On determination of the Curie point from thermomagnetic curves

Abstract

[1] In many rock magnetic studies, information on magnetic mineralogy is of crucial importance. Besides standard analytical methods, such as X-ray spectroscopy, more sensitive thermomagnetic analyses are often used. Temperature dependence of magnetic parameters can serve as basis for determination of magnetic second-order phase transition temperatures. Although limited by several drawbacks, the most serious being thermally induced transformations of the original minerals, this method provides useful information not only about the presence of magnetic minerals, but also additional knowledge on, e.g., the prevailing grain size distribution or degree of substitution. In thermomagnetic analysis, temperature dependence of two parameters, induced magnetization and magnetic susceptibility, is mostly used. However, let us say because of historical reasons, the same approach for the Curie point determination has been often used in analyzing the two parameters. In our contribution, we discuss the physical principles of the two parameters, showing that the methods developed and used for induced magnetization cannot be used also for temperature dependence of magnetic susceptibility because there is no physical justification to do so. Otherwise, the error in determining the Curie point can be some few degrees but can reach also several tens of degrees. Such an error has serious consequences for further interpretation of the data, e.g., in terms of degree of Ti substitution in Ti magnetite.

Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

“…The stepwise experiments NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2938 ARTICLE involved a set of heating/cooling cycles with increases in T max for each cycle, usually in increments of 50 or 100°C. Curie temperatures were calculated from both the warming and cooling curves by finding inflection points of the w(T) or M S (T) curves, where the downward curvature of the ferrimagnetic state gives way to the upward curvature of the paramagnetic state 55,56 . In practice this was done by finding the temperature for which the first derivative of the (threepoint moving average) curve has a minimum (that is, greatest negative) value.…”

Investigation of polluted alluvial soils by magnetic susceptibility methods: a case study of the Litavka River

“…The identification of Fe oxides is based on the determination of Curie temperature (Tc) that is characteristic for individual oxides. The Tc of studied samples are close to 585°C (assessed according to Petrovský & Kapička 2006) pointing to the presence of magnetite. In some cases, a shift to higher Curie point temperature is likely due to a more significant contribution of maghemite.…”

Thermomagnetic Behavior of Magnetic Susceptibility—Heating Rate and Sample Size Effects

“…In contrast with the former examples from soil samples, here we have stable mineralogy and similar heating-cooling curves for the two different heating rates and the two different sample sizes. It can be noticed that small sample size results in bigger differences in the calculated Curie temperatures on the cooling curves, using the method of first derivative (Petrovsky and Kapicka, 2006). Considering only heating curves, it appears that Tc (small sample size) < Tc (large sample size), while for the cooling curves there is no such a tendency.…”

“…The example presented here gives such a preliminary data. According to the calculated Tcs' for the two sample sizes and two heating rates following the procedure in Petrovsky and Kapicka (2006), it is observed (Figure 6) that at slow heating rates, highest differences in the Curie temperatures are present. At the same time, the difference in Tcs for small and large sample size is also detected.…”

“…Cooling curve is indicative for the occurrence of mineralogical changes caused by laboratory heating. Considerable effort has been put on theoretical and experimental studies, defining physical basis of the observed susceptibility changes (Egli, 2009;Fabian et al, 2013;Dunlop, 2014) and methodologies for estimation of the Curie temperature (Gromme et al, 1969;Petrovsky and Kapicka, 2006). The shape and the observed peaks on the heating and cooling curves of k-T cycles are interpreted as a reflection of different environmental processes [e.g., degree of soil formation and synthesis of pedogenic strongly magnetic fraction; presence of substitutions; reactions of Fe minerals with organic matter; destruction of thermally unstable minerals such as carbonates, organic matter, clay minerals (Liu et al, 2005;Hanesch et al, 2006;Jiang et al, 2015)].…”