R&D: Study Uncovers New Materials Interactions That Could Improve Storage

From University of Tennessee, Knoxville, TN, USA

Close up reflection of open hard drive disk

A study (see below) provides insight into multiferroic materials, which could have substantive implications in fields such as data storage.

The study looked at lanthanum cobaltite (LaCoO₃ or LCO), a thin crystalline film that, once grown on a substrate, can be analyzed through electron microscopy and polarized neutron reflectometry to measure electron density and differences in magnetization, respectively.

LCO is special because it is a ferroelastic material, meaning that its properties will change in response to a stressor and retain the changes after the stressor has been removed.

An ultrathin film of LCO – one whose thickness is about 12 nanometers, or twelve thousand-millionths of a meter – is unique because it is also a ferromagnet. The combination of being ferroelastic and a ferromagnet means ultrathin LCO is a multiferroic – a material with elastic and magnetic properties that can change under stress or by magnetic fields. This means the material could, in principle, record the stress of its environment as magnetic information.

“An important finding was that by growing the LCO films on chemically different substrates, or bases, we could change the magnetic properties of the film,” said Michael Fitzsimmons, joint physics professor, University of Tennessee, Knoxville, TN, USA, and Oak Ridge National Laboratory (ORNL), and leader, Thin Films and Nanostructures Group in ORNL’s Neutron Scattering Division.

Being able to easily manipulate a substance’s ferromagnetic properties is an important step in creating devices that require less energy to operate. In the case of LCO, the connection between its ferroelastic and ferromagnetic properties would drastically cut down on the amount of energy currently required by current magnetic technology.

“An example is a magnetic read head, a piece used in digital storage units,” Fitzsimmons said. “A magnetic field changes the alignment of a small region of magnetic material—its direction represents some information.”

This type of magnetic field is produced by a current pulse, which takes a significant amount of energy.

“If instead we could change the direction of magnetization by applying electric charge without passing current, then we wouldn’t need so much energy,” Fitzsimmons said. “One aim is to create devices that can do new things like sense light, chemical composition, magnetic fields, or heat, or manipulate and store data in compact objects that do not require much energy to operate.”

Article: Switchable orbital polarization and magnetization in strained LaCoO3 films

Physical Review Materials has published an article written by Er-Jia Guo, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China, and 3Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China, Ryan D. Desautels, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, David Keavney, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA, Andreas Herklotz, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, and Institute for Physics, Martin-Luther-University Halle-Wittenberg, Halle (Saale) 06120, Germany,T. Zac Ward, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, Michael R. Fitzsimmons, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, and Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA, and Ho Nyung Lee, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.

Structural properties of LCO films grown on various substrates. (a) Lattice parameters of the LCO bulk and various single-crystal substrates [SLAO:SrLaAlO4,LAO:LaAlO3,NGO:NdGaO3,LSAT:(LaAlO3)0.3−(Sr1Al0.5Ta0.5O3)0.7,STO:SrTiO3,DSO:DyScO3, and GSO:GdScO3]. Pseudocubic lattice constants were used for noncubic substrates. Insets indicate the in-plane bond angle (Co–O–Co) β and the bond length (Co–O) d. (b) XRD θ–2θ scans around the 002 reflection of LCO films grown on various substrates. The 002 peak of LCO films is indicated with “▼.” The misfit strain of LCO films gradually changes from the compressive (SLAO, LAO) to the tensile strain (NGO, LSAT, STO, DSO, and GSO). (c) Reciprocal space maps around substrate’s 103 or 103pc reflections (or the SLAO substrate’s 1110 reflection). All films are coherently grown on the substrates. (d) M-T curves and (e) M-H hysteresis loops of LCO films grown on SLAO, LAO, STO, and LSAT substrates. The linear (or nonlinear) diamagnetic backgrounds from these substrates were subtracted. For the M-T curves, a magnetic field of 0.1 T was applied along the in-plane direction during the cooling and warm-up measurements. The magnetic hysteresis loops were measured at 10 K and the maximum magnetic fields of ±5 T.

Abstract: “Strain engineering of epitaxial heterostructures offers opportunities to control the orbital degree of freedom by lifting the degeneracy of eg states. Here, we show that the orbital occupation in LaCoO3 (LCO) films can be switched between two degenerate eg bands with epitaxial strain. The orbital polarization of nearly −100% (or 100%) is controlled by depleting occupation of the dx2−y2(ord3z2−r2) orbital entirely in LCO for large compressive (or moderate tensile) strain. The change of electronic configuration associated with the spin-state transition modulates the magnetization of strained LCO films. Under compressive strain, LCO films exhibit a small magnetization without long-range ferromagnetic ordering. With tensile-strain increases, the magnetization of LCO films increases and reaches the maximum value when the bonding angle (Co–O–Co) is close to 180° and the in-plane bond length (Co–O) is unstretched. Our results highlight the role of octahedral distortion and spin-state crossover in tailoring the magnetic properties of cobaltite thin films, suggesting an attractive route to deliberately control the orbital polarization that can be tuned to maximize the functionality of oxide heterostructures.“