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PubMed1989

[Sandwich type dental magnetic devices of Nd-Fe-B magnet and permendur].

Nd-Fe-B magnets have a very high maximum energy product (BH max), which is defined as the attractive strength between a magnet and opposing magnetic materials. Permendur (Fe-49 Co-2 V) has the greatest magnetic saturation (Bs), which makes it strongly magnetized. If magnetic retainers were made with Nd-Fe-B magnets and a permendur yoke, they would be small and have strong retention. The purpose of this investigation was to develop small dental magnetic devices with Nd-Fe-B magnets and permendur yokes. The magnetic devices form sandwich-type magnetic circuits with magnetic stainless steel keepers. A 4 x 3 x 2 mm rectangular prism Nd-Fe-B magnet was used. The magnet was sandwitched between the semi-columnar yokes. To protect the magnet from corrosion, the devices were encapsulated with 304 stainless steel by silver brazing and adhesion bonding of a stainless steel tube and foil. The optimum cross-sectional area of the yoke was determined experimentally. The dimensions of the devices were phi 5 x 3.5 mm for 4 x 3 x 2 magnet. The breakaway retention for a keeper of magnetic stainless steel (Type XM 27) was 852 g on average. This breakaway retention is sufficient for dental prosthetic applications.

Beryllium↗
PubMed2004

Systematic correlation of transcranial magnetic stimulation and magnetic resonance imaging in cervical spondylotic myelopathy.

STUDY DESIGN: A prospective study over a 3.5-year period involving transcranial magnetic stimulation and magnetic resonance imaging. OBJECTIVES: To assess the correlation of transcranial magnetic stimulation and magnetic resonance imaging in cervical spondylotic myelopathy qualitatively and statistically. SUMMARY OF BACKGROUND DATA: Cervical spondylotic myelopathy presents with different degrees of cord compression, which can be assessed by magnetic resonance imaging. There are no large studies correlating transcranial magnetic stimulation and magnetic resonance imaging findings in this condition. METHODS: A total of 141 patients with a clinical diagnosis of cervical spondylotic myelopathy were prospectively studied over a 3.5-year period. They were classified into Groups 1 to 4 based on severity of cervical cord changes on magnetic resonance imaging. All had transcranial magnetic stimulation and central motor conduction time measurements within 2 months of the magnetic resonance imaging study. RESULTS: Twenty-eight, 49, 28, and 36 patients were classified into Groups 1 to 4, respectively. Mean upper limb and lower limb central motor conduction times correlated with the severity of magnetic resonance cord compression. The absence of central motor conduction time abnormalities correlated reliably with the absence of cervical cord impingement as in Group 1. Statistically significant right left difference in central motor conduction time in the lower limbs was seen between Groups 1 (no cord changes) and Group 2 (mild cord impingement). Eight other patients with diagnoses other than cervical spondylotic myelopathy all showed central motor conduction time abnormalities. The sensitivity and specificity for transcranial magnetic stimulation for differentiating the presence from absence of magnetic resonance imaging cord abnormality were 100% and 84.8%, respectively. CONCLUSIONS: Transcranial magnetic stimulation showed excellent correlation with magnetic resonance imaging findings and can be considered as an effective technique for screening patients for cervical cord abnormalities before magnetic resonance imaging in the clinical setting. The findings in this study have relevant implications in the pathophysiology, management, and health costs of cervical spondylotic myelopathy.

Adult↗
PubMed2002

Nonequilibrium Brownian dynamics analysis of negative viscosity induced in a magnetic fluid subjected to both ac magnetic and shear flow fields.

We study the rheological and magnetic characteristics of a magnetic fluid. The system, which we investigate, is as follows. Ferromagnetic particles are dispersed in a solvent, which is subjected to both ac magnetic and shear flow fields. The translational and rotational motions of particles are calculated by the Brownian dynamics method based on Langevin equations and the rheological and magnetic characteristics of the magnetic fluid system are estimated. First, we investigate the rheological and magnetic characteristics of the system in a dc magnetic field and then we analyze the effect of an ac magnetic field on those characteristics. We find that the negative viscosity effect is induced at a certain frequency range of the ac magnetic field. We also find that there are two main mechanisms responsible for the occurrence of negative viscosity. (1) Resonance between the rotational motions of the dipoles of particles and the fluctuation of ac magnetic fields occurs when applied magnetic fields are weak compared to the shear rate, in which case particles can still rotate in magnetic fields. Beyond this resonance frequency, negative viscosity appears. (2) The magnetic dipole moments of particles are forced to stay in the direction of the magnetic field when strong magnetic fields are applied in relatively low shear flow fields. However, negative viscosity occurs when the frequency of external magnetic fields exceeds a critical value, in which case the dipoles rotate continuously in a shear flow without stopping. In both cases, the mean angular velocity of the particles becomes higher than that of the solvent.

Journal Article↗
PubMed1995

Extent and flux density of static magnetic fields generated by orthodontic samarium-cobalt magnets.

The aim of this study was to measure and to analyze the extent and flux density of static magnetic fields generated by commercially available samarium-cobalt magnets used in orthodontics. The flux density was measured with a gaussmeter and a Hall probe with the magnets mounted in clinically relevant positions, i.e., in attractive and in repelling positions and also in the single position. Furthermore, the flux density between new and clinically used and recycled magnets was compared. It was found that the maximum flux density was generated at the pole faces and that magnets in attractive positions produced the highest flux density (2.2 kG), followed by the single magnet (2.0 kG) and the repelling magnets (1.7 kG). The flux density decreased rapidly (exponentially) with increased distance from the magnets. The flux density was approximately the same or less than the flux density of the earth magnetism (0.3 to 0.7 G) 60 mm from the attractive magnets, 50 mm from the single magnets, and 35 mm from the repelling magnets. The difference in flux density between new and clinically used and recycled samarium-cobalt magnets was negligible. Thus, the static magnetic field exposure of surrounding tissues can be assumed to be low, and the conceivable risk of harmful biologic effects must be regarded as small and limited when the tested orthodontic magnets are used clinically.

Cobalt↗
PubMed1995

[Clinical investigation of transcranial magnetic stimulation of the facial nerve--an early prognostic diagnosis of patients with peripheral facial palsy and the facial nerve magnetic stimulation site].

To obtain an early prognostic diagnosis of patients with peripheral facial palsy, a magnetic stimulator (Dantec Mag 2) was used to directly stimulate the intracranial portion of the facial nerve in 15 normal subjects and 108 patients with peripheral facial palsy. In normal subjects and patients with facial palsy, compound muscle action potentials (CMAPs) of the orbicularis oris muscle elicited by transcranial magnetic stimulation were compared with CMAPs elicited by electrical stimulation at a peripheral site of the stylomastoid foramen. This technique is similar to electroneurography (ENoG) and is regularly used in our department. In normal subjects, the latency of magnetically evoked CMAPs was longer (1.0ms, SD 0.39ms) than that of CMAPs evoked by electrical stimulation. There were two categories of patients; the first group consisted of patients who visited our hospital within 2 weeks after palsy onset with a record of electrically evoked CMAPs (ENoG) and magnetically evoked CMAPs, the second group consisted of all others. The first group was then divided into four subgroups based on minimal ENoG values obtained within 2 weeks after the onset of palsy. In patients, ENoG values declined until the seventh day after palsy onset, and then plateaued. However, the amplitude ratio of magnetically evoked CMAPs between the affected side and normal side showed no tendency to deline until the seventh day after palsy onset. Thus, whether magnetically evoked CMAPs could be recorded must be discussed in relation to the prognosis of facial palsy. The patients in whom magnetically evoked CMAPs could be recorded within the seven days after the onset of palsy were classified into a group in which the minimal ENoG value was greater than 20%. These patients recovered almost 2 months after the onset of palsy, and were significantly better than the recovery rates of those patients in whom magnetically evoked CMAPs could not be recorded. The site at which the facial nerve is magnetically stimulated remains controversial. In patients with peripheral facial palsy, recovery of the stapedial reflex, blink reflex and magnetically evoked CMAPs were examined to investigate the site of magnetic stimulation. From the clinical perspective, the facial nerve is thought to be magnetically stimulated near the meatal foramen that Fisch reported the site of damage in Bell's palsy. This stimulation site was almost the same point as that calculated from the mean latency difference between magnetically evoked CMAPs and ENoG in normal controls.

Adolescent↗
PubMed2001

Effects of paramagnetic ferrocenium cations on the magnetic properties of the anionic single-molecule magnet [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)]-.

The preparation and physical characterization are reported for the single-molecule magnet salts [M(Cp')(2)](n)()[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (M = Fe, n = 1, Cp' = C(5)Me(5) (2a), C(5)H(5) (2b); M = Co, n = 1, Cp' = C(5)Me(5) (2c), C(5)H(5) (2d); M = Fe, n = 2, Cp' = C(5)Me(5) (2e), C(5)H(5) (2f)) to investigate the effects of paramagnetic cations on the magnetization relaxation behavior of [Mn(12)]- anionic single-molecule magnets. Complex 2a.2H(2)O crystallizes in the orthorhombic space group Aba2, with cell dimensions at 173 K of a = 25.6292(2) A, b = 25.4201(3) A, c = 29.1915(2) A, and Z = 4. Complex 2c.2CH(2)Cl(2).C(6)H(14) crystallizes in the monoclinic space group P2(1)/c, with cell dimensions at 173 K of a = 17.8332(6) A, b = 26.2661(9) A, c = 36.0781(11) A, beta = 92.8907(3) degrees, and Z = 4. These two salts consist of either paramagnetic [Fe(C(5)Me(5))(2)]+ cations or diamagnetic [Co(C(5)Me(5))(2)]+ cations, and [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)]- anions. The structures of the anions in the two salts are similar, consisting of a central Mn(4)O(4) cubane moiety, surrounded by a nonplanar ring of eight Mn atoms that are bridged by and connected to the cube via mu(3)-O(2)- ions. The oxidation states of four Mn sites out of eight outer Mn ions in complex 2a were assigned to be +2.75 from the valence bond sum analysis although the disordering of bridging carboxylates prevents more precise determination. On the other hand in complex 2c, one Mn site out of eight outer Mn ions was identified as a Mn(II) ion, accommodating the "extra" electron; this was deduced by a valence bond sum analysis. Thus, the anion in complex 2c has a Mn(II)(1)Mn(III)(7)Mn(IV)(4) oxidation state description. The Jahn-Teller axes of the Mn(III) ions in both anions are roughly aligned in one direction. All complexes studied exhibit a single out-of-phase ac magnetic susceptibility (chi"(M)) signal in the 4.6-4.8 K range for complexes 2a-2d and in the 2.8-2.9 K range for complexes 2e and 2f at 1 kHz ac frequency. The temperature of the chi"(M) peaks is frequency dependent, as expected for single-molecule magnets. From Arrhenius plots of the frequency dependence of the temperature of the chi"(M) maxima, the effective energy barriers U(eff) for changing spin from "up" to spin "down" were estimated to be 50-54 K for complexes 2a-2d and 27-28 K for complexes 2e and 2f. The least-squares fits of the reduced magnetization data indicate that both complexes 2a and 2d have ground states of S = (21)/(2). High-frequency EPR spectra were recorded for complex 2a at frequencies of 217, 327, and 434 GHz in the 4.5-30 K range. The observed transition fields were least-squares fit to give g = 1.91, D = -0.35 cm(-1), and B(4)(0) = -3.6 x 10(-7) cm(-1) for the S = (21)/(2) ground state. The effective energy barrier U(eff) is slightly lower than U estimated from D, which is consistent with the thermally assisted tunneling model. Magnetization hysteresis loops were observed for complexes 2a and 2c. Although 2a was oriented in a different manner as expected by strong magnetic field, both complexes show clear hysteresis loops with some steps on them, indicating that the effect of the magnetic cation on the magnetization relaxation of the anionic [Mn(12)]- complex is rather small. An 11% (57)Fe enriched complex 2b was studied by means of Mössbauer spectroscopy down to as low as 1.7 K. Slow paramagnetic relaxation broadening and magnetic hyperfine splitting were evident in the low-temperature spectra, indicating that the iron atoms feel a growing magnetic field owing to slow magnetization reversal in the [Mn(12)]- anions.

Journal Article↗
PubMed1997

Magnetic orientation and the magnetic sense in arthropods.

The physical properties of the earth's magnetic field are summarized with the aim of emphasizing their significance as cues that can be exploited in orientational tasks. Past work has revealed magnetic orientation in vertebrates as well as invertebrates, including arthropods. The key finding to date has been that, as opposed to many vertebrates, the magnetic compass of arthropods responds to the polarity, rather than to the inclination of the earth's magnetic field. As in the case of vertebrates, the debate over how arthropods detect magnetic fields has yet to be resolved. Currently, evidence has been reported in support of a detection system based on magnetite crystals together with a variety of detection systems based on events occurring at the molecular level. Interactions between the magnetic and other compasses in orientation experiments suggest the existence of an area in the brain where spatial orientation information from magnetic and other stimuli converges. The slow advance of our knowledge on magnetic orientation in arthropods, as opposed to the much better understanding of magnetic orientation in vertebrates, arises from difficulties in identifying the appropriate behavioural contexts in which arthropods respond to magnetic fields in both laboratory and field situations. Arthropods thus present challenges not only in demonstrating magnetic orientation, but also in elucidating the sensory mechanisms involved in the perception of magnetic fields.

Animals↗
PubMed2000

Study of magnetic particles pulse-injected into an annular SPLITT-like channel inside a quadrupole magnetic field.

Advantages of the continuous magnetic flow sorting for biomedical applications over current, batch-wise magnetic separations include high throughput and a potential for scale-up operations. A continuous magnetic sorting process has been developed based on the quadrupole magnetic field centered on an annular flow channel. The performance of the sorter has been described using the conceptual framework of split-flow thin (SPLITT) fractionation, a derivative of field-flow fractionation (FFF). To eliminate the variability inherent in working with a heterogenous cell population, we developed a set of monodisperse magnetic microspheres of a characteristic magnetization, and a magnetophoretic mobility, similar to those of the cells labeled with a magnetic colloid. The theory of the magnetic sorting process has been tested by injecting a suspension of the magnetic beads into the carrier fluid flowing through the sorter and by comparing the theoretical and experimental recovery versus total flow-rate profiles. The position of the recovery maxima along the total flow-rate axis was a function of the average bead magnetophoretic mobility and the magnetic field intensity. The theory has correctly predicted the position of the peak maxima on the total flow-rate axis and the dependence on the bead mobility and the field intensity, but has not correctly predicted the peak heights. The differences between the calculated and the measured peak heights were a function of the total flow-rate through the system, indicating a fluid-mechanical origin of the deviations from the theory (such as expected of the lift force effects in the system). The well-controlled elution studies using the monodisperse magnetic beads, and the SPLITT theory, provided us with a firm basis for the future sorter evaluation using cell mixtures.

Magnetics↗
PubMed2000

A hybrid, inverse approach to the design of magnetic resonance imaging magnets.

This paper describes a hybrid numerical method of an inverse approach to the design of compact magnetic resonance imaging magnets. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. The emphasis of this work is on the optimal design of short MRI magnets. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric MRI magnets as well as asymmetric magnets. The results highlight that the method can be used to obtain a compact MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1 m in length, significantly shorter than current designs. Viable asymmetric magnet designs, in which the edge of the homogeneous region is very close to one end of the magnet system are also presented. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries.

Algorithms↗
PubMed2000

Extremely low frequency magnetic fields in residences in Germany. Distribution of measurements, comparison of two methods for assessing exposure, and predictors for the occurrence of magnetic fields above background level.

We examined the results of 1,835 magnetic field measurements in German residences conducted between November 1997 and September 1999. The measurements were part of an epidemiological study on the relationship between magnetic fields and childhood leukemia. We performed a fixed-location measurement of the magnetic field at 50 Hz and 16 2/3 Hz (frequency of the German railway system) over 24 h in the child's bedroom in the residence of each study participant. In addition, we conducted a second 24 h-measurement in the living room at 50 Hz, and spot measurements while walking through all rooms of the respective dwelling. Median 50 Hz magnetic fields above 0.2 muT were found to be infrequent in Germany (only 1.4% of all residences). Fields produced by high-voltage power lines (123-420 kV) were lower than expected: the median magnetic field was above 0.2 muT in only 8 (32.0%) of 25 residences located 50 m or closer to a high-voltage power line indicating that power lines in Germany are usually run well below the maximum power load. We found that magnetic fields were correlated with the type of residence and higher magnetic fields were measured in apartment buildings. There was also some evidence for a positive correlation between magnetic fields and traffic density and an inverse association between magnetic fields and family net income. The 24 h-magnetic field measurements correlated well with the spot measurements (r>0.7). However, when dichotomized with a cut-off point of 0.2 muT, there was only a poor agreement between the two measurement methods. A loss of the strength of the association after categorization was also observed when comparing the arithmetic mean and median of the same 24 h-measurement. In summary, these analyses give a valuable overview of magnetic field distributions in German residences.

Child↗
PubMed2002

Magnetic fluctuations with a zero mean field in a random fluid flow with a finite correlation time and a small magnetic diffusion.

Magnetic fluctuations with a zero mean field in a random flow with a finite correlation time and a small yet finite magnetic diffusion are studied. Equation for the second-order correlation function of a magnetic field is derived. This equation comprises spatial derivatives of high orders due to a nonlocal nature of magnetic field transport in a random velocity field with a finite correlation time. For a random Gaussian velocity field with a small correlation time the equation for the second-order correlation function of the magnetic field is a third-order partial differential equation. For this velocity field and a small magnetic diffusion with large magnetic Prandtl numbers the growth rate of the second moment of magnetic field is estimated. The finite correlation time of a turbulent velocity field causes an increase of the growth rate of magnetic fluctuations. It is demonstrated that the results obtained for the cases of a small yet finite magnetic diffusion and a zero magnetic diffusion are different.

Journal Article↗
PubMed1999

High-resolution magnetic relaxation dispersion measurements of solute spin probes using a dual-magnet system.

The magnetic field dependence of the nuclear spin-lattice relaxation rate provides a detailed report of the spectral density functions that characterize the intra- and intermolecular fluctuations that drive magnetic relaxation. We have addressed the difficult sensitivity and resolution problems associated with low magnetic field strengths by using two magnets in close proximity and shielded from each other. The sample is stored in the high magnetic field, pneumatically driven to the variable satellite field, then returned to the high field for detection at high resolution. A magnetic shield effectively decouples the two magnets so that varying the satellite field strength has minimal effect on the field strength and shim of the high field magnet. The disadvantage of the sample-shuttle magnet-pair system is the restriction imposed on the relaxation times by the finite shuttle times. Experiments not described here have shown this rate maximum to be about 20 s(-1) for most practical solutions. However, we demonstrate here that the sensitivity gains over switched-current magnet systems permit characterization of solute inter- and intramolecular dynamics over the time scale range from tens of microseconds to less than a picosecond. This range permits investigation of a number of crucial chemical dynamics questions, while high sensitivity permits examination of a variety of solute spins. Representative data are presented for (1)H, (111)Cd, and (7)Li.

Cadmium↗
PubMed1995

[A functional orthodontic magnetic appliance (FOMA) after Vardimon. 1. A three-dimensional analysis of the force system of the attractive magnets].

The functional magnetic system (FMS) is a removable functional appliance which induces mandibular advance by means of mandibular and maxillary magnets in an attracting configuration. The maxillary and mandibular plates are each equipped with 2 cylindrically shaped cobalt-samarium magnets, 4 mm in diameter and 3 mm in height, which are welded into stainless steel housings. The force system of this magnetic configuration was analyzed using the orthodontic measurement and simulation system (OMSS). OMSS simulated the mandibular jaw movements by separating the installed magnets vertically, corresponding to a mouth opening of X = -10 mm, transversally (right excursion, +/left excursion, -) at Y = +/- 10 mm and sagittally (anterior displacement, +/posterior displacement, -) at Z = +/- 10 mm. The resulting 2D and 3D force/displacement diagrams elucidate the outstanding centripetal-spatial orientation characteristics of the functional magnetic appliance in reference to the full overlap brought about by the attraction of the mandibular magnet by the maxillary magnet. The maximum centripetal forces reached a value of approximately FY, max = 0.65 N for the vertical attracting force at full overlap of the mandibular and maxillary magnets (X = 0.55 mm, Y = Z = 0 mm), a value of FY, max = 0.65 N for the medial shearing force at a partial transversal overlap Z = 0, Y = +/- 2 mm and Y = +/- 6 mm), and for the sagittal shearing force a value of FZ, max = 1.2 N at a partial sagittal overlap of the magnets (Y = 0 mm, Z = +/- 2 mm).(ABSTRACT TRUNCATED AT 250 WORDS)

Cobalt↗
PubMed2003

Magnetic drug targeting--biodistribution of the magnetic carrier and the chemotherapeutic agent mitoxantrone after locoregional cancer treatment.

Magnetic Drug Targeting means the specific delivery of chemotherapeutic agents to their desired targets, e.g. tumors, by using magnetic nanoparticles (ferrofluids) bound to these agents and an external magnetic field which is focused on the tumor. This type of target directed drug injection attempts to concentrate a pharmacologic agent by enhancing its efficacy while simultaneously minimizing deleterious side effects. In previous studies, we have been able to demonstrate the efficacy of this type of localized intraarterial chemotherapy in VX2 squamous cell carcinoma among rabbits [Alexiou, C., Arnold, W., Klein, R.J., Parak, F.G., Hulin, P., Bergemann, C., Erhardt, W., Wagenpfeil, S. and Lübbe, A.S. "Locoregional cancer treatment with Magnetic Drug Targeting", Cancer Res. 60 (2000) 6641-6648]. In the present investigation, we have studied the biodistribution of ferrofluids and chemotherapeutic agent by measuring the amount in the tumor, peritumoral area, various organs and body fluids (e.g. blood and urine), with and without Magnetic Drug Targeting. We compared results to that of administering a chemotherapeutic agent soley. An external magnetic field was directed toward the tumor for 60 min. Biodistribution of ferrofluids in the tumor was investigated using histological cross sections and measured semi-quantitatively using 123I-labeled nanoparticles and quantitatively by the use of radioactive 59Fe-ferrofluids. Mitoxantrone was quantitatively measured using HPLC-analysis. The strength of the external magnetic field was 0.6 Tesla (permanent magnet) in the 123iodine study and 1.7 Tesla (electromagnet) in the 59Fe-study and HPLC-analysis. The concentration of the ferrofluids (FFs) in the tumor region i.e. the tumor tissue and the surrounding area, which was under the influence of an external magnetic field, was found to be much higher than in the absence of one. In contrast to systemic chemotherapy, a much higher concentration of mitoxantrone in the tumor and the peritumoral area (region surrounding the tumor < or = 1 cm), by using only 50% and 20% of the normal dose was seen. Thus, the higher concentration of mitoxantrone could explain the therapeutic efficacy of Magnetic Drug Targeting in treatment of VX2 squamous cell carcinoma in rabbits in our previous studies with the advantage of no adverse clinical side effects.

Animals↗
PubMed2002

[Establishment of two-dimensional magnetic field finite element analysis model of cup-yoke-type magnetic attachment].

OBJECTIVE: The purpose of this study was to establish two-dimensional magnetic field finite element analysis model of cup-yoke-type magnetic attachment for optmizing the design of magnetic attachments. METHODS: Because the magnetic field of cup-yoke-type magnetic attachment is stable axial-symmetrical, the authors only analyzed two-dimensional magnetic field of 1/2 section. The Maxwell stress between magnet and keeper was integral analyzed using the finite element method, and the attractive force between magnetic attachments was obtained. RESULTS: Compared the value of calculated attractive force with that of examined, the authors found that the procedure veritably reflected influencing trend of variable factors on attractive force, and the value of former one was 10% less than the later one. CONCLUSION: This procedure can be used in magnetic field calculation of cup-yoke-type magnetic attachments.

Computer Simulation↗
PubMed2004

Mixed-valence MnIIIMnIV clusters [Mn7O8(O2SePh)8(O2CMe)(H2O)] and [Mn7O8(O2SePh)9(H2O)]: single-chain magnets exhibiting quantum tunneling of magnetization.

The syntheses, structures, and magnetic properties of two new Mn7 complexes containing phenylseleninate ligands are reported. [Mn7O8(O2SePh)8(O2CMe)(H2O)] (1) and [Mn7O8(O2SePh)9(H2O)] (2) were both prepared by the reaction of 18 equiv of benzeneseleninic acid (PhSeO2H) with [Mn12O12(O2CMe)16(H2O)4] in MeCN. Complex 1 x 6MeCN crystallizes in the triclinic space group P, and complex 2 x 2CH2Cl2 crystallizes in the monoclinic space group P2(1)/m. Both compounds possess an unprecedented [Mn7O8]9+ core comprising a central [MnIII3(micro3-O)4]+ unit attached to [MnIV2(micro-O)2]4+ and [MnIV2(micro-O)(micro3-O)]4+ units on either side. In each cluster, the PhSeO2- groups function as bridging ligands between adjacent Mn centers. The structure reveals strong Se.O intermolecular contacts between Mn7 units to give a one-dimensional chain structure, with weak interchain interactions. Solid-state DC magnetic susceptibility measurements of complexes 1 and 2 reveal that they have very similar properties, and detailed studies on 1 by AC susceptibility measurements confirm an S = 2 ground-state spin value. In addition, out-of-phase AC signals are observed, suggesting slow magnetization relaxation. Magnetization versus DC field sweeps down to 0.04 K reveals hysteresis loops, but the temperature dependence of the coercivity is not what is expected of a single-molecule magnet. Instead, the behavior is due to single-chain magnetism, albeit with weak antiferromagnetic interactions between the chains, with the barrier to relaxation arising from a combination of molecular anisotropy and ferromagnetic intermolecular exchange interactions mediated by the Se...O contacts. An Arrhenius plot was constructed from the magnetization versus time decay data. The thermally activated region at > 0.5 K gave an effective relaxation barrier (Ueff) of 14.2 K. Below approximately 0.1 K, the relaxation is independent of temperature, which is characteristic of magnetization quantum tunneling through the anisotropy barrier. These Mn7 compounds are thus the first single-chain magnets to comprise polynuclear metal clusters and also the first for which the temperature-independent relaxation characteristic of tunneling has been identified. The work also emphasizes that out-of-phase AC signals for ostensibly molecular compounds are not sufficient proof by themselves of a single-molecule magnet.

Journal Article↗
PubMed2002

Ultrafast precessional magnetization reversal by picosecond magnetic field pulse shaping.

Since the invention of the first magnetic memory disk in 1954, much effort has been put into enhancing the speed, bit density and reliability of magnetic memory devices. In the case of magnetic random access memory (MRAM) devices, fast coherent magnetization rotation by precession of the entire memory cell is desired, because reversal by domain-wall motion is much too slow. In principle, the fundamental limit of the switching speed via precession is given by half of the precession period. However, under-critically damped systems exhibit severe ringing and simulations show that, as a consequence, undesired back-switching of magnetic elements of an MRAM can easily be initiated by subsequent write pulses, threatening data integrity. We present a method to reverse the magnetization in under-critically damped systems by coherent rotation of the magnetization while avoiding any ringing. This is achieved by applying specifically shaped magnetic field pulses that match the intrinsic properties of the magnetic elements. We demonstrate, by probing all three magnetization components, that reliable precessional reversal in lithographically structured micrometre-sized elliptical permalloy elements is possible at switching times of about 200 ps, which is ten times faster than the natural damping time constant.

Journal Article↗
PubMed2004

Effect of magnetic resonance imaging on internal magnet strength in Med-El Combi 40+ cochlear implants.

OBJECTIVE: Magnetic resonance imaging (MRI) has been contraindicated when cochlear implants containing an internal magnet are in place because of concerns regarding torque, force, demagnetization, artifacts, induced voltages, and heating. The objective was to determine the magnetic field strength of Med-El Combi 40+ cochlear implant internal magnets after MRI studies. STUDY DESIGN/METHODS: Two fresh cadavers were used to study demagnetization using a repeated measures design and a magnetometer. Pre- and postMRI measurement of magnetic field strength was completed. Five sets of sagittal T1-weighted, axial T1-weighted, and axial T2-weighted sequences were performed on a cadaver at 0.2 Tesla in the device-up and device-down positions. In the other cadaver, 15 sets of sagittal T1-weighted, axial T1-weighted, and axial T2-weighted sequences were performed on a cadaver at 1.5 Tesla were conducted, 5 each with the head oriented at 80, 90, and 100 degrees rotated around the yaw plane (rotated around the z-axis). Subsequently, three cochlear implant patients completed 0.2 Tesla MRIs. For these patients, subjective and objective assessment of cochlear implant performance was performed. SETTING: Academic medical center. RESULTS: In the cadaver studies, analysis of variance showed no significant difference in the magnetic field strength after the 0.2 or 1.5 Tesla scans. There was no significant difference in the magnetic field strength for the three patients undergoing 0.2 Tesla MRIs and no adverse consequences, including no changes in telemetry, auditory sensations, nonauditory sensations, and sound quality. CONCLUSIONS: No significant demagnetization of the internal magnet occurred during repeated 1.5 Tesla MRI scans with the head orientations used in this study. In the cochlear implant patients, no significant demagnetization of the internal magnet occurred after a 0.2 Tesla MRI.

Artifacts↗