Adrian R. Muxworthy

First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature

Abstract The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Neel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.

Magnetostatic interaction fields in first-order-reversal-curve diagrams

The contribution of magnetostatic interaction fields in magnetic systems during first-order-reversal-curve (FORC) simulations has been systematically addressed using a dynamic micromagnetic algorithm. The interaction field distributions (IFD) display a nonlinear dependency on the field history and intergrain spacing, and are commonly asymmetric. The IFDs tend to be more Gaussian on average than Cauchian as predicted analytically for disordered systems, due to ordering during FORC diagram determination. The spreading of the FORC distribution in the vertical direction of the FORC diagram is shown to be directly related to the mean standard deviation of the IFD during the FORC measurement, with a small offset related to the smoothing factor.

Micromagnetic modeling of first-order reversal curve (FORC) diagrams for single-domain and pseudo-single-domain magnetite

First-order reversal curve (FORC) diagrams have been experimentally shown to be a better way of discriminating domain states in a sample compared to the straightforward use of major hysteresis loops. In order to better understand the fundamental behavior of assemblages of single-domain (SD) grains, we used a micromagnetic model witha conjugate gradient algorith m to calculate FORC diagrams for isolated grains of magnetite as well as for arrays of grains. In the case of individual elongated grains, we found that the FORC diagram consists of a single peak centered on the coercive force Hc if the grain is SD. For a pseudo-single-domain (PSD) grain with vortex structure, we observe multiple peaks on the FORC diagram. The modeling of arrays of elongated SD particles reveals two distinct types of patterns depending on the spacing between particles. In a 2U2U2 array of particles, a secondary branchon the reversal curves appears if the spacing between particles is less than about twice the particle length. This feature translates into the appearance of one negative and three positive peaks on the FORC diagram. In the case of a 3U3U3 array of particles, we again observe several secondary branches when the spacing between grains is less than about twice the particle length, leading to the appearance of multiple peaks on the FORC diagram. Splitting of the central peak on the Hu axis when particles interact could explain the vertical spread of FORC distributions of natural interacting samples as an effect of superposition of multiple peaks caused by the random orientation and distributions of particle spacing and switching fields of a large number of grains. The presence of symmetric peaks on a FORC diagram can be an indicator of the presence of either small PSD grains or magnetic interactions in an ensemble of grains.

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

Domain structures of small pseudo-single domain (PSD) magnetite near the Verwey transition (Tυ) at ≈120 K were modeled using an unconstrained three-dimensional micromagnetic algorithm. The single-domain (SD) threshold (d0) for the monoclinic phase below Tυ was calculated to be ≈0.14 μm at 110 K. However, it is postulated that as a result of the very high energy barriers in the monoclinic phase, grains near d0 in size and in vortex states are unlikely to denucleate domain walls to become SD. Low-temperature cycling of saturation isothermal remanence (SIRM), thermoremanence (TRM), and partial TRM (pTRM) through Tυ was simulated. Domain structures were found to align along the monoclinic “easy” magnetocrystalline anisotropy axis, i.e., the c axis, on simulated cooling through Tυ. This process was found to “destroy” SIRM structures giving rise to demagnetization; however, for TRM and pTRM structures only “closure” domains were removed increasing magnetostatic leakage giving rise to a reversible anomaly in rough agreement with experimental studies. SIRM displayed a smaller anomaly at Tυ, in agreement with experimental studies.

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

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.

Distribution anisotropy: the influence of magnetic interactions on the anisotropy of magnetic remanence

Abstract The anisotropy of magnetic remanence (AMR) is often used as a tool for examining magnetic anisotropy of rocks. However, the influence of magnetostatic interactions on AMR has not been previously rigorously addressed either theoretically or experimentally, though it is widely thought to be highly significant. Using a three-dimensional micromagnetic algorithm, we have conducted a systematic numerical study of the role of magnetostatic interactions on AMR. We have considered both lineation and foliation, by modelling assemblages of ideal single domain grains and magnetically non-uniform magnetite-like cubic grains. We show that magnetostatic interactions strongly affect the measured AMR signal. It is found that depending on the orientation of the single-grain anisotropy and grain spacing it is possible for the AMR signal from a chain or grid of grains to be either oblate or prolate. For non-uniform grains, the degree of anisotropy generally increases with increasing interactions. In the modelling of AMR anisotropy, saturation isothermal remanence was chosen for numerical tractability. The influence of interactions on other types of more commonly measured AMR, are considered in light of the results in this paper.