In-situ high-temperature domain structures of die-upset Nd–Fe–B magnets

Abstract

a b s t r a c tMagnetic domains of a die-upset Nd-Fe-B magnet with an energy product of about 50 MGOe are investigated by using in-situ magnetic force microscopy (MFM), varying temperature from room temperature to 240°C. For both as-prepared and magnetized Nd-Fe-B samples, the domain-pattern evolution during thermal demagnetization is monitored by MFM. The temperature dependence of the evolution of reverse domains is explained by micromagnetic models. At all temperatures, the magnetization reversal in the technological relevant magnetized limit proceeds by nucleation.The large demand for hybrid electric vehicles and wind turbines for electricity generation has led to renewed interest in the development of Nd-Fe-B magnets, particularly concerning their performance at elevated temperatures (T > 150°C) [1]. The understanding of the temperature dependence of domain structures in Nd 2 Fe 14 B is of great importance, because Nd 2 Fe 14 B has a relatively low Curie temperature (T c = 312°C). The magnetocystalline anisotropy decreases rapidly as the temperature approaches T c [2], accompanied by a reduction in coercivity and by changes in domain structure and reversal mechanism. It is great of interest to investigate the high temperature domain structures in these materials from both fundamental research and practical application points of views.To fully exploit the potential of Nd 2 Fe 14 B [3-6], it is necessary to have the magnetic grain c-axis aligned. One method of achieving this goal is die-upsetting, where the c-axis of the grains is stressinduced and parallel to the press direction [7-11] and where the texture formation is caused by preferential grain growth via dissolution and precipitation [12][13][14].Previous studies of the die-upset Nd-Fe-B have shown that the magnetic domains at room temperature, referred to as interaction domains, are much larger than the grain size [15][16][17], because the grains are coupled due to intergranular exchange. The grains included in the interaction domain are almost in single-domain because their size (50-400 nm) is comparable to the single domain grain size for Nd 2 Fe 14 B [18].Many high-performance permanent magnets, for example in motors, operate above room temperature. It is therefore desirable to directly observe domain structures at elevated temperatures, rather than making indirect conclusions, for example from hysteresis loops. Magnetic force microscopy (MFM), a surface technique, has been widely used to reveal the magnetic domains of bulk permanent materials [17,[19][20][21][22][23][24]. Surprisingly, it has not been possible so far to record in-situ MFM images of die-upset Nd-Fe-B magnet above room temperature. In this paper, we directly monitor the magnetic domains of a bulk die-upset Nd-Fe-B magnet at elevated temperature.The die-upset Nd-Fe-B magnets were prepared by using hot compaction and subsequent hot deformation. Melt-spun powder of Nd 13.62 Fe 75.70 Co 4.45 B 5.76 Ga 0.47 was hot-pressed in vacuum at 550°C and 113 MPa to obtain pieces of fully dense isotr...

Investigation of thermally evaporated nanocrystalline thin cobalt films

“…In consequence of the magnetic anisotropy reduction, an increase of magnetic domains width can be observed, as shown in Figs. 5a-f and 7a in which the in-plane domain structure with large domains of the order of 10 µm in size can be observed in Refs [7,8,35,51,52].…”