Unusual rotating magnetocaloric effect in the hexagonal $\mathrm{ErMn}{\mathrm{O}}_3$ single crystal

It is known that orthorhombic $R\mathrm{Mn}{\mathrm{O}}_3$ multiferroics $\left(R=\mathrm{magnetic}\mathrm{rare}\mathrm{earth}\right)$ with low symmetry exhibit a large rotating magnetocaloric effect because of their strong magnetocrystalline anisotropy. In this paper, we demonstrate that the hexagonal $\mathrm{ErMn}{\mathrm{O}}_3$ single crystals also unveil a giant rotating magnetocaloric effect that can be obtained by spinning them in constant magnetic fields around their $a$ or $b$ axes. When the $\mathrm{ErMn}{\mathrm{O}}_3$ crystal is rotated with the magnetic field initially parallel to the $c$ axis, the resulting entropy change reaches maximum values of 7, 17, and 20 J/kg K under 2, 5, and 7 T, respectively. These values are comparable to or even larger than those shown by some of the best orthorhombic phases. More interestingly, the generated anisotropic thermal effect is about three times larger than that exhibited by the hexagonal $\mathrm{HoMn}{\mathrm{O}}_3$ single crystal. The enhancement of the rotating magnetocaloric effect in the hexagonal $\mathrm{ErMn}{\mathrm{O}}_3$ compound arises from the unique features of the $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic sublattice. In fact, the $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic moments located at $2a$ sites experience a first-order metamagnetic transition close to 3 K along the $c$ axis resulting in a peaked magnetocaloric effect over a narrower temperature range. In contrast, the “paramagnetic” behavior of $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic moments within the $ab$ plane produces a larger magnetocaloric effect over a wider temperature range. Therefore, the magnetocaloric effect anisotropy is maximized between the $c$ and the $ab$ directions, leading to a giant rotating magnetocaloric effect.

Figure 1

Left: Micro-Raman spectra at 10 K for $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$. Those of $\mathrm{YMn}{\mathrm{O}}_3$ considered as a reference for hexagonal $R\mathrm{Mn}{\mathrm{O}}_3$ phases are also reported for comparison. The backscattering and light polarization configurations $b\left(cc\right)b$ and $c\left(ab\right)c$ allow the observation of $A_1$ and $E_2$ phonon symmetries, respectively. Right: Crystallographic structure of $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$. $R_1$ and $R_2$ correspond to $2a$ and $4b$ sites, respectively.

Figure 2

(a) Temperature dependence of zero-field cooled and field cooled magnetizations of $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ under a magnetic field of 0.1 T along the $ab$ plane and the $c$ axis. (b) Thermomagnetic curves of $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ under magnetic fields from 0.2 to 4 T $\left(H//c\right)$.

Figure 3

(a) Isothermal magnetization curves of the single crystal $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ at 2 K along the $ab$ plane and the $c$ axis. (b) Collected magnetic isotherms along the $c$ axis at very low temperatures and, under low magnetic fields.

Figure 4

Isothermal magnetization curves of the single crystal $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ for (a) $H//ab$, (b) $H//c$, and (c) $H//c$.

Figure 5

Temperature dependence of the isothermal entropy change in $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ under different magnetic fields for (a) $H//ab$ and (b) $H//c$.

Figure 6

(a) Temperature dependence of the critical field $B_C$ (along the $c$ axis) corresponding to the metamagnetic transition taking place in the single crystal $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$. (b) The associated entropy change deduced from the Clausius-Clapeyron equation.

Figure 7

Temperature dependence of the Raman-active phonons $E_2$(5) (a) and $A_1$(9) (b) for both $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ and $\mathrm{YMn}{\mathrm{O}}_3$ single crystals. Right: Description of ion relative motions.

Figure 8

(a) Comparison between the $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ entropy changes related to $H//ab$ and $H//c$ for 7 T. (b) Adiabatic temperature change of $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ as a function of magnetic field at 19 K applied within the $ab$ plane. (c, d) Arrott plots of $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ close to the ordering point of $\mathrm{E}{\mathrm{r}}^{3+}$ magnetic moments along the $c$ axis and the $ab$ plane, respectively.

Figure 9

(a) Entropy changes related to the rotation of $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ from the $c$ to the $a$ (or $b)$ direction by 90° in different constant magnetic fields, with magnetic field initially parallel to the $c$ axis. (b) Associated adiabatic temperature change as a function of magnetic field at 19 K.

Figure 10

(a) Temperature dependence of the rotating entropy change in the hexagonal $\mathrm{ErMn}{\mathrm{O}}_3$ and $\mathrm{HoMn}{\mathrm{O}}_3$ [16] single crystals for 7 T. (b)The calculated rotating entropy change from the coherent rotational model for $\mathrm{h}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$. The magnetic field is initially parallel to the $c$ axis. β is the angle between the easy direction $(a$ or $b$ axes) and the magnetic field.