Magnetic and magnetocaloric properties of $\mathrm{HoCr}{\mathrm{O}}_3$ tuned by selective rare-earth doping

Rare-earth chromites ($R\mathrm{Cr}{\mathrm{O}}_3$) are an important subclass of functional materials with interesting magnetic, electrical, and catalytic properties that make them promising for various applications. Here, we report comparisons on the structural and magnetic properties of $\mathrm{HoCr}{\mathrm{O}}_3$ with those of $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ powder samples, which were doped by $\mathrm{T}{\mathrm{m}}^{3+}$ (or $\mathrm{G}{\mathrm{d}}^{3+}$) ions at the $A$-site with ionic radius smaller (or larger) than that of $\mathrm{H}{\mathrm{o}}^{3+}$ ion. The structural properties of the samples were characterized by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy carried out at ambient. The band gaps of the three samples were determined by analyzing their ultraviolet Vis spectra. Magnetic studies carried out from 5 K to 300 K show that the Néel temperature ($T_{\mathrm{N}}^{\mathrm{Cr}}$, where $\mathrm{C}{\mathrm{r}}^{3+}$ ions order) increases with an increase in the average $A$-site ($R$-site) ionic radius, tolerance factor, and Cr1-O1-Cr1 bond angle but decreases with an increase in the orthorhombic strain factor. In addition, the application of external hydrostatic pressure was found to enhance $T_{\mathrm{N}}^{\mathrm{Cr}}$ of $\mathrm{HoCr}{\mathrm{O}}_3$, similar to the effect observed by doping $\mathrm{HoCr}{\mathrm{O}}_3$ with $\mathrm{G}{\mathrm{d}}^{3+}$ ions. These changes in $T_{\mathrm{N}}^{\mathrm{Cr}}$ with $A$-site doping and hydrostatic pressure are related to changes in $\mathrm{C}{\mathrm{r}}^{3+}-\mathrm{C}{\mathrm{r}}^{3+}$ exchange coupling resulting from changes in the Cr1-O1-Cr1 bond angle and Cr1-O1 bond lengths, respectively. Temperature dependent paramagnetic susceptibility data of the samples was fitted to the modified Curie-Weiss law that included the Dzyloshinskii-Moriya interaction. Isothermal magnetization data showed that the magnetic behavior of the samples changes from canted antiferromagnetic at low temperatures to paramagnetic at higher temperatures. The large magnetocaloric entropy change was observed in $\mathrm{HoCr}{\mathrm{O}}_3$, which was enhanced by Gd doping but lowered by Tm doping, showing its tunability by $A$-site doping. Correlation between magnetoelectric properties and magnetization is discussed.

Figure 1

(a) The x-ray diffraction data of $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples. (b) Representative x-ray diffraction data and Rietveld refinement results of HTCO; $A$-site ionic radius ($R_{\mathrm{avg}}$) dependence of the lattice parameters $a$ (squares), $b$ (circles), and $c$ (triangles) is shown in (c) and that of the volume of the unit cell ($V$) in (d) with the lines connecting the data points as visual guides.

Figure 2

Scanning electron microscopy images of (a) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), (b) $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and (c) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples. (d) Energy-dispersive x-ray spectroscopy of the HTCO sample shows the presence of Ho, Tm, Cr, and O elements.

Figure 3

(a) Raman spectra of $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples. (b) Raman shift of phonon modes as a function of $A$-site ionic radius ($R_{\mathrm{avg}}$, in angstrom) of HTCO, HCO, and HGCO. The dashed lines are visual guides connecting the data points with the number next to each line representing the percentage decrease of the line positions with increase in $R_{\mathrm{avg}}$.

Figure 4

Optical absorption spectra of (a) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), (b) $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and (c) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples along with the respective (d)–(f) plots of ${(\alpha h\nu )}^2$ versus $h\nu$ for the samples following Eq. (1) with the extrapolated red line determining the direct energy gap.

Figure 5

(a) Temperature dependence of the magnetic susceptibility (χ) of $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) measured in $H=50\mathrm{Oe}$. (b) Plots of [$d$(χT)/dT] versus $T$ from 5 to 200 K to determine $T_{\mathrm{N}}^{\mathrm{Cr}},$ the ordering temperature of $\mathrm{C}{\mathrm{r}}^{3+}$ moments.

Figure 6

The dependence of Néel temperature ($T_{\mathrm{N}}^{\mathrm{Cr}}$) on (a) average radii of $A$-site ions ($R_{\mathrm{avg}}$), (b) tolerance factor ($t$), (c) orthorhombic strain (S), and (d) the in-plane Cr1-O1-Cr1 bond angle. The black triangles represent the data of the present $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3, \mathrm{HoCr}{\mathrm{O}}_3$, and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ samples, and the red squares represent the data of $\mathrm{GdCr}{\mathrm{O}}_3, \mathrm{NdCr}{\mathrm{O}}_3$, and $\mathrm{LaCr}{\mathrm{O}}_3$ taken from the literature [36, 40].

Figure 7

(a) The temperature dependence of mutual inductance of $\mathrm{HoCr}{\mathrm{O}}_3$ pellet sample under different hydrostatic pressure (0–16.5 kbar); (b) Néel temperature ($T_{\mathrm{N}}^{\mathrm{Cr}}$) of $\mathrm{HoCr}{\mathrm{O}}_3$ sample as a function of the hydrostatic pressure. The line joining the points in (b) is drawn for visual clarity.

Figure 8

Temperature dependence of inverse susceptibility (1/χ) of (a) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), (b) $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and (c) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples, with the black squares as data points and the red solid line fits to the Curie-Weiss law. In (d)–(f), the red lines are the corresponding fits to the modified Curie-Weiss law of Eq. (4), which includes the DM interaction.

Figure 9

Isothermal magnetization ($M$) data versus applied magnetic field (in first quadrant) at select temperatures between 5 K and 145 K for (a) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), (b) $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and (c) $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples. The lines joining the data points are visual guides.

Figure 10

The temperature dependence of (a) entropy change ($\mathrm{\Delta }S$) and (b) refrigeration capacity ($RC$) of $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO) sample in magnetic fields up to 7T (70 kOe).

Figure 11

(a) The maximum of magnetic entropy change $(-\mathrm{\Delta }S)$, (b) refrigeration capacity ($RC$), and (c) adiabatic temperature change $\left(\mathrm{\Delta }{T}_{\mathrm{ad}}\right)$ as a function of magnetic field for $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{T}{\mathrm{m}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HTCO), $\mathrm{HoCr}{\mathrm{O}}_3$ (HCO), and $\mathrm{H}{\mathrm{o}}_{0.67}\mathrm{G}{\mathrm{d}}_{0.33}\mathrm{Cr}{\mathrm{O}}_3$ (HGCO) samples. The lines joining the data points are visual guides.