ALBERT

Notes: We have measured the anisotropic magnetoresistance of Fe films exchange coupled to antiferromagnetic MnF2 layers. Exchange bias and coercivity obtained from magnetoresistance are in close agreement with superconducting quantum interference device magnetometry data. In addition the magnetoresistance reveals an asymmetry in the magnetization reversal process, despite the fact that the magnetization hysteresis loops show little shape asymmetry. These results correlate well with an earlier study of magnetization reversal asymmetry by polarized neutron reflectometry. The data imply that the magnetization reverses by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other. © 2000 American Institute of Physics.

Notes: Large exchange bias effects were measured in antiferromagnetic FeF2-ferromagnetic Fe bilayers. The interface spin structure and roughness was controlled by using different substrates and growth temperatures. The samples were characterized ex situ using low and high angle x-ray diffraction, in-plane x-ray diffraction, and atomic force microscopy. We discovered that the exchange bias is largest in the (110) FeF2 spin-compensated surface films, in contradiction with simple models that predict a low exchange anisotropy for spin-compensated surfaces. For these samples, the exchange anisotropy decreases as the interface roughness increases. © 1996 American Institute of Physics.

Notes: It is well known that competing magnetic interactions in amorphous Fe-rich Fe-Zr alloys result in a ferromagnetic-like transition below room temperature (for example, Tc≈235 K for a Fe90Zr10 alloy). The field dependence of the ac response of such a system with competing interactions is thus complex, with the possible coexistence of ferromagnetic domains and spin freezing processes. One consequence of this is the observed1 Hopkinson peak in ac susceptibility and its strong field dependence when measured at ordinary fields ((approximately-greater-than)80 A/m). The existence of such a complex Hopkinson peak, which arises from the contribution of domain wall motion and hysteretic effects, makes it very difficult to identify, unambiguously, a spin freezing temperature. The purpose of this paper is to clarify this point. Complex ac susceptibility [χ'(T), χ‘(T)] measurements have been carried out over an extended range of field amplitude (0.08–8000 A/m) and three decades of frequency (1–1000 Hz) along the axis of a single sample of melt spun Fe90Zr10 ribbon. We find that, below the threshold field, there is a unique spin freezing temperature with almost no frequency or field dependence. At higher fields ((approximately-greater-than)80 A/m) the viscous susceptibility, χ‘(T), peak at low temperatures shows a strong temperature, frequency, and field dependence, indicating a superposition of the effects of "induced'' long range order with the spinglass state. The ferro-to-spin glass transition temperature using this approach has been found to be Tsf=25 K.

Notes: We demonstrate a new and versatile method to measure, on a nanometric scale directly, the magnetostrictive properties during the magnetization process to saturation, using a modified scanning tunneling microscope (STM). Both positive and negative magnetostrictive amorphous as-quenched wires have been studied. The studied samples are single pieces of amorphous wires typically 125 μm in diameter and 10–15 mm long. The magnetostriction data are then correlated with the longitudinal magnetization process, measured by a conventional induction technique and by SQUID magnetometry. The longitudinal magnetization process measurements have been performed in the same wires used in the STM studies. The field dependence of the magnetostriction helps to discern the operative magnetization process in the wires. In a 12-mm-long Co-based amorphous wire we observe a continuous rotation of the magnetization from zero field to saturation. The field dependence of the magnetization is a linear process reaching a value of about 5100 G at 8 Oe applied field. The magnetostrictive process for the same wire measured with a STM shows a continuous shrinking of the samples as a function of the applied field, reaching a value of −280 A(ring) for 8 Oe applied field. The obtained δl/l||sat, −2.3×10−6, agrees well with reported values of the saturation magnetostriction constant for this wire. In a 15-mm-long Fe-based amorphous wire we observe a more complicated field dependence of both the magnetization and the magnetostriction processes from zero-field to saturation.The longitudinal magnetization saturates at about 14000 G for 60 Oe applied field. The magnetostrictive process for the same wire measured with STM shows an elongation onset at about 1 Oe, pointing unambiguously to a 180° domain wall movement as the operative magnetization process below this field value. The field dependence of the magnetostriction saturates at about +5000 A(ring) for 60 Oe applied field. The obtained δl/l||sat, about +3.3×10−5, is again consistent with reported values of the saturation magnetostriction constant. Thus the STM approach to determine λ, on an A(ring) length scale, gives us a distinct great advantage in studying the magnetization process towards saturation.

Notes: Submicrometric magnetic structures have been fabricated by electron-beam lithography on Si substrates. High-quality patterns have been obtained with typical length scale of the structures in the range of 100 nm. The designed geometrical configurations are suitable for investigation of their physical properties by transport measurements in a controlled way. In particular, long chains of connected magnetic dots are useful to analyze magnetization reversal processes, whereas ordered arrays of isolated dots can be used to study pinning effects in superconducting films. © 1998 American Institute of Physics.

Notes: The real and imaginary parts of ac susceptibility of a sintered Y1Ba2Cu3O7−δ superconductor were measured before and after powdering. The temperature-dependent susceptibility may be separated into two contributions, one sensitive and the other relatively insensitive to the magnitude of the measuring field. The former is partially suppressed by coarsely crushing the sample. It is completely suppressed after finely powdering, whereupon the susceptibility curves become insensitive to the magnitude of the measuring field. Several models apparently consistent with the results are discussed.

Notes: ac and dc magnetic susceptibilities were studied for relatively dense single-phase YBa2Cu3O7−x samples. The data are discussed in terms of sample defects.

Notes: We have studied the physical properties of high Tc Y-Ba-Cu-Ox superconducting materials with levitation, ac susceptibility, macroscopic resistivity, resistivity as measured by scanning tunneling microscopy (STM) and Hall effect. Levitation experiments show that the powder of the as-prepared material does not levitate at liquid nitrogen while pellets and powders that have been heated above 450 °C do levitate (are superconducting). These experiments seem to indicate that clustering and intimate contact of fine grains are necessary for levitating. The ac susceptibility experiments show that diamagnetism is extremely sensitive to pellet density. The higher the density and the smaller the field amplitude the less diamagnetic is the system. This is interpreted as evidence for a surface, not a bulk effect. Resistivity at 17 °C measured with macroscopic contacts is of the order of 10 Ω cm. When measured with microscopic STM contacts, a clear semiconducting behavior is observed. This observation does not preclude grain boundary regions with metallic conductivity. We conjecture that the bulk of the grain is semiconducting with a conducting percolative network of grain boundaries that is the source of the superconductivity. We propose a model of a superconductor-metal-superconductor percolative network based on the excitonic model of Allender, Bray, and Bardeen to explain the high values of Tc. Hall measurements show large effective masses in agreement with an excitonic model.

Notes: We demonstrate a versatile capability to measure directly the magnetostrictive properties through the magnetization process on a nanometric scale using a modified scanning tunneling microscope. Single 10 mm long, 125 μm diam amorphous wires of both positive and negative magnetostriction have been studied and the data are compared with the hysteretic loops determined by both ac and SQUID magnetic measurements. This improved technique promises interesting possibilities, from both fundamental and applications points of view, in a number of scientific disciplines especially of interest in life and environmental sciences. © 1995 American Institute of Physics.

Notes: We have studied the exchange anisotropy of ferromagnetic Fe films grown on antiferromagnetic FeF2 single crystals. The behavior of the hysteresis loops of the Fe above and below the Néel temperature TN of FeF2 indicates a 90° rotation of the ferromagnetic easy axis due to the antiferromagnetic ordering. By examining the Fe hysteresis loops together with the FeF2 susceptibility behavior we infer that below TN the ferromagnetic and antiferromagnetic spins are coupled perpendicular to each other. This behavior can be explained by recent micromagnetic calculations on exchange bias systems, or by magnetoelastic effects. © 1998 American Institute of Physics.