Measurement of magnetization at high temperatures and the origin of thermoremanent magnetization. A review.

Abstract

Recent studies on the measurements of magnetization at high temperatures are reviewed. Use of a second gradient magnetometer is the main advance in instrumentation. Direct observation of how remanent magnetization is acquired and/ or lost by changing temperature provides strong evidence about the origin of thermoremanent magnetization. According to such observation, Neel's single domain theory seems to be correct. Neel's multi domain theory (which is originally a two domain theory, but is frequently applied to grains with many domains), however, cannot explain the behavior of magnetization at high temperatures. The behavior of magnetization in pseudo single domain (psd) grains is strongly dependent on the grain size. It seems that the heterogeneity in the magnetic grains and the screening effect are the main effects which characterize the psd moment.It should be emphasized that at high temperatures, there are still many interesting phenomena which have not been adequately described and/ or explained. Some of these interesting phenomena are shown as a guide for future studies.The high temperature measurement of magnetization is undoubtedly important for understanding TRM (thermoremanentrnagnetization), because TRM is acquired at high temperatures. In the 1950's and 1960's, when astatic magnetometers were the main instruments for the measurements of magnetization, there were many studies which reported measurements of magnetization at high temperatures and important advancements were made (e.g., NAGATA and OZIMA, 1955). In the 1970's and early 1980's, however, relatively few such studies were done. This is probably because of the difficulty in using spinner magnetometers and SQUID magnetometers at high temperatures (COLLINSON, 1983). Although some spinner magnetometers can be used at high temperatures, it is difficult to control the atmosphere, and induced magnetization cannot be measured. For rock magnetic purposes, it is important to control the atmosphere so that the magnetic grains are not altered during high temperature measurements. SQUID magnetometers of ordinary design cannot be used with electronic furnaces (CoLLINSON,1983). Such disadvantage has prevented the use of these modern instruments for high temperature measurements. Recently, however, second gradient SQUID magnetometers were put to use for high

Continuous and stepwise thermal demagnetization: are they equivalent?

“…The decreases are claimed not to be due to thermoviscous decay. The cause is not entirely clear but the phenomenon is well substantiated (Sugiura 1989; Muxworthy 2000).…”

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

“…Further major problems with applying both the hysteretic and thermofluctuafional models to MD TRM have recently come to light following experiments on properties of partial TRM (pTRM) [Sugiura, 1989]. These experiments suggest that the position of key domain walls is not blocked or fixed at a blocking temperature, contradicting the fundamental basis of Ntel's theoretical approach [McClelland and Sugiura, 1987].…”

Paleointensity determination using continuous thermal measurements by a high‐temperature vibrating thermomagnetometer

“…Even with negative feedback amplification [ Dunlop and West , 1969], ballistic measurements lack the sensitivity needed for most paleointensity work. Parry [1975, 1979] measured TRM and pTRM development during cooling using an astatic system and Sugiura [1989] reviews similar studies using SQUID gradiometer and vibrating‐sample magnetometer (VSM) systems. These studies were not aimed at paleointensity determination, although the detectors do have the requisite sensitivity.…”