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  • 1.
    Hudl, Matthias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mathieu, Roland
    Nordblad, Per
    Tunable exchange bias in dilute magnetic alloys - chiral spin glasses2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 19964Article in journal (Refereed)
    Abstract [en]

    A unidirectional anisotropy appears in field cooled samples of dilute magnetic alloys at temperatures well below the cusp temperature of the zero field cooled magnetization curve. Magnetization measurements on a Cu(13.5 at% Mn) sample show that this anisotropy is essentially temperature independent and acts on a temperature dependent excess magnetization, Delta M. The anisotropy can be partially or fully transferred from being locked to the direction of the cooling field at lower fields to becoming locked to the direction of Delta M at larger fields, thus instead appearing as a uniaxial anisotropy. This introduces a deceiving division of the anisotropy into a superposition of a unidirectional and a uniaxial part. This two faced nature of the anisotropy has been empirically scrutinized and concluded to originate from one and the same exchange mechanism: the Dzyaloshinsky-Moriya interaction.

  • 2. Ivanov, S. A.
    et al.
    Bush, A. A.
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Stash, A. I.
    Andre, G.
    Tellgren, R.
    Cherepanov, V. M.
    Stepanov, A. V.
    Kamentsev, K. E.
    Tokunaga, Y.
    Taguchi, Y.
    Tokura, Y.
    Nordblad, P.
    Mathieu, R.
    Spin and dipole order in geometrically frustrated mixed-valence manganite Pb3Mn7O152016In: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 27, no 12, p. 12562-12573Article in journal (Refereed)
    Abstract [en]

    The structural, magnetic, and dielectric properties of Pb3Mn7O15 have been investigated using high-quality single crystals. Pb3Mn7O15 adopts a pseudo-hexagonal orthorhombic structure, with partially filled Kagom, layers connected by ribbons of edge-sharing MnO6 octahedra and intercalated Pb cations. There are 9 inequivalent sites in the structure for the Mn ions, which exist both as Mn3+ and Mn4+. Pb3Mn7O15 undergoes an antiferromagnetic transition below T-N similar to 67 K, with significant geometric frustration. Neutron powder diffraction on crushed single crystals allowed us to determine the low-temperature antiferromagnetic magnetic structure. We discuss the magnetic interaction pathways in the structure and possible interplay between the structural distortions imprinted by the lone-electron pair of Pb2+ cations and Mn3+/Mn4+ charge ordering.

  • 3. Leo, N.
    et al.
    Carolus, V.
    White, J. S.
    Kenzelmann, M.
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Tolédano, P.
    Honda, T.
    Kimura, T.
    Ivanov, S. A.
    Weil, M.
    Lottermoser, Th.
    Meier, D.
    Fiebig, M.
    Magnetoelectric inversion of domain patterns2018In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 560, no 7719, p. 466-+Article in journal (Refereed)
    Abstract [en]

    The inversion of inhomogeneous physical states has great technological importance; for example, active noise reduction relies on the emission of an inverted sound wave that interferes destructively with the noise of the emitter(1), and inverting the evolution of a spin system by using a magnetic-field pulse enables magnetic resonance tomography(2). In contrast to these examples, inversion of a distribution of ferromagnetic or ferroelectric domains within a material is surprisingly difficult: field poling creates a single-domain state, and piece-by-piece inversion using a scanning tip is impractical. Here we report inversion of entire ferromagnetic and ferroelectric domain patterns in the magnetoelectric material Co3TeO6 and the multiferroic material Mn2GeO4, respectively. In these materials, an applied magnetic field reverses the magnetization or polarization, respectively, of each domain, but leaves the domain pattern intact. Landau theory indicates that this type of magnetoelectric inversion is universal across materials that exhibit complex ordering, with one order parameter holding the memory of the domain structure and another setting its overall sign. Domain-pattern inversion is only one example of a previously unnoticed effect in systems such as multiferroics, in which several order parameters are available for combination. Exploring these effects could therefore advance multiferroics towards new levels of functionality.

  • 4.
    Pancaldi, Matteo
    et al.
    Stockholm University, Faculty of Science, Department of Physics. CIC nanoGUNE, Spain.
    Freeman, Ryan
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Hoffmann, Matthias C.
    Urazhdin, Sergei
    Vavassori, Paolo
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Anti-reflection coating design for metallic terahertz meta-materials2018In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 3, p. 2917-2927Article in journal (Refereed)
    Abstract [en]

    We demonstrate a silicon-based, single-layer anti-reflection coating that suppresses the reflectivity of metals at near-infrared frequencies, enabling optical probing of nano-scale structures embedded in highly reflective surroundings. Our design does not affect the interaction of terahertz radiation with metallic structures that can be used to achieve terahertz near-field enhancement. We have verified the functionality of the design by calculating and measuring the reflectivity of both infrared and terahertz radiation from a silicon/gold double layer as a function of the silicon thickness. We have also fabricated the unit cell of a terahertz meta-material, a dipole antenna comprising two 20-nm thick extended gold plates separated by a 2 mu m gap, where the terahertz field is locally enhanced. We used the time-domain finite element method to demonstrate that such near-field enhancement is preserved in the presence of the anti-reflection coating. Finally, we performed magneto-optical Kerr effect measurements on a single 3-nm thick, 1-mu m wide magnetic wire placed in the gap of such a dipole antenna. The wire only occupies 2% of the area probed by the laser beam, but its magneto-optical response can be clearly detected. Our design paves the way for ultrafast time-resolved studies, using table-top femtosecond near-infrared lasers, of dynamics in nano-structures driven by strong terahertz radiation.

  • 5.
    Polley, Debanjan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Pancaldi, Matteo
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Vavassori, Paolo
    Urazhdin, Sergei
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    THz-driven demagnetization with perpendicular magnetic anisotropy: towards ultrafast ballistic switching2018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 8, article id 084001Article in journal (Refereed)
    Abstract [en]

    We study THz-driven spin dynamics in thin CoPt films with perpendicular magnetic anisotropy. Femtosecond magneto-optical Kerr effect measurements show that demagnetization amplitude of about 1% can be achieved with a peak THz electric field of 300 kV cm(-1), and a corresponding peak magnetic field of 0.1 T. The effect is more than an order of magnitude larger than observed in samples with easy-plane anisotropy irradiated with the same field strength. We also utilize finite-element simulations to design a meta-material structure that can enhance the THz magnetic field by more than an order of magnitude, over an area of several tens of square micrometers. Magnetic fields exceeding 1 Tesla, generated in such meta-materials with the available laser-based THz sources, are expected to produce full magnetization reversal via ultrafast ballistic precession driven by the THz radiation. Our results demonstrate the possibility of table-top ultrafast magnetization reversal induced by THz radiation.

  • 6. Willa, K.
    et al.
    Diao, Zhu
    Stockholm University, Faculty of Science, Department of Physics. Halmstad University, Sweden.
    Campanini, Donato
    Stockholm University, Faculty of Science, Department of Physics.
    Welp, U.
    Divan, R.
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Islam, Z.
    Kwok, W. -K.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Nanocalorimeter platform for in situ specific heat measurements and x-ray diffraction at low temperature2017In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 88, no 12, article id 125108Article in journal (Refereed)
    Abstract [en]

    Recent advances in electronics and nanofabrication have enabled membrane-based nanocalorimetry for measurements of the specific heat of microgram-sized samples. We have integrated a nanocalorimeter platform into a 4.5 T split-pair vertical-field magnet to allow for the simultaneous measurement of the specific heat and x-ray scattering in magnetic fields and at temperatures as low as 4 K. This multi-modal approach empowers researchers to directly correlate scattering experiments with insights from thermodynamic properties including structural, electronic, orbital, and magnetic phase transitions. The use of a nanocalorimeter sample platform enables numerous technical advantages: precise measurement and control of the sample temperature, quantification of beam heating effects, fast and precise positioning of the sample in the x-ray beam, and fast acquisition of x-ray scans over a wide temperature range without the need for time-consuming re-centering and re-alignment. Furthermore, on an YBa2Cu3O7-delta crystal and a copper foil, we demonstrate a novel approach to x-ray absorption spectroscopy by monitoring the change in sample temperature as a function of incident photon energy. Finally, we illustrate the new insights that can be gained from in situ structural and thermodynamic measurements by investigating the superheated state occurring at the first-order magneto-elastic phase transition of Fe2P, a material that is of interest for magnetocaloric applications.

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