Effect of exchange anisotropy on the hysteresis behavior of Co particles

NANoSTRUCTURED MATERIALS VOL. 1, PP. 449-456, 1992 COPYRIGHT ©1993 PERGAMONPRESS LTD. ALL RIGHTSRESERVED

0965-9773/92 $5.00 + .00 PRINTED IN THE USA

EFFECT OF EXCHANGE ANISOTROPY ON THE HYSTERESIS BEHAVIOR OF Co PARTICLES

S. Gangopadhyay(a), G.C. Hadjipanayis(a), C.M. Sorensenfo) and K j . Klabunde(c) (a)Department of Physics and Astronomy, University of Delaware, Newark, DE 19716. (b)Department of Physics, (c)Department of Chemistry, Kansas State University, Manhattan, KS 66506.

(Accepted December 1992) Abstract--Fine particles of fcc Co have been prepared in the size range of 50-350 f~ by using the vapor deposition technique. The dependence of coercivity on particle size and temperature has been studied. The effect of surface passivation on the coercivity and its temperature dependence has also been investigated. A maximum coercivity of 1650 Oe and a saturation magnetization of 125 emu/g was obtained for a particle size of 275,4. A very strong exchange anisotropy (shift -10.7 kOe) at cryogenic temperatures was found due to the core-shell morphology of the particles. The effect of particle size and temperature has been studied on the exchange anisotropy and on the hysteresis behavior of the particles. INTRODUCTION Research in the field of fine magnetic particles has been very active in the last several decades due to their potential applications in many areas of technology including high density magnetic recording media (1,2). Their small size (10 to few 100 A) not only makes them a better recording media due to their superior magnetic properties, but also opens up an interesting scientific problem on finite size effects. The magnetization (3A), coercivity (5), and magnetic transition temperatures (6) are influenced by the size of the particles. Enhancement of coercivity, particularly at cryogenic temperatures, has been observed in fine Fe particles (7,8). This enhancement is found to be dependent on the size and the surface properties of the particles, and it is attributed to the large magnetic anisotropies observed in these particles. Exchange anisotropy was first discovered by Meiklejohn and Bean (9) in compacted oxidecoated Co particles. Such exchange interaction has been found in antiferro-ferromagnetic, ferriferromagnetic, and ferri-antiferromagneticaUy coupled systems (10-12). The unidirectional anisotropy gives rise to shifted hysteresis loops when the sample is cooled in a field (in addition to sin0 torque curves and rotational hysteresis at even very high fields). The energy due to this anisotropy is given by the expression, E = -K. cos O, 449

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where 0 is the angle between the easy axis and the magnetization, and Ku is the unidirectional anisotropy constant. In this paper we will report on the influence of particle size, surface oxidation and temperature on the exchange interaction between the ferromagnetic Co-core and its antiferromagnetic oxide coating. A correlation between the large coercivities observed and the exchange anisotropy will be shown. EXPERIMENTAL METHODS Co particles were prepared by the vapor deposition technique (13). Base pressure of 10-510.6 ton" was acquired prior to the evaporation of the metal in an inert gas (argon) atmosphere. By varying the argon pressure from 1 to 30 ton",particles in the size range of 50-350 A were obtained. Particles were collected on a water-cooled Cu plate above the tungsten crucible. Before the particles were exposed to atmosphere, they were passivated with an argon-air mixture for few hours to obtain the "core-shell" structure. Particles without an oxide coating were prepared by first evaporating Co particles onto a water-cooled Kapton substrate and then a thin film (N200 A)ofAg on their surface to protect them from further oxidation. The structure and morphology of the samples were studied by X-ray diffraction, selected area diffraction (SAD) and transmission electron microscopy (TEM). Magnetic studies were performed using a SQUID magnetometer in the 10-300 K temperature range and with a maximum field of 55 kOe.

Figure 1. Bright field transmission electron micrograph of passivated Co particles, of median diameter 130 A.

EFFECT OF EXCHANGEANISOTROPYON Co PARTICLES

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MAGNETIC AND STRUCTURAL RESULTS The particles obtained had a face-centered-cubic structure. The shape of the particles was nearly spherical, as shown in Figure 1. The deviation from sphericity became higher as the particle size increased. The diffraction lines due to the oxides were broad and faint, indicative of a small crystallite size. CoO was always present in addition to Co304. The saturation magnetization of the particles increased with particle size (14), reaching a maximum of 125 emu/g for a particle size of 275 A. Particles below 100 A had a very low magnetization (10-20 emu/g) due to higher oxidation and surface canting of moments (3). Smaller particles had very large coercivities (6-7 kOe) at cryogenic temperatures (Figure 2). As the smaller particle samples (< 100 A) aged, the coercivity at lower temperatures decreased. This is attributed to the higher anisotropy caused by the thicker oxide shell, which results in nonsaturation of the moments even at fields as high as 55 kOe. The temperature dependence of coercivity for particles of different sizes is shown in Figure 3. At room temperature the coercivity increased with particle size, reaching a maximum of 1650 Oe for a particle size of 275/k and then decreased slightly with further particle size increase. Particles below 70 A were superparamagnetic at room temperature. The drop in coercivity at low temperatures for the 70 and 100 A particles is due to non-saturation effects. In an attempt to distinguish between the dependence of coercivity on particle size and surface oxidation, Co particles of different sizes with minimal surface oxidation were prepared. The temperature dependence of coercivity of Ag coated Co particles was measured, and the results are shown in Figure 4. The room temperature coercivities were not drastically affected, but the

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coercivities at cryogenic temperatures were much lowered. Both at 10 and 300 K the coercivity increased with particle size, until a size o f - 130/k was reached, beyond which the coercivity started decreasing again, signifying the onset of a multi-domain behavior. To prove the existence of exchange anisotropy between the antiferromagnetic Co-oxide shell and the ferromagnetic Co core, the sample was field-cooled (FC) in an applied field of 20 kOe from room temperature to 10 K where its hysteresis loop was measured. The loop was found to be shifted from the origin and with respect to the corresponding zero-field-cooled (ZFC) loop (shown in Figure 5). The extent of this shift was found to be proportional to the particle size. The shift defined as H FC- HZFC(indicated in Figure 5) is a measure of the unidirectional anisotropy induced through exchange coupling between the core and the shell moments. Shifts as large as 10.7 kOe were observed in particles of 80/k. The existence of unidirectional exchange anisotropy Ku, in the material has been shown (9) to cause an increase in the coercivity of the material by an amount Ku/Ms. The magnitude of Ku for different particle sizes was calculated and is shown in Figure 6. For a sample with 100/k particle size, Ku exhibited a maximum (2.1 x 106 ergs/cc). The temperature domain of this anisotropy was investigated. Samples were FC from 300 to 10 K and their loops were measured at various temperatures through subsequent warming to 300 K. The FC coercivity at these temperatures was compared with that of the ZFC coercivity and the shift was determined. Figure 7 shows the shift as a function of temperature for various particle sizes. Regardless of the particle size the shift always disappeared at 150 K. This behavior is different from the results of Meiklejohn and Bean (9).

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DISCUSSION The high coercivities observed in the smaller particle samples (Figure 3) disappeared as the sample was covered with Ag film (Figure 4). This indicates that the larger coercivities associated with small passivated particles at cryogenic temperatures are due to the presence of the Co-oxide coating. Passivated Co particles with a size of 80 A are superparamagnetic at room temperature whereas an 80 ,h, Co/Ag sample is not. This is because the oxide-shell thickness is ~ 2 0 A (observed

by HRTEM) and this leaves only a 40 A Co panicle in the core which is expected to be superparamagnetic at room temperature. The maximum observed in Ku as a function of particle size (Figure 6) could be explained by the following arguments. When the particles are very small, a major fraction of them is completely oxidized due to their small size, and therefore the density of antiferro-ferromagnetic interfaces which cause the unidirectional anisotropy is much reduced. As the particle size approaches 100 A, a large number of particles have the core/shell morphology,

and therefore a large volume fraction of antiferro- to ferromagnetic phase, resulting in a maximum exchange anisotropy. Any further increase in particle size would cause a decrease in the volume fraction o f antiferromagnetic phase and hence in the areal density of antiferro-ferromagnetic interfaces leading to a reduced unidirectional anisotropy.

The significance of Figure 7 is the disappearance of the shift at about 150 K in all the samples irrespective of their particle size. This indicates the loss of exchange interaction, which could result from any of the following conditions: a) the Co-oxide surface coating loses its magnetic order (by reaching its N6el temperature); b) the Co-oxide becomes superparamagnetic due to its fine crystallite size and therefore magnetically soft; c) the magnetocrystalline anisotropy o f Co-oxide

decreases rapidly at high temperatures; (d) the Co-oxide is off stoichiometry (a mixture of C0304 and C o O ) (15). i

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t e m p e r a t u r e (K) Figure 7. Temperature dependence of shift for samples of different panicle size. The arrow near 150 K indicates the disappearance of the shift. In order to check for the first possibility, fine panicles of CoO and Co304 were prepared and their N6el temperature was measured for different panicle sizes. A 60 A CoO powder had a N6el temperature of 270 K (only 20 K less than the bulk CoO) and a 80 A Co304 powder had a N6el temperature of 30 K (only 10 K lower than the bulk). The size of the small crystallites of Cooxide found in the surface layer of our Co panicles is about 20 A. It would be hard to imagine that the effect of finite size could reduce the N6el temperature of CoO particles from 270 K to 150 K when the size is decreased from 80 to 20 A. Thus, the first possibility is ruled out. An estimate of the Blocking temperature of a 20 A spherical CoO particle with a magnetocrystalline anisotropy of 5 x 106 ergs/cc (1) is 150 K, which is the temperature where the shift disappears in all of our samples. Thus, the superparamagnetism of the oxide shell can explain the loss of exchange interaction in passivated Co panicles. A fast decrease in the crystal anisotropy (condition (c)) could also result in the loss of shifted loop, as has beenobserved previously in Ag-Mn and Cu-Mn alloys (16) and still needs to be investigated in the present system. We also plan to examine condition (d) through XPS measurements to determine the stoichiometry of Co-oxide. CONCLUSIONS Optimum values of saturation magnetization (125 emu/g) and coercivity (1650 Oe) were obtained in passivated fcc Co particles of diameter 275 A. The hysteresis loop shift, which is proportional to the interface exchange interaction, is found to depend on panicle size, which is consistent with the exchange anisotropy model. A comparable volume ratio of antiferro- to ferromagnetic phases and a high density of interfaces are required to produce large exchange interaction. The presence of large unidirectional exchange anisotropy confirmed the core-shell structure in Co-Co-oxide particles. The large coercivities (about 6 kOe) observed at cryogenic temperatures can be explained in pan by the existence of the interface exchange anisotropy.

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ACKNOWLEDGEMENT This work was supported by NSF CHE-9013930.

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