Intertwined magnetization and exchange bias reversals across compensation temperature in $\mathrm{YbCr}{\mathrm{O}}_3$ compound

We have investigated the origin of magnetization and exchange bias (EB) reversals in the $\mathrm{YbCr}{\mathrm{O}}_3$ compound at the magnetization compensation temperature, $T_{\mathrm{COMP}}$ of 16.5 K by dc magnetization, ac susceptibility, neutron diffraction (ND), neutron depolarization, specific heat, and dielectric measurements. A weak magnetization $\left(M_{\mathrm{Cr}}\right)$ is observed below the Néel temperature $T_{\mathrm{N}}$ (∼120 K) in dc magnetization, which is consistent with a canted antiferromagnetic (AFM) state due to ordering of ${\mathrm{Cr}}^{3+}$ moments. Specific heat data exhibit a λ-shaped peak at $T_{\mathrm{N}}$. Further, a monotonic increase of ac susceptibility at low temperatures and the Schottky anomaly in the specific heat data account for the polarized nature of the ${\mathrm{Yb}}^{3+}$ moment $\left(M_{\mathrm{Yb}}\right)$ under the molecular field of the AFM ${\mathrm{Cr}}^{3+}$ sublattice. Our ND study has confirmed a finite polarized ${\mathrm{Yb}}^{3+}$ ordered moment, and the $G_{\mathrm{z}}$-type AFM ordering of ${\mathrm{Cr}}^{3+}$ moments below $T_{\mathrm{N}}$. The temperature variations of the lattice constants $a$ and $b$ show a crossover from positive to negative thermal expansion (NTE) across the $T_{\mathrm{COMP}}$ while cooling. Below $T_{\mathrm{COMP}}$, the separation between Cr-Cr atoms lying in the $ab$ plane increases with decrease in temperature, which corroborates with the observed NTE of the lattice constants. In the present compound, the sign change of the net magnetization arising out of the AFM coupled polarized ${\mathrm{Yb}}^{3+}$ $\left(M_{\mathrm{Yb}}\right)$ and the ferromagnetic ${\mathrm{Cr}}^{3+}$ $\left(M_{\mathrm{Cr}}\right)$ sublattice moments results in the sign reversal of magnetization. Interestingly, anomalous behavior of the coercive field with its minimum value at $T\sim T_{\mathrm{COMP}}$ is observed. EB also changes sign at $T\sim T_{\mathrm{COMP}}$. Moreover, the temperature variation of the real part of the dielectric constant reveals an anomaly at $T_{\mathrm{COMP}}$, indicating a weak magnetodielectric coupling in the $\mathrm{YbCr}{\mathrm{O}}_3$ compound. A training effect analysis ensures the conventional nature of the observed EB. Neutron depolarization study sheds light on the temperature-dependent domain magnetization with its zero value at the $T_{\mathrm{COMP}}$. The presence of the observed important phenomena viz. magnetization and EB reversals in a single-phase compound suggests their possible use in making magnetization switching, spin-value, thermomagnetic, and other spintronic devices.

Figure 1

Rietveld refined x-ray diffraction pattern of $\mathrm{YbCr}{\mathrm{O}}_3$ at 300 K. All Bragg peaks have been indexed with respective (hkl) values for the Pbnm space group. The observed and calculated patterns are represented by black symbols and a solid red line, respectively. The difference pattern has been shown at the bottom by a blue line, while Bragg peaks have been shown by the vertical green lines. (b) Crystal structure of the $\mathrm{YbCr}{\mathrm{O}}_3$ compound along with $\mathrm{Cr}{\mathrm{O}}_6$ octahedra. Here, ${\mathrm{O}}_1$ is the apical oxygen along the $c$ axis, while equatorial oxygen ${\mathrm{O}}_2$ atoms lie in the $ab$ plane.

Figure 2

(a) Field-cooled cooling (FCC) $M$(T) curves recorded under various cooling magnetic fields. Inset shows the derivative of the dc magnetization curve measured under $H=50\mathrm{Oe}$. (b) The real part of the ac susceptibility $\left({\chi }_{\mathrm{ac}}\right)$ measured at 987 Hz. Inset shows the $d{\chi }_{\mathrm{ac}}/dT$ vs $T$ curve.

Figure 3

Thermal variation of transmitted neutron beam polarization ($P_f$) under 50 Oe magnetic field for $\mathrm{YbCr}{\mathrm{O}}_3$ compound.

Figure 4

(a) Rietveld refined neutron diffraction patterns at various temperatures. All Bragg peaks have been indexed with respective (hkl) values. The observed and calculated data are shown by black symbols and a solid red line, respectively. The difference of these two patterns is shown at the bottom by a blue line. The upper and lower vertical lines (green) represent the position of nuclear and magnetic Bragg peaks, respectively. The low angle magnetic and nuclear-cum-magnetic Bragg peaks are shown by the shaded area. Here, data have been recorded in the angular range of $3^{\circ }<2\theta <{130}^{\circ }$, but for clarity, only data up to $2\theta ={40}^{\circ }$ are shown. (b) Thermal variations of $G_z$ (Cr) and $F_x$ (Yb) magnetic moments.

Figure 5

Schematic representation of the magnetic structure of the $\mathrm{YbCr}{\mathrm{O}}_3$ compound as obtained from neutron diffraction study at 1.5 K.

Figure 6

Thermal variations of the (a) lattice constants, (b) unit cell volume, and (c) Cr-Cr distance via O1 (along the $c$ direction) and O2 (in the $ab$ plane), respectively. The solid lines are guides to the eye.

Figure 7

$M$ vs $T$ curve measured under 50 Oe magnetic field. Solid line is the fitted curve as per the model proposed by Cooke et al. [34]. Inset shows the variation of $M_{\mathrm{Cr}}$ and $H_{\mathrm{I}}$ as a function of applied magnetic field, where solid lines are guides to the eye.

Figure 8

(a) Measured field-cooled (FC) $M$($H$) curves under $H_{\mathrm{COOL}}=10\mathrm{kOe}$ at various temperatures. Inset shows the enlarge view of the $M$($H$) curves at low magnetic fields. (b) and (c) Nonmonotonic variation of $H_{\mathrm{C}}$ and $M_{\mathrm{R}}$ with temperature. Inset shows the enlarge views of $H_C$ and $M_R$ in low temperature range. (d) and (e) Thermal variations of $H_{\mathrm{EB}}$ and $M_{\mathrm{EB}}$. (f) $M$($H$) curves recorded at 60 K under $H_{\mathrm{COOL}}=\pm 10\mathrm{kOe}$. The measurement range was ±50 kOe. For clarity, only the data for ±20 kOe are shown.

Figure 9

Variation of $-H_{\mathrm{EB}}$ and $M_{\mathrm{EB}}$ at 60 K with $H_{\mathrm{COOL}}$. Inset shows the dependence of $-H_{\mathrm{EB}}$ on the number of hysteresis loops ($n$) recorded under $H_{\mathrm{COOL}}=10\mathrm{kOe}$ at 60 K. Square (red) symbol shows the experimental data, and solid black line is a fit to the experimental data.

Figure 10

Temperature dependence of the specific heat capacity ($C_{\mathrm{P}}$) under zero applied magnetic field. Inset shows the fitting of low temperature hump as per Eq. (6).

Figure 11

Variation of real part of dielectric constant (${\varepsilon }^{\prime }$) as a function of temperature at various frequencies. Inset shows the dielectric anomaly at $T_{\mathrm{COMP}}$.