Instability of magnetism in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3(x=0.015,0.03)$: Competition between pressure and thermal cycling effects

Magnetic properties of polycrystalline ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3(x=0.015,0.03)$ samples have been investigated in the temperature range $1.5–300\mathrm{K}$, magnetic field up to $16\mathrm{kOe}$ and under hydrostatic pressures up to $11\mathrm{kbar}$. The studies involved sequential zero field cooled (ZFC) magnetization $\left(M\right)$ measurements followed by measurements during cooling in the same magnetic field $\left(H\right)$. Additional measurements of magnetization vs $H$ were carried out at ambient and applied pressure. The volume of the ferromagnetic phase for $x=0.015$ sample exhibits a considerable decrease upon thermal cycling even after many cycles. On the other hand, both the Curie temperature $\left(T_{\mathrm{C}}\right)$ and the width of thermal hysteresis remain practically unchanged during the thermal cycling. In contrary to the effect of thermal cycling, applied pressure enhances $T_{\mathrm{C}}$ and the volume of the ferromagnetic phase below $T_{\mathrm{C}}$ and reduces considerably the width of thermal hysteresis. A ferromagnetic contribution to magnetization due to short range ferromagnetic coupling was observed in the vicinity of the charge order transition temperature $T_{\mathrm{CO}}\approx 220\mathrm{K}$; it appears to enhance under pressure. In the case of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Cr}}_{0.03}{\mathrm{O}}_3$ an applied pressure also increases the magnetization and $T_{\mathrm{C}}$ with a pressure coefficient of $d{T}_{\mathrm{C}}∕dP\approx 1.8\mathrm{K}∕\mathrm{kbar}$, at the same time the hysteresis in the vicinity of $T_{\mathrm{C}}$ narrows. The results observed are related to the interfacial elastic energy at the phase boundaries of the phase separated Cr doped manganites. The effect of the suppression of the Jahn-Teller distortions and of the electron-phonon coupling in interfacial regions and in ferromagnetic domains under an applied hydrostatic pressure is also discussed.

Figure 1

(Color online) Temperature dependence of magnetization recorded in $H=14.5\mathrm{kOe}$ after zero field cooling for ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.985}{\mathrm{Cr}}_{0.015}{\mathrm{O}}_3$ and ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Cr}}_{0.03}{\mathrm{O}}_3$.

Figure 2

(Color online) Neutron diffraction patterns of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Cr}}_{0.03}{\mathrm{O}}_3$ sample at various temperatures between 1.5 and $300\mathrm{K}$; stars indicate the magnetic peaks. Inset shows an enlargement of the small angle part of the 1.5 and of the $300\mathrm{K}$ neutron diffraction patterns.

Figure 3

Temperature variation of the lattice parameters for ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Cr}}_{0.03}{\mathrm{O}}_3$ compound, the dashed area indicates the temperature range where the phase separation is important.

Figure 4

(Color online) Measurements of zero field cooled magnetization $\left(M_{\mathrm{ZFC}}\right)$ followed by magnetization measurements upon cooling in the same field $\left(M_{\mathrm{FCC}}\right)$ of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.985}{\mathrm{Cr}}_{0.015}{\mathrm{O}}_3$. Measurements of $M_{\mathrm{ZFC}}$ and $M_{\mathrm{FCC}}$ after three sequential thermal cycles $\left(300\text{−}5\text{−}300\mathrm{K}\right)$ are also presented.

Figure 5

(Color online) Measurements of zero field cooled magnetization $\left(M_{\mathrm{ZFC}}\right)$ followed by magnetization measurements upon cooling in the same field $\left(M_{\mathrm{FCC}}\right)$ of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.985}{\mathrm{Cr}}_{0.015}{\mathrm{O}}_3$ taken after the first cycle of an applied pressure $\left(0\text{−}10.5\text{−}0\mathrm{kbar}\right)$ at ambient pressure and under $10.5\mathrm{kbar}$. The measurement was carried out also after three cycles of the applied pressure. All of the measurements presented here were carried out after $>30$ thermal sequential cycles $\left(300\text{−}5\text{−}300\mathrm{K}\right)$. Inset shows a comparison of the curves of magnetization recorded at $1^{\mathrm{st}}$ and $3^{\mathrm{rd}}$ application of pressure in the vicinity of $T_{\mathrm{C}}$. The values recorded at $3^{\mathrm{rd}}$ application of pressure are magnified by factor of 1.5.

Figure 6

(Color online) Measurements of $M_{\mathrm{ZFC}}$ of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.985}{\mathrm{Cr}}_{0.015}{\mathrm{O}}_3$ followed by magnetization measurements of this sample upon cooling $\left(M_{\mathrm{FCC}}\right)$ in the same field $H=15\mathrm{kOe}$. The measurements were carried out at ambient pressure $P=0$ for a pristine state and under a pressure of $10.5\mathrm{kbar}$. The same measurements performed at the third cycle of applied pressure are also presented. The measurements under pressure were carried out after $>30$ thermal sequential cycles $\left(300\text{−}5\text{−}300\mathrm{K}\right)$.

Figure 7

(Color online) Magnetization curves $M$ vs $H$ of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.985}{\mathrm{Cr}}_{0.015}{\mathrm{O}}_3$ measured after $>30$ thermal sequential cycles: (a) at ambient pressure $P=0$ and under $P=10.6\mathrm{kbar}$ following the $1^{\mathrm{st}}$ and $2^{\mathrm{nd}}$ pressure cycle $\left(0\text{−}10.6\text{−}0\mathrm{kbar}\right)$ as well as after $12\mathrm{h}$ of released pressure; (b) at ambient pressure $(P=0)$ and at various cycles of pressure.

Figure 8

(Color online) Measurements of zero field cooled magnetization $\left(M_{\mathrm{ZFC}}\right)$ followed by magnetization measurements upon cooling in the same field $\left(M_{\mathrm{FCC}}\right)$ of ${\mathrm{Pr}}_{05}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Cr}}_{0.03}{\mathrm{O}}_3$ measured at various cycles of applied pressure $P$. The results obtained at $P=0$ after releasing pressure are also presented. Inset shows the pressure dependence of $T_{\mathrm{C}}$.

Figure 9

(Color online) Measurements of $M$ vs $H$ of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.97}{\mathrm{Cr}}_{0.03}{\mathrm{O}}_3$ at temperature $T=5\mathrm{K}$ and at various cycles of applied pressure. The $M$ vs $H$ curve measured after releasing pressure is also shown.