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Wyświetlanie 1-10 z 47
Tytuł:
Magnetic Interaction by Exchange of Field Bosons
Autorzy:
Köbler, U.
Hoser, A.
Tematy:
75.10.-b
75.30.Ds
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Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:
https://bibliotekanauki.pl/articles/1427529.pdf  Link otwiera się w nowym oknie
Opis:
It is shown that atomistic spin wave theory gives no correct account of the temperature dependence of the magnetic order parameter. The experimentally observed universal temperature dependence can be explained only by a field theory of magnetism. This means that instead by interacting spins (magnons) the dynamics is controlled by a boson field. The field quanta can be supposed to be magnetic density waves with dispersions that are simple power function of wave vector. This results in the observed universality. In three dimensions the field quanta have no mass and linear dispersion and cannot be observed using inelastic neutron scattering. Experiments on standing magnetic waves in thin ferromagnetic films provide direct information on the dispersion of the field quanta. A careful analysis of the available experimental data indicates that the dispersion of the field bosons is ım q, ım $q^2,$ and ım $q^{3//2}$ in three, two, and one dimensions.
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Dimensionality in Field Theory and in Spin Wave Theory
Autorzy:
Köbler, U.
Hoser, A.
Tematy:
75.10.-b
75.30.Ds
Pokaż więcej
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:
https://bibliotekanauki.pl/articles/1385629.pdf  Link otwiera się w nowym oknie
Opis:
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.
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Boson Fields in Ordered Magnets
Autorzy:
Hoser, A.
Köbler, U.
Tematy:
75.10.-b
75.30.Ds
Pokaż więcej
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:
https://bibliotekanauki.pl/articles/1385608.pdf  Link otwiera się w nowym oknie
Opis:
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.
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Neutron diffraction studies of PrNi₅Sn
Autorzy:
Penc, B.
Hoser, A.
Szytuła, A.
Tematy:
61.05.F-
75.20.En
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Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:
https://bibliotekanauki.pl/articles/1075355.pdf  Link otwiera się w nowym oknie
Opis:
Powder neutron diffraction measurements of PrNi₅Sn performed in the temperature range 1.5-76 K indicate that the compound crystallizes in a hexagonal CeNi₅Sn-type crystal structure (space group P6₃/mmc). The a lattice parameter and the unit cell volume V increase while the c lattice parameter does not change with increasing temperature. No long range magnetic ordering was detected down to 1.5 K, in contradiction to bulk magnetometric results.
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Neutron diffraction studies of PrNi_{5}Sn
Autorzy:
Penc, Bogusław
Szytuła, Andrzej
Hoser, A.
Opis:
Powder neutron diffraction measurements of PrNi_{5}Sn performed in the temperature range 1.5-76 K indicate that the compound crystallizes in a hexagonal CeNi_{5}Sn-type crystal structure (space group P6_{3}/mmc). The a lattice parameter and the unit cell volume V increase while the c lattice parameter does not change with increasing temperature. No long range magnetic ordering was detected down to 1.5 K, in contradiction to bulk magnetometric results.
Dostawca treści:
Repozytorium Uniwersytetu Jagiellońskiego
Artykuł
Tytuł:
Size effects in antiferromagnetic NiO nanoparticles
Autorzy:
Baran, S.
Hoser, A.
Penc, B.
Szytuła, A.
Tematy:
61.05.F-
75.30.Cr
75.47.Lx
75.50.Ee
Pokaż więcej
Wydawca:
Polska Akademia Nauk. Instytut Fizyki PAN
Powiązania:
https://bibliotekanauki.pl/articles/1075485.pdf  Link otwiera się w nowym oknie
Opis:
X-ray and neutron diffraction as well as magnetometric methods were used in order to investigate crystal and magnetic structure together with magnetic properties of nickel oxide NiO obtained from thermal decomposition of Ni(OH)₂. It has been found that crystal unit cell volume and crystal unit cell deformation parameter decrease with increasing decomposition temperature T_{d} while grain size increases. The results of magnetization, magnetic susceptibility and neutron diffraction measurements reveal a formation of antiferromagnetic order with uncompensated magnetic moment below the Néel temperature. Magnetization together with coercive field decreases with increasing T_{d}. The neutron diffractogram of sample obtained at 240°C indicates broadening of both the peaks of nuclear and magnetic origin. The magnetic ordering may be described by a propagation vector k=[½,½,½].
Dostawca treści:
Biblioteka Nauki
Artykuł
Tytuł:
Size effects in antiferromagnetic NiO nanoparticles
Autorzy:
Penc, Bogusław
Szytuła, Andrzej
Baran, Stanisław
Hoser, A.
Opis:
X-ray and neutron diffraction as well as magnetometric methods were used in order to investigate crystal and magnetic structure together with magnetic properties of nickel oxide NiO obtained from thermal decomposition of Ni(OH)$_{2}$. It has been found that crystal unit cell volume and crystal unit cell deformation parameter decrease with increasing decomposition temperature $T_{d}$ while grain size increases. The results of magnetization, magnetic susceptibility and neutron diffraction measurements reveal a formation of antiferromagnetic order with uncompensated magnetic moment below the Néel temperature. Magnetization together with coercive field decreases with increasing $T_{d}$. The neutron diffractogram of sample obtained at 240°C indicates broadening of both the peaks of nuclear and magnetic origin. The magnetic ordering may be described by a propagation vector $k = \left [ \frac{1}{2}, \frac{1}{2}, \frac{1}{2} \right ]$.
Dostawca treści:
Repozytorium Uniwersytetu Jagiellońskiego
Artykuł
Wyświetlanie 1-10 z 47

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