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Giant magnetoresistance in a spinodally decomposed, bulk 60 Cu‐20 Ni‐20 Fe alloy is reported. An annealed, quenched, and heat‐treated sample with a compositional modulation of ≲50 A size exhibits a ΔR/R value as high as 9% at 4.2 K. Optimization of the ferromagnetic phase particle geometry through a combination of spinodal decomposition and uniaxial deformation led to a locally multilayered, superlattice‐like structure and a dramatic increase in room‐temperature magnetoresistance from ∼0.6 to ∼5%. This improvement in magnetoresistance is accompanied by a decrease in coercivity from ∼620 Oe in the fully decomposed material to ∼45 Oe in the optimized structure. Interestingly, this structure no longer exhibits the commonly observed temperature‐dependent behavior of ΔR/R increasing at low temperatures, but rather shows a decrease at 4.2 K.

The Mossbauer effect has been used to measure the hyperfine field of 57Fe nuclei in the layered antiferromagnet Ba2FeF6. The temperature dependence of the sublattice magnetization was measured an order of magnitude closer to the Neel temperature than previous studies. A range of fit analysis to the power law equation h(t)=Btβ, yields β=0.135(3) and TN=47.96(4) K for 4.31×10−3

The Mossbauer effect of the 37.2 keV γ transition of 121Sb has been employed to study the electronic configuration of Sb ions in perovskite superconducting materials. The isomer shift (IS) changes with Sb content and falls in the region of the Sb(V) valence state. The IS increases from 7.00 mm/s for BaPb0.75Sb0.25O3 to 7.25 mm/s for BaPb0.75Bi0.25O3 and to 7.70 mm/s for (Ba0.6K0.4)BiO3 compounds, indicating a high degree of covalency, corresponding to changes in the 5sm5pn hybridization.

The Mossbauer effect of the 37.2 keV γ transition of 121Sb has been employed to study the electronic configuration of Sb ions in BaBi1−xSbxO3 with x=0.2, 0.4, and 0.5. A single resonance absorption line is observed. The isomer shift changes slightly with Sb content and falls in the region corresponding to an Sb(V) valence state. The isomer shift of 7.6 mm/s is significantly depressed (∼1 mm/s) relative to its value in a typical Sb(V)O6 chromophore and indicates a high degree of covalency, corresponding to significant 5sm5pn hybridization.

The reduction of V ions in FeV3O8 from V5+(3d0) and V4+(3d1) to V4+(3d1) and V3+(3d2) by lithiation with n‐BuLi has been inferred by 57Fe Mossbauer effect. The Fe is localized and in the Fe3+ state for all x.

Iron has been successfully incorporated in the quasicrystal i‐Al74Si6Mn20−xFex up to a concentration x=7. A combination of magnetic susceptibility and Mossbauer study establishes the fact that there are at least two ‘‘classes’’ in Mn sites and that Fe substitutes for only one of them. High‐field Mossbauer measurements at 4.2 K show that the substituted iron carries no local moment, while low‐temperature susceptibility data indicate that its presence does not perturb the Curie–Weiss amplitude of the sample. The inference is that both magnetic and nonmagnetic Mn sites must be present in the quasicrystal and that iron can substitute only from among the latter. The simplest interpretation would be in terms of two ‘‘classes’’ on Mn sites (one supporting a local moment, one not), although the possibility of there being more than one class of nonmagnetic site cannot yet be excluded.

Magnetic susceptibility, electrical resistivity, Mossbauer effect (ME) and transmission electron microscopy measurements in single crystals of the metal intercalated layer compounds FexTaS2 (0.20⩽x⩽0.34) are reported. The compounds are metallic, ferromagnetic and have large coercive force. ME measurements show the iron to be high‐spin Fe2+, with a distribution of hyperfine fields at low temperatures. Transmission electron microscopy suggests the existence of clusters. These results may explain the diversity of magnetic preperties on nonstoichiometric intercalated compounds such as FexTaS2.

Lorentz microscopy and magnetization and Mossbauer effect measurements have been performed on a Cr‐Co‐Fe alloy having a two‐phase microstructure. Domains spanning many particles have been seen in samples whose Hc’s vary between 200 and 420 Oe. Walls appear to be ragged and are held back by particles. Magnetization and Mossbauer effect measurements indicate that the two phases constituting the microstructure in each of the three situations examined are ferromagnetic at ambient temperature. Based on these results it is inferred that domain‐wall–particle interaction is responsible for Hc.

The orthorhombic compounds BaMF4 (M=Mn, Fe, Co, or Ni) have recently been found to undergo antiferromagnetic transitions at low temperatures. Susceptibility measurements on BaNiF4 reveal that a broad maximum occurs at about 150°K similar to that observed in a number of layer‐type structures. As the temperature is lowered, χ∥ along b decreases rapidly, while χ⊥ goes through a shallow minimum around 50°K. At 77°K, no magnetic order can be detected in powder neutron diffraction patterns of BaNiF4, but at 4.2°K there are magnetic peaks which can be indexed on a unit cell doubled along the b and c directions (A21am orientation). The magnetic structure consists of puckered (010) sheets within which the moments are coupled antiparallel to neighboring moments about 4‐A apart. However, the crystal structure is such that the net interaction between adjacent sheets is zero, and ordering in the b direction depends upon interactions between second nearest sheets 14‐A apart. This is very similar to the situation in K2N...

We have demonstrated the presence of circulating circumferential currents in toroids of Tl‐Ba‐Ca‐Cu‐O superconductors at 77.3 and 90.2 K. The nominal composition was Tl1.5Ba2Ca2Cu3Ox; however, both the x‐ray diffraction pattern and the Meissner effect indicate the presence of more than one phase. The circulating current was found to decrease by 27% in 3.8×103 s. The nature of the above time dependence is not consistent with the presence of a simple ohmic resistance but rather indicates that a process such as flux creep is the cause of this decay. An estimated upper limit of the resistivity is 1.3×10−14 Ω cm at 77.3 K.

Mossbauer effect measurements of 57Fe in a Cr‐Co‐Fe alloy indicate the existence of two ferromagnetic phases at room temperature. The hyperfine field of the two phases differ by ∼11%. This result implies that domain‐wall–particle interaction is responsible for the coercive force, which is a different mechanism than in some Alnico alloys.

A new superconducting phase with an onset of 123 K and zero resistance at 121 K has been detected in the Tl0.5Ba2Ca2Cu3Oy multiphase system. The composition is reproducible and stable.

Magnetic measurements and Mossbauer effect (ME) studies of 57Fe in CsFeF3 are reported. The compound is hexagonal at room temperature, but undergoes a crystallographic transformation at 83°K to a structure with lower symmetry. Magnetization data show a spontaneous magnetization appearing at TC=62°K, with a saturation magnetization at low temperatures of 1.72 μB per formula unit. In the magnetic state ME yields well resolved hfs corresponding to the two magnetically inequivalent iron sites. At 4.2°K the following hyperfine structure parameters have been obtained: Heff(A)=318 kOe, Heff(B)=274 kOe; Q.S. (A)=−2.85 mm/sec, and Q.S. (B)=−1.9 mm/sec, with the axes of the EFG tensors at 40° to the direction of the magnetic moments.

Mossbauer absorption spectra of 57Fe in ultrafine powders of NiFe2O4 were taken in external magnetic fields from 0 to 20 kOe at room temperature. A transition from a spectrum with no hyperfine field, Hhpf, to a Zeeman split spectrum of Hhpf=500 kOe has been observed. Comparison of the results with magnetization measurements suggest that the drastic effect of the external field is due to the polarization of the superparamagnetic particles by the field.

The unusual magnetic and electrical properties of the semiconductor SmB6 have been closely investigated recently. Primarily from magnetic susceptibility measurements, it has been hypothesized that the Sm valence changes from 3 + (4f5, 6H5/2) at high temperatures to 2 + (4f6, 7F0) at low temperatures. We have used the Mossbauer effect in 149Sm to make a direct determination of the valence state by means of the ``isomer shift,'' which measures the s‐electron density at the Sm nucleus. This density changes with the 4f configuration because of coulombic shielding effects. Unfortunately, the change between 4f5 and 4f6 is too small to resolve resonance lines from the two charge states. The observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0. mm/sec−1 respectively) obtained for well characterized ionic di‐ and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K. A new model is presented to explain both the magnetic susceptibility data and the Mossbauer results.

The orthorhombic compound RbFe3+F4 has been studied by magnetic susceptibility and Mossbauer‐effect (ME) measurements. The magnetic susceptibilities for H∥b and H⊥b are characterized by a broad maximum in χ∥ and χ⊥ at Tmax=(215±10)°K, followed by a rapid decrease in χ∥ and a shallow minimum in χ⊥ below 135°K. The susceptibility behavior is similar to that observed in a number of antiferromagnetic layer‐type structures. In the antiferromagnetic state, ME yields well‐resolved hyperfine spectra, revealing a unique magnetic field and EFG at all iron nuclei. The temperature dependence of the sublattice magnetization in powder samples has been determined by measuring the internal fields at the iron sites, over the range 4.2°–300°K. At low temperatures the results are found to agree with the simple two‐dimensional spin‐wave theory for T≤TN/2. From about 0.40

Mossbauer effect (ME) measurements of the hyperfine (hf) interaction in single crystals of FeBO3, have been used to study the behavior of the sublattice magnetization near the Neel temperature. The ME spectra show that the two nearly antiferromagnetic sublattice moments lie within the easy plane, in agreement with symmetry considerations for weak ferromagnets with calcite structure. From about 0.94≤T/TN≤0.9996 the hf field fits the equation H(T)=H0D(1‐T/TN)β with β=0.354±0.005, D=1.208±0.001 and TN=348.35°K to ±0.01°K precision and to ±0.2°K absolute accuracy.

A field‐induced spin reorientation has been observed in the weak ferromagnet, ErCrO3, at temperatures below the Neel point, TN=133.2°K. With a magnetic field, H, along the a axis of this orthorhombic compound, the spontaneous magnetization rotates from the c axis, at H=0, to the a axis, at H=Ht. The ``transition'' field, Ht, increases from zero at TN to 11 kOe at T=22°K. The temperature dependence is discussed in terms of a formula which gives the first‐order Er3+ contributions to Ht. Above 120°K, the data can be fitted to the curve Ht=a(1‐T/TN)1/2, with a=8.1±0.5 kOe. The canting angle for the Cr3+ sublattices and the effective field, Heff, on the Er3+ ions have been estimated from spontaneous‐magnetization data. These data were obtained by extrapolating from high magnetic fields to H=0 for both the c and a axes. A canting angle of 35 mrad gives reasonable agreement with both sets of data, but Heff reverses sign on reorientation of the spins. At low temperatures, Heff≈−10 kOe along the c axis.

Single crystals of FexTaS2 (x≈0.28) are metallic, ferromagnetic (Tc=73±5 °K), and have a large coercive force of 55 kOe at 4.2 °K. Mossbauer measurements show the iron to be high‐spin Fe2+, with a distribution of hyperfine fields at low temperatures. Electron microscopic observations, together with these results, suggest the presence of iron clusters.