Spontaneous Magnetization — Techniques and Measurements

Abstract

Measurements on the temperature variation of spontaneous magnetization by both the magnetothermal and magnetic methods have hitherto been con6ned to nickel; the extension of these measurements, calhng for greater sensitivity, is discussed.To the direct method devised by Weiss and I'orrer has been added a direct current ampli6er, which allows a small magnetocaloric rise of temperature to be measured accurately. The original ferromagnetic balance devised by Sucksmith has been improved and now. is capable of wider application to measurements of magnetic intensity in 6elds which are due to either air core coils or electromagnets. The method is not complicated by the presence of any image effect so that measurements of do/dP in high 6elds can be made with considerable precision.Measurements on a copper-nickel alloy (27.5 percent Cu) have been made, and it is shown that the two methods give identical results except in the immediate neighborhood of the Curie temperature, where the thermal measurements give a steeper descent to the temperature axis with little "tail, " while the magnetic data indicate a higher Curie temperature with a signi6cant tail. Below the Curie temperature the linear relationship between the magnetocaloric rise of temperature and the square of the intensity magnetization is found as in nickel and iron; above this temperature there is pronounced curvature.Measurements have also been carried out on a mixed ferrite (MgO ZnO 2Fe203). Satisfactory agreement between the results by the two methods are obtained, both curves diGering markedly from the law of corresponding states for the ferromagnetic elements.

Calculating the spontaneous magnetization and defining the Curie temperature using a positive-feedback model

“…(18), can affect both the shape and area of the loop. For a given spontaneous magnetization m sp ¼ M sp /(aM s ), the corresponding temperature can be calculated by setting either h s rev or h s irr (depending on whether m sp occurs on a reversible or irreversible loop segment) to zero, and then solving for t. Whereas irreversible hysteresis loop segments are normally ignored in spontaneous-magnetization studies, we find that these are important when t approaches 1, since they can explain the "tail" or "foot" that has been observed experimentally, [12][13][14]18,29 theoretically in ferromagnetic Ising systems 30 and in micromagnetic simulations. 31 The literature contains several alternative explanations of these tails.…”

“…(29) to actual measurements. 9 These extrapolations are also used in magnetocaloric 13,14,18 and isothermal 14 definitions of T c . A further definition 44 uses measurements of @T/@H, while a pulsed-field determination employs susceptibility changes near T c .…”

“…31 The literature contains several alternative explanations of these tails. 5,16,18,[32][33][34] Depending on t, we find the theoretical spontaneous-magnetization behavior from the appropriate one of two possible situations. In Fig.…”

“…In practice such extrapolations are imprecise because the tail occurs in nearly all cases. [12][13][14][15][16] For this reason, a magnetocaloric measurement procedure that minimizes the tail phenomenon has been widely advocated, [12][13][14]17,18 even though the physics involved is not strictly magnetic. A survey of the nature of the tail problem has been given in the seminal works of Belov.…”

Estimating spontaneous magnetization from a mean field analysis of the magnetic entropy change

“…Early work on the study of the spontaneous magnetization of ferromagnets [11], consisted on directly measuring the adiabatic temperature change DT ad of a material, since within a molecular field approach the relation between DT ad and the spontaneous magnetization is established. For a general presentation, see Refs.…”