Low-temperature properties of a single crystal of magnetite oriented along principal magnetic axes

https://doi.org/10.1016/S0012-821X(98)00269-6Get rights and content

Abstract

We have measured saturation induced and remanent magnetizations and induced magnetization as a function of field at low temperatures, between 300 K and 10 K, on an oriented 1.5-mm single crystal of magnetite. The induced magnetization curves along the cubic [001], [11̄0], and [110] axes at 10 K have very different approaches to saturation. The crystal is easy to magnetize along [001] but difficult along [11̄0] and [110], the hard directions of magnetization for monoclinic magnetite. The temperature dependence of saturation magnetization between the Verwey transition temperature, Tv = 119 K, and 10 K is also different along the three axes, indicating that below Tv the crystal has uniaxial symmetry. The room-temperature saturation remanence (SIRM) produced along [001] decreases continuously in the course of zero-field cooling, levelling out at the isotropic temperature, Ti = 130 K, where the first magnetocrystalline anisotropy constant becomes zero. At Ti, 86% of the initial SIRM was demagnetized. The domain wall pinning responsible for this soft remanence fraction must be magnetocrystalline controlled. The remaining 14% of the SIRM is temperature independent between Ti and Tv and must be magnetoelastically pinned. This surviving hard remanence is the core of the stable magnetic memory. The Verwey transition at 119 K, where the crystal structure changes from cubic to monoclinic, is marked by a discontinuous increase in remanence, indicating that the cubic [001] direction suddenly becomes an easy direction of magnetization. The formation of monoclinic twins may also affect the intensity of remanence below Tv. Reheating from 10 K retraces the cooling curve, with a decrease at Tv back to the original remanence level, which is maintained to 300 K. When SIRM is not along [001], the initial SIRM is larger but the reversible changes across the Verwey transition are much smaller. The SIRM produced at 20 K is an order of magnitude larger than the 300 K SIRM, but the only change during warming is a discontinuous and irreversible drop to zero at Tv.

Introduction

Magnetite (Fe3O4) has two magnetic transitions below room temperature. At the isotropic point Ti around 130 K, the first magnetocrystalline anisotropy constant (K1) passes through zero, causing domain walls to unpin. At the Verwey transition, a crystallographic phase transition variously reported to occur at Tv = 110–125 K, the structure changes from cubic to monoclinic. In most previous studies, it has been difficult to separate with certainty the magnetic effects of the two transitions. For example, in the technique of low-temperature demagnetization (LTD), rocks containing magnetite are cooled to 77 K and rewarmed to room temperature in zero field in order to erase less stable and reliable components of remanent magnetization. It is unclear whether all the loss of remanence is due to the vanishing of K1 at Ti or whether the change in crystallographic structure at Tv, accompanied by switching of magnetic easy axes, also plays a role, perhaps even a controlling role. In the present study, by using a carefully oriented crystal of magnetite, we have been able to clearly separate the effects occurring at the two transitions.

The inverse technique to LTD is often used for identifying magnetite in sediments and soils. It consists of producing a synthetic remanence (usually a saturation isothermal remanent magnetization or SIRM) at very low temperature and continuously monitoring the remanence in a zero-field warming–recooling cycle to 300 K. A sudden loss of remanence in warming through Tv, sometimes with a partial recovery on recooling, is diagnostic of magnetite. There is usually little if any remanence change around Ti in this case.

LTD is of considerable interest as a laboratory cleaning technique in paleomagnetism 1, 2, 3. It serves to remove a large part of the remanence due to loosely pinned domain walls in multidomain (MD) grains, thereby isolating the stable remanence [4]. LTD experiments on synthetic magnetites 5, 6, crushed natural magnetites [7], natural single crystals 2, 8, and magnetite-bearing rocks [9]have shown that the surviving remanence (or magnetic memory) after LTD has single-domain (SD) like properties with high coercivities. As would be expected, memory ratios of both SIRM and thermoremanent magnetization (TRM) increase as the grain size decreases [6]. Crystal defects that strongly pin domain walls also increase the memory and its stability. A comprehensive review appears in Ref. [10].

Recent work has shown that even a large (3 mm) single crystal of magnetite contains distinct SD-like and MD remanence components [11]. The magnetic memory following LTD has the following properties: (1) the memories of SIRM and TRM have SD-like alternating field (AF) decay curves; (2) the SIRM and TRM memories have practically no unblocking temperatures below 560°C; (3) the memory fraction of high-temperature partial TRM also has mainly blocking temperatures above 565°C, very similar to those of the total TRM memory. Crystal defects and resulting stress centres, rather than separate SD regions in the crystal, seem to be responsible for the stable SD-like memory.

In many of the LTD experiments described above, measurements were made only at room temperature, before and after cycling the sample to 77 K. In this approach, the data between 300 K and 77 K are lost, including the vitally important properties at the isotropic point and the Verwey transition. In the present study, we therefore continuously monitored the remanent and induced magnetizations of a well-characterized single crystal of magnetite as a function of temperature in both cooling–warming and warming–cooling cycles between 300 K and 10 K. Our purposes were: (1) to clearly separate the isotropic and Verwey transitions and better define their temperatures, Ti and Tv; (2) to understand the effects of crossing Ti and Tv on both room-temperature and low-temperature remanence; (3) to document how the remanences of the high-temperature (cubic) and low-temperature (monoclinic) phases of magnetite change with temperature after crossing the transitions; and (4) to understand the origin of the SD-like magnetic memory of cubic magnetite at room temperature after an LTD cooling–heating cycle.

Our experiments were carried out on a pure, stoichiometric natural single crystal of magnetite, carefully oriented so that the magnetic field was applied along crystallographic principal axes. The crystal quality and purity significantly affect the crystallographic phase transition. Slight deviations from stoichiometry [12]and the presence of cation impurities 13, 14cause broadening and eventual suppression of the Verwey transition. The remanence and other magnetic properties of magnetite also depend strongly on crystallographic orientation. Upon cooling through Tv, the structure changes from cubic spinel to monoclinic. At the same time, the easy directions of magnetization change from <111> to one of the <001> axes.

Section snippets

Characterization of the crystal and experimental procedure

Magnetic measurements were carried out on a museum-quality 1.5 mm octahedral crystal of magnetite. The {111} crystal faces were flat and smooth, with no striations indicative of deformational twinning. X-ray diffraction using a Debye–Scherrer camera with Cu-Kα radiation and a silicon standard was carried out on a small chip of the crystal. The spinel unit cell edge was 8.402±0.002 Å, in good agreement with the standard value 8.396 Å for magnetite.

The composition of the crystal was determined

Induced magnetization curves along principal axes

Fig. 1 shows induced magnetization M as a function of applied field H measured at 10 K along the a, b, and c principal axes. The three magnetization curves have quite different approaches to saturation. Saturation was achieved in relatively low fields of ≈0.2 T in the [001] easy direction but required much higher fields in the [11̄0] and [110] directions perpendicular to the c axis. Thus the crystal below Tv has essentially uniaxial symmetry. The magnetization curves along the three principal

Discussion

Cooling our crystal in zero field through the isotropic point at Ti = 130 K resulted in the permanent loss of 86% of the room-temperature SIRM (Fig. 4). This decrease in remanence with cooling below 300 K is mainly due to progressive jumps of loosely pinned domain walls [4]. According to Kittel [23], the 180° Bloch wall thickness w is given byw=π(A/Ku)1/2where A is the exchange constant and Ku is the effective magnetic anisotropy, which is the sum of the magnetocrystalline and magnetoelastic

Conclusions

SIRM cooling/rewarming curves provide important information about low-temperature demagnetization (LTD). LTD destroys remanence due to the loosely pinned domain walls (magnetocrystalline pinning), leaving strongly pinned walls (magnetoelastic pinning) and possibly other sources of remanence as the carriers of low-temperature memory. All unpinning occurs at Ti in our crystal.

There is a clean separation between the two remanence transitions in our magnetite crystal. The magnetic isotropic point,

Acknowledgements

We thank Malcolm Back of the Royal Ontario Museum, Toronto for donating the natural single crystal of magnetite and Sue Halgedahl and Ron Merrill for helpful reviews. These measurements were carried out at the Institute for Rock Magnetism at the University of Minnesota, which is operated with funding from the National Science Foundation and the Keck Foundation. We are grateful to Drs. Subir Banerjee and Bruce Moskowitz for welcoming us Visiting Fellows and to Jim Marvin and Mike Jackson for

References (28)

  • S.L. Halgedahl et al.

    Low-temperature behavior of single-domain through multidomain magnetite

    Earth Planet. Sci. Lett.

    (1995)
  • A. Kozlowski et al.

    Specific heat of low doped magnetite, Fe3−xTixO4 and Fe3−yZnyO4

    J. Magn. Magn. Mat.

    (1995)
  • N. Tsuya et al.

    Effect of magnetoelastic coupling on the anisotropy of magnetite below the transition temperature

    Physica B

    (1977)
  • M. Ozima et al.

    Low temperature treatment as an effective means of `magnetic cleaning' of natural remanent magnetization

    J. Geomagn. Geoelectr.

    (1964)
  • K. Kobayashi et al.

    Stable remanence and memory of multidomain materials with special reference to magnetite

    Philos. Mag.

    (1968)
  • R.T. Merrill

    Low-temperature treatments of magnetite and magnetite-bearing rocks

    J. Geophys. Res.

    (1970)
  • D.J. Dunlop et al.

    Separating multidomain and single-domain-like remanences in pseudo-single-domain magnetites (215–540 nm) by low-temperature demagnetization

    J. Geophys. Res.

    (1991)
  • F. Heider et al.

    Low-temperature and alternating field demagnetization of saturation remanence and thermoremanence in magnetite grains (0.037 μm to 5 mm)

    J. Geophys. Res.

    (1992)
  • E. McClelland et al.

    Metastability of domain state in multidomain magnetite: consequences for remanence acquisition

    J. Geophys. Res.

    (1995)
  • M. Ozima et al.

    Low-temperature characteristics of remanent magnetization of magnetite −− self reversal and recovery phenomena of remanent magnetization

    J. Geophys. Res.

    (1964)
  • J.P. Hodych et al.

    Low-temperature demagnetization of saturation remanence in magnetite-bearing dolerites of high coercivity

    Geophys. J. Int.

    (1998)
  • D.J. Dunlop, Ö. Özdemir, Rock Magnetism: Fundamentals and Frontiers, Cambridge University Press, New York, 1997, 573...
  • Ö. Özdemir et al.

    Single-domain-like behavior in a 3-mm natural single crystal of magnetite

    J. Geophys. Res.

    (1998)
  • Ö. Özdemir et al.

    The effect of oxidation on the Verwey transition in magnetite

    Geophys. Res. Lett.

    (1993)
  • Cited by (0)

    View full text