Spontaneous Exchange Bias in High Energy Ball Milled Mnbi Alloys

Low temperature phase (LTP) MnBi has received considerable interest as a prospective "Gap Magnet" between the ferrites and rare-earth magnets. The ball-milled Mn55+xBi45-x (x = 0, 1, 3, 5, 8) powders exhibited a positive spontaneous exchange bias (SEB) at room temperature, which is yet to be reported for this system. The vacuum arc melted alloys were homogenized and then subjected to high-energy ball milling. The pinning of ferromagnetic MnBi spins by localized antiferromagnetic clusters (due to Mn interstitials, anti-site disorder, and compositional fluctuations) is proposed to be the cause for SEB phenomenon. A noticeable coercivity of around 7.5 kOe at ambient temperature was achieved for x = 8 sample after 8 h of milling. The maximum exchange bias field (H-EB) of around 1.2 kOe at room temperature has been observed for x = 2 after 4 h and x = 8 after 8 h of milling. Apart from hard magnets, this phenomenon opens up the possibility of its application in magnetoresistive random access memory (MRRAM) devices and spintronics.