Two-Stage Magnetization Reversal in Exchange Biased Bilayers

Abstract

MnF 2 ͞Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops. DOI: 10.1103/PhysRevLett.86.4394 PACS numbers: 75.70.Cn, 75.30.Gw Despite recently renewed interest in the phenomenon of exchange anisotropy [1] at the interface between antiferromagnets (AF) and ferromagnets (F), a full understanding is elusive [2]. A complete description of exchange anisotropy would incorporate an explanation of the mechanism for exchange biasing [2][3][4][5][6][7][8], the effects of interface disorder [9][10][11], the relationship between the exchange bias ͑H E ͒ and the coercivity ͑H C ͒ [6,12,13], as well as an understanding of the magnetization reversal mechanisms [14][15][16]. In an attempt to address these issues, we have focused on a model thin film system TMF 2 ͞Fe (TM transition metal), with which we have been able to elucidate perpendicular coupling [17], effects of compensation [18] and interfacial disorder [9,10], and the relationship between H E and H C [12].A recent thrust has been to understand the mechanisms by which the magnetization reverses in such systems [15]. This is a fundamental point, clearly related to the behavior of H C (zero moment half-width of the hysteresis loop), which is considered important [6]. Although there are few more fundamental questions about the nature of a hysteresis loop than the issue of the magnetization reversal mechanism, it has largely escaped investigation in exchange biased systems. Recent papers [14,15] have contributed to the realization that the magnetization reversal is intrinsically asymmetric under certain conditions. This naturally explains the often observed [1,[18][19][20] [22]. In summary, it is clear that reversal asymmetry in exchange biased systems is a general phenomenon requiring immediate attention.In the MnF 2 ͞Fe and FeF 2 ͞Fe systems, we have shown that for certain cooling field orientations the magnetization reverses by coherent rotation on the high field side of the loop (i.e., where the coercive field is jH E j 1 H C ) and by domain wall nucleation and propagation on the low field side of the loop (i.e., where the coercive field is H C 2 jH E j), a very clear asymmetry [15]. The magnetization hysteresis loops show very slight shape asymmetry, which must be associated with this effect [23]. The asymmetry is easily understood on consideration of the twinned nature of the AF films (Fig. 1). When the cooling field bisects the twin anisotropy axes (as shown), the frustration of the coupling between the individual AF twins and the ferromagnetic overlayer leads to an effective "45 ± coupling," r...

Domain structures during magnetization reversal in exchange-biased layers

“…This is in most cases contrary to the experimental observation of magnetic domains in such layers, 4 -6 although for some cases, a coherent magnetization rotation has been also reported. 7 Measurements of the exchange bias and coercive field as a function of the applied field angle 8,9 are very informative and provide a good test for the validity of existing theoretical models. 10 This article presents a study of the domain patterns during the magnetization reversal in exchange-biased Co/NiO bilayers, for different angles between the applied magnetic field and the exchange biasing axis.…”

Magneto-optical study of magnetization reversal asymmetry in exchange bias

“…The most prominent example of an asymmetric reversal occurs in Fe/ MnF 2 , which exhibits a pronounced step on only one side of the hysteresis loop. 3 Polarized neutron reflectometry ͑PNR͒ showed that this step is related to coherent rotation of the magnetization, 3,4 which is pinned in a potential minimum transverse to the applied magnetic field. As PNR is not sensitive to the direction of the transverse moment it cannot determine the chirality of the magnetization vector upon magnetization reversal.…”

“…Details of the sample preparation and structural characterization are given elsewhere. 3 MOKE measurements were performed with an in-plane magnetic field H ext oriented horizontally ͓Fig. 1͑a͔͒ and at 45°with respect to the ͓001͔ direction of the AFM twins.…”

“…This is crucial to observe the asymmetry in the hysteresis loop. 3 Kerr loops were taken in two separate configurations ͓Fig. 1͑a͔͒.…”

Neutron scattering studies of nanomagnetism and artificially structured materials

“…To date, neutron scattering has been used extensively to investigate exchange-biased systems. These measurements provide insight into the magnetization reversal processes of the ferromagnet [38,296] and the spin structure of the antiferromagnet [31,39,227]. Neutron diffraction has also been used to identify and characterize new antiferromagnetic materials, such as MnN [30,297] and MnPd [40] that may exhibit better performance in spin-valve structures.…”