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Structural peculiarities in magnetic small particles
Nuclear Instruments North-Holland
and Methods
in Physics Research
B76 (1993) 132-137
NllNlB
Beam Interactions with Materials 8 Atoms
Structural peculiarities
in magnetic small particles
K. Haneda a and A.H. Morrish b aDepartment of Electronic Materials, Ishinomaki Senshu University, Ishinomaki 986, Japan b Department of Physics, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
Nanostructured magnetic materials, consisting of nanometer-sized crystallites, are currently a developing subject. Evidence has been accumulating that they possess properties that can differ substantially from those of bulk materials. This paper illustrates how Miissbauer spectroscopy can yield useful information on the structural peculiarities associated with these small particles. As illustrations, metallic iron and iron-oxide systems are considered in detail. The subjects discussed include: (1) phase stabilities in small particles, (2) deformed or nonsymmetrical atomic arrangements in small particles, and (3) peculiar magnetic structures or non-collinear spin arrangements in small magnetic oxide particles that are correlated with lower specific magnetizations as compared to the bulk values.
1. Introduction
2. Phase stability in small particles
Nanostructured materials or mesoscopic-scale systems, proposed by some to represent a new solid state structure, are currently a developing subject. The major constituent of these materials is the nanometer-sized crystallites themselves. These crystalline cells can exist in a variety of phases including not only the ordinary crystalline structure but also nonequilibrium states. This article is concerned with some of the structural peculiarities associated with magnetic small particles including those in a nonequilibrium state. On many occasions magnetic small particles have proved to be of great practical use. Examples are ferrofluids, particulate magneticrecording media, microwave devices, permanent magnets, catalysts, biomagnetic applications and so on. The exploration of the physical nature of magnetic small particles should provide the key for their full utilization and perhaps for completely new developments. This paper illustrates how Miissbauer spectroscopy can yield useful information on structural peculiarities associated with these small particles. Earlier, in several reports we reviewed the experimental results for many materials in fine particle form from the same point of view [l-5]; a body of evidence has been accumulated that demonstrates that small particles do possess identifiable unique properties. This present report then is supplementary to our earlier articles. The focus is on metallic iron small particles, including our recent results on fee Fe nanometer particles that are paramagnets even down to 1.8 K, and on iron-oxide small particles with emphasis on those with the rare-earth iron garnet structure.
The phase transformation or the phase stability in a mesoscopic-scale system will be important not only for the understanding of the physical nature of the system, but also for the practical use of the small particles. These particles can exist in a variety of phases including the ordinary crystalline state characteristic of the bulk material as well as non-equilbrium states. To date the phase most familiar to us is the one in which the particles are in their equilibrium crystalline form. In addition non-equilibrium phases occur in materials that are on the way towards mature, well-defined bulk crystallites or grains in equilibrium. Structural phase transformations of several magnetic iron oxides in small particle form in the Fe,O,-yFe,O,-a-Fe20, system have been studied in our laboratory [6,7], primarily from a kinetic viewpoint. One conclusion reached so far is that phase transformations in small particle systems take place at temperatures that are significantly lower than those required for the bulk material; these transformations occur most easily for the smallest particles in a system. This is advantageous for the production of small particles at relatively low temperatures. Magnetite, Fe,O,, is a spine1 with one ferrous ion and one ferric ion per formula unit located at B-sites and one ferric ion located at an A site. The material undergoes a phase transition from cubic to a structure with lower symmetry at 119 K, below that point fast electron hopping between the Fe2+ and Fe3+ ions on B-sites ceases. The MGssbauer spectrum changes abruptly at this temperature; this phase change is called the Verwey transition. Topsae and M@rup IS] studied
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K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
the Verwey transition in microcrystals (10 nm) of Fe,O, by Miissbauer spectroscopy and found that the transition temperature is around 100 K. They deduced that the particle size dependence of the Verwey temperature had its origin in partial oxidization of the Fe,O, through the oxygen covering the surface of the particles. This then led to an excess of ferric ions and vacancies in the particle as a whole. The depression of the Verwey temperature observed was indeed in accord with earlier results for bulk samples which had variable, non-stoichiometric ratios of ferric to ferrous ions [9]. Very small iron particles are usually pyrophoric. However, once passivated by a proper method, they can remain fairly stable for several years or even longer. The passivation process largely affects the morphology of the oxide layers formed on the surface of the ultrafine iron particles [lO,ll]. Here we shall describe one example of small iron particles in a crystalline nonequilibrium phase normally non-existent at room temperature, prepared by a rapid-queching technique, namely -y-Fe small particles. Metallic iron exists in two different crystal structures. At low temperatures below 1183 K (a-Fe phase) and at high temperatures above 1663 K up to the melting point of 1807 K (&Fe phase), iron has the body-centered cubic (bee) structure; between 1183 and 1663 K, the face-centered (fee) structure (-r-Fe phase) is stable. Recently, ultrafine iron particles (8 nm diameter) were made by a new technique, the TEA CO, laser induced breakdown of Fe(CO), [12,13]. The Mossbauer spectra shown in fig. 1 indicates that, although (Y-Fe and Fe-oxides are present, about 30 at.% of these particles have the fee structure, that is, the -y-phase; this is one of highest yields reported to date for isolated y-Fe particles [14]. In the formation process during rapid cooling, it is likely that the particle size is closely correlated with how easy the martensitic phase-transformation from the y- to the a-phase takes place. The smaller particles, cooling faster, would remain as y-Fe and the larger ones, experiencing slower cooling, would have a greater chance to be transformed to a-Fe. Another possibility is that the surface of a particle, cooling faster, would remain as y-Fe and the inner core of a particle, experiencing slower cooling, would have a greater chance to be transformed to a-Fe. In any event, the pyrophoric nature of these particles would inevitably lead to the presence of a surface oxidation layer.
3. Atomic arrangements
in small particles
Sometimes the atomic arrangements in small particles may differ from those in bulk materials. Examples will be given for the cation distributions in ZnFe,O, small particles, structural-vacancy arrangements in y-
2.251
g x 3
2.523
: d
2.488
2.431
VELOCITY
(mm/s)
Fig. 1. “Fe MGssbauer spectra of nanometer particles (Fe-system) at various temperatures prepared by the TEA CO, laser induced breakdown of Fe(CO),.
Fe,O, small particles, and atomic arrangements in fee Fe small particles. Cation distributions in ZnFe20, small particles prepared by a chemical coprecipitation method have been studied by time-of-flight neutron powder diffraction at room temperature in order to seek the origin of the anomalously high magnetization observed [15]. The occupancy probability of an Fe3+ ion in a tetrahedral A-site is 0.108 for 96 nm and 0.142 for 27 nm diameter particles. This is contrary to the expectation for the normal spine1 structure for which the zinc A-site occupancy is 1. In other words, the Fe3’ occupation in an A-site increases with decreasing particle size. Therefore, the large magnetization observed in ZnFezO, fine particles as compared to bulk materials can be ascribed to a strong coupling between Fe3+ ions at Aand B-sites via the well-known superexchange interaction. It is well known that y-Fe,O, contains structural vacancies exclusively occupying B-sites to form an ordered superlattice [16]. The correct chemical formula is therefore (Fe) [Fe,,, 0 1,3]04r where 0 denotes a vacancy. However the vacancy ordering is missing in small y-Fe203 particles. Miissbauer spectroscopy is useful in determining the location of these vacancies.
134
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
The conclusion we reached is that the vacancies are disordered in y-Fe,O, small particles less than about 20 nm in diameter although all the vacancies still occupy B-sites [17]. If the surface layer is responsible, the thickness required is approximately 2-3 nm; however, whether the phenomenon is a surface or an intrinsic size effect has yet to be studied. Back to the y-Fe particles of fig. 1. The central absorption attributed to y-Fe is almost unchanged with temperature. For a satisfactory spectral fit, one singlet and doublet were requred for the fee-Fe phase. The central singlet can be attributed to a fee-Fe phase in which each iron atom has 12 nearest neighboring Fe atoms as in the bulk; then the quadrupole interaction will be zero. The doublet could correspond to iron atoms at the surface or interface between the fee-Fe phase and the iron-oxide or cu-Fe phases where the number of nearest Fe atoms is different. This lack of local cubic symmetry at the iron sites will lead to a non-zero electric-field gradient (EFG), hence a doublet. On the magnetic ordering scheme in the y-Fe phase, although the details will be published elsewhere [18], the essence is the following. Additional Mossbauer measurements at various temperatures down to 1.8 K with using a smaller velocity scale with and without an
external magnetic field of 50 kOe show that y-Fe is a paramagnet over the entire temperature range from room temperature down to 1.8 K. For reference purposes, the lattice 5onstant obtained for these y-Fe particles was 3.616 A. A portion at least of these peculiar atomic arrangments in small particles must certainly have their source in the surface layer. For iron-oxide systems, non-symmetric suprexchange interactions can also be expected near the surface.
4. Magnetic structure in small particles Normally the specific saturation magnetization, cS, is considered to be an intrinsic property of a particular magnetic material; in other words values of a, measured for bulk materials are expected to be applicable to small particles regardless of size and morphology. However the magnetization of ferromagnetic or ferrimagnetic oxide small particles is generally observed to be less than that of the bulk. For these samples, “Fe Mossbauer spectra in applied magnetic fields up to 10 T usually show a lack of dipole alignment (that is, a non-collinear structure) over a wide range of temperatures [1,2]. Examples, in recent reports, are hexagonal
03)
(b)
!I 60
40
28 Fig. 2. X-ray
diffraction
pattern
taken
.
.A
L ..
n
I. 60
(deg)
using Cu Ka radiation for small particles of rare-earth Dy,BiFe,O,,. Here 20 is twice the Bragg angle.
iron garnets;
(a) Y,Fe,012
and (b)
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
crystalline BaFe,,O,, [19] and BaFeiz_,,Co,Ti,O,, [20] and the cubic spine1 Y-Fe20, 1211; earlier examples are given in our several reviews [l-S]. New results for cubic materials with the rare-earth iron garnet (RIG) structure as well as some additional recent data will be presented here. Although noncollinearity has been observed in several kinds of oxide particles, it has not been detected either for metallic iron particles [ 10,221 or for FeNi and FeCo alloys [23]. Our reasonable deduction reached at this time is that the surface of small magnetic particles does possess some properties that differ from the bulk, so that a particle in general consists of two components, an inner core and an outer shell. For oxide particles with a noncollinear magnetic structure, nonsymmetrical superexchange interactions near the surface may play a role, whereas for metallic iron alloy particles with a collinear structure, itinerent electrons may act to reduce any nonsymmetric interactions near the surface. The magnetism of ultrafine garnet particles, R,Fe,O,,, has not attracted much attention to date in the literature. Recently however a new application has been proposed in which these particles would be used in coated-type magneto-optical recording media [24.25]. To be practicable, the particle size must be small enough compared to a light wavelength (< 50 nm) in order to suppress light scattering by the particles. The crystal structure of the rare-earth iron garnets (RIG) is isomorphous to that of the mineral grossularite, One cubic unit cell contains eight forCa,A1zSi,0,2. mula units of R,Fe,O,?. The magnetic structure of is somewhat complicated since there are RjFeSOiZ three sublattices arranged ferrimagnetically. Of the five Fe ions per formula unit, two Fe ions occupy octahedral surroundings (16a) and couple tightly antiparallel to three Fe ions in tetrahedral surroundings (24d). The three R ions lie in dodecahedral sites (24~) and couple weakly antiparallel to the resultant moment of the Fe sublattices. Most of the R ions have large magnetic moments at low temperatures; then they provide the dominant contribution to the net magnetization. With the increase in the temperature, the c-sublattice magnetization decreases more rapidly than the resultant Fe-sublattice (ad) magnetization. Therefore a compensation temperature appears at which the net magnetization becomes zero. Above this compensation temperature the net magnetization of the iron sublattices is predominant. Fine particles (40-50 nm in size) of the crystalline garnets Y,Fe,O,, (YIG) and Dy,BiFe,O,z (DyBiIG, a bismuth substituted rareearth garnet) have been prepared by the chemical coprecipitation method. X-ray diffractograms, shown in fig. 2, indicate that the samples are essentially single-phase with the garnet structure. Saturation magnetization data obtained by using
135
Temn [Kl Fig. 3. Saturation magnetization for small RIG particles (40-50
as a function of temperature nm); (a) Dy,BiFe,O,, and (b)
YjFcP,,.
a VSM with a maximum applied field of 1.5 kOe at various temperatures from 4.2 to 300 K are shown in fig. 3; the numerical values at 4.2 K are 19.3 emu/g for YIG and 46.5 emu/g for DyBiIG. These are substantially lower than the values either reported or calculated for bulk materials [26], namely 26.2 emu/g (300 K) and 38.0 emu/g or 5.0 pa/moI (0 K) for YIG and 56.4 emu/g or 9.6 pa/mol (0 K) for DyBiIG. It should be noted that the compensation point for DyBiIG small particles seems to be close to the value for bulk Dy,Fe,O,, (226 K), but the magnetization is never completely compensated. In order to examine the magnetic structure of these small particles, “Fe Mossbauer spectra were taken in a longitudinal 5 T magnetic field at 4.2 K. The results, together with some additional spectra, are shown in figs. 4 and 5 for YIG and DyBiIG small particles, respectively. At a glance, the presence of a non-collinear spin structure is obvious for both samples, since the 2 and 5 line are clearly visible for the in-field spectra. For detailed analyses, computer curve-fittings were made by assuming two six-line patterns, one for the a-sublattice and the other for the d-sublattice. The area ratio of the d-sublattice to the a-sublattice is close to 1.5 for all temperatues for both samples, indicating that the cation arrangements are close to those in the bulk. The essence is summarized in table 1. Although the presence of recalcitrant spins will of course produce a reduction in the magnetization, still some additional factor seems to be required to account for the reductions observed. Fig. 6 shows the thermo-magnetization curve for y-Fe,O, particles prepared from acicular-shaped yFeOOH (around 0.3 urn length) by dehydration at 400°C for 1 h. As is well known, this method produces exremely fine crystallites in a particle [27]. The crystallite size, based on the X-ray line broadening, is 6.5 nm.
K. Haneda, A.H. Morrish / Structural peculiatities in magnetic small particles
136
Table 1 Parameters deduced particles at 4.2 K Area ratio 24d/16a
l.llZ-
from
Mossbauer
H,,
spectra
of small
Average angle
a
Mel
RIG
canting
kd 0.328
-
YIG DyBiIG rn 0.320-
1.5 1.5
a No external
16a
24d
16a
24d
541 536
470 472
3-5 19
19 25
field.
is $ 2
0.332
-
0.325
-
0.422
-
z z
0.402-
VELOCITY
(mm/s)
Fig. 4. 57Fe Miissbauer spectra of small YsFesO,, particles taken at various temperatures; (a) room temperature, (b) 77 K, (c) 4.2 K and (d) 4.2 K in a longitudinal magnetic field of 50 kOe.
It is interesting to note that the saturation magnetization first goes up with increasing temperature from 4.2 K and then after reaching a plateau it goes down. This
observation is fully consistent both with our previous finding that the spin canting is temperature-dependent and with our previous suggestion that the surface layer of the crystallites making up the particle contributes to the spin canting. Of special note is the investigation of the surface magnetism of NiFe,O, small particles with the TOP spectrometer (time-of-flight spectrometer with optical polarizer) by Itoh et al. [28]. They confirmed that the magnetic structure in the surface layers differs from the one in the inner core. Currently topical in the area of particulate magnetic-recording media is hexagonal Co-Ti substituted small particles [29]. Mossbauer spectra BaFei,Oi, taken with high magnetic fields applied establish that a noncollinear spin configuration is present in this material [20]. It has been deduced that part of this noncollinearity is a bulk property in which the spin structure is a cone of easy directions and another part is a surface effect involving the recalcitrant surface spins. Bulk hematite, a-Fe,O,, which has the hexagonal structure, undergoes a phase transtion at around 263 K, called the Morin temperature. Above this temperature the Fe spin directions lie in the basal plane forming a canted antiferromagnet (sometimes called
0.267
0.265
I-15
I -10
I
I
-5
0
VELOCITY
I
I
5
10
15
(mm/s)
Fig 5 . 57Fe Mossbauer spectra of small Dy,BiFesO,, particles taken at 4.2 K in a longitudinal magnetic field of (a) 0 and (b) 50 kOe.
,,5 0
i 200
100
Temp
300
lK1
Fig. 6. Saturation magnetization as a function of temperature for y-Fe,O, prepared from dehydration of y-FeOOH.
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
weak ferromagnetism). Below this temperature the spins he along the c-axis forming a simple collinear antiferromagnet. It has been reported that the Morin temperature decreases with decreasing particle size; no transition is observed for small particles below 25 nm or so [30]. It would be interesting to consider this phenomenon with a two-component model of a particle. If a particle is covered by recalcitrant spins on the surface, this structure may predominate when the particle size becomes small enough. Eventually when the particle is all surface, no transition may occur anymore. In addition, it is possible that the thickness of the surface component is a function of temperature. Small (Y-Fe particles, useful in magnetic recording, are usually passivated by an outer Fe-oxide layer, consisting of tightly packed extremely fine crystallites of Fe,O, and y-Fe,O, a few nanometers in size [lO,ll]. Mossbauer spectra under strong magnetic fields of different samples have indicated that the surface oxide layer is hardly magnetized at all; therefore, it makes almost no contribution to the total magnetization of a particle. This is primarily why the saturation magnetization value usually obtained for acicular-shaped Fe particles (0.2-0.3 urn length, axial ratio 10) is only 140 emu/g at most, far less than the value for pure metallic iron (218 emu/g). Furubayashi et al. [31] studied 2 nm iron particles dispersed in oil with a surface-active agent. They found that the hyperfine field at 4.2 K was distributed over the range from 300 to 400 kOe, with an average of 350 kOe. Although the authors suggest that adsorbed organic molecules are responsible for the effect, it is also conceivable that the lattice constant on the surface layer has a distribution. Then this variety of atomic distances may produce a range in the hyperfine fields.
5. Concluding
remarks
Although small particles of magnetic materials have been investigated for decades, interest in them continues apace. In spite of various experimental difficuties and ambiguities in interpretation of the data, it has become clear that small magnetic particles do possess identifiable unique properties. New ways to make these particles have opened doors for new discoveries and applications. As more sensitive techniques are developed hopefully new interesting properties will become detectable. It seems inevitable however that Mossbauer spectroscopy will continue to play a major role in future investigations.
References [ll A.H. Morrish and K. Haneda. 31-34
(1983) 951.
J. Magn. Magn. Mat.
[21K. Haneda, [31 K. Haneda,
Canadian J. Phys. 65 (1987) 1233. J. Surf. Sci. Sot. Japan (Hyomen Kagaku)
137
8 (1987) 427. [41 K. Haneda and A.H. Morrish, Phase Transitions 24-26 (1990) 661. [51 A.H. Morrish and Z.X. Zhou. in: Science and Technology of Nanostructured Magnetic Materials, NATO AS1 Series, vol. 259, eds. G.C. Hadjipanayis and G.A. Prinz (Plenum Press, New York, 1991) pp. 511-537. [61 A.H. Morrish and G.A. Sawatzky, Ferrites, Proc lnt. Conf. (University of Tokyo Press, Tokyo, 1971) pp. 144147. [71 K. Haneda and A.H. Morrish. J. Phys. Colloq. 38 (1977) Cl-321. [81 H. Topsoe and S. Morup, Proc. lnt. Conf. Mijssbauer Spectroscopy. Acad. Gorniczo-Hutnicza lm. S. Staszica Karkow, 1975 pp. 321-324. [9] E.J.W. Vetwey and P.W. Haayman. Physica 8 (1941) 979. [lo] K. Haneda and A.H. Morrish, Surf. Sci. 77 (1978) 584. [ll] K. Haneda and A.H. Morrish, Nature 282 (1979) 186. [12] T. Majima. T. lshii, Y. Matsumoto and M. Takami, J. Am. Chem. Sot. 111 (1989) 2417. [13] Y. Langsam and A.M. Ronn, Chem. Phys. 54 (1981) 277. [14] M. Kusunoki and T. lchihashi, Jpn. J. Appl. Phys. 35 (1986) L219. [15] T. Kamiyama, K. Haneda, T. Sato, S. lkeda and H. Asano. Solid State Commun. 81 (1992) 563. [16] G.W. van Oosterhout and C.J.M Rooijmans. Nature 181 (1958) 44. [171 K. Haneda and A.H. Morrish, Solid State Commun. 23 (1977) 779. [181 K. Haneda. X.Z. Zhou, A.H. Morrish. T. Majima and T. Miyahara. Phys. Rev. B46 (1992) 13832. and A.H. Morrish, IEEE Trans. Magn. [191 K. Haneda MAC-25 (1989) 2597. [201 K. Haneda. X.Z. Zhou and A.H. Morrish, to be published. and Pll P.M. de Bakker. E. DeGrave, R.E. Vandenberghe L.H. Bowen, Hyperfine Interactions 54 (1990) 493. [221 A.H. Morrish and K. Haneda, J. Magn. Magn. Mat. 15-18 (1980) 1089. [231 A.H. Morrish and R.J. Pollard. Adv. Ceramics 16 (1986) 393. [241 T. Fumoto, Y. Kumura, M. Gomi and M. Abe, J. Magn. Sot. Jpn. 15 suppl. (1991) 263. [251 Y. Kumura. T. Fujimoto, M. Gomi and M. Abe, J. Magn. Sot. Jpn. 15 suppl. (1991) 267. [261 F. Bertaut and R. Pauthenet, Proc. IEE Suppl. (London) BlOJ (1957) 261. [271 A.H. Morrish, in: Crystals. vol. 2 (Springer. Berlin, 1980) pp. 1976-1997. [281 S. ltoh, K. Haneda and Y. Endoh, KEK progress report 90-2 (1990) 142. [291 0. Kubo, T. ldo and H. Yokoyama, IEEE Trans. Magn. MAG-18 (1982) 1122. [301 T. Takada, N. Yamamoto, T. Shinjo, M. Kiyama and Y. Bando, Bull. Inst. Chem. Res. Kyoto Univ. 43 (1965) 406. I. Nakatani and N. Saegusa, J. Phys. Sot. [311 T. Furubayashi, Jpn. 56 (1987) 1855.
and Methods
in Physics Research
B76 (1993) 132-137
NllNlB
Beam Interactions with Materials 8 Atoms
Structural peculiarities
in magnetic small particles
K. Haneda a and A.H. Morrish b aDepartment of Electronic Materials, Ishinomaki Senshu University, Ishinomaki 986, Japan b Department of Physics, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
Nanostructured magnetic materials, consisting of nanometer-sized crystallites, are currently a developing subject. Evidence has been accumulating that they possess properties that can differ substantially from those of bulk materials. This paper illustrates how Miissbauer spectroscopy can yield useful information on the structural peculiarities associated with these small particles. As illustrations, metallic iron and iron-oxide systems are considered in detail. The subjects discussed include: (1) phase stabilities in small particles, (2) deformed or nonsymmetrical atomic arrangements in small particles, and (3) peculiar magnetic structures or non-collinear spin arrangements in small magnetic oxide particles that are correlated with lower specific magnetizations as compared to the bulk values.
1. Introduction
2. Phase stability in small particles
Nanostructured materials or mesoscopic-scale systems, proposed by some to represent a new solid state structure, are currently a developing subject. The major constituent of these materials is the nanometer-sized crystallites themselves. These crystalline cells can exist in a variety of phases including not only the ordinary crystalline structure but also nonequilibrium states. This article is concerned with some of the structural peculiarities associated with magnetic small particles including those in a nonequilibrium state. On many occasions magnetic small particles have proved to be of great practical use. Examples are ferrofluids, particulate magneticrecording media, microwave devices, permanent magnets, catalysts, biomagnetic applications and so on. The exploration of the physical nature of magnetic small particles should provide the key for their full utilization and perhaps for completely new developments. This paper illustrates how Miissbauer spectroscopy can yield useful information on structural peculiarities associated with these small particles. Earlier, in several reports we reviewed the experimental results for many materials in fine particle form from the same point of view [l-5]; a body of evidence has been accumulated that demonstrates that small particles do possess identifiable unique properties. This present report then is supplementary to our earlier articles. The focus is on metallic iron small particles, including our recent results on fee Fe nanometer particles that are paramagnets even down to 1.8 K, and on iron-oxide small particles with emphasis on those with the rare-earth iron garnet structure.
The phase transformation or the phase stability in a mesoscopic-scale system will be important not only for the understanding of the physical nature of the system, but also for the practical use of the small particles. These particles can exist in a variety of phases including the ordinary crystalline state characteristic of the bulk material as well as non-equilbrium states. To date the phase most familiar to us is the one in which the particles are in their equilibrium crystalline form. In addition non-equilibrium phases occur in materials that are on the way towards mature, well-defined bulk crystallites or grains in equilibrium. Structural phase transformations of several magnetic iron oxides in small particle form in the Fe,O,-yFe,O,-a-Fe20, system have been studied in our laboratory [6,7], primarily from a kinetic viewpoint. One conclusion reached so far is that phase transformations in small particle systems take place at temperatures that are significantly lower than those required for the bulk material; these transformations occur most easily for the smallest particles in a system. This is advantageous for the production of small particles at relatively low temperatures. Magnetite, Fe,O,, is a spine1 with one ferrous ion and one ferric ion per formula unit located at B-sites and one ferric ion located at an A site. The material undergoes a phase transition from cubic to a structure with lower symmetry at 119 K, below that point fast electron hopping between the Fe2+ and Fe3+ ions on B-sites ceases. The MGssbauer spectrum changes abruptly at this temperature; this phase change is called the Verwey transition. Topsae and M@rup IS] studied
0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
B.V. All rights reserved
133
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
the Verwey transition in microcrystals (10 nm) of Fe,O, by Miissbauer spectroscopy and found that the transition temperature is around 100 K. They deduced that the particle size dependence of the Verwey temperature had its origin in partial oxidization of the Fe,O, through the oxygen covering the surface of the particles. This then led to an excess of ferric ions and vacancies in the particle as a whole. The depression of the Verwey temperature observed was indeed in accord with earlier results for bulk samples which had variable, non-stoichiometric ratios of ferric to ferrous ions [9]. Very small iron particles are usually pyrophoric. However, once passivated by a proper method, they can remain fairly stable for several years or even longer. The passivation process largely affects the morphology of the oxide layers formed on the surface of the ultrafine iron particles [lO,ll]. Here we shall describe one example of small iron particles in a crystalline nonequilibrium phase normally non-existent at room temperature, prepared by a rapid-queching technique, namely -y-Fe small particles. Metallic iron exists in two different crystal structures. At low temperatures below 1183 K (a-Fe phase) and at high temperatures above 1663 K up to the melting point of 1807 K (&Fe phase), iron has the body-centered cubic (bee) structure; between 1183 and 1663 K, the face-centered (fee) structure (-r-Fe phase) is stable. Recently, ultrafine iron particles (8 nm diameter) were made by a new technique, the TEA CO, laser induced breakdown of Fe(CO), [12,13]. The Mossbauer spectra shown in fig. 1 indicates that, although (Y-Fe and Fe-oxides are present, about 30 at.% of these particles have the fee structure, that is, the -y-phase; this is one of highest yields reported to date for isolated y-Fe particles [14]. In the formation process during rapid cooling, it is likely that the particle size is closely correlated with how easy the martensitic phase-transformation from the y- to the a-phase takes place. The smaller particles, cooling faster, would remain as y-Fe and the larger ones, experiencing slower cooling, would have a greater chance to be transformed to a-Fe. Another possibility is that the surface of a particle, cooling faster, would remain as y-Fe and the inner core of a particle, experiencing slower cooling, would have a greater chance to be transformed to a-Fe. In any event, the pyrophoric nature of these particles would inevitably lead to the presence of a surface oxidation layer.
3. Atomic arrangements
in small particles
Sometimes the atomic arrangements in small particles may differ from those in bulk materials. Examples will be given for the cation distributions in ZnFe,O, small particles, structural-vacancy arrangements in y-
2.251
g x 3
2.523
: d
2.488
2.431
VELOCITY
(mm/s)
Fig. 1. “Fe MGssbauer spectra of nanometer particles (Fe-system) at various temperatures prepared by the TEA CO, laser induced breakdown of Fe(CO),.
Fe,O, small particles, and atomic arrangements in fee Fe small particles. Cation distributions in ZnFe20, small particles prepared by a chemical coprecipitation method have been studied by time-of-flight neutron powder diffraction at room temperature in order to seek the origin of the anomalously high magnetization observed [15]. The occupancy probability of an Fe3+ ion in a tetrahedral A-site is 0.108 for 96 nm and 0.142 for 27 nm diameter particles. This is contrary to the expectation for the normal spine1 structure for which the zinc A-site occupancy is 1. In other words, the Fe3’ occupation in an A-site increases with decreasing particle size. Therefore, the large magnetization observed in ZnFezO, fine particles as compared to bulk materials can be ascribed to a strong coupling between Fe3+ ions at Aand B-sites via the well-known superexchange interaction. It is well known that y-Fe,O, contains structural vacancies exclusively occupying B-sites to form an ordered superlattice [16]. The correct chemical formula is therefore (Fe) [Fe,,, 0 1,3]04r where 0 denotes a vacancy. However the vacancy ordering is missing in small y-Fe203 particles. Miissbauer spectroscopy is useful in determining the location of these vacancies.
134
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
The conclusion we reached is that the vacancies are disordered in y-Fe,O, small particles less than about 20 nm in diameter although all the vacancies still occupy B-sites [17]. If the surface layer is responsible, the thickness required is approximately 2-3 nm; however, whether the phenomenon is a surface or an intrinsic size effect has yet to be studied. Back to the y-Fe particles of fig. 1. The central absorption attributed to y-Fe is almost unchanged with temperature. For a satisfactory spectral fit, one singlet and doublet were requred for the fee-Fe phase. The central singlet can be attributed to a fee-Fe phase in which each iron atom has 12 nearest neighboring Fe atoms as in the bulk; then the quadrupole interaction will be zero. The doublet could correspond to iron atoms at the surface or interface between the fee-Fe phase and the iron-oxide or cu-Fe phases where the number of nearest Fe atoms is different. This lack of local cubic symmetry at the iron sites will lead to a non-zero electric-field gradient (EFG), hence a doublet. On the magnetic ordering scheme in the y-Fe phase, although the details will be published elsewhere [18], the essence is the following. Additional Mossbauer measurements at various temperatures down to 1.8 K with using a smaller velocity scale with and without an
external magnetic field of 50 kOe show that y-Fe is a paramagnet over the entire temperature range from room temperature down to 1.8 K. For reference purposes, the lattice 5onstant obtained for these y-Fe particles was 3.616 A. A portion at least of these peculiar atomic arrangments in small particles must certainly have their source in the surface layer. For iron-oxide systems, non-symmetric suprexchange interactions can also be expected near the surface.
4. Magnetic structure in small particles Normally the specific saturation magnetization, cS, is considered to be an intrinsic property of a particular magnetic material; in other words values of a, measured for bulk materials are expected to be applicable to small particles regardless of size and morphology. However the magnetization of ferromagnetic or ferrimagnetic oxide small particles is generally observed to be less than that of the bulk. For these samples, “Fe Mossbauer spectra in applied magnetic fields up to 10 T usually show a lack of dipole alignment (that is, a non-collinear structure) over a wide range of temperatures [1,2]. Examples, in recent reports, are hexagonal
03)
(b)
!I 60
40
28 Fig. 2. X-ray
diffraction
pattern
taken
.
.A
L ..
n
I. 60
(deg)
using Cu Ka radiation for small particles of rare-earth Dy,BiFe,O,,. Here 20 is twice the Bragg angle.
iron garnets;
(a) Y,Fe,012
and (b)
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
crystalline BaFe,,O,, [19] and BaFeiz_,,Co,Ti,O,, [20] and the cubic spine1 Y-Fe20, 1211; earlier examples are given in our several reviews [l-S]. New results for cubic materials with the rare-earth iron garnet (RIG) structure as well as some additional recent data will be presented here. Although noncollinearity has been observed in several kinds of oxide particles, it has not been detected either for metallic iron particles [ 10,221 or for FeNi and FeCo alloys [23]. Our reasonable deduction reached at this time is that the surface of small magnetic particles does possess some properties that differ from the bulk, so that a particle in general consists of two components, an inner core and an outer shell. For oxide particles with a noncollinear magnetic structure, nonsymmetrical superexchange interactions near the surface may play a role, whereas for metallic iron alloy particles with a collinear structure, itinerent electrons may act to reduce any nonsymmetric interactions near the surface. The magnetism of ultrafine garnet particles, R,Fe,O,,, has not attracted much attention to date in the literature. Recently however a new application has been proposed in which these particles would be used in coated-type magneto-optical recording media [24.25]. To be practicable, the particle size must be small enough compared to a light wavelength (< 50 nm) in order to suppress light scattering by the particles. The crystal structure of the rare-earth iron garnets (RIG) is isomorphous to that of the mineral grossularite, One cubic unit cell contains eight forCa,A1zSi,0,2. mula units of R,Fe,O,?. The magnetic structure of is somewhat complicated since there are RjFeSOiZ three sublattices arranged ferrimagnetically. Of the five Fe ions per formula unit, two Fe ions occupy octahedral surroundings (16a) and couple tightly antiparallel to three Fe ions in tetrahedral surroundings (24d). The three R ions lie in dodecahedral sites (24~) and couple weakly antiparallel to the resultant moment of the Fe sublattices. Most of the R ions have large magnetic moments at low temperatures; then they provide the dominant contribution to the net magnetization. With the increase in the temperature, the c-sublattice magnetization decreases more rapidly than the resultant Fe-sublattice (ad) magnetization. Therefore a compensation temperature appears at which the net magnetization becomes zero. Above this compensation temperature the net magnetization of the iron sublattices is predominant. Fine particles (40-50 nm in size) of the crystalline garnets Y,Fe,O,, (YIG) and Dy,BiFe,O,z (DyBiIG, a bismuth substituted rareearth garnet) have been prepared by the chemical coprecipitation method. X-ray diffractograms, shown in fig. 2, indicate that the samples are essentially single-phase with the garnet structure. Saturation magnetization data obtained by using
135
Temn [Kl Fig. 3. Saturation magnetization for small RIG particles (40-50
as a function of temperature nm); (a) Dy,BiFe,O,, and (b)
YjFcP,,.
a VSM with a maximum applied field of 1.5 kOe at various temperatures from 4.2 to 300 K are shown in fig. 3; the numerical values at 4.2 K are 19.3 emu/g for YIG and 46.5 emu/g for DyBiIG. These are substantially lower than the values either reported or calculated for bulk materials [26], namely 26.2 emu/g (300 K) and 38.0 emu/g or 5.0 pa/moI (0 K) for YIG and 56.4 emu/g or 9.6 pa/mol (0 K) for DyBiIG. It should be noted that the compensation point for DyBiIG small particles seems to be close to the value for bulk Dy,Fe,O,, (226 K), but the magnetization is never completely compensated. In order to examine the magnetic structure of these small particles, “Fe Mossbauer spectra were taken in a longitudinal 5 T magnetic field at 4.2 K. The results, together with some additional spectra, are shown in figs. 4 and 5 for YIG and DyBiIG small particles, respectively. At a glance, the presence of a non-collinear spin structure is obvious for both samples, since the 2 and 5 line are clearly visible for the in-field spectra. For detailed analyses, computer curve-fittings were made by assuming two six-line patterns, one for the a-sublattice and the other for the d-sublattice. The area ratio of the d-sublattice to the a-sublattice is close to 1.5 for all temperatues for both samples, indicating that the cation arrangements are close to those in the bulk. The essence is summarized in table 1. Although the presence of recalcitrant spins will of course produce a reduction in the magnetization, still some additional factor seems to be required to account for the reductions observed. Fig. 6 shows the thermo-magnetization curve for y-Fe,O, particles prepared from acicular-shaped yFeOOH (around 0.3 urn length) by dehydration at 400°C for 1 h. As is well known, this method produces exremely fine crystallites in a particle [27]. The crystallite size, based on the X-ray line broadening, is 6.5 nm.
K. Haneda, A.H. Morrish / Structural peculiatities in magnetic small particles
136
Table 1 Parameters deduced particles at 4.2 K Area ratio 24d/16a
l.llZ-
from
Mossbauer
H,,
spectra
of small
Average angle
a
Mel
RIG
canting
kd 0.328
-
YIG DyBiIG rn 0.320-
1.5 1.5
a No external
16a
24d
16a
24d
541 536
470 472
3-5 19
19 25
field.
is $ 2
0.332
-
0.325
-
0.422
-
z z
0.402-
VELOCITY
(mm/s)
Fig. 4. 57Fe Miissbauer spectra of small YsFesO,, particles taken at various temperatures; (a) room temperature, (b) 77 K, (c) 4.2 K and (d) 4.2 K in a longitudinal magnetic field of 50 kOe.
It is interesting to note that the saturation magnetization first goes up with increasing temperature from 4.2 K and then after reaching a plateau it goes down. This
observation is fully consistent both with our previous finding that the spin canting is temperature-dependent and with our previous suggestion that the surface layer of the crystallites making up the particle contributes to the spin canting. Of special note is the investigation of the surface magnetism of NiFe,O, small particles with the TOP spectrometer (time-of-flight spectrometer with optical polarizer) by Itoh et al. [28]. They confirmed that the magnetic structure in the surface layers differs from the one in the inner core. Currently topical in the area of particulate magnetic-recording media is hexagonal Co-Ti substituted small particles [29]. Mossbauer spectra BaFei,Oi, taken with high magnetic fields applied establish that a noncollinear spin configuration is present in this material [20]. It has been deduced that part of this noncollinearity is a bulk property in which the spin structure is a cone of easy directions and another part is a surface effect involving the recalcitrant surface spins. Bulk hematite, a-Fe,O,, which has the hexagonal structure, undergoes a phase transtion at around 263 K, called the Morin temperature. Above this temperature the Fe spin directions lie in the basal plane forming a canted antiferromagnet (sometimes called
0.267
0.265
I-15
I -10
I
I
-5
0
VELOCITY
I
I
5
10
15
(mm/s)
Fig 5 . 57Fe Mossbauer spectra of small Dy,BiFesO,, particles taken at 4.2 K in a longitudinal magnetic field of (a) 0 and (b) 50 kOe.
,,5 0
i 200
100
Temp
300
lK1
Fig. 6. Saturation magnetization as a function of temperature for y-Fe,O, prepared from dehydration of y-FeOOH.
K. Haneda, A.H. Morrish / Structural peculiarities in magnetic small particles
weak ferromagnetism). Below this temperature the spins he along the c-axis forming a simple collinear antiferromagnet. It has been reported that the Morin temperature decreases with decreasing particle size; no transition is observed for small particles below 25 nm or so [30]. It would be interesting to consider this phenomenon with a two-component model of a particle. If a particle is covered by recalcitrant spins on the surface, this structure may predominate when the particle size becomes small enough. Eventually when the particle is all surface, no transition may occur anymore. In addition, it is possible that the thickness of the surface component is a function of temperature. Small (Y-Fe particles, useful in magnetic recording, are usually passivated by an outer Fe-oxide layer, consisting of tightly packed extremely fine crystallites of Fe,O, and y-Fe,O, a few nanometers in size [lO,ll]. Mossbauer spectra under strong magnetic fields of different samples have indicated that the surface oxide layer is hardly magnetized at all; therefore, it makes almost no contribution to the total magnetization of a particle. This is primarily why the saturation magnetization value usually obtained for acicular-shaped Fe particles (0.2-0.3 urn length, axial ratio 10) is only 140 emu/g at most, far less than the value for pure metallic iron (218 emu/g). Furubayashi et al. [31] studied 2 nm iron particles dispersed in oil with a surface-active agent. They found that the hyperfine field at 4.2 K was distributed over the range from 300 to 400 kOe, with an average of 350 kOe. Although the authors suggest that adsorbed organic molecules are responsible for the effect, it is also conceivable that the lattice constant on the surface layer has a distribution. Then this variety of atomic distances may produce a range in the hyperfine fields.
5. Concluding
remarks
Although small particles of magnetic materials have been investigated for decades, interest in them continues apace. In spite of various experimental difficuties and ambiguities in interpretation of the data, it has become clear that small magnetic particles do possess identifiable unique properties. New ways to make these particles have opened doors for new discoveries and applications. As more sensitive techniques are developed hopefully new interesting properties will become detectable. It seems inevitable however that Mossbauer spectroscopy will continue to play a major role in future investigations.
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(1983) 951.
J. Magn. Magn. Mat.
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137
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