Electronic components made of several layers with different magnetic orders are frequently used in various devices such as in read heads for hard drives, which read the data stored, or in highly sensitive magnetic field sensors, which are read electrically. An international team of researchers from the Paul Scherrer Institute (PSI), Switzerland; École Polytechnique Fédérale de Lausanne, Switzerland; the Institut Laue-Langevin, France; the University of Fribourg, Switzerland; and ETH Zürich have now found a material that combines different magnetic properties. The material used is lutetium manganese oxide (LuMnO3), which has a perovskite structure much like high-temperature superconductors. The thin, single crystalline layers were grown using pulsed laser deposition on a nonmagnetic, single crystalline carrier crystal (YAlO3).
Normally, single crystalline LuMnO3 with an orthorhombic structure exhibits an antiferromagnetic order where two spins always point in one direction, and the next two in the opposite direction (E-type antiferromagnet). In this study, however, a ferromagnetic order (where all spins point in the same direction) was observed in the direct vicinity of the surface of the carrier crystal. “Normally, you can’t convert an antiferromagnet into a ferromagnet—for reasons of symmetry apart from anything else. Something special must have happened here,” said Christof Schneider, one of the researchers involved at PSI.
As reported in the July 19 issue of Physical Review Letters (DOI: 10.1103/PhysRevLett.111.037201), the most likely explanation for the effect is that the crystal structure of the material becomes distorted because it adapts to the structure of the carrier crystal and is therefore highly strained. The ferromagnetic order is preferred in the distorted structure. The crystalline structure relaxes as the distance to the YAlO3–LuMnO3 interface increases, but complete relaxation is not achieved even in films ∼200 nm thick. The expectation is that the bulk E-type antiferromagnetic order should appear from a certain distance. Instead, an antiferromagnetic spin spiral is observed where the spins are arranged in the form of a spiral staircase.
Experiments with neutrons to study the magnetic structure on LuMnO3 demonstrate these spin spirals. The measurement results suggest that there is also the expected antiferromagnetic order. “To unravel the mystery of the magnetism in the thin films, we realized that we had to apply multiple neutron scattering techniques. Particularly crucial were novel neutron reflectivity measurements which allowed us to identify the exact location of ferromagnetism,” said Christof Niedermayer, who conducted part of the neutron experiments at PSI.
