Giant moment and ferromagnetism in dilute Pd(Mn) alloys

The magnetization $\sigma$ of a series of Pd(Mn) alloys has been measured as a function of concentration $c$ ($0.05-\mathrm{at}.\% \mathrm{Mn}\le c\le 2.45-\mathrm{at}.\% \mathrm{Mn}$), temperature ($1.4\le T\le 260$ K) and applied field ($0\le H\le 210$ kOe). The average saturation moment (for $c\le 0.49$-at% Mn) is ${\mu }_{\mathrm{sat}}=7.5\mathrm{}{\mu }_B/\mathrm{Mn}$. The saturation moment is consistent with the effective moment obtained from the low-field susceptibility (which obeys the Curie-Weiss law) provided that we take $S=\frac{5}{2}$ (from the specific heat) and $g_{\mathrm{eff}}=\frac{\mu }{S}\approx 3$. For $c\lesssim 0.23$-at.% Mn and $T\gtrsim 3.5\ominus$ the magnetization as a function of $T$ and $H$ is well described by the molecular-field model (with $S=\frac{5}{2}$ and $g_{\mathrm{eff}}\approx 3$) without any adjustable parameters. The magnetic field dependence of the specific heat yields an independent measure of the magnetic moment of Mn, which agrees with the magnetization data, and thus also leads to an enhanced effective $g$ value. For the most concentrated alloys 210 kOe is not quite sufficient to saturate the solute magnetization. This is attributed to near-neighbor (Mn-Mn) antiferromagnetic interactions. Curie temperatures ($T_C$) and Curie-Weiss temperatures ($\ominus$) were measured and compared with data from the literature and with corresponding data on Pd(Fe) and Pd(Co). Up to 1 at.% the variations of $T_C$ vs $c$ are similar for Pd(Mn), Pd(Fe), and Pd(Co), and the variations of $\ominus$ vs $c$ are also similar. A plot of ${\sigma }^2$ vs $\frac{H_i}{\sigma }$ ($H_i$ is the internal field) for Pd(1.3-at.% Mn) shows that the magnetic transition is very sharp with a width comparable to that of the specific-heat cusp (i.e., less than 1% of $T$). The spontaneous magnetization of three Pd(Mn) alloys was measured as a function of $T$ and the coefficient of the low-temperature ${\left(\frac{T}{T_C}\right)}^{\frac{3}{2}}$ term yielded a value for the spin-wave stiffness constant $D\approx 0.7T_C$ Å${}_{}{}^2{}_{}{}^{}\mathrm{K}$, in agreement with the results obtained by others from the magnetoresistance. The spontaneous magnetization and the high-field saturation behavior are used to estimate the energy of direct antiferromagnetic interactions between the neighboring Mn atoms ($\frac{E_{\mathrm{int}}}{k_B}\approx 55$ K) and the number $n$ of sites involved ($n\approx 35$). The magnetic susceptibility of Pd(1-at.% Re) = 4.30×${10}^{-6\mathrm{}}$ emu/g at 4.2 K was determined for comparison with isoelectronic Pd(Mn) alloys.