Generation of 360° domain walls in magnetic tunnel junctions

F. Montaigne , M. Hehn¹, N. Rougemaille², S. El Moussaoui³, F. Maccherozzi³, R. Belkhou3, D. Lacour¹

¹Laboratoire de Physique des Matériaux, Nancy-Université, CNRS, Boulevard des Aiguillettes, B.P. 239, F-54506 Vandœuvre lès Nancy, France
²Institut Néel, CNRS and Université Joseph Fourier, 25 Rue des Martyrs, B.P. 166, F-38042 Grenoble Cedex 9, France
³Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France and Synchrotron ELETTRA, AREA Science Park, 34012 Basovizza, Trieste, Italy

Magnetic tunnel junctions (MTJ) are probably one of the most studied devices in the so called spintronics research field. Their active part is composed of two ferromagnetic electrodes separated by an insulating layer thinner than 3 nm that a tunnel current exists between the two electrodes. The most attractive property of MTJs concerns their strong electrical resistance variation with the magnetic configuration of the electrodes. The resistance can vary from more than 100%. The Magnetic Random Access Memories (MRAM) and the read heads of hard drive disks take benefits from this property. Mainly to increase storage capacity, smaller and smaller MTJs have to be realized. Size effect starts then to play an important role and can affect drastically the electrodes magnetic properties. Combining high resolution magnetic imaging at ELETTRA and micromagnetic simulations, we have demonstrated the strong influence of a dipolar magnetic coupling on the magnetization reversal of MTJs [1]. Understanding the domain wall formation process is crucial since it affects the magnetic - electrical properties of MTJs.

Figure 2: XMCD-PEEM images of 4 ellipses recorded at the Co and at the Ni edges. The photons incidence direction is aligned along the ellipses long axis from the top left corner. The white and black contrasts correspond to magnetization components aligned along the long ellipses axis. XMCD-PEEM images of 2 ellipses (1 µm x 3 µm) measured at the Ni edge. The gray level distribution corresponds to the scalar projection of the local magnetization with respect to the light incidence: (b) parallel and (c) perpendicular to the ellipses long axis.

Figure 1: Schematic representation of our MTJs. The two magnetic electrodes, composed of a nickel/iron alloy and of cobalt, are respectively represented in green and purple. They are separated by 2 nm thick layer of oxidized aluminium (not represented for clarity reason). The magnetic stray field sketched in red tends to align the magnetization each electrode in an antiparallel configuration.   When the MTJs are structured in micrometer sized elements, an antiferromagnetic coupling tends to align the magnetization of each electrode in an antiparallel configuration. This coupling originates from the magnetic stray field at the nanostructures edges (see red arrows in fig. 1). We have studied the case of MTJs patterned in ellipses with the following stack: Ta(5nm)/Co(4nm)/Al2O3(2nm)/Fe20Ni80(4nm)/Ru(2nm). We have used the high spatial resolution of the X-ray Photoemission Electron Microscopy (X-PEEM) combined to X-ray Magnetic Circular Dichroism (XMCD) to image the magnetic configuration of our MTJs. This powerful technique allows to image independently the magnetic configuration in each electrodes if composed of different elements. Fig 2.(a) shows that, when no magnetic field is applied, both layer magnetizations are mainly in an antiparallel configuration. Moreover, if the Co layer magnetization is uniform, surprisingly this is not always the case in the NiFe layer. The (b) and (c) parts of fig. 2 present images obtained in geometries where the technique is sensitive to the magnetization component either along the ellipses long axis (b) or along the ellipses short axis (c). The combination of this imaging technique with in-silico modelling (right part of the fig. 2b and 2c) allows to understand the nature and the formation process of the non uniform magnetization distribution presents in the NiFe layer. In this region, the magnetization rotates continuously by 360°, forming an object separating two regions of uniform magnetization. This magnetic object is called a 360° domain wall. By simulations, only one chirality is obtained for the wall, although three different cases can be observed experimentally: no wall and two chiralities (fig. 2). This difference has been attributed to local magnetic anisotropy fluctuations at the ellipses extremities which drive the magnetization curling direction during the reversal process.     Reference: M. Hehn et al. Applied Physics Letters 92, 072501 (2008).