The Invar Problem

Abstract

Anomalous thermal expansion and its relation to magnetic properties are surveyed for various Invar alloys. Saturation magnetization was measured for alloy systems (Fe"Nh.-z)92MS, where M = Cu, Co, Mn, Cr, and V, and also for Fe70(RexPtl_x)ao at 4.2°K. Sharp drop of the sa!uratlOn moment oc:curs at the average electron concentration, N =8.13 to 8.52, for all the Invar alloys inc1udm~ the above-mentlO~ed alloy systems and N =8.21 to 8.52 for all the Fe-Ni base alloys. When, however, It was plotted agamst the electron concentration of the matrix surrounding less electropositive impurity atoms such as ~n, Cr, or V, all the alloy systems were found to exhibit a sharp drop of the moment i~ narrower ~o~~ent.ratlOn range ~f 8.48 to 8.60 for all the Fe-Ni base alloys. Saturation moment and a reversible sust:eptlblhty m.the saturatIOn range were also measured at room temperature for (110) [001] and (001) [110] rolled smgle crystals of the composition 29 at.% Ni-Fe. It was found that saturation moment was reduced from.79 to 72 emu/g, and the reversible susceptibility increased from 4.0 to 4.6X10-4 emu/g by 70~ reductIOn of the thlckn~ss. The mechanism was discussed in relation to the well-known effect of cold rollmg on the thermal expansIOn coefficient of Invar.

Fe–Ni Alloys at High Pressures and Temperatures: Statistical Thermodynamics and Kinetics of the $L1_{2}$ or $D0_{19}$ Atomic Order

“…для парамагнетної сприйнятливости (з «парамагнетною точкою Кюрі» T p ) було зроблено в ряді робіт, зокрема, в [131,132]; тут k B -Больцманнова стала, ν -число атомів в одиниці об'єму, J і s -квантові числа, які характеризують повний механічний мо-мент та власний спіновий момент атому відповідно, g -фактор Лянде, тобто магнетомеханічне відношення, що приблизно дорів-нює 2, а J = s у випадку, коли магнетний момент атому створю- [91,[124][125][126][127]. На рисунку 3.4, б заштрихована ділянка відповідає змішано-магнетній фазі з неколі-неарною конфіґурацією спінів [128]; вертикальні пунктирні лінії позна-чають інварну ділянку, а суцільні криві [133,134]).…”

Thermodynamic Properties of Invar at Low Temperatures

“…The original theory was based on localization of the spins. More recently, emphasis has been placed on the itinerant aspects, 16,17 the neutron scattering experiments have been re-assessed, 21 and additional experiments 22 have not supported the Kondorskii model. On the other hand, recent studies of the Mossbauer effect in very iron-rich small particles 23 and films 24 confirm the existance of antiferromagnetic regions.…”

“…The spin-wave excitations will not affect (21) and (22) to low order in the temperature, 28 ' 29 and since they are not needed for our discussion, we will not consider them further. We consider the low-temperature region where a has essentially its zero-temperature value.…”

“…The indication is that da/dV for Invar is large because of an approach to instability such that (22) is nearly violated as the Ni concentration is reduced. Instabilities of this general type have been discussed by Shimizu 80 and applied to the Invar problem by Shimizu and Hirooka 17 and Mizoguchi.…”

“…1 for two possible variations of A with a, I and II, both leading to the same r = 0 magnetization. The net area beneath A must be less than that beneath 21 a, and (22) demands that the intersection of A and 21<j be such that A crosses 21a fromjbelow. When la or A or both are slightly changed due to changes in temperature and volume, case I will lead to a first-order transition to o*=0 if the new intersection is unfavorable.…”