Nanoscale physics

Spin-reorientation phase transition and nonequilibrium thermodynamics of compensated ferrimagnet ErFeO3
Zubov E.E.1,2
1Kyiv Academic University, National Academy of Sciences of Ukraine, 36 Vernadsky blvd., 03142 Kyiv, Ukraine
E-mail: eezubov@ukr.net
2Vasyl’ Stus Donetsk National University, 21 600-Richchya Str., 21021 Vinnytsia, Ukraine

Orthoferrites with a general formula RMO3, where R and M are rare-earth and transition metal ions, are studied during a long time as physical objects with pronounced quantum properties of their thermodynamical behavior. Currently, it became clear that these compounds have good perspectives for applications in ultrafast spin switching devices, sensors, high-frequency generators due to their unique characteristics associated with the presence of magnetoelectric coupling, negative magnetization, spin flips, and exchange bias.
In this work, in the framework of quantum mechanical theory with anisotropic Hamiltonian an expression for the free energy F(α) as a function of antiferromagnetic axis direction was obtained. The detailed F(α) analysis for ErFeO3 shows availability of two minima with α = ±π/2 and a potential barrier near the compensation temperature Tcomp. The anisotropy of the R-M exchange interaction plays an important role in realization of spin-reorientation phase transition at temperature T = TSR =-d2(+1)(Jx2-Jz2)/(bJ2), where d, Ji and b are Dzyaloshinskii–Moriya parameters, Er3+-Fe3+ and Fe3+-Fe3+ anisotropic parts of exchange interactions, respectively, with an isotropic part J for Fe3+ - Fe3+ exchange and effective spin . Therefore, orthoferrites with low TSR point out on a weak anisotropy of the R-M exchange interaction.
Using the experimental data for magnetization, Tcomp, TSR and the Neel temperature, microscopic parameters of interactions in ErFeO3 have been determined. One can say that the studied othoferrites are related to systems with competing interactions. The Neel-Brown model for magnetic nanoparticles is used to explain the experimentally observed effects of the irreversibility, spin overturns and exchange bias in the hysteresis phenomena.