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Register a new userMagnetic anisotropy of BaCu2Si2O7: theory and antiferromagnetic resonance
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Antiferromagnetic resonance (AFMR) of BaCu2Si2O7 and a microscopic theory of the magnetic anisotropy of spin 1/2 chain compounds with folded CuO3 geometry being in good agreement with the available data are presented. The AFMR studies at 4.2 K show the existence of two gaps (40 and 76 GHz) at zero magnetic field and of two spin re-orientation transitions for H||c. The microscopic origin of the two gaps is shown to be Hunds rule coupling which leads to a residual anisotropy beyond the compensation of the Dzyaloshinskii-Moriya term by the symmetric anisotropy which would be valid without Hunds coupling.
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We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below the phase transition at 40 K is evidence for an antiferromagnetically ordered ground state. In the normal state, above 40 K, the temperature dependence of the spin-susceptibilty measured by ESR agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic resonance line width has an unusual magnetic-field dependence which is large and temperature independent in the magnetically ordered state and decreases rapidly above the transition. These observations agree with the suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.
Quasi-one-dimensional magnet NiCl$_2cdot$4SC(NH$_2$)$_2$, usually abbreviated as DTN, does not order at zero field down to $T=0$: due to the strong single-ion anisotropy of the easy plane type acting on $S=1$ Ni$^{2+}$ ions, the $S_z=0$ ground state is separated from $S_z=pm 1$ excitations by an energy gap. Once the magnetic field is applied along the main anisotropy axis, the gap closes at $B_{c1}=2.18$ T and the field-induced antiferromagnetic order arises. The low-energy excitations spectrum of this field-induced ordered state includes two branches of excitations, one of them have to be a gapless Goldstone mode. Recent studies of excitations spectrum in a field-induced ordered state of DTN (T.Soldatov et.al, Phys.Rev.B 101, 104410 (2020)) have revealed that Goldstone mode became gapped as magnetic field deviates from the main symmetry axis. This paper proposes simple description of antiferromagnetic resonance modes of quasi-one-dimensional quantum $S=1$ magnet with strong single-ion anisotropy. The approach used is based on a combination of the strong coupling model for the anisotropic spin chain with the conventional mean-field model of antiferromagnetic resonance. The resulting model fits to the known experimental results without additional tuning parameters.
The polarization dependence of magnetic excitations in the quasi one-dimensional antiferromagnet BaCu2Si2O7 is studied as a function of momentum and energy transfer. The results of inelastic neutron scattering measurements are directly compared to semi-analytical calculations based on the chain-Mean Field and Random Phase approximations. A quantitative agreement between theoretically calculated and experimentally measured dynamic structure factors of transverse spin fluctuations is obtained. In contrast, substantial discrepancies are found for longitudinal polarization. This behavior is attributed to intrinsic limitations of the RPA that ignores correlation effects.
We present a dynamical mean-field study of antiferromagnetic magnons in one-, two- and three-orbital Hubbard model of square and bcc cubic lattice at intermediate coupling strength. Weinvestigate the effect of anisotropy introduced by an external magnetic field or single-ion anisotropy.For the latter we tune continuously between the easy-axis and easy-plane models. We also analyzea model with spin-orbit coupling in cubic site-symmetry setting. The ordered states as well as themagnetic excitations are sensitive to even a small breaking ofSU(2)symmetry of the model andfollow the expectations of spin-wave theory as well as general symmetry considerations.
Soft x-ray absorption experiments on the transition metal L2,3 edge of multiferroic MnWO4 and antiferromagnetic CoWO4 are presented. The observed linear polarization dependence, analyzed by full-multiplet calculations, is used to determine the ground state wave function of the magnetic Mn2+ and Co2+ ions. The impact of the local structure and the spin-orbit coupling on the orbital moment is discussed in terms of the single-ion anisotropy. It is shown that the orbital moment in CoWO4 is responsible for the collinear antiferromagnetism, while the small size of spin-orbit coupling effects make spiral magnetic order in MnWO4 possible, enabling the material to be multiferroic.
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