| High-performance permanent magnet materials have attracted considerable attention due to their wide application in various aspects,such as magnetic storage,wind power generation,magnetic levitation transportation systems and intelligent robots.At present,the relatively mature permanent magnet materials are mostly concentrated on rare earth permanent magnet materials.However,as a non-renewable energy source,the total amount of rare-earth materials is limited,at the same time,the demand for high-performance permanent magnet materials is increasing.Therefore,rare-earth-free permanent magnet materials are getting more and more attention.Among these materials,low-temperature phase Mn Bi(LTP-Mn Bi)has become one of the most potential candidates due to its unique advantages.LTP-Mn Bi has very high uniaxial magnetocrystalline anisotropy,and has a positive coercivity-temperature coefficient between 150 K and 550 K,which means that the coercivity will increase with the increase of temperature.In this high temperature range,the coercivity of LTP-Mn Bi is much larger than those of the current commonly used permanent magnet materials(such as rare-earth Nd-Fe-B and Sm-Co materials and other candidate materials that do not contain rare earth elements).In addition,the permanent magnetic materials based on thin films are easy to integrate with other materials,which is in line with the current development trend of high performance,miniaturization and light weight of components.Therefore,in this paper,the LTP-Mn Bi films were prepared by magnetron co-sputtering method,and the effects of initial atomic ratio,cooling rate and thickness of the films on Mn Bi properties were studied.The main research contents are as follows:(1)Films with Mn/Bi atomic ratio(R)in the range of 0.84-1.32 were prepared,and it was found that the atomic ratios have significant effects on the properties of LTP-Mn Bi films.The easy magnetization axis of the films was observed to transform from the in-plane direction to the out-of-plane direction as the Mn/Bi ratio increased from R < 1 to R > 1.And the spontaneous exchange bias(EB)phenomenon at room temperature was found in the Mn Bi film with R ? 1.24.When the atomic ratio increases to 1.32,a maximum EB field about 1032 Oe was obtained at room temperature,which is the largest EB magnitude thus far reported for Mn Bi materials at room temperature.(2)In order to study the effect of the cooling rate on the properties of the Mn Bi materials during the post-annealing process,Mn Bi films with different Mn/Bi atomic ratios were prepared and post annealed.The effect of cooling rates on the coercivity of LTP-Mn Bi films was studied by changing the cooling time in annealing process under the same other conditions,and the corresponding mechanism of coercivity was analyzed.The results show that the rapid cooling is beneficial to increase the coercivity of the film,and after analysis,it is concluded that the coercivity of the LTP-Mn Bi films are mainly controlled by the pinning mechanism,And,the rapid cooling during the film annealing process is more beneficial to the grain refinement than the slow cooling,which increases the number of pinning points,thereby increasing the coercivity of the films.(3)LTP-MnBi films with different thicknesses were prepared,and the influence of thickness on the morphology,structure and magnetic properties of the films before and after annealing were studied.The results show that it is easily oxidized when the the film thickness is too small,while the grain size will increase when the the film thickness is too large,which will result in a decrease in coercivity.Therefore,the proper thickness is conducive to the formation of the thin film with smaller grain size,better compactness,higher vertical orientation,and greater coercivity,and thereby improving the overall magnetic properties of the MnBi thin films. |
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