The martensitic structures, exchange bias effect and training effect in Ni-Mn-Ga ribbons

The exchange bias effect is the physical basics of spin valves and magnetic recording devices. A stable exchange bias effect would be suitable for practical application. In this work, Ni50Mn50-xGax (11 <= x <= 16) ribbons were obtained by melt spinning. The grain size and microstructure have been tuned through melt spinning, which changes the exchange interaction in ribbons and thus gets a giant and stable exchange bias effect. As the increase of x, the structure of Ni50Mn50-xGax (11 <= x <= 16) ribbons undergoes a transition from non-modulated martensite to non-modulated martensite &10 layers modulated martensite to 10 layers modulated martensite. The average grain size of all ribbons is less than 10 mu m, and the martensitic plate widths are smaller than 200 nm. With the increasing x or the cooling fields, the magnetization of the ribbons has been enhanced. As a result, the competition between ferromagnetic and antiferromagnetic interactions has been changed and thus influencing spin glass behavior. Furthermore, a low degree of attenuation for the exchange bias effect after continuous cycles has been achieved which arises from the stable spin configuration with a larger contribution of frozen spin which has a slow relaxation rate. For example, the exchange bias field of 8.78 kOe in Ni50Mn38Ga12 ribbons can be achieved under the 40 kOe field cooling at 10 K, and the exchange bias field after infinite cycles can be kept at 8.58 kOe, the degree of attenuation is only 3.4 %. This work enriches the research on the exchange bias effect of Heusler alloys and provides the experimental basis and theoretical support for designing a stable exchange bias effect.