NMR investigation of spin fluctuations in the itinerant-electron magnetic compound ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$

We took ${}^{31}\text{P}$ NMR measurements of mainly paramagnetic phase ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$ ($0\le x\le 0.5$) to reveal the itinerant-electron metamagnetic transition, and of its magnetically ordered phase ($0.7\le x\le 1$), and characterized their spin fluctuations by estimating the spin fluctuation parameter $T_0$ corresponding to the width of the spin fluctuation in the spectrum in frequency space. ${\mathrm{SrCo}}_2{\mathrm{P}}_2$ has a quasi-two-dimensional uncollapsed tetragonal (ucT) cell without interlayer P-P bonds, whereas ${\mathrm{CaCo}}_2{\mathrm{P}}_2$ has a three-dimensional collapsed tetragonal (cT) cell with P-P bonds. The $ab$-in-plane component of $T_0$ is much larger than the out-of-plane component in ${\mathrm{SrCo}}_2{\mathrm{P}}_2$. As $x$ increases from 0 to 0.5, the in-plane component of $T_0$ decreases proportionally with the metamagnetic transition field. In the antiferromagnetic cT phase ($0.7\le x\le 1$), $T_0$ is constant and spin fluctuations show an isotropic character in contrast to their behavior in the paramagnetic ucT phase ($0\le x\le 0.5$). These results indicate that the in-plane spin fluctuations due to the quasi-two-dimensional crystal structure play a significant role in the metamagnetic transition of this system.

Figure 1

NMR results for static susceptibility in single crystals of ${\mathrm{SrCo}}_2{\mathrm{P}}_2$. (a) Normalized field-swept ${}^{31}\text{P}$-NMR spectra with the applied field $H\parallel a$ and $H\parallel c$ at 60.20 MHz. (b) Temperature dependence of the Knight shift and magnetic susceptibility. The red circles and blue squares distinguish ${}^{31}{K}_a$ and ${}^{31}{K}_c$, respectively. The red and blue lines mark ${\chi }_a$ and ${\chi }_c$. (c) $K$ versus $\chi$ plots for each field direction. The red and blue lines represent linear fits over ranges 4.2–200 K and 130–200 K, respectively.

Figure 2

Dynamical magnetic properties in single crystals of ${\mathrm{SrCo}}_2{\mathrm{P}}_2$. (a) Temperature dependence of $1/T_{1}T$. The open red circles, the solid blue squares, and the solid red circles represent ${(1/T_{1}T)}_a, {(1/T_{1}T)}_{\mathrm{in}}={(1/T_{1}T)}_c$, and ${(1/T_{1}T)}_{\mathrm{out}}=2{(1/T_{1}T)}_a-{(1/T_{1}T)}_c$, respectively. (b) $1/T_{1}T$ versus $K$ plots. The solid red circles and solid blue squares distinguish ${(1/T_{1}T)}_{\mathrm{in}}$ versus $K_a, {(1/T_{1}T)}_{\mathrm{out}}$ versus $K_c$ (bottom scale), and the open blue squares mark ${(1/T_{1}T)}_{\mathrm{in}}$ versus $K_a^{3/2}$ (top scale), respectively. The red, blue, and black lines represent linear fits to the data. (c) Temperature dependence of $1/T_{1}TK$ (left scale) and $1/T_{1}T{K}^{3/2}$ (right scale).

Figure 3

NMR results for the static susceptibility in single crystals of ${\mathrm{CaCo}}_2{\mathrm{P}}_2$. (a) Field-swept ${}^{31}\text{P}$-NMR spectra with the applied field $H\parallel a$ and $H\parallel c$ at 60.20 MHz. (b) Temperature dependence of the Knight shift, magnetic susceptibility ($M/H$), and the full width at half maximum of the spectra for each field direction. The solid red circles and blue squares represent ${}^{31}{K}_a$ and ${}^{31}{K}_c$, respectively. The red and blue lines represent $H/M_a$ and $H/M_c$ at 1 T. The open red circles and blue squares denote the full width at half maximum of the spectra for $H\parallel a$ and $H\parallel c$ (c) $K-\chi$ plots for each field direction. The red and blue lines represent linear fits in the range from 130–200 K.

Figure 4

Dynamical magnetic properties in single crystals of ${\mathrm{CaCo}}_2{\mathrm{P}}_2$. (a) Temperature dependence of ${(1/T_1)}_a$ and ${(1/T_{1}T)}_c$. (b) Temperature dependence of $1/T_{1}T$. The solid blue squares and the solid red circles distinguish ${(1/T_{1}T)}_{\mathrm{in}}={(1/T_{1}T)}_c$ and ${(1/T_{1}T)}_{\mathrm{out}}=2{(1/T_{1}T)}_a-{(1/T_{1}T)}_c$, respectively. (c) $1/T_{1}T$ versus $K$ plots. The solid red circles, the solid blue squares, and the open blue squares represent ${(1/T_{1}T)}_{\mathrm{in}}$ versus $K_a, {(1/T_{1}T)}_{\mathrm{out}}$ versus $K_c$, and ${(1/T_{1}T)}_{\mathrm{in}}$ versus $K_a^{1.5}$, respectively. The red, blue, and black lines denote linear fits to the data. (d) Temperature versus $1/T_{1}TK$ and $1/T_{1}T{K}^{3/2}$.

Figure 5

Temperature dependence of $M/H$ for ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$ in a field of 1 T. The inset is a magnification the plot at low susceptibility

Figure 6

Temperature dependence of the Knight shift and $1/T_{1}T$ in ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$ for various compositions. (a) Enhanced Pauli paramagnetic regime. (b) Antiferromagnetic regime. The inset in (b) shows the $x$ dependence of the size of ordered Co moment estimated from NMR results (circles), the Arrott plot using magnetization curve (squares), and $\mu \mathrm{SR}$ (triangle) in Ref. [25].

Figure 7

Spin fluctuation parameter $T_0$ of ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{\mathrm{P}}_2$. (a) $T_0$ versus Ca content $x$. (b) $T_0$ versus lattice parameter $c$. The circles and squares represent the in-plane and out-of-plane components, respectively, of $T_0$. The triangles and diamonds represent the metamagnetic transition field $H_c$ and the maximum temperature of the susceptibility $T_{\max }$ as in Ref. [13]. The shaded region represents the antiferromagnetic metal (AFM) phase with the collapsed tetragonal (cT) cell.