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5 Acta Physica Polonica A

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Dimensionality in Field Theory and in Spin Wave Theory

100%

Köbler U. , Hoser A.

nr 2

356-358

The different meaning of dimensionality and universality in field theory and in spin wave theory is illustrated on account of experimental examples. In spin wave theory it is distinguished between the dimensionality of the spin and the dimensionality of the exchange interactions. According to Renormalization Group (RG) theory, these atomistic characteristics are unimportant for the critical dynamics. Instead by inter-atomic interactions the dynamics of the ordered state is determined by the excitations of the continuous magnetic medium. These excitations are bosons. Consequently, the dimensionality of ordered magnets has to be assessed to the dimensionality of the relevant boson field. The most serious consequence of RG theory is that the magnetic ordering transition also is executed by the boson field. Typical for boson dynamics is a finite width of the critical range. In the atomistic models universality applies asymptotically at T_{c} only. It is evident that the critical power functions of the field dynamics are different from those of the atomistic dynamics.

Boson Fields in Ordered Magnets

100%

Hoser A. , Köbler U.

nr 2

350-352

Spin wave theory of magnetism reveals two severe shortcomings. First, it considers non quantized classical spins and, second, the predicted temperature power functions for the thermal decrease of the magnetic order parameter hold asymptotically at T = 0 only. As experiments unambiguously show the dynamics is different for magnets with integer and half-integer spin and the "critical" power functions at T = 0 of type ΔM ≈ T^{ε} or at T = T_{c} of type ≈(T_{c}-T)^β hold over a finite temperature range, independent of spin structure. The finite critical range unequivocally indicates that the dynamics of the spins is controlled by a field of freely propagating bosons instead by exchange interactions. Consequently, field theories are necessary for description of the thermodynamics of ordered magnets. The experimental indications will be discussed that the field quanta are essentially magnetic dipole radiation emitted by the precessing magnetic moments. Since integer and half-integer spins precess differently the generated field quanta and the dynamics of the field are correspondingly different.

Crystal Structure and Magnetic Properties of Tb_{11}O_{20}

71%

Baran S. , Duraj R. , Hoser A. , Penc B. , Szytuła A.

nr 1

98-100

Magnetic and neutron diffraction data for Tb_{11}O_{20} compound are reported. This compound crystallizes in a triclinic crystal structure described by the space group P1 and is antiferromagnetic with the Néel temperature 5.1 K.

Neutron Diffraction Studies of Nanoparticle DyMnO_3 Compound

71%

Szytuła A. , Penc B. , Dyakonov V. , Baran S. , Hoser A.

nr 1

65-66

The neutron powder diffraction (NPD) measurements of the nano-size DyMnO_3 manganite have been performed. The obtained results indicate that this compound crystallizes in the orthorhombic crystal structure described by the space group Pnma. The Mn and Dy moments order antiferromagnetically at different temperatures and form modulated magnetic structure described by the propagation vector k=(k_x, 0, 0) with the different values of k_x component for the Mn and Dy sublattices. The values of k_x component for Mn sublattice increase with decreasing of the temperature and are smaller that in bulk compound. The wide Bragg peaks related to the Dy sublattice suggest that the magnetic order in this sublattice has the cluster-like character.

Neutron Diffraction Studies of Tb_2Ni_{2-x}In Intermetallic Compounds

61%

Szytuła A. , Baran S. , Hoser A. , Kalychak Y. M. , Penc B. , Tyvanchuk Y.

nr 6

994-997

The magnetic ordering of the Tb_2Ni_{1.78}In and Tb_2Ni_2In have been studied by neutron diffraction measurements. Tb_2Ni_{1.78}In with the tetragonal Mo_2FeB_2-type (space group P4/mbm, tP10) is antiferromagnet with the Néel temperature equal to 20 K. Below this temperature Tb moments form collinear magnetic structure commensurate with the crystal, described by the propagation vectors equal to (1/4, 1/4, 1/2). Magnetic moment equal to 7.60(6) μ_{B} is parallel to c-axis. The Tb_2Ni_2In in the orthorhombic Mn_2 lB_2-type (space group Cmmm, oC10) was detected as an impurity in the studied sample. It orders antiferromagnetically below ≈ 100 K with collinear moment arrangement described by the propagation vector (1/2, 1/2, 1/2). t 1.6 K μ_{Tb} = 6.33(14) μ_{B} and is parallel to the c-axis.