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At least 145 records · Page 8Linked to original sources
PubMed2003

Single-molecule magnets: two-electron reduced version of a Mn12 complex and environmental influences on the magnetization relaxation of (PPh4)(2)[Mn(12)O(12)(O(2)CCHCl2)(16)(H2O)4].

The complex [Mn(12)O(12)(O(2)CCHCl(2))(16)(H(2)O)(4)] (2) in MeCN exhibits three quasi-reversible one-electron reduction processes at significantly higher potentials than [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)] (1). This has allowed the two-electron reduced version of 2 to be generated and isolated. Reaction of 2 with one and two equivalents of PPh(4)I led to isolation of (PPh(4))[Mn(12)O(12)(O(2)CCHCl(2))(16)(H(2)O)(4)] (3) and (PPh(4))(2)[Mn(12)O(12)(O(2)CCHCl(2))(16)(H(2)O)(4)] (4), respectively. The latter represents a new isolated oxidation level of the Mn(12) family of single-molecule magnets (SMMs). Crystallization from CH(2)Cl(2)/hexanes yields a mixture of two crystal forms, 4.4CH(2)Cl(2).H(2)O (4a) and 4.6CH(2)Cl(2) (4b), both of which have been structurally characterized as triclinic and monoclinic, respectively. The molecular structures are very similar, with the added electrons localized on former Mn(III) ions to give a trapped-valence 2Mn(II), 6Mn(III), 4Mn(IV) oxidation state description. Dried solid analyzed as unsolvated 4. (1)H NMR spectral data in CD(2)Cl(2) confirm that 4 retains its solid-state structure in solution. Bulk DC magnetization data for dried 4 in the 1.80-4.00 K and 10-70 kG ranges were fit to give S = 10, D = -0.275 cm(-1), g = 2.00 and |D|/g = 0.14 cm(-1), where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4a and 4b give resolvable frequency-dependent out-of-phase (chi(M)'') signals in AC susceptibility studies resulting from the magnetization relaxation of SMMs. Relaxation rate vs T data to 1.8 K obtained from the chi(M)'' vs temperature studies were supplemented with rate vs T data measured to lower temperatures via magnetization vs time decay data, and these were fit to the Arrhenius equation to give the effective barrier to relaxation (U(eff)). The U(eff) values are 18.5 and 30.3 K for 4a and 4b, respectively. A similar analysis for dried 4 using AC data gave U(eff) = 32 K. Magnetization vs DC field sweeps on single crystals of 4a and 4b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded |D|/g values of 0.087 and 0.14 cm(-1) for 4a and 4b, respectively, suggesting that the differences in U(eff) are primarily caused by changes to D. The combined work demonstrates the sensitivity of the magnetic properties of these new [Mn(12)](2-) SMMs to subtle differences in their environment as determined by the precise packing, solvent molecules, and overall crystal symmetry (space group) and represents an important caveat to workers in the field.

Journal Article↗
PubMed2002

A cyano-bridged single-molecule magnet: slow magnetic relaxation in a trigonal prismatic MnMo(6)(CN)(18) cluster.

We report the synthesis of the first well-documented example of a cyano-bridged single-molecule magnet. An assembly reaction parallel to that employed in producing the trigonal prismatic [(Me(3)tacn)(6)MnCr(6)(CN)(18)](2+) (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane) cluster affords K[(Me(3)tacn)(6)MnMo(6)(CN)(18)](ClO(4))(3) (1), containing an analogous molybdenum(III)-substituted cluster. Fits to the DC magnetic susceptibility and magnetization data for 1 show that the MnMo(6) cluster possesses weak antiferromagnetic coupling (J = -6.7 cm(-1)), leading to an S = (13)/(2) ground state with significantly enhanced magnetic anisotropy (D = -0.33 cm(-1) and E = -0.018 cm(-1)). Consistent with these results, AC magnetic susceptibility measurements show the molecule to exhibit slow magnetic relaxation indicative of a single-molecule magnet with an energy barrier of 10 cm(-1) for spin reversal.

Journal Article↗
PubMed2003

Sub-100 nm confinement of magnetic nanoparticles using localized magnetic field gradients.

Ferromagnetic rods containing thin sections of diamagnetic metal create intense magnetic field gradients that attract and confine magnetic nanoparticles to regions of space as small as 20 nm. The rods (80 nm diameter) comprised alternating sections of CoNi ( approximately 350 nm) and Au (20-160 nm) formed by electrodeposition into porous polycarbonate membranes. Upon magnetizing the rods, large magnetic gradients (106-107 T/m) form at the boundaries between ferromagnetic and diamagnetic sections. These gradients attract and confine magnetic nanoparticles to attoliter volumes of space surrounding the rod. This method provides a new tool for generating intense, highly localized magnetic field gradients, by design, and confining magnetic nanoparticles in these gradients.

Cobalt↗
PubMed2006

Two-dimensional networks based on Mn4 complex linked by dicyanamide anion: from single-molecule magnet to classical magnet behavior.

Three two-dimensional (2D) network compounds based on Mn(III)/Mn(II) tetranuclear single-molecule magnets (SMMs) connected by dicyanamide (dcn-) linkers have been synthesized: [Mn4(hmp)4(Hpdm)2(dcn)2](ClO4)2 x 2 H2O x 2 MeCN (2), [Mn4(hmp)4Br2(OMe)2(dcn)2] x 0.5 H2O x 2 THF (3), [Mn4(hmp)6(dcn)2](ClO4)2 (4), where Hhmp and H2pdm are 2-hydroxymethylpyridine and pyridine-2,6-dimethanol, respectively. The [Mn4]/dcn- system appears very versatile, but enables its chemistry to be rationalized by a fine-tune of the synthetic conditions. The double cuboidal [Mn4] unit is preserved in the whole family of compounds, despite strong modifications of its Mn(II) coordination sphere. The chemical control of the coordination number of dcn- on the Mn(II) sites has been the key to obtain the following series of compounds: a discrete cluster, [Mn4(hmp)6(NO3)2(dcn)2] x 2 MeCN (1), 2D networks (2, 3, and 4), and the previously reported 3D compound, [Mn4(hmp)4(mu3-OH)2][Mn(II)(dcn)6] x 2 MeCN x THF. Direct current magnetic measurements show that both Mn2+-Mn3+ and Mn3+-Mn3+ intra-[Mn4] magnetic interactions are ferromagnetic leading to an S(T) = 9 ground state for the [Mn4] unit. Despite the very similar 2D lattices in 2-4, the two kinds of orientation of the [Mn4] unit (i.e., angle variations between the two easy axes) lead to different magnetic properties ranging from SMM behavior for 2 and 1 to a long-range canted antiferromagnetic order for 4. Compound 3 is more complicated as the magnetic measurements strongly suggest the presence of a canted antiferromagnetic order below 2.1 K, although the magnetization slow relaxation is simultaneously observed. Heat capacity measurements confirm the long-range magnetic order in 4, while in 3, the critical behavior is frozen by the slow relaxation of the anisotropic [Mn4] units.

Journal Article↗
PubMed2006

What tunes the structural anisotropy of magnetic fluids under a magnetic field?

In the present study, the structure of monophasic ionic magnetic fluids under a static magnetic field is explored. In these aqueous electrostatically stabilized ferrofluids, we vary both the isotropic interparticle interactions and the anisotropic dipolar magnetic interaction by tuning the ionic strength and the size of the nanoparticles. Small angle neutron scattering measurements carried out on nanoparticles dispersed in light water exhibit miscellaneous 2D nuclear patterns under a magnetic field with various q-dependent anisotropies. In this nondeuterated solvent where the magnetic scattering is negligible, this anisotropy originates from an anisotropy of the structure of the dispersions. Both the low q region and the peak of the structure factor can be anisotropic. On the scale of the interparticle distance, the structure is better defined in the direction perpendicular to the field. In the thermodynamic limit (q-->0), the model previously described in ref 10 matches the data without any fitting parameters: the interparticle interaction is more repulsive in the direction parallel to the magnetic field. At low q, the amplitude of the anisotropy of the pattern is governed by the ratio of two interaction parameters: the reduced parameter of the anisotropic magnetic dipolar interaction, gamma/Phi, over the isotropic interaction parameter, , in zero field, which is proportional to the second virial coefficient.

Journal Article↗
PubMed2003

High-temperature superconductor bulk magnets that can trap magnetic fields of over 17 tesla at 29 K.

Large-grain high-temperature superconductors of the form RE-Ba-Cu-O (where RE is a rare-earth element) can trap magnetic fields of several tesla at low temperatures, and so can be used for permanent magnet applications. The magnitude of the trapped field is proportional to the critical current density and the volume of the superconductor. Various potential engineering applications for such magnets have emerged, and some have already been commercialized. However, the range of applications is limited by poor mechanical stability and low thermal conductivity of the bulk superconductors; RE-Ba-Cu-O magnets have been found to fracture during high-field activation, owing to magnetic pressure. Here we present a post-fabrication treatment that improves the mechanical properties as well as thermal conductivity of a bulk Y-Ba-Cu-O magnet, thereby increasing its field-trapping capacity. First, resin impregnation and wrapping the materials in carbon fibre improves the mechanical properties. Second, a small hole drilled into the centre of the magnet allows impregnation of Bi-Pb-Sn-Cd alloy into the superconductor and inclusion of an aluminium wire support, which results in a significant enhancement of thermal stability and internal mechanical strength. As a result, 17.24 T could be trapped, without fracturing, in a bulk Y-Ba-Cu-O sample of 2.65 cm diameter at 29 K.

Journal Article↗
PubMed1999

Propagation of a magnetic domain wall in a submicrometer magnetic wire

The motion of a magnetic domain wall in a submicrometer magnetic wire was detected by use of the giant magnetoresistance effect. Magnetization reversal in a submicrometer magnetic wire takes place by the propagation of a magnetic domain wall, which can be treated as a "particle." The propagation velocity of the magnetic domain wall was determined as a function of the applied magnetic field.

Journal Article↗
PubMed1991

Alteration of the Magnetic Properties of Aquaspirillum magnetotacticum by a Pulse Magnetization Technique.

The presence of a narrow shape and size distribution for magnetite crystals within magnetotactic organisms suggests strongly that there are species-specific mechanisms that control the process of biomineralization. In order to explore the extent of this control, cultures of Aquaspirillum magnetotacticum in the exponential growth phase were exposed to increasing magnetic pulses with the aim of separating cell populations on the basis of their magnetic coercivities. Isothermal remanent magnetization and anhysteretic remanent magnetization studies were performed with freeze-dried magnetic cells after the remagnetization treatment. Subpopulations of A. magnetotacticum that showed an increase in coercivity correlated with the intensity of the magnetic pulses were isolated. After successive subcultures of the remaining north-seeking cells, a maximum bulk coercivity (H(b)) of 40 mT was obtained after treatment with a 55-mT pulse. Although we obtained A. magnetotacticum variants displaying higher coercivities than the wild-type strain, changes in crystal size or shape of the magnetite crystals were below reliable detection limits with transmission electron microscopy. Attempts to shift the coercivity towards higher values caused it to decrease, a change which was accompanied by an increase in magnetostatic interactions of the magnetosome chains as well as an increase in the cell population displaying an abnormal distribution of the magnetosome chains. Ultrastructural analyses of cells and magnetosomes revealed the appearance of cystlike bodies which occasionally contained magnetosomes. The increase in cystlike cells and abnormal magnetosome chains when higher magnetic pulses were used suggested that magnetosomes were collapsing because of stronger interparticle magnetostatic forces.

Journal Article↗
PubMed2006

[Effects of small interfering RNA magnetic nanoparticles combination with external magnetic fields on survivin gene expression of bladder cancer cells and apoptosis].

OBJECTIVE: To investigate the effects of the small interfering RNA plasmid-dextran magnetic nanoparticles (siRNA-DMN) combination with external magnetic fields on silencing survivin gene expression of bladder cancer cells and apoptosis when DMN used as gene carrier to transfer siRNA-survivin recombinant plasmid in vivo. METHODS: The siRNA-survivin recombinant plasmid specific targeted survivin was synthesized in previous experiment. DMN were prepared by chemical coprecipitation method and used as gene carrier. The siRNA-DMN were constructed by static electricity of polylysine and transferred into human bladder cancer BIU-87 cells with the help of external magnetic fields. The growth inhibiting rate (IR) of BIU-87 cells was observed by 3-[4, 5-dimethylthiazol-2-yl]-2, 5-diphenyl tetrazolium bromide's test and the apoptosis index (AI) was detected by transferase-mediated dUTP nick end labeling method. The relatively transcription levels of survivin mRNA and protein expression were respectively detected by semi-quantitive reverse transcription polymerase chain reaction and Western Blotting techniques. RESULTS: The diameter, effective diameter and saturation magnetization of DMN-siRNA were about 10 - 12 nm, 94.8 nm and 0.19 emu/g, respectively. The IR (39.60%) and AI (28.72%), the relative expression of survivin mRNA and protein of siRNA-DMN combination with external magnetic fields on BIU-87 cells were significantly higher and lower than those in the control group and single siRNA-DMN group, respectively (P < 0.05). CONCLUSIONS: The siRNA-survivin plasmid-DMN combination with external magnetic fields could effectively inhibit survivin expression and induce BIU-87 cells apoptosis which provided experimental basis for the magnetic targeting gene therapy of bladder tumor.

Apoptosis↗
PubMed1999

Increased DNA synthesis in INIT/10T1/2 cells after exposure to a 60 Hz magnetic field: a magnetic-field or a thermal effect?

This study was designed to test the hypothesis that a 0.1-0.8-mT 60 Hz magnetic field may act as a promoter of carcinogenesis. C3H 10T1/2 mouse fibroblasts initiated with the carcinogen methylcholanthrene (INIT/10T1/2 cells) were used; in these cells, expression of the carcinogenic phenotype is suppressed indefinitely by the presence of retinyl acetate in the culture medium. After withdrawal of retinyl acetate, expression of the carcinogenic phenotype may be observed as the loss of contact inhibition. Cells grown without retinyl acetate were exposed to 0.1-0.8-mT (rms) 60 Hz magnetic fields or to sham fields. Eight days after exposure, magnetic-field and sham-exposed cells showed the same levels of incorporation of [3H]thymidine, and both had counts significantly higher than those of unexposed cells. The rate of incorporation of [3H]thymidine was very sensitive to small (0.1-0.8 degrees C) and transient (60 min) increases in incubation temperature during the first few days of withdrawal of retinyl acetate. Exposure of Jurkat (human acute T-cell lymphoma) and GH3 (rat pituitary tumor) cells to magnetic fields and sham conditions yielded similar results. INIT/10T1/2 cells cultured in the presence of retinyl acetate showed no effect of exposure conditions. Both magnetic-field and sham exposures caused a slight increase in temperature within the exposure zone in the incubator. Thus the differences between rates of incorporation of [3H]thymidine in magnetic field-exposed, sham-exposed and unexposed cells seem to be attributable at least in part to a slight elevation in temperature during exposure. Since some cells appear to be extremely sensitive to small increases in temperature, measurements of magnetic-field effects must be made and interpreted with caution.

Animals↗
PubMedDate not supplied

Static magnetic field effects on human subjects related to magnetic resonance imaging systems.

GOAL: This paper reviews recent studies evaluating human subjects for physiologic or neuro-cognitive function adverse effects resulting from exposure to static magnetic fields of magnetic resonance imaging systems. MATERIALS AND METHODS: The results of three studies are summarized. Two studies evaluated exposure to a maximum of 8 Tesla (T). The first series studied 25 normal human subjects' sequential vital signs (heart rate, blood pressure, blood oxygenation, core temperature, ECG, respiratory rate) measured at different magnetic field strengths to a maximum of 8 T. A second series of 25 subjects were studied at 0.05 and 8 T (out and in the bore of the magnet), performing 12 different standardized neuro-psychological tests and auditory-motor reaction times. The subjects' comments were recorded immediately following the study and after a three-month interval. The third study contained 17 subjects, placed near the bore of a 1.5 T magnet, and it used six different cognitive, cognitive-motor, or sensory tests. RESULTS: There were no clinically significant changes in the subjects' physiologic measurements at 8 T. There was a slight increase in the systolic blood pressure with increasing magnetic field strength. There did not appear to be any adverse effect on the cognitive performance of the subjects at 8 T. A few subjects commented at the time of initial exposure on dizziness, metallic taste in the mouth, or discomfort related to the measurement instruments or the head coil. There were no adverse comments at 3 months. The 1.5 T study had two of the four neuro-behavioral domains exhibiting adverse effects (sensory and cognitive-motor). CONCLUSIONS: These studies did not demonstrate any clinically relevant adverse effects on neuro-cognitive testing or vital sign changes. One short-term memory, one sensory, and one cognitive-motor test demonstrated adverse effects, but the significance is not clear.

Blood Pressure↗
PubMed2002

Oriented immobilization of galactose oxidase to bead and magnetic bead cellulose and poly(HEMA-co-EDMA) and magnetic poly(HEMA-co-EDMA) microspheres.

In order to obtain an active and stable oxidation reactor for daily use in biochemical laboratory we decided to immobilize galactose oxidase orientedly through a carbohydrate chain to the magnetic carriers. We used hydrazide derivatives of non-magnetic and magnetic bead cellulose and of magnetic and non-magnetic poly(HEMA-co-EDMA) microspheres. Activation of the enzyme molecules was done by sodium periodate in the presence of supplements (fucose, CuSO4, catalase). Orientedly immobilized galactose oxidase presents high storage stability and lower susceptibility to inappropriate microenvironmental conditions. Reactor reactivated by three pulses of D-galactose retained practically 100% of its native activity after 6 months. The positive properties of both magnetic carriers were entirely confirmed.

Animals↗
PubMed1996

Measurements of magnetism-related forces and torque moments affecting medical instruments, implants, and foreign objects during magnetic resonance imaging at all degrees of freedom.

We designed a novel force/torque moment transducer with six degrees of freedom. This new sensor is used for measuring the forces and torque moments exerted by static magnetic resonance imaging (MRI) fields on various medical implants, instruments, and ballistic shell fragments. To determine the location of strongest effect the inhomogeneity of the magnetic field between the entrance of the MRI-tube (portal), and the imaging section was measured. Force and torque moments of a sample of 45 objects were measured at two positions, at the portal, and in the center of the imaging section. We classified the objects regarding to the magnetism-related forces measured in the homogeneous part of the MRI tube. About one-third of the objects was non- or weakly magnetic, one-third moderately, and the last third was strongly magnetic. Forces within the inhomogeneous area of the magnetic field at the entrance part of the tube were up to half a magnitude larger than forces inside the homogeneous imaging section. Here the greatest potential risks to patients might occur.

Biophysical Phenomena↗
PubMed2001

Magnetic resonance spectroscopy and magnetic resonance imaging findings in Krabbe's disease.

Two twins with late infantile globoid cell leukodystrophy of Krabbe's disease were studied with conventional magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopy. Brain MRI demonstrated brain atrophy with extensive bilateral symmetric abnormal T2 signal in the posterior periventricular white matter, parietal lobes, corona radiata, centrum semiovale, and splenium of the corpus callosum. Magnetic resonance imaging-guided proton magnetic resonance spectroscopy revealed prominent peaks from choline-containing compounds, total creatine, and inositols. The N-acetylaspartate peak was markedly reduced, and the choline-to-N-acetylaspartate ratio was abnormally high; in one of the twins, lactic acid was also detected. The constellation of magnetic resonance spectroscopy findings is indicative of extensive demyelination, gliosis, and loss of axons in the involved white matter; the latter two events occur in the later stages of globoid cell leukodystrophy. In conjunction with brain MRI, these magnetic resonance spectroscopy findings may alert clinicians to the possibility of leukodystrophy in children with progressive encephalopathy.

Aspartic Acid↗
PubMed2006

Short communication: assessment of environmental disturbances to the static magnetic field in magnetic resonance installations.

The static magnetic field of MRI scanners can be affected by environmental factors. Magnetic resonance spectroscopy and functional imaging with single-shot echo-planar imaging (EPI) are particularly vulnerable to the movement of lifts, vehicles, trains and other large metallic masses in the vicinity. This work investigates the sensitivity of two different imaging techniques to assess disturbances of the static magnetic field: (i) phase changes in gradient-echo images of a uniform test object; and (ii) image displacement along the phase encoding direction in single-shot EPI images. For the latter a hexane sample was used, and the separation between CH2 and CH3 signals was taken as a reference. Both techniques were evaluated in a site known to be free of any significant environmental disturbances and validated by inducing a magnetic field disturbance. Both techniques provide valuable information in acceptance tests, allowing MRI users to evaluate and manage the environmental conditions surrounding a scanner. The single-shot EPI technique was found to be highly sensitive, being expected to detect magnetic field fluctuations down to 0.005 parts per million (ppm). The phase images method was found to be less sensitive (0.02 ppm) but is more easily available. The single-shot EPI technique was used in acceptance tests and environmental disturbances to the magnetic field of the order of 0.04 ppm were measured at the isocentre on two separate occasions.

Artifacts↗
PubMed1994

Magnetic resonance imaging artifacts and the magnetic attachment system.

The use of a rare earth magnetic attachment system as a means of retaining dentures or maxillofacial prostheses results in artifacts, when magnetic resonance imaging (MRI) is used as a diagnostic tool. In such cases, the artifact is not caused by the magnet itself, but by the ferromagnetic stainless steel keeper that is placed in the body. This report evaluates such ferromagnetic stainless steel devices with respect to magnetic resonance imaging artifacts. A grid phantom and a 0.2 Tesla superconducting system were used to assess the imaging artifacts. The magnetic properties, shape, composition and size of ferromagnetic stainless steel devices were analyzed to study the relation MRI artifacts and ferromagnetic materials. The higher the magnetic permeability was, the greater the artifact produced. The size and volume of the material directly influence the artifact produced. The artifact size can be attenuated by the sequence used to obtain the images.

Adult↗
PubMed2000

Sextant localization of prostate cancer: comparison of sextant biopsy, magnetic resonance imaging and magnetic resonance spectroscopic imaging with step section histology.

PURPOSE: We compared the accuracy of endorectal magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging with that of sextant biopsy for the sextant localization of prostate cancer. MATERIALS AND METHODS: Sextant biopsy, MRI, magnetic resonance spectroscopic imaging and radical prostatectomy with step section histology were done in 47 patients with prostate cancer. For each sextant we categorized biopsy and imaging results as positive or negative for cancer. Step section histology was used as the standard of reference. RESULTS: For sextant localization of prostate cancer MRI and magnetic resonance spectroscopic imaging were more sensitive but less specific than biopsy (67% and 76% versus 50%, and 69% and 68% versus 82%, respectively). The sensitivity of sextant biopsy was significantly less in the prostate apex than in the mid prostate or prostate base (38% versus 52% and 62%, respectively). MRI and magnetic resonance spectroscopic imaging had similar efficacy throughout the prostate compared with biopsy only as well as better sensitivity and specificity in the prostate apex (60% and 75%, and 86% and 68%, respectively). A positive biopsy or imaging result had 94% sensitivity for cancer and concordant positivity by all 3 tests was highly specific at 98%. CONCLUSIONS: Overall MRI and magnetic resonance spectroscopic imaging have accuracy similar to biopsy for intraprostatic localization of cancer and they are more accurate than biopsy in the prostate apex. These 2 imaging modalities may supplement biopsy results by increasing physician confidence when evaluating intraprostatic tumor location, which may be important for planning disease targeted therapy.

Aged↗
PubMed1996

[From nuclear magnetic resonance to magnetic resonance imaging].

Magnetic resonance imaging is based upon the phenomenon of nuclear magnetic resonance. The patient is placed in a magnet the provides a strong magnetic field inducing a net magnetization due to hydrogen nuclei inside biological tissues. The image is formed from the localized measurement of this magnetization by means of radio-frequency waves. The contrast is a function of the number of hydrogen nuclei and of the time characteristics T1 and T2 of the return to equilibrium. Many other parameters can be measured by this method and the permanent improvement of the technique will extend further the field of its medical application.

Forecasting↗