@article{9cc7d19117c2406891f29a7f31328858,
title = "Magnetic reversal in three-dimensional exchange-spring permanent magnets",
abstract = "In this paper, we investigate the magnetization reversal in single-phase RE2 Fe14 B and two-phase α-Fe RE2 Fe14 B with varying nanoscale grain structures and intergranular exchange interactions produced via controlled segregation during crystallization. We show that the loss of coercivity arises because domain-wall processes dominate the magnetic reversal as the exchange interactions increase. Micromagnetic modeling corroborates a transition to strongly cooperative magnetic reversal as the exchange interactions increase. The magnetic reversal is controlled by the growth of interaction domains via discrete domain-wall motion, and the coercivity is intrinsically limited by the presence of interaction domains. To alleviate this problem, we have built an additional length scale into the structure that is below the interaction domain size but above the limit for intergranular exchange interactions to be significant. These {"}single-interaction domain{"} structures retain nucleation-type magnetic reversal and high coercivity. We show experimentally that nanocomposite Sm-Co/Co with this additional length scale has excellent coercivity and nucleation-controlled reversal.",
author = "Shield, {J. E.} and J. Zhou and S. Aich and Ravindran, {V. K.} and R. Skomski and Sellmyer, {D. J.}",
note = "Funding Information: In this paper, we have shown that the magnetic reversal processes in exchange-spring permanent magnets are dominated by domain-wall processes. The presence of interaction domains is largely responsible for the domain-controlled demagnetization behavior. We have also proposed grain structures that can effectively eliminate interaction domains by generating an additional length scale larger than the nanoscale but below the single domain limit. A model system with ∼ 150 nm SmCo 7 grains with intragranular Co precipitates and exchange-spring characteristics has been shown to have nucleation-controlled magnetization processes. As a result, improved materials can be developed that will approach theoretical predictions for exchange-spring nanocomposite permanent magnets. The authors appreciate useful discussions with D.C. Crew, and sample preparation from B. Kappes and D.J. Branagan. The compositional profiles were obtained through the SHaRE program at Oak Ridge National Laboratory, and assistance from J. Bentley is greatly appreciated. This project was supported by the NSF-MRSEC QSPINS, the Nebraska Research Initiative, the Center for Materials Research and Analysis, the National Science Foundation through grant No. DMR0305354, and the Department of Energy. FIG. 1. Reversible magnetization as a function of irreversible magnetization for single-phase (a) Nd-Pr-Dy-Fe-Co-B-Ti-Zr-C and (b) Nd-Fe-B, and for (c) two-phase nanocomposite Pr-Fe-Co-B-Nb. All curves are at a reverse field of 6 kOe. FIG. 2. Transmission electron micrograph of a rapidly solidified Sm-Co alloy showing soft magnetic fcc Co precipitates imbedded in a hard magnetic SmCo 7 grains. The SmCo 7 grains are below the single domain limit but above the size necessary for the exchange-spring effect. FIG. 3. Reversible magnetization ( M rev ) as a function of irreversible magnetization ( M irr ) for the Sm-Co alloy. ",
year = "2006",
doi = "10.1063/1.2163837",
language = "English (US)",
volume = "99",
journal = "Journal of Applied Physics",
issn = "0021-8979",
publisher = "American Institute of Physics",
number = "8",
}