Magnetic Susceptibility of Collinear and Noncollinear Heisenberg Antiferromagnets

Predictions of the anisotropic magnetic susceptibility chi below the antiferromagnetic (AFM) ordering temperatures TN of local moment Heisenberg AFMs have been made previously using molecular field theory (MFT) but are very limited in their applicability. Here a MFT calculation of chi(T<=TN) is presented for a wide variety of collinear and noncollinear Heisenberg AFMs containing identical crystallographically equivalent spins without recourse to magnetic sublattices. The results are expressed in terms of directly measurable experimental parameters and are fitted with no adjustable parameters to experimental chi(T<=TN) data from the literature for several collinear and noncollinear AFMs. The influence of spin correlations and fluctuations beyond MFT is quantified by the deviation of the theory from the data. The origin of the universal chi(T<=TN) observed for triangular lattice AFMs exhibiting coplanar noncollinear 120 degree AFM ordering is clarified.Comment: 5 pages, 5 figure

Alternating commensurate-incommensurate structures in the magnetic phase diagram of CsNiF3

The magnetic phase diagram of the quasi one-dimensional spinchain system CsNiF$_3$ below the N\'eel temperature is determined. For magnetic fields perpendicular to the spin chains incommensurate phases are predicted. From linear spin-wave theory we obtain the instability line of the paramagnetic phase as a function of the strength and the direction of the field. The system undergoes a transition to a commensurate or an incommensurate phase depending on the direction of the magnetic field. In the commensurate phase the characterizing wave vector is locked to values describing a two-sublattice structure, whereas in the incommensurate phase the wave vector changes continuously between the corresponding two-sublattice wave vectors.Comment: 11 pages, LaTeX, 5 figures, sent to PRB Rapid Communicatio

Renormalization of the spin-wave spectrum in three-dimentional ferromagnets with dipolar interaction

Renormalization of the spin-wave spectrum is discussed in a cubic ferromagnet with dipolar forces at $T_C\gg T\ge0$. First 1/S-corrections are considered in detail to the bare spectrum $\epsilon_{\bf k} = \sqrt{Dk^2 (Dk^2 + S\omega_0\sin^2\theta_{\bf k})}$, where $D$ is the spin-wave stiffness, $\theta_{\bf k}$ is the angle between $\bf k$ and the magnetization and $\omega_0$ is the characteristic dipolar energy. In accordance with previous results we obtain the thermal renormalization of constants $D$ and $\omega_0$ in the expression for the bare spectrum. Besides, a number of previously unknown features are revealed. We observe terms which depend on azimuthal angle of the momentum $\bf k$. It is obtained an isotropic term proportional to $k$ which makes the spectrum linear rather than quadratic when $\sin\theta_{\bf k}=0$ and $k \ll \omega_0/D$. In particular a spin-wave gap proportional to $\sin\theta_{\bf k}$ is observed. Essentially, thermal contribution from the Hartree-Fock diagram to the isotropic correction as well as to the spin-wave gap are proportional to the demagnetizing factor in the direction of domain magnetization. This nontrivial behavior is attributed to the long-range nature of the dipolar interaction. It is shown that the gap screens infrared singularities of the first 1/S-corrections to the spin-wave stiffness and longitudinal dynamical spin susceptibility (LDSS) obtained before. We demonstrate that higher order 1/S-corrections to these quantities are small at $T\ll\omega_0$. However the analysis of the entire perturbation series is still required to derive the spectrum and LDSS when $T\gg\omega_0$.Comment: 11 pages, 1 figur

A vacuum ultraviolet spectrophotometric system

The development of a vacuum ultraviolet spectrophotometric system for measuring transmittance and reflectance at variable angles is presented. Using various detectors and sources, the spectrophotometric system has been used for wavelengths from 80 nm to 300 nm with optical components up to 80 mm in diameter. The capability exists to make measurements through the visible range

Controlled switching of intrinsic localized modes in a 1-D antiferromagnet

Nearly steady-state locked intrinsic localized modes (ILMs) in the quasi-1d antiferromagnet (C2H5NH3)2CuCl4 are detected via four-wave mixing emission or the uniform mode absorption. Exploiting the long-time stability of these locked ILMs, repeatable nonlinear switching is observed by varying the sample temperature, and localized modes with various amplitudes are created by modulation of the microwave driver power. This steady-state ILM locking technique could be used to produce energy localization in other atomic lattices.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. v.2 : clarifications of text and figures in response to comment

Order of Two-Dimensional Isotropic Dipolar Antiferromagnets

The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets is studied. It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter. The resulting finite N\'eel-temperature is calculated for a square lattice by means of linear spin-wave theory.Comment: 10 pages, REVTEX, 1 figure available upon request, TUM-CP-93-0

Bose-Einstein condensation in antiferromagnets close to the saturation field

At zero temperature and strong applied magnetic fields the ground sate of an anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins. We study the quantum phase transition as the field is reduced below an upper critical $H_{c2}$ and the system enters a XY-antiferromagnetic phase. Using a bond operator representation we consider a model spin-1 Heisenberg antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under strong magnetic fields. We show that the transition at $H_{c2}$ can be interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical results are used to analyze our magnetization versus field data in the organic compound $NiCl_2$-$4SC(NH_2)_2$ (DTN) at very low temperatures. This is the ideal BEC system to study this transition since $H_{c2}$ is sufficiently low to be reached with static magnetic fields (as opposed to pulsed fields). The scaling of the magnetization as a function of field and temperature close to $H_{c2}$ shows excellent agreement with the theoretical predictions. It allows to obtain the quantum critical exponents and confirm the BEC nature of the transition at $H_{c2}$.Comment: 4 pages, 1 figure. Accepted for publication in PRB

Radiation Damage Effects in Far Ultraviolet Filters and Substrates

New advances in VUV thin film filter technology have been made using filter designs with multilayers of materials such as Al2O3, BaF2, CaF2, HfO2, LaF3, MgF2, and SiO2. Our immediate application for these filters will be in an imaging system to be flown on a satellite where a 2 X 9 R(sub E) orbit will expose the instrument to approximately 275 krads of radiation. In view of the fact that no previous studies have been made on potential radiation damage of these materials in the thin film format, we report on such an assessment here. Transmittances and reflectances of BaF2, CaF2, HfO2, LaF3, MgF2, and SiO2 thin films on MgF2 substrates, Al2O3 thin films on fused silica substrates, uncoated fused silica and MgF2, and four multilayer filters made from these materials were measured from 120 nm to 180 nm before and after irradiation by 250 krads from a Co-60 gamma radiation source. No radiation-induced losses in transmittance or reflectance occurred in this wavelength range. Additional postradiation measurements from 160 nm to 300 nm indicated a 3 - 5% radiation-induced absorption near 260 nm in some of the samples with MgF2 substrates. From these measurements it is concluded that far ultraviolet filters made from the materials tested should experience less that 5% change from exposure to up to 250 krads of high energy radiation in space applications

A Spin Model for Investigating Chirality

Spin chirality has generated great interest recently both from possible applications to flux phases and intrinsically, as an example of a several-site magnetic order parameter that can be long-ranged even where simpler order parameters are not. Previous work (motivated by the flux phases) has focused on antiferromagnetic chiral order; we construct a model in which the chirality orders ferromagnetically and investigate the model's behavior as a function of spin. Enlisting the aid of exact diagonalization, spin-waves, perturbation theory, and mean fields, we conclude that the model likely has long-ranged chiral order for spins 1 and greater and no non-trivial chiral order for spin 1/2.Comment: uuencoded gzipped tarred plain tex fil