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Direct Imaging of Asymmetric Magnetization Reversal Print

The phenomenon of exchange bias has transformed how data is read on magnetic hard disks and created an explosion in their information storage density. However, it remains poorly understood, and even the fundamental mechanism of magnetic reversal for exchange-biased systems in changing magnetic fields is unclear. By using x-ray photoemission electron microscopy at the ALS to directly image the magnetic structure of an exchange-biased film, a team from the University of Washington and the Stanford Synchrotron Radiation Laboratory has identified separate magnetic-reversal mechanisms in the two branches of a hysteresis loop. This advance in fundamental understanding will provide new insights for developing the next generation of information storage and sensing devices where exchange bias is expected to play a critical role.

Magnetic Comings and Goings

Each atom in a magnetic material like iron is itself a tiny magnet represented by a magnetic moment. If the atomic moments are pointing in random directions, they cancel each other out. So, to bring about the magnetic state, one applies a strong external magnetic field to force the individual moments into alignment. On removing the external field, the moments remain aligned, and it takes a second equally strong field in the opposite direction to reverse the alignment. In so-called exchange-biased magnetic layers, the situation changes in the important respect that the field required to reverse the moments is higher than is then required to reestablish them in the original direction. This preferential magnetization effect provides a useful magnetic “reference” for modern magnetic devices, such as read heads and storage disks, based on thin layers.

Remarkably, despite its widespread use in today’s technology, exchange bias is not well understood at the atomic level. Krishnan et al. have used an x-ray microscopy technique to show that there is an asymmetry in the manner by which the moments align, with one mechanism known as coherent rotation (because the moments rotate together) dominating in one direction and another known as nucleation and growth (because islands of opposite orientation arise and grow) dominating in the other. This information will be a useful guide to those designing new information storage and sensing devices.

There are two basic energies involved in the manipulation and control of the magnetic properties of materials. Exchange controls magnetic order, and anisotropy controls magnetic orientation. A soft ferromagnet such as iron has a large exchange parameter but a small anisotropy, making ferromagnetic order stable at higher temperatures but with an unpredictable orientation of the magnetization, especially in structures of nanoscale dimensions. On the other hand, many antiferromagnets have weak exchange interactions (low ordering or Néel temperatures) but large anisotropies that result in very stable orientations.

Magnetic hysteresis takes place by one of two fundamental mechanisms: coherent rotation (left) or by the nucleation and growth of reverse domains (right). Notice the symmetry of the mechanism: it is the same in both directions of the applied field.

Exchange bias arises when a thin ferromagnetic film is grown on an antiferromagnet and the resulting heterostructure is cooled in a magnetic field through the Néel temperature of the antiferromagnet. As a result of exchange coupling between the layers, the ferromagnet both retains a stable order and gains a higher anisotropy at room temperature. Moreover, the unidirectional character of the anisotropy results in a shifted hysteresis loop that is now centered on a non-zero magnetic field. This exchange bias makes the ferromagnet an excellent magnetic reference layer in modern nanolayer magnetic devices because it is very difficult to demagnetize it.

More than fifty years of research has provided varying insight into the exchange-bias phenomenon but not yet a comprehensive description of all its salient features. To gain more insight, the Washington–Stanford team resorted to x-ray photoemission electron microscopy (PEEM) imaging of high-quality single-crystal ferromagnetic iron epitaxially grown on antiferromagnetic MnPd (all on an MgO substrate), samples that had been previously well-characterized magnetically and structurally.

At an iron absorption resonance, absorption of circularly polarized x rays at ALS Beamline is sensitive to the angle between the magnetization within a ferromagnetic domain and the polarization vector. With the PEEM-2 microscope, this x-ray magnetic circular dichroism (XMCD) effect allows an exact determination of the direction of the local domain magnetization at the surface of ferromagnets with a spatial resolution of 50 nm or less.

By means of XMCD measurements taken at points in hysteresis loops with the applied field in different crystallographic directions of the iron ferromagnet, the team has accumulated the first direct imaging evidence for an asymmetry in the magnetic-reversal mechanism in exchange-biased systems, evidence that until now has only been inferred indirectly by measurements such as neutron scattering.

Hysteresis loops measured with a vibrating-sample magnetometer with the applied magnetic field in different crystallographic directions. The loops are shifted when the field is applied in the bias direction (left) and 45 degrees to the bias direction (center). When it is applied perpendicular to the bias direction, an intermediate state results (right).

PEEM images of an exchange-biased sample at (top) point A on the descending and (center) point B on the ascending hysteresis loops for H applied in the iron [110] direction. Crystallographic orientations are also shown (bottom). The gray-scale circle links the direction of magnetization (M) in the domains to their brightness in the images. The domainless structure at point A is consistent with a coherent rotation in the descending branch of the hysteresis loop, whereas the domain structure visible at point B suggests reversal by nucleation and growth in the ascending branch.

Normally, magnetic reversal in ferromagnets occurs either by coherent rotation of magnetic moments in the domain or by nucleation and growth of reverse domains. Generally, the mechanism is determined by the material microstructure and is symmetric with respect to the applied field, i.e., it is the same in both branches of the hysteresis loop. However, the team found that in exchanged-biased ferromagnetic iron, the magnetization reversal occurs by moment rotation for decreasing fields, while it proceeds by domain nucleation and growth for increasing fields. The observed domains are also consistent with the crystallography of the bilayers and favor a configuration that minimizes the overall magnetostatic energy of the ferromagnetic layer.

Research conducted by P. Blomqvist and K.M. Krishnan (University of Washington) and H. Ohldag (Stanford Synchrotron Radiation Laboratory).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: P. Blomqvist, K.M. Krishnan, and H. Ohldag, "Direct imaging of asymmetric magnetization reversal in exchange-biased Fe/MnPd bilayers by x-ray photoemission electron microscopy," Phys. Rev. Lett. 94, 107203 (2005).