Domain Walls in Antiferromagnets and the Weak Ferromagnetism of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>α</mml:mi></mml:math>-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Fe</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Abstract

At the N\'eel temperature local nucleations of the antiferromagnetic order and their subsequent growth lead to the formation of domain walls. The domains in an antiferromagnet are thermodynamically stable only when the anisotropy force opposing the gradual switch of spins in the Bloch zone is small such that the wall energy is offset by the gain in entropy. However, in most cases the domain wall would owe its stability to the presence of lattice imperfections, such as interstitial atoms or dislocations. A typical magnetization curve of an antiferromagnet with ferromagnetic domain walls is dipicted.The weak ferromagnetism observed in the (111) plane of $\ensuremath{\alpha}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ (hematite) is identified with the magnetization in domain walls pinned down by lattice imperfections. An assumption that the linear dimension of domains is, on the average, ${10}^{4}$ atomic spaces gives the observed strength of the weak ferromagnetism. The disappearance of this ferromagnetism at $\mathrm{ca}$ -20\ifmmode^\circ\else\textdegree\fi{}C when the magnetic axis switches from [$11\overline{2}$] direction to the [111] direction is due to the extreme difference in the anisotropy force in the (111) plane and that in a plane containing the [111] axis. The following experimental findings on $\ensuremath{\alpha}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ are interpreted: (a) The variation of the ferromagnetism as the temperature increases shows the general feature of the decrease of long-range order in cooperative phenomena, being very gradual at lower temperatures and growing sharper and sharper as the temperature approaches the N\'eel point. (b) In the transition region of the two antiferromagnetic states of $\ensuremath{\alpha}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ an applied field in the (111) plane causes a decrease of the temperature at which the ferromagnetic effect and the large magnetostriction effect disappear but produces no change in the neutron diffraction intensity of the (111) line.

Keywords

Affiliated Institutions

• Carnegie Institution for Science US

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