Strontium-Induced Oxygen Defect Structure and Hole Doping in La2 — Sr Cu04 Zhengquan Tan, ' M. E. Filipkowski, ' J. I. Budnick, ' E. K. Heller, ' D. L. Brewe, ' B. L. Chamberland, ' C. E. Bouldin, J. C. Woicik, and D. Shi "Uni Uersity of Connecticut, Storrs, Connecticut 06269 ' 'National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ' Argonne National Laboratory, Argonne, Illinois 60439 (Received 29 November 1989)

Coupling mechanisms between Co/sub 0.75/Fe/sub 0.25/ films separated by Mn layers are investigated using Brillouin light scattering from thermal spin waves, Observed applied field and Mn thickness dependences of acoustic and optic type spin wave frequencies are discussed in terms of exchange coupling and magnetic order in the Mn.

Coupling mechanisms between Co0.75Fe0.25 films separated by Mn layers are investigated using Brillouin light scattering from thermal spin waves. Trilayers of CoFe/Mn/CoFe, grown by molecular-beam epitaxy, were studied with Mn thicknesses between 6 and 30 Angstrom. Spin-wave excitations corresponding to acoustic and optic modes in double layer magnetic structures were observed, and allowed a determination of a strong magnetic coupling between the CoFe films. Spin-wave frequencies were examined as functions of applied field, Mn film thickness, and temperature. A microscopic model is used to examine magnetic order in a ferromagnet/antiferromagnet/ferromagnet structure. Discrepancies between the data and predictions of the microscopic model provide evidence for a twist in the FeCo spins that extends into the ferromagnet layer. [S0163-1829(98)10117-0].

We report on Brillouin scattering measurements on a CoFe/Mn/CoFe trilayer film characterized by unusually large biquadratic coupling. The magnetic field dependence of the exchange coupled in- and out-of-phase magnons as well as their in-plane directional dependence are determined. The saturation magnetization of the trilayer was measured independently through superconducting quantum interference device magnetometry. The spin wave data is well represented by a generalization of the model that takes into account the antiferromagnetic order in the Mn layer.

Trilayers of CoFe/Mn/CoFe(001) have been prepared by molecular beam epitaxy and their magnetic properties measured by magnetometry and ferromagnetic resonance. Very strong near-90° coupling between the CoFe layers, with no evidence for 180° coupling, was found in all but the thickest Mn-layer samples. The coupling energy has the form suggested recently for the case when the interlayer itself is antiferromagnetic. An analysis of the ferromagnetic resonance data indicates that the magnitude of the coupling oscillates with the Mn thickness.

Trilayer samples of CoFe/Mn/CoFe(001) have been prepared by molecular beam epitaxy and investigated by magnetometry and ferromagnetic resonance (FMR). Very strong near-90 degrees coupling (up to 2.5 ergs/cm(2)) between the CoFe layers, with no evidence for 180 degrees coupling, was found in all but the thickest Mn-layer samples. The field dependence of the magnetization requires that the coupling energy have the algebraic form F-c = C-+(phi(1) - phi(2))(2) + C--(phi(1) - phi(2) - pi)(2) suggested recently for antiferromagnetic interlayers. The observed angular dependence of both the acoustic and optical FMR modes found can be described by this model.

Novel electronic properties are observed in metals in contact with the ferromagnetic insulator europium monoxide (EuO). We have successfully prepared epitaxial bilayers and multilayers involving EuO and the metals V or Ag. With metal layers of 100 angstroms thickness or less, these structures exhibit a slope discontinuity in their resistance versus temperature at 90 K for V/EuO interfaces and at 50 K for Ag/EuO interfaces. For metal layers less than about 40 angstroms, a feature is also apparent in R(T) at 70 K, the Curie temperature of bulk EuO. The slope discontinuities are reminiscent of those observed at the Curie temperature of ferromagnetic metals. Kerr effect measurements confirm that the EuO exhibits ferromagnetic ordering at 70 K. In V/EuO structures, the V still displays superconductivity with a slightly depressed transition temperature. This suggests that such metal/ferromagnetic insulator structures may be used to prepare artificial ferromagnetic superconductors. The resistive transition is incomplete, suggesting small regions of isolated superconductivity within the sample, as has been predicted for certain special cases when a superconductor is in proximity to a ferromagnet.

Magnetoresistance measurements have been performed on epitaxial metal/ferromagnetic insulator bilayers. They are more sensitive to magnetic behavior at the interface of such structures than magnetization measurements. It is clear from the magnetoresistance data that previously reported slope discontinuities in the resistance versus temperature of such heterostructures are magnetic in origin. These studies demonstrate that the conduction electrons of the metal are coupled to the spins in the magnetic insulator, and act as probes of the magnetic state at the interface between the two materials. An example of the usefulness of this probe is shown by magnetoresistance measurements on a Ag/EuO bilayer.

A new system, exhibiting unusually large interlayer biquadratic exchange coupling, has been discovered, consisting of single crystal FeCo/Mn/FeCo sandwiches epitaxially grown on GaAs substrates. Normalized m(H) curves yield a remanent moment of approximately 0.5 for all cases studied, with a number of examples having saturation fields as large as several Tesla. This implies a biquadratic coupling constant as large or larger than many known bilinear coupling constants. None of the normalized m(H) data exhibits a remanence of less than 0.5, indicating the absence of comparable contributions from bilinear coupling. Angular dependent FMR at 9 GHz implies a fourfold anisotropy of opposite sign to that measured for single FeCo layers, whereas FMR at 35 GHz agrees in sign with the single layer anisotropy. Detailed analysis of this contradiction shows that this is an apparent anisotropy reversal which emerges within the theory of FMR in the presence of large biquadratic coupling, requiring values of the coupling constant in excess of −j2=2 ergs/cm2, where the coupling term in the energy is written J2[(m1⋅m2)/m1m2]2. This large biquadratic coupling, together with the absence of bilinear coupling, appears to contradict existing theories of interlayer exchange.

Sensitivity to magnetic atoms and low intrinsic absorption characterize the interaction of neutrons with matter. Consequently, polarized neutron reflectivity provides a unique means of performing depth-resolved vector magnetometry. We have used this technique to determine the magnetization depth profiles of Fe/Cr superlattices. Superlattices of bilayer composition [55 Å Fe/17 Å Cr], grown at 523 K, exhibit biquadratic coupling with large saturation fields (∼3 kOe), while those grown at 293 K are ferromagnetically ordered. We have directly measured the evolution of the coupling angle between adjacent Fe layers as a function of applied field and will discuss how bilinear, biquadratic, and external field terms produce the observed order. The weaker coupling found in the Fe/Al system makes possible the investigation of a range of spin configurations at temperatures that do not endanger the sample. We have mapped the phase diagram of a [42 Å Fe/12 Å Al/39 Å Fe] (100) trilayer and find evidence of biquadratic coupling at low temperatures and fields (e.g., when H=180 Oe, the Fe layer spins relax away from ferromagnetic alignment below T≊170 K). Our measurements agree qualitatively with energy minimization calculations and the results of bulk magnetometry.

Molecular beam epitaxy has been used to fabricate Fe/Al/Fe(001) trilayers and FexCo1-x(001) alloy films on ZnSe-epilayered GaAs(001) substrates. A temperature dependence study of the 90° coupling in Fe/Al/Fe trilayers indicated a rapid decrease in coupling strength (B12) with increasing temperature. Also, the magnetic properties of (001) FexCo1-x alloy films spanning both the thermodynamically allowed bcc regime from 0.25 < x ≤ 1 and the epitaxy-extended metastable bcc regime from 0 ≤ x ≤ 0.25 are reported.

The temperature dependence of the unusual magnetic 90-degrees coupling in epitaxial Fe/Al/Fe(001) trilayers has been determined. For specific values of applied field and temperature, in-plane anisotropy and the interlayer coupling drive abrupt first-order changes in the temperature-dependent moment of these trilayers. The occurrence of such a first-order transition can be used to determine the value of the interlayer coupling constant, J(Q), at the transition temperature. We show that Fe/Al/Fe(001) trilayers also exhibit distinct second-order transitions in the temperature-dependent moment, and that the occurrence of a second-order transition can similarly be used to specify J(Q)(T). A comparison is made of the values of J(Q)(T) determined by these two approaches, and the dependence of J(Q) on T is analyzed.

The temperature dependence of the unusual magnetic 90° coupling in epitaxial Fe/Al/Fe(001) trilayers has been determined. Trilayers with Al thickness near 15 Å exhibit a rapid decrease of coupling strength (B12) with increasing temperature. This temperature dependence is well described by the relations B12 = B012(1 - TT0)2 or B12 = B012 exp(- TT0) which contrast with the approximately linear temperature dependence previously reported for 180° coupled systems.

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The muon spin rotation (MUSR) study of local magnetism of Sr-doped La2CrO4 is reviewed. Emphasis is placed on magnetic order as detected by local and bulk probes with local atomic environments studies by x ray absorption fine structure (XAFS). Correlations between the MUSR study of local magnetic ordering and the bulk magnetization study are presented along with a discussion of the dependence upon oxygen stoichiometry. Results are presented for both superconducting phases and magnetic phases. Recent data which reveals the existence of local magnetic ordering in the hydrogen-doped YBa2Cu3O7 system are also discussed.