Spontaneous magnetization above $T_C$ in polycrystalline ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Ba}}_{0.3}{\mathrm{MnO}}_3$

In the present work, spontaneous magnetization is observed in the inverse magnetic susceptibility of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Ba}}_{0.3}{\mathrm{MnO}}_3$ compounds above $T_C$ up to a temperature $T^{*}$. From information gathered from neutron diffraction, dilatometry, and high-field magnetization data, we suggest that $T^{*}$ is related to the transition temperature of the low-temperature (high magnetic field) magnetic phase. In the temperature region between $T^{*}$ and $T_C$, the application of a magnetic field drives the system from the high-temperature to low-temperature magnetic phases, the latter possessing a higher magnetization.

Figure 1

Main graph: Field-cooled magnetization of LCMO (continuous line) and LBMO (dotted line) as a function of reduced temperature in a 0.01-T (left axis) and a 1-T (right axis) magnetic field. $T_C$ is defined as the temperature where the derivative of magnetization with respect to temperature is maximal under a 0.01-T field. Inset: Saturation magnetization at 10 K for LCMO and LBMO in a magnetic field of 5 T.

Figure 2

Main graphs: Experimentally measured field-cooled inverse magnetic susceptibility of LCMO (a) and LBMO (b) as a function of the reduced temperature for increasing applied magnetic field. Insets (a) and (b) are the simulated field-cooled inverse magnetic susceptibility using parameter values for disorder and magnetic clustering from a mean-field and Bean-Rodbell-based analysis via a dedicated software package. (c) Spontaneous magnetization extracted from Arrott plots (inset), as a function of the reduced temperature for both samples. The vertical dashed lines are guides for the eye to indicate for each sample the coincidence of the transition temperature suggested by the spontaneous magnetization extracted from the Arrott plots, and $T^{*}$ extracted from the inverse susceptibility.

Figure 3

LBMO experimental (black lines) and simulated (red dots) curves showing (a) magnetization as a function of applied magnetic field, at some representative temperature 300, 310, 320, 330, 340, and 350 K (with lowest temperature at bottom of graph) and (b) typical $H/M$ vs $M^2$ Arrot plots (for the same temperatures).

Figure 4

Linear parallel magnetostriction of LCMO (a) and LBMO (b). (c) Isothermal volume magnetostriction, $\omega$, of LCMO (open black squares) and LBMO (open red triangles) as a function of the temperature under the application of a 1-T magnetic field.

Figure 5

Seebeck coefficient as a function of the reduced temperature for LBMO (red triangles) and LCMO (black squares). Around $T_C$, a sharp change of the Seebeck coefficient can be seen for LCMO only.

Figure 6

(a) Refined reduced volume (red triangles and left axis) from neutron diffraction data and thermal expansion (red dotted line and right axis) of LBMO measured with a capacitance dilatometer as a function of reduced temperature. (b) Refined reduced volume (black squares and left axis) from neutron diffraction data and thermal expansion (black dotted line and right axis) of LCMO measured with a capacitance dilatometer as a function of reduced temperature.

Figure 7

Average Mn-O bond length (a) and Mn-O-Mn bond angles (b) for LCMO (black squares) and LBMO (red triangles) in comparison with ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ data (blue stars) from the literature [31] as a function of reduced temperature. Mn-O bond length and angles are changing significantly at the transition in LCMO as in ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ and less so in LBMO.

Figure 8

Graph adapted from Ref. [31] (Radaelli et al.) showing the bandwidths and transition temperatures of $A_{0.7}{A}_{0.3}^{\prime }{\mathrm{MnO}}_3$ compounds. The black line is a quadratic fit used to correlate $T_C$ with $W$. LCMO (black square) has been added and LBMO (red triangle) has been identified in this graph using neutron diffraction data above $T_C$ (1.1$T_C)$. Bandwidth values for low-temperature phases, calculated using neutron diffraction data below $T_C$ (0.9$T_C)$, are shown in dashed lines in black for LCMO and red for LBMO. The transition temperatures of the low-temperature phase of both LCMO and LBMO each coincide with the $T^{*}$ determined in the Griffiths-like phase using magnetization data at high field and the inverse susceptibility deviation from the Curie-Weiss law in the PM regime.