Exchange Coupling in the Paramagnetic State

J. W. Cai, Kai Liu, and C. L. Chien, The Johns Hopkins University, Baltimore, MD 21218

I. Introduction

When a ferromagnet (FM)/ antiferromagnet (AF) bilayer, with the Curie temperature (T_C) of the FM higher than the Néel temperature (T_N) of the AF, has been field-cooled across T_N, an exchange bias is set in. The resultant hysteresis loop of the FM is now shifted by an amount termed the exchange field (H_E), accompanied by an enhanced coercivity (H_C). In the cases thus far reported, T_C has always been much higher than T_N. During field cooling across T_N, the FM layer is in the single-domain state while the exchange coupling is being locked in. It has been generally accepted that T_C > T_N is a prerequisite for establishing FM/AF exchange coupling. In this work, we have studied an FM/AF bilayer of a-Fe₄Ni₇₆B₂₀ (T_C ~ 150 K) and CoO (T_N = 291 K) with T_C much lower than T_N, a hitherto unexplored regime where the FM ordering is absent when the exchange coupling is being established. We have observed exchange coupling in this system, which persists well into the paramagnetic (PM) state (T >T_C).

II. Results

The hysteresis loop of a single 300 Å a-Fe₄Ni₇₆B₂₀ layer at 80 K is shown in Fig. 1a, exhibiting a square loop with a small coercivity of only 0.4 Oe, which are characteristics of a soft FM. However, a bilayer of a-Fe₄Ni₇₆B₂₀(300 Å)/CoO (250 Å), field-cooled in a field of 10 kOe to 80 K, shows a shifted hysteresis loop with large values of H_E and H_C, which are clear signatures of exchange coupling. The hysteresis loops measured at successively higher temperature from 80 K to 290 K are shown in Fig. 1c — 1h. At higher temperatures, the coercivity progressively decreases and vanishes near T_C. Most strikingly, the collapsed loop at T > T_C continues to be shifted with an exchange field H_E, which first increases to a maximum before decreasing progressively to zero at 290 K, the T_N of CoO. Thus, we not only have observed exchange coupling at T < T_C in a bilayer where T_C is much less than T_N, but also at T > T_C, when the FM layer is in the PM state.

Figure 1: Hysteresis loops of a single layer a-Fe₄Ni₇₆B₂₀ at 80 K (a) and a bilayer of a-Fe₄Ni₇₆B₂₀(300 Å)/CoO (250 Å) at 80 K after zero-field cooling to 80 K (b), after field cooling in 10 kOe to 80 K and measured at 80 K (c), 120 K (d), 150 K (e), 160 K (f), 220 K (g), and 290 K (h). The temperature dependence of H_E and H_C, obtained from the hysteresis loops shown in Fig. 1c-1h, are presented in Fig. 2. A number of striking features are evident. First of all, H_E and the enhanced H_C do not both vanish at T_N, completely different from what has been universally observed in bilayers with T_C > T_N. Instead, while H_E vanishes at T_N, H_C vanishes at a lower temperature near T_C. This indicates vividly that in exchange-coupled FM/AF bilayers, the exchange field is dictated by the AF ordering, but the coercivity, although significantly enhanced by the exchange coupling, is intrinsic to the FM ordering. Most importantly, the collapsed loop continues to be shifted from H = 0 at T > T_C, i.e., the exchange coupling persists when the FM layer is already in the PM state. It is noted in Fig. 2 that, while H_C decreases monotonically with temperature and reaches the terminal value at T_C, H_E shows a sharp rise near T_C before decreasing towards zero at T_N.

Finally, the realization of exchange coupling in bilayers with T_C<< T_N also has important implication in technological application of exchange coupling in spin-valve devices. For FM/AF bilayers with optimized performance, one can broaden the search to a greater variety of FM and AF materials to realize suitable values of H_E and H_C near room temperature without regard to the condition of T_C > T_N.

Figure 2: Temperature dependence of exchange field H_E and coercivity H_C of a-Fe₄Ni₇₆B₂₀(300 Å)/CoO (250 Å) after field cooling in 10 kOe to 80 K.

III. Summary

Contrary to the common perception of T_C > T_N as a prerequisite for exchange coupling between a FM and an AF layers, we have demonstrated exchange coupling where T_C << T_N. The exchange coupling exists not only in T < T_C, but also in T_C < T < T_N, where the bulk of the FM layer is in the PM state. With increasing temperature, H_C vanishes at T_C, whereas H_E persists to T_N. The results show that the exchange coupling can be established in FM/AF bilayers regardless of the relative values of T_C and T_N.

Reference

J. W. Cai, Kai Liu, and C. L. Chien, Phys. Rev. B 60, 72 (1999).