M.B. Brodsky

Layered magnetic structures: evidence for antiferromagnetic coupling of Fe layers across Cr interlayers

Double layers of (100)‐oriented Fe with an individual thickness of ≊100 A, separated by interlayers of Cr or Au with variable thickness were grown epitaxially on Au (100) surfaces. The spin‐wave mode spectra of these double layers as detected by means of light scattering were used to measure the effective exchange coupling of the Fe films across the interlayer. For Au interlayers it decreases monotonically with increasing Au thickness dAu and disappears at dAu≳20 A. For Cr interlayers of proper thickness dCr, i.e., 4 A≲dCr≲9 A we obtain antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field in analogy to the spin‐flop phase of antiferromagnets. This is revealed by the behavior of the static magnetization of such samples as a function of external field and the spectrum of spin‐wave modes with the wave vector parallel to a small external field.

Magnetothermopower of Fe / Cr superlattices

Abstract We report magnetothermopower S(H) measurements at room temperature for sputtered [Fe(32 A) / Cr(x)]30 superlattices with x ≈ 5–50 A. The thermopower in zero field S(H = 0) appears to extrapolate to the known values for Fe and Cr in the x = 0 and large-x limits, respectively, and drops below these values at intermediate x. S follows the same trend with H and x as the magnetoresistance (MR), which shows oscillatory behavior as a function of x, and large field effects for antiferromagnetic Fe interlayer couplings. Both S(H) and the MR saturate when the applied field aligns the magnetization of the Fe-layers parallel to each other, but the thermopower is enhanced by application of a magnetic field, while the resistivity decreases, in accord with expectation.

Anomalous L3L2 white line ratios and spin pairing in 3d transition metals and alloys: Cr metal and Cr20Au80

Abstract It is shown by transmission electron energy loss spectroscopy that a large difference exists between the L 3 L 2 ratio obtain for chromium metal and that obtained for chromium in Cr20Au80 alloy for which chromium exists in a highly spin-aligned state. An argument is given that the anomalous L 3 L 2 white line ratios observed for 3d transition metals may be correlated strongly with the degree of spin pairing in the 3d band.

Surface phonons in Cu/Ni superlattices

The modulation wavelength dependence of the surface‐wave velocity in Cu/Ni superlattices has been investigated using Brillouin scattering. It is the first compositionally modulated system to be studied by this technique and the first for which elastic properties have been determined by both ‘‘macroscopic’’ methods and Brillouin scattering. Contrary to the enhancements previously reported in the biaxial modulus and in the Young’s modulus, the modulus associated with the surface‐wave velocity shows no evidence of anomalous behavior.

Magnetization of very thin Ni films in epitaxial metal film sandwiches of Cu

Single‐crystal nickel films varying in thickness from 2‐40 A were prepared by epitaxial growth between thicker layers (500 A) of single‐crystal copper (001). Their magnetization was measured as a function of field up to 10 kOe using a dc SQUID magnetometer. The saturation magnetization of these films was determined as a function of Ni thickness at liquid helium temperatures. A decrease in saturation moment from that of bulk nickel was observed for films of thickness less than 18 A. The apparent Curie temperature also decreases with decreasing thickness for these films. The magnetization at maximum field as a function of temperature does not follow a Brillouin function.

Hyperfine Interaction and Susceptibility in Some Actinide Metals and Intermetallic Compounds

Magnetic effects in a number of actinide metals and intermetallic compounds are being investigated utilizing the 59.6 keV Mossbauer resonance of 237Np in conjunction with bulk susceptibility measurements. The cubic Laves phase compound NpAl2 is found to be ferromagnetic. The paramagnetic moment is 2.30 μB, somewhat lower than in other cubic Np magnetic materials. The saturation magnetic hyperfine field is found to be (3155±50) kOe. Taking H(T)/H(0) = D[1−T/Tc]β, we find D = 1.48±0.10, Tc = 55.8±0.1°K and β = 0.355±0.017 for 0.994>T/Tc>0.83. At lower temperatures, departure from molecular field behavior is seen. In cubic actinide magnetic materials, an electric‐quadrupole hyperfine interaction will be observed due to alignment of the unquenched orbital momentum. In NpAl2 we find e2qQ = − (27±1) MHz. For NpC, which has the NaCl structure, we obtain the hyperfine parameters Hi = (4789±50) kOe and e2qQ = (577±58) MHz at 4.2°K. Metallic neptunium (orthorhombic) and americium (hexagonal) show no magnetic hyperf...

In-situ HVEM studies of the early stages of oxidation of nickel and nickel-chromium alloys

Abstract In-situ oxidation of Ni, Ni-5at%Cr and Ni-25at%Cr single crystals of 〈100〉 orientation was studied by electron diffraction and contrast experiments both in a high voltage electron microscope environmental cell, and in conventional 100 keV transmission electron microscopes. Environmental cell studies were conducted at temperatures between 500 and 700 ° C, and at oxygen partial pressures of 2.7 Pa and 2.4 kPa Torr for periods up to 90 min. The oxidation process was characterized using bright-field, dark-field imaging and selected-area diffraction techniques, as well as X-ray energy-dispersive and electron energy loss spectroscopy. Oxidation was observed to begin within 40 s. Simultaneous nucleation of small NiO crystallites of 5 to 10 nm diameter and growth of bright rectangular facets were observed, eventually resulting in non-uniform sample thickness. The epitaxial relationships were determined for Ni 〈100〉 and Ni-5at%Cr 〈100〉. These results and the conditions for the formation of Cr 2 O 3 are discussed and compared with those for oxidation processes in bulk alloys.

Magnetotransport studies of epitaxial Cr thin films

We report the results of transport measurements on epitaxial Cr thin films. The variation of TN (The Neel transition temperature) with respect to the film thickness is discussed in terms of the stress in the film. In addition, the temperature dependence of the magnetoresistance and Hall resistance for temperatures from 4 to 150 K are measured and discussed, with particular attention to the temperature dependence of the nonlinear portion of the Hall data.

Magnetic property changes in epitaxial metal film sandwiches

Abstract Epitaxial metal film sandwiches (EMFS) containing Pd or Cr, have been prepared between single crystal Ag or Au. The Modified Pd/Cr show major changes in physical properties. Pd has a stretched lattice parameter in Au-Pd-Au, which combines with a tetragonal distortion to cause exchange enhancements up to 28 000 and spin fluctuation temperatures of 1–10 K. In Au-Cr-Au, Cr takes up the fcc structure, leading to superconductivity due to a high N ( E F ). These results are contrasted to data for Ag-Pd-Ag and Ag-Cr-Ag EMFS.

Enhanced magnetism in metal film sandwiches (invited)

The magnetic susceptibilities, χ, of Pd and Cr have been found to show very large enhancements in metal‐film sandwiches of Au/Pd/Au and Au/Cr/Au. The enhancement arises from a combination of two factors: a narrowing of the density of states due to the thinness (∼1.5 nm) of the Pd and Cr films, and a stretching of the lattice by the Au. Stretching has been verified for the Au/Pd/Au films. Although the low density of states for Cr should limit such an enhancement, resistivity and susceptibility data are consistent with an antiferromagnetic transition at 2.4 K. Between 3 and 10 K, the temperature dependences of χ are approximately Curie‐Weiss like for both Au/Pd/Au and Au/Cr/Au. The Au/Pd/Au and Au/Cr/Au data yield exchange enhancements of 400–28000 and ∼200, respectively. This makes Au/Pd/Au a possible candidate for p‐wave‐pairing superconductivity. Measurements seeking superconductivity at 0.010 K for Au/Pd/Au are described. No measurable χ is observed with Au/V/Au sandwiches, although surface spin fluctua...