Creation of an Antiferromagnetic Exchange Spring
In the ongoing quest for faster and more efficient magnetic data storage, designs for devices such as read heads in computer hard drives are mostly produced through a trial-and-error process, combining thin magnetic films with different properties. To speed up this search for better materials, researchers are striving for a better understanding of the microscopic structure and interactions between ferromagnet and antiferromagnet layers. Researchers from the ALS, Stanford University, and Italy have now solved a piece of this puzzle using an x-ray magnetometer at the ALS. They proved that antiferromagnets in contact with ferromagnets form an exchange spring system. An exchange spring combines the maneuverability of magnetically soft materials with the permanence of magnetically hard materials.
Exchange springs are structures consisting of magnetically hard materials and magnetically soft materials. Magnetically soft materials can be magnetized very easily, but their orientation remains sensitive to magnetic fields. Magnetically hard materials retain their magnetic orientation, even in strong magnetic fields. The ultimate hard-magnetic material is the antiferromagnet, because extremely high fields are required to change its magnetism. The benefit of an exchange spring is that the magnetism of the magnetically soft material is reinforced by the magnetically hard material. When an antiferromagnet and a ferromagnet are combined in a layered structure—such structures are part of the read heads in computer hard drives—the hard antiferromagnet pins and holds the magnetization of the ferromagnet across the interface in the presence of an applied magnetic field, up to a certain field threshold. This pinning, known as exchange bias, results from atomic exchange forces across ferromagnet–antiferromagnet interfaces, which tend to align the magnetization of nearby atoms. When a stronger magnetic field above the threshold is applied, abrupt movement of the ferromagnet is expected, leaving the hard antiferromagnet relatively unaffected. In reality, as the recent ALS experiments showed, the behavior is different. In these experiments, the magnetization of the soft layer dragged the magnetization of the antiferromagnet, winding it like a clock spring. The result is creation of a domain wall between the rotated region at the surface of the sample and the unrotated region below. This behavior is common with ferromagnets but was unknown for antiferromagnets. The experiments were conducted at ALS Beamline 4.0.2, an elliptically polarizing undulator (EPU), complemented by measurements at Beamline 7.3.1.1, a photoemission electron microscope (PEEM-2). A recently built octupole magnet allowed rotation of a magnetic field of up to one Tesla in any direction in space. The team studied thin cobalt (ferromagnet) layers on nickel oxide (antiferromagnet) single crystals because their magnetic properties are well known. 
The researchers measured electron yield spectra of the topmost 5 nanometers of the antiferromagnet surface, expecting that this region would most strongly show a twisting of the magnetic structure and the formation of a domain wall. The response of the antiferromagnet was recorded using x-ray magnetic linear dichroism (XMLD), a spectroscopic technique that quantitatively measures the angle between the magnetic moments of the antiferromagnet and the linear polarization direction of the x rays.
As expected, spectra measured on a pure nickel oxide showed no rotation, because the antiferromagnet alone was unaffected by magnetic fields. However, the cobalt–nickel oxide sample showed a clear rotation of more than 45 degrees when a magnetic field was applied. These results demonstrate that antiferromagnets in contact with ferromagnets form an exchange spring system, which is instrumental in explaining exchange bias and understanding the behavior of this complex magnetic system. Research conducted by A. Scholl and E. Arenholz (ALS), M. Liberati (Istituto Nazionale per la Fisica della Materia, Italy, and ALS), H. Ohldag (Stanford Synchrotron Radiation Laboratory and ALS), and J. Stöhr (Stanford Synchrotron Radiation Laboratory). Research funding: The U.S. Department of Energy, Office of Basic Energy Sciences (BES) and Ministero dell'Istruzione, dell'Università e della Ricerca (Italy). Operation of the ALS is supported by BES. Publication about this research: A. Scholl, M. Liberati, E. Arenholz, H. Ohldag, and J. Stöhr, "Creation of an Antiferromagnetic Exchange Spring," Phys. Rev. Lett. 92, 247201 (2004). |
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