MAGNETOSTRICTION AND SPIN-FLOPPING OF UNIAXIALLY COMPRESSED ANTIFERROMAGNETS

摘要

1. Magnetostriction of MnF_2 is determined by the interactions of two types: the exchange and anisotropic ones. Their contributions may be separated. The absolute value of exchange interaction grows at hydrostatic compression which agrees with an increase of the Neel tempearture at hydrostatic pressure and with volume reduction of the crystal below T_N resulted from magnetic ordering. Magnetic anisotropy of MnF_2 has purely magnetodipole origin. 2. Indirect exchange interaction decreases as R~(-14) where R s the distance between the ions Mn~(2+) and F. The dependence of exchange interaction on angle φ, which is composed by the bond-lines in the chains Mn~(2+)―F ―Mn~(2+), is described semiempiricaly by a function cosφ. So the relationship between the indirect exchange interaction and the lattice characteristics R and φ is as follows: W～R~(-14) cosφ. 3. Uniaxial pressure effect on the critical field of spin-flop in α-Fe_2O_3 is explained by a lattice strain dependence of magnetic anisotropy. The calculated derivatives of magnetodipole interactions on strain do not explain the observed dependence H_c(p_(33)). The strain dependence of the spin-orbit contribu ion in energy of magnetic anisotropy determines both the value and the sign of the magnetoelastic properties of hematite Fe_2O_3. 4. Investigation of uniaxial compression effect on the critical field H_c has revealed the sharp dependence of anisotropy energy on pressure for Cr_2O_3. The effect excludes the trivial explanation by the peculiarities of magnetodipole interactions. It is attributed to the characteristics of spin orbit-coupling in this crystal. 5. Magnetoelastic properties of Cr_2O_3 are determined by spin-orbit coupling which is the source both of the single-ion anisotropy and of the exchange interaction anisotropy. The total anisotropy energy is the sum of the significantly higher components, characterized by the close magnitudes but of the opposite sign. Even their small variation with crystal deformation may result in a drastic changes of the low total energy. 6. The strain jumps in Cr_2O_3 and MnF_2 at phase transition in the field H_c on the one hand and the independently measured derivatives dH/dσ_(33) on the other are in agreement with the thermodynamic equation, which relates them along the curve of phase equilibrium. 7. Some similarities with the superconductors are traced.