Mechanical and Civil Engineering Seminar

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In 1914, a magnetic alloy with an unusually high magnetic permeability and low hysteresis was discovered at the Bell Telephone Laboratories. This magnetic alloy resulted from a series of investigations on the iron-nickel alloy system, in which the nickel content and manufacturing conditions, such as heat treatment, magnetic annealing and mechanical loads, were systematically varied. Under a very specific combination of the alloy composition (78.5% nickel content) and manufacturing conditions, the magnetic alloy (now known as the permalloy) demonstrates a drastic increase in the magnetic permeability and a decrease in hysteresis. In the past, this unusual behavior of the permalloy has been attributed to the anisotropy constant, however, there is still no theory that explains this drastic decrease in coercivity in magnetic alloys. Our goal is to identify a mathematical relation between the magnetic material constants that is key in reducing magnetic hysteresis. We hypothesize that a combination of a large local disturbance (material microstructure) and material constants, such as anisotropy coefficient and magnetostriction coefficients, contribute to the drastic decrease in hysteresis in magnetic alloys. We formalize this idea using the micro-magnetics theory in a phase-field framework and use energy minimization methods to investigate the links between microstructures and material constants in nickel-iron alloys. Our results demonstrate agreement with the permalloy experiments and provide theoretical insights into developing novel magnetic alloys with negligible hysteresis.