Magnetic studies of oxidation characteristics of fine particle LaNi5

Journal of the Less-Common

Metals,

79 (1981)

297 - 309

297

MAGNETIC STUDIES OF OXIDATION CHARACTERISTICS PARTICLE LaNis

F. T. PARKER

Department (U.S.A.)

OF FINE

and H. OESTERREICHER

of Chemistry,

University

of California,

San Diego, La Jolla, CA 92093

(Received January 7,198l)

Summary Magnetic properties of fine particles of rare earth-transition metal compounds (in particular LaNis) were studied with respect to oxidation at various temperatures. The relevance of oxidation processes to permanent magnet technology and applications of hydrogen storage is indicated. For pulverized and dehydrided LaNis a nearly linear decrease in magnetic moment was observed between 77 K and room temperature. The characteristics of superparamagnetism are apparent. The stability of fine particle LaNis was found to be fairly good in air but poor in hardened epoxy resin. The parabolic oxidation rate characteristics were determined for LaNis heated in oxygen.

1. Introduction Several compounds in the CaCus family have achieved commercial interest. Two based on the rare earth (R) family are SmCos for permanent magnet materials and LaNis for hydrogen storage applications. In both cases the materials are, at some stage of their preparation, in the form of small particles with a typical size of about 5 pm. Because of the strongly electropositive nature of the rare earth, oxidation is a serious technical problem. Not only are the relevant properties reduced because of volume losses to the decomposed material but surface characteristics are crucial to the performance of both SmCo, and LaNis. In SmCos, surface oxidation can strongly reduce the magnetic coercivity of the material; in LaNis, activation to achieve admittance of the hydrogen gas can be altered. Thus studies of the oxidation processes are necessary for a better understanding of these technologically important materials. Surface studies on LaNis have shown decomposition driven by oxygen. X-ray photoelectron spectroscopy [l] of an air-exposed sample indicates that lanthanum on the surface is in the form of La(OH), and that nickel is 0022-5088/81/0000-0000/$02.50

@ Elsevier Sequoia/Printed

in The Netherlands

298

about 90% in the metallic form. Auger electron spectroscopy [2] on singlecrystal LaNir, cleaved in ultrahigh vacuum and then exposed to oxygen indicates that the surface is depleted in nickel, is essentially constant in lanthanum and is of course oxygen rich. This suggests a covering layer rich in LazOs and a subsurface layer of nickel metallic precipitates. Magnetization studies at room temperature [l] show a steady increase in magnetic moment with the number of hydriding cycles. Since the bulk LaNi, is Pauli paramagnetic, this indicates that additional nickel is forming with each cycle. Bartlett and Jorgensen [ 31 have extensively examined the oxidation properties of the commercial magnet material SmCoB above room temperature. They found that SmCor, oxidizes to form two layers: (1) a thin porous outer scale composed of cobalt oxide and SmzOs; (2) a tight inner subscale consisting of parallel Sm,Os platelets surrounded by cobalt metal. The dominant subscale is produced by internal oxidation of samarium, with the oxidation kinetics controlled by diffusion of oxygen along the interface between the SmzOs and cobalt in the subscale at a rate several orders of magnitude greater than for the rate of oxygen diffusion in Sm,Os or cobalt. There is no substantial difference in the Sm:Co concentration ratio in the subscale after oxidation, indicating no counterdiffusion of the reactive rare earth constituent. In the limit of oxidation thickness 6 much less than particle size, we have a planar diffusion-limited kinetics problem with the parabolic solution 6 = h, tllz. In this paper we present a study of the magnetic properties of some small particle RN&, materials after exposure to oxidizing environments for long periods of time at room temperature and short periods at elevated temperatures. As has been shown elsewhere [ 41, dehydrided LaNir, exhibits nearly stabilized magnetic properties after about a day in air. The magnetic characteristics of this quasi-stable state are examined here. Also examined are the stabilities of small particle RNir, in a commercial epoxy resin. The analog permanent magnet material SmCo, is often sealed in organic binders such as epoxy resins [ 51. Also, anisotropy measurements are sometimes made on magnetically aligned powders sealed in such resins. In both cases the moment of the original material obscures the magnetic properties of any decomposition products, and the analog RNis compounds are good substitutes for studying the oxidation properties. 2. Experimental details Some materials were prepared by induction melting 99.9% rare earth metals with 99.9% pure nickel metal on a water-cooled copper boat in an argon atmosphere. Except for SmNi,, these were single-phase materials as evidenced by X-ray diffraction with Cu Ko radiation. SmNis was annealed in tantalum foil under argon for 8 days at 750 “C. Most samples of LaNi, came directly from a commercial supplier (Hy-Stor). Since some rejected pieces were slightly magnetic (up to about 1% Ni), one portion was remelted (hereafter denoted by P3 and P4).

299

Most samples were pulverized in an agate ball mill under hexane for 10 min. They were then either stored in air or potted in an ambient-curing epoxy resin (CIBA 506) which appeared stable to 200 “C after hardening. Some samples of dehydrided LaNiB (1 or 2 cycles) were also studied for comparison. Magnetization measurements were obtained with a vibrating-sample magnetometer. Generally, we shall quote a percentage conversion value, obtained as the ratio of the observed moment at 77 K in a 10 kOe field to the value which would occur if all the nickel were converted to the bulk elemental form. The slight decrease in the moment of bulk nickel to room temperature is ignored. Subtraction of the bulk paramagnetism will be performed unless stated otherwise .

3. Structural results X-ray diffractometer patterns indicated that the pulverized LaNi, samples were of a single-phase CaCu, structure. Line broadening independent of (hkl) was observed. In the isostructural CeCo, a similar isotropic strain broadening was shown to correlate well with particle size [6] . Assuming that the same mechanism is valid here, we can estimate an average particle diameter of 4 ,um from a typical broadening b, of 0.42” for the (301) peak (P5). Other (hkf) reflections and other samples gave similar results. After heating the pulverized samples of LaNi, in oxygen or epoxy resin, reflections from impurity phases were absent in the X-ray diffraction patterns. For example, a sample (P5) was heated in 1 atm O2 for 2 h at 160 “C. The magnetic moment at 77 K was 11% of the maximum possible if all nickel were in the elemental form. The X-ray peak intensity of any second phase was less than 1% of the dominant (111) diffraction peak of the CaCu, structure. Careful scans of the areas corresponding to the (111) peak of nickel, the (101) and (102) peaks of LasOs and the (110) peak of LaNiOs showed that the relative X-ray intensity of each of these possible phases was below 0.3%. Similarly, another sample heated in hardened epoxy resin (Pl) for 7 h at 140 “C exhibited 15% magnetic conversion at 77 K, and the upper limits on the nickel and LazOs X-ray peak intensities were about 1%. 4. Ma~etiza~on

results

The bulk remelted LaNis exhibited Pauli paramagnetic behavior, with a magnetization in a 10 kOe field of 23 emu mol-l at 77 K and 21 emu mol-’ at room temperature. This corresponds to an effective conversion of about 0.1% and is unimportant except for the freshly pulverized samples. All pulverized and dehydrided materials exhibited similar initial M uersus 2’ characteristics. A representative sample (P3) is shown in Fig. 1, corrected for bulk pammagnetism. In applied fields greater than a few kilooersteds,

300

000

T

P E

2 P

000

ot/

I

’

200

100 TiKl

_

I

I

300

100

I

200

3

3

T(K)

Fig. 1. Magnetization us. temperature for a pulverized LaNi5 sample (P3), corrected for bulk paramagnetism. The data were taken on slow warming of the sample from 77 K in applied fields of 10, 7.2, 3 and 0.2 kOe from upper to lower curves. As with the other figures, the magnetization is expressed as the percentage of the total elemental nickel moment possible at 0 K. Fig. 2. Magnetization us. temperature in H, = 7.2 kOe for pulverized .LaNis (P3) after heating in 1 atm 02 for 0.5 h at the temperature in degrees Celsius designated (ap, as pulverized). Each curve corresponds to a separate portion of the sample.

magnetization for all samples decreased nearly linearly with temperature (except for samples with strongly paramagnetic rare earths contributing to the magnetization). All samples appeared to have a kink in M uersus 2’ near T, = 200 K in applied fields of 0.2 kOe, a feature similar to the inflection point in simple ferromagnets. The observed magnetization at 77 K in Fig. 1 corresponds to about 1.6% of the possible nickel moment for the sample shown. Other pulverized LaNi, samples were typically between 0.8% and 1.2% at 77 K; a dehydrided sample was about 0.4% at 77 K. Straight-line extrapolation of the Fig. 1 data to 0 K indicates a conversion of about 2%. These 77 K results are only qualitative because of the temperature and field dependences of the observed magnetization. The larger field M uersus T data in Fig. 1 suggest that, even after correction for bulk paramagnetism, the pulverized material exhibited Pauli paramagnetism. The bulk-corrected differential susceptibility for P3 at room temperature was about equal to that of the bulk material (the uncorrected

301

..

T(K)

.

Fig. 3. Magnetization vs. applied field for LaNi6 at 295 K: curve a, sample P3 heated for 0.5 h in 1 atm Oz at 161 “C; curve b, uncorrected data for as-pulverized P3; curve c, curve b corrected for bulk paramagnetiim; curve d, bulk paramagnetism. Fig. 4. Magnetization vs. temperature in H, = 7.2 kOe for pulverized La& (P2) heated under 1 atm 02 at 120 “C for the times in hours shown on the curves. Each curve corresponds to a separate sample of P2.

susceptibility was twice that of the bulk). Considering the different samples, the bulk-corrected differential susceptibility at room temperature was approximately proportional to the conversion measured at 77 K. The dehydrided (2 cycles) sample had essentially no excess susceptibility at room temperature, and the other pulverized samples exhibited 50% - 75% of the excess susceptibility of P3. From Fig. 1 a shallow peak in the differential susceptibility associated with !I’, (H) is seen near 215 K, with a magnitude about 50% greater than for other temperatures. Figure 2 exhibits the M uersus T curves (Ha = 7.2 kOe) for pulverized LaNis (P3) heated in 1 atm 0s for 0.5 h at various temperatures. Partial decomposition occurred below 100 “C. The ratio M(295 K)/M(77 K) increased steadily with further oxidation, although it never reached the 0.95 value for elemental nickel. The field dependence of M at 295 K is shown in Fig. 3 for pulverized LaNia (P3). Also shown is the curve for the sample heated at 161 “C under oxygen (scaled to 0.1 of the original). Even at room temperature the magnetization appears to saturate quite readily. Generally, we were unable to determine a difference in magnetization for fields between 7 and 10 kOe below room temperature for the heated samples. It should be noted that, at least for the heated sample shown in Fig. 3, a demagnetizing field of

302

about 2 kOe is evident, the same as for spherical particles of elemental nickel. For longer periods of time in the oxygen atmosphere, the oxidation increased, as shown for pulverized LaNi, (P2) at 1 atm Oz at 120 “C in Fig. 4. Again the ratio M(295 K)/M(77 K) increased steadily with oxidation. From the relative magnetization of P2 and P3 heated in oxygen at 120 “C for 0.5 h, we can estimate the surface area of P2 to be about 70% of that of P3 per unit volume, and thus the inverse particle radii should also have the same ratio. Similar tests on Pl, P4 and P5 gave estimates of 50% - 60% of the surface area of P3, suggesting that the average particle diameter for P3 is about 2 E.tm. The effect of the oxygen pressure dependence on the reaction rate was also examined. Portions of P4 were treated for 0.5 h at 120 “C under 1 atm Oz and 0.22 atm Oz. The percentage conversion at 77 K was 2.7% for the samples treated under 1 atm Oz and 2.6% for the samples treated under 0.22 atm Oz, with the initial material 1% converted. A sample of dehydrided (2 cycles) LaNi, was heated in 1 atm Oz at 160 “C for 0.5 h. The conversion at 77 K was 2.4%; before heating, the conversion was 0.7%. (This material had been in air for several months and exhibited some further oxidation relative to the conversion given earlier in this work.) The sample of P3 similarly heated exhibited 12.9% conversion. If the difference in rates arises from a smaller surface area for the dehydrided sample, this implies that the radii of the dehydrided particles are about five times the P3 average (ignoring the original conversion, as will be shown to be reasonable later). If we use the published distribution (1 cycle) of particle sizes [ 71, we find an average diameter of about 11 E.trn which, at five times the estimated size of P3 particles found earlier, is in good agreement. A dehydrided (1 cycle) sample heated in 1 atm Oz at 160 “C for 3 h exhibited 4.7% conversion at 77 K, in good agreement with the data above if the parabolic law is employed.

5. Stability in epoxy resin Since ail the samples studied were sealed in epoxy resin, the stability of the pulverized LaNi, samples in liquid resin was considered. Therefore samples (Pl) were encapsulated both in the ambient temperature resin and in a plastic screw fitting. Both exhibited 1.2% conversion at 77 K. Long-term stability in the epoxy resin was poor, even at room temperature. Little change was noted in the conversion of the P3 samples after 10 days. However, 4 months later the results presented in Table 1 were obtained. Although the lesser-oxidized materials might be expected to decompose proportionately more, the relatively large decomposition of the pulverized-only sample and the lack of further decomposition in the heavily oxidized samples seem unusual.

303 TABLE

1

Percentage conversion at 77 K of pulverized La&, temperature in hardened epoxy resin Sample

Amount converted original week (k)

Pulverized only 99 “C 119 “C 139 “C 161 “C

1.6 2.9 4.9 8.6 12.9

after

(P3) stored at room

Amount converted after im elapsed time of 4 months (%) 6.5 5.8 6.2 8.6 12.9

The sample heading denotes the original treatment in gaseous oxygen (see Fig. 2).

Fig. 5. Uncorrected magnetization us. temperature in H, = 7.2 kOe for pulverized La&, (Pl) sealed in hardentq epoxy resin. Above room temperature the temperature increase was about 0.5 “C min (curve a) until the maximum temperature was obtained. The sample was then cooled slowly towards room temperature and rapidly to 77 K; then a second cycle (curve b) was begun.

Other pulverized samples showed a decomposition dependence roughly proportional to the relative surface area as determined from the oxidation results. For example, P4 (pulverized only) initially had a 1.0% conversion at 77 K, and after 3.5 months in epoxy resin a 3.5% conversion. As an interesting comparison a sample of P4 stored in air for 3.5 months exhibited little change, with 1.2% conversion at 77 K. The dehydrided (2 cycles) material exhibited 0.7% conversion when initially sealed and 1.0% conversion at 77 K after 6 months in epoxy resin. In air the original material exhibited 0.4% conversion (in air about 12 h before measurement), 0.7% conversion after 4 days and 0.8% conversion after 5 months. The relative ease of decomposition of powdered LaNis in hardened epoxy resin at room temperature suggests that even slight warming of the samples could provoke a rapid decomposition. This can be seen in Fig. 5

304 TABLE 2 Effective decomposition temperatures Td defined as the temperature where M increases to 1.05 times the minimum m~etization in 7.2 kOe

Sample

Td WI

Encapsulation

Formation

LaNie (Pl) LaNis (Pl) LaNib SrnNib

342 357 331 345 374 351

EPOXY Plastic, screw Epoxy Epoxy Epoxy Epoxy

P

%.75Gdo,5Nk SmNia

P D P P P

All samples were pulverized (P) for 10 min, except for a dehydrided (D) (2 cycles) LaNiesample.

where pulverized LaNi, (Pl) in hardened epoxy resin was warmed at about 0.5 “C min-’ above room temperature. A sudden surge in magnetic moment is noted at about 350 K. The sample was then cooled slowly towards room temperature, cooled rapidly to 77 K and then put through another cycle. Another sharp increase in moment is seen at about 350 K. This sample was then heated at 140 “C for various elapsed times. After 1 h the conversion at 77 K had jumped to about 11%. Further conversion was nearly linear with time, reaching 15% after another 6 h at 140 “C. As noted previously, no impurity peaks were then seen in the X-ray diffraction pattern. The dehydrided (1 cycle) LaNis exhibited 9% conversion at 77 K after 6 h at 140 “C in hardened epoxy resin. Several materials were examined for the onset temperature of the surge in moment. As a qualitative measure, Td was defined as the temperature at which the moment in a 7.2 kOe applied field increased to 1.05 times the minimum value. This depends on factors such as particle size, base magnetization and the presence of Curie-Weiss moments (e.g. gadolinium) but gives a good estimate of the rapid decomposition onset. The results are shown in Table 2. The LaNi5 sample with an air atmosphere (plastic screw holder) is seen to exhibit the moment surge at a slightly higher temperature than the epoxy-resin-encapsulated material. Little increased stability is seen with the more oxidation-resistant heavier rare earths in the RN& structure. The lowest Td occurs for the material dehydrided for 2 cycles. This materM was sealed quite soon after exposure to air and exhibited only 0.4% conversion at 77 K. If the decomposition products for the gadolinium-based RNi5 series contained gadolinium magnetically coupled to nickel, we might expect evidence of moment compensation with the typical antiparallel arrangement of gadolinium and transition metal moments. However, none of the materials at different decomposition stages exhibited even partial compensation, as determined from M uersus 2’ in a 1 kOe applied field.

305 TABLE 3 Kink temperature T, in M versus T curves for various applied fields in pulverized LaNie (P3)

0.2 0.9 3.0 7.2 10.0

185 200 230 245 260

6. Discussion The data presented give evidence about the details of decomposition of the rare earth-transition metal compounds after diminution. There is substantial demonstration that the M uersus T curves characteristic of the initially pulverized material are not representative of magnetic ordering in a particular compound but arise from superparamagnetic clustering of the nickel particles. The shape of the curves and the previously mentioned kink temperature T, vary somewhat between samples. There are no known [8] magnetic critical temperatures in the LaNi binary series as high as the typical T, of 200 K, although presumably there could exist an oxygenstabilized compound. The heavier RsNi,, compounds have Curie temperatures approaching 200 K, so presumably the lanthanum analog might be responsible for the initial magnetic properties if it formed during low temperature oxidation. Other evidence against magnetic compound formation includes the observation that T, increases steadily with further decomposition (see Fig. 5). In the as-pulverized material, T, varies markedly with applied field. This can be seen in Table 3, with the P3 data showing an overall shift in T, of 75 K with applied fields of 0.2 - 10 kOe. For comparison we generated M uersus T curves on the molecular field model for a simple ferromagnet (g = 2; S = * ), with T, = 200 K. The shift in T, with field is only about 6 K from H,=OtoH,=lOkOe. The data thus suggest a predominately superparamagnetic nickel moment in the as-pulverized materials. Unfortunately, a wide range of nickel cluster sizes were presumably present in these materials. The high temperature data in a 0.2 kOe field (Fig. 1) suggest some large clusters yielding little variation in M with temperature, as well as clusters with a pronounced change in M above 77 K. Thus we shall try to model the superparamagnetic region so as to reproduce some of the properties observed in the nearly featureless curves seen in Fig. 1. The equation describing superparamagnetic behavior will be employed in the normalized form

306

O.! b

I.! -ii 3i I.C

O.!

C (b)

, 100

I

TlKl

IO

100

Fig. 6, (a) Reduced cluster moment u us. temperature for superparamagnetic clusters of only one size with a cluster moment & = 76~~ and intercluster interaction h = 120 kOe (the different curves correspond to applied fields of 10, 7.2, 3 and 0.2 kOe from upper to lower curves (demagnetizing effects are ignored)); (b) magnetization us. temperature for a dist~bution of predomin~tly small cluster sizes as described in the text with H, = 3 kOe and an intercluster interaction A = 730 kOe (-, self-consistently determined moment for these clusters, normalized to the data (‘(r ) taken arbitrarily from the corresponding curve in Fig. 1).

u = L(a) =

coth a - f

a

where

u is the reduced m~etization, L(a) is the Langevin function, perthe nickel cluster moment (assumed to be independent of temperature), H,, the applied field and X the intercluster exchange coupling constant. In weak fields H, this leads to an apparent Curie temperature T: given by 3kT, = &=h, or nX = 44.71‘, (kOe) where n is the number of Bohr magnetone per cluster. With the experimental T, at about 200 K, the product nX is fixed. To satisfy approximately the spread in T, with variation in applied field H,, n must be about 75, as can be seen in Fig. 6. Curves generated with smaller cluster sizes exhibit less of a spread near T,. The nearly constant separation of the various curves in Fig. 1 is not reproduced well with this model of only one cluster size. An alternative is to ignore the slight variation in bulk-corrected differential susceptibility with temperature and to assume that this susceptibility

307

arises mainly from increased Pauli paramagnetism as suggested earlier. The temperature dependence of it4 then arises mainly from small clusters. We arbitrarily chose the following model. Cluster sizes are 4/1,, @a, . . . ,2Oy a and lOOpa, the intercluster interaction h is 730 kOe, ai =

L+ {Ha

+ X(L(Ui))}

(determined self-consistently)

and

(thus the total moment at 0 K is the same for each cluster). The normalized results are compared with representative data points from P3 in Fig. 6 for H, = 3 kOe. The fit is quite good. For H, = 10 kOe there is only a slight difference in the curves at higher temperatures. The small increase in differential susceptibility near 200 K is not generated in this model, however. There is another determination of the magnetic characteristics of small particle LaNi, which was obtained by repeated hydriding cycles [ 91. The magnetization curves at room temperature gave a cluster moment of about 36OOpa. For a cluster of this size there would be less than 10% change in magnetization in a 10 kOe field between 77 K and room temperature (h = 0); this does not agree with the present data. Ultrathin nickel films exhibit similar quasi-linear M uersus T curves below room temperature [lo] . For a nominal film thickness of 14 A the moment is about 50% of the 0 K value at 300 K. The thin film tends to break up into superparamagnetic islands. However, T, and the moment at 0 K are comparable with those of bulk nickel. The assumption of constant moment per nickel ion at 0 K independent of the fractional superparamagnetic content was implicitly assumed in this work. The parabolic oxidation law can be shown to hold for the data in Fig. 4 (P2), as shown in Fig. 7. Assuming that the original as-formed conversion is a,,, then after a time t at temperature T the conversion 6 is given by d = (S2 - &,e2)l12= k, t”‘. There is a relatively large uncertainty in 6e, because of the field and temperature dependences of the magnetization at 77 K. However, because of the form of the expression for d, this quantity is insensitive to large deviations in 6c. In Fig. 8 we show the rates h,(T) as determined from the conversion percentage at 77 K in Fig. 2 (P3) for heating in 1 atm O2 at various temperatures for 0.5 h. These are determined through k, = ((6 2 - &e2)/t} 1’2. As discussed above, 6,, is relatively unimportant in determining k, . Also shown in Fig. 8 are the extrapolated rates based on the Bartlett and Jorgensen data for SmCo,, assuming a particle diameter of 2 pm. Especially at low temperatures, LaNi, seems to oxidize much more readily than the extrapolations from SmCor,. The activation energy for k, is 8.8 kcal mol-l; for SmCo5 the rate below 1000 K is 14 kcaI mol- ‘. PrCos is reported to show the same oxidation behavior as SmCos .

308

20 .

E - IO .

0

1/1

?O

4

tl/2;h,l/2)

I OOQ’T

[K-’ 1

Fig. 7. Magnetic conversion d = (S2 - 602)1’2 vs. t112 (where t is the time in 1 atm 02 at 120 “C) for pulverized LaNiz (P2) as determined from the moments at 77 K (see Fig. 4). Fig. 8. Arrhenius plot for the magnetic conversion rate k, = ((6 2 - 6 o2)/t }I’ vs. 1000/T (where T is the temperature at which the sample was heated in 1 atm 02 for 0.5 h) for pulverized LaNia (P3) as determined from the moments at 77 K (see Fig. 2). The lower curve corresponds to the extrapolated values for SmCos (r = 1 pm [ 31).

Part of the enhanced rates in LaNi, could be due to the lattice disruption evidenced by the X-ray diffraction line broadening. However, the dehydrided materials oxidize similarly to the pulverized materials when adjusted for particle size differences. Dehydrided LaNi, exhibits only (MO) line broadening [ 111. Actually, the rates for LaNie seen in Fig. 8 are lower limits. In one scanning electron micrograph, Barrett shows a scale thickness of 40 pm with the subscale thickness about three times greater. The scale is composed of cobalt oxide and Sm20s; in LaNi, we would expect La20s and NiO. NiO is antiferromagnetic below room temperature and therefore exhibits no moment. (Part of the enhanced moment seen with the samples sealed in epoxy resin may be due to the suppression of the formation of NiO because of the lower effective oxygen pressure.) Additionally, we would expect some saturation effects on the oxidation process [ 121. Assuming a uniform particle size, we can estimate that E, would increase by about 5% after correction for saturation. Incidentally, similar corrections on Fig. 7 would essentially result in a small change in the slope of d versus t ‘j2. 7. Conclusion A study of some RNi, compounds in the form of smaIl particles showed that they tend to decompose near room temperature. The original

309

pulverized and dehydrided materials exhibited similar magnetization uersus temperature curves, the moment decreasing nearly linearly from 77 K to room temperature. A superparamagnetic model can account for most of the features of this temperature dependence. The stability of the fine particle LaNis was relatively good in air and poor in hardened epoxy resin. The latter phenomenon has special relevance to the pulverized permanent magnet material SmCo, sealed in organic binders. No X-ray diffraction evidence of decomposition products was seen. By heating pulverized LaNis in oxygen, the activation energy for the parabolic rate constant k, was determined to be about 9 kcal mole1 , substantially less than a previous result in the analog SmCoe.

Acknowledgments This work was supported in part by the National Science Foundation under Grant DMR 79-20268 and by the Basic Energy Sciences Division, U.S. Department of Energy.

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