The surface roughness of orthodontic archwires is an essential factor that determines the effectiveness of arch-guided tooth movement. Using the non-destructive techniques of atomic force microscopy (AFM) and of laser specular reflectance, the surface roughness of 11 nickel-titanium orthodontic wires, a stainless steel and a beta-titanium wire was measured. The results were compared with those obtained using surface profilometry. The smoothest wire, stainless steel, had an optical roughness of 0.10 micron, compared with 0.09 micron from AFM and 0.06 from profilometry. The surface roughness for the beta-titanium wire measured by all three methods was approximately 0.21 micron, while that of the NiTi wires ranged from 0.10 to 1.30 microns. As the surface roughness not only affects the effectiveness of sliding mechanics, but also the corrosion behaviour and the aesthetics of orthodontic components, the manufacturers of orthodontic wires should make an effort to improve the surface quality of their products.
Magnetization and neutron-diffraction measurements were performed on a single crystal of Cu 2 MnSnS 4. This quartenary magnetic semiconductor has the stannite structure ͑derived from the zinc-blende structure which is common to many II-VI dilute magnetic semiconductors͒, and it orders antiferromagnetically at low temperature. The neutron data for the nuclear structure confirm that the space group is I42m. Both the neutron and magnetization data give T N ϭ8.8 K for the Néel temperature. The neutron data show a collinear antiferromagnetic ͑AF͒ structure with a propagation vector kϭ͓1/2,0,1/2͔, in agreement with earlier neutron data on a powder. However, the deduced angle between the spin axis and the crystallographic c direction is between 6°and 16°, in contrast to the earlier value of 40°. The magnetization curve at TӶT N shows the presence of a spin rotation ͑analogous to a spin flop͒, which indicates that the spin axis is indeed close to the c direction. The deduced magnetic anisotropy gives an anisotropy field H A Х2 kOe. At high magnetic fields the magnetization curve at TӶT N shows the transition between the canted ͑spin-flop͒ phase and the paramagnetic phase. The transition field, Hϭ245.5 kOe, yields an intersublattice exchange field H E ϭ124 kOe. The exchange constants deduced from H E and the Curie-Weiss temperature ⌰ϭϪ25 K show that the antiferromagnetic interactions are an order of magnitude smaller than in II-VI dilute magnetic semiconductors ͑DMS's͒. The much weaker antiferromagnetic interactions are expected from the difference in the crystal structures ͑stannite versus zinc-blende͒. A more surprising result is that the exchange constant which controls the AF order below T N is not between Mn ions with the smallest separation. This result contrasts with a prediction made for the related II-VI DMS, according to which the exchange constants decrease rapidly with distance. ͓S0163-1829͑97͒04234-3͔
The magnetization of Mn1-xZnxF2 single crystals with x = 0.25 and 0.51 was studied in low magnetic fields directed along the easy axis (c axis). Data were taken for axial fields Haxial between approximately 10-3 Oe and approximately 10 Oe. Below the Neel temperature TN, the magnetization M for given H,axial and temperature T depended on the history of the sample. Data obtained while cooling in a constant Haxial (FC procedure) show a dramatic rise of M below TN. This rise corresponds to the development of a thermoremanent magnetization M,. The sign and magnitude of M, depend on the axial field present while cooling; Mr is proportional to Haxial at low axial fields, but approaches saturation at Haxial approximately 1 Oe. The measured values of Mr(t)/Mr(0), where t = T/TN is the reduced temperature, are independent of Haxial and are the same for both samples. They also agree with values of Mr(t)/Mr(0) measured in K2Fe1-xInxCl5.H20, but not with those measured in K2Fe(Cl1-xBrx)5.H20. Fits of the temperature dependence of M, near TN yield an effective critical exponent B,. Extrapolated values of B, very close to TN are between 0.35 and 0.40. Experiments in which Haxial is changed below TN show that the remanent magnetization is controlled only by the field Haxial(TN) present while cooling through TN. The observed properties of Mr, particularly the saturation in axial fields of approximately 1 Oe, cannot be explained in terms of domains caused by random fields. Another explanation that fails is based on the statistical difference between the number of up and down spins in each of the antiferromagnetic domains, which exist even in the absence of random fields. It is possible that Mr arises from the walls between such domains. but since this explanation is yet to be tested other possible explanations cannot be ruled out.
The magnetic properties of a new diluted magnetic semiconductor Zn1−xCrxSe are reported. Specific heat was measured for 1.5 K≤T≤20 K and B≤3 T, whereas magnetization data were taken at T=2 and 4.2 K for magnetic field (B≤6 T) along (100), (110), and (111) crystallographic directions. The data are interpreted using the crystal-field model for the Cr++ ion, including static Jahn–Teller distortion.
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