Materials Science for High-Performance Permanent Magnets: Coercivity Mechanism
Sponsored by: TMS Functional Materials Division, TMS: Magnetic Materials Committee
Program Organizers: Satoshi Hirosawa, National Institute for Material Science; Matthew Kramer, Iowa State University; Oliver Gutfleisch, Technische Universität Darmstadt; Hae-Woong Kwon, Pukyong National University

Tuesday 8:30 AM
February 28, 2017
Room: 24C
Location: San Diego Convention Ctr

Funding support provided by: Elements Strategy Initiative Center for Magnetic Materials

Session Chair: Akimasa Sakuma, Tohoku Umiversity; Gino Hrkac, University of Exeter

8:30 AM  Invited
Tailoring the Coercive Field of Grain Boundary Engineered Magnets: A Nanoanalytical TEM and Micromagnetic Simulation Study: Josef Fidler1; Gregor Alexander Zickler1; Ahmad Asali1; 1TU Wien
    The role of grain boundary (GB) phases of Nd-Fe-B based magnets, whose composition, thickness and magnetic properties primarily control the coercive field, have extensively been studied during the last 30 years. Local changes of the anisotropy field and demagnetizing field at/near intergranular phases considerably reduce the overall coercive field. Nowadays, new nanoanalytical TEM/STEM techniques with atomic resolution allow the creation of precise microstructural models suitable for the numerical micromagnetic calculation of the coercive field of an individual magnet whose microstructure strongly depends on the processing parameters and nominal composition. The present study with special emphasis on nanoanalytical, high resolution EELS characterization compares different microstructures: large grained sintered heavy rare earth free magnets with an anisotropic compositional behaviour of GBs parallel and perpendicular to the alignment direction and nanocrystalline magnets with directly de-coupled or coupled grains obtained by rapidly quenching, which are widely used for bonded and hot deformed type magnets.

9:00 AM  Invited
Demagnetizing Fields and Magnetization Reversal in Permanent Magnets: Johann Fischbacher1; Lukas Exl2; Thomas Schrefl1; 1Danube University Krems; 2Vienna University
    Micromagnetic simulations give a deep insight into the mechanisms that cause magnetization reversal at external fields well below the anisotropy field. Comparing finite element micromagnetic simulations with a simple model for magnetization reversal, we show how magnetostatic interactions trigger magnetization reversal. Magnetization reversal starts, when the total internal field exceeds the Stoner-Wohlfarth switching field locally. Scanning the magnet grain by grain, we create maps of the local coercive field within a permanent magnet. The position of the weak spots within a magnet depends on the nature of the grain surfaces and on the temperature. Grain boundary diffusion shifts the nucleation site from the grain surface towards to core of the grain. With increasing temperature, the magneto-crystalline anisotropy decreases faster than the demagnetizing field. Consequently, demagnetizing effects dominate magnetization reversal at high temperature. Work supported by the Austrian Science Fund (F41-12), EC (H2020 Novamag), NEDO (MagHEM), JST (CREST).

9:30 AM  Invited
Analyses on Magnetization Reversal Process of Nd-Fe-B Hot-deformed Magnets: Satoshi Okamoto1; Takahiro Yomogita1; Luran Zhang1; Nobuaki Kikuchi1; Osamu Kitakami1; Hossein Sepehri-Amin2; Tadakatsu Ohkubo2; Kazuhiro Hono2; Takahiro Akiya3; Keiko Hioki4; Atsushi Hattori4; 1Tohoku University; 2ESICMM-NIMS; 3 Daido Steel Co., LTD; 4Daido Steel Co., LTD
    We have explored the magnetization reversal process of Nd-Fe-B hot-deformed magnets with and without eutectic alloy grain boundary diffusion processes through FORC diagram measurement and energy barrier analysis based on magnetic viscosity measurement. Although the values of coercivity for the magnets with and without the grain boundary diffusion processes are quite different, it is surprising that there are very little differences on the FORC diagrams and energy barrier analysis among these magnets. The FORC diagram measurement reveals very narrow switching field distribution and very small effect of the local demagnetization field in the magnets. The energy barrier analysis indicates that nucleation of reversed domains takes place at the beginning of the magnetization reversal and then domain wall propagation becomes the dominant magnetization reversal process for the most part of the demagnetization process. The barrier heights and the intrinsic reversal fields for the nucleation and the wall pinning are quantitatively discussed.

10:00 AM Break

10:20 AM  Invited
Theoretical Study on Atomic Structures and Coercivity in Nd-Fe-B Magnets: Hiroki Tsuchiura1; 1Tohoku University
    Several recent experimental studies for sintered Nd-Fe-B magnets have shown that atomic structures around the grain boundaries of the magnets significantly affect their coercivity. Theoretically, we have found based on first-principles calculations that the Nd ions exposed on the (001) surface not only lose their uniaxial local magnetic anisotropy but also exhibit in-plane anisotropy. It has been also shown that those Nd ions can cause a considerable reduction of coercivity by using a Landau-Lifshitz-Gilbert (LLG) type simulation technique for a classical spin model on a cubic lattice. In this contribution, we consider the next question whether such coercivity reduction mechanism can be relevant for the real Nd-Fe-B magnets. To this end, we study magnetization reversal process based on a microscopic spin model with realistic interfacial atomic structures around the grain boundaries found experimentally or by using first-principles molecular dynamics.

10:50 AM  
Grain Boundary Diffusion of Different Rare Earth Elements in Nd-Fe-B Sintered Magnets by Experiment and FEM Simulation: Konrad Löwe1; Dimitri Benke1; Tim Lienig1; Michael Duerrschnabel1; Leopoldo Molina-Luna1; Konstantin Skokov1; Oliver Gutfleisch1; 1Technische Universität Darmstadt
    In the present work we explore the influence of a coercivity gradient in Nd-Fe-B magnets produced by the Grain Boundary Diffusion Process (GBDP) on the overall coercivity. We diffused four different rare earth elements (Dy, Tb, Ce and Gd) in commercial Nd-Fe-B magnets. By means of cutting the magnets into thin slices we obtain lateral coercivity profiles, from which diffusion constants are extracted. We find that in sintered magnets Tb diffuses significantly faster than Dy. Contrary to the magnets diffused with the heavy rare earths, the microstructure in the magnets treated with Ce show no (Nd,Ce)-Fe-B shells in the surface regions. Finally, a simple model for the magnetization reversal in grain boundary diffusion processed gradient Nd-Fe-B magnets was developed and implemented into an FEM software. The simulation indicates that the macroscopic coercivity of a gradient magnet depends mostly on the coercivity of its surface region.

11:10 AM  
Temperature Dependence of Threshold of Magnetic Fields for Nucleation and Domain Wall Propagation: Seiji Miyashita1; Masamichi Nishino2; 1The University of Tokyo; 2National Institue for Material Science
    In order to study coercive mechanism of permanent magnets at finite temperatures, we studied temperature dependence of threshold of magnetic fields for nucleation and domain wall propagation in a sandwich structure of hard-soft-hard magnet. We found general features of the reduction of the threshold with the temperature. In particular, we found novel dependences due to the narrow domain wall effect due to the reduction of the exchange energy. We also studied the nucleation and domain wall propagation in a realistic modeling of Nd-Fe-B magnets by making use of real structure and parameters which are obtained in literatures and also from the first-priciple calculations. There we reproduced experimentally-observed properties, and also we found some microscopic properties related to the magnetization reversal in the material.

11:30 AM  Invited
Theoretical Study on the Temperature Dependence of Magnetic Anisotropy Constants of Rare Earth Permanent Magnets: Akimasa Sakuma1; Daisuke Miura1; Yuta Toga2; 1Tohoku University; 2National Institute for Materials Science
     For permanent magnets, magnetic anisotropy is the most important property because of its close connection with the coercivity. Recent development of electric vehicles stimulates much interest in the temperature dependence of this property of magnets especially above room temperatures where the motors operate. In this symposium, we will first overview the general theory for the magnetic anisotropy at finite temperature and show how the magnetic anisotropy constants (MAC) vary as temperature increases. Next, we will show some calculated results for the temperature dependence of MAC of Nd2Fe14B using the constrained Monte Carlo method [1]. Here we mainly discuss the effects of exchange field on the MAC and the site dependence of the MAC’s at around the room temperature.References: [1] P. Asselin, et al., Phys. Rev. B 82, 054415 (2010).