr5B20 alloys has been studied. The dependences ΔR/R0 = f(Ta, ta) and the Mössbauer spectra of annealed samples have been measured as well. The results indicate two annealing temperature intervals where both R(T) and ΔR/R0 behave in a different way. The observed changes are connected with various structural relaxation types: “compositional” and “topological” ordering, respectively.

Analysis of ferrite circulators by 2-D finite-element and recursive Green's function techniques

Ferrite circulator operation is analyzed here by two techniques. The first employs a two-dimensional (2-D) finite-element (FE) technique, using a publicly available FE package. We show how to adapt this code to the solution of the magnetostatic equations and solve for the distribution of internal magnetic field inside a round ferrite puck of finite thickness, and use it to verify existing approximations for the demagnetizing fields. Additionally, the 2-D FE method has also been used to calculate the RF fields and scattering parameters in circulators having noncircular shapes, as well as nonuniform material properties and bias conditions. We have also investigated the field solutions for round circulators, calculated using a recursive Green's function (RGF) technique. This technique allows for radially varying properties in the material or bias fields, and thus accommodates the nonuniform demagnetizing field distribution in finite pucks. A comparison of the results of this technique with experiment is made. We show how the impedance-matching structures attached to the circulator affect the field distributions inside, and present plots of the field distributions as a function of frequency, which provide insight into circulator operation.

Muon-Spin Rotation Measurements of the Magnetic Field Dependence of the Vortex-Core Radius and Magnetic Penetration Depth inNbSe2

Muon-spin rotation spectroscopy has been used to measure the internal magnetic field distribution in NbSe2 for Hc1 << H < 0.25 Hc2. The deduced profiles of the supercurrent density indicate that the vortex-core radius in the bulk decreases sharply with increasing magnetic field. This effect, which is attributed to increased vortex-vortex interactions, does not agree with the dirty-limit microscopic theory. A simple phenomenological equation in which the core radius depends on the intervortex spacing is used to model this behaviour. In addition, we find for the first time that the in-plane magnetic penetration depth increases linearly with H in the vortex state of a conventional superconductor.

μSR studies of the metallic alkali fullerides

We report ZF μSR measurements in Cs1C60 at low temperature and LF μSR in the superconductor Rb3C60. In Cs1C60, the internal magnetic field distribution is broad and static, and the relaxation at 1.9 K resembles that of a spin density wave. In Rb3C60, we observe a strong field dependence to the coherence peak in the LF relaxation rate of endohedral muonium ( Mu@C60) at anomalously low field and a residual low temperature relaxation.

Zero Field Muon Spin Relaxation Study of the Low Temperature State inα-(BEDT-TTF)2KHg(SCN)4

Zero field muon spin relaxation measurements are reported for the low temperature magnetic phase of the organic metal (BEDT-TTF${)}_{2}$KHg(SCN${)}_{4}$. Steplike changes in the relaxation rate indicate two transitions; the first occurs around 12 K and is accompanied by a change in the shape of the relaxation function. A further increase in the relaxation rate occurs below 8 K. These changes signify the onset of an internal magnetic field distribution below the transition, which is assigned to a spin-density-wave (SDW) ordering with an estimated amplitude of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{\ensuremath{\mu}}_{B}$ below 8 K. This very small value is consistent with the upper limits set by previous magnetic resonance studies and also with a simple mean field description of the SDW transition.

Thermal remagnetization effect in REFeB permanent magnets

Thermal remagnetization was studied on samples with different MHc distribution. The process is computer simulated on the basis of a model for which the internal magnetic field distribution is calculated with given experimental MHc distribution and MHc and Ms temperature dependence. Peculiarities of the open-circuit thermal remagnetization of non-ellipsoidal magnets are examined.

2相ステンレス鋳鋼におけるフェライトの相分離過程のメスバウアー分光測定

Duplex stainless steels become brittle at medium temperatures, owing to the phase decomposition of a ferrite into a Cr-rich α′ phase and an Fe-rich α phase. A cast CF3M duplex stainless steel containing 25% volume fraction of ferrite was aged at 723 K for time periods up to 10000 h to investigate the phase decomposition mechanism and to evaluate the composition of the decomposition products using Mössbauer spectroscopy at room temperature. The half width of the internal magnetic field distribution curve estimated from the experimental Mössbauer spectra increases in the initial stage of aging, suggesting compositional fluctuation due to spinodal decomposition. The ferrite phase finally decomposes into the Fe-rich α and Cr-rich α′ phases after prolonged aging. The composition of the Fe-rich α phase is estimated to be Fe-11 at%Cr-5 at%Ni by numerically analyzing the internal magnetic field distribution based upon the local environmental effect of solute atoms on the central Fe nucleus. The result is consistent with the one previously obtained by atom-probe analysis. The isomer shift of 57Fe in the α′ phase is estimated to be −0.114±0.002 mm/s at room temperature by decomposing an overlapped paramagnetic absorption peak into component peaks. The Cr content of the phase is estimated to be about 85 at% by plotting the isomer shift on an experimental isomer shift versus Cr content diagram.