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Langmuir 1998, 14, 1984-1989
Synthesis and Magnetic Properties of Elongated Fe-Cu Alloys N. Duxin,†,‡ N. Brun,§ C. Colliex,§ and M. P. Pileni†,‡,* Universite´ Pierre et Marie Curie, Laboratoire “Structure et Re´ activite´ des Syste` mes Interfaciaux”, URA CNRS 1662, BP 52, 4 place Jussieu, 75005 Paris, France, CEA-DSM-DRECAM Service de Chimie Mole´ culaire, CEA Saclay, 91191 Gif sur Yvette Cedex, France, and Universite´ Paris-Sud, Laboratoire de Physique des Solides, URA CNRS 002, Baˆ t 510, 91450 Orsay Cedex, France Received August 12, 1997. In Final Form: January 8, 1998 Colloidal copper (Fe-Cu) alloy was prepared in aqueous solution via sodium borohydride reduction of Cu and Fe dodecyl sulfate [Cu(DS)2 and Fe(DS)2]. The synthesis performed at the critical micellar concentration induced formation of an interconnected network of Fe-Cu alloys. The material was characterized by transmission electron microscopy, electron diffraction, Mo¨ssbauer spectroscopy, and electron energy loss spectroscopy. The magnetization properties of the Fe-Cu alloy are presented.
Introduction Fine metal structures constitute a wide class of active catalysts because of their extremely large surface area. Nanostructured materials have attracted a great deal of attention because of their unique characteristics for producing ductile materials in intermetallic compounds and that allows development of high-strain-state superplastics.1-3 In the equilibrium states, the solid solubility between Fe and Cu is negligibly small. The mixing enthalpy of Fe and Cu is positive, and these elements form no intermetallic compounds even though their atomic radii are quite similar. Several methods have been used to produce metastable crystalline and amorphous alloys.4-12 In our previous paper13 we showed that a rather homogeneous alloy of immiscible Fe and Cu could be made from reverse micelles as the starting point of the chemical reaction. The nanosized alloys are fairly polydispersed, with sizes ranging from 2 to 12 nm. In the present paper, we report the reduction of Cu and Fe ions in aqueous dodecyl sulfate oil-in-water micellar solution. Surfactant aggregation favors the formation of Fe-Cu alloy particles forming an * Author to whom correspondence should be addressed. † Universite ´ Pierre et Marie Curie. ‡ CEA-DSM-DRECAM Service de Chimie Mole ´ culaire. § Universite ´ Paris-Sud. (1) Tracy, M. J.; Groza, J. R. Nanostruct. Mater. 1992, 1, 369. (2) Siegel, R. Nanostruct. Mater. 1993, 3, 1. (3) Higashi, K.; Mukai, T.; Tanimura, S.; Inoue, A.; Masumoto, T.; Kita, K.; 4htera, K.; Nagahora, J. Scr. Metall. 1992, 26, 191. (4) Sumiyama, K.; Nakamura, Y. J. Magn. Magn. Mater. 1983, 35, 219. (5) Chien, C. L.; Liou, S. H.; Kofalt, D.; Yu, W.; Egami, T.; McGuire, T. R. Phys. Rev. B. 1986, 33, 3247. (6) Uenishi, K.; Kobayashi, K. F.; Nasu, S.; Hatano, H.; Ishihara, K. N.; Shingu, P. H. Z. Metallkd. 1992, 83, 132. (7) Yavari, A.; Desre, J.; Benameur, T. Phys. Rev. Lett. 1992, 8, 2235. (8) Jiang, J. Z.; Gonser, U.; Gente, C.; Bormann, R. Appl. Phys. Lett. 1993, 63, 1056. (9) Jiang, J. Z.; Gonser, U.; Gente, C.; Bormann, R. Appl. Phys. Lett. 1993, 63, 2768. (10) Jiang, J. Z.; Chen, F. T. J. Phys.: Condens. Matter 1994, 6, L343. (11) Jiang, J. Z.; Pankhurst, Q. A.; Johnson, C. E.; Gentes, C.; Bormann, R. J. Phys.: Condens. Matter 1994, 6, L227. (12) Chow, G. M.; Ambrose, T.; Xiao, J. Q.; Twigg, M. E.; Baral, S.; Ervin, A. M.; Qadri, S. B.; Feng, C. R. Nanostuct. mater. 1992, 1, 361. (13) Duxin, N.; Brun, N.; Bonville, P.; Colliex, C.; Pileni, M. P. J. Phys. Chem. 1997, 101, 8907.
interconnected network. The characterization and magnetic properties of the elongated Fe-Cu alloy are presented. Experimental Section Sodium dodecyl sulfate (SDS) was from Fluka, and copper chloride (CuCl2), iron chloride (FeCl2), and sodium borohydride (NaBH4) were from Prolabo and Sigma, respectively. These products were used without further purification. Mo1 ssbauer Experiments. The Mo¨ssbauer spectra were recorded using a 57Co* :Rh γ-ray source (E0 ) 14.4 keV) mounted on an electromagnetic drive with a triangular velocity signal. The spectra were fitted by least-squares analysis to obtain the hyperfine parameters (isomer shift, δ; quadrupolar splitting, ∆; and hyperfine field, Hhf). In some cases, a broad hyperfine field distribution was observed, which was fitted by using either a histogram of hyperfine fields with free weights or a Gaussian distribution. Briefly, the main characteristics of the 57Fe Mo¨ssbauer spectra of Fe oxides and Fe metallic alloys are as follows. The isomer shift values (δ) are given with respect to R-Fe. The parameter δ is very sensitive to the Fe oxidation state; for metallic Fe, ionic Fe3+, and ionic Fe2+, typical values of δ are 0, 0.5, and 1.2 mm/s, respectively. The Hhf value at saturation also depends on the oxidation state: for Fe in a metallic environment, Hhf(T ) 0) ) 250-350 kOe (340 kOe for R-Fe) and for Fe3+ in an insulating oxide, Hhf(T ) 0) ) 450-550 kOe. All the spectra reported are zero external field spectra and were recorded at 4.2 K. To record the Mo¨ssbauer spectrum, the particles were encapsulated in poly(vinyl alcohol) (PVA). Magnetization Curves. Magnetization curves were created with a SQUID magnetometer. To prevent agglomeration, magnetic particles were dispersed in PVA. The volume fraction of magnetic particles was kept at 1%. The Langevin function is given by:
M ) coth(R) - 1/R with R ) µH/kT
(1)
where M, µ, K, and T are the magnetization of the collective particles, the magnetic moment of one particle, the Boltzmann constant, and temperature, respectively. The anisotropy constant, K, was evaluated from the Ne´el equation14:
τ ) τ0 exp(KV/kTB)
(2)
where τ, τ0, and V are the relaxation time (τ ) 100 s), the (14) Ne´el, L. C. R. Acad. Sc. Paris 1949, 228, 664.
S0743-7463(97)00909-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/28/1998
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microscopic relaxation time (τ0 ) 10-9 s), and the average volume of the material, respectively. The anisotropy energy varies with the applied field as follows: 15
E(H) ) KV sin2 Θ - µH cos Θ
(3)
where Θ and µ are the angle between the easy axis magnetization and the direction of the applied field and the magnetic moment of the particles, respectively (µ ) MSV, where MS is the saturation magnetization of the particles). At zero field, this curve presents two minima of equal energy. As the field increases, the energy barrier is perturbed and the moment relaxes more easily from one minimum to the other, on the other side of the anisotropy barrier. At a given value of the field, HK, there is only one stable location of the moment, and then no maximum in the zero field cool (ZFC) curve is observed. In the general case of random orientations of the easy axes of particles, the question of the field dependence E(H) of the anisotropy barriers cannot be solved analytically. If the easy axes are parallel to the field, the anisotropy barrier, E(H), is described as follows:16
E(H) ) KV(1 - H/HK)3/2
(4)
where HK is the coercive field at which the given barrier vanishes. From the Ne´el equation (eq 1), it can be shown that:
(() ) ln
τ kTB τ0 KV
2/3
)1-
H HK
(5)
where TB is the blocking temperature. The susceptibility can be deduced from the Curie-Weiss law: 17,18
The relevant characterization edges are the Fe L2,3 edge at 708 eV, the Cu L2,3 edge at 931 eV, and the C at 284 eV. To check the homogeneity in composition of the material at a nanoscale level, we acquired several sequences of spectra in the linespectrum mode. In the line-spectrum mode, the 1-nm diameter electron probe is scanned automatically across the sample, and for each position of the probe, an EELS spectrum is acquired. There are typically 64 or 128 spectra in the series with the probe step being determined by the magnification. In the present case, the probe step is ∼1 nm between each spectrum. The series of spectra can be processed a posteriori.20 With the conventional quantification process (modeling and subtracting background under a characteristic edge), the Fe/Cu ratios can be evaluated. Synthesis of Iron and Copper Dodecyl Sulfate. Copper and Fe dodecyl sulfate [Cu(DS)2 and Fe(DS)2, respectively] are made as described in the literature. An aqueous solution of 0.2 M SDS was mixed with 0.3 M of either ferrous or copper chloride. The solution was kept at 2 °C, and a precipitate appeared. The precipitate was washed several times with a 0.1 M iron or copper chloride solution and recrystallized in distilled water. The Fe(DS)2 and Cu(DS)2 form micellar aggregates of mixed surfactant with critical micellar concentrations (CMC), determined from conductivity, of 1.2 × 10-3 and 1.39 × 10-3 M, respectively. The CMC of the mixed surfactant used for the synthesis [30% Fe(DS)2 and 70% Cu(DS)2] was 1.34 × 10-3 M. These values are in good agreement with that given in the literature.21 Because the Fe(DS)2 and Cu(DS)2 CMC values are very close, formation of mixed micelles is assumed as a first approximation. The shape and the size of Fe(DS)2 and Cu(DS)2 micellar solutions were determined by small-angle X-ray scattering and by light scattering.22 The micelles are prolate ellipsoidal in shape, with a hydrodynamic radius of 2.7 nm. These results are in good agreement with those obtained with the cadmium derivative Cd(DS)2.23
Results and Discussion χ ) C/(T - T0)
(6)
where C and T0 are the Curie constant and the ordering temperature, respectively. X-ray Diffraction. A Stoe Stadi P goniometer with a Siemen Kristalloflex-X-ray generator using cobalt anticathode driven by a personal computer through the Daco-PM interface was used for X-ray diffraction (XRD) studies. Transmission Electron Microscopy (TEM) and Electron Diffraction. A JEOL electron microscope (JEOL. 100 CX. 2) was used. Energy Dispersive Spectrometry (EDS). The spectra were obtained with a transmission electron microscope (JEOL 100CXII) equipped with a ASID 4D in the STEM mode and a Link AN 10,000. This equipment cannot detect boron or oxygen. The relative amounts of Fe and Cu atoms were determined with a rather large probe size (35 nm). Electron Energy Loss Spectrometry (EELS). The EELS data were recorded with a scanning transmission electron microscope (STEM; VG HB 501 equipped with a Gatan 666 parallel electron energy loss spectrometer) for the same sample as previously used for the normal TEM work. EELS measures the energy loss suffered by high-energy-incident electrons transmitted across the specimen and can be used to identify and measure the concentration of different atoms revealed by their characteristic core-ionization edges. Furthermore, we benefited from the spectrum-line acquisition mode installed on this machine,19 which has been shown to provide a well-defined relationship between topography and chemistry at the nanometer scale. (15) Bacri, J. C.; Perzinski, R.; Salin, D. J. Magn. Magn. Mater. 1988, 71, 246. (16) Victora, R. H. Phys. Rev. Lett. 1989, 63, 457. (17) Bradbury, A.; Menear, S.; O′Grady, K.; Chantrell, R. W. IEEE Trans. Magn. 1984, 20, 1846. (18) Holmes, M.; O′Grady, K.; Popplewell, J. J. Magn. Magn. Mater. 1990, 85, 47. (19) Colliex, C.; Tence´, M.; Lefe`vre, E.; Mory, C.; Gu, H.; Bouchet, D.; Jeanguillaume, C. Mikrochim Acta 1994, 114/115, 71.
Synthesis and Characterization of Copper Metal Particles in Copper Dodecyl Sulfate Micelles. Syntheses of Fe-Cu alloys were performed by reduction of mixed micelles made of Cu(DS)2 and Fe(DS)2, with sodium borohydride (NaBH4) as the reducing agent. To prevent oxidation, the synthesis was performed under a glovebox in a nitrogen atmosphere. Immediately after the NaBH4 addition, the solution turned black, with formation of dispersed nanoparticles. The relative Fe(DS)2/[Cu(DS)2 + Fe(DS)2] and Fe(DS)2/NaBH4 ratios were kept to 30% and 0.15, respectively. Syntheses were performed above the cmc and at various micellar concentrations. For each synthesis, a drop of the solution was left on a carbon grid. The TEM patterns differ with the micellar concentration (Figure 1). At low micellar concentration, the TEM images show formation, at the cmc, of a large interconnected network. By increasing the micellar concentration, the number of spherical particles increased, with a decrease in the number of elongated nanoparticles. Figure 1 shows that electron diffractograms are in good agreement with the simulated diffractogram of bulk metal Cu, indicating a face-centered cubic structure (FCC) with a lattice constant of 3.61 Å. In this paper, we chose to carefully characterize the nanomaterial obtained close to the cmc with 1.05 × 10-3 M Cu(DS)2, 4.5 × 10-4 M Fe(DS)2, and 3 × 10-3 M NaBH4. The TEM image (Figure 2A) shows formation of elongated nanoparticles. With high resolution (Figure 2B), the distance between two planes of ≈2 Å was determined and (20) Tence´, M.; Quartuccio, M.; Colliex, C. Ultramicroscopy 1995, 58, 42. (21) Moroi, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1974, 46, 111. (22) Lisiecki, I.; Dias, O.; Pileni, M. P., to be submitted. (23) Petit, C.; Jain, T. K.; Billoudet, F.; Pileni, M. P. Langmuir 1994, 10, 4446.
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Figure 2. Transmission electron microscopy patterns of a colloidal dispersion observed for a reaction at 1.05 × 10-3 M Cu(DS)2 and 4.5 × 10-4 M Fe(DS)2. (A) overview of the sample; (B) high-resolution image. The distance between [111] planes are indicated by arrows.
Figure 1. Transmission electron microscopy patterns of colloidal dispersion observed for various concentrations of reactants and the corresponding electron diffraction patterns. (A) 1.05 × 10-3 M Cu(DS)2 and 4.5 × 10-4 M Fe(DS)2; (B) 1.4 × 10-3 M Cu(DS)2 and 6 × 10-4 M Fe(DS)2; (C) 2.1 × 10-3 M Cu(DS)2 and 9 × 10-4 M Fe(DS)2; (D) 7 × 10-3 M Cu(DS)2 and 3 × 10-3 M Fe(DS)2.
is in good agreement with the distance between the [111] planes in the FCC copper bulk phase. The FCC structure was confirmed by XRD (Figure 3). Neither an oxide phase nor a body-core-centered (BCC) structure due to an R-Fe phase was detected. The EDS measurements in various regions of the carbon grid indicate that the estimated average composition of the material is 14% Fe and 86% Cu atoms. Comparison of the percentages of Fe and Cu atoms determined by EDS with those used to make the material (30% Fe and 70% Cu) indicates that part of the Fe(DS)2 was not reduced by NaBH4. This conclusion was confirmed by Mo¨ssbauer spectroscopy (shown later). These data indicate the formation of Fe-Cu alloys. However, the EDS technique gives only an average value on the composition. To get information on the nanoscopic scale, EELS studies were performed. Several sequences of spectra in the line-spectrum mode were acquired across various aggregates or single nanoparticles (Figures 4A and D). The corresponding profiles (Figures 4B and E) indicate the simultaneous presence of Fe and Cu. However, the Fe/(Fe + Cu) ratios show a relative heterogeneity of Fe inside Cu matrixes. Figures 4C and F show that the Fe composition varies from 1 to 37%, whereas the Cu composition varies from 99 to 63%, indicating formation of heterogeneous alloys. The peaks due to Fe correspond
Figure 3. X-ray diffraction pattern of powder made from synthesis of 1.05 × 10-3 M Cu(DS)2 and 4.5 × 10-4 M Fe(DS)2 micelles.
to the surface of the particles (comparison between Figures 4A and D and Figures 4B and E). To confirm such behavior, another line spectrum was recorded across an aggregate (Figure 5A). The corresponding chemical profile, diplayed in Figure 5B, shows a plateau in the Fe profile that could be attributed to a gradient of Fe atoms from the center to the surface of the material. Simulations of the chemical profile were performed by assuming that a sphere with a 16-nm diameter is either homogeneously composed of 9.6% Fe and 90.4% Cu or is made of two concentrical shells differing by their compositions; the internal one is composed of 89.5% Cu and 4.4% Fe, whereas the external one is made of 89.5% Cu and 10.5% Fe. The full line in Figure 5B shows the presence of a plateau of Fe composition for a heterogeneous sphere. Hence, it seems reasonable to conclude that the Fe composition is lower in the core than at the surface of the particle. A 57Fe Mo¨ssbauer spectra recorded at 4.2 K (Figure 6) shows two components that can be resolved as follows: (i) A broad six-line pattern with a mean Hhf value of 209
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Figure 4. (A, D) STEM bright field images of aggregates. (B, E) The corresponding EELS chemical profiles in the line-spectrum mode recorded in various regions of the aggregate. (C, F) Fe/Cu ratios of the aggregates displayed in A and D, respectively. Key: (b) copper; (9) iron.
kOe and a root-mean-square deviation of 100 kOe, which is attributed to an Fe-Cu alloy. The relative intensity of this component is 75%. (ii) A quadrupolar doublet with δ ) 1.45 mm/s and ∆ ) 2 mm/s attributed to Fe2+. The relative intensity of this component is 25%. This result confirms the EDS measurements from which a part of the Fe(DS)2 was not reduced by NaBH4. No hyperfine fields in the range of 300 kOe or 450-550 kOe were detected, confirming the absence of oxides or R-Fe phases, respectively. From these data, formation of an Fe-Cu alloy can be concluded, where the Fe atoms replace Cu atoms in FCC matrix of the alloy. However, the number of Fe atoms per Cu atom is not constant. A gradient of Fe from the core to the surface is demonstrated. This disorder of Fe atoms in an FCC Cu structure and the absence of an R-Fe phase is supported by the broad Mo¨ssbauer six-line spectra and by the XRD pattern. The presence of interstitial boron in the material is expected, as shown in our previous paper.13 Magnetic Properties. The variation of magnetization, M (emu‚g-1), with applied field, H (kOe), was recorded at various temperatures (Figure 7A). The saturation magnetization was never reached. As expected, the magnetization increased with decreasing temperature as a result of the competition between the magnetic order and the thermal disorder. At 1.8 K, the saturation magnetization was estimated as 40 emu‚g-1 by zero extrapolation of the M versus 1/H curve. In the bulk Fe-Cu alloy phase, it was demonstrated that the magnetization linearly decreased with increasing the percentage of Cu atoms participating to the formation of alloy.4 In the absence of
Figure 5. (A) STEM bright field image of aggregates. (B) The corresponding EELS chemical profile in the line-spectrum mode. Key: (0) iron; (O) copper.
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Figure 6. The 57Fe Mo¨ssbauer spectra at T ) 4.2 K. The material was encapsulated in PVA. The doublet was asigned to residual Fe2+ and the broad sextet to metallic iron (Fe0) in the FCC Fe-Cu phase.
Figure 7. (A) Variation of the magnetization with the applied field at various temperatures: (O) 1.8 K; (×) 5 K; (+) 10 K; (]) 15 K; (0) 20 K. (B) Variation of the magnetization with the ratio of applied field to temperature, at various temperatures: (O) 1.8 K; (×) 5 K; (+) 10 K; (]) 15 K; (0) 20 K. The material was encapsulated in PVA.
Cu, the magnetization is that of the R-Fe phase. Assuming a similar behavior on the nanoscale level, the saturation magnetization of the Fe0.14-Cu0.86 alloy is estimated as 30 emu‚g-1. This value is in good agreement with that deduced at 1.8 K (40 emu‚g-1). Figure 7B shows the variation of magnetization with the ratio of applied field over temperature. Above 10 K, the curves are superimposed. According to the Langevin
Duxin et al.
function (eq 1), this behavior is a direct measurement of superparamagnetic behavior of the nanosized particles.24 Below 10 K, the magnetization differs with temperature, indicating that the magnetic moments of the Fe-Cu alloy are blocked. The ZFC curves were recorded at various applied fields (Figure 8). At and below 100 Oe, a blocking temperature (TB) was observed, whereas above 100 Oe, a constant decrease in the magnetization with increasing temperature was observed. The TB values were 9.5, 8.5, and 7.5 K for applied fields of 20, 50, and 100 Oe, respectively. These results are consistent with the variation of the anisotropy energy with application of an external field.15 According to eq 5, a linear variation of TB2/3 as a function of the applied field takes place, and the anisotropy field can be estimated by extrapolation at zero TB to be 575 Oe. This value is assigned to the coercive field of a magnetization curve M versus H. The magnetization curve (Figure 9) shows a coercive field, Hc, equal to 250 Oe. Hence, the experimental value of the coercive field and that deduced from zero extrapolation of the blocking temperature with the applied magnetic field are of the same order of magnitude. The difference between the two values could be attributed to the heterogeneities of the material. As a matter of fact, the anisotropy constant changes with composition. Furthermore, the model developed by Victora16 assumes spherical and homogeneous particles. The interactions between the particles (see below) may also play a role in the HK value. The TB is determined from the maximum of the ZFC curve at low applied field (20 Oe). The insert in Figure 8 shows that the maximum in the ZFC curve is observed at 9.5 K. To determine the anisotropy constant from the Ne´el equation, we need to determine the volume of the particles. Because of the shape of the material (Figure 2A), this evaluation is rather difficult. To give an order of magnitude to the anisotropy constant, we decided to use the volume of the particles deduced from the XRD spectrum, assuming spherical particles. The best simulation of the XRD spectrum is obtained for 6.6 nm as diameter. This simulation permits estimations from eq 2, of the anisotropy constant in the range of 10+5 erg‚cm-3. The exact value obtained by calculation is 2 × 10+5 erg‚cm-3. Figure 10 shows the variation of the inverse of initial susceptibility, χi, versus temperature. When the magnetic energy is significantly lower than the thermic energy, the variation of the initial susceptibility obeys the CurieWeiss law: χi-1 ) (T - T0)/C, where C is the Curie constant and To is the ordering temperature.16,17 This equation describes the linear domain of χi-1 versus T, where the switch of the magnetic moments is thermally activated. The linear domain is expected to be observed when µH/kT < 0.1. The superparamagnetic behavior of the particles strongly depends on the applied field. By assuming, as already mentioned, that the particle diameter is equal to 6.6 nm, the particles would have a superparamagnetic behavior at 61, 152, 300, and 1500 K when the applied field is 20, 50, 100, and 500 Oe, respectively. In our experimental conditions, the Curie-Weiss law can only be applied for applied fields equal to 20 and 50 Oe. In these two cases, the ordering temperature T0 is deduced from extrapolation to zero 1/χ as 50 and 115 K, respectively (Figure 10). Because the ordering temperature increases with the applied field and does not remain at zero, it can be concluded that interactions between particle take (24) Bean, C. P.; Livingston, J. D. J. Appl. Phys., Supp. 30 1959, 120S.
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Figure 8. Zero field cooled (ZFC) magnetization as a function of temperature for various applied fields: (O) 20 Oe; (0) 50 Oe; (]) 100 Oe; (4) 500 Oe; (+) 1000 Oe. The material was encapsulated in PVA. Insert: Enhancement of the ZFC for applied fields of 20, 50, and 100 Oe.
Figure 9. Magnetization curve as a function of the applied field at T ) 1.8 K. Insert: Enhancement at low fields. The material was encapsulated in PVA.
place.18 This interaction could be due to some aggregation when the sample has been fixed in the polymer. Hence, the elongated alloys are characterized by a superparamagnetic behavior above 10 K. The saturation magnetization is in good agreement with that obtained in the bulk phase. A rather good agreement between the coercive field deduced from the variation of the blocking temperature and the applied field is observed. Even at rather low volume fraction (1%), some interactions between particles are observed. Conclusion Fe-Cu nanoparticles were obtained from chemical reduction of Fe(DS)2 and Cu(DS)2 by NaBH4. At a given
Figure 10. Variation of the inverse of susceptibility with temperature for various applied fields: (O) 20 Oe; (0) 50 Oe; (]) 100 Oe; (4) 500 Oe.
concentration of reactants, close to the cmc elongated nanoparticles were obtained. Cu was heterogeneously substituted by Fe in the FCC lattice and no oxide phase was observed. The magnetic properties of the material exhibit superparamagnetism with a blocking temperature dependence on the applied field and an anisotropy constant of ≈2 × 105 erg/cm3. Acknowledgment. We acknowledge Dr. P. Bonville for providing the Mo¨ssbauer spectrum, Dr. I. Rosenman for the SQUID measurements, and Dr. E. Vincent for fruitful discussions. LA9709094
