Modification of magnetic anisotropy through $3d-4f$ coupling in La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$

Magnetic behavior of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ was investigated by a combination of magnetic measurements, magneto-optical imaging, neutron diffraction, and x-ray absorption spectroscopy, including x-ray magnetic circular dichroism. The material crystallizes in the ThCr${}_2$Si${}_2$ structure type and exhibits three consecutive magnetic phase transitions. At 167 K, the Co magnetic moments order ferromagnetically in the $ab$ plane of the tetragonal crystal structure. At 66 K, a ferromagnetic ordering of Pr(4$f$) moments parallel to the $c$ axis causes a rotation of the Co(3$d$) moments towards the $c$ axis in the direction opposite to the Pr moments, thus forming a noncollinear ferrimagnetically ordered structure and switching the direction of the total magnetization from the $ab$ plane to the $c$ axis. The third magnetic transition observed at 35 K is likely associated with the establishment of the collinear ferrimagnetic order along the $c$ axis.

Figure 1

(a) Crystal structure of $A{T}_2{X}_2$ that belongs to the ThCr${}_2$Si${}_2$ structure type. (b) Top view of the square planar net of $T$ atoms, with $X$ atoms positioned above and below the net in a chessboardlike fashion.

Figure 2

Temperature dependence of unit cell parameters $a$ and $c$ in the crystal structure of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$.

Figure 3

(a) Temperature dependence of magnetization measured on a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ in the field-cooled mode, with the magnetic field of 0.01 T applied parallel (red curve) and perpendicular (black curve) to the $c$ axis. The inset shows the surface of the single crystal viewed along the $c$ axis. This crystal was used for MOI. (b) Temperature dependences of the magnetic component of the heat capacity and the first derivative of electrical resistivity for La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$.

Figure 4

Isothermal field dependences of magnetization for a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ measured with the magnetic field parallel (a) and perpendicular (b) to the $c$ axis.

Figure 5

Magnetic hysteresis loops measured on a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ at 100 K (a) and 5 K (b) with the magnetic field applied parallel (red curves) and perpendicular (black curves) to the $c$ axis.

Figure 6

Magneto-optical images taken on a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ oriented with the $c$ axis along the optical axis of the microscope. Temperature-dependent patterns were recorded upon cooling from room temperature in a zero magnetic field (a–d) or in a parallel applied field of 0.18 T (e). (f) Pattern obtained after applying a parallel ac magnetic field of 0.010 T at a frequency of 100 Hz and a temperature of 45 K.

Figure 7

The 5-K XANES spectra (a) and variable-temperature XMCD spectra (b) measured at Co-$L_{2,3}$ and Pr-$M_{4,5}$ edges on a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ with a 1-T magnetic field applied along the $c$ axis.

Figure 8

Field dependences of the integrated intensities of Co-$L_3$ and Pr-$M_4$ XMCD spectra measured on a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ at 5 K (left) and 100 K (right), with the magnetic field direction parallel to the $c$ axis (a) and inclined 70° or 60° to the $c$ axis (b).

Figure 9

Temperature dependence of normalized intensity of NPD magnetic peaks (002) and (101) of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$. Solid lines are visual guides.

Figure 10

Rietveld refinement of the NPD pattern of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ collected at 4 K ($R_{\mathrm{F}}$ $=$ 0.0675, ${\chi }^2$ $=$ 1.85).

Figure 11

The typical experimental setup in the scattering experiments on the HB-2A neutron powder diffractometer.

Figure 12

Fragments of variable-temperature NPD patterns collected on loosely packed powder of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ in an applied magnetic field of 0.25 T. Note the crossover point at 59 K (highlighted in red).

Figure 13

Isofield temperature dependence of magnetization measured on a single crystal of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ at various magnetic fields applied parallel to the $c$ axis in the field-cooled regime.

Figure 14

Proposed magnetic structures of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ in different temperature ranges between the critical points of magnetic phase transitions.