Dilatometry has been used to confirm that the reversion of deformation-induced martensite in Type 304 stainless steel occurs principally sin the temperature range 425 to 650°C, and to show that the reversion is a thermal or not diffusion controlled. An attempt to correlate the change in length of cold-drawn 304 wire samples on heating to 800°C with the martensite content determined from the measured saturation magnetization was only qualitatively successful. The heat of reversion of martensite in cold-drawn wire samples was found by differential scanning calorimetry to be in the range 1800 to 2200 J/mol in cold-drawn 304 wires, in reasonable agreement with values for the heat of formation of martensite in 4340 steel determined by a completely different method. We also observe an exothermic reaction near 650°C of a few hundred J/mol which we ascribe to the recrystallization of cold-worked austenite.

Using a vibrating-sample magnetometer with a maximum field of 20.5 kOe, we have measured over 50 samples of annealed 304 stainless steel, which is usually considered to be non-magnetic. In almost every case, we observe the presence of a small, usually less than 0.01, fraction of a ferromagnetic phase, which we believe to be equilibrium bcc delta ferrite. The consequences of this observation for the measurement and specification of the magnetic properties of annealed 304 stainless are discussed. Our measurements also establish the most likely value for the magnetic permeability of the fcc austenitic phase in 304 stainless steel austenite as 1.0033 +/- 0.0003. (C) 2018 Elsevier B.V. All rights reserved.

From measurements of the magnetic field produced in the air gap of an electromagnet by a small permanent magnet sample, we can predict the relative sensitivity of a vibrating-sample magnetometer (VSM) for any practical location of the pickup coils. By repeating the measurements with the pole pieces removed, we can simulate the case of complete magnetic saturation (permeability μ = 1) of the pole pieces, and thus predict the relative magnitude of the image effect for any coil location. Under some conditions, a coil position can be chosen that eliminates the image effect.

Closed-circuit magnetic measurements using a hysteresisgraph have generally been considered free from errors associated with the demagnetizing factor, as well as from the image effect, both of which can occur in open-circuit measurements. However, measurements on magnetic samples clamped between the pole pieces of an electromagnet may show an apparent drop in magnetization with increasing applied field, similar to the image effect found in open-circuit measurements. We have shown that as the saturation magnetization of the sample increases, the drop in apparent magnetization becomes greater and appears at lower fields; the effect also increases as the length-to-diameter ratio L/D of the sample decreases. This behavior has been attributed to distortion of the magnetic field distribution around the sample resulting from localized saturation of the electromagnet pole pieces. This paper presents the results of computer modeling using finite element method, FEM, which confirm this explanation and show that the field acting on the sample is highly non-uniform when the sample L/D is small. The modeling results are in good agreement with experimental data, and show that the apparent drop in magnetization is caused by measurement of the applied field that does not accurately reflect the field acting on the sample.

To find out whether experimental magnetometric demagnetizing factors for disk samples of permalloy are valid for other magnetic materials with significant values of coercive field, we measured demagnetizing factors for disk samples of 1095 (high carbon) steel shim stock (Hc≅45 Oe), and for cold-rolled pure nickel (Hc≅23 Oe). The observed demagnetizing factors for steel agree well with those for permalloy but the values for cold-rolled nickel do not, at least for disk samples with diameter-to-thickness ratios d/t greater than 100. The criterion for agreement appears to be a hysteresis loop (M versus H) with straight, parallel sides up to at least half the saturation magnetization, implying a magnetization process controlled by the magnitude of the demagnetizing field.

This chapter contains sections titled: Introduction Domain Wall Structure Domain Wall Observation Magnetostatic Energy and Domain Structure Single-Domain Particles Micromagnetics Domain Wall Motion Hindrances to Wall Motion (Inclusions) Residual Stress Hindrances to Wall Motion (Microstress) Hindrances to Wall Motion (General) Magnetization by Rotation Magnetization in Low Fields Magnetization in High Fields Shapes of Hysteresis Loops Effect of Plastic Deformation (Cold Work) Problems

This chapter contains sections titled: Introduction Magnetic Moments of Electrons Magnetic Moments of Atoms Theory of Diamagnetism Diamagnetic Substances Classical Theory of Paramagnetism Quantum Theory of Paramagnetism Paramagnetic Substances Problems

Magnetic measurements made under closed-circuit conditions are generally considered to be free from errors associated with the image effect. However, magnetic measurements on permanent magnet sample cylinders clamped between the pole pieces of an electromagnet show an apparent drop in magnetization with increasing applied field that is similar to the image effect found in open-circuit measurements. The closed-circuit effect has been measured for a range of sample geometries and materials, and occurs in both hard and soft magnetic materials. The drop in apparent magnetization is greater, and appears at lower fields, as the saturation magnetization of the sample increases. The effect increases as the length-to-diameter ratio L/D of the sample decreases, and becomes minimal for L/D > 1.8 . In addition to the apparent decrease in measured magnetization, there is a decrease in the measured magnetic field. We attribute the effect to a non-uniform magnetization of the electromagnet pole pieces, where local saturation distorts the magnetic flux distribution around the sample.

Introduction to Magnetic Materials, 2nd Edition covers the basics of magnetic quantities, magnetic devices, and materials used in practice. While retaining much of the original, this revision now covers SQUID and alternating gradient magnetometers, magnetic force microscope, Kerr effect, amorphous alloys, rare-earth magnets, SI Units alongside cgs units, and other up-to-date topics. In addition, the authors have added an entirely new chapter on information materials. The text presents materials at the practical rather than theoretical level, allowing for a physical, quantitative, measurement-based understanding of magnetism among readers, be they professional engineers or graduate-level students.

This chapter contains sections titled: Introduction Single-Domain vs Multi-Domain Behavior Coercivity of Fine Particles Magnetization Reversal by Spin Rotation Magnetization Reversal by Wall Motion Superparamagnetism in Fine Particles Superparamagnetism in Alloys Exchange Anisotropy Preparation and Structure of Thin Films Induced Anisotropy in Films Domain Walls in Films Domains in Films Problems