Magneto-optical study of magnetization reversal asymmetry in exchange bias

Applied Physics Letters, 2007

Magnetization reversal via rotation is typical in ferromagnet/antiferromagnet exchange biased systems. The reversibility of the rotation is a manifestation of the microscopic reversal process. The authors have investigated the magnetization reversal in Fe/epitaxial-FeF2 thin films using vector magnetometry and first-order reversal curves. The reversal is predominantly by rotation as the applied field makes an angle with the antiferromagnet spin axis, mostly irreversible at small angles and reversible at larger angles. A modified Stoner-Wohlfarth model reproduces the overall trend of the irreversibility evolution. The remaining discrepancies between the modeled and measured irreversibilities may be attributed to local incomplete domain walls. (

Physical Review B, 2008

We measured directly the depth-dependent Fe spin rotation upon magnetization reversal in exchangecoupled Fe/ MnF 2 bilayers using nuclear resonant scattering of synchrotron radiation from an 57 Fe-probe layer buried at different depths within the Fe film. Our results show that the exchange-biased ferromagnetic layer develops a noncollinear spin structure along the film normal direction, reminiscent of a partial domain wall parallel to the Fe/ MnF 2 interface. This is contrary to most theoretical models of exchange bias which assume a collinear spin structure in the ferromagnetic layer.

Applied Physics Letters, 2005

A detailed study of exchange-biased Fe∕MnF2 bilayers using magneto-optical Kerr effect shows that the magnetization reversal occurs almost fully through domain wall nucleation and propagation for external fields parallel to the exchange-bias direction. For finite angles ϕ between bias and external field, the magnetization is aligned perpendicular to the cooling-field direction for a limited field range for decreasing fields. For external fields perpendicular to the bias direction, the magnetization aligns with the cooling-field direction for descending and ascending fields before fully reversing. The field range for which the magnetization is close to perpendicular to the external field can be estimated using a simple effective-field model.

2004

Depth-dependent Fe spin structures of the remanent state in exchange-coupled Fe/MnF 2 films have been probed using 57 Fe conversion electron Mössbauer spectroscopy, both above and well below the MnF 2 Néel temperature. 57 Fe probe layers were embedded either at the Fe/MnF 2 interface or in the center of the Fe film. Remarkably, exchange bias induces a significant change of the in-plane angular distribution of the Fe magnetic moments at the interface and inside the Fe film, away from the saturation magnetization direction. Results from vector magnetometry support these conclusions.

Physical Review Letters, 2001

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.

Physical Review Letters, 2005

The magnetization reversal in exchange-biased ferromagnetic-antiferromagnetic (FM-AFM) bilayers is investigated. Different reversal pathways on each branch of the hysteresis loop, i.e., asymmetry, are obtained both experimentally and theoretically when the magnetic field is applied at certain angles from the anisotropy direction. The range of angles and the magnitude of this asymmetry are determined by the ratio between the FM anisotropy and the interfacial FM-AFM exchange anisotropy. The occurrence of asymmetry is linked with the appearance of irreversibility, i.e., finite coercivity, as well as with the maximum of exchange bias, increasing for larger anisotropy ratios. Our results indicate that asymmetric hysteresis loops are intrinsic to exchange-biased systems and the competition between anisotropies determines the asymmetric behavior of the magnetization reversal.

Journal of Magnetism and Magnetic Materials, 2013

We report an asymmetry of magnetic disorder in exchange-biased IrMn(t IrMn ¼ 5-20 nm)/CoFe(50 nm) films observed by means of a Kerr microscope, capable of direct domain observation. From the correlation between the magnetization half-reversal time and applied magnetic field, we find that the magnetization switching in all the films occurs via a thermally activated reversal mechanism for both branches of hysteresis loops. Surprisingly, in the forward branch reversal where the applied magnetic field is antiparallel to the direction of exchange-bias field, degree of magnetic disorder decreases as exchange-bias field increases, which is definitely contrasted with the case of backward branch reversal. This result is likely ascribed to the fact that the local values of exchange-bias field and coercive field are oppositely fluctuating with each other in the film.

arXiv (Cornell University), 2022

Magnetization reversal in ferro-and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.

Physical Review B, 2002

We present a method to determine all the components of the magnetization vector in ultrathin ferromagnetic films using magneto-optical Kerr effects of either both p-and s-polarization waves or each polarization wave. The technique is applied to an in situ study of magnetization reversal and spin-reorientation transition ͑SRT͒ in Co films grown on a Pt͑111͒ single-crystal substrate. The thickness-driven SRT from perpendicular to in-plane magnetization in Co/Pt͑111͒ occurs in the film thickness range of 10-15 ML. This transition proceeds via a stable state of the canted phase exhibiting a typical second-order behavior. The second-and fourth-order surface anisotropy constants K 2s ϭ1.8 mJ/m 2 and K 4s ϭϪ0.034 mJ/m 2 , are determined from the theoretical fit to the magnetization orientation in the canted phase. The large second-order surface anisotropy is interpreted to be responsible for the later onset of transition, while the small fourth-order surface anisotropy results in a stable canted phase during the SRT.

Applied Physics Letters, 2006

Journal of Applied Physics, 2007

We have investigated the magnetization reversal for exchange coupled polycrystalline [IrMn/CoFe]N multilayers. Polarized neutron reflectivity (PNR) data indicate a simultaneous coherent rotation of all ferromagnetic layers for a sample with N =10 and angles of 45° and 90° between the applied field and the exchange bias direction. On the other hand, magneto-optic Kerr effect (MOKE) measurements, which are sensitive mainly to the two topmost bilayers, reveal a variation of the strength of the exchange bias and the uniaxial anisotropy as a function of N for multilayers with N =1 up to 10. The MOKE data thus indicate the direction of the magnetization to vary from layer to layer for intermediate fields. PNR was found to be insensitive to this variation as the deviation of the layer magnetization directions from its mean value is relatively small (≈10°). These studies demonstrate how the complementary techniques PNR and MOKE can be used to obtain a layer-by-layer vector magnetometry of multilayer stacks.

Journal of Magnetics

Magneto-optical Kerr effect (MOKE) magnetometry was used to investigate magnetization reversal dynamics in 30-nm NiFe/15-nm FeMn, 15-nm FeMn/30-nm CoFe bilayers, and 30-nm NiFe/(2,10)-nm FeMn/30-nm CoFe trilayers. The in-plane magnetization components of each ferromagnetic layer, both parallel and perpendicular to the applied field, were separately determined by measuring the longitudinal and transverse MOKE hysteresis loops from both the front and back sides of the film for an oblique incident s-polarized beam. The magnetization of the FeMn/CoFe bilayer was reversed abruptly and symmetrically through nucleation and domain wall propagation, while that of the NiFe/FeMn bilayer was reversed asymmetrically with a dominant rotation. In the NiFe/FeMn/CoFe trilayers, the magnetic reversal of the two ferromagnetic layers proceeded via nucleation and domain wall propagation for 2-nm FeMn, but via asymmetric rotation for 10-nm FeMn. The exchange-biased ferromagnetic layers showed the magnetization reversal along the same path in the film plane for the decreasing and increasing field branches from transverse MOKE hysteresis loops, which can be qualitatively explained by the theoretical model of the exchange-biased ferromagnetic/antiferromagnetic systems.

Applied Physics Letters, 2007

Physical Review B

Magneto-optical Kerr effect, normally found in magnetic materials with nonzero magnetization such as ferromagnets and ferrimagnets, has been known for more than a century. Here, using first-principles density functional theory, we demonstrate large magneto-optical Kerr effect in high temperature noncollinear antiferromagnets Mn3X (X = Rh, Ir, or Pt), in contrast to usual wisdom. The calculated Kerr rotation angles are large, being comparable to that of transition metal magnets such as bcc Fe. The large Kerr rotation angles and ellipticities are found to originate from the lifting of the band double-degeneracy due to the absence of spatial symmetry in the Mn3X noncollinear antiferromagnets which together with the time-reversal symmetry would preserve the Kramers theorem. Our results indicate that Mn3X would provide a rare material platform for exploration of subtle magneto-optical phenomena in noncollinear magnetic materials without net magnetization.

Physical Review B, 2011

Comprehension of the origins of asymmetric behavior during the magnetization reversal is an outstanding issue in the studies of exchange bias (EB). By far, two intrinsic origins of the asymmetric magnetization reversal behavior (AMRB) have been found in different systems separately. The compatibility of the two origins has not been proved---yet. Here, we report on the AMRBs derived from cooperation