Demagnetization curves and coercivity mechanism in Sm(CoFeCuZr)z magnets

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Abstract

The hysteresis behavior of Sm(CoFeCuZr)z magnets aged at 850°C and slowly cooled to temperatures down to 400°C has been examined. All the samples show two-step demagnetization curves. Field dependence of irreversible susceptibility χirr shows two peaks, namely Hci1 and Hci2.Hci1 increases from almost zero to 21 kOe when the end-ageing-temperature (Tend) decreases from 600 to 400°C, while Hci2 remains virtually unchanged. This behavior is attributed to the inhomogeneous distribution of Cu in the magnet. The change of Hci1 is believed to be related to the segregation of Cu into the cell boundary phase during the slow cooling process. Hci2 does not change with Tend, which may suggest that the Cu content of the cell boundary in the Cu-rich region may be high enough to make it non-ferromagnetic at room temperature. Further Cu segregation during slow cooling to a lower temperature would not increase Hci2 because the difference of anisotropy constant between the matrix and cell boundary phase ΔK1 has already reached its maximum. Temperature dependence of χirr shows that the higher the temperature, the smaller the difference between Hci1 and Hci2. When the temperature reaches 320°C, Hci1=Hci2, which might suggest that 320°C is higher than the Curie temperature of the cell boundary phase in the Cu-poor region. Then the contribution of the cell boundary phase to domain wall pinning becomes almost the same in both the Cu-rich and -poor regions.

Introduction

Sm(CoFeCuZr)z magnets attracted considerable attention recently due to their high-Curie temperature (∼825°C) [1]. SmCo 2 : 17 magnets are well established to be domain-wall pinning type magnets [2], [3]. Their microstructure consists of a mixture of cellular and lamella structures. The cell interior is a rhombohedral Sm2(CoFe)17 phase surrounded by a hexagonal Sm(CoCu)5 cell boundary phase, with a hexagonal Th2Ni17-type Zr-rich lamella phase superimposed on the cellular microstructure. The rhombohedral Th2Zn17-type Sm2(CoFe)17 matrix phase is responsible for the high saturation magnetization, and the CaCu5-type Cu-rich Sm(CoCu)5 cell boundary phase for the high intrinsic coercivity by pinning the domain walls. The lamella phase is supposed to provide easy diffusion paths for the copper segregation [4], [5]. Sm(CoFeCuZr)z magnets are complex metallurgical systems with four compositional and at least an equal number of heat treating variables [6]. A metallurgical model has been proposed to describe the functions of each element [6]. The heat treatment procedure of these alloys is very complicated, which involves homogenization at 1150–1180°C followed by isothermal ageing at about 800–860°C for up to 12 h, and a slow cooling or step ageing from the ageing temperature to 400°C. Without the slow cooling or step ageing process, the intrinsic coercivity is extremely low. The effect of slow cooling is not yet very well understood.

In this paper, we studied the demagnetization curves of Sm(CoFeCuZr)z magnets, which were aged at 850°C and then slowly cooled to various temperatures, and discussed the mechanism of coercivity development during the slow cooling.

Section snippets

Experimental

Sm(CoFeCuZr)z commercial magnets were sealed into quartz tubes under vacuum, and solutionized at 1170°C for up to 12 h, then quenched to room temperature in a water bath in order to obtain a single 2 : 17 phase. The solutionized samples were aged at 850°C for 12 h, followed by a slow cooling at 0.7°C/min to 600, 500 and 400°C, respectively, and then quenched to room temperature. Magnetic properties were measured using an Oxford vibrating sample magnetometer (VSM) with a maximum applied field of 50 

Results and discussion

Fig. 1 shows the hysteresis loops of Sm(CoFeCuZr)z magnets aged at 850°C for 12 h and slowly cooled to an end-ageing-temperature (Tend) of 600 (a), 500 (b) and 400°C (c), respectively. It can be clearly seen that the curve shape becomes better when the magnets are slowly cooled to a lower temperature.

Fig. 2 shows the demagnetization curves of these samples. The magnet slowly cooled to 600°C exhibits a clear two step demagnetization curve. Even in the magnet slowly cooled to 400°C, the feature of

Conclusions

By fitting the χirr versus H curves, two peaks at different fields, namely Hci1 and Hci2, were found to exist for all the Sm(CoFeCuZr)z magnets that were slowly cooled to 600, 500 and 400°C, respectively. The Hci1 was found to increase from almost zero to 20 kOe when the end-ageing temperature decreases from 600 to 400°C, while Hci2 remains almost unchanged. This phenomenon has been interpreted in terms of the inhomogeneity of Cu distribution and cell incompleteness. For the fully processed

Acknowledgements

This work is supported by United States Air Force under Grant No. MURI F49620-96-1-0403.

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