Evidence For Antiferromagnetic Coupling Research Articles

Properties of Heusler alloy Co2Cr1−xFexAl superlattices and spin valves

We investigate the properties of epitaxial Co2Cr1−xFexAl-based multilayers grown by dc magnetron sputtering upon MgO substrates. Co2Cr1−xFexAl/Cr superlattices of high epitaxial quality can be grown, though no evidence for antiferromagnetic coupling is observed. Simple spin-valve trilayers of the form Co2Cr1−xFexAl/Cu/Co90Fe10 show large giant magnetoresistances of up to 6.8% at room temperature, comparable to the highest achieved in conventional exchange-biased spin valves.

Magnetization and magnetotransport studies ofY Ba2Cu3O7−δ/La1−xPbxMnO3 heterostructures

We report magnetotransport and magnetization studies of heterostructures comprising alternately stackedY Ba2Cu3O7−δ (YBCO) andLa1−xPbxMnO3 (LPMO)layers. The c-axis-oriented bi-layer and four-layer YBCO/LPMO heterostructures were prepared in situ on (100)SrTiO3 substrates using pulsed laser deposition. Zero-field-cooled (ZFC) and field-cooled (FC)magnetization data show that in a four-layer structure the LPMO layers becomeantiferromagnetically coupled when the thermodynamic-fluctuation-induced Cooper pairingbegins in the YBCO layers, which is well supported by the magnetization versus appliedfield data. Evidence of antiferromagnetic coupling is also reflected in the temperaturedependence of the magnetoresistance.

Antiferromagnetic interlayer coupling in epitaxial Fe/EuS ( 100) bilayers

The magnetic coupling of Fe/EuS (100) epitaxial bilayers has been investigated. Evidence of antiferromagnetic coupling has been found, both when the samples are cooled below the Curie temperature of the EuS layer, and in the hysteresis loops at low temperatures. The coupling strength has been measured as a function of temperature.

Giant magnetoresistance of electrodeposited Co/Cu multilayers

We report on the structural and electrical characterization of electrodeposited Co/Cu multilayers grown in a single electrolyte based on CoSO4 and CuSO4. A high degree of crystallographic orientation and superlattice coherence is found in the growth on (100)- and (111)-oriented substrates. The magnetoresistance (MR), measured in the current-in-plane configuration at room temperature, is dominated by the giant MR effect for Cu-layer thicknesses dCu ≿ 3 nm and by the anisotropic MR effect for dCu ≾ 2.5 nm. A maximum of 14% is measured for dCu ≈ 4 nm. No evidence for antiferromagnetic coupling is found. Instead, the giant MR gradually diminishes with decreasing dCu < 4 nm which is attributed to ferromagnetic coupling due to magnetic pinholes. The influence of the Cu2+-ion concentration, the addition of levelling agents, and the Co-and Cu-layer thicknesses on the structure and magnetoresistance is systematically investigated. Especially the use of levelling agents has a catastrophic effect on the structural quality of the multilayers and on the magnitude of the MR.

Magnetic properties of Fe/Pd multilayers grown by electron-beam evaporation

The magnetic properties of [Fe(20 Å)/Pd(x Å)]25 multilayers prepared by ultrahigh vacuum electron-beam evaporation are presented. It is found that the crystal structure of Fe layers change from bcc to fcc when the thickness of Pd layers dPd≥36 Å. The saturation magnetization per unit Fe volume at 5 K is enhanced and oscillates with the increment of the thickness of Pd layers, due to the polarization of Pd atoms. The magnetic hysteresis loops of samples indicate low coercive forces, and ferromagnetic coupling between the Fe layers for all Pd thicknesses (6–60 Å). The conversion electron Mössbauer spectra measurements proved that the magnetic moment of fcc Fe is the same as that of the bcc Fe. It is also found that the magnetic anisotropy dependence on dPd is similar to that of the saturation magnetization, and relates to the Fe layer structure transition. The Curie temperature of Fe/Pd multilayers decreases monotonously with the increasing of dPd. The low temperature magnetization measurement of Fe/Pd multilayers suggests that the interlayer coupling between Fe layers and polarization of Pd layers influences the temperature dependence of saturation magnetization. No evidence of antiferromagnetic coupling between Fe layers and giant magnetoresistance effect is found. The relationship among structure, polarization of Pd layers, and magnetic coupling is discussed.

Giant Magnetoresistance in Co/Ag Multilayers

ABSTRACTThe Magnetoresistance (MR), Magnetic properties, and crystal structure of dc magnetron sputtered CO/Ag periodic multilayers have been investigated. The Co layer thickness was fixed at -30 Å while the thickness of the Ag layer was systematically varied. ‘Giant’ magnetoresistance was observed. The MR ratio has been found to decline monotonically with increasing Ag thickness in the range 30 Å to 107 Å. Although the maximum room temperature MR ratio is a Modest 4.78%, a more technologically significant measurement of field sensitivity (MR ratio/FWHM of the MR vs. H peak) is a promising 0.1%/Oe at its best. The effect of the number of bilayer units has also been examined and no substantial differences were noted between multilayers containing 8, 9, and 10 bilayer units. Coercivities as determined by both magnetometer and the splitting of the MR peaks are in agreement and increase from 25 to 38 Oe with increasing Ag thickness. Evidence for antiferromagnetic coupling is apparent in the hysteresis loops. High angle X-ray diffractometry (HXRD) in the θ-2θ mode revealed a strong Ag (111) texture in the film, with satellite peaks indicating a layered structure. Low angle XRD (LXRD) also yielded broad superlattice peaks in all samples at least to the second order.

Evidence for Antiferromagnetic Coupling between Fe Layers through Cr from Neutron Diffraction

Small-angle neutron diffraction measurements have been made for a multilayer, [Fe(27 A)/Cr(12 A)]×30, at room temperature. It was found that the magnetizations of adjacent Fe layers are coupled in antiparallel. The decrease of antiferromagnetic peak intensity as a function of external field was observed. It is confirmed that the giant magnetoresistance in Fe/Cr multilayers is due to the antiferromagnetic ordering of Fe layers caused by the interlayer coupling across an intervening Cr layer.

Physicochemical characterization of the four-iron-four-sulphide ferredoxin from Bacillus stearothermophilus

1. A stable ferredoxin was prepared from Bacillus stearothermophilus and purified by chromatography on DEAE-cellulose and by electrophoresis. 2. The minimum molecular weight determined from the amino acid composition was about 7900 and this was in reasonable agreement with a value of 8500 determined by polyacrylamide-gel electrophoresis. The ferredoxin contained four iron atoms and four labile sulphide groups per molecule. 3. The optical absorption, optical-rotatory-dispersion and circular-dichroism spectra are typical of ferredoxins containing 4Fe-4S clusters. 4. Oxidation-reduction titrations, combined with electron-paramagnetic-resonance (e.p.r.) spectroscopy, showed that the protein has a mid-point potential, at pH8, of -280 +/- 10mV, and that only one electron-accepting paramagnetic species is present. 5. The e.p.r. spectrum of the reduced ferredoxin is more readily saturated with microwave power at low temperatures than those of the eight-iron ferredoxins, indicating that there is another mechanism of electron-spin relaxation in the latter. 6. Mossbauer spectra of both redox states were observed over a range of temperatures and in magnetic fields. At high temperatures (77 degrees K and above) both redox states appear as quadrupole-split doublets; in the reduced state two resolved doublets are seen, suggesting appreciable localization of the additional reducing electron. 7. The average chemical shift indicates formal valences of two Fe3+ and two Fe2+ in the oxidized state and three Fe2+ and one Fe3+ in the reduced state. However, the spectra indicate that there are differing degrees of electron delocalization over the iron atoms. 8. At low temperatures (4.2 degrees K) the oxidized form shows no hyperfine magnetic interaction, even in an applied magnetic field, evidence that the oxidized ferredoxin is in a non-magnetic state as a result of antiferromagnetic coupling between the iron atoms. 9. At 4.2 degrees K the reduced form shows a broad asymmetric pattern resulting from magnetic hyperfine interaction. This contrasts with the reduced ferredoxin of Clostridium pasteurianum, which shows a doublet, suggesting that in the latter there may be interaction between the two 4Fe-4S centres. 10. In large applied magnetic fields, positive and negative hyperfine fields are seen in the Mossbauer spectra of the reduced ferredoxin, evidence for antiferromagnetic coupling between the iron atoms in the 4Fe-4S centre. The high-field spectra of the reduced ferredoxin of B. stearothermophilus are similar to those of the reduced ferredoxin of C. pasteurianum.

MÖSSBAUER EFFECT STUDIES ON THE FOUR-IRON CENTRES OF TWO IRON-SULPHUR PROTEINS

MÖSSBAUER EFFECT STUDIES ON THE FOUR-IRON CENTRES OF TWO IRON-SULPHUR PROTEINS

Mössbauer effect in the high-potential iron-sulphur protein from Chromatium. Evidence for the state of the iron atoms.

1. The previous Mössbauer work on Chromatium high-potential iron-sulphur protein by Moss et al. (1968) and Evans et al. (1970) was extended to high applied magnetic fields. 2. Measurements of the reduced protein confirm that it is non-magnetic. 3. Spectra of the oxidized protein in applied magnetic fields clearly indicate that some iron atoms have a positive hyperfine field, which is evidence for antiferromagnetic coupling. 4. The spectra can be interpreted in terms of two types of iron atom with positive and negative hyperfine fields of 9 and 12T respectively. 5. A consideration of the chemical shifts and other evidence suggests formal valences of two Fe(3+) and two Fe(2+) atoms in the non-magnetic reduced state, and three Fe(3+) atoms and one Fe(2+) atom in the oxidized state. 6. However, no separate Fe(3+) and Fe(2+) spectra are seen, suggesting that the d electrons are not localized on particular iron atoms.

Low-Temperature Magnetic and Thermal Properties of NiSnCl6·6H2O

NiSn${\mathrm{Cl}}_{6}$\ifmmode\cdot\else\textperiodcentered\fi{}6${\mathrm{H}}_{2}$O forms rhombohedral (trigonal) crystals containing one axially distorted octahedral ${[\mathrm{N}\mathrm{i}\phantom{\rule{0ex}{0ex}}{({\mathrm{H}}_{2}\mathrm{O})}_{6}]}^{++}$ complex per unit cell. Measurements of (a) the magnetic susceptibility, (b) isothermal magnetization, and (c) magnetic heat capacity between \ensuremath{\sim} 0.35 and 4.2 K can be described by the spin Hamiltonian $\mathcal{H}=D{S}_{z}^{2}+g{\ensuremath{\mu}}_{B}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ for $S=1$ with $\frac{D}{k}=+0.64$ K and $g=2.25$. The ground-state triplet is thus split in zero field into a lower singlet and an upper doublet. There is also evidence of antiferromagnetic coupling among ${\mathrm{Ni}}^{++}$ ions which is weak in comparison with this splitting. A system of this type should exhibit unusual properties for $T&lt;\frac{D}{k}$. Exploratory magnetocaloric observations with $H\ensuremath{\parallel}\mathrm{trigonal}\mathrm{axis}$ reveal cooling by adiabatic magnetization for $H\ensuremath{\le}\frac{D}{g{\ensuremath{\mu}}_{B}}$. They also suggest the establishment of spin order at sufficiently low temperatures for $H\ensuremath{\sim}\frac{D}{g{\ensuremath{\mu}}_{B}}$.