9988
J. Phys. Chem. C 2008, 112, 9988–9995
Multifrequency EMR and Magnetic Characterization of Synthetic Powdered Hematite Cristina Carbone,† Francesco Di Benedetto,*,‡ Claudio Sangregorio,§ Pietro Marescotti,† Luca A. Pardi,| and Lorenzo Sorace*,§ DIPTERIS, UniVersity of GenoVa, Corso Europa 26, 16162 GenoVa, Italy, Dipartimento di Chimica, UniVersity of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy, Dipartimento di Scienze della Terra, UniVersity of Florence, Via G. La Pira, 4, 50121 Firenze, Italy, UdR INSTM, UniVersity of Florence, Via della Lastruccia 3 Sesto Fiorentino, 50019 Sesto Fiorentino (FI), Italy, and CNR-IPCF, Via Giuseppe Moruzzi 1, 56124 Pisa, Italy ReceiVed: December 23, 2007; ReVised Manuscript ReceiVed: March 31, 2008
The present study reports the results of a combined approach which uses morphological characterization, magnetic measurements, and multifrequency electron magnetic resonance (EMR; 9-285 GHz) to identify and characterize both the bulk and the nanoscale magnetic properties of a sample of synthetic hematite, R-Fe2O3. We show here that the heterogeneous nature of the investigated sample is reflected in some peculiarities of the EMR spectra. The use of multifrequency EMR spectroscopy allowed us to assign, for the first time, the signals due to different magnetic modes of the bulk hematite on a powdered sample, indicating a large predominance of this behavior in the considered sample. At the same time, however, we observed a large decrease of the Morin transition temperature for a part of the sample, which we attributed to the presence of a fraction of smaller particles. Moreover, the presence of a single domain superparamagnetic phase was evidenced, due to the fraction of nanometer-size particles. These results are discussed with relation to the magnetic and morphological characterization undertaken by TEM showing the complementarity of the different techniques. Introduction Hematite, R-Fe2O3, is an iron oxide mineral of the oxides and hydroxides group, and it is, together with goethite, the most common iron oxide in Earth’s crust and on the Martian surface. Its name originates from the Greek word for blood, haima, which refers to the dark red color of the mineral’s streak, causing for the red and red-brown staining of soils and surface rocks. Hematite is present in most magmatic, metamorphic, and sedimentary rocks, showing an extreme variability of mineral associations, textures, and crystal size (ranging from a few nanometers up to several centimeters). Hematite weathering, when hematite is exposed to the atmosphere, allows iron to enter in a global cycle, where secondary iron oxyhydroxides play a major role, especially in the pedosphere.1 The release of iron in the environment affects also the cycling of many other elements.2 A recent rise in interest involved the discovery of hematite on the surface of Mars, due to the possible relationship between hematite and a former presence of water.3 Its ubiquitous presence in Earth’s crust4 makes this mineral the object of many studies. The knowledge of the formation and of the specific chemical and physical properties of hematite in each peculiar context can reveal valuable genetic and environmental information.5–8 Hematite can host in structural and interstitial positions of its lattice many heteroatoms, among which are aluminum, transition metal elements, and some * Corresponding authors. Telephone: +390554573336 (L.S.); +390552756349 (F.D.B). Fax: +390554573372 (L.S.); +39055284571 (F.D.B). E-mail:
[email protected] (L.S.);
[email protected] (F.D.B). † DIPTERIS, University of Genova. ‡ Dipartimento di Chimica and Dipartimento di Scienze della Terra, University of Florence. § Dipartimento di Chimica and UdR INSTM, University of Florence. | CNR-IPCF.
semimetals.1 This substitutional capability has a relevant interest for pedogenetic studies, as hematite can control the mobility of several toxic elements.9 The structure of hematite is similar to that of corundum, and consists essentially of a dense arrangement of Fe3+ ions in octahedral coordination with oxygen in hexagonal closest packing. The structure can also be described as a stacking of sheets of octahedrally coordinated Fe3+ ions between two closed-packed layers of oxygen. The Fe-O sheets are held together by strong covalent bonds giving rise to a very hard and dense structure. The magnetic properties of hematite have been the object of many systematic studies over many decades.10–14 The main achieved results can be summarized as follows: (1) Hematite is paramagnetic above 956 K (TN) and presents a first-order magnetic transition at TM ) 263 K called the Morin transition.10b Above this temperature the Fe3+ ions are antiferromagnetically coupled across the shared octahedral faces along the c-axis, giving rise to two interpenetrating sublattices antiferromagnetically coupled. However, as the magnetization vectors of these sublattices are not exactly antiparallel, showing a canting angle of 0.1°, a weak ferromagnetic interaction results.14 At the Morin temperature competition between the weak magnetic anisotropy of the Fe3+ ion and the dipolar anisotropy causes the electron spins to reorient from the basal plane to the c-axis. In this state the spins are exactly antiparallel and hematite is antiferromagnetic. In other words, TM is the temperature where the spins flip from the ab-plane to the c-axis. (2) The magnetic properties of hematite were found to be strongly dependent on the particle size and on the presence of strains and crystal defects.11,15 These studies show that TM decreases with the decrease of particle size down to about 8
10.1021/jp712045s CCC: $40.75 2008 American Chemical Society Published on Web 06/10/2008
Magnetic Properties of Synthetic Hematite nm, where it disappears; TM also decreases with the increase of number of strains and defects within the crystal structure. (3) The magnetic properties were also found to be dependent on the morphology of the particle; indeed for polyhedral and plate-like crystals, TM is shifted to lower values, whereas for needle-like and disk-shaped crystals it is shifted to higher temperatures.16 More recently, the interest toward hematite magnetic properties has been renewed due to the discovery of 5-10 µm crystalline hematite on the Martian surface.17 This has been considered as a candidate source for the magnetic anomalies on the Martian southern hemisphere4,18 and prompted a series of characterizations of different size single-domain particles, with particular attention to their thermoremanent magnetization properties after cycling through the Morin transition.19,20 Furthermore, the general interest in the magnetic properties of nanostructured iron oxide based materials led different groups to perform accurate magnetic studies on monodispersed nanopowders and shape-selected nanoparticles.11,13,21,22 However, the easiness of occurrence of hematite in different textures and morphologies, as in most of the rock and soil natural samples, requires investigation methods able to discriminate properties of mineral assemblages where bulk and polydispersed nanosized hematite can occur. In this respect, a crucial role can be played by electron magnetic resonance (EMR) techniques, which act as a local probe and can give useful information about size and shape distributions in nanostructured materials. As an example, a recent study by one of the authors23 suggested the feasibility of the use of magnetic phases as “tracers” of heteroions, i.e., minor to trace substituents of the main elements constituting the host mineral, using combined electron magnetic resonance and electron spin-echo coupled investigations. The reliability and accuracy of the spectroscopic results require, however, a deep knowledge of the magnetic and magnetic resonance features of the host. In the literature, EMR studies on hematite were quite frequent in the past,24 but they were applied to specific or “ideal” cases, i.e., bulk single crystals, and only in relatively narrow frequency ranges. A systematic multifrequency (from 9.5 to 285 GHz) EMR study was therefore undertaken to verify if the information available in literature on the different hematite typologies can be properly assessed by studying an hematite sample synthesized without any particular control of the shape and size of crystallites. Our main focus was indeed the investigation of a polydisperse material ranging in size from nano- to micrometers. Synthetic hematite was chosen to avoid the effects of heteroatom substitution. To reduce possible ambiguities arising from EMR investigation, this was paralleled by a detailed characterization of the magnetic properties of the sample. Further, a transmission electron microscopy (TEM) study was performed to better establish a correspondence between EMR results and the microstructural characteristics of hematite. Experimental Section The analyzed sample is a synthetic red powder produced by Pfizer Inc. (purity 99.9%). Structural and Morphological Characterization. The powder X-ray diffraction (XRD) analysis was carried out using a Philips PW1710 diffractometer equipped with a Co anode (Co KR radiation, graphite monochromator; current 20 mA, voltage 40 kV, time per step 2.000 s, step size 0.030, start angle 17° 2θ, end angle 130° 2θ) and interfaced with PC-APD software for data acquisition and processing. Structural characterizations were obtained from Rietveld refinement using Fullprof 2000 software.
J. Phys. Chem. C, Vol. 112, No. 27, 2008 9989 Transmission electron microscopic (TEM) analyses were carried out with a Jeol JEM-2010 TEM at 200 kV. The samples were prepared by grinding selected amounts of the specimens, which were ultrasonically dispersed in alcohol and then deposited onto “holey” carbon coated copper grids. Analytical electron microscopic (AEM) investigations were performed using an X-ray EDS system (Oxford Pentafet). Magnetic Characterization. Magnetization measurements were performed using a Cryogenic S600 and MPMS Quantum Design superconducting quantum interference device (SQUID) magnetometers in the 2-300 K temperature range on a powder sample. The powder was either measured as is, or was pressed in a pellet to prevent preferential orientation of the crystallites in the magnetic field. High field magnetization measurements (up to 120 KOe) at different temperatures (10-290 K) were performed on a vibrating sample magnetometer (Oxford Instruments) on a single crystal of natural hematite (Sample No. M0019, mineralogical collection, DIPTERIS) with the magnetic field oriented along and perpendicularly to [001]. Electron Magnetic Resonance. X-Band EMR (ca. 9.4 GHz) spectra were measured with a Bruker Elexsys E500 equipped with liquid N2 (ER4131VT, Bruker) and 4He flux (Oxford Instruments) cryostats to work at variable temperatures (298-120 K and 120-4 K, respectively). W-Band (ca. 94 GHz) EMR spectra were measured with a Bruker Elexsys E600 CW spectrometer equipped with a 60 KOe split-coil superconducting magnet (Oxford Instruments). We used a Teraflex resonator having a cylindrical cavity operating in the TE011 mode. A continuous flow ESR910 4He cryostat by Oxford Instruments was used to work at variable temperatures between 4 and 300 K. Samples were embedded in wax to avoid preferential orientation. HF-EMR spectra (ν ) 190-285 GHz, H ) 0-120 KOe, T ) 30-300 K) were recorded on a home-built spectrometer at the IPCF-CNR in Pisa working in single pass mode with oversized wave guides. Samples were pressed in a pellet mixed with n-eicosane to prevent preferential orientation of the microcrystallites. However, spectra recorded on loose powder did not show any significant differences. Computational Details Calculations of the resonant fields were performed through a self-assembled workspace, where the equations describing them were implemented without approximations.25 The weak ferromagnetic (WF) modes were calculated in the two main parallel and perpendicular orientations, i.e., ξ ) 0, 90°, and in selected orientations in the range between these two extreme values. Due to the complexity of establishing a general H(T,ω,ξ) expression describing the antiferromagnetic (AF) modes, H(T,ω,0) and H(T,ω,90) were implemented separately according to the procedure followed by Morrish.25 It is to be noted here that the angle θ (Table 1) represents the angle between the z-direction and the vector difference of the two sublattice magnetizations. All theoretical resonant fields (see Table 1), except those arising from eqs 4 and 5, were calculated through numerical solution of the characteristic polynomial expressions arising from the combination of eqs 1, 2, and 3 (WF modes) and of eqs 6, 7, 8, and 9 (AF2 mode). Uncertainty in the evaluated fields is on the order of 10 Oe. Equations 4 and 5 were exactly solved as a function of temperature. Results and Discussion Structural and Microfeatures. X-ray diffraction analyses confirm that the analyzed sample is entirely constituted by
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TABLE 1: Equations 1-9 Used for the Calculation of the Temperature Dependence of the Different Modes of the EMR Spectra of “Bulk” Hematitea equation
mode
Weak Ferromagnetic Modes
H sin ξ(H sin ξ + HD) + 36HEHB cos 6β + 2HEHME -
( )
ω1 2 )0 γ
-2HEHK1(T))H2 cos2 ξ + HD(H sin ξ + HD) + 6HEHB cos 6β + 2HEHME -
( )
ω2 2 )0 γ
( ωγ ) - ( ωγ ) [( γ ) + ( γ ) ] + ( γ ) ( γ ) - H sin ξ cos ξ ) 0 4
2
ω1
2
ω2
2
ω1
2
ω2
2
4
2
2
(1)
WF1
(2)
WF2
(3)
Antiferromagnetic Modes ξ ) 0°
√2HE(HK1(T) + 2HK2(T)) - HD2 + H - ωγ ) 0
(4)
AF1
√2HE(HK1(T) + 2HK2(T)) - HD2 - H - ωγ ) 0
(5)
AF2
(6)
AF1
H2 +
ξ ) 90°
( )
ω0 2 ω2 - 4HBHK2(T) sin2 θ + 30HEHB sin4 θ cos 6β )0 γ γ
()
{( )
cos2 θ
}
ω0 2 - 12HEHK2(T) sin2 θ - 21HEHB sin4 θ cos 6β - 3HEHME + 12HEHB sin4 θ cos 6β + γ ω2 ) 0 (7) 4HEHME γ
()
2HE(HK1(T) + 2HK2(T)) - HD2 )
ω0 γ
(8)
sin θ[2HEHK1(T) - HD2 + 4HEHK1(T) cos2 θ-6HEHB sin4 θ cos 6β-2HEHME] ) HHD a
AF2
(9)
Equations 8 and 9 are needed to obtain the values of some parameters of eqs 4–7; eq 3 generalizes eqs 1 and 2 for a given angle, ζ.
hematite. A good agreement between the structural data resulting from Rietveld refinement (Rwp ) 13.4 and Rp ) 12.5) and those reported in the ICSD-15840 card (Figure 1a) is obtained. No preferred orientation is observed. TEM images show that the synthetic hematite powder is mainly composed by aggregates of heterogeneous nanoparticles varying in size from 20 to 100 nm. Moreover, minor amounts (about 30%) of larger aggregates of up to 400 nm were also observed (Figure 1c). Individual particles within aggregates are characterized by several habits varying from elongated prisms to subrounded and disk-shaped grains and polyhedral and hexagonal plates (Figure 1b,c). No evidence of particles smaller than 20 nm was obtained by highresolution TEM (HRTEM). Moreover, the latter technique allows obtaining well-defined lattice images of single particles confirming their good crystallinity even in the smaller particles
(Figure 1d). Finally, the AEM analyses confirm that only iron and no other metal is present in all of the investigated crystals. Magnetic Characterization. The temperature dependence of the magnetization was investigated with an applied field of 50 Oe after a zero field cooling (ZFC) and a field cooling (FC) procedure (Figure 2). The main features of the curves are the two thermal irreversibilities observed at T < 150 K and for 230 K < T < 300 K. The first one depends on the cooling procedure (zero field or field cooling) and is characteristic of the blocking process of the magnetic moment of single domain particles. In the case of antiferromagnetic materials such as hematite, the nonzero magnetic moment which blocks at low temperature arises from the uncompensated spins on the surface of the particles.26 Therefore, this behavior suggests the presence of small particles with size below the single domain threshold
Magnetic Properties of Synthetic Hematite
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Figure 1. (a) Experimental powder diffraction pattern and difference between Rietveld refinement calculated (XRDc) and observed (XRDo) patterns of synthetic hematite. (b, c) TEM images acquired at (b) 600× and (c) 50× magnification showing the different habits. (d) HRTEM image at 800× magnification. The white square evidences the lattice fringes that testify to the good crystallinity of the particle even at nanometer scale.
Figure 2. Temperature dependence of the magnetization of the sample on heating after zero field cooling (filled squares) and on field cooling (open circles) The inset shows a detail of the hysteresis cycle at 2.5 K.
(ca. 30-80 nm depending on the particle shape). The particle size distribution is large and produces a smearing of the blocking temperature over a large temperature range below 150 K. This interpretation is further confirmed by the observation of an open hysteresis loop at 2.5 K (Hc ) 0.25 kOe, inset of Figure 2) contrary to what is expected for highly crystalline multidomain antiferromagnetic particles. However, it must be noted that a contribution to the opening of the M vs H loops may also come from a defect moment independent of the spin-canted one of the WF state. The second thermal irreversibility depends not on the cooling procedure but rather on the thermal cycle, i.e., heating or cooling, independently of the applied field. Indeed, the FC curve measured on heating the sample coincides above 150 K with
the ZFC one. This thermal irreversibility occurs around the Morin transition and is accordingly accompanied by a large increase in the observed magnetic moment due to the AF to WF transition. Above the transition, all the particles are in the weak ferromagnetic phase, and accordingly a much larger coercivity is observed (Hc ) 1.36 kOe at room temperature). The spontaneous magnetization, M0, evaluated from the linear fit of the high field part of the curve (25 kOe < H < 120 kOe), is 0.27 emu/g, in the range expected for hematite powders, confirming the good quality of our sample. Nanocrystals of size comparable to those observed by TEM in our samples are expected, on the basis of literature data, to exhibit the Morin temperature in the 200-255 K range.21 In our case TM, taken as the inflection point in the M vs T curve on heating the sample, is observed at 260 K, close to the bulk value.21,25 This suggests that our sample contains high crystallinity particles and that effects due to the reduced size of the particles on the Morin transition temperature are hidden behind the dominating “bulk” behavior. The hysteresis observed on thermally cycling the sample through the Morin temperature, expressed as the difference ∆TM, between TM values measured on heating and cooling the sample, is 22 K when a 25 Oe field is applied. Neither TM nor ∆TM changes, within the experimental accuracy, for applied field up to 1 kOe; then for larger fields they both decrease with increasing field strength, reaching the values of TM ) 207 K and ∆TM ) 15 K for H ) 65 kOe. The trend qualitatively follows the expected temperature dependence of the critical fields required for the spin-flopping transition.25 This hysteretic behavior has been already reported for other polycrystalline hematite samples, and its appearance and width
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Figure 3. Temperature dependence of the EMR spectra at different frequencies. (a) At 9.4 GHz: the dashed line shows the disappearance of the signal attributed to the WF phase at the Morin transition. The arrow marks the shift and the broadening of the g ) 2.00 line below 30 K. (b) At 95 GHz: the dashed and dotted lines evidence the temperature dependence of the transition attributed to different modes. (c) At 190 GHz: the dashed and dotted lines evidence the temperature dependence of the transition attributed to different modes. (d) At 285 GHz: the dashed and dotted lines evidence the temperature dependence of the transition attributed to different modes. For the two latter frequencies, the sharp signal around g ) 2.00 (Hr ) 68 and 102 kOe, respectively) is the signal from Mn2+ used as a field marker.
were found to be strongly dependent on the crystal size and shape, strains, crystal defects, and impurities.11,13,19,20,21c,22,27 For example, Williamson et al. observed by Mo¨ssbauer spectroscopy a 12 K hysteresis on shock-modified hematite grains with size lower than 200 nm, but no hysteretic behavior was reported in the magnetic data27 and a similar behavior was reported for 200 nm particles prepared by the sol-gel method.21c On the contrary, Bercoff et al. reported ∆TM lower than 10 K in 55 nm R-Fe2O3 particles prepared by ball milling.20 The presence of a thermal hysteretic behavior arises from the firstorder character of the Morin transition and implies the stabilization of a metastable magnetic state beyond TM. Following Goya et al.,21c as crystal defects act as nucleation centers for the AF and WF phases, the observation of the AF/WF state beyond TM requires almost perfect crystals, suggesting once again the high crystallinity of our sample. Electron Magnetic Resonance. X-Band EMR spectra of a powder sample of synthetic hematite measured as a function of temperature clearly evidence different regimes (Figure 3a). At room temperature a broad and asymmetric band, centered at g ) 2.00 with a clear shoulder at very low field, is observed. The shoulder disappears at temperatures below the Morin transition, thus indicating that it is strictly connected with the weak ferromagnetic structure. This is in agreement with an early report of hematite single crystal measurements of zero field absorption at the X-band frequency.24a On the other hand, the broad peak around g ) 2.00 just gains intensity down to 50 K. On further lowering the temperature below 50 K, an interesting phenomenon is observed, namely the shifting and broadening of the g ) 2.00 band: this is a signature for superparamagnetic behavior.28–30
At 95 GHz a much more rich EMR spectral behavior as a function of temperature is observed (see Figure 3b). At room temperature a strong signal at Hr ) 25 kOe is accompanied by a weaker one, around 33.3 kOe, corresponding to g ) 2.00 for this frequency. On lowering the temperature down to TM, both signals remain unaltered in position and line width. However, below 250 K the peak at 25 kOe rapidly disappears, while the g ) 2.00 signal gains intensity, but no shift or broadening is observed. At 240 K a new signal is observed, with Hr ) 11.6 kOe, which moves toward higher values on lowering the temperature, reaching the maximum of Hr ) 40.3 kOe at 100 K and then remaining constant down to 10 K. Finally, at higher frequencies three distinct resonances with different thermal behaviors are observed in the whole temperature range: at 190 GHz the resonance field of the main signal occurring around 57 kOe is constant in the whole temperature range investigated, whereas a marked temperature dependence is observed for the low field signals, visible at 235 K, with Hr(235 K) ) 17 and 46.4 kOe, respectively, which move downfield on lowering temperature. A similar behavior is observed for the 285 GHz spectra, featuring a strong temperature-independent signal at 91 kOe and two further signals which appear above 280 K and below 220 K and shift downward on decreasing and increasing the temperature, respectively. An important point to be noted is that the signals at 58 kOe (190 GHz) and 91 kOe (285 GHz) are strongly reduced in intensity below the Morin transition temperature of bulk hematite (about 260 K) but disappear only at lower temperatures (around 100 K). Interestingly, while a weak signal is observed at g ) 2.00 for 190 GHz, this is completely absent at 285 GHz.
Magnetic Properties of Synthetic Hematite
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TABLE 2: Values of the Parameters Used for the Calculation of the Temperature Dependence of the Different Modes of the EMR Spectra of Hematitea
a
symbol
description
value
HE HD HB HME HK1(T), HK2(T) ω γ ξ β
exchange field Dzialoshinskii field basal plane anisotropy field magnetostriction field first-order and second-order crystalline anisotropy fields experimental frequency gyromagnetic ratio angle between the applied magnetic field and the c-axis of hematite angle between relevant directions (01j0 and 100) in the basal plane
9.2 × 106 Oe 2.22 × 104 Oe 6.8 × 10-4 Oe 7.5 × 10-1 Oe variable 95, 190, 285 GHz 176.1 GHz variable 30°
The calculated modes are labeled as follows: WF1ξ and WF2ξ for the WF state and AF1ξ and AF2ξ for the AF state.
This very rich spectral behavior is not unexpected and explains the long-standing interest in the EMR studies of hematite since the earliest days of this technique.24 However, as mentioned in the Introduction, virtually all of the studies reported up to now concerned angular-dependent single crystal spectra with field applied either in plane or along the symmetry axis, and only very few works were concerned with multifrequency spectra. These contributions pointed out the existence of different collective modes, in both the antiferromagnetic and the weak ferromagnetic phases, with peculiar temperature dependence. The global temperature dependence of the signals assigned to bulk hematite was reproduced by taking into account the theoretical modeling summarized more than a decade ago by Morrish.25 For both AF and WF states two distinct modes, which depend on the structural and magnetic characteristics of hematite and on T, on the instrumental frequency ω, and on the angle ξ between the direction of the external applied field and the c-axis, should be observed. In this approach the observed temperature dependence for each mode is attributed to the terms related to the first-order and second-order crystalline anisotropy energy constants (HK1 and HK2, respectively). The resonant fields of the four modes, expressed as H(T,ω,ξ) relations, were thus obtained from the equations reported in Table 1 using the values reported in Table 2. It is evident from Figure 4 that all the observed signals with the exception of the one at g ) 2.00 can be nicely reproduced by the model, indicating a substantial persistence of bulk-like properties. When compared to single crystal spectra, it is evident that one major difference resides in the weaker intensity of the parallel-type transition with respect to perpendicular ones, as expected for a spherical distribution of the microcrystals, and as usually observed in normal EPR spectroscopy. However, the most important discrepancy from the behavior predicted on the basis of bulk magnetic parameters is the observation of the WF1 mode at 190 and 285 GHz below the TM, even if of decreased intensity. This is very important from a diagnostic point of view as it suggests the presence of a fraction of material with dimensions large enough to undergo the Morin transition but at a largely reduced temperature. This confirms the presence in the sample of a considerable fraction of material of reduced size and/or of different morphologies. Indeed, as already mentioned in the Introduction and in the Magnetic Characterization sections, TM has been reported to decrease with the size of the particle and to critically depend on the morphology.21 In particular, Amin and Arajs11 derived for spherical particles an inverse linear dependence of the Morin temperature on the dimension of the particles, TM ) 264.2 - 2194/d, where d is the diameter of the particle expressed in nanometers and 264.2 is the Morin temperature of the bulk phase. If we assume that
Figure 4. (a) Temperature dependence of the observed (fillled squares) and calculated (continuous line) resonance fields for the AF and WF phases of hematite with applied field along the c-axis at different frequencies. The filled gray circles evidence the temperature dependence of the critical field between the WF and AF phases measured on a single crystal of hematite by VSM with the field along the c-axis. (b) Temperature dependence of the observed (filled squares) and calculated (continuous line) resonance fields for the AF and WF phases of hematite with the applied field perpendicular to the c-axis at different frequencies. The open squares identify the weaker signals attributed to the WF mode below the TM. The filled gray circles evidence the temperature dependence of the critical field between the WF and AF phases measured on a single crystal of hematite by VSM with the field perpendicular to the c-axis. For the identification of the different modes, refer to Table 2.
the temperature at which the signal of the WF phase completely disappears, i.e., 100 K, is the Morin transition temperature for a fraction of our sample, this model would imply an average size of 13 nm for this fraction. This would be much smaller than that observed by TEM, and further, it must be considered that the morphological characterization did not evidence the presence of spherical particles whereas the shape of the grains
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has been reported to play a relevant role in determining the value of the Morin temperature. In particular, on the basis of Mo¨ssbauer spectroscopy results, it was reported13 that the Morin temperature is shifted to lower values for the polyhedral and plate-like hematites, whereas the Morin temperature corresponding to needle-like and disk-shaped hematites is shifted to higher temperatures. These studies also pointed out that, apart from spherical and disk-shaped hematite, a relevant fraction of the WF phase is preserved at low temperature. More recently, a TM of 122 K has been reported for hematite nanorods21f with diameters of 20-100 nm and lengths of 1-2 µm. The comparison of our results with TEM analysis and literature data seems then to suggest the presence in our sample of a fraction of polyhedral and plate-like hematites of about 50 nm which are responsible for the permanence of the WF phase signal even below TM. This fraction is not evidenced in the magnetic characterization because it is has a very low mass and its magnetic contribution is then hidden. In this respect EMR appears more powerful as no other signal is superimposing on that of the persistent WF phase, which is then easily discernible. A final point which is worth discussing is the observation of a signal centered at g ) 2.00 for the lowest-frequency spectra. We assign this signal to the superparamagnetic fraction of the investigated sample, as witnessed by the peculiar thermal behavior of the corresponding signal in the X-band below 50 K and further confirmed by magnetic measurements (see above). The broadening of the line and the shift of the apparent resonance field to lower field occur because on cooling the magnetic moments cannot freely fluctuate anymore. As fluctuations tend to reduce the anisotropic contribution to the free energy and the effective magnetic field sensed by the sample, at higher temperature narrow spectra with larger resonance field are observed. On the other hand, at low temperature the thermal fluctuations are progressively frozen and the spectra are broadened and observed at lower field. In the description of the temperature dependence of the EMR spectra of superparamagnetic nanoparticles, a crucial role is played by the individual nanoparticle line width, which depends on temperature and volume according to31
∆H ) ∆HTL(MVHeff/kT)
(10)
where ∆ΗT is a temperature-dependent saturation line width of the largest particle, which in turns depends on the anisotropy constant of the material, and L(x) is the Langevin function. For ultrafine hematite nanoparticles of estimated diameter of 3.2 nm dispersed in an alumina matrix, a variation of the apparent magnetic resonance field as well as a broadening of the line has been reported, quite surprisingly, already below 350 K.32 The observation for our sample of a broadening at the X-band below 50 K may indicate either a reduced volume of our particles or, more reasonably, a reduced value of the anisotropy constant. As a matter of fact, Morup et al.15,22 estimated a value for nanosized hematite reduced by 1-2 orders of magnitude with respect to that reported by Fiorani et al.32 which could perfectly explain the observed behavior. At any rate, the observation of the superparamagnetic effect in the EMR spectra only at low temperature is perfectly consistent with the TB of the ZFC/FC curve (see above). The absence of the thermal dependence of the superparamagnetic signal and its reduction in relative intensity compared to the bulk ones for the correspondingly higher frequency signal can also be explained in this framework. Indeed, as the resonance magnetic field is increased, the individual nanoparticle line width increases so that the particles responsible for the
superparamagnetic behavior observed at the X-band at higher frequency give so broad a signal as to be unobservable, even at room temperature. The signal observed at higher frequencies should then be attributed to the smallest particles, as this leads to a reduction in the argument of the Langevin function in eq 10. This process reaches a limit at 285 GHz, where no signal at g ) 2.00 is observed. This approach confirms the powerfulness of a multifrequency approach to evidence resonance due to particles of different sizes.33 Conclusions We reported for the first time a multifrequency EMR analysis of a polydisperse synthetic hematite powder, showing that many different behaviors can be observed at the same time on the same sample. In particular, we have shown that the coexistence of different magnetic behaviors, ranging from bulk behavior to superparamagnetism, can be discerned and attributed to particles with different sizes and morphologies. This result has been obtained thanks to the multifrequency approach we adopted. Indeed, spectra recorded at different frequencies evidence the response of particles of different sizes and/or anisotropy (and then of different morphologies). In particular, we stress here the importance of observing the superparamagnetic behavior of hematite nanoparticles in a variable-size sample through electron magnetic resonance. We have shown how the presence of smaller particles, even in a small amount, which escapes detection by the classic morphological analysis, can be evidenced through the help of magnetic and EMR characterization. We think that this combined approach can be successfully applied to an accurate characterization of both natural samples containing hematite in different shapes and dimensions and of polydisperse materials ranging from nano- to microscale. Acknowledgment. We acknowledge the financial support from ItalianMIUR (FIRB and PRIN grants), from the EC through the NE-MAG-MANET (NMP3-CT-2005-515767), and of Ente Cassa di Risparmio di Firenze. We are grateful to Prof. M. Romanelli for fruitful discussions. References and Notes (1) Cornell, R. M.; Schwertmann, U. The Iron oxides. Structure, properties, reactions, occurrence and uses; VCH: Weinheim, 1996. (2) Zhang, W.; Yu, L.; Hutchinson, S. M. Sci. Total EnViron. 2001, 266, 169. (3) Jakosky, B. M. Science 1999, 283, 648. (4) Kletetschka, G.; Wasilewski, P. J.; Taylor, P. T. Earth Planet. Sci. Lett. 2000, 176, 469. (5) Murad, E.; Rojik, P. Am. Mineral. 2003, 88, 1915. (6) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (7) Carbone, C.; Di Benedetto, F.; Marescotti, P.; Martinelli, A.; Sangregorio, C.; Cipriani, C.; Lucchetti, G.; Romanelli, M. Eur. J. Mineral. 2005, 17, 785. (8) Carbone, C.; Di Benedetto, F.; Marescotti, P.; Sangregorio, C.; Sorace, L.; Lima, N.; Romanelli, M.; Lucchetti, G.; Cipriani, C. Mineral. Petrol. 2005, 85, 19. (9) Romero, F. M.; Armienta, M. A.; Gonzalez-Hernandez, G. Appl. Geochem. 2007, 22, 109. (10) (a) Smith, T. T. Phys. ReV. 1916, 8, 721. (b) Morin, F. J. Phys. ReV. 1950, 78, 819. (c) Nee´l, L. ReV. Mod. Phys. 1953, 25, 58. (d) Lin, S. T. Phys. ReV. 1959, 116, 1447. (11) Amin, N.; Arajs, S. Phys. ReV. B 1987, 35, 4810. (12) Mørup, S.; Tronc, E. Phys. ReV. Lett. 1994, 72, 3278. (13) Sorescu, M.; Brand, R. A.; Mihaila-Tarabasanu, D.; Diamandescu, L. J. Appl. Phys. 1999, 85, 5546. (14) Dzyaloshinsky, I. E. J. Phys. Chem. Solids 1958, 4, 241. (15) Bodker, F.; Hansen, M. F.; Koch, C. B.; Lefmann, K.; Mørup, S. Phys. ReV. B 2000, 61, 6826. (16) Sorescu, M.; Brand, R. A.; Mihaila-Tarabasanu, D.; Diamandescu, L. J. Alloys Compd. 1998, 280, 273.
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