Effect of Magnetic Vacancies on the Spontaneous Spin-Reorientation Transition in HoFe${}_{1-x}$Al${}_x$O${}_3$ Single Crystals

Abstract

In this work, we report the first growth of single crystals of the substitution series HoFe${}_{1-x}$Al${}_x$O${}_3$ with aluminium concentrations up to $x=0.2$ and investigate the evolution of their spontaneous spin-reorientation transition (SRT). Among rare-earth orthoferrites, HoFeO${}_3$ exhibits a distinctive sequence of magnetic phases (${\Gamma }_4$-${\Gamma }_{24}$-${\Gamma }_{12}$-${\Gamma }_2$). This complex sequence arises from the competition between the $K_{ac}$ and $K_{ab}$ anisotropies associated with the Ho${}^{3+}$ ions and the effective magnetic field produced by the weak ferromagnetic moment of the canted Fe${}^{3+}$ sublattice. Introducing magnetic vacancies perturbs the antiferromagnetic compensation of the Fe${}^{3+}$ subsystem in the $ab$ plane and generates an additional effective magnetic field acting on the Ho${}^{3+}$ ions. This field alters the balance between the $K_{ac}$ and $K_{ab}$ anisotropies within the SRT temperature range and thereby broadens the stability range of the ${\Gamma }_{12}$ phase in the magnetic phase diagram.