Design of materials with small magnetic hysteresis: the unexpected role of magnetostriction.

Numerous studies in the 1950s-1970s based on linear stability analysis of the single domainstate on the shoulder of the hysteresis loop show that this method fails to predict the width of the hysteresis loop, a conclusion referred to as the ’coercivity paradox.’ We argue that the basic idea of using micromagnetics to predict coercivity is reasonable, but the fault lies with presumptions; one needs to account for large but localized disturbances arising from potent defects. To investigate the prediction of coercivity in this context, we develop an implementation of micromagnetics using the magnetization and vector potential as basic unknowns. We have developed a strategy with minimal assumptions for a soft magnetic material, like FeNi, to predict the coercivity and the evolution of magnetic domain patterns under the varying applied field at nano- and macroscales. Various material parameters control the value of coercivity such as the anisotropy constant, magnetostrictive constants, and elastic constants. Introducing a largely localized disturbance through defects, a nucleation site by lowering the energy barrier for the reversal of the magnetization is provided. We have demonstrated that variations in the size of localized disturbances significantly influ-ence coercivity in the presence of magnetostriction, primarily because larger defects introduce stronger localized stress fields that provide more hindrance to domain-wall motion, and thus increase the coercivity. Additionally, we showed that the inclusion of magnetostriction (i.e. magnetoelastic energy) alters the nucleation barrier, compelling the magnetization to follow alternative reversal paths that comply with strain compatibility conditions and this results in reduced coercivity. These alternative magnetization paths manifest as distinct magnetic domain patterns, which emerge as local energy minimizers under varying applied fields. Notably, even small values of magnetostriction have a significant effect on coercivity, and our predictions offer a pathway for designing soft magnetic alloys. Most materials used in everyday applications exist on a macroscale. However, predictinghysteresis on this scale presents significant challenges. Capturing the necessary physical phenomena requires simulations with approximately 10²¹ nodes, often leading to computation times that span days or even months for a single run. To overcome this, we developed a novel strategy that uses nanoscale simulations to predict hysteresis at the macroscale. Our approach incorporates ellipsoidal geometries and strategic modifications to the micromagnetic energy formulation. Using this method, we successfully predicted coercivity values that are consistent with the experimental observations. Magnetostriction plays a critical role in determining the width of the hysteresis loop inmagnetic alloys. In our prior simulation work, we relied on magnetoelastic energy expressions derived from the existing literature. Although these studies provided valuable insights into the fundamentals of magnetoelastic energy, we identified several issues in their formulations and assumptions. These disagreements motivated us to pursue a new approach for deriving the magnetoelastic energy. Specifically, we used reference configuration at Curie temperature, and then employed a change of reference configuration to room temperature in the presence of a strong magnetic field to evaluate the magnetoelastic energy.

Author dc:creator

• Singh, Anjanroop

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/11299/279781

OAI identifier oai:identifier

oai:conservancy.umn.edu:11299/279781