Magnetic behavior of $R{\mathrm{Mn}}_{2+x}{\mathrm{Al}}_{10-x}$ $(R=\mathrm{La},\mathrm{Gd})$ crystals

The crystallographic and magnetic properties of the solution grown $R{\mathrm{Mn}}_{2+x}{\mathrm{Al}}_{10-x}$ ($R=\mathrm{Gd}$ and La) crystals with tetragonal $(P4∕nmm)$ structure are investigated. For these, single-crystal x-ray diffraction results have shown the preferential occupation of excess manganese on the aluminum $8i$ crystallographic site. Due to excess Mn, there is evidence of local-moment magnetism in $\mathrm{La}{\mathrm{Mn}}_{2+x}{\mathrm{Al}}_{10-x}$ and their random distribution is thought to be responsible for the magnetic correlations below $\sim 50\mathrm{K}$ and the spin-glass behavior below $\sim 3\mathrm{K}$. For $\mathrm{Gd}{\mathrm{Mn}}_{2+x}{\mathrm{Al}}_{10-x}$, the extra manganese influences temperature below which the sample enters into a state with a net ferromagnetic component: $T_C\approx 16\mathrm{K}$ for $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and $25.5\mathrm{K}$ for $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$. Assuming a linear dependence between $T_C$ and excess Mn concentration, the fully stoichiometric and ordered $\mathrm{Gd}{\mathrm{Mn}}_2{\mathrm{Al}}_{10}$ should have $T_C\approx 5\mathrm{K}$.

Figure 1

(Color online) As grown $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ crystal on a millimeter scale paper, depicting typical dimensions of the $R{\mathrm{Mn}}_{2+x}{\mathrm{Al}}_{10-x}$ crystals and the crystallographic direction.

Figure 2

(Color online) The tetragonal crystal structure of (a) $R{\mathrm{Mn}}_4{\mathrm{Al}}_8$ $(I4∕mmm)$ with Mn-Mn chains and (b) $R{\mathrm{Mn}}_2{\mathrm{Al}}_{10}$ $(P4∕nmm)$ with Mn-Al3 chains running along the $c$ axis. The bottom of (b) depicts only the Mn and Al3 sublattice (three unit cells along the $c$ axis). Coordination polyhedra in $R{\mathrm{Mn}}_2{\mathrm{Al}}_{10}$ are $4mm$ for $R1$, $-42m$ for $R2$, and $m$ for Mn.

Figure 3

Temperature dependence of magnetization in zero-field- (zfc) and field-cooled (fc) forms along two crystallographic directions for (a) $\mathrm{La}{\mathrm{Mn}}_{2.26\left(1\right)}{\mathrm{Al}}_{9.74\left(1\right)}$ and (b) $\mathrm{La}{\mathrm{Mn}}_{2.41\left(3\right)}{\mathrm{Al}}_{9.59\left(3\right)}$. Insets are the low-temperature regions.

Figure 4

(Color online) For $\mathrm{La}{\mathrm{Mn}}_{2.41\left(3\right)}{\mathrm{Al}}_{9.59\left(3\right)}$, the zero field- (zfc) and field-cooled (fc) magnetization results vs temperature at chosen applied fields along the easy axis of magnetization, $M_{\perp c}$. The inset is a plot of magnetic field vs the irreversibility temperature $\left(T_{\mathrm{ir}}\right)$ found at zfc/fc separation curves.

Figure 5

Along the two crystallographic directions and for (a) $\mathrm{La}{\mathrm{Mn}}_{2.26\left(1\right)}{\mathrm{Al}}_{9.74\left(1\right)}$ and (b) $\mathrm{La}{\mathrm{Mn}}_{2.41\left(3\right)}{\mathrm{Al}}_{9.59\left(3\right)}$, the plots of inverse magnetization with fits to modified Curie-Weiss above $50\mathrm{K}$. The insets are the field-dependent magnetization results at $1.8\mathrm{K}$.

Figure 6

For $\mathrm{La}{\mathrm{Mn}}_{2.26\left(1\right)}{\mathrm{Al}}_{9.74\left(1\right)}$ and $\mathrm{La}{\mathrm{Mn}}_{2.41\left(3\right)}{\mathrm{Al}}_{9.59\left(3\right)}$, the temperature dependence of specific heat. The insets are the enlarged low-temperature region (bottom) and the data in the form of $C∕T$ vs $T^2$ (top).

Figure 7

For $\mathrm{La}{\mathrm{Mn}}_{2.26\left(1\right)}{\mathrm{Al}}_{9.74\left(1\right)}$ and $\mathrm{La}{\mathrm{Mn}}_{2.41\left(3\right)}{\mathrm{Al}}_{9.59\left(3\right)}$, (a) specific heat in the form of $C∕T$ vs $T$, along with the difference in the two data ($\Delta C∕T)$, (b) the difference in entropy between the two samples $(\Delta S∕R)$.

Figure 8

(a) For $\mathrm{La}{\mathrm{Mn}}_{2.26\left(1\right)}{\mathrm{Al}}_{9.74\left(1\right)}$ and $\mathrm{La}{\mathrm{Mn}}_{2.41\left(3\right)}{\mathrm{Al}}_{9.59\left(3\right)}$, the temperature dependence of electrical resistivity at zero applied field. The insets represent the enlarged low-temperature regions (top) and also the $\rho$ vs $\ln T$ (bottom). (b) The temperature dependence of resistivity for $\mathrm{La}{\mathrm{Mn}}_{2.26\left(1\right)}{\mathrm{Al}}_{9.74\left(1\right)}$ at $0\mathrm{Oe}$ and $50\mathrm{kOe}$ applied fields, with the inset representing the enlarged low-temperature regions.

Figure 9

Temperature dependence of magnetization in zero field- (zfc) and field-cooled (fc) forms for (a) $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and (b) $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$, along two crystallographic directions.

Figure 10

The inverse of temperature-dependent magnetization $\left(1\mathrm{kOe}\right)$ and the fits above $150\mathrm{K}$ to Curie-Weiss for (a) $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and (b) $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$.

Figure 11

Magnetization vs applied field of (a) $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and (b) $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$, along two different crystallographic axes, at $1.8\mathrm{K}$. The insets are the low field regions.

Figure 12

The Arrott plots in the form of $M^2$ vs $H_{\mathrm{eff}}∕M$ for $M_{\Vert c}$ and (a) $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and (b) $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$, where $H_{\mathrm{eff}}$ is the effective field estimates acting in each sample.

Figure 13

Temperature dependence of specific heat for $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$.

Figure 14

Temperature dependence of magnetic specific heat in the form of $C_m∕T$ and entropy change $(S∕R)$ for (a) $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and (b) $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$.

Figure 15

Temperature dependence of electrical resistivity for $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$. The inset is the enlarged low-temperature data, with shifted $\rho$ scales.

Figure 16

For (a) $\mathrm{Gd}{\mathrm{Mn}}_{2.21\left(4\right)}{\mathrm{Al}}_{9.79\left(4\right)}$ and (b) $\mathrm{Gd}{\mathrm{Mn}}_{2.39\left(2\right)}{\mathrm{Al}}_{9.61\left(2\right)}$, temperature dependence of specific heat are shown along with resistivity and tetragonal average of magnetization by $M=[M_{\Vert c}+2M_{\perp c}]∕3$. The location of Curie temperature $\left(T_C\right)$ for each is shown with a vertical arrow.