Anomalously large anisotropic magnetoresistance in a perovskite manganite

Abstract

The signature of correlated electron materials (CEMs) is the coupling between spin, charge, orbital and lattice resulting in exotic functionality. This complexity is directly responsible for their tunability. We demonstrate here that the broken symmetry, through cubic to orthorhombic distortion in the lattice structure in a prototype manganite single crystal, La(0.69)Ca(0.31)MnO(3), leads to an anisotropic magneto-elastic response to an external field, and consequently to remarkable magneto-transport behavior. An anomalous anisotropic magnetoresistance (AMR) effect occurs close to the metal-insulator transition (MIT) in the system, showing a direct correlation with the anisotropic field-tuned MIT in the system and can be understood by means of a simple phenomenological model. A small crystalline anisotropy stimulates a "colossal" AMR near the MIT phase boundary of the system, thus revealing the intimate interplay between magneto- and electronic-crystalline couplings.

Fig. 1.

Magnetic and magneto-transport properties of La_0.69Ca_0.31MnO₃. (A) T-dependence of the magnetization with an Inset showing the unit cell structure as well as the crystalline axes. (B) T-dependence of the resistivity measured from a slab of a single crystal under different fields. The Inset shows a schematic arrangement of the measurements from the crystal with a slab sample size: 3.8 × 0.85 × 0.15 mm³. The sample c-axis is normal to the slab plane. (C) Angle- and T-dependences of the normalized resistivity, ρ(Θ)/ρ(Θ) = 0°, measured under a magnetic field rotating about the x-direction (i.e., the current (I) direction).

Fig. 2.

Correlation between MIT and AMR of La_0.69Ca_0.31MnO₃. (A) T- and H- (along c-axis) dependence of the resistivity (ρ) and T_MI defined as the peak temperature in dρ/dT − T curve (see the Inset). (B) A contour plot and line plots of (C) the T- and (D) H-dependence of the measured R_ρ from the slab single crystal as the field was applied in the xz plane (thus including Lorentzian MR). The solid lines through the data points are guides to the eye.

Fig. 3.

Field-orientation dependence of magnetoresistance and MIT in La_0.69Ca_0.31MnO₃. (A) Field strength and orientation dependence of resistivity normalized to its zero-field value at the temperatures near T_MI and (B) T_MI measured from the slab single crystal (see the Inset of Fig. 1B). The Inset presents the T-dependence of resistivity on warming under different values of magnetic field along z(c)-axis (solid circle) and in xy(ab)-plane. The solid lines are guides to the eye. The dashed arrows indicate the horizontal and vertical cuts for a fixed temperature and field, respectively.

Fig. 4.

Crystal twinning behavior of La_0.69Ca_0.31MnO₃. Lorentzian TEM images with the selected-area electron diffraction patterns of corresponding areas at T = 80 K with different crystalline orientations. (A) The 45° crystal twinning between a- and b-axis. (B) The 90° crystal twinning in the c-axis. The T dependence of the anisotropic R_ρ at H = 1.0 T measured from (C) the slab and (D) a square-rod La_0.69Ca_0.31MnO₃ single crystal. The Inset shows a schematic arrangement of the measurements from the square-rod sample size: 7.0 × 0.85 × 0.85 mm³. The c-axis is in the plane perpendicular to the current direction.

Fig. 5.

Phenomenological uniaxial anisotropy model for the AMR of La_0.69Ca_0.31MnO₃. (A) Calculated normalized resistivity at a given temperature as a function of magnetic field strength and orientation with respect to the crystal c-axis (Θ), based upon the uniaxial anisotropic model (see the Inset for the uniaxial resistivity ellipsoid). (B) Calculated (solid lines) Θ-dependence of normalized resistivity for three values of fields (corresponding to the three sheets in A) rotating in the yz plane, compared with the experimental results (symbols) measured from the slab sample at T = 225 K. (C and D) Calculated angle dependence of normalized resistivity for a field of H = 1.0 T rotating in three perpendicular planes (from y to x, z to y, and z to x direction), compared with the experimental results (symbols) measured at T = 225 K from the slab (see Fig. 1) and square-rod (see Fig. 4.) sample, respectively.

Fig. 6.

The magnetization of La_0.69Ca_0.31MnO₃ as a function of magnetic field for the two distinct field directions measured at different temperature, H//z(c)- and H⊥z(c)-axis, respectively.

Fig. 7.

Schematic view of the relative orientation relation between magnetic field (H), measured current (I), and crystal axis of square-rod sample existing two c-axis components due to twinning domains.