Common exchange-biased spin switching mechanism in orthoferrites

The unconventional exchange-bias (EB) effect in single crystals of $R\mathrm{Fe}{\mathrm{O}}_3\left(R=\mathrm{Nd},\mathrm{Sm}\right)$ compensated ferrimagnets (fMs), composed of two antiferromagnetically (AFM) coupled $R$ and Fe sublattices with opposite ferromagnetic (FM) moments, exhibiting the FM moment reversal via a fast spin switching is reported. In ${\mathrm{NdFeO}}_3$, the EB anisotropy field emerges and diverges upon approaching the compensation temperature $T_{\mathrm{comp}}^{\mathrm{Nd}}=9.2\mathrm{K}$, and changes sign with crossing $T_{\mathrm{comp}}^{\mathrm{Nd}}$, in similarity to the behavior observed recently in ${\mathrm{ErFeO}}_3$. In contrast, ${\mathrm{SmFeO}}_3$ exhibits a substantial EB, not only at its $T_{\mathrm{comp}}^{\mathrm{Sm}}=4.8\mathrm{K}$, but also at higher temperatures up to 100 K, and the EB changes its sign with increasing cooling field. It is shown that in all known $R{\mathrm{FeO}}_3$ ($R=\mathrm{Nd},\mathrm{Sm},\mathrm{Er}$) compensated fMs the field-induced FM moment reversal is similarly exchange biased near $T_{\mathrm{comp}}$.

Figure 1

(a) Magnetization hysteresis loops of ${\mathrm{NdFeO}}_3$ and ${\mathrm{SmFeO}}_3$ single crystals, measured along both the $c$ and $a$ axes at 300 K. (b) Temperature dependence of magnetization of ${\mathrm{SmFeO}}_3$ single crystal measured along the $a$ axis upon cooling (FC) at 100 Oe and on warming (FW) at several applied fields. The inset shows the sign change in magnetization at the compensation point $T_{\mathrm{comp}}^{\mathrm{Sm}}=4.8\mathrm{K}$. (c) Magnetization of ${\mathrm{NdFeO}}_3$ measured with FC mode along both the $a$ and $c$ axes at 100 Oe, and with FW mode at $H=2\mathrm{Oe}$ after cooling at 10 kOe. The shadow marks the area of the Fe spin reorientation. The inset shows the sign change in magnetization at $T_{\mathrm{comp}}^{\mathrm{Nd}}=9.2\mathrm{K}$. (d) ZFC magnetization of ${\mathrm{NdFeO}}_3$ measured along the $a$ axis on warming at several applied fields. The possible spin configurations below and above the spin switching temperature $T_{\mathrm{ssw}}$ are shown. The inset presents the $T_{\mathrm{ssw}}$ vs $H$ dependence in both Nd and Sm compounds.

Figure 2

Magnetization hysteresis loops of ${\mathrm{NdFeO}}_3$, in an extended scale, measured between ±15 kOe and with cooling field 15 kOe along the $a$ axis at temperatures $T>T_{\mathrm{comp}}=9.2\mathrm{K}$ (a, b, c) and at $T<T_{\mathrm{comp}}$ (d, e, f). The dashed lines show the AFM contribution, and the solid lines present the FM contribution to the loops, calculated by extracting the AFM contribution from the original $M$($H$) loops. Arrows indicate the fields of the first and the second magnetization reversals, $H_1$ and $H_2$, respectively, and EB fields, $H_{\mathrm{EB}}$.

Figure 3

(a) Magnetization hysteresis loops of ${\mathrm{SmFeO}}_3$, measured in ZFC mode along the $a$ axis at various temperatures. (b) Cooling-field dependence of EB field $H_{\mathrm{EB}}$ at 10 and 50 K. Hysteresis loops measured along the $a$ axis in both ZFC and FC ($H_{\mathrm{cool}}=10\mathrm{kOe}$) modes at (c) 50 K, (d) 10 K, and (e) 5.5 K.

Figure 4

Temperature dependences of exchange-bias field $H_{\mathrm{EB}}$ (a, c) and average coercive field $H_{\mathrm{C}}$ (b, d) of Nd, Sm, and Er orthoferrites. (Data for ${\mathrm{ErFeO}}_3$ are taken from Ref. [9].) Remarkably, the EB increases on approaching the compensation point $T_{\mathrm{comp}}$ and changes sign across $T_{\mathrm{comp}}$. The inset of (c) shows positive and negative $H_{\mathrm{EB}}$ of ${\mathrm{SmFeO}}_3$ corresponding to the FC and ZFC modes, respectively, between 6 and 100 K.

Figure 5

Temperature dependence of net FM moment $M_{\mathrm{FM}}$ derived from the $M$($H$) loops for Nd, Sm, and Er orthoferrites. The inset shows the $M_{\mathrm{FM}}$ as a function of reduced temperature $T/T_{\mathrm{comp}}$. The bold lines are the best fit with Eq. (1).