Low-temperature cycling of isothermal and anhysteretic remanence: microcoercivity and magnetic memory

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Abstract

This paper reports low-temperature cycling (LTC) through the Verwey transition of anhysteretic remanence (ARM), partial ARMs and partially demagnetised saturation isothermal remanence (SIRM) induced at room temperature in pseudo-single-domain and multidomain (MD) magnetite. The remanences were cooled in zero field to 50 K and then heated back to room temperature. By inducing partial ARMs over different field ranges and by partially alternating field demagnetising SIRMs, it was possible to isolate both low-coercive-force and high-coercive-force fractions of remanence. On cooling through the Verwey transition, a sharp increase in the remanence was observed. The relative size of the jump increased as the high-coercive-force fraction was increasingly isolated. This behaviour is interpreted as being due to both an increase in the single-domain/multidomain threshold size on cooling through the Verwey transition and to the reduction or elimination of closure domains in the low-temperature phase. In addition, the memory ratio, i.e. the fraction of remanence remaining after LTC divided by the initial remanence, was found to be higher for the high-coercive-force fraction than the low-coercive-force fraction. In our interpretation, the high-coercivity fraction behaviour is associated with reversible domain re-organisation effects, whilst the low-coercive force fraction’s behaviour is associated with irreversible domain re-organisation and (de-)nucleation processes. Due to the decrease in magnetocrystalline anisotropy on cooling to the Verwey transition, the high-coercive-force fraction is likely to be magnetoelastically controlled. Thus, a rock displaying high-coercive-force behaviour is likely to carry a palaeomagnetically meaningful remanence with high unblocking temperatures. In addition, LTC analysis can be used to identify the domain state dominating the natural remanence in magnetite-bearing rocks.

Introduction

Zero-field low-temperature cycling (LTC) of magnetite-bearing rocks from room temperature to liquid nitrogen temperature causes varying degrees of partial demagnetisation of remanence depending on domain state [1], [2], [3], [4], [5], [6], [7], [8]. The partial demagnetisation is thought to be primarily related to changes in the magnetocrystalline anisotropy, but the exact processes governing LTC behaviour are still unresolved. There is a need for greater understanding if LTC is to become a reliable palaeomagnetic or environmental magnetic technique.

To understand the mechanisms controlling demagnetisation, it is revealing to study the LTC behaviour of various initial remanence states. The direct measurement of remanence during LTC has not been extensive; previous studies have predominantly considered saturation isothermal remanence (SIRM) [1], [2], [3], [4], [6], [7], [8]. It is found that SIRM induced in a multidomain (MD) sample at room temperature gradually decreases on cooling to the magnetocrystalline energy isotropic point TK at 130 K and the Verwey transition TV at 120–124 K [9], [10]. This decrease in remanence has been associated with domain re-ordering effects, i.e. domain wall re-equilibration or domain nucleation [4], [7]. On cooling through TV, an increase in magnetisation or ‘jump’ is observed. The size of the jump for SIRM carried by assemblages of crystals is relatively small compared to that of SIRM induced in orientated single crystals ∼1–4 mm in diameter [6], [8]. Below the monoclinic/triclinic phase of magnetite, SIRM displays thermally reversible behaviour. On warming through TV, the jump is found to be mainly, though not always, reversible. On warming from TK to room temperature, SIRM displays irreversible behaviour, with some increase in magnetisation (recovery). The degree of recovery is found to be dependent on internal stress [11] and has been associated with the stiffening of domain walls on heating [7].

There are a limited number of studies of the low-temperature cycling of thermoremanent magnetisation (TRM) [1], [7], [12], of which by far the most extensive is the paper of Muxworthy and McClelland [7], who measured LTC behaviour of TRM and partial TRMs (pTRMs) induced in well characterised synthetic and natural stoichiometric pseudo-single-domain (PSD) and MD magnetites. TRM and pTRM carried by assemblages of magnetite crystals consistently displayed a large jump on cooling through TV. The size of the jump was influenced by grain size, the temperature range over which the pTRM was acquired and inducing field intensity, e.g. high-temperature pTRMs induced in small fields in PSD samples displayed larger jumps than pTRMs induced in larger fields in large MD samples at lower temperatures. The size of the jump was on average larger than that observed for the same samples carrying an SIRM [7].

In addition to experiments, micromagnetic and other numerical models of single sub-micrometre crystals predict the jump behaviour at TV for simulated SIRM, TRM and pTRM structures [5], [13]. However, due to the simplifications of the model the size of the simulated jump was larger than that observed for assemblages of small PSD magnetites.

The behaviour at TV was explained by a shift in domain state to a more single domain-like (SD) structure and the removal of closure domains in the monoclinic phase [5], [7], [8]. This was concluded by examining hysteresis data, micromagnetic solutions and by considering the relative anisotropy which indicates the favourability of closure domains [5], [8], [14]. It was suggested that closure domains play a more important role in reducing the demagnetising energy in small grains than larger grains and in pTRM structures acquired at high temperatures.

The results of TRM LTC curves have been revealing, but our current knowledge of MD TRM acquisition is incomplete, making it difficult to fully interpret such LTC data. To quantify and improve our understanding of the behaviour of remanence at low temperatures, in this paper LTC curves of partially alternating-field (AF) demagnetised SIRM, anhysteretic remanence (ARM) and partial ARM (pARM) are presented. It is the first time LTC curves for such remanences have been reported. Although these remanences in MD magnetite are not fully understood [15], our knowledge is better than that of TRM as we can more accurately identify which part of the coercivity spectrum is affected during the induction.

Section snippets

Sample description

Magnetite samples from two different origins were utilised in this study; three small commercial MD samples W(1.7 μm), W(7.0 μm) and W(11 μm), acquired from Wright Industries, and two MD synthetic samples made by hydrothermal recrystallisation (H (39 μm)) and (H (108 μm)).

The Wright samples were obtained 6 months before the experimentation and stored in a desiccator. Grain size distributions were found to be log-normal from scanning electron microscope photographs (Table 1). X-ray diffraction

Experimental procedures

A series of LTC experiments were made; samples were given either an ARM, pARM or partially demagnetised SIRM, and the behaviour of the remanence was measured during LTC. These experiments were designed to examine how different fractions of the coercivity spectrum behave during LTC.

ARMs and pARMs were induced with a DTech pARM inducer/demagnetiser. The maximum AC field was 200 mT. The effect of applying different biasing DC fields over different field ranges was examined. Partially demagnetised

Low-temperature cycling of partially AF-demagnetised SIRMs

For three samples, W(7.0 μm), H(39 μm) and H(108 μm), LTC curves were measured for partially AF-demagnetised SIRMs (Fig. 3). The general shape of the undemagnetised SIRM LTC curves was in good agreement with previously published data for assemblages of PSD and MD magnetite [2], [3], [4], [7]. As the peak AF demagnetisation field increases, several features are observed. First, the demagnetisation which occurs on cooling to TK, associated with domain wall re-ordering, decreases. Second, a sudden

Discussion

There are large variations in both the initial remanences and LTC curves shown in this paper. However, there are two clear trends. First, initial remanences associated with higher-coercivity fractions display the highest ΔVJ values, e.g. ΔVJ is largest for SIRM AF-demagnetised to 100 mT (Fig. 3) and pARM180200 (Fig. 6). Secondly, the high-coercivity fraction is generally reversible during LTC, i.e. the high-coercivity remanences have the highest memory ratios (Fig. 4, Fig. 7). Irreversible

Conclusions

This study clearly demonstrates that LTC behaviour is influenced by the coercive force distribution of the remanence. It is shown that key features such as ΔVJ and high memory ratios are associated with the highest coercivity fraction of remanence. Some of the LTC behaviour can be explained by the application of previously reported theories [5], [6], [7], [8].

The identification of high memory ratios or a significant ΔVJ peak for a natural remanent magnetisation (NRM) would indicate that the

Acknowledgments

We benefited from fruitful discussions with Yongjae Yu and Allan Jacobs. Mike Jackson and an anonymous reviewer made constructive comments. We thank Wright Industries for providing the samples. Fred Neub of the Materials Engineering Department, University of Toronto, helped with the SEM work. The magnetic experiments were made at the Institute for Rock Magnetism, University of Minnesota, which is funded by the National Science Foundation, the W.M. Keck Foundation and the University of

References (32)

  • A.R. Muxworthy et al.

    Micromagnetic models of pseudo-single domain grains of magnetite near the Verwey transition

    J. Geophys. Res.

    (1999)
  • A.R. Muxworthy et al.

    The causes of low-temperature demagnetisation of remanence in multidomain magnetite

    Geophys. J. Int.

    (2000)
  • A.R. Muxworthy et al.

    Review of the low-temperature magnetic properties of magnetite from a rock magnetic perspective

    Geophys. J. Int.

    (2000)
  • R.J. Harrison, Magnetic transitions in minerals, in: S.A.T. Redfern, M.A. Carpenter (Eds.), Reviews in Mineralogy and...
  • J. King et al.

    Low-temperature properties of magnetite

    J. Geophys. Res.

    (2000)
  • R.L. Hartstra

    TRM, ARM and Isr of two natural magnetites of MD and PSD grain size

    Geophys. J. R. Astron. Soc.

    (1983)
  • Cited by (0)

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