Collapse of the Small‐Angle Magnon Scattering in Fe as a Function of Magnetic Field

The dependence of the spin wave energy on the magnetization M⇒ and the applied magnetic field H⇒ in Fe (and other ferromagnets) has not been very well investigated with neutrons. According to the Holstein‐Primakoff dispersion relation, the contributions of the Zeeman energy gμBHgμBH and the dipole‐dipole interactions 4π gμBM sin2θq4πgμBMsin2θq do not simply add linearly to the exchange energy Dq2. However, in order to see these contributions, one must observe the very low energy (.01 – .1 mev) spin waves. One of the predictions of this dispersion relation is that the scattering of neutrons by spin waves near the origin should disappear as the magnetic field is increased. This is a consequence of the kinematics of the scattering process. Using our double‐Si crystal technique for small angle scattering we have experimentally observed this collapse at a field of about 8 kG in Fe at room temperature as predicted by theory. We have also measured the scattering due to these very low energy spin waves at temperatures up to .7 Tc and compared the data on an absolute scale with the theoretical cross section. The agreement is reasonably good.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87920/2/1340_1.pd

Science and peace: Coping with our creations

Item consists of a digitized copy of a video recording of a Vancouver Institute lecture given by Anthony Arrott on October 1, 1988. Original video recording available in the University Archives (UBC VT 156).Non UBCUnreviewedUnknow

Pattern and rhythm in physics and art

Item consists of a digitized copy of an audio recording of a Vancouver Institute lecture given by Anthony Arrott on November 22, 1980. Original audio recording available in the University Archives (UBC AT 944).Other UBCUnreviewedOthe

Suppression of anisotropy and inversion symmetry effects by magnetic charge density points to better motors

It is proposed to use periodically patterned polycrystalline Iron in motors, generators and transformers to operate at a higher fraction of the saturation magnetization [Schäfer, Soldatov, and Araid, Journal of Magnetism and Magnetic Material 474, 221 (2019)]

Using magnetic charge to understand soft-magnetic materials

This is an overview of what the Landau-Lifshitz-Gilbert equations are doing in soft-magnetic materials with dimensions large compared to the exchange length. The surface magnetic charges try to cancel applied magnetic fields inside the soft magnetic material. The exchange energy tries to reach a minimum while meeting the boundary conditions set by the magnetic charges by using magnetization patterns that have a curl but no divergence. It can almost do this, but it still pays to add some divergence to further lower the exchange energy. There are then both positively and negatively charged regions in the bulk. The unlike charges attract one another, but do not annihilate because they are paid for by the reduction in exchange energy. The micromagnetics of soft magnetic materials is about how those charges rearrange themselves. The topology of magnetic charge distributions presents challenges for mathematicians. No one guessed that they like to form helical patterns of extended multiples of charge density

Spin Directions in Pure Chromium

We have carried out a triple‐axis polarized‐neutron‐beam experiment with polarization analysis of the final beam and magnetic fields to 15 kG applied to a pure Cr single crystal. The purpose was to determine whether the spin axis in the transversely polarized spin‐density wave state (122°K–38.5°C) is confined to the cube edges or whether in sufficient fields it can be made to lie in an arbitrary direction (perpendicular to the wave vector). The experiments show unambiguously that the latter is so. At 25°C, it is slightly more difficult to confine the spins to a single 110 axis than it is to a single 100 axis. At lower temperatures this anisotropy is enhanced. These results along with our previous results for the field dependence of the cube‐edge components using unpolarized neutrons have been analyzed in terms of two different models. Both models have the spins in all directions perpendicular to the wave vector of the spin‐density wave. One is the model of thermal activation of small domains. The other considers a domain structure with wall motion. In both models ansiotropy and magnetic field influence the net number of spins in any given direction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70250/2/JAPIAU-40-3-1447-1.pd

Neutron—Diffraction Study of Cr and Cr Alloys

The principle quantities which parameterize the antiferromagnetism of Cr are studied by neutron diffraction as a function of temperature, pressure, magnetic field, and concentration of solute atoms. In order to account for the observed intensities of the magnetic reflections, their dependence on magnetic field and temperature, the torque measurements of Montalvo and Marcus, and the creation of a ``single‐Q'' state in field‐cooling, a model is proposed based on the presumed existence of thermally active polarization domains within a ``single‐Q'' region of the crystal. The variation of the polarization axis from place to place and with time lowers the free energy by an increase in entropy. The pressure dependence of the first‐order phase change at 38.5°C is given as dTN∕dP=−5.4 deg∕kbar. The temperature dependence of the length of the wave vector below TN is given as (1∕Q) (dQ∕dT)=6.3×10−5 deg−1. Alloys with 0.5 and 0.78 wt% Fe and with 0.9 wt% Co show a decrease in TN of ∼20°K per at.% of solute. The amplitudes of the magnetization waves increase and the wave vector Q approaches commensurateness with the lattice periodicity with increasing solute concentration in contrast to results for other solutes. Some unusual effects were observed for 2.3 wt% Fe samples.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70981/2/JAPIAU-38-3-1243-1.pd

Magnetic charges suppress effects of anisotropy in polycrystalline soft ferromagnetic materials

Micromagnetic simulations of polycrystalline iron washers show that grain boundary charges, ρ = -div M, suppress bad effects of magnetocrystalline anisotropy. A single domain wall divides the washer into two domains with opposite magnetization; M is almost = ± Ms ϕ, where ϕ circulates about the hole in the washer. There is a ripple structure. M tilts back and forth toward the inner and outer surfaces. Magnetic charge densities, σm = n·M, on the surfaces keep M at the surfaces very close to lying in the surfaces. The exchange εx and magnetostatic εd energy densities try to keep M parallel to the surfaces throughout the washer, except at the domain wall. An anisotropy energy in each grain is reduced linearly in the angle of rotation away from the circulating pattern towards the nearest anisotropy axis. Both εx and εd near grain boundaries increase as the square of these angles. Anisotropy wins for small rotations. However, the coefficients of the positive quadratic terms are so much larger than the coefficients of the negative linear terms that the rotations are quite small. If the height of the washer is sufficiently greater than 300 nm, M in the washer no longer acts as it would in a thin film. If 300 nm washers are stacked with a spacing of 4 nm, the ripple structure is not lost. The stacked washers can then be used as the core of a transformer. The most remarkable effect is that starting with M = Ms ϕ everywhere, the reversal of M by the field from a current along the z-axis produces a single domain wall. It is stable even in zero field because the wall has Néel caps that act as springs against the surfaces. The suppression of crystalline anisotropy in polycrystalline iron also occurs for geometries other than the toroid; some might be better for creating transformers