[en] Spin tunneling in molecular magnets has attracted much attention, however theoretical considerations of this phenomenon up to now have not taken into account the many-spin nature of molecular magnets. We present, to our knowledge, the first successful attempt of a realistic calculation of tunneling splittings for Mn₁₂ molecules, thus achieving a quantitatively accurate many-spin description of a real molecular magnet in the energy interval ranging from about 100 K down to 10^-12 K. Comparison with the results of the standard single-spin model shows that many-spin effects affect the tunneling splittings considerably. The values of ground state splitting given by single-spin and many-spin models differ from each other by a factor of 5. (c) 2000 American Institute of Physics

[en] An approach to image the domains and domain walls of small ferromagnetic entities using atomic force microscopy (AFM), with a nonmagnetic AFM probe, has been developed. Exciting the sample in an external ac magnetic field, the distribution of magnetostrictive response at the surface is detected. By this technique, the domains and domain walls of submicron Co dots have been imaged with a 1 nm lateral resolution. In elliptical Co dots with a 350-nm-long axis on a triangular lattice array with 400 nm periodicity, we find evidence for two domains with opposite magnetization orientation across a wall. The domain-wall width in these dots is found to be about 35 nm. Furthermore, we observe a ferromagnetic alignment of the domains in the neighboring dots, which suggests a magnetostatic interaction among the dots. (c) 2000 American Institute of Physics

[en] Theoretical studies of the equilibrium geometries, electronic structure, and magnetic properties of Cr₈ and Cr₁₃ clusters have been carried out using the local-spin-density approximation in the density-functional theory. Several nearly degenerate isomers have been identified for both these clusters. In contrast to the recent studies of Cheng and Wang [Phys. Rev. Lett. 77, 51 (1996)], no indication of dimerization was found in the ground-state geometry of Cr₈. The isomers have different magnetic properties. Among the three nearly degenerate isomers of Cr₈, two are antiferromagnetic while the other is ferromagnetic with a net moment of 4.0μ_B. Cr₁₃, on the other hand, has two nearly degenerate isomers--a ferrimagnetic one with a net moment of 14.0μ_B and a ferromagnetic one with a net moment of 2.0μ_B. These results are consistent with collision-induced dissociation and magnetic deflection experiments. (c) 1999 The American Physical Society

[en] Starting from rapidly quenched Nd_δFe_13.1B(2.05≤δ≤147.6) alloys and by appropriate annealing, the microstructure was tailored from strongly interacting Nd₂Fe₁₄B grains to magnetically isolated single domain Nd₂Fe₁₄B grains embedded in a nonmagnetic Nd-rich matrix (α-Nd and γ-Nd). This change in microstructure was found to have a large effect on coercivity, i.e., coercivity, μ₀H_c, increases with an increase of the Nd concentration from 1.25 T in Nd_2.05Fe_13.1B to 2.75 T in Nd_147.6Fe_13.1B at 290 K. Using transmission electron microscopy, the Nd₂Fe₁₄B grains in Nd_147.6Fe_13.1B were confirmed to be randomly oriented platelets with the c axis normal to the plate and an average size of 100x40x25 nm. For these randomly oriented, noninteracting, single domain Nd₂Fe₁₄B grains, the coercivity was calculated using a Stoner-Wohlfarth model which included the shape anisotropy of the grains. The observed coercivity of Nd₂Fe₁₄B in such nanocomposite Nd_147.6Fe_13.1B alloys is ∼83% of its theoretical value. (c) 2000 American Institute of Physics

[en] Numerical simulations of submicron Co extruded elliptical dots were performed to illustrate the relative importance of different physical parameters on the switching behavior in the easy direction. Shape, size, magnetic moment magnitude, and the magnitude and distribution of the crystalline anisotropicity were varied. The simulation represents magnetostatic, exchange, and crystalline anisotropicity fields on a structured mesh using finite difference techniques. The smooth boundary of the dots is accurately represented by use of the embedded curve boundary method. Agreement with experimental hysteresis measurements of submicron dot arrays is obtained when an appropriate angular distribution of the grain anisotropicity axes is invoked. (c) 2000 American Institute of Physics

[en] Magneto-optically active nanocomposite structures have been created by using ion implantation and thermal processing to form precipitated layers of ferromagnetic α-Fe or ferrimagnetic Fe₃O₄ that are embedded in the near-surface region of (100)-oriented yttrium stabilized ZrO₂ (YSZ). When Fe-implanted YSZ is annealed at 1100 degree sign C in Ar+4%H₂, the redox conditions are sufficiently reducing so that metallic Fe is the stable phase. At lower temperatures the annealing conditions become less reducing and Fe₃O₄ becomes the stable phase. Transmission electron microscopy and x-ray diffraction studies established that each α-Fe or Fe₃O₄ particle is a single crystal that is crystallographically aligned with respect to the YSZ host. Magneto-optical effects due to both the α-Fe and Fe₃O₄ nanophase precipitates were found and characterized using magnetic circular dichroism. These magneto-optical effects have potential applications in integrated-optical devices. (c) 2000 American Institute of Physics

[en] A synergistic approach that combines first-principles theory and electron photodetachment experiment is shown to be able to uniquely identify the ground state of a nearly degenerate cluster in the gas phase. Additionally, this approach can complement the Stern-Gerlach technique in determining the magnetic moment of small clusters unambiguously. The method, applied to a Fe₃ cluster, reveals its ground state to have a magnetic moment of 10μ_B--in contrast with earlier predictions. (c) 2000 The American Physical Society

[en] A spin-wave Brillouin scattering study of a CoPt-SiO₂ granular magnetic recording medium was made. This film contains ferromagnetic CoPt particles in a SiO₂ matrix, and has an extremely low medium noise property due to little exchange coupling between magnetic grains. Spin waves of both the propagating surface mode and standing wave mode were found to be excited in granular magnetic films with various microstructures. A possible origin of the spin wave is a magnetostatic coupling between regularly ordered CoPt grains, as reported for artificially patterned magnetic thin films. This result shows two promising features of the CoPt-SiO₂ granular film for high density recording medium: It is an ordered media obtained in a self-organizing manner, and it is less influenced by the thermal fluctuation effect, which is a serious problem for current high density magnetic recording. (c) 2000 American Institute of Physics

[en] Nanocomposite FePt:SiO₂ thin films consisting of high anisotropy FePt particles embedded in a SiO₂ matrix have been successfully fabricated by annealing the as-deposited FePt/SiO₂ multilayers. By adjusting the annealing temperatures and compositions, films were obtained with coercivity of 3.8 kOe and grain size of 10 nm, which are suitable for high-density magnetic recording. Magnetic activation volumes were measured and thermal stability is discussed. (c) 2000 American Institute of Physics

[en] The soft magnetic properties of the nanocrystalline Fe endash M endash B (M=Zr,Nb) alloys, which exhibit a high saturation flux density (B_s) above 1.5 T as well as a high effective permeability (μ_e) above 30000 at 1 kHz, were found to be improved by adding small amounts of Cu and by optimizing the chemical composition. The addition of Cu to the alloys decreases the bcc grain size. The excellent soft magnetic properties (a high μ_e of 100000 at 1 kHz combined with a high B_s of 1.53 T) can be achieved in the region where small grain size, as well as nearly zero-magnetostriction are obtained, which is attained in the compositional range around Fe₈₄Nb_3.5Zr_3.5B₈Cu₁. The soft magnetic properties can be further improved by low temperature annealing before the crystallization treatment, probably as a result of a decreased grain size distribution in the crystallized state. Consequently, the μ_e reaches the maximum value of 120000 for the nanocrystalline Fe₈₄Nb_3.5Zr_3.5B₈Cu₁ alloy. copyright 1997 American Institute of Physics