No abstract

We describe the magnetic and transport properties of Fe(OOI )/Cr(OOI) superlattices grown on GaAs (001) by molecular-beam epitaxy and characterized by reflection high-energy electron diffraction (RHEED), Auger spectroscopy, x-ray diffraction, and electron microscopy. For Cr layers thinner than about 30 A the magnetic behavior reveals strong antiferromagnetic couplings between the Fe layers across the Cr layers. Polarized neutron diffraction experiments confirm the existence of an antiferromagnetic superstructure. We discuss the origin of the antiferromagnetic (AF) coupling. The Fe/Cr superlattices with AF interlayer coupling exhibit a giant magnetoresistance: when an applied field aligns the magnetizations of the Fe layers, the resistivity drops by a factor of 2 for some samples. This giant magnetoresistance can be ascribed to the spin dependence of the electron scattering by interfaces. We compare our results with the predictions of two recent theoretical models.

Direct observations of the magnetization structure in two reentrant spin-glass alloys [polycrystalline Nii-xMn* and amorphous (Fe7sMn22)75Pi6B6Al3] near liquid-helium temperature are reported. Magnetic domains several hundred /im long are seen. This structure hardly changes upon cooling through the reentrant transition (7/ -35 K). In situ observation of the domain-wall motions reveals a gradual crossover to a regime of strong irreversibilities. Reentrant spin-glass systems are also characterized by transverse fluctuations within the domain.PACS numbers: 75.60. Ch, 75.30.Kz, 75.60.Ej Many crystalline and amorphous alloys with a random mixture of ferromagnetic and antiferromagnetic interactions undergo a paramagnetic-ferromagnetic-like transition at a certain temperature T c , but at a lower temperature Tf the ferromagnetism seems to disappear giving rise to a spin-glass-like or reentrant state. During the last decade, reentrant spin-glass (R-SG) transition has been extensively investigated both theoretically and experimentally by means of many different techniques, including not only global magnetic measurements but also other methods probing the magnetic structure on microscopic scales. The most common of these are smallangle neutron-scattering experiments 1 " 4 (few hundred angstroms), transport measurements 5 (scale of the electron mean free path -5 to 50 A), and Mossbauer spectroscopy 6 (atomic scale).However, up to now, the complexity of the R-SG problem has made it very difficult to interpret the experimental data unambiguously, so that the exact nature of the reentrant transition is still unclear and highly controversial. Some of the debated questions are as follows: (l) What is the structure of the magnetization in the so-called ferromagnetic phase 7 (7>< T< T c )? (2) What then happens to it in the reentrant phase (i.e., for T?In particular, does the well-known sudden drop of the zero-field-cooled magnetization mzFc(T) below 7/ correspond to the vanishing of the spontaneous magnetization and consequently to a well-defined phase transition as has often been claimed? In an effort to clarify this situation we have carried out direct investigations of the domain structure by means of Lorentz transmission electron microscopy 8 . Three representative R-SG alloys [polycrystalline Nii-x Mn x with x^O.lS, 0.19, 0.21, and 0.25, amorphous (Fe7sMn22)75Pi6B6Al3, and amorphous FewZriol were examined as functions of temperature (10 < T< 290 K) and field (0 < 7/ < 1 kOe). Because of the lack of space the results on the FeZr alloy will be reported elsewhere.Two different instruments were used in this investigation, the high-voltage transmission electron microscope of the Laboratoire d'Optique Electronique du Centre National de la Recherche Scientifique de Toulouse (operated at 2 MV) and a Philips model EM400 (biased at 120 kV) equipped with a field-effect filament. Each of the two instru...

We report a detailed investigation of the magnetic hysteresis of YBa2Cu3O7 oxide in fields ranging from ∼0.2 Oe to 35 kOe and at different temperatures (4.2≤T≤100 K). It is shown that the magnetic behavior is that of a granular type-II superconductor in which strongly superconducting grains are coupled via less superconducting links. As a result, at low-H (H≲20 Oe at 4.2 K) the sample appears like a single superconducting filament while at high enough field (H≳5 kOe) it behaves like a granular superconductor with most of the weak links being broken by the applied field. Five threshold fields are identified charaterizing the above magnetic regimes and the transition between them. It is concluded that the critical current density within the individual grains could attain 107 A/cm2.

Lorentz electron transmission microscopy investigations of the magnetization structure as a function of temperature (6 K≲T≲290 K) in amorphous films of Fe90Zr10 are reported. Magnetic domains as large as ∼50 μm are observed. It is found that the domain configuration stays essentially unchanged upon cooling through the so-called reentrant spin-glass phase. However, direct magnetic measurements on the same sputtered films show that the degree of spin-glass order which sets in below∼30 K is rather low, whereas the Curie temperature exceeds 290 K.

No abstract

We present magnetization, torque and magnetoresistance measurements on (001)Fe/(001)Cr multilayers prepared by MBE. Our main results are : a) evidence of antiferromagnetic interlayer couplings, b) the observation of a huge magnetoresistance

No abstract