2, Institut NEEL, Grenoble, , France
3, CEA Grenoble, Grenoble, , France
MnBi has attracted much interest within the past decade for its potential use in rare-earth free magnets. When stabilized in the so-called low temperature phase, MnBi exhibits an intriguing magnetic behavior with an unusual positive-dependence of anisotropy constant on temperature, reaching a maximum of 2.2 MJ.m-3 at 490 K . On the other hand, two magnetic phase nanocomposites, made of a fine mixture of nanosized grains with respectively high magnetization and high anisotropy, are good candidates to build high performance magnets . In this context, we studied the structural and magnetic properties of MnBi nanosized grains.
MnBi nanoclusters, with a mean size of 5 nm, were produced by gas aggregation in a low energy cluster beam deposition system, and protected with an amorphous carbon layer. We investigated their structure in a transmission electron microscope and by X-ray diffraction, and their magnetic response in a superconducting quantum interference device (SQUID) magnetometer. In this study, we focused on the onset of magnetic ordering after annealing at various temperatures, ranging from 250°C to 500°C.
In the as-deposited state, MnBi clusters, with nearly equiatomic composition, present a low degree of crystallinity. They show a paramagnetic-like response, with relatively large magnetic moment, which contrasts with a recent report on as-prepared MnBi 10-nm clusters made by sputtering . Low temperature annealing leads to magnetic ordering with temperature independent magnetization below room temperature. Annealing at a temperature ranging between 300°C and 400°C leads to the formation of a hard magnetic phase with low thermal stability, which disappears after annealing at 500°C. We will discuss the evolution of structural and magnetic properties upon annealing, considering finite size effects.
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