Change In Coercive Force Research Articles

Uniaxial anisotropy by "radiomagnetic" treatment; controlling factors in a new process

A new process-an electron-"radiomagnetic" treatment-for obtaining high-remanence, low-coercive-force loops in magnetic alloys was recently announced. As an example, 2-MeV electron irradiation of 6-mil-thick ring laminations of polycrystalline 5-80 Mo Permalloy with 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> e/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in an applied circumferential magnetic field of 0.2 Oe at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim100\deg</tex> C produced record highs in remanence (∼6700 G) for this material. Additional studies of this process have been made to determine some of the controlling factors and the range of application. In particular, the effects of the dose (number of e/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and of the preirradiation magnetic properties were examined. The results show that: 1) for a given dose of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.1 \times 10^{17}</tex> 2-MeV e/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the relative change in remanence ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaB_{r}/B_{r}</tex> ) is always positive, ranging from 10 to 50 percent, but varies inversely with the preirradiation value of remanence (B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> ); 2) for the same dose, the relative change in coercive force ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaH_{c}/H_{c}</tex> ) also depends upon the preirradiation value of remanence, but in a different way. For <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B_{r} &lt; 5000</tex> G, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaH_{c}/H_{c}</tex> is either negative or zero. For <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B_{r} &gt; 5000</tex> G, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaH_{c}/H_{c}</tex> is positive, ranging from 20 to 150 percent, and increases linearly with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B_{r}; 3</tex> ) if the dose is reduced to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim0.8 \times 10^{17}</tex> e/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , then <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltaH_{c}/H_{c}</tex> is reduced to a tolerable level (∼10 percent) with no significant sacrifice in the positive gain in remanence and rectangularity. Hence, there are optimum dose ranges in the "radio-magnetic" treatments of alloys, where significant gains in remanence may be obtained without appreciable increases in coercive force.

The use of coercive forces in the investigation of the process of pressing ferromagnetic powders

1. It is shown that the dependence of the coercive force on density in the range 2.75–7.25 g/cm3 has a distinct maximum. 2. It is found that the character of function Hc=f(d) sharply changes during short annealing. 3. A curve is plotted to show the dependence of the change of coercive force (†Hc) on density; it shows the presence of work hardening at various stages of pressing and its monotonie increase with increasing density, 4. It is shown that the use of coercive force in the investigation of the process of pressing can offer certain advantages.

Effect of Cobalt Content on the Properties of Elongated Iron Particles

The dependence of the magnetic properties of 200-Å-diam elongated particles of iron on cobalt alloy content is reported over the range from 0% to 90% cobalt. In particular, the compositional dependence of coercive force, corrected to zero packing density, is examined and compared to that expected theoretically for shape anisotropy. To a first approximation theory and experiment agree. Differences between theory and experiment in the compositional behavior of coercive force as well as temperature coefficient of coercive force and coercive force change with packing are attributed to a small crystal anisotropy contribution. However, the crystal anisotropies required do not agree with bulk measurements. Magnet energy was constant and at a maximum from 20 to 70% cobalt rather than only at 38% cobalt as predicted from considering shape anisotropy energy alone.

Influence of Particle Size on the Coercive Force of Barium Ferrite Powders

The influence of milling on the intrinsic coercive force (Hci) of different ferrite powders has been studied recently by several investigators. These have ascribed changes of coercive force to dislocations which are introduced through milling and are removed through subsequent annealing. In our own studies of barium ferrite powders we relate the changes of Hci to changes in particle size. This interpretation is supported by experimental data and electron micrographs. The large particles of barium ferrite material (calcined at 1300°C) change during ball milling from predominantly multidomain size (several microns) to nearly superparamagnetic size (0.02–0.5 microns). Accordingly, coercive force increases at first and then decreases as milling is extended. Annealing of ball milled material between 800° and 1100°C results in slight sintering. Since the sintering reduces the number of nearly superparamagnetic particles, considerable increases in coercive force occur. These interpretations are further supported by the temperature dependence of the coercive force. After short milling, when multidomain particles are still predominant, Hci decreases as temperature of powder is lowered. After extended milling, however, this effect changes sign which is attributed to thermal fluctuations of the magnetization in nearly superparamagnetic particles.

Irradiation of Magnetic Materials with 1.5- and 4-MeV Protons

Pure iron and 5% molybdenum Permalloy samples, 0.01 in. thick, were irradiated with 1.5- and 4-MeV protons at total incident integrated fluxes of 1016 and 1017 protons per cm2. The actual shapes used were a ring 1 31 6−in. o.d.×⅝-in. i.d. and a disk of ⅝-in. diameter. The ring was utilized for dc magnetic property measurements (before and after irradiation) while the disk was used for magnetic anisotropy torque measurements. The remanence (Br) and maximum permeability (μm) of molybdenum Permalloy decreased by about 35%. This is of the same order of change produced by neutron irradiation of 1017 neutrons/cm2 in this material. However, measurements to date indicate that changes in coercive force and initial permeability due to proton irradiation are relatively small as compared with neutron irradiation effects. The decreases in Br and μm of the pure iron samples were of the order of 10 to 20% for irradiations of both ∼1016 and ∼1017 protons/cm2. Both materials were essentially isotropic initially. Post irradiation torque measurements showed that the materials became slightly more isotropic. No changes were observed in saturation magnetization. Heating of control samples to 250°C did not produce changes in magnetic properties, showing that temperature alone is not a factor in producing the permanent changes which occurred under irradiation. All irradiations were performed at temperatures below 250°C. The changes due to proton irradiation are thought to be due to proton-induced disordering.

The Magnetic Properties of Evaporated Nickel and Iron Films

Coercive force, retentivity, and hysteresis loop forms for evaporated films of Ni and Fe.---Films of nickel and iron, produced by evaporation at low pressure, were studied with the object of verifying the reported existence of an abrupt change of coercive force at a critical thickness, and also to discover the cause of certain peculiar hysteresis loop forms. Critical thickness was found for iron films at approximately 50 m\ensuremath{\mu}, in satisfactory agreement with Sorensen. At this thickness the coercive force changes abruptly from the high value of approximately 100 for thinner films. The observations were made upon films deposited on a base of aluminum foil,.0025 cm in thickness. The foil was heated previous to and during deposit. No such critical thickness was observed in nickel. Peculiar hysteresis loops were found. Nickel films unheated have very narrow loops, with magnetic induction nearly proportional to the field up to 139 gauss, where the induction is one-third to one-half that of the metal in bulk. Films heated previous to and during deposit give a magnetic induction at 139 gauss that is approximately three times as great, or like that of metal in bulk. But the retentivity and coercive force increase five-fold and become less like those of the metal in bulk. In similarly heated films of iron, these last two magnetic properties are also much greater than with metal in bulk. The peculiarities of the films are probably caused by the nature of the crystalline state. To what extent the phenomena depend upon the presence of the aluminum base is yet to be ascertained. The present view is that the presence of gas alters the crystal growth. On the whole the experiments seem to emphasize the importance of the influence of the crystalline state upon magnetic properties.