Magnetic interaction effects on the hard magnetic properties of ball-milled SmCo 5 + NiO and SmCo 5 + CoO composites: A M plot study

The effects of magnetic interactions on the hard magnetic properties of SmCo5 ball milled with NiO [antiferromagnetic (AFM) at room temperature] or with CoO [paramagnetic (PM) at room temperature] have been studied. The FM–AFM system exhibits improved magnetic properties (coercivity and squareness) for all compositions. The effects of magnetic interactions on the magnetic properties are analyzed in terms of classical ΔM plots. The plots show that in both systems magnetizing-like FM–FM exchange interactions are predominant for fields μ0H μ0HC. The different types of magnetic interactions are found to depend on the degree of SmCo5 dispersion in the AFM (NiO) or PM (CoO) matrices. Moreover, the role of the AFM appears to be to enhance both the dipolar and exchange-like interactions, although the exchange effects appear to be responsible for the improvement of the magnetic properties.

The effects of magnetic interactions on the hard magnetic properties of SmCo 5 ball milled with NiO ͓antiferromagnetic ͑AFM͒ at room temperature͔ or with CoO ͓paramagnetic ͑PM͒ at room temperature͔ have been studied. The FM-AFM system exhibits improved magnetic properties ͑coercivity and squareness͒ for all compositions. The effects of magnetic interactions on the magnetic properties are analyzed in terms of classical ⌬M plots. The plots show that in both systems magnetizing-like FM-FM exchange interactions are predominant for fields 0 H Ͻ 0 H C , while long-range dipolar interactions prevail for 0 HϾ 0 H C . The different types of magnetic interactions are found to depend on the degree of SmCo 5 dispersion in the AFM ͑NiO͒ or PM ͑CoO͒ matrices. Moreover, the role of the AFM appears to be to enhance both the dipolar and exchange-like interactions, although the exchange effects appear to be responsible for the improvement of the magnetic properties. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1541650͔

I. INTRODUCTION
Recently, it has been demonstrated that by ball milling antiferromagnetic ͑AFM͒ and ferromagnetic ͑FM͒ materials it is possible to produce a microstructure of FM particles embedded in an AFM matrix. This microstructure favors exchange interactions between FM and AFM phases, which are found to induce coercivity, 0 H C , and squareness, M R /M S , enhancements. 1,2 In FM materials, a procedure, known as ⌬M plots, was developed to evaluate the strength of the different types of magnetic interactions. 3 This technique differentiates between two types of remanence curves: ͑i͒ the isothermal remanent magnetization M r ( 0 H), which is obtained by measuring the remanent magnetization after progressively magnetizing a fully demagnetized sample, using increasingly larger positive magnetic fields; and ͑ii͒ the demagnetizing remanent magnetization M d ( 0 H), which is obtained by progressively demagnetizing a fully saturated sample in increasingly negative magnetic fields. Usually, both M r ( 0 H) and M d ( 0 H) are normalized to the value obtained after saturating the sample ͑i.e., M r (ϱ)), so that m r ( 0 H)ϭM r ( 0 H)/M r (ϱ) and m d ( 0 H)ϭM d ( 0 H)/M d (ϱ). From the normalized remanence curves ⌬M is defined as Positive ⌬M values are interpreted as magnetizing shortrange exchange interactions, while negative ⌬M values cor-respond to demagnetizing long-range dipolar interactions. However, note that the effects of FM-AFM exchange interactions on ⌬M plots have not been studied systematically so far. 4 In this work, a comparative study of the magnetic properties of SmCo 5 ball milled with NiO and CoO is presented. Since the Néel temperature of CoO is below room temperature, only in SmCo 5 ϩNiO composites some effects due to FM-AFM exchange interactions are expected. The results indicate that the observed enhancements of 0 H C and M R /M S in the FM-AFM system are related to the exchange-like interactions induced by the AFM-FM coupling.

II. EXPERIMENTAL PROCEDURE
Ball milling of SmCo 5 ͑99%, Ͻ500m͒ alone and together with NiO ͑99%, Ͻ44 m, T N ϭ590 K) and CoO ͑99%, Ͻ44 m, T N ϭ290 K) powders, in the weight ratios of 4:1, 3:2, 2:3, and 1:3, was carried out in a planetary mill, under argon atmosphere, at 500 rpm, using agate vials and balls, in a ball-to-powder weight ratio of 2:1. The milling time used in the present study was 16 h. Morphological and structural characterizations were performed by scanning electron microscopy ͑SEM͒ and x-ray diffraction ͑XRD͒, where microstructural parameters were evaluated using a full pattern fitting procedure ͑Rietveld method͒. 5 Magnetic hysteresis loops and remanence curves were carried out on tightly packed isotropic powders by means of an extracting magnetometer, with a maximum applied field of 20 T.
a͒ Author to whom correspondence should be addressed; electronic mail: dolors.baro@uab.es

III. RESULTS AND DISCUSSION
The SEM images of SmCo 5 powders milled together with NiO or CoO reveal that, during the milling, both components tend to solder together, forming aggregates of a few m in size, in which SmCo 5 grains become embedded in the AFM ͑NiO͒ or paramagnetic ͑PM͒ ͑CoO͒ matrices. 2,6 Moreover, the analysis of the XRD spectra shows that for a fixed milling time ͑e.g., 16 h͒, the milling is more aggressive ͑i.e., more structural disorder͒ for mixtures with larger FM content. 2,6 For example, SmCo 5 crystallite size, ͗D͘ SmCo 5 , reduces progressively with the SmCo 5 content, from 15.7 nm in SmCo 5 (1):CoO(3) to 5.8 nm in SmCo 5 milled alone. In addition, for all FM percentages, the difference of ͗D͘ SmCo 5 between the SmCo 5 ϩNiO and the SmCo 5 ϩCoO systems are found to be small ͑i.e., less than 6%͒. This reveals that, in fact, the microstructure developed during ball milling is very similar in both cases.
Shown in Fig. 1, is the dependence of 0 H C with the FM:AFM ͑or PM͒ ratio ͑i.e., FM percentage͒ for SmCo 5 ϩNiO and SmCo 5 ϩCoO. As can be seen in the figure, both systems exhibit a similar behavior, i.e., there is a small maximum for intermediate FM percentages and a clear decrease of coercivity for increasing FM contents. However, 0 H C is larger for SmCo 5 ϩNiO than for SmCo 5 ϩCoO, especially for small FM contents. A similar behavior is observed for the squareness ratio, M R /M S ͑see Fig. 1 inset͒. These effects are the interplay between structural and magnetic effects ͑e.g., particle size, crystallite size, and packing density͒. 7 For example, diluting the FM in the AFM or PM matrix results in the reduction of the milling-induced structural disorder as well as the reduction of the dipolar and FM-FM exchange interactions. 2,6 However, due to the antiferromagnetic character of NiO, AFM-FM exchange interactions are also induced, which lead to the observed differences between SmCo 5 ϩNiO and SmCo 5 ϩCoO.  (3) composites. Note that the curves were measured up to 20 T. Although both systems have very similar morphological and structural properties, m r ( 0 H) and m d ( 0 H) are found to be somewhat different. Namely, as can be seen in the figure, m r ( 0 H) increases more rapidly with field in SmCo 5 ϩCoO than in SmCo 5 ϩNiO. This means that after applying a certain positive magnetic field the overall magnetization retained in the direction of the field, once the field is removed, is higher for FM:PM than for FM:AFM composites. Moreover, m d ( 0 H) decreases more slowly in SmCo 5 ϩNiO than in SmCo 5 ϩCoO, i.e., the latter system is easier to demagnetize. A similar trend has also been observed for the other FM:AFM ͑or PM͒ weight ratios and even for particles obtained after milling for different times ͑e.g., 8 or 32 h͒. Actually, FM-AFM exchange interactions, which influence the magnetization reversal of SmCo 5 ϩNiO composites, are probably responsible for these differences. For instance, the AFM spins could exert, during magnetization and demagnetization, a microscopic torque on the spins of the FM, which would make them difficult to reverse.
The ⌬M plots corresponding to SmCo 5 (1):NiO(3) and SmCo 5 (1):CoO(3) are shown in Fig. 3. It can be seen that both types of interactions ͑e.g., dipolar and exchange͒ are present in both systems. Positive ⌬M values ͑exchange in-teractions͒ are obtained when the applied field is lower than 0 H C , while dipolar interactions are predominant for 0 H Ͼ 0 H C . Similar ⌬M plots are obtained for the other FM:AFM ͑or PM͒ weight ratios, except for as-milled SmCo 5 powders alone, where the positive peak due to FM exchange interactions is not observed. This is because demagnetizing ͑i.e., dipolar͒ interactions predominate in FM particles milled alone, probably due to the large stray fields that each FM particle creates on its neighboring particles. It can also be Thus, although AFM-FM coupling appears to induce an increase of both dipolar-like and exchange-like interactions, the trend of the interaction field indicates that the relative importance of the exchange interactions increases for the AFM-FM system. Thus, exchangelike interactions appear to be responsible for the improvement of the hard magnetic properties of the composites. Note that when comparing the normalized loops of SmCo 5 ϩNiO and SmCo 5 ϩCoO ͑inset of Fig. 4͒ the former has a squarer loop, which is also an indication of enhanced exchange coupling.

IV. CONCLUSIONS
A systematic study of the different types of magnetic interactions present in SmCo 5 ϩNiO ͑AFM͒ and SmCo 5 ϩCoO ͑PM͒ composites, synthesized by ball milling and their effects on the magnetic properties, has been carried out by means of classical ⌬M plots. It has been shown that FM:AFM exchange coupling generates an increase of the interactions, both magnetizing and demagnetizing-like, in the system. However, the enhanced coercivity, 0 H C and remanence M R /M S observed for SmCo 5 ϩNiO ͑AFM͒ appear to be controlled by the increase of the exchange interactions.