Characteristic temperatures of exchange biased systems

Abstract

Characteristic temperatures in ferromagnetic -antiferromagnetic exchange biased systems are analyzed. In addition to usual blocking temperature of exchange bias TB, and the Néel temperature of an antiferromagnet TN , the inducing temperature T ind , i.e., the temperature, at which the direction of exchange anisotropy is established, has been recently proposed. We demonstrate that this temperature is in general case different from TB and TN . Physics and experimental approaches to measure the inducing temperature are discussed. Measurements of T ind , in addition to TB, and TN , provide important information about exchange interactions in ferromagnetic -antiferromagnetic heterostructures.PACS numbers: 75.30. Gw, 75.70.Cn, 75.30.Et, 75.50.Ee Exchange anisotropy appears in hybrid ferromagnetic (F) -antiferromagnetic (AF) systems due to exchange interactions at the F-AF interface [1]. The interfacial exchange creates an additional energy barrier, which F magnetic moments have to overcome during the magnetization reversal. The exchange anisotropy is unidirectional and shows up as a horizontal shift of the magnetic hysteresis loop after field cooling, and the exchange bias field is determined as the value to which the center of the hysteresis loop is shifted with respect to the zero field [2]. This assumes that the AF structure stays stable, which is valid unless the total AF magnetocrystalline anisotropy is too low, when AF spins rotate coherently with F spins, and the exchange bias vanishes [3,4,5]. The loop is normally shifted in the direction opposite to the cooling field, which indicates that the interfacial exchange coupling is ferromagnetic, i.e., it favors parallel orientation of the interfacial F and AF spins. The case of "positive" loop shifts, which may assume antiferromagnetic coupling at the F-AF interface (favoring antiparallel alignment of the interfacial F and AF spins), was described by Nogués et al. [6,7].It is a "common knowledge" that the exchange anisotropy is established when field cooling a F-AF system through the Néel temperature T N of the antiferromagnet [8,9]. The blocking temperature of exchange bias T B is the temperature, at which exchange bias disappears. It has been recently demonstrated that the direction of exchange anisotropy can be established at a temperature larger than T B , which is determined as exchange bias inducing temperature T ind [10].The procedure of measuring T ind is as follows. At first the sample is field cooled in a "negative" field −H F C from temperature T M (T M > T N ), to a certain temperature T switch , where the sign of the cooling field is changing. The further cooling to the temperature T m * Electronic address: dobrynin@ameslab. gov FIG. 1: Fig. 1. Exchange bias field H eb , measured at temperature Tm as a function of temperature T switch , at which the direction of the cooling field is changed from −HF C to +HF C . The temperature is scanning from TM down to Tm. In case of T ind < T switch < TM the exchange bias is negative (−Hm), since it's induced ...

Stabilization of Antiferromagnetic Order in FeO Nanolayers

“…In exchange bias systems, one usually distinguishes the Néel (ordering) temperature T N of the antiferromagnet and the blocking temperature T B below which exchange bias occurs [7]. For thick AFM layers (> 50 nm), T N and T B are usually found to be equivalent.…”