Strain Induced Anisotropic Magnetic Behaviour and Exchange Coupling Effect in Fe-SmCo5 Permanent Magnets Generated by High Pressure Torsion

Lukas Weissitsch,Martin Stückler,Stefan Wurster,Heinz Krenn,Andrea Bachmaier,Peter Knoll,Reinhard Pippan

doi:10.3390/cryst10111026

Abstract

High-pressure torsion (HPT), a technique of severe plastic deformation (SPD), is shown as a promising processing method for exchange-spring magnetic materials in bulk form. Powder mixtures of Fe and SmCo5 are consolidated and deformed by HPT exhibiting sample dimensions of several millimetres, being essential for bulky magnetic applications. The structural evolution during HPT deformation of Fe-SmCo5 compounds at room- and elevated- temperatures of chemical compositions consisting of 87, 47, 24 and 10 wt.% Fe is studied and microstructurally analysed. Electron microscopy and synchrotron X-ray diffraction reveal a dual-phase nanostructured composite for the as-deformed samples with grain refinement after HPT deformation. SQUID magnetometry measurements show hysteresis curves of an exchange coupled nanocomposite at room temperature, while for low temperatures a decoupling of Fe and SmCo5 is observed. Furthermore, exchange interactions between the hard- and soft-magnetic phase can explain a shift of the hysteresis curve. Strong emphasis is devoted to the correlation between the magnetic properties and the evolving nano-structure during HPT deformation, which is conducted for a 1:1 composition ratio of Fe to SmCo5. SQUID magnetometry measurements show an increasing saturation magnetisation for increasing strain γ and a maximum of the coercive field strength at a shear strain of γ = 75.

Highlights

Permanent magnets are designed for a high saturation magnetisation, a high Curie temperature and a high anisotropy resulting in a strong coercivity and yielding a high energy product
We demonstrate the feasibility of High-pressure torsion (HPT)-deformation to process exchange coupled spring magnets
Plastic deformation is mainly observed in the Fe phase, limited plastic deformation, a splitting and localized shearing of the SmCo5 phases is found

Summary

Introduction

Permanent magnets are designed for a high saturation magnetisation, a high Curie temperature and a high anisotropy resulting in a strong coercivity and yielding a high energy product. The maximal achievable energy product of a magnet is a widely used and crucial parameter to compare different magnetic materials or processing routes. Compared to Nd-Fe-B based magnets, the intermetallic Sm-Co compounds offer a relatively low saturation magnetisation. Sm-Co based magnets instead provide the highest Curie temperatures (up to 1190 K), which is essential for applications at elevated temperatures [1]. State-of-the-art magnetic materials, which provide a maximum magnetic field, require supplementary. The demand for permanent magnetic materials is strongly increasing, no matter if implemented in electric motors for vehicles or in windmills producing renewable energy. Mining and production processes of rare-earth elements are often correlated to political instabilities in the countries of origin and to monopolistic situations, which influences the price of elements on the world market [4]

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