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Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Santa Cruz Electronic Theses and Dissertations (2018)

Recent computational work predicted orthorhombic antiferromagnetic NaMnF3 to undergo a ferroelectric transition if an a = c structural distortion is imposed. Such a distortion may be achieved by epitaxial growth of NaMnF3 on a cubic substrate in thin film form. It has been shown that the a = c distortion leads to the freezing of a soft polar mode resulting in a structural transformation from a non-polar Pnma to a polar Pna21 space group with a polarization of P = 6 μ C/cm2 along the long axis. A weak ferromagnetic phase and an amplification of magnetoelectric coupling are also expected in the strained films, with a magnetoelectric response comparable to Cr2O3.

In this work, I investigate multiferroicity in NaMnF3 thin films. Epitaxial NaMnF3 thin films were grown on cubic SrTiO3 (001) single crystal substrates via molecular beam epitaxy (MBE). Thin film structural quality was investigated as a function of the substrate temperature and film thickness using X-ray diffraction (XRD), in-situ reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The best films were smooth single phase grown at 250 − 300 ◦C substrate temperatures with four different twin domains. In-plane magnetization measure- ments using a superconducting quantum interference device (SQUID) magnetometer revealed antiferromagnetic ordering with weak ferromagnetism below the Neel tem- perature TN = 66 K. Reproducible room temperature ferroelectric switching by a

voltage-biased scanning probe was detected with preferred up (out-of-plane) polar- ization using piezoresponse force microscopy. An interesting 180◦ out-of-plane polar- ization flip by the application of an in-plane electric field was also observed.

For the exchange bias and dielectric studies, NaMnF3 films were grown on 30 nm SrRuO3 (001) layer, which was used as a ferromagnetic layer as well as a bottom electrode, and grown via pulsed laser deposition. After field cooling below the Neel temperature of NaMnF3, hysteresis loops were measured as the function of tempera- ture. A negative exchange bias was found with the blocking temperature (TB) ∼ 45 K. The complex permittivity as a function of frequency measurements indicated a strong Debye-like relaxation. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K.

    Cover page: Multiferrocity In Fluoroperovskite Thin FilmsCreative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    UC Santa Cruz Previously Published Works (2017)
    The NaMnF3 fluoride-perovskite has been found, theoretically, to be ferroelectric under epitaxial strain becoming a promising alternative to conventional oxides for multiferroic applications. Nevertheless, this fluoroperovskite has not been experimentally verified to be ferroelectric so far. Here we report signatures of room temperature ferroelectricity observed in perovskite NaMnF3 thin films grown on SrTiO3. Using piezoresponse force microscopy, we studied the evolution of ferroelectric polarization in response to external and built-in electric fields. Density functional theory calculations were also performed to help understand the strong competition between ferroelectric and paraelectric phases as well as the profound influences of strain. These results, together with the magnetic order previously reported in the same material, pave the way to future multiferroic and magnetoelectric investigations in fluoroperovskites.
      Cover page: Room temperature ferroelectricity in fluoroperovskite thin films
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2016)
      Multiferroic materials have simultaneous magnetic and ferroelectric long-range orders and can be potentially useful for a wide range of applications. Conventional ferroelectricity in oxide perovskites favors nonmagnetic electronic configurations of transition metal ions, thus limiting the number of intrinsic multiferroic materials. On the other hand, this is not necessarily true for multiferroic fluorides. Using molecular beam epitaxy, we demonstrate for the first time that the multiferroic orthorhombic fluoride BaCoF4 can be synthesized in thin film form. Ferroelectric hysteresis measurements and piezoresponse force microscopy show that the films are indeed ferroelectric. From structural information, magnetic measurements, and first-principles calculations, a modified magnetic ground state is identified which can be represented as a combination of bulk collinear antiferromagnetism with two additional canted spin orders oriented along orthogonal axes of the BaCoF4 unit cell. The calculations indicate that an anisotropic epitaxial strain is responsible for this unusual magnetic ground state.
        Cover page: Multiferroic BaCoF4 in Thin Film Form: Ferroelectricity, Magnetic Ordering, and Strain
        • Article
        • Peer Reviewed
        UC Santa Cruz Previously Published Works (2016)
        We report on the observation of the spin Seebeck effect in antiferromagnetic MnF_{2}. A device scale on-chip heater is deposited on a bilayer of MnF_{2} (110) (30  nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF_{2} (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF_{2} through the inverse spin Hall effect. The low temperature (2-80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9  T) are applied parallel to the easy axis of the MnF_{2} thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.
          Cover page: Antiferromagnetic Spin Seebeck Effect
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