摘要:

The principal parameters characterizing the low‐temperature magnetic and thermal properties of Tb(OH)3have been inferred from measurements of optical fluorescence and absorption spectra of single crystals. Zeeman effect measurements in fields up to 36 kOe have determinedg‐values for various single ion levels; in particularg∥=17.7±0.2 for the ground state. This compares well with the value obtained from magnetization measurements and with the value predicted by a crystal field analysis based on a fit of the observed zero field energy levels. The onset of magnetic order near the Curie point (3.7°K) produces shifts and splittings of the observed spectra. At 1.3°K in zero external magnetic field, the ground state is split by 6.4±0.2 cm−1, which together with the observedg‐value implies an effective internal field of 7.8±0.2 kOe parallel to thecaxis. This value is close to other estimates based on bulk measurements and shows that the interaction is predominantly, but not exclusively due to magnetic dipole coupling. The existence of a small, additional antiferromagnetic interaction is inferred, and this is supported by the zero‐field splittings observed in other levels.

ISSN:0021-8979    

DOI:10.1063/1.1657518

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The antiferromagnetic spin flop transition in CuCl2·2H2O has been studied in the temperature region 1°–4.2°K by means of adiabatic measurements on a single crystal. The phase boundary obtained from isentropes in theH‐Tplane, which show a characteristic anomaly at the transition, conforms closely with that observed by NMR. Isentropic rotations of an external field in theabandacplanes of the crystal confirm a flopping of the magnetic moments in theabplane. When the applied field exceeds the critical spin‐flop field by more than 50%, however, it appears that spin flopping no longer occurs in this plane. The forms of the isentropic curves are in general agreement with those predicted from the molecular‐field model. Specific‐heat measurements in applied fields yielded the paramagnetic phase boundary up to 22 kOe, the sharpness of the specific heat singularity being maintained in the highest field applied. The Ne´el temperature atH=0 was 4.357±0.010°K with the triple point at 4.31°K, 8.50 kOe. Comparison of the sublattice magnetization vs temperature curve derived from the specific‐heat data with the predictions of the molecular field and Ising models suggests that neither of these models accurately represents the magnetic behavior of CuCl2·2H2O.

ISSN:0021-8979    

DOI:10.1063/1.1657519

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The low‐temperature experimental study of the antiferromagnet FeCl2in pulsed fields up to nearly 500 kOe has given evidence of a nonlinear behavior of the magnetization when the field is applied perpendicular to the anisotropy axis, the magnetization versus field increasing more steeply near the saturation than one would expect from the usual molecular field theory alone. A simple microscopic explanation of this phenomenon (which has already been observed in many antiferromagnets) can be put forward in this particular case: the essential in FeCl2lies in the existence of comparable orders of magnitude for crystal anisotropy and exchange interactions. At 0°K, by using a crystal‐field approximation (strong cubic potential and weak trigonal distortion), and a molecular‐field approximation (Heisenberg Hamiltonian with two exchange parameters |JF|≫|JAF|), we get a nonlinear magnetization law before the saturation. The calculation has been carried out thoroughly only in the case where the ferromagnetic exchange is large compared with the crystal anisotropy. The value of the exchange and crystal‐field parameters of FeCl2lead to a satisfactory quantitative agreement between experiment and theory.

ISSN:0021-8979    

DOI:10.1063/1.1657520

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Theb‐axis magnetization has been studied at various temperatures under various values of external magnetic field applied along thebaxis, which is the intermediate direction of magnetization. The transformations to the fan structure from the screw and ferromagnetic spin structures have been observed at various temperatures. From these results a spin structure diagram is made as a function of temperature and magnetic field applied along thebaxis.

ISSN:0021-8979    

DOI:10.1063/1.1657521

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Continuing a study of the magnetic rare‐earth hydroxides,1we have measured the specific heats of hydrothermally crystallized powder samples of Tb(OH)3, Dy(OH)3and Ho(OH)3between 0.6° and 18°K. Sharp anomalies associated with paramagnetic‐ferromagnetic phase transitions were found at 3.71° Tb(OH)3; 3.48° Dy(OH)3; and 2.54°K Ho(OH)3, in good agreement with the Curie temperatures deduced from magnetic measurements.1Analysis of the specific heats for Dy(OH)3and Ho(OH)3is complicated by Schottky contributions from low‐lying crystal‐field levels, and in the latter compound also by a large hyperfine contribution,Chf. However, for Tb(OH)3the ground state doublet is far below the first excited state which is at 170°K,2and the magnetic specific heatCmcould therefore be isolated by a simple analysis of the ``high'' temperature measurements to determine the lattice contribution, and a small calculated correction forChfat the lowest temperatures. The corresponding total entropy change is estimated to be (0.69±0.05)R, in good agreement with the expected valueRIn2. The highly anisotropic ground state1−3makes Tb(OH)3an unusually good approximation to an Ising ferromagnet, and this was confirmed by an analysis of the low‐temperature tail ofCm. A value for the energy of a single‐spin reversal atT=0°K is deduced: &Dgr;0=8.4±0.3°K, in reasonable agreement with the value 9.2±0.8°K estimated from the integrated specific heat and with the value 9.4±0.2°K obtained spectroscopically.2The reason for the apparent small discrepancy is not clear. Comparison of the experimental &Dgr;0with the calculated magnetic dipole contribution (&Dgr;0)dip=11.8°K shows that the ordering in Tb(OH)3is predominantly, but not exclusively, due to dipolar interactions. A similar situation was found for Dy(OH)3, where &Dgr;0=8.8±0.3°K was determined from the low‐temperature specific heat while the calculated (&Dgr;0)dip=13.3°K. For Ho(OH)3an accurate value for &Dgr;0could not be estimated, but it seems likely that the interactions are similar to those in Tb‐ and Dy(OH)3. Full details of this work are being published elsewhere.

ISSN:0021-8979    

DOI:10.1063/1.1657522

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The force that a magnetic field gradient can exert on a ferromagnetic material can be utilized in a number of medical applications. This force is proportional to the spatial gradient of the ambient magnetic field times the magnetic moment of the material; the magnetic moment is a function of the magnitude of the ambient field and the geometry and material used. This paper discusses the importance of each of these factors and discusses methods for calculating magnetic fields and gradients produced by permanent magnets, electromagnets, and superconducting magnets. Examples of an electromagnet and a superconducting magnet for medical applications are presented.

ISSN:0021-8979    

DOI:10.1063/1.1657523

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Normal function was simulated in nonfunctioning heart and bladder of dogs by implanting ferromagnetic material on or in the walls of these organs and applying a magnetic field to produce compression against the body wall. A field of 0.1 T and a 0.05‐T/cm gradient applied to three strips of C1010 steel (each 8.5 g and 5.1×2.5×0.08 cm), sutured to the base of the right ventricle of the 165‐g heart of a 17.5‐kg dog, produced normal left and right intraventricular pressures. At higher field gradients, less metal was needed. A magnetic field of 0.17 T and 0.11 T/cm applied to a steel strip of 0.22 g, or to 0.48 g of sintered iron on or in the posterior wall of the bladder of a 19‐kg dog containing 75‐cc saline produced a rise in intravesical pressure of up to 16 mm Hg, adequate for complete emptying. The forces produced by application of high magnetic fields to ferromagnetic material on or in dog heart or bladder appeared to be sufficient to restore function in man.

ISSN:0021-8979    

DOI:10.1063/1.1657524

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The rupture of intracranial aneurysms is a major cause of strokes, but present operations to repair aneurysms and to prevent their recurrent rupture are plagued by high morbidity and mortality rates. A simpler and safer operation is proposed utilizing two permanent magnet‐tipped probes introduced through a small opening in the skull and guided under x‐ray control to lie near the aneurysm. The magnets attract ferromagnetic emboli injected into the cerebral circulation through the internal carotid artery in the neck, eventually filling the aneurysm. Following the formation of a blood clot which would reinforce the aneurysm wall against rupture, the probes would be withdrawn. Simulated aneurysms were occluded with three‐micron carbonyl‐iron particles injected upstream from two 5‐mm‐diameter, bullet‐shaped Alnico V magnets applied at the aneurysmal opening. The same particles injected into the intracranial circulation of animals did not obstruct the capillaries and produced no detectable clinical, physiologic, or pathologic changes in the brains. A clinical trial using this technique was successful in occluding an aneurysm in a patient.

ISSN:0021-8979    

DOI:10.1063/1.1657525

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Measurements of the ac and dc magnetic fields produced by natural internal ion currents in living systems are useful in principle because they can reduce or bypass the limitations of surface voltage measurements. The magnetic field from the normal human heart was first detected by two groups using gradient detectors and little or no shielding. The fields arose from the same ion currents seen on the ECG, and were ∼5×10−7G at QRS maximum. Using a well shielded room and a detector consisting of a compact coil feeding a parametric amplifier, the following measurements have been made by the author: (1) mapping the heart's magnetic field around the torsos of one abnormal and six normal human subjects, verifying a previously proposed model; (2) mapping a coarse distribution of the magnetic field (∼10−9G) around the human head produced by the same alpha‐rhythm currents conventionally seen on the EEG; (3) exploring the dc field around the abdomen, which has a maximum of ∼10−6G and is of undetermined origin. Further evaluation of data and experiments are necessary to see if the theoretical benefits of surface magnetic field measurements can be conveniently realized in practice.

ISSN:0021-8979    

DOI:10.1063/1.1657527

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Measuring the magnetization of various alloys in the Ni3Al and Ni3Ga phases, we found that going to higher Ni concentrations within these phases, a transition from strongly exchange enhanced paramagnetism to weak ferromagnetism takes place at 74.5 and 76 at.% Ni, respectively. The shape of the reciprocal magnetic susceptibility vs temperature curves measured between 4.2° and 300°K is of the Stoner type and suggests for these alloys a density‐of‐states curve varying appreciably in the Fermi region leading to a low effective Fermi energy. This conclusion is confirmed by the strong curvature of the magnetization in high magnetic fields. These results and the fact that an increase of the magnetic field applied on ferro‐magnetic samples effects a large increase of the magnetization even at the lowest temperatures (4.2°K) and in the highest fields (200 kOe) led us to a description in terms of the itinerant electron model. The use of this model seems justified by the fact that in agreement with calculations by Edwards and Wohlfarth, the magnetic data of both paramagnetic and ferromagnetic compositions fit nicely on straight lines in &sgr;2vsH/&sgr; plots, this being so for all temperatures (down to 4.2°K) and all magnetic fields between 4 and 200 kOe. Combining the magnetic data and low‐temperature specific‐heat data, we obtained some information on the bandshape at the Fermi level. We cannot conclude, however, to more than a nice qualitative agreement with the Stoner model at this moment. As would be expected in highly polarizable metals and alloys, small amounts of Fe dissolved both in paramagnetic and in ferromagnetic alloys in the Ni3Al and Ni3Ga phases lead to giant moment inducement.

ISSN:0021-8979    

DOI:10.1063/1.1657528

出版商:AIP    

年代:1969

数据来源: AIP