Anisotropic Magnetoresistance in Perovskite Manganites

Abstract

We have summarized some important results of the anisotropic transport properties of the prototypical manganite La 1−x CaxMnO 3 as well as a few others. The temperature dependence of anisotropic magnetoresistance in manganites exhibits a peak near the magnetic ordering temperature which differs dramatically from the ones in 3d ferromagnetic metals and alloys. Depending on the strain-driven orbital state, the AMR in manganites could be enhanced dramatically. The AMR in manganites is much larger than in ferromagnetic metals, where its magnitude is only of the order of a few percent. At low fields (below few kG) the intrinsic magnetocrystalline anisotropy becomes important and the dependence of resistance on angle changes to a more switching-like behavior. The magnitude of the switching is sufficiently large such that this phenomenon could be useful in spintronics for magnetic field sensing and nonvolatile memory applications. Moreover, at temperatures far below the metal insulator transition temperatures, the AMR in LCMO/LAO also flips and changes a sign in contrast to LCMO/STO films. Also sign flip takes place in charge and orbital ordered manganites exhibiting field induced ferromagnetism. Polycrystalline manganite samples also exhibit strong AMR. At lower temperatures the dependence of the AMR on temperature is dramatically different in polycrystalline samples than the epitaxial or single crystal samples. The behavior in the the polycrystalline samples could be governed by spin-polarized transport across the grain boundaries.

Resonance spin–charge phenomena and mechanism of magnetoresistance anisotropy in manganite/metal bilayer structures

“…At the first glance, this is consistent with the modern consideration [25,31] which predicts that the AMR effect in manganites depends on the field direction relative to the [100] crystallographic axis. This approach, however, is restricted by low temperatures (T<<T C ) and does not take into account the in-plane magnetic anisotropy; thus, it is hardly applicable to our data.…”

Magnetotransport properties of La 1−x Ca x MnO 3 (0.52 ≤ x ≤ 0.75): Signature of phase coexistence

“…In short, we will mention the most important results obtained on La 1-x Ca x MnO 3 manganites related to bulk material and films. In the bulk material for x < 0.5 (where the comparison with films was investigated) and at the metal-insulator transition (MIT) a resistivity increase is expected because the charge carriers are scattered by grain boundaries [15]. On the other hand, the MIT temperatures in bulk materials are lower than in thin film samples (of the same composition) suggesting that the epitaxial strain in the films modifies the in-plane and out-of-plane hopping amplitudes that consequently enhance the ferromagnetic transition temperature [16].…”

“…were found to decrease with a decreasing film thickness while a reduction in the film thickness increases the magnetoresistance of these films as well as the anisotropy of magnetoresistance [15]. At this point, it is also important to mention that the optical properties study was performed comparing bulk material and films in overdoped region for x > 0.5, with the additional motivation to clarify the role of disorder in the physics of overdoped La 1−x Ca x MnO 3 manganites [23].…”

Sign reversal of anisotropic magnetoresistance in La0.7Ca0.3MnO3/SrTiO3 ultrathin films

“…It is found that the othorhombic La 0.7 Ca 0.3 MnO 3 (LCMO) with an in-plane lattice constant 3.863 Å can be grown on SrTiO 3 (001) substrate (STO), which has the cubic structure with a lattice constant 3.905 Å such that the lattice mismatch between the STO and the LCMO is only ~1%. AMR has been found to be remarkably large in highly strained ultrathin films as compared to the AMR in thicker films 1,5,6,[9][10][11][12] at certain temperatures below their ferromagnetic Curie temperature (T C ) which is attributed to the increase in the strain as the thickness of the film decreases 12, 13 . In this paper, anisotropy in epitaxial LCMO ultrathin films (4 nm, 6 nm and 8 nm) grown on STO (001) substrate is investigated.…”

“…al., 9,11 . They could not observe a sign change in the AMR for the LCMO/STO films (down to 7 nm) even though they could observe a change in the sign of the AMR at temperatures below T C in the LCMO/LAO thin films.…”