Showing papers in "IEEE Transactions on Magnetics in 1981"
TL;DR: In this article, the results of work on the preparation of aqueous magnetic liquids without using organic stabilizing agents are presented, and the results are compared to those of the same authors.
Abstract: The results of work on the preparation of aqueous magnetic liquids without using organic stabilizing agents is presented.
2,527 citations
IBM1
TL;DR: In this paper, a planar, ungrounded, inductive loop of the dc SQUID acts as the wide single turn primary of a thin-film transformer, and two tunnel junctions are located at the outside edge of the SQUID loop.
Abstract: We have devised and tested a planar coupling scheme in which the technology used to produce ultra low noise tunnel junction dc SQUIDs is employed to achieve tight coupling between the SQUID loop and and the input coil. In our scheme the planar, ungroundplaned, inductive loop of the dc SQUID acts as the wide single turn primary of a thin-film transformer. The input coil consists of a multiple-turn secondary in the form of a spiral stripline fabricated directly above the primary. The two tunnel junctions are located at the outside edge of the SQUID loop. A low inductance stripline structure connects the junctions to the region of high current flow in the SQUID loop. We have evaluated this coupling scheme experimentally in SQUIDs with 10-turn, 19-turn, 50-turn, and 100-turn input coils. All have mutual inductances per turn of approximately 80 pH, in good agreement with numerical calculations. Detailed measurements on the 50-turn SQUID gave a mutual inductance M of 3.8 nH, an input coil inductance L i of 190 nH, a SQUID self inductance of L of 89 pH, and a coupling constant k2= M2/L i L of 0.86. The 100- turn version is estimated to have similar coupling performance with an input coil inductance of approximately 0.8 μH.
303 citations
TL;DR: In this paper, the coupling energy between elementary dipoles enables the calculation of forces and stiffnesses between small permanent magnets, and the results are used to synthesize all the different basic configurations of magnetic bearings and magnetic couplings.
Abstract: The coupling energy between elementary dipoles enables the calculation of forces and stiffnesses between small permanent magnets. The results are used to synthesize all the different basic configurations of magnetic bearings and magnetic couplings. This method enables us to predict some of their specific properties and the approximate calculation of their characteristics. It can be extended to all devices where forces are created between permanent magnets.
215 citations
TL;DR: In this paper, the authors studied the effect of the time scale of magnetization changes on the coercivity of a magnetic recording material and found that the difference between the short and long-term coercivities becomes more pronounced as the size of the magnetic switching unit becomes smaller.
Abstract: The coercivity of a magnetic material is a crucial parameter in determining its use in recording. The apparent value of this quantity is found to depend upon the time scale of interest. A method of measurement that observes changes of magnetization on a short time scale will yield a larger coercivity than one that measures changes over a longer time scale. The cause of this phenomenon is thermally assisted reversal of magnetic domains. Thus, for a magnetic recording material, the coercivity relevant to a high-speed writing process is greater than that relevant to transition broadening and long-term storage stability. The difference between the short-and long-term coercivities becomes more pronounced as the size of the magnetic switching unit becomes smaller. Therefore, these kinetic effects will become increasingly important in determining the ultimate limitations of future high-density media. Data are presented that illustrate the variation of coercivity with time scale in presently used and experimental recording materials.
203 citations
TL;DR: In this paper, the magnetomechanical coupling factor, relative permeability, and magnetization as a function of annealing temperature and applied field were measured in transversely annealed samples of Metglas 2605 CO (Fe 67 CO 18 B 14 Si 1 ) and Metglas2605 SC (Fe 81 B 13.5 Si 3.5 C 2 ).
Abstract: We have measured the magnetomechanical coupling factor, relative permeability, and magnetization as a function of annealing temperature and applied field in transversely annealed samples of Metglas 2605 CO (Fe 67 CO 18 B 14 Si 1 ) and Metglas 2605 SC (Fe 81 B 13.5 Si 3.5 C 2 ). In 2605 CO the maximum coupling factor of 0.71 was obtained for ribbons annealed between 360 and 375°C in fields greater than 1 k Oe. Plots of the impedance and admittance circles indicated a very low loss material. In samples with a coupling factor of 0.71, the Q value from the impedance circle was 35, while the Q value from the admittance circle was 120. In the 2605 SC alloy considerably higher coupling factors were calculated (0.9 to 0.96) due to the much lower anisotropy. The magnetization of the samples are mainly rotational in nature.
151 citations
TL;DR: In this article, a rational function approximation for magnetization curves and hysteresis loops is presented, which leads to Rayleigh relations and the law of approach to saturation as limit forms.
Abstract: A rational function approximation for magnetization curves and hysteresis loops is presented. This approximation leads to Rayleigh relations and the law of approach to saturation as limit forms. Furthermore, the magnetic permeability and the magnetic energy density is computed. Finally, this analysis is applied to commercial ferrite cores and pure ceramic samples.
120 citations
TL;DR: In biology, the study of geomagnetic orientation has gained new momentum since the discovery of magnetic field detectors in aquatic organisms and the ability of sharks and rays to orient to the earth's magnetic field has been demonstrated in behavioral experiments as discussed by the authors.
Abstract: In biology, the study of geomagnetic orientation has gained new momentum since the discovery of magnetic field detectors in aquatic organisms. Sharks and rays respond to dc and low frequency voltage gradients of 0.005 μV/cm. By moving through the earth's magnetic field, they induce electric fields well within the sensitivity range of their keen electric sense. As these fields depend on the direction in which the animal is heading, the induced voltage gradients may serve as the biophysical basis of an electromagnetic compass sense. The ability of sharks and rays to orient to the earth's magnetic field has been demonstrated in behavioral experiments. Also, various marine and freshwater mud bacteria are endowed with permanent magnetic dipole moments, directed parallel to the axis of motility. When separated from the sediments, these bacteria return to the mud by migrating downward along the earth's inclined magnetic field lines. Their orientation is largely determined by the principles of statistical mechanics and may be expressed in terms of the directive magnetic force, the randomizing effect of thermal agitation, and the cells' flagellar thrust. Observations on live bacteria yield individual dipole moments circa 15 × kT/G.
113 citations
TL;DR: In this paper, the variational formulations of magnetostatic scalar and vector potentials are reviewed and an original energy functional for non linear anisotropic vector potential with a proof of uniqueness of solution is proposed.
Abstract: 3D modeling becomes necessary for analysis of special electromagnetic systems and also for testing the consistency of two dimensional models In this paper, the variational formulations of magnetostatic scalar and vector potentials are reviewed An original energy functional for non linear anisotropic vector potential with a proof of uniqueness of solution is proposed The biggest problems remain data input and results reduction; these two problems are analysed and solutions are given with some illustrated examples
95 citations
IBM1
TL;DR: In this paper, the specific capacitance of tunnel junctions was measured by observing resonances in lightly-damped 2-junction interferometers, using the resonance voltage, obtained by analyzing steps in the I-V characteristics, and the interferometer inductance, which was measured directly by injection of a control current.
Abstract: The specific capacitance of several types of Josephson tunnel junctions has been measured by observing resonances in lightly-damped 2-junction interferometers. The capacitance was calculated using the resonance voltage, obtained by analyzing steps in the I-V characteristics, and the interferometer inductance, which was measured directly by the injection of a control current. Using this technique, the specific capacitance C s was determined for tunnel junctions with Pb-In-Au alloy base electrodes and Pb-Bi counterelectrodes, as well as for junctions made on Nb films with Pb-In-Au counterelectrodes. In both cases, barriers were produced by rf plasma oxidation. Junctions with Josephson current densities j 1 between 200 and 5000 A/cm2were investigated. It was found that 1/C s decreased with \log j_{1} , and that at j 1 = 1000 A/cm2, C s was 4.2 ±0.3 μF/cm2for the Pb-alloy junctions and 13.4 ±1 μF/cm2for the Nb junctions. These results are discussed in relation to available data on oxide thickness and dielectric constant.
95 citations
IBM1
TL;DR: In this article, the intrinsic energy sensitivity of dc SQUID was shown to scale as (I 0 R)2 for tunnel junctions, where I o is the critical current per junction and R is the shunt resistance per junction.
Abstract: The four primary areas of concern in the development of high performance dc SQUIDs are white noise, 1/f noise, readout schemes, and coupling. For an optimized dc SQUID the intrinsic energy sensitivity in the white noise region is given by e w =γ 1 hk B T(eI 0 R)-1+γ 2 h, where I o is the critical current per junction and R is the shunt resistance per junction. γ 1 , which multiplies the thermal noise contribution, and γ 2 , which multiplies the shot noise/zero point fluctuation contribution, are numerical factors of order unity. Values of e w approaching h have recently been measured for several members of a new generation of low noise dc SQUIDs. The intrinsic energy sensitivity in the 1/f noise region, e f , is predicted to scale as (I 0 R)2for tunnel junctions. This may impose significant low frequency limitations on SQUIDs with very low values of e w . Readout schemes for high sensitivity dc SQUIDs will require further development. At the moment primarily small signal amplifier readout schemes are being used to evaluate the new generation of low noise SQUIDs. Planar thin-film coupling schemes are about to have a big impact on dc SQUID design. Such schemes can achieve tight coupling between SQUID and input coil. and can be implemented using the same fabrication techniques that produce SQUIDs with low values of e w .
86 citations
TL;DR: In this article, an entirely new formulation of the classical steady-state skin effect problem is presented, where the magnetic vector potential is obtained directly from the projective solution of a single integrodifferential equation.
Abstract: An entirely new formulation of the classical steady-state skin effect problem is presented. The magnetic vector potential is obtained directly from the projective solution of a single integrodifferential equation. In this new form of the steady-state diffusion equation, the known measurable total current in conductors replaces the usual unknown source current density vector. The validity and correctness of the new formulation are demonstrated by a simple, easily verifiable example based on finite element discretization.
TL;DR: Based on the dependence of critical current, Nb 3 Sn area and grain size on the heat treatment conditions, a two-stage reaction process is proposed to improve the critical current carrying capacity in magnetic fields greater than 10 Tesla as mentioned in this paper.
Abstract: The grain structure of a commercial multifilamentary Nb 3 Sn wire has been investigated as a function of reaction temperature, T R , and time, t R . Results allow a quantitative description of grain growth. Based on the dependence of critical current, Nb 3 Sn area and grain size on the heat treatment conditions a two-stage reaction process is proposed to improve the critical current carrying capacity in magnetic fields greater than 10 Tesla. The first reaction converts the major part of the Nb to fine grained Nb 3 Sn at low T R . Subsequent high temperature ( T_{R}\simeq 800°C) heat treatment for a short time ( t_{R}\leq1 h) increases both T c and B c2 due to composition approaching stoichiometry (or a higher degree of order) without detrimental grain growth. First experimental results show a remarkable improvement in the high field critical current for this two stage-reaction process.
TL;DR: Barium ferrite thin films with perpendicular magnetic anisotropy have been prepared by DC diode sputtering as discussed by the authors, which can be used as a perpendicular magnetic recording medium.
Abstract: Barium ferrite thin films with perpendicular magnetic anisotropy have been prepared by DC diode sputtering. When a sintered target with appropriate composition was used, c-axis oriented Ba ferrite film was obtained whose easy direction of magnetization was normal to the film plane. Its saturation magnetization Ms, coercive force Hc\perp and magnetic anisotropy constant Ku were 400 emu/cc, 680 Oe and 1.67 × 106erg/cc, respectively. Ba ferrite films may be usable as are Co-Cr films as a perpendicular magnetic recording medium.
TL;DR: In this paper, the formation of the magnetic clusters is a spontaneous process resulting from intercrystalline interactions and local inhomogeneities in the anisotropy of a thin Co-Re film.
Abstract: The micromagnetic structures of the high-coercivity, isotropic, and high-squareness thin films of sputtered Co-Re have been investigated using transmission electron microscope (TEM) Lorentz imaging and electron deflection methods. From the behavior of the magnetic ripple structure under applied field and the configuration of the local surface fields observed in these experiments, the existence of magnetic clusters in these films was verified. Based on the interpretation of the field dependence of the ripple formation and the hysteretic properties of the film, it is concluded that the formation of the magnetic clusters is a spontaneous process resulting from intercrystalline interactions and local inhomogeneities in the anisotropy. The effects of such cluster formation on longitudinal magnetic recording were investigated. The results show that the reduction of dipole energy at the transition region between two oppositely magnetized regions can be achieved by a stepwise rotation of the magnetization vector of an individual cluster in the form of a vortex. This type of rotation creates a finite transition length which is limited by the size of the magnetic cluster of the film. Consequently, it is concluded that the maximum packing density for saturation recording in these types of films would be less than that predicted by the phenomenological equation, which was derived based solely on considerations of the demagnetization field and the coercivity of the film.
TL;DR: In this article, a method for the calculation of the electromagnetic torque in electric machines is proposed based on an analytical field calculation in the air-gap combined with finite element field calculations in the saturated rotor and stator iron.
Abstract: In many engineering applications the calculation of forces and torques is subject of great importance [7] , [8]. A method for the calculation of the electromagnetic torque in electric machines is proposed in the paper. This method is based on an analytical field calculation in the air-gap combined with finite element field calculations in the saturated rotor and stator iron. The results of field calculation obtained using such a method are more precise than those calculated by the classical finite element method; thus the accuracy of the calculated torque will be better. An application for the case of synchronous machine is given at the end of the paper.
IBM1
TL;DR: In this article, a variational numerical method is described to calculate the inductances of a multi-superconductor transmission line system, in which the currents in the superconductors are distributed in a way that the sum of the total magnetostatic energy and the total kinetic energy of the system is a minimum.
Abstract: A variational numerical method is described in this paper to calculate the inductances of a multi-superconductor transmission line system. We show that the currents in the superconductors are distributed in a way that the sum of the total magnetostatic energy and the total kinetic energy of the system is a minimum. Using this principle, a variational technique is formulated to calculate the current distribution in the conductors and to calculate the inductances of the conductor system, in particular, we subdivide the conductor into small rectangular sub-conductors and assume a uniform trial current in each of the sub-conductor. The total energy of the system is obtained through Green's function and is minimized by applying the Lagrange multiplier technique. Once the current distribution is obtained, the inductances can be obtained easily. The method is implemented numerically and proved to be fast and accurate.
TL;DR: In this article, the applicability of vacuum deposition to the preparation of Co-Cr perpendicular anisotropy films at a deposition rate up to 6000 A/sec was confirmed.
Abstract: Magnetic and structural properties, crystal orientation, resistance to atmospheric corrosion, and depth profile of vacuum deposited Co-Cr perpendicular anisotropy films are presented. In contrast to the RF sputtered Co-Cr films having a rather small deposition rate, the present study confirms the applicability of vacuum deposition to the preparation of Co-Cr perpendicular anisotropy films at a deposition rate up to 6000 A/sec. It was found that films with relatively high magnetization and coercive force could be made which at the same time exhibited good corrosion resistance.
TL;DR: Several kinds of Fe-base amorphous alloys with transition metals were prepared by means of a co-sputtering technique as discussed by the authors, and the reduced magnetization curves of the present alloys are flatter than those of crystalline Ni and Fe.
Abstract: Several kinds of Fe-base amorphous alloys with transition metals were prepared by means of a co-sputtering technique. The reduction of the magnetic moment of Fe-Ta and Fe-Nb amorphous alloys is much more pronounced than that of Fe-Hf and Fe-Zr amorphous alloys. Fe-Mo, Fe-V and Fe-W amorphous alloys are paramagnetic. The concentration dependence of the Curie temperature also exhibits a similar tendency. Such behavior can be explained by a d-band filling model. The magnetic-carrier ratio is larger than unity, indicating itinerant magnetic behavior. The reduced magnetization curves of the present alloys are flatter than those of crystalline Ni and Fe, and Co-base amorphous alloys.
TL;DR: The methods of information theory are applied to run-length limited codes with charge constraints, useful in several areas of information storage, including magnetic recording, where it may be desirable to eliminate the dc component of the frequency spectrum.
Abstract: The methods of information theory are applied to run-length limited codes with charge constraints. These self-clocking codes are useful in several areas of information storage, including magnetic recording, where it may be desirable to eliminate the dc component of the frequency spectrum. The channel capacity of run-length limited codes, with and without charge constraints, is derived and tabulated. The channel capacity specifies the maximum ratio of data/message bits achievable in implementing these codes and gives insight into the choice of codes for a particular task. The well-known frequency modulation (FM) code provides a simple example of these techniques.
TL;DR: In this article, the influence of dislocation structures on the motion of magnetic domain walls was investigated in high-purity polycrystalline iron by measuring the magnetomechanical damping and the magnetic properties (coercive field and parameters of the Rayleigh law).
Abstract: The influence of dislocation structures on the motion of magnetic domain walls (DW) was investigated in high-purity polycrystalline iron by measuring the magnetomechanical damping and the magnetic properties (coercive field and parameters of the Rayleigh law). The dislocation structures created by strain-hardening were observed by high-voltage (800 kV), transmission electron microscopy (TEM). As the flow stress increases from 25 to 128 N.mm-2, the four quantities under study exhibit three successive stages related to different dislocation distributions. They reflect different weighted averages of the DW interactions with isolated dislocations (stage I), dislocation tangles (stage II), and dislocation cells (stage III).
TL;DR: In this paper, it was shown that the principal cause of the excess eddy current loss in amorphous ribbons is the non-sinusoidal non-uniform and non-repetitive domain wall motion.
Abstract: The total power loss under alternating flux at power frequencies and zero applied stress conditions is an important criterion for the manufacturer and user of soft magnetic materials. The total power loss may be divided into three major types, namely the static hysteresis loss, classical eddy current loss and excess eddy current loss. It has been shown that the excess eddy current loss is responsible for approximately 90% and 50% of the total power loss of wide amorphous ribbon and grain-oriented silicon-iron respectively. Therefore it is possible to make comparisons of the causes and magnitude of the excess eddy current loss in these materials. The origin of the excess eddy current loss in grain-oriented 3% silicon-iron has been attributed to many causes. The feasibility of each of the suggestions as a possible cause of the excess eddy current loss of amorphous ribbon material is considered. It is concluded that the principal cause of the excess eddy current loss in amorphous ribbons is the non-sinusoidal non-uniform and non-repetitive domain wall motion which is the source of the observed non-sinusoidal and localised variations of the flux density.
TL;DR: In this article, the effects of variations of Nb and Al powder size, composition, reduction ratio, reaction temperature, and reaction time on J c vs appiied field are presented.
Abstract: Fabrication of multifilamentary Nb-Al superconducting wires with high overall critical current densities at high fields is discussed. Powder metallurgy processed materials have been made with low reaction temperatures and with J c > 104A/cm2at 14 T. The effects of variations of Nb and Al powder size, composition, reduction ratio, reaction temperature, and reaction time on J c vs appiied field are presented. All the Nb-Al materials show good strain tolerance at high J c and high field. The results also show that scaleup is promising.
TL;DR: In this article, the magnetic properties of the implanted layer in thin garnet films can be obtained from ferromagnetic resonance experiments using parallel spectra alone, and an approximate profile of implantation induced anisotropy is obtained using perpendicular resonance alone.
Abstract: Magnetic properties of the implanted layer in thin garnet films can be obtained from ferromagnetic resonance experiments. Approximate profiles of implantation induced anisotropy can be obtained using perpendicular resonance alone. For maximum information and accuracy, both perpendicular and parallel resonance spectra are needed, and measurements should be made on a number of samples from which varying amounts of the implanted layer have been removed by ion milling. For narrow linewidth materials it is possible to deduce profiles of H k , 4πM, and A and to determine the value of g in the implanted layer. Methods are presented for the accurate calculation of parallel ferromagnetic resonance (FMR) spectra with depth varying magnetic parameters. This method of analysis has been successfully applied to a yttrium iron garnet (YIG) film substituted with Gd, Tm, Ga, and implanted with He ions at 140 keV with a density of 3 × 1015cm-2.
TL;DR: In this paper, the authors report on reactively sputtered NbN films up to 5 μm thick and 2.2 cm wide which have ρ n > 200 μΩ cm and a self-field J c up to 106A/cm2.
Abstract: Power switches such as a Superconducting Fault Current Limiter require large cross sectional area superconductors with both high critical current density J c and normal state resistivity ρ n . Large values of J c and ρ n have been previously reported in small cross sectional area "weak links" of NbN. We report on reactively sputtered NbN films up to 5 μm thick and 2.2 cm wide which have ρ n > 200 μΩ cm and a self-field J c up to 106A/cm2. Severe degradation in J c was observed with increasing film width and for millisecond current pulses. This degradation could be substantially reduced by stabilization with either low ρ n normal metal or the use of a sapphire substrate. The resistivity and critical current dependence both imply Josephson coupled grains and the results will be discussed within that model.
TL;DR: In this paper, the authors present a review of developments in magnetometry since the symposium of the 12th Annual Conference on Magnetism and Magnetic Materials in 1966, and discuss the basic principles and selected examples of some of these methods.
Abstract: This article reviews developments in magnetometry since the symposium of the 12th Annual Conference on Magnetism and Magnetic Materials in 1966. No new principles have been discovered since then. However, many variations of force, magnetic induction and indirect techniques have been published which satisfy a variety of different applications. The basic principles and selected examples of some of these are discussed. The advantages and limitations of each method are outlined. The choice of a particular technique often depends on the constraints imposed by the specimen environment. Measurements by ac and dc force and magnetic induction, SQUID detectors, moving sample, rotating sample and field, and some indirect techniques are examined.
TL;DR: In this paper, the coupling losses between strands in cables of LCT conductor type in changing transverse magnetic fields were calculated for two situations envisaged in a. c. loss measurements: a) magnetic field applied on finite length of an infinite cable (infinite as well as finite short circuited cables were considered); b) finite length cable in spatially independent magnetic field.
Abstract: The coupling losses between strands in cables of LCT conductor type in changing transverse magnetic fields were calculated for two situations envisaged in a. c. loss measurements: a) magnetic field applied on finite length of an infinite cable (infinite as well as finite short circuited cables were considered); b) finite length of cable in spatially independent magnetic field. It is found that in the first case the losses per length are enhanced considerably above loss generation e. g. in a magnet. If one cabling length l o is exposed to the field, the losses are enhanced by a factor about 2. This factor decreases only very slowly towards 1, if the field region increases. The results are explained by considering short circuited cables of different length with one cabling length in applied magnetic field. On the other side, the coupling losses of a finite piece of cable with length l \geq l_{o} are close to those in an infinite cable in magnetlc fleld, being identical with them for l = nl o (n-integer). The results for both cases mentioned above are discussed in the view of different loss measurement arrengements for cables.
TL;DR: In this paper, the formation of the Nb 3 Al A15 phase under equilibrium conditions was studied on bulk, homogenized samples, and the T c of samples annealed during long periods at temperatures below 900°C was found to be close to 12 K.
Abstract: The formation of the Nb 3 Al A15 phase under equilibrium conditions is studied on bulk, homogenized samples. Due to compositional effects, T c of samples annealed during long periods at temperatures below 900°C is found to be close to 12 K. It follows that the high T c values (15-16 K) observed on multifilamentary Nb 3 Al wires reacted at these temperatures cannot be interpreted on the basis of the equilibrium phase diagram.
TL;DR: In this article, a review of the work on the measurement and interpretation of the properties of individual particles, of the type incorporated into recording tape, is presented, where it is shown that the remanent coercive force H r (O) of particles of any of the materials measured covered a range of several hundred Oersted.
Abstract: A review is made of the work on the measurement and interpretation of the properties of individual particles, of the type incorporated into recording tape. It was shown that the remanent coercive force H r (O) of particles of any of the materials measured covered a range of several hundred Oersted. A high proportion of the particies were multiple, that is they comprised several particles lying side by side: it is known that most of the particles in recording tape are of this type. An investigation was made of the nature of the reversal process, and of the origin of the range in H r (O), for particles of γ-Fe 2 O 3 , CrO 2 and metal powder (iron). The variation of H r (0) with angle between the field and particle axis was determined for γ-Fe 2 O 3 and CrO 2 . The results of the above work were used in an attempt to synthesize the remanent loop of a tape from the knowledge of the properties of its constituent particles.
TL;DR: In this article, the authors used the reciprocity theorem to analyze the reproducing performance of a reproducing head by introducing thickness loss of the main pole of the head and obtaining a satisfactory coincidence between the calculated and the measured results in the wavelength response even at the densities of more than 150 kMRPI.
Abstract: From the measurements of the surface field of a recording medium, it was obtained that an almost ideal step change distribution of magnetization is formed in perpendicular recording for digital signals. The fact represents that the upper limit of usable recording density is determined only by the resolution of a reproducing head. Therefore, the reproduction by a perpendicular head was successfully analyzed by using the reciprocity theorem. By introducing thickness loss of main pole of the head, the satisfactory coincidence was obtained between the calculated and the measured results in the wavelength response even at the densities of more than 150 kMRPI. In reproducing process, the magnetic interaction between the main pole and the medium acts so as to decrease the effective spacing between them.
TL;DR: In this paper, the magnetisation of a system of fine cobalt particles at 77°K is time dependent and the time decay of magnetisation has been found to be logarithmic.
Abstract: Measurements are presented which show that the magnetisation of a system of fine cobalt particles at 77°K is time dependent. The system examined in this work is of fine cobalt particles with median diameter D v = 70 A dispersed in toluene. The particles have been found to be in the fcc phase. The time decay of magnetisation has been found to be logarithmic. The time dependence coefficient A = -d\bar{I}/dLnt varies with the applied field H and is a maximum for H = H c the coercive force. These observations are in good agreement with the theory presented in this paper.