Coercivity and squareness enhancement in ball-milled hard magnetic – antiferromagnetic composites

The room-temperature coercivity, HC , and squareness, MR / MS ~remanence/saturation magnetizations!, of permanent magnet, SmCo 5 powders have been enhanced by ball milling with antiferromagnetic NiO ~with Neel temperature, T N 5590 K!. This enhancement is observed in the as-milled state. However, when the milling of SmCo 5 is carried out with an antiferromagnet with T N below room temperature ~e.g., for CoO, TN5290 K!, the coercivity enhancement is only observed at low temperatures after field cooling throughTN . The ferromagnetic-antiferromagnetic exchange coupling induced either by local heating during milling (SmCo51NiO) or field cooling (SmCo51CoO) is shown to be the origin of the HC increase. © 2001 American Institute of Physics. @DOI: 10.1063/1.1392308#

The room-temperature coercivity, H C , and squareness, M R /M S ͑remanence/saturation magnetizations͒, of permanent magnet, SmCo 5 powders have been enhanced by ball milling with antiferromagnetic NiO ͑with Ne ´el temperature, T N ϭ590 K͒.This enhancement is observed in the as-milled state.However, when the milling of SmCo 5 is carried out with an antiferromagnet with T N below room temperature ͑e.g., for CoO, T N ϭ290 K͒, the coercivity enhancement is only observed at low temperatures after field cooling through T N .The ferromagnetic-antiferromagnetic exchange coupling induced either by local heating during milling (SmCo 5 ϩNiO) or field cooling (SmCo 5 ϩCoO) is shown to be the origin of the H C increase.© 2001 American Institute of Physics.͓DOI: 10.1063/1.1392308͔ During the last few decades permanent magnet development has been centered on the production of highly anisotropic materials 1 and nanocomposite magnets consisting of a mixture of exchange coupled hard and soft magnetic components, commonly known as spring magnets. 2 In the latter, a remanence enhancement is induced by the ferromagnetic ͑FM͒-ferromagnetic exchange interaction.However, usually in these systems a reduction of coercivity, H C , cannot be avoided. 2Conversely, an enhancement of H C and a shift of the hysteresis loops along the field axis ͑exchange bias͒ are well known effects of antiferromagnetic ͑AFM͒-FM exchange coupling. 3,4Exchange bias has been extensively studied in thin films, because of its role in spin-valve devices. 5owever, the coercivity enhancement associated with exchange bias has been investigated less. 6In the case of powders, usually a widening of the loop is observed far below room temperature ͑RT͒, either because the Ne ´el temperature, T N , of the AFM is below RT or the AFM grains are so small that they behave superparamagnetically at RT. 3,4,7,8 Furthermore, in powders, the AFM phase is usually obtained by oxidation or sulfuration of the FM ͑e.g., Co-CoO, Fe-FeS or Ni-NiO͒. 3,4,7,8In the case of rare-earth permanent magnets ͑e.g., SmCo 5 or Nd 2 Fe 14 B͒ in general it is not possible to obtain AFM phases by oxidation, since it is mainly selective for rare-earth and the oxides obtained ͑e.g., Sm 2 O 3 or Nd 2 O 3 ) are not antiferromagnetic.However, it has been demonstrated recently that it is possible to induce FM-AFM coupling when FM and AFM powders are milled together. 9,10n this letter we report the enhancement of H C and M R /M S in SmCo 5 due to AFM-FM exchange interactions when ball milled with NiO and CoO.
The milling was carried out for different times ͑0.25-32 h͒ using a planetary mill. 10he microstructure of the as-milled powders was studied by x-ray diffraction ͑XRD͒.XRD patterns were fitted using the Rietveld method from which the crystallite size, ͗D͘, was evaluated for each component.Morphological characterization was performed using a scanning electron microscope ͑SEM͒.Magnetic hysteresis loops of tightly packed isotropic powders were carried out at RT with a maximum field of 0 H max ϭ23 T, by means of an extraction magnetometer.Hysteresis loops after zero-field cooling ͑ZFC͒ and field cooling ( 0 H FC ϭ5 T) of SmCo 5 and SmCo 5 ϩCoO were also carried out at Tϭ30 and 100 K.
For the three systems studied ͑SmCo 5 , SmCo 5 ϩNiO and SmCo 5 ϩCoO͒ the SmCo 5 crystallite size, ͗D͘, is a decreasing function of the milling time, especially during the first 4 h of milling.However, this reduction is somewhat steeper when milling SmCo 5 alone.For long milling times the crystallite size stabilizes to a nanometric range ͑e.g., ͗D͘ ϭ10 nm in SmCo 5 ϩCoO͒, but ͗D͘ remains larger in SmCo 5 ϩCoO and SmCo 5 ϩNiO than in SmCo 5 alone.
SEM micrographs of ball-milled SmCo 5 also reveal a reduction of the particle size and changes in shape with an increase in the milling time, from about 500 m irregular and sharp-edged particles to roughly spherical particles of about 5 m in the 32 h ball-milled SmCo 5 .A different microstructure is encountered in ball-milled SmCo 5 ϩNiO and SmCo 5 ϩCoO.In both cases, in addition to the SmCo 5 particle size reduction, observed in SmCo 5 alone, the SmCo 5 particles in SmCo 5 ϩAFM become progressively surrounded and soldered to NiO or CoO.After 32 h of milling they form aggregates of up to 10 m in size composed of several SmCo 5 particles embedded in a NiO or CoO ''matrix.''Shown in Fig. 1 is the milling time dependence of the coercivity, H C , for the three series of powders, measured at RT. SmCo 5 exhibits typical behavior with milling time, i.e., a sharp increase of H C for short milling times, a maximum in H C ͑ 0 H C ϭ1.1 T after 4 h of milling͒, followed by a gradual decrease of H C for long milling times. 12,13Although a͒ Electronic mail: dolors.baro@uab.esthe behavior of the three systems is similar for short milling times, a maximum value of H C is obtained for SmCo 5 ϩNiO, 0 H C ϭ1.5 T.Moreover, in contrast to what is observed for SmCo 5 alone, the H C for SmCo 5 ϩNiO and SmCo 5 ϩCoO levels off for long milling times.It is also worth noting that even from the early stages of milling an enhancement of H C is observed in ball-milled SmCo 5 ϩNiO in comparison with H C values of ball-milled SmCo 5 and SmCo 5 ϩCoO.
As shown in Fig. 2, the coercivities of SmCo 5 ͑milled 4 h͒ and SmCo 5 ϩCoO ͑milled 32 h͒ are both found to increase at low temperatures.Note that milling times exhibiting maximum RT H C were chosen for each system for the field cooling experiments.However, although the RT H C of both systems is similar, the low temperature coercivity increases further in the SmCo 5 ϩCoO system after field cooling ( 0 H FC ϭ5 T) to below T N than in SmCo 5 alone.Moreover, if SmCo 5 ϩCoO is ZFC to low temperatures, the coercivity obtained ͑ 0 H C ϭ2.02 T at Tϭ100 K͒ is clearly smaller than the one after field cooling ͑ 0 H C ϭ2.19 T at Tϭ100 K͒.Heat treatments above the T N of NiO and subsequent field cooling to RT were also carried out for the SmCo 5 ϩNiO.However, they resulted in a significant reduction of H C .Note that small shifts, H E , of the hysteresis loops in the field axis were often observed for both SmCo 5 and SmCo 5 ϩCoO ͑ 0 H E ϳ0.05 T at RT and 0 H E ϳ0.1 T at Tϭ100 K͒.
The milling time dependence of the squareness, M R /M S , is shown in Fig. 3 for the three systems.It can be seen in Fig. 3 that the squareness of the three systems increases sharply for short milling times.However, the largest M R /M S ratio is obtained for SmCo 5 ϩNiO, with M R /M S ϭ0.98 after 1 h of milling.It should also be noted that M R /M S for SmCo 5 ϩNiO remains high ͑Ͼ0.85͒even for moderate milling times.
For example, the decrease of H C observed in SmCo 5 for long milling times is known to be due to the tendency towards amorphization of SmCo 5 induced by the large amounts of defects introduced after long milling times. 13As evidenced by XRD results, milling is less aggressive to SmCo 5 when it is milled together with CoO or NiO.This could explain why H C remains large for SmCo 5 ϩNiO and SmCo 5 ϩCoO even for long milling times.However, although the microstructure and morphology of SmCo 5 ϩNiO and SmCo 5 ϩCoO are rather similar, the former exhibits a much larger H C , ͑Fig.1͒.Since NiO is antiferromagnetic at RT (T N ϭ590 K) while CoO is paramagnetic (T N ϭ290 K), this allows the separation of morphological-structural effects from magnetic coupling ones.Thus, the enhanced H C should be attributed to the existence of FM-AFM exchange coupling in the SmCo 5 ϩNiO as-milled powders.Usually, to induce such coupling, a field cooling process through T N is required. 4In our case, exchange interactions between the FM and the AFM grains are introduced during milling, without the need of heat treatments.This can be understood because in a planetary mill the temperature can be locally increased in excess of 600 K, due to impacts between the powder and balls. 14The local field created by the SmCo 5 particles plays the role of cooling field during ball-particle impact and induces AFM-FM exchange coupling.Note that it has been demonstrated in thin films that it is the FM moment at the interface rather than the cooling field that controls AFM-FM interface coupling. 15Since T N in CoO is lower than RT, no coupling is induced during milling in SmCo 5 ϩCoO, thus the RT H C remains similar to the maximum H C for ball-milled SmCo 5 alone ( 0 H C ϭ1.1 T).
However, as shown in Fig. 2, when SmCo 5 ϩCoO is field cooled to low temperatures H C increases substantially.Part of this increase is due to the changes in magnetocrystalline anisotropy of SmCo 5 , since a similar increase in H C is ob- served for SmCo 5 alone.Nevertheless, as expected from the FM-AFM coupling, SmCo 5 ϩCoO exhibits extra H C enhancement at low temperatures with respect to single SmCo 5 after the same field cooling procedure.Further proof of the effect of FM-AFM coupling comes from H C in SmCo 5 ϩCoO after ZFC.Although the local field of the SmCo 5 particles can induce AFM-FM coupling to the CoO even after ZFC from a demagnetized state, only those SmCo 5 particles which are single domain will fully contribute to it.In a field cooling experiment ( 0 H FC ϭ5 T) the total magnetic moment of nearly all SmCo 5 particle spins is aligned parallel to the applied field direction, thus all particles contribute to the coupling.Hence, one would expect smaller coupling and consequently reduced H C enhancement after ZFC, as is observed experimentally.
Unfortunately, field cooling SmCo 5 ϩNiO from above T N of NiO does not result in enhancement of H C as would be expected from AFM-FM coupling.This is because of the rapid decrease of H C of SmCo 5 when submitted to moderate annealing temperatures, due to the segregation of softer phases ͑Sm 2 Co 7 and Sm 2 Co 17 ͒. 12,16In other words, the decrease of H C at Tϭ600 K ͑before the field cooling proce-dure͒ is more important than the possible gain due AFM-FM coupling.Note that the local temperature reached during milling can be above the temperature at which soft phases segregate.Nevertheless, the duration of local heating ͑only effective for a few s͒ is exceedingly short to allow diffusion to induce segregation.Hence, the negative effects of the temperature are not observed during milling.
The existence of loop shifts is usually linked to AFM-FM exchange coupling, which strengthens our argument.However, loop shifts have also been observed in SmCo 5 alone, which is usually related to interface spin-glass states due to milling induced surface disorder. 13lthough the Stoner-Wolfarth model 17 for isotropic, single domain, noninteracting particles predicts a squareness of M R /M S ϭ0.5, small particle hard magnets are known to usually exhibit rather large squareness, 18,19 M R /M S ϳ0.8, similar to the values for SmCo 5 shown in Fig. 3.These high M R /M S values are due to short-range exchange interactions among SmCo 5 particles. 20Thus, isolating the SmCo 5 particles should result in a reduction of M R /M S , as observed for SmCo 5 ϩCoO after short milling times.Since CoO is paramagnetic at RT, essentially its role is simply to separate the SmCo 5 particles.The crossover at moderate milling times between the M R /M S of SmCo 5 ϩCoO and SmCo 5 alone is probably due to the more aggressive effects of milling on SmCo 5 alone.Contrary to what is observed in ball-milled SmCo 5 ϩCoO, in SmCo 5 ϩNiO, even higher M R /M S values are obtained in comparison with ball-milled SmCo 5 .Hence, the presence of the AFM NiO phase surrounding SmCo 5 seems to play an important role in further enhancing M R /M S .Despite the fact that M R /M S enhancement has also been observed in other AFM-FM systems, 10,21 its origin, although clearly related to AFM-FM interaction, is not well understood.
Finally, note that, although the effects described appear to be clearly linked to AFM-FM exchange interactions, some effects from the differences in microstructure and surface disorder cannot be completely ruled out.
In conclusion, we have shown that the coercivity and squareness of permanent magnet powders ͑e.g., SmCo 5 ͒ can be enhanced after milling them with an antiferromagnet.To obtain these enhancements at RT and above it is necessary to induce exchange coupling between the permanent magnet and an antiferromagnet with T N ϾRT.Hence, this study opens up new possibilities for improvement of permanent magnet's magnetic properties.