$4f$ spin driven ferroelectric-ferromagnetic multiferroicity in ${\mathrm{PrMn}}_2{\mathrm{O}}_5$ under a magnetic field

In contrast to all other members of the $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ family with nonzero $4f$ electrons ($R$ = Nd to Lu), ${\mathrm{PrMn}}_2{\mathrm{O}}_5$ does not show any spin driven ferroelectricity in the magnetically ordered phase. By means of high-field electric polarization measurements up to 45 T, we have found that this exceptional candidate undergoes a spin driven multiferroic phase under magnetic field. X-ray magnetic circular dichroism studies up to 30 T at the Pr $L_2$ edge show that this ferroelectricity originates from and directly couples to the ferromagnetic component of the ${\mathrm{Pr}}^{3+}$ spins. Experimental observations along with our generalized gradient-approximation $+U$ calculations reveal that this exotic ferroelectric-ferromagnetic combination stabilizes through the exchange-striction mechanism solely driven by a $3d-4f$-type coupling, as opposed to the other $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ members with $3d-3d$ driven ferroelectric-antiferromagnetic-type conventional type-II multiferroicity.

Figure 1

The top panel shows the magnetic field $H$ dependence of the pyrocurrent $I$ up to 45 T at different temperatures using the $H\parallel I\parallel b$ configuration. The inset shows a schematic diagram of the pyroelectric technique used to record the field dependence of the pyrocurrent applying pulsed magnetic fields. The bottom panel depicts electric polarization $P$ along the $b$ axis as a function of magnetic field.

Figure 2

Field dependence $H$ of the magnetization $M$ up to 42 T at 1.5 K with $H\parallel b$.

Figure 3

Time profile of the magnetic field pulse used for the x-ray magnetic circular dichroism measurements.

Figure 4

(a) X-ray absorption spectrum (XAS) and (b) x-ray magnetic circular dichroism (XMCD) spectra at the Pr $L_2$ edge for ${\mathrm{PrMn}}_2{\mathrm{O}}_5$ with magnetic field applied parallel to the $b$ axis at $T$ = 2 K. Data sets for different magnetic fields are offset along the vertical axis for clarity.

Figure 5

Field dependence of the XMCD integrated intensity over the entire Pr $L_2$ edge range at 2 K compared with the field dependence of the electric polarization along the $b$ axis at $T$ = 1.5 K shown in Fig. 4. The dotted line is a guide to the eyes.

Figure 6

Theoretically calculated partial density of states for Pr $f$ and Mn $d$ in the magnetic ground state.

Figure 7

(a) Schematic magnetic structure at zero field (magnetic space group $P_{a}b{2}_{1}a$) [30]. (b) Proposed schematic magnetic structure at high field in the ferroelectric phase (magnetic space group $P{b}^{\prime }{2}_1{m}^{\prime }$). The amplitudes of the arrows showing the magnetic moments are not to the scale for the sake of clarity. ${\mathrm{Mn}}^{3+}$, ${\mathrm{Mn}}^{4+}$, and ${\mathrm{Pr}}^{3+}$ ions are represented in blue, green, and yellow, respectively.