Magnetic Hardening Induced by Exchange Coupling in Mechanically Milled Antiferromagnetic - Ferromagnetic Composites
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magnetoresistance materials (milling magnetic-nonmagnetic phases) [6]. However, studies of exchange interaction between ferromagnetic and antiferromagnetic materials induced by ball milling are scarce. Nevertheless, recently the possibility of increasing the room temperature coercivity by mechanical alloying transition metals (Ni, Co, Fe) and their own antiferromagnetic oxides (NiO or CoO) or sulfides (FeS) has been proven [7]. Moreover, it is well known that oxidized ferromagnetic fine particles can also exhibit coercivity enhancements [8]. However, this enhancement is mainly observed far below room temperature, which makes this property not useful for applications. It is noteworthy that coercivity increases due to AFM-FM exchange coupling are also being studied in thin film systems [9,10]. In this paper we show the possibility of increasing the room temperature coercivity and energy product by ball milling FM and AFM composed of different transition metals, e.g. Co and NiO, after adequate magnetic field heat treatments. The hardening of the FM phase is optimized by varying the AFM:FM ratio. EXPERIMENTAL Different AFM:FM weight ratios (0:1, 3:7, 2:3, 1:1 and 3:2) of gas - atomized powders of NiO (99%, TD remain unchanged. Consequently, the HE obtained after warming to each TD gives a measure of the number of crystallites with TBi < TD [15]. As can be seen in fig. 4, already at TD = 350 K a reduction in HE can be observed. Moreover, for TD = 500 K HE is reduced to zero (i.e. 50 % of the crystallites have TBi < 500 K). These results indicate a broad distribution of blocking temperatures in this system. However, although the thermal stability of HE is strongly influenced by the distribution of TB, we observed that Hc is rather insensitive to it. This is probably because although the overall HE depends on the strength and sign of the coupling for each crystallite, the enhancement of Hc depends only on the strength of the coupling. TD,
CONCLUSIONS In conclusion, we have shown that mechanically milled AFM(NiO) - FM(Co) composites display significant enhancements of the room temperature coercivity and energy product due to the exchange coupling induced after heat treating the samples at TANN > TN under magnetic fields. The as-milled powders exhibit a peculiar microstructure (Co lamellae surrounded by refined NiO particles), which allows the existence of a large interface area between the FM and the AFM. The maximum magnetic hardening is shown to be a trade off between the coercivity enhancement produced by exchange coupling and the overall reduction of saturation magnetization due to the antiferromagnet. All the effects (Hc, HE and BHMax) decrease with increasing temperature and the NiO-Co samples become uncoupled when the
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measuring temperature is larger than the antiferromagnetic N6el temperature. Although HE is strongly influenced by the distribution of TB, Hc remains rather insensitive to it. ACKNOWLEDGEMENTS This work was supported in part by DGESEIC under contract MAT98-0730 and by CICYT under contract PETRI
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