Y. W. Du

Exchange coupling in Fe50Mn50/Ni81Fe19 bilayer: Dependence on antiferromagnetic layer thickness

We have determined the dependence of exchange coupling on the antiferromagnetic layer thickness (tAF) in FeMn/NiFe bilayers, where the FeMn layer varies from 0.5 to 32 nm, and the NiFe is constant at 30 nm. For tAF>3 nm, the exchange field Hex varies as (1/tAF)0.3 at both 80 and 300 K, whereas the coercivity is insensitive to tAF. The value of Hex displays a simple angular dependence of cos θ.We have determined the dependence of exchange coupling on the antiferromagnetic layer thickness (tAF) in FeMn/NiFe bilayers, where the FeMn layer varies from 0.5 to 32 nm, and the NiFe is constant at 30 nm. For tAF>3 nm, the exchange field Hex varies as (1/tAF)0.3 at both 80 and 300 K, whereas the coercivity is insensitive to tAF. The value of Hex displays a simple angular dependence of cos θ.

Mössbauer investigation of zinc ferrite particles

Mossbauer spectra of Zn0.2Fe2.3O4 particles at different temperatures have been obtained. They have been analyzed with five components, one due to the A‐site Fe3+ ions, and the others due to the B‐site Fe3+ and Fe2+ ions. Magnetic and electronic properties have been discussed with regard to zinc substitution and temperature dependence.

The coexistence of magnetic ordering and superconductivity in YBa2(Cu0.94Fe0.06)3O9−σ

Abstract We have made superconducting samples of the Y-Ba-Cu-O system with 6% Fe substituted for the Cu. The sample has been verified by X-ray diffraction to be a single-phased 1-2-3 compound. The 6% Fe substitution reduces T c from 94 to 80 K. The room temperature Mossbauer spectrum shows two pairs of doublets, indicating there are two distinct Cu sites in the sample. The most surprising result is that magnetic ordering of the Fe coexists with superconductivity of the material at 4.2 K.

The antiferromagnetism in YBa2Cu3O6+δ studied by Mössbauer spectroscopy

Abstract A small amount of 57 Fe (about 0.3%) was used to substitute for Cu in the Y-Ba-Cu-O 1-2-3 compounds. With this small amount of Fe, the possibility of Fe-Fe interactions can be ignored and the 57 Fe ions serve only as a probe to study the magnetic structure of the high T c superconductor compounds. As a result of annealing samples in flowing Ar gas at 750°C for 10 h, about 30% of the iron ions migrate from Cu(1) sites to Cu(2) sites. The Mossbauer spectrum of the Fe at the Cu(2) site shows that the Cu(2)-O 2 is antiferromagnetically ordered which agrees with the neutron diffraction result. The temperature dependence of the hyperfine field of the magnetic component follows a T 3/2 law, indicating a 3 dimensional spin wave excitation. This is consistent with the magnetic ordering of the Fe probe at the Cu(1) site which we observe at 4.2 K. The analysis of the Mossbauer spectra at different temperatures and with different oxygen deficiencies are also discussed.

Magnetic ordering and superconducting in Fe‐doped 1‐2‐3 compounds

We have doped Fe into the ground YBa2Cu3O7 and have discovered that the Fe goes primarily into one of the two Cu sites. At least 95% of the Fe substitutes into the Cu(1) site in the Cu‐O layers between Ba layers. A high valence state of iron, Fe4+, is found to exist inside this compound. For even small amounts of Fe we see a spin‐glass type of magnetic ordering of the Fe spins which we feel is associated with a tendency for antiferromagnetic couplings of the Cu(1) ions. We conclude that the Cu(2)‐O2 plane is more important than the Cu(1)‐O chain for the superconductivity in the 1‐2‐3 compounds.

A Mössbauer study of fine iron particles (invited)

Mossbauer spectroscopic measurements have been carried out on uniform‐size Fe particles over a size range from approximately 150 to 400 A. The particles were produced by evaporation into a nitrogen atmosphere. We have discovered that the thin oxide coating on these particles results in a significantly altered surface‐phonon spectrum as well as increased coercivity and a larger magnetic hyperfine field for the Fe core. These results correlate well with studies on thin epitaxial films.

The effect of Fe in Fe‐doped YBa2Cu3O7 superconductors

A series of samples of YBa2(Cu1−xFex)3O7 with x ranging from 0 to 0.13 was made and investigated by x‐ray diffraction, dc SQUID magnetometry, and Mossbauer spectroscopy. In the samples made by the ordinary process, Fe mainly occupies Cu(1) sites. The orthorhombic‐to‐tetragonal structural transition occurs at x≂0.03, and the superconducting transition temperature is depressed to zero at x≂0.17. By annealing the samples in Ar gas at 750 °C for 24 h and then reloading the lost oxygen in O2 gas at 210 °C for 48 h, we were able to draw about 30% of the Fe from Cu(1) sites into Cu(2) sites. This was verified by the enhancement of the component in the Mossbauer spectrum corresponding to Fe in the Cu(2) sites. The structural and superconducting properties of this new series of samples were also investigated. Comparison with the ordinary Fe‐doped samples was also made.

Structural and superconducting properties of Bi-Sr-Ca-Cu superconductors studied by Fe doping

1.5% of Fe has been substituted for Cu in several ‘‘2212’’ and ‘‘2223’’ Bi‐Sr‐Ca‐Cu superconductors. All of the samples show a reduction of Tc by about 13 K due to the Fe impurities. Mossbauer measurements at room temperature reveal structural characteristics such as stacking faults and intergrowth of different phases in these Bi‐based compounds on the microscopic scale. The suppression of Tc due to Fe doping in the Bi ‘‘2212’’ or ‘‘ 2223’’ system is comparable to that of the ‘‘123’’ system, but much smaller than that of the ‘‘214’’ system. The interplanar correlation existing in the ‘‘123’’ and the Bi ‘‘2212’’ and ‘‘2223’’systems seems to play an important role in sustaining the high‐temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.

Substitution of Fe for Cu in the 1-2-3 system and its implication on the nature of the high-Tc superconductivity

Abstract We have doped Fe into Gd-based 1-2-3 compounds with dopant concentration in the range of 0.003 to 0.15. The structural and superconducting properties of all samples are determined with X-ray diffraction and dc SQUID magnetometry. Careful analysis of the Mossbauer spectra for all the samples leads to the determination of the oxygen coordinations around the Fe dopants. We then propose a model that describes the substitution of Fe for Cu in the 1-2-3 system. Our model suggests that the linear Cu(1)-O chains are important to the 90 K superconductivity in the 1-2-3 system.

Superconductivity and spin‐glass ordering in RBa2 (Cu1−xFex)3Oz: R=Y,Gd; 0≤x≤0.12

Samples of oxygen deficient perovskites, YBa2 (Cu1−x Fex )3Oz and GdBa2 (Cu1−x Fex )3 Oz (z≂7) were prepared with up to 12% Fe substituting for Cu. A novel variation of the microwave absorption technique was employed to establish the superconducting transition in these samples and to show that the reduction in the transition temperature with increasing Fe concentration is faster in the Gd samples than in the Y ones. In both series of samples, the room‐temperature Mossbauer spectra reveal two distinct Fe sites corresponding to two local oxygen coordinations, and more significantly the spectra at 4.2 K for specimens with x≥0.03 show that the Fe moments are magnetically ordered, leading to the coexistence of superconductivity and magnetism.