Measurement of Magnetic Exchange in Ferromagnet-Superconductor La_2/3Ca_1/3MnO₃ YBa₂Cu₃O₇

Phys. Rev. Lett. 109, 137005 (2012)

Author(s) S. R. Giblin, J. W. Taylor, J. A. Duffy, M. W. Butchers, C. Utfeld, S. B. Dugdale, T. Nakamura, C. Visani and J. Sanatamaria

Abstract: The existence of coherence magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult t omeasure directly. Here we report on a study of ferromagnetic-superconductor bilayers La_2/3Ca_1/3MnO₃/YBa₂Cu₃O₇ (LCMO/YBCO) with varying YBCO layer thicknesses. Using X-ray magnetic circular dichroism, we demonstrate that th ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperature below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker than 12 unit cells, the Cu moment is suppressed for temperatures above the superconducting transition, suggesting this to be a direct measurement of magnetic coherence in the normal state of a superconducting oxide.

Ferrimagnetism in Fe-rich NbFe₂

Phys. Rev. B. 85, 115137 (2012)

Author(s): T. D. Haynes, I. Maskery, M. W. Butchers, J. A. Duffy, J. W. Taylor, S. R. Giblin, C. Utfeld, J. Laverock, S. B. Dugdale, Y. Sakurai, M. Itou, C. Pfleiderer, M. Hirschberger, A. Neubauer, W. Duncan, and F. M. Grosche

Abstract: We report a study of the spin moment in single crystal Nb_1-yFe_2+y with y=0.015 using spin-dependent Compton scattering in conjunction with ab-initio electronic structure calculations. From the experiment, we find that the total spin moment is 0.245 ± 0.004 μ_B. Comparison of the measured spin density with theoretical results from LMTO calculations determines there to be a ferrimagnetic arrangement of Fe moments, with the 2a Fe site aligned antiparallel to the bulk moment. There is a small moment on the Nb site, also antiparallel to the net moment. The orbital moment has been determined to be 0.02 ± 0.02μ_B.

Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning

J. Phys.: Condens. Matter 24, 146002 (2012)

Author(s): J. D. Aldous, C. W. Burrows, I. Maskery, M. S. Brewer, T. P. A. Hase, J. A. Duffy, M. R. Lees, C. Sáchez-Hanke, T. Decoster, W. Theis, A. Quesada, A. K. Schmid, G. R. Bell

Abstract: Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide 4.5 nm thick covers air-exposed samples which increases the film's coercivity. HCl etching efficiently removes this oxide in situ surface treatment of etched samples enables surface magnetic contrast to be observed SPLEEM. A thin Sb capping layer prevents oxidation and preserves ferromagnetism throughout the MnSb film. The interpretation of Mn L_3,2 edge XMCD data is discussed.

Cubic MnSb: Epitaxial growth of a predicted room temperature half-metal

Phys. Rev. B. 85, 060403(R) (2012)

Author(s): J. D. Aldous, C. W. Burrows, A. M. Sánchez, R. Beanland, I. Maskery, M. K. Bradley, M. d S. Dias, J. B. Staunton, and G. R. Bell

Abstract: Epitaxial films including bulk-like cubic and wurtzite polymorphs of MnSb have been grown by molecular beam epitaxy on GaAs via careful control of the Sb4/Mn flux ratio. Nonzero-temperature density functional theory was used to predict ab initio the half metallicity of the cubic polymorph and compare its spin polarization as a function of reduced magnetization with that of the well known half-metal NiMnSb. In both cases, half-metallicity is lost at a threshold magnetization reduction, corresponding to a temperature T* < T_C. This threshold is far higher for cubic MnSb compared to NiMnSb and corresponds to T* > 350K, making epitaxial cubic MnSb a promising candidate for efficient room temperature spin injection into semiconductors.

Spin and orbital Ti magnetism at LaMnO₃/SrTiO₃ interfaces

Nature. Comm. 1, 82 (2010)

Author(s): J. Garcia-Barriocanal, J. C. Cezar, F. Y. Bruno, P. Thakur, N. B. Brookes, C. Utfeld, A. Rivera-Calzada, S. R. Giblin, J. W. Taylor, J. A. Duffy, S. B. Dugdale, T. Nakamura, K. Kodama, C. Leon, S. Okamoto and J. Santamaria

Abstract: In systems with strong electron-lattice coupling, such as manganites, orbital degeneracy is lifted, causing a null expectation value of the orbital magnetic moment. Magnetic structure is thus determined by spin–spin superexchange. In titanates, however, with much smaller Jahn–Teller distortions, orbital degeneracy might allow non-zero values of the orbital magnetic moment, and novel forms of ferromagnetic superexchange interaction unique to t_2g electron systems have been theoretically predicted, although their experimental observation has remained elusive. In this paper, we report a new kind of Ti³⁺ ferromagnetism at LaMnO₃/SrTiO₃ epitaxial interfaces. It results from charge transfer to the empty conduction band of the titanate and has spin and orbital contributions evidencing the role of orbital degeneracy. The possibility of tuning magnetic alignment (ferromagnetic or antiferromagnetic) of Ti and Mn moments by structural parameters is demonstrated. This result will provide important clues for understanding the effects of orbital degeneracy in superexchange coupling.

Spin and orbital moments in Fe₃O₄

Phys. Rev. B. 81, 134424 (2010)

Author(s): J. A. Duffy, J. W. Taylor, S. B. Dugdale, C. Shenton-Taylor, M. W. Butchers, S. R. Giblin, M. J. Cooper, Y. Sakurai and M. Itou

Abstract: We report a study of the spin moment in Fe₃O₄ as a function of temperature, using spin-dependent Compton scattering. Data were obtained for applied magnetic fields of 2.5 T and 7 T. We find that there is a spin moment of 4 μ_B for all temperatures and crystallographic directions. The orbital moment, determined by comparison with bulk magnetometry is less than 0.1 μ_B. The magnetic Compton profiles have the characteristic shape for Fe 3d electrons as all temperatures. No evidence for any anomalies in the spin moment at the Verwey transition was observed. Our data are consistent with the highly spin polarized electronic struture expected for bulk Fe₃O₄.

Bulk electronic structure of optimally doped Ba(Fe_1-xCo_x)₂As₂

Phys. Rev. B. 101, 064509 (2010)

Author(s): C. Utfeld, J. Laverock, T. D. Haynes, S. B. Dugdale, J. A. Duffy, M. W. Butchers, J. W. Taylor, S. R. Giblin, J. G. Analytis, J. Chu, I. R. Fisher, M. Itou, and Y. Sakurai

Abstract: We report high-resolution, bulk Compton scattering measurements unveiling the Fermi surface of an optimally-doped iron-arsenide superconductor, Ba(Fe_0.93Co_0.07)₂As₂. Our measurements are in agreement with first-principles calculations of the electronic structure, revealing both the X-centered electron pockets and the Γ-centered hole pockets. Moreover, our data are consistent with the strong three-dimensionality of one of these sheets that has been predicted by electronic structure calculations at the LDA-minimum As position. Complementary calculations of the non-interacting susceptibility, χ₀(q, ω), suggest that the broad peak that develops due to interband Fermi surface nesting, and which has motivated several theories of superconductivity in this class of material, survives the measured three-dimensionality of the Fermi surface in this family.

Bulk spin polarization of Co_1-xFe_xS₂

Phys. Rev. Lett. 103, 226403 (2009)

Author(s): Utfeld, C., Giblin, S. R., Taylor, J. W., Duffy, J. A., Shenton-Taylor, C., Laverock, J., Dugdale, S. B., Manno, M., Leighton, C., Itou, M. and Sakurai, Y.

Abstract: We report on a new method to determine the degree of bulk spin polarization in single crystal Co_(1-x)Fe_xS₂ by modeling magnetic Compton scattering with ab initio calculations. Spin-dependent Compton profiles were measured for CoS₂ and Co_0.9Fe_0.1S₂. The ab initio calculations were then refined by rigidly shifting the bands to provide the best fit between the calculated and experimental directional profiles for each sample. The bulk spin polarizations, P, corresponding to the spin-polarized density of states at the Fermi level, were then extracted from the refined calculations. The values were found to be P=-72 ± 6% and P=18±7% for CoS₂ and Co_0.9Fe_0.1S₂, respectively. Furthermore, determinations of P weighted by the Fermi velocity (v_F or v_F²) were obtained, permitting a rigorous comparison with other experimental data and highlighting the experimental dependence of P on v_F.

Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

Phys. Rev. B. 76, 052509 (2007)

Author(s): J. Laverock, S. B. Dugdale, J. A. Duffy, J. Wooldridge, G. Balakrishnan, M. R. Lees, G.-q. Zheng, D. Chen, C. T. Lin, A. Andrejczuk, M. Itou, and Y. Sakurai

Abstract: The surprise discovery of superconductivity below 5 K in sodium cobalt oxides when hydrated with water has caught the attention of experimentalists and theorists alike. Most explanations for its occurence have focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, but direct attempts to look for them have instead cast serious doubts over their existence. Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodium cobalt oxides using the technique of x-ray Compton scattering.