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Data Brief Volume 7, Number 9 7 September 2006 Q09003, doi:10.1029/2006GC001299
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525-2027
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Low-temperature first-order reversal curve (FORC) diagrams for synthetic and natural samples Claire Carvallo Institut de Mine´ralogie et de Physique des Milieux Condense´s, 140 Rue de Lourmel, F-75014 Paris, France (
[email protected])
Adrian Muxworthy GeoForschungsZentrum Potsdam, Section 3.3, Telegrafenberg, D-14473 Potsdam, Germany Permanently at Department of Earth Science and Engineering, South Kensington Campus, Imperial College, London SW7 2AZ, UK
[1] First-order reversal curve (FORC) diagrams were measured on a variety of synthetic and natural (submarine basalts and potsherds) samples, as well as for a mixture, between room temperature and 10 K. Measuring FORC diagrams allowed us to examine the behavior of the coercivity field and interaction field distributions with decreasing temperature. Generally, as the temperature is decreased, FORC contours were observed to expand in all directions, e.g., the maximum coercivity peak migrates toward a higher coercivity. For synthetic magnetite samples, there were abrupt changes in the FORC distribution at the Verwey transition (120 K), while in the more complex natural magnetite samples the FORC distribution gradually progressed with temperature. As the temperature decreased, FORC diagrams were found to display different domain state characteristics. For example, samples with dominant superparamagnetic signals became single-domain (SD)-like, a SD-like sample stayed SD-like, and a PSD-like sample became more SD-like. In addition to these general features, we also observed some more specialized features. First, in both synthetic and natural magnetite samples, a secondary higher-coercivity peak is occasionally present below the Verwey transition, which we suggest is a twinning contribution. Second, the coercivity increases by a factor 15 between 300 K and 20 K in some of the seamount samples. Third, the effect of field cooling/ zero-field cooling on FORC diagrams is negligible, with the hysteresis parameters HC and HCR displaying a greater dependency. Finally, it is shown that in mixtures of magnetite and hematite, the hematite contribution disappears on cooling below the Morin transition, leaving a strong, well-defined magnetite signal. Components: 5385 words, 11 figures, 2 tables. Keywords: FORC diagram; low temperature; ODP Leg 197. Index Terms: 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques. Received 7 March 2006; Revised 4 May 2006; Accepted 20 June 2006; Published 7 September 2006. Carvallo, C., and A. Muxworthy (2006), Low-temperature first-order reversal curve (FORC) diagrams for synthetic and natural samples, Geochem. Geophys. Geosyst., 7, Q09003, doi:10.1029/2006GC001299.
Copyright 2006 by the American Geophysical Union
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1. Introduction [2] In the last five years, first-order reversal curve (FORC) diagrams, calculated from partial hysteresis curves, have become an efficient way of characterizing magnetic domain-state in grain assemblages [Roberts et al., 2000]. A complete description of the measurement method, analysis and interpretations of FORC diagrams is given by Roberts et al. [2000] and Muxworthy and Roberts [2006]. As a first approximation, FORC distributions can be interpreted as the coercivity distribution along the horizontal axis and the interaction field distribution, along the vertical axis [Muxworthy and Williams, 2005]. Our further understanding of FORC diagrams has greatly improved in the last few years, thanks to micromagnetic modeling [Stancu et al., 2003; Muxworthy and Williams, 2005] and measurements on both synthetic and natural samples [Roberts et al., 2000; Muxworthy et al., 2005; Carvallo et al., 2006]. [3] The vast majority of FORC diagrams have been measured at room temperature, while a few have been measured at high temperature [Muxworthy and Dunlop, 2002], with only one published FORC diagram below room temperature published [Carvallo et al., 2004a]. A number of magnetic properties, e.g., susceptibility and coercivity field, vary with temperature, due to the temperature dependency of the controlling magnetic energies, e.g., the exchange coupling. In addition to gradual changes, low-temperature transitions, such as the Verwey transition for magnetite and the Morin transition for hematite, are often used to characterize magnetic mineralogy in natural samples. [4] In this paper we present FORC diagrams measured between room temperature and 10 K, for both synthetic and natural samples, and for a mixture of synthetic magnetite and natural powdered hematite. The main aim of this study was to observe the response of FORC diagrams to the low-temperature changes, and to assess the suitability of low-temperature FORC magnetometry in rock magnetism.
2. Samples 2.1. Synthetic Samples [5] The synthetic samples consist of four powdered pseudo-single-domain (PSD) magnetite samples (W(0.3 mm), W(11 mm), W3006 and W5000) manufactured by the Wright Industries
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Inc. Samples W5000 and W(0.3 mm) were similar, however, W(0.3 mm) was stored in a dessicator unlike W5000, and the two samples come from different batches. As such we treat the two samples separately. [ 6] Samples W(0.3 mm) and W(11 mm) have been previously described in detail (including FORC diagrams) [Muxworthy and Dunlop, 2002; Muxworthy et al., 2003, 2005]. The hysteresis parameters measured for this study for W(0.3 mm) and W(11 mm) are similar to those reported previously (Table 1). W(0.3 mm) and W(11 mm) were found to be near stoichiometric when their Mo¨ssbauer spectra were measured [Muxworthy et al., 2003]. SIRM cooling and warming curves indicated Verwey transition temperatures of 118 K in both samples (Table 1). Curie temperatures for sister samples were determined to be 583C [Muxworthy and Dunlop, 2002], indicating a small degree of oxidation. These samples were stored in a dessicator. No noticeable change in their FORC diagrams was observed before and after storage. [7] Both W3006 and W5000 powders are almost pure magnetite, with Curie temperatures close to 580C, and a secondary Curie temperature around 300C, probably due to some oxidation, which disappears upon cooling. From the warming curve in zero-field of saturation isothermal remanent magnetization (SIRM) produced at 20 K, Verwey transitions are clearly visible at 115 K. SIRM drops by about 80% for W3006 and 25% for W5000 when going through the Verwey transition. After being heated to 600C, W5000 does not show the secondary Curie temperature and the SIRM drop at the Verwey transition is almost 60%.
2.2. Natural Samples [8] Some magnetic properties (Mrs/Ms, Hc, Verwey temperature when applicable and Curie temperature) are given in Table 2. [9] Six of the natural samples in this study come from the Emperor Seamounts and were recovered during Leg 197 [Tarduno et al., 2003]. Titanomagnetite Fe3-xTixO4 and titanomaghemite, with an ulvo¨spinel fraction x varying between 0.6 and 0.03, are the main magnetic carriers in these samples [Carvallo et al., 2004b]. [10] Two other natural samples are potsherd fragments from a native site in Southern Ontario and were used for paleointensity determinations [Carvallo and Dunlop, 2001]. The Curie temperature, calculated from the saturation magnetization 2 of 12
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Table 1. Physical, Chemical, and Magnetic Properties of the Synthetic Samplesa Sample Name
Grain Size, mm
m0HC, mT
MRS/MS
TV or Morin, K
Chemical Description
W(0.3 mm) W(11 mm) W3006 W5000 LH6
0.3 (0.2) 11 (3) 1.06(0.71)c 0.3(0.2)c 75 – 100
30 4.2 11.8 34.2 180d
0.29 0.06 0.17 0.38 0.77d
118b 118 115 116 210 – 230
magnetite magnetite magnetite magnetite hematitee
a The grain-size distributions for samples W(0.3 mm), W(11 mm), W(5000), and W3006 were determined from scanning electron micrographs. The grain size standard deviations are shown in brackets. The grain size for sample LH6 is nominal. The hysteresis parameters were determined using a maximum field of 1 T, the same as the FORC method maximum field. The chemical compositions for W(0.3 mm), W(11 mm), and LH6 were determined from Mo¨ssbauer analysis, XRD analysis, microprobe analysis, and quantitative Rietveld analysis [de Boer and Dekkers, 1996, 1998; Muxworthy et al., 2005]. b Weak Verwey transition, probably suppressed by grain size. c Measured on a sister sample by Yu et al. [2002]. d Measurement in a field of 5 T did not significantly change the hysteresis parameters [Muxworthy et al., 2005]. e Evidence for traces of magnetite and/or maghemite in the sample.
(Ms (T)) curve around 565C and the absence of Verwey transition on the SIRM warming curve indicate that the magnetic carrier is titanomagnetite (x 0.05) [Hunt et al., 1995], or magnetite substituted with other elements (e.g., aluminum).
(FWHM) of the main peak of the FORC distribution parallel to the Hu axis through the peak Hc [Muxworthy and Williams, 2005].
[ 11 ] A natural powered hematite containing maghemite (LH6) was considered. LH6 was crushed from a massive hematite aggregate [Hartstra, 1982], with nominal size range 75 – 100 mm, and was stored in air for 24 years. X-ray diffraction (XRD) and Mo¨ssbauer analysis conducted shortly before the FORC diagrams were measured indicated that the sample was 98% hematite with a trace of either magnetite and/or maghemite [Muxworthy et al., 2005]. The hysteresis parameters (Table 1) are in good agreement with those previously reported for LH6 samples [de Boer and Dekkers, 1998]. SIRM cooling and warming curves found a Morin transition [Morin, 1950] in the range 210–230 K (258 K for stoichiometric hematite), suggesting that the hematite in LH6 is slightly nonstoichiometric. Measurement of the temperature dependence of susceptibility indicated a small percentage of magnetite in LH6 through the identification of the Verwey transition [Muxworthy et al., 2005].
3.1.1. W3006
3. FORC Diagrams as a Function of Temperature [12] For each sample and at each temperature where a FORC diagram was measured, we determined the main FORC peak (MFP), which is the coercivity field corresponding to the maximum of the FORC distribution. The interaction field is quantified with the full width at half-maximum
3.1. Synthetic Samples [13] The FORC diagram at 300 K is characteristic of a PSD grain distribution, with the innermost contour being closed and the outermost contours intersecting the Hc = 0 axis (Figure 1a). FORC diagrams and hysteresis parameters do not show any significant variations for temperatures 100 K. The effect of the Verwey transition is seen between 130 and 100 K, where the FORC distribution expands along both axis, and the MFP moves toward high coercivities (Figure 1b). At 50 K, the MFP increases and strong asymmetries develop: a positive one, at Hc = 30 mT, Hu = 25 mT, and a negative one, along the Hc = Hu axis (Figure 1c).
Table 2. Magnetic Properties of the Natural Samplesa Sample Name
m0HC, mT
MRS/MS
TV, K
Tc, C
6A2 5A3 4A2 5G1 5D3 6C1 Potsherd C Potsherd X
25.3 33.3 7.7 9.1 30.6 5.9 7.7 7.5
0.781 0.417 0.669 0.507 0.459 0.210 0.248 0.221
none 110 none none 110 none none none
180 211 262 260 575 565 565
a
The properties of LH6 although of natural origin are shown in Table 1. The sample was in powder form, making it easier to conduct a more detailed analysis. The Curie temperature was not measured for sample 6C1.
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Figure 1. FORC diagrams for sample W3006 at various temperatures, (a) room temperature, (b) 100 K, and (c) 50 K, and (d) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature. In all three FORC diagrams, 80 FORCs were measured, smoothing factor SF = 3, averaging time is 0.2 s, and field step is 5 mT.
These features are also present when the temperature is decreased to 20 K and 10 K. [14] The Verwey transition is well marked in the variation of the MFP as a function of temperature, which stays constant until 120 K and then increases up to 4 times the room temperature, reached for T = 10 K. The FWHM is also constant until 120 K, then jumps by about 30% and does not vary when the temperature is decreased further to 10 K (Figure 1d).
3.1.2. W5000 [15] The closed contours on the FORC diagram at room temperature are consistent with the grain size
in the small PSD range (Figure 2a). When the temperature is decreased down to 20 K, the FORC distribution becomes more SD-like, with more closed contours, and spreads more toward high coercivities (Figures 2b and 2c). The position of the MFP as well as the FWHM do not show any significant variations dependent on temperature, but only a slight progressive increase with decreasing temperature. There is only a slight increase in the coercivity of the MFP (by about 20%) when crossing the Verwey transition (Figure 2d). [16] We remeasured the same sample after it had been heated once to the Curie temperature. Now we only have pure magnetite in the sample, as
Figure 2. FORC diagrams for sample W5000 at various temperatures, (a) room temperature (field step is 10 mT), (b) 100 K (field step is 65 mT), and (c) 20 K (field step is 130 mT), and (d) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature. In all FORC diagrams, 80 FORCs were measured, SF = 3, and averaging time is 0.2 s. 4 of 12
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Figure 3. FORC diagrams at various temperatures for samples W5000 reheated, (a) room temperature and (b) 100 K, and (c) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature. In all FORC diagrams, 80 FORCs were measured, SF = 3, averaging time is 0.2 s, and field step is 5 mT.
Figure 4. FORC diagrams at various temperatures for a potsherd sample (sample labeled C by Carvallo and Dunlop [2001]), (a) room temperature (field step is 1.7 mT), (b) 200 K (field step is 1.3 mT), (c) 130 K (field step is 1.6 mT), (d) 100 K (field step is 2 mT), (e) 50 K (field step is 4 mT), and (f) 20 K (field step is 4 mT), (g) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature, and (h) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature for the second potsherd sample (sample labeled X by Carvallo and Dunlop [2001]). In all FORC diagrams, 80 FORCs were measured, SF = 5, and averaging time is 0.2 s. 5 of 12
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Figure 5. FORC diagrams for sample 6A2. (a) Room temperature (field step is 1.3 mT), (b) 150 K (field step is 13 mT), (c) 20 K (field step is 18 mT), and (d) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature. In all FORC diagrams, 100 FORCs were measured, SF = 5, and averaging time is 0.4 s. Note the scale difference between Figures 5a and 5b.
indicated by the large drop in SIRM at the Verwey transition and by the single Curie point at 580C on the saturation magnetization variation. The room temperature FORC diagram is similar to that of the nonheated sample (Figure 3a). The effect of the Verwey transition is more dramatic than for the nonheated sample. Between 200 and 100 K, the FORC distribution spreads in both directions (Figure 3b), then stays constant when the temperature is further decreased to 50 K. This evolution is reflected in the behavior of the MFP, which remains constant between 300 K and 200 K and then increases progressively until 20 K, where its value is more than the double of the room temper-
ature value. The FWHM shows a similar behavior. It increases by 40% in one step between 200 and 100 K (Figure 3c).
3.2. Natural Samples 3.2.1. Potsherd Samples [17] Both pottery samples have the same behavior on the FORC diagrams. The room temperature FORC diagram is characteristic of SP behavior (Figure 4a) [Roberts et al., 2000]. When the temperature is decreased, a SD peak appears, characterized by a higher coercivity and closed contours. This SD peak progressively replaces the
Figure 6. FORC diagrams for sample 5A3. (a) Room temperature (field step is 5 mT), (b) 100 K (field step is 6.5 mT), (c) 10 K (field step is 9.8 mT), and (d) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature. In all FORC diagrams, 80 FORCs were measured, SF = 5, and averaging time is 0.2 s. 6 of 12
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Figure 7. FORC diagrams for sample 5G1. (a) Room temperature (field step is 1 mT), (b) 20 K (field step is 4.9 mT), and (c) variation of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature. In all FORC diagrams, 80 FORCs were measured, SF = 5, and averaging time is 0.2 s.
SP peak. The peaks are clearly separately seen at 200, 130 and 100 K (Figures 4b, 4c, and 4d). At 20 K, only the SD contribution is left on the FORC diagram (Figure 4f). In the same time, the FWHM increases between 200 K and 50 K, as well as the spread of the FORC distribution along the Hc axis (Figure 4g). This behavior is caused by the fact that the SP/SD size threshold varies as a function of the temperature and the magnetocrystalline anisotropy magnitude is also temperature dependent. When the temperature decreases, the threshold increases, so grains that were SP at room temperature become stable SD. The evolution of MFP with temperature follows the same trend (Figure 4g): as long as the SP grains dominate the magnetic properties, the MFP is zero. When the SD behavior appears, the MFP increases until T = 20 K. The FWHM also increases in the range 200 –20 K. The second potsherd sample (labeled X by Carvallo and Dunlop [2001]) shows a very similar evolution for the FORC diagrams as well as for the MFP and FWHM (Figure 4h).
3.2.2. Seamount Samples [18] These samples have a complex magnetic mineralogy. Tarduno and Cottrell [2005] measured FORC diagrams on some of the samples from site 1205 (both whole rock and single crystal) used for paleointensity measurements and found a mixture of MD and SD grain sizes. In our study, several types of behaviors were observed. Three representative examples of behavior are shown in Figures 5, 6, and 7.
[19] From the MS(T) curve, sample 6A2 (ODP name: 1206A, 22R1, 127–129) is composed of titanomaghemite with a TC of 180C and an ulvo¨spinel ratio x 0.6. The room temperature FORC diagram indicates the presence of SD grains, with almost all the contours being closed and the MFP is around 30 mT (Figure 5a). When the temperature is decreased, the MFP increases dramatically. At 150 K, the MFP is an order of magnitude larger than the room temperature MFP (Figure 5b). Between 150 and 50 K, the coercivity doubles (Figure 5c). In the same time, the FORC contours expand in both directions, but the shape of the FORC distribution stays roughly constant. The MFP and the FWHM also show a strong increase (at least an order of magnitude) between 200 and 20 K (Figure 5d). [20] In some cases, the FORC distribution varies much less with temperature. For example, sample 5A3 (ODP name: 1205A, 18R2, 16–18, titanomaghemite with x 0.55) is PSD according to the room temperature FORC distribution (Figure 6a). When the temperature decreases, the only visible evolution is a progressive spread in the same proportions along both axes (Figures 6b and 6c). The coercivity decreases slightly until 100 K, and then increases sharply between 50 and 20 K (Figure 6d). The FWHM increases linearly by about 40% over the temperature range studied. [21] The FORC diagram evolution of sample 5G1 (ODP name: 1205A, 27R4, 60–62) resembles that of sample 5A3, with an important spread of the 7 of 12
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Figure 8. Variations of main FORC peak (plus symbols) and FWHM (cross symbols) with temperature for seamount samples: (a) 4A2, (b) 5D3, and (c) 6C1.
contours, especially along the Hc axis (Figure 7). However, the most interesting feature is the appearance of a secondary peak close to the Hc = 0 axis, in the positive Hu region, which displaces the main peak in the negative Hu region, at 20 K (Figure 7b). This pattern is very similar to the asymmetry observed for the W3006 synthetic sample (Figure 1c). Both the MFP and the FWHM increase when the temperature decreases (Figure 7c). [22] For the other three samples, the FORC diagram evolution resembles one of these two typical behaviors. Sample 4A2 (ODP name: 1204B, 13R3, 46–48, Figure 8a) is similar to 6A2 (Figure 6), and show a fairly strong global increase in MFP and FWHM. The zero-field warming curve of SIRM of sample 4A2 is an N-type curve with a compensation point around 60K, which indicates that a selfreversal of the magnetization may have occurred in this sample. Such self-reversal has been observed in basalts from ODP Site 883, which is 0.5 km from site 1204 [Doubrovine and Tarduno, 2004a, 2004b]. 5D3 (ODP name: 1205A, 33R3, 30 – 32, Figure 8b) and 6C1 (ODP name: 1206A, 41R1, 35–37, Figure 8c) are still characterized by an increase in FWHM, especially at temperatures lower than 50 K, but their MFP does not vary consistently.
3.3. Magnetic Mixture [23] We measured FORC diagrams as a function of temperature for a mixture made with the natural powder samples LH6 (hematite) + W(0.3 mm) magnetite. The end-members have very different coercivities (Figures 9a and 9e) and are therefore clearly resolved on the room temperature FORC diagrams (Figure 9c). When the temperature is decreased below the Morin transition, only the magnetite component can be seen (Figure 9d). The Morin transition marks the change from a
canted antiferromagnetic (above the transition) to a perfect antiferromagnetic (below the transition) magnetic structure, which explains why the hematite signature disappears on the FORC diagram below the Morin transition (Figure 9b).
3.4. Influence of FC-ZFC on the Low-Temperature FORC Diagrams [24] The magnetic behavior of magnetite below the Verwey transition is known to be strongly affected by the presence of an external field on cooling [Moskowitz et al., 1993; Kosterov, 2001, 2002]. In multidomain magnetite, this dependency is due to the interaction and alignment of the low-temperature monoclinic crystallographic axes with the applied external field; the large magnetocrystalline anisotropy in the monoclinic phase is aligned to accommodate the magnetic structure. Normally two regimes are considered, zero-field cooled (ZFC) regime and a field cooled (FC) regime, where the applied field is usually the maximum available, i.e., a saturating field >0.5 T. All the measurements presented so far were done in ZFC conditions. [25] The affect of zero-field/field cooling (1.8 T) on the FORC diagrams of sample W(0.3 mm) is found to be very subtle (Figure 10), the differences on the FORC diagram being primarily a small shift in the main peak to higher values of HC. Between the FORC diagram at 140 K, and the two FORC diagrams at 80 K temperature, the differences are remarkably small. [26] Basic hysteresis parameters such as HC and HCR appear to be more revealing as to the importance of field during cooling through the Verwey transition (Figure 11). In this figure, the applied field during FC is varied. It is seen that below an applied field of 20–30 mT, FC and ZFC behavior 8 of 12
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Figure 9. FORC diagrams for mixtures of magnetite sample W(0.3 mm) and hematite-rich sample LH6 at 300 K and at 140 K below the Morin transition. (a) LH6 at 300 K, (b) LH6 at 140 K, (c) 2.5% W(0.3 mm) and 97.5% LH6 at 300 K, (d) 2.5% W(0.3 mm) and 97.5% LH6 at 140 K, (e) W(0.3 mm) at 300 K, and (f) W(0.3 mm) at 140 K. One hundred twenty FORCs were measured, SF = 4, and the averaging time is 0.25 s. A major hysteresis loop for LH6 at room temperature is depicted by Muxworthy et al. [2005]. At 140 K, LH6’s major hysteresis curve appears as a reversible ‘‘paramagnetic’’ straight line. 9 of 12
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is essentially the same. Above 100 mT, HC and HCR are independent of applied field during cooling. In-between (30–100 mT), however, HC and HCR display a dependency on the applied field. This range is approximately the same range as the coercivity distribution of the sample. It is suggested therefore that to reach the ‘‘maximum FC state,’’ i.e., for applied fields >100 mT, the applied field has only to be sufficient to overcome the coercivity spectrum.
4. Discussion and Conclusions [27] The main patterns of FORC diagrams with temperature are in agreement with the expected evolution of grain size: when the temperature decreases, a sample which is SD-like at room temperature stays SD-like at low-temperature (Figures 2 and 3), and a sample which is PSD-like at room temperature becomes SD-like at lowtemperature (Figure 1). This is due to both a reduction in temperature, and also crystallographic changes in magnetite. Micromagnetic models predict that the critical SD size increases on cooling through the Verwey transition from 0.07 to 0.14 mm [Muxworthy and Williams, 1999]. It has also been shown that a sample dominated by SP grains at room temperature becomes stable SD at low-temperature (Figure 4), because the thermal energy is lower.
Figure 10. FORC diagrams for sample W(0.3 mm) at (a) 140 K, (b) 80 K after zero-field cooling, and (c) 80 K after field cooling in 1.8 T. The field applied during cooling was parallel to the direction of the FORC measurement. One hundred twenty FORCs were measured, SF = 4, and the averaging time is 0.25 s.
[28] The position of the coercivity peak (MFP) is also consistent with the modeled and experimental studies of the variation of coercivity with ¨ zdemir et al., 2002]. The increase temperature [O in coercivity in the synthetic magnetite samples at the Verwey transition (Figures 1 and 2), corresponding to a displacement of the main FORC peak toward higher coercivities, is in agreement with the large changes in magnetocrystalline anisotropies when magnetite transforms from cubic to monoclinic crystallographic structure at the Verwey transition. The natural samples display a more gradual increase in coercivity. These samples have a more complex mineralogy, and their coercivities are governed by the various contributions of magnetocrystalline, magnetoelastic and shape anisotropies, which are all related to Ms and anisotropy constants (K1 for magnetocrystalline and ls for magnetoelastic anisotropy), themselves depending on the temperature. [29] The FORC diagrams of the mixture studied is also consistent with the evolution we expected,
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Figure 11. HC and HCR versus applied cooling field for sample W(0.3 mm) at 80 K. The ZFC data is plotted at a cooling field value of 1 mT.
showing in particular that the hematite contribution disappears below the Morin transition. [30] We also observed a few intriguing features. On several occasions, a secondary peak appears under 70 K, both in synthetic (Figure 1 for the W3006 PSD magnetite) and natural (Figure 7 for an Emperor Seamount sample). The peak is above the main peak, in the positive Hu region and it displaces the main peak slightly below the Hu = 0 axis. It is not an artifact of the processing but a real feature. It may be caused by crystallographic twinning effects. [31] Another surprising result is the very large increase in coercivity (by a factor 15) between 300 and 20 K for some of the SD seamount samples (Figure 5). Most of the increase takes place between 300 and 150 K. Yu et al. [2004] observed the same dramatic increase in submarine basaltic glass, though in the 100–50 K temperature range. Reduction in thermally activated switching in SD grains is likely to be the cause of this increase. [32] We investigated the effect of FC/ZFC on FORC diagrams. This effect is negligible, however the variation with temperature of the hysteresis parameters HC and HCR shows some dependency on the FC/ZFC regime. A field of about the same strength as HC is enough to switch on FC and ZFC effect.
[33] Compared to other low-temperature methods for identifying magnetic minerals, low-temperature FORC diagrams are not very efficient. In particular, low-temperature SIRM curves have proved to be a very reliable tool of magnetometry [e.g., Carvallo et al., 2006]. However, FORC diagrams are useful for investigating changes in grain structure and anisotropies at low temperature.
Acknowledgments [34] FORC diagrams were measured at the Institute for Rock Magnetism, which is supported by the University of Minnesota, the Keck Foundation, and the NSF Earth Sciences Division. We thank Mike Jackson, Jim Marvin, and Peat Sølheid for helping with the measurements. This research used samples provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. A.R.M. is funded by the Royal Society. Reviews from John Tarduno and Andrei Kosterov helped to improve the paper.
References Carvallo, C., and D. J. Dunlop (2001), Archeomagnetism of potsherds from Grand Banks, Ontario: A test of low paleofield intensities in Ontario around 1000 A. D., Earth Planet. Sci. Lett., 186, 437 – 450. ¨. O ¨ zdemir, and D. J. Dunlop (2004a), FirstCarvallo, C., O order reversal curve (FORC) diagrams of elongated singledomain grains at high and low temperatures, J. Geophys. Res., 109, B04105, doi:10.1029/2003JB002539. 11 of 12
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