Complex interplay of $3d$ and $4f$ magnetism in ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$

We report on structural, magnetic, electrical, and thermodynamic properties of Gd-doped $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_3$ single crystals for Gd doping levels $0⩽x⩽1$. At room temperature, for all doping levels the orthorhombic O’ phase is indicative of a strong Jahn-Teller distortion. All compositions are insulating. The magnetism of ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$ is dominated by the relatively strong $\mathrm{Mn}\mathrm{O}\mathrm{Mn}$ superexchange. For increasing Gd doping, the weakening of the nearest-neighbor exchange interactions due to the significant decrease of the $\mathrm{Mn}\mathrm{O}\mathrm{Mn}$ bond angles leads to the continuous suppression of the magnetic phase-transition temperature into the A-type antiferromagnetic low-temperature phase. The temperature dependence of the magnetization can only be explained assuming canting of the manganese spins. The magnetic moments of Gd are weakly antiferromagnetically coupled within the sublattice and are antiferromagnetically coupled to the Mn moments. For intermediate concentrations compensation points are found, below which the spontaneous magnetization becomes negative. In pure $\mathrm{Gd}\mathrm{Mn}{\mathrm{O}}_3$ the Mn spins undergo a transition into a complex, probably incommensurate magnetic structure at $41.5\mathrm{K}$, followed by a further ordering transition at $18–20\mathrm{K}$ revealing weak ferromagnetism due to canting and finally by the onset of magnetic order in the Gd sublattice at $6.5\mathrm{K}$. At the lowest temperatures and low external fields, both magnetic sublattices reveal a canted structure with antiparallel ferromagnetic components.

Figure 1

Lattice constants $a$, $b$, and $c∕\sqrt{2}$ (upper frame), volume of the unit cell (middle frame), and in-plane $\mathrm{Mn}\mathrm{O}\mathrm{Mn}$ bond angle $\varphi$ in ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$ versus Gd concentration $x$.

Figure 2

(Color online) Temperature dependence of the resistivity in ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$ for various compounds plotted in an Arrhenius representation. (The $x=1$ curve is shifted for better clarity.) The solid lines at high temperatures represent Arrhenius fits indicating a purely activated type of behavior of the charge carriers. The dashed lines at low temperatures represent variable-range-hopping conductivity. Inset: Resistivity above $400\mathrm{K}$.

Figure 3

(Color online) Inverse dc susceptibility $(\chi =M∕H)$ versus temperature as measured in an external magnetic field of $10\mathrm{Oe}$. The solid lines through the data for $x=0$, 0.25, 0.5, and 0.75 represent fits to Curie-Weiss laws. The solid line through the data for $x=1$ indicates the result of a fit taking into account two magnetic sublattices coupled ferrimagnetically.

Figure 4

Concentration dependencies of the parameters as obtained from the analysis of the susceptibility measurements: Néel temperatures (asterixes: upper frame), CW temperatures (open rhombs: middle frame), and paramagnetic moments (solid circles: lower frame). The solid line through the concentration dependence is a fit as described in the text. The dashed lines are drawn to guide the eye.

Figure 5

Magnetization $M$ versus temperature in ${\mathrm{La}}_{1-x}{\mathrm{Gd}}_x\mathrm{Mn}{\mathrm{O}}_3$ for $x=0.25$ (triangles up), 0.5 (stars), and 0.75 (triangles down). The data were recorded on cooling from room temperature in an external magnetic field of $10\mathrm{Oe}$. The solid line represents a fit that takes into account the paramagnetic contribution of the Gd subsystem polarized by the internal field of the manganese moments due to $\mathrm{Gd}\mathrm{Mn}$ coupling. Inset: Data and fit for $x=0.75$ on an enlarged scale.

Figure 6

(Color online) Magnetization $M$ versus temperature for ${\mathrm{La}}_{0.5}{\mathrm{Gd}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ as measured in an external field of $10\mathrm{Oe}$. The data were taken on cooling from room temperature (triangles down). At the lowest temperatures the magnetization was switched in an external field of $50\mathrm{kOe}$. Then $M$ was measured on heating (triangles up). The insets a, b, and c indicate the spin structure at the respective temperatures and cycle positions.

Figure 7

(Color online) Magnetization $M$ versus temperature for $\mathrm{Gd}\mathrm{Mn}{\mathrm{O}}_3$ plotted on a double-logarithmic scale as measured in an external field of $1000\mathrm{Oe}$. The easy (triangles up) and the hard direction (triangles down), which split approximately at $20\mathrm{K}$, are shown. The inset shows the ac susceptibility measured along $c$ and within the $ab$-plane at a dc field of zero and $1000\mathrm{Oe}$ at 3 measuring frequencies of 1 (solid line), 35 (dashed line), and $1000\mathrm{Hz}$ (dotted line). Perpendicular to the c-axis, no dependence on magnetic bias field or frequency can be detected.

Figure 8

(Color online) Magnetization along $c$ as function of the external magnetic field for a series of temperatures between $1.8\mathrm{K}$ and $30\mathrm{K}$. The inset shows the hysteresis loops at low fields on an enlarged scale.

Figure 9

(Color online) Heat capacity of $\mathrm{Gd}\mathrm{Mn}{\mathrm{O}}_3$ plotted as $C∕T$ as a function of temperature, measured at different external magnetic fields up to $50\mathrm{kOe}$. The dashed line represents data for ${\mathrm{La}}_{0.25}{\mathrm{Gd}}_{0.75}\mathrm{Mn}{\mathrm{O}}_3$ in zero external field. The inset shows the field dependence of the heat capacity of $\mathrm{Gd}\mathrm{Mn}{\mathrm{O}}_3$ at $T=26.5\mathrm{K}$.

Figure 10

(Color online) $(H,T)$-phase diagram of $\mathrm{Gd}\mathrm{Mn}{\mathrm{O}}_3$. The spin structures of the two magnetic sublattices are indicated.