Room-temperature coercivity enhancement in mechanically alloyed antiferromagnetic-ferromagnetic powders

Enhancements in coercivity, squareness and energy product have been obtained at room-temperature by mechanically milling mixtures of ferromagnetic (FM) (Co) and antiferromagnetic (AFM) (NiO) powders. All these e!ects decrease with increasing temperatures, up to 1"600K, just above the NeH el temperature of the antiferromagnet. Due to the competition between coercivity enhancement and reduction of saturation magnetization in AFM/FM mixtures, although the maximum coercivity enhancement has been found for (AFM)1 : 1(FM) weight ratio, the maximum energy product is obtained for composites with larger FM content, i.e. (AFM)2 : 3(FM). ( 2000 Elsevier Science B.V. All rights

T N ϭ590 K and T N ϭ610 K, respectively. 15The weight ratio of FM to AFM was usually kept to 1:1.Ball milling of pure Co powder was also carried out in the same conditions.The as-milled powders were annealed for different times in vacuum, under different magnetic fields (Hϭ0 -10 kOe), in the range of 300-850 K and field cooled to room temperature.The samples were structurally characterized by x-ray diffraction, using a Cu K␣ incident radiation.Magnetic hysteresis loops, up to 10 kOe, were carried out in loosely packed powders at room temperature by means of vibrating sample magnetometry ͑VSM͒.
As can be seen in Fig. 1, as the milling time increases, the coercivity (H C ) of the pure Co samples increases, up to 1 h.This initial H C increase is due to the transformation of the original fccϩhcp powder to hcp, as observed by x-ray diffraction, 16,17 where pure hcp-Co is known to have larger H C than fcc-Co. 18Further increase of the milling time causes a͒ Electronic mail: Dolors.Baro@UAB.ES FIG. 1. Dependence of the coercivity, H C , on the milling time for Co powders for as milled ͑full symbols͒ and milled and annealed at T ANN ϭ600 K for 0.5 h at Hϭ5 kOe ͑open symbols͒.Note that the circles are for the original Co powders, as-obtained ͑full symbol͒ and annealed at T ANN ϭ600 K for 0.5 h ͑open symbols͒.The lines are a guide to the eye.a reduction of H C ͑Fig.1͒.This effect is probably linked with the transformation of hcp-Co to a random close-packed phase upon long term milling. 16,17Annealing the as milled powders at 600 K for 0.5 h produces a slight decrease in H C ͑Fig.1͒ which is probably related to grain growth and/or initial transformation of the hcp-Co to fcc-Co during the annealing.
Similar to what is observed for ball milled pure Co, a maximum in H C is also obtained for mechanically alloyed Co-NiO particles when increasing the milling time, as shown in Fig. 2. Nevertheless, the maximum appears now for much longer milling times, i.e., 20 h.Hence, NiO slows down the Co phase transformation during mechanical alloying.Contrary to what we observe for ball milled Co, annealing at 600 K for 0.5 h in a field of Hϭ5 kOe, produces a significant increase in H C .Thus, the addition of NiO particles in the mechanical alloying plays an important role in the enhancement of H C .To better understand the role of NiO, annealings at other temperatures have been carried out.As can be observed in Fig. 3, the maximum effect is found around T ANN ϭ600 K, i.e., for T ANN ϾT N .However, if T ANN is too high there is a transformation from hcp-Co to fcc-Co, causing the reduction of H C .The enhancement of H C after field cooling through the Ne ´el temperature is in agreement with what is found for fine powder systems at low temperatures. 6Therefore, these results seem to indicate that it is the exchange interaction-exchange bias between the AFM and FM powders which induces the coercivity enhancement.
To confirm this assumption, we carried out annealings at different fields.As expected, annealing at Hϭ0 kOe also produces an increase in H C due to the remanence of the Co powders which create a field to the NiO powders, in agreement with previous FM-AFM ball milling studies. 14However, the increase in H C is 50% smaller than the one obtained for annealing at Hϭ5 kOe.Annealing time is also found to play an important role.Long annealing times ͑several hours͒ tend to either slowly transform hcp-Co towards fcc-Co and/or probably induce excessive interdiffusion, leading to a decrease of the H C enhancement.In our case, for the sample mechanically alloyed for 20 h annealing for more than 3 h was found to deteriorate the properties.The maximum coercivity increase found in this study was around 125% (H C,Max ϭ370 Oe), with respect to the original Co powder.
Further evidence for the origin of the increase in H C comes from the shift, towards negative fields, in the hysteresis loop along the field axis, H E ͑Fig.3͒, which is typically induced by AFM-FM exchange coupling. 6In agreement with Ni-NiO fine powder studies 9,11 and Co-NiO thin film studies, 19 the loop shifts observed for the Co-NiO samples mechanically alloyed for 20 h are rather small as compared to their coercivity ͑see inset of Fig. 3͒.This is probably due to the rather small anisotropy of NiO, which causes the AFM spins to be easily rotated by the FM spins, thus inducing a small loop shift but a large coercivity. 5,6The system exhibits its maximum loop shift at T ANN ϭ600 K in agreement with H C ͑Fig.3͒.
An enhancement in the squareness ratio, M R /M S , where M R and M S are the remanent and saturation magnetization, is also observed after field annealing the Co-NiO powders.The increase in M R /M S follows the same behavior as H C with annealing ͑see Fig. 3 inset͒, with a maximum enhancement of remanence of M R /M S Ϸ70% as compared to the original Co powder.The origin of this enhancement of the remanence is still not well understood.However, its T ANN dependence indicates that it is also related to exchange bias.
Finally, we have also observed that all the above effects ͑enhancement of H C and M R and loop shift͒ obtained after annealing at T ANN ϭ600 K and field cooling to room temperature, decrease for increasing measuring temperatures, disappearing above T N , as expected for exchange bias effects. 6 Mechanical alloying of Co-FeS leads to a similar increase in the H C and the presence of loop shifts ͑an example of the comparison of the hysteresis loops before and after field annealing is shown in the inset of Fig. 4͒.However, the maximum in H C is not observed after field annealing at T ANN ϭ600 K, as for NiO, but at T ANN ϭ650 K ͑see Fig. 4͒, i.e., for T ANN ϾT N ϭ613 K.This provides further evidence for the exchange interaction between the FM and AFM induced when field cooling through the Ne ´el temperature of the AFM.
In conclusion, we have shown an enhancement of the coercivity and squareness ratio of Co powders at room temperature and above, by mechanically alloying them with NiO and FeS.The coercivity of the Co-NiO powders exhibits a maximum at longer milling times than for pure Co.Moreover, field annealing increases H C rather than decreasing it, as observed for pure Co.The maximum enhancement of H C is found for field annealing at T ANN ϾT N and posterior field cooling to room temperature.The hysteresis loops also ex-hibit shifts in the field axis.The effects disappear for measuring temperatures above T N .This behavior indicates that the H C enhancement is caused by exchange bias effects.In agreement with thin film systems, exchange bias properties can be obtained in fine powders composed of FM and AFM of different transition metals.Moreover, being able to increase and control H C at room temperature and above makes these materials interesting for magnetic applications.In particular, hard magnets ͑FeNdB, SmFe͒ could be mixed ͑e.g., mechanically alloyed͒ with AFMs with T N above room temperature to further enhance their coercivity.
Mechanical alloying of soft and hard magnetic materials introduces exchange interactions between both materials, which have been used to induce magnetic hardening in magnetic materials [1,2].The exchange interaction creates a remanence increase in the hard magnetic materials, which leads to the enhancement of the energy product, BH .Similar e!ects have also been found in soft}hard bilayer and multilayer systems [3].Exchange interactions between ferromagnetic (FM) and antiferromagnetic (AFM) materials have been mainly studied in thin "lm systems [4}6] and transition metal particles embedded in their native oxides [4,7}9].These systems exhibit loop shifts in the "eld axis and a coercivity enhancement, as they are "eld cooled through the NeH el temperature of the AFM, ¹ , [4].In "ne particle systems, these e!ects are usually present only far below room temperature [4,7}9].Although the microscopic origin of the loop shifts and coercivity enhancements is not clear at present, simple models [7,8] suggest that AFM materials with larger anisotropies tend to induce larger loop shifts, while AFM with smaller anisotropy bring about important coercivity enhancements [4].
We have recently shown that it is possible to enhance the room-temperature coercivity of FM materials by heat treating mechanically milled fer-romagnetic}antiferromagnetic composites, such as Co}NiO or Co}FeS [10].
In this paper we study the optimal conditions (AFM : FM ratio, milling time) for room-temperature enhancement of the energy product, BH .The BH enhancement is found to be maximum for (AFM) 2 : 3 (FM) and disappears above the AFM NeH el temperature, as expected for AFM}FM exchange interaction e!ects.
The samples were structurally characterized by X-ray di!raction (XRD) (Cu-K -radiation).Their morphological characterization was performed by using scanning electron microscopy (SEM), equipped with energy dispersive X-ray analysis (EDX).Magnetic hysteresis loops, up to 10 kOe, were measured in loosely packed powders, at temperatures between room temperature and 750 K, by means of vibrating sample magnetometry (VSM).
Fig. 1 shows the dependence of the room-temperature coercivity, H ! , on the milling time for the NiO : Co weight ratios of 1 : 1 and 2 : 3 (before and after "eld annealing).In both the cases, as the milling time increases, a maximum in H ! is obtained.The initial H ! rise is attributed to the transformation of the original mixture of FCC and HCP-Co to HCP at the "rst stages of milling, as has been observed by XRD [11,12].The reduction of H ! after the long-term milling is probably linked with the transformation of the HCP-Co to a random close packed phase [11,12].As can be observed in Fig. 1, after "eld annealing at ¹ ,, " 600 K in H"5 kOe and "eld cooling to room temperature, both AFM : FM ratios exhibit an enhancement of the coercivity (de"ned as the di!erence of H ! before and after annealing, H ! ).A summary of the room-temperature data for the di!erent NiO : Co exchange couples is presented in Table 1.Coercivity enhancements and loop shifts in the "eld axis, H # , towards the negative "eld axis, are observed for all AFM/FM ratios (except in pure Co) after annealing at ¹ ,, "600 K and "eld cooling to room temperature.This is expected from exchange interactions between the FM and the AFM.Simple models [4,7,8] suggest that when a system consisting of an FM and an AFM is "eld cooled through ¹ , , the spins of the AFM tend to align ferromagnetically to the spins of the FM, inducing an interface coupling.Consequently, when a hysteresis loop is performed on the AFM}FM coupled system, as the FM spins try to rotate they have to &drag' irreversibly by the AFM spins due to the coupling (for systems with small AFM anisotropy); thus increasing H ! .In addition, due to the AFM}FM coupling, an unidirectional anisotropy is introduced in the system, which is the origin of the shift of the hysteresis loop [4,7,8].As can be seen in Table 1, the maximum coercivity enhancement, H ! , is obtained for the 1 : 1 weight ratio, where H ! is about 45 Oe.Note that the maximum H ! after milling was also observed for the 1 : 1 weight ratio (Table 1).Also shown in Table 1 is the required milling time so as to achieve the largest coercivities, t , for the di!erent NiO : Co weight ratios.As the Co content !, are attributed to the rather small anisotropy of NiO, which causes the antiferromagnet spins to be easily rotated by the FM spins, thus inducing a small loop shift but a large coercivity [4,7,8].Moreover, no signi"cant training e!ect of the loops (i.e.reduction of H # or H ! for consecutive loops [13]) was observed.
The microstructure of the ball-milled powders helps to better understand the magnetic properties of the FM}AFM system.Short milling times induce mainly mixing of the constituents with only limited contact between NiO and Co.At intermediate milling times, a large number of Co lamellae are formed, which are surrounded by "ne particles of NiO.This microstructure induces an increase of the interface area and thus the coupling between the FM and the AFM components [14].Long milling times probably start to create excessive amounts of defects and intermixing between the AFM and FM phases.It is noteworthy that X-ray results indicate that "eld annealing (¹ ,, (600 K) does not induce any phase changes and promotes only minor crystallite growth.
The "gure of merit of hard magnetic materials is not so much their coercivity, H ! , as their energy product, BH .Note that the quality of a permanent magnet is determined by the magnetostatic energy stored in a magnetic circuit containing the magnet.This energy is given by the total area enclosed by the hysteresis loop and is proportional to the area of the maximum rectangle which can be "tted inside the second quadrant of the B}H hysteresis loop.Usually, this &maximum rectangle' (denoted as BH ) and not the area of the loop, is given as a quantitative measure of the quality of a hard magnet.In Fig. 2, the room-temperature energy products, BH , are shown as a function of the Co weight percent (AFM/FM ratio).In the as-milled powders, BH tends to increase as the Co content is raised.Although the maximum H ! after milling remains almost constant for all compositions (see Table 1), the lower saturation magnetization, M 1 (de"ned as the measured magnetic moment divided by the total mass (Co#NiO)), for composites with smaller Co content drastically reduces BH .However, after "eld annealing (H"5 kOe) the powders at ¹ ,, "600 K and "eld cooling to room temperature, when exchange interaction is present, an enhancement of the energy product, BH is obtained.It is remarkable that the highest BH is found for the 2 : 3 ratio, although H ! was further enhanced in the 1 : 1 samples.The maximum increase of BH is found to be 83% with respect to the original unmilled Co powder.If the amount of NiO is not large enough (e.g. when the ratio is 3 : 7), the coupling is not so e!ective in increasing BH , as can be seen in Fig. 2. If the NiO amount is exceedingly large (e.g. in the ratios 1 : 1 and 3 : 2), the saturation magnetization is excessively reduced, therefore the e!ect of the increase in H ! due to the coupling is not enough to enhance BH .Note that to isolate the e!ects of the coupling induced by the "eld cooling, H ! and BH are given in Fig. 3, for the 1 : 1 ratio, as the di!erence of  .As can be seen in Fig. 3, as temperature increases, H ! , H # and (BH) all decreases and become almost zero at ¹"600 K.As expected from the nature of the exchange bias coupling, the magnetic hardening (H ! and BH ) deteriorates as the temperature is increased.The decrease of these properties with temperature can be understood as a result of a reduction in the AFM/FM interface coupling and as a consequence of either the loss of the magnetic order in the AFM and/or the reduction of the AFM magnetocrystalline anisotropy.The critical temperature at which exchange coupling e!ects disappear is known as the blocking temperature, usually designed by ¹ , and is generally close to or below ¹ , [4,15}18].In most studies concerning exchange coupling in oxidized "ne particle systems, ¹ is far below room temperature.This means that H ! and H # are generally only observed at low temperatures [4,6}8].This is either because ¹ , is below room temperature (e.g.¹ , (CoO)"290 K or ¹ , (FeO)"200 K) or because the oxidation of small FM particles leads to surface AFM layers which are either very thin or with very small grains, causing the reduction of ¹ , and thus ¹ due to "nite size e!ects [4,15}18].In our case, the NiO crystallites, as deduced from X-ray di!raction, remain rather large (d&15 nm [14]), hence ¹ +¹ , .In conclusion, we have shown that it is possible to enhance the room-temperature coercivity and energy product of a ferromagnet (in our case, Co), by ball milling it with an antiferromagnet, with NeH el temperature above room temperature (in our case, NiO).The maximum coercivity, H ! , and enhancement of the coercivity, H ! , are found for NiO 1 : 1 Co weight ratio, i.e. when a maximum coupling is present in the system.However, the addition of AFM reduces the overall saturation magnetization in AFM : FM mixtures.As a result, the maximum BH is found for NiO 2 : 3 Co weight ratio and not for NiO 1 : 1 Co.The enhancement of H ! and BH are found to decrease with increasing temperature, vanishing above ¹ , (NiO)"590 K, as expected for an exchange coupled FM}AFM system.

FIG. 2 .
FIG.2.Dependence of the coercivity, H C , on the milling time for Co-NiO 1:1 powders for as milled ͑full symbols͒ and milled and annealed at T ANN ϭ600 K for 0.5 h at Hϭ5 kOe ͑open symbols͒.The lines are a guide to the eye.

Table 1
Summary of the room-temperature magnetic properties of ball-milled NiO : Co mixtures, for di!erent % Co content (in brackets, the NiO : Co weight ratio), where t is the optimum milling time as to achieve the maximum H ! .All values before annealing (b.a.) and after annealing (a.a.) at ¹ ,, "600 K are given for the optimum t