Toward a better understanding of magnetic screening in multidomain grains.

Xu, Song; Merrill, Ronald T.

doi:10.5636/jgg.42.637

Cited by 12 publications

(12 citation statements)

References 13 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This means that lnagnetite may be incipiently transforming its space group, leading to structural in h olno g e nei ties. Stable re•nanence carried by pinned walls in an MD grain can be screened by soft dolnain walls which are fi'ee to lnove; this could either enhance or reduce the remanence of the stable part [Xu and Merrill, 1990]. In an earlier section, we have made the assumption that the true self-reversed component of the TRM can be described as the higher coercivity part where soft walls which contribute to the screening effect are not involved.…”

Section: Incipient Exsolution By Spinodal Decomposition At Low-ts Woumentioning

confidence: 95%

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

McClelland¹,

Щербаков²

1995

J. Geophys. Res.

View full text Add to dashboard Cite

Experimental studies on isothermal remanent magnetization (IRM) acquisition of synthetic specimens containing crushed natural magnetite in seven grain sizes from 5–10 μm to 100–150 μm show that the initial demagnetization state before IRM acquisition strongly affects the samples' ability to acquire IRM. It is easier to acquire IRM from an initially thermally demagnetized state than from an alternating field (AF) demagnetized state, and low‐temperature demagnetization of the thermally demagnetized state increases resistance to IRM acquisition. These data are interpreted as confirmation of the existence of a number of possible domain states (metastable states) which have variable capacity for remanence acquisition. Our results also demonstrate that activation of domain structure is much more effective by AF than by direct (dc) fields. We suggest that the continually changing external field experienced during AF demagnetization allows the magnetic structure to reorganize incrementally towards the demagnetized state, requiring a considerably lower peak field (about 3 times lower) to achieve the same demagnetization effect. Thermoremanent magnetization (TRM) in larger grain sizes is partially self‐reversed. A difference in grain size dependence is observed for a number of magnetic parameters. All parameters vary with grain size below about 50 μm. Above 50 μm, parameters associated with coercivity, AF demagnetization, or low‐temperature treatment (Hcr; saturation IRM (SIRM) memory; TRM memory; median acquisition field (MAF) of IRM after AF; median destructive fields (MDF) of SIRM) are independent of grain size; parameters associated with TRM or thermal demagnetization (MDF ofTRM; MAF of IRM after thermal demagnetization; MAF of IRM after thermal demagnetization followed by low temperature demagnetization; self‐reversal as percentage of TRM) show grain size dependency throughout the grain size range. Our interpretation of the data invokes the existence of subtle material inhomogeneities in both natural and synthetic magnetites, giving rise to a limited range of Curie temperatures below 580°C (perhaps down to 560°C). We suggest that their control over the local energy minima states available to the grain is more significant in the TRM or thermally demagnetized state, while shielding by “soft” domain walls is dominant in the AF demagnetized state where walls are in the simplest domain configuration, leading to differing grain size dependencies of these different states.

show abstract

Section: Incipient Exsolution By Spinodal Decomposition At Low-ts Woumentioning

confidence: 95%

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

McClelland¹,

Щербаков²

1995

J. Geophys. Res.

View full text Add to dashboard Cite

show abstract

“…If bad palaeointensity determination is due to the presence of MD grains, the high coercivity and low X-values for samples from Table 1 must be explained. One possible answer could be found in analytical and numerical models, developed by Xu and Merrill (1990) and Xu and Dunlop (1993). These authors show that due to screening by soft walls in MD grains, the AF-stability of saturation remanence is usually higher than the expected maximum microcoercivity associated with the domain wall pinning.…”

Section: Rock Magnetic Properties Of the Archaeological Samples And Tmentioning

confidence: 99%

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

Jordanova

Petrovský²,

Kovacheva

1997

J. geomagn. geoelec

View full text Add to dashboard Cite

Rock magnetic properties of archaeomagnetic samples taken from ovens are studied. Thermal demagnetization of saturation remanences and other studies reveal the presence of: (titano)magnetite, iron oxyhydroxides, and maghemite with Tb -650°C which is stable with respect to inversion. During thermal demagnetization the soft IRM component (0.06 T) is always the strongest one, indicating the importance of the (titano)magnetite contribution. A break-up in the values of coercivity H with respect to initial mass-specific magnetic susceptibility (X) occurs at X -1.5 x 10-6 m3/kg and separates poorer from better quality samples in the palaeointensity experiment. Separation of bad and good Thellier experiments is also observed in the relation between the coercivities and the concentration-independent parameter SIRM/X. Thus, the poor quality of the palaeointensity experiments for some samples can probably be ascribed to the presence of weathering products and MD magnetite grains while the good quality of these experiments is related to "soft" hysteresis properties and a broad unblocking temperature spectrum.

show abstract

“…A simple explanation for the phenomenon described above might be the self-shielding effect of "soft" domain walls [Moon and Merrill, 1986a]. However, McClelland and Sugiura [1987] and Xu and Merrill [1990a] found the self-shielding effect to be unsatisfactory as an explanation for some pTRM data, and we will present the arguments against self-shielding later in the paper.…”

Section: Despite Point 3 Thettier's Law Of Additivity Of Ptrm (That mentioning

confidence: 99%

A model of multidomain thermoremanent magnetization incorporating temperature‐variable domain structure

Щербаков¹,

McClelland²,

Щербакова³

1993

J. Geophys. Res.

View full text Add to dashboard Cite

There are some fundamental experimental observations of properties of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) magnetite that cannot be explained by Ntel's theories of TRM. We present experimental results that show (1) that pTRMs are additive at any temperature, (2) that a pTRM acquired in field H between temperatures T1 and T 2 decreases on zero-field cooling below T 2 when normalized by M s (T), (3) that thermal pre-history has a strong effect on the intensity of a pTRM. These results strongly point to reorganization of domain structure during cooling being the dominant controlling factor in TRM acquisition in MD material. We further develop the approach of McClelland and Sugiura [ 1987] where TRM and pTRM are considered to be nonequilibrium states, and change in domain structure with changing temperature provides the driving force to allow a pTRM to shift toward the demagnetized state on zero-field cooling, for example. A random element is essential in such a kinetically controlled system; in this paper we consider the physical mechanism providing this random element to be the variation of direction of the easy axis of magnetization throughout the grain due to local crystal defects, or stress effects due to the domains themselves, for example. Thermally driven domain structure changes then cause essentially random local changes of magnetization, which are governed by kinetic equations. Our model is developed by considering the magnetization of discrete cells within a cubic grain chosen to have reasonably uniform magnetic properties within the cell but probably different between cells, and the model satisfactorily explains our experimental observations. The strong effect of thermal prehistory is ascribed to the existence of a spectrum of local energy minima states, and the behavior of an MD grain is likened to that of a spin glass.grains are adequate to explain experimental observations, at least qualitatively. However, theories of TRM in multidomain (MD) grains grossly•fail to explain experimental observations in some fundamental areas. Ndel [1955] laid the foundations for the present theories of TRM in iron oxides. He pointed out that remanent magnetization exists because of barriers to change in magnetization.He postulated that in large multidomain grains these barriers prevent the movement of domain walls and that they are proportional to the coercive force H c. In small single domain grains, he suggested that barriers are due to anisotropic magnetic properties. In iron oxides which have uniaxial shape anisotropy, there are two antiparallel low energy favourable states with an energy barrier Kv (K is anisotropy constant; v is volume) which has to be surmounted to change state. A particular SD grain will "block" in its contribution to the total TRM when the thermal fluctuations due to the falling temperature no longer exceed a critical value necessary to change the magnetization direction. We will call this the "thermofiuctuational" model, which has sucessfully predicted...

show abstract

Toward a better understanding of magnetic screening in multidomain grains.

Cited by 12 publications

References 13 publications

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

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

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

A model of multidomain thermoremanent magnetization incorporating temperature‐variable domain structure

Contact Info

Product

Resources

About