Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets.

A. Pantelica,I. Dan,A. Leca,A. D. Crisan,P. Palade,O. Crisan

doi:10.3390/nano10081618

Abstract

With the aim of demonstrating phase coexistence of two magnetic phases in an intermediate annealing regime and obtaining highly coercive FePt nanocomposite magnets, two alloys of slightly off-equiatomic composition of a binary Fe-Pt system were prepared by dynamic rotation switching and ball milling. The alloys, with a composition Fe53Pt47 and Fe55Pt45, were subsequently annealed at 400 °C and 550 °C and structurally and magnetically characterized by means of X-ray diffraction, 57Fe Mössbauer spectrometry and Superconducting Quantum Interference Device (SQUID) magnetometry measurements. Gradual disorder–order phase transformation and temperature-dependent evolution of the phase structure were monitored using X-ray diffraction of synchrotron radiation. It was shown that for annealing temperatures as low as 400 °C, a predominant, highly ordered L10 phase is formed in both alloys, coexisting with a cubic L12 soft magnetic FePt phase. The coexistence of the two phases is evidenced through all the investigating techniques that we employed. SQUID magnetometry hysteresis loops of samples annealed at 400 °C exhibit inflection points that witness the coexistence of the soft and hard magnetic phases and high values of coercivity and remanence are obtained. For the samples annealed at 500 °C, the hysteresis loops are continuous, without inflection points, witnessing complete exchange coupling of the hard and soft magnetic phases and further enhancement of the coercive field. Maximum energy products comparable with values of current permanent magnets are found for both samples for annealing temperatures as low as 500 °C. These findings demonstrate an interesting method to obtain rare earth-free permanent nanocomposite magnets with hard–soft exchange-coupled magnetic phases.

Highlights

During the last decade, a lot of research effort has been dedicated to the quest for new classes of permanent magnets, rare earth (RE)-free and capable of operating at high temperatures
There is much more versatility in metallurgical methods since they allow for slight changes in composition and doping with a small amount of other elements in order to promote the formation of the L10 phase directly from the synthesis without the need for additional annealing, or to promote the formation of additional magnetic phases, needed for instance to construct a nanocomposite exchange-spring FePt-based magnet
We report on the coexistence of hard L10 and soft L12 FePt phases, we follow the evolution with the temperature of the structural phase transition with occurrence of the L10 phase at early stages of annealing by means of temperature-dependent X-ray diffraction of synchrotron radiation, and we show that the coexistence of the two phases, partially exchange-coupled at 400 ◦C, leads to a fully coupled exchange spring magnet at temperatures as low as 500 ◦C

Summary

Introduction

A lot of research effort has been dedicated to the quest for new classes of permanent magnets, RE-free and capable of operating at high temperatures. With the purpose of following the disorder–order transformation that conveys the formation of the hard magnetic tetragonal L10 phase, the as-milled powders were subsequently annealed under argon flow (99.9999% purity, 100 mL/ min) for 1.5 h at 400 ◦C and 500 ◦C for samples Fe53Pt47 and Fe55Pt45, respectively.

Results

Conclusion