Structure and magnetic properties of $R{\mathrm{Cu}}_4\mathrm{Mn}$ $(R=\mathrm{La}–\mathrm{Gd})$

We report the synthesis of new alloys of composition $R{\mathrm{Cu}}_4\mathrm{Mn}$ for $R=\mathrm{La}$, Ce, Pr, Nd, Sm, and Gd. The ternary compounds are found to crystallize in the hexagonal $\mathrm{Ca}{\mathrm{Cu}}_5$-type structure of the $R{\mathrm{Cu}}_5$ parent compounds, but for the Gd compound, this proved to be the high-temperature form. X-ray diffraction on a single crystal of $\mathrm{Gd}{\mathrm{Cu}}_4\mathrm{Mn}$ was used to determine the site occupancies. The Mn ions substitute randomly on the Cu sublattice. The magnetization, electrical resistivity, and the heat capacity data show the absence of long range magnetic order in all the alloys in marked contrast to the parent $R{\mathrm{Cu}}_5$ ($R=\mathrm{Ce}$, Nd, Sm, and Gd) compounds which order magnetically. A complex, inhomogeneous magnetic state arising from the Cu-Mn disorder and $3d\text{−}3d$ and $3d\text{−}4f$ interactions is proposed.

Figure 1

The DTA curve of $\mathrm{La}{\mathrm{Cu}}_4\mathrm{Mn}$ recorded on cooling at $10°\mathrm{C}∕\min$. The pronounced peak at $825°\mathrm{C}$ is due to the congruent solidification of $\mathrm{La}{\mathrm{Cu}}_4\mathrm{Mn}$. The small peak at $735°\mathrm{C}$ can be ascribed to the formation of a phase with approximate composition $\mathrm{La}{\mathrm{Cu}}_{4.7}{\mathrm{Mn}}_{1.3}$.

Figure 2

Powder diffraction pattern of $\mathrm{Sm}{\mathrm{Cu}}_4\mathrm{Mn}$.

Figure 3

The ZFC and FC magnetizations of $\mathrm{La}{\mathrm{Cu}}_4\mathrm{Mn}$ in a field of $100\mathrm{Oe}$. ${\chi }_{\mathrm{ac}}$ is also plotted. The inset shows the ${\chi }^{-1}$ measured in applied fields of 0.1 and $3\mathrm{kOe}$. The solid line is a fit to the data.

Figure 4

The magnetization of $\mathrm{La}{\mathrm{Cu}}_4\mathrm{Mn}$ at $4.5\mathrm{K}$ up to $70\mathrm{kOe}$. The upper inset shows the low-field region on an expanded scale. The lower inset shows the isothermal magnetization at 75, 100, 125, 150, 200, 250, and $300\mathrm{K}$.

Figure 5

The ${\chi }^{-1}$ of $\mathrm{Ce}{\mathrm{Cu}}_4\mathrm{Mn}$ between 50 and $300\mathrm{K}$ in $1\mathrm{kOe}$. The upper inset shows in-field magnetization at $1.8\mathrm{K}$. The lower inset shows ZFC and FC dc magnetizations in 50 and $500\mathrm{Oe}$ and ${\chi }_{\mathrm{ac}}$.

Figure 6

The ${\chi }^{-1}$ of $\mathrm{Pr}{\mathrm{Cu}}_4\mathrm{Mn}$ in $3\mathrm{kOe}$. The ZFC and FC dc magnetizations in $100\mathrm{Oe}$ are shown in the upper inset. The lower inset shows a portion of the hysteresis curve at $4.5\mathrm{K}$.

Figure 7

The ${\chi }^{-1}$ of $\mathrm{Nd}{\mathrm{Cu}}_4\mathrm{Mn}$ in $3\mathrm{kOe}$. The ZFC and FC dc magnetizations in $100\mathrm{Oe}$ and ${\chi }_{\mathrm{ac}}$ are plotted in the upper inset. The lower inset shows the hysteresis curve at $5\mathrm{K}$.

Figure 8

The ZFC and FC magnetizations of $\mathrm{Gd}{\mathrm{Cu}}_4\mathrm{Mn}$ in a field of $1\mathrm{kOe}$. The lower inset shows the ac susceptibility at 31 and $310\mathrm{Hz}$. The ${\chi }^{-1}$ up to $300\mathrm{K}$ is shown in the upper inset.

Figure 9

The ZFC and FC magnetizations of $\mathrm{Sm}{\mathrm{Cu}}_4\mathrm{Mn}$ in a field of $1\mathrm{kOe}$. The inset shows the ac susceptibility at 11 and $112\mathrm{Hz}$.

Figure 10

The hysteresis loop of $\mathrm{Sm}{\mathrm{Cu}}_4\mathrm{Mn}$ at $1.8\mathrm{K}$. The inset shows the magnetization near the origin on an expanded scale.

Figure 11

The hysteresis loop of $\mathrm{Gd}{\mathrm{Cu}}_4\mathrm{Mn}$ at 1.8 and $60\mathrm{K}$. The inset shows the in-field magnetization at 75, 100, 125, 150, 200, 250, and $300\mathrm{K}$.

Figure 12

The heat capacity $C$ of $\mathrm{La}{\mathrm{Cu}}_4\mathrm{Mn}$ and $\mathrm{Gd}{\mathrm{Cu}}_4\mathrm{Mn}$ between 20 and $50\mathrm{K}$. The lower inset shows the data for $\mathrm{Sm}{\mathrm{Cu}}_4\mathrm{Mn}$ between 100 and $125\mathrm{K}$; $C∕T$ vs $T^2$ up to $10\mathrm{K}$ on the three compounds is shown in the upper inset.

Figure 13

The electrical resistivity of $R{\mathrm{Cu}}_4\mathrm{Mn}$ ($R=\mathrm{La}$, Sm, and Gd).