MAGNETIC NANOSTRUCTURES
MAGNETIC PROPERTIES OF LITHOGRAPHED NANOSTRUCTURES
It is well known that as the physical dimensions of a system become comparable to characteristic length scales of a given physical property (e.g. mean free path for electrical systems or wavelength in optical systems) its properties can be severely affected. In the case of magnetism the main characteristic length scales are the exchange length, lFM, and the magnetic domain wall width, dFM, which are in the order of tens of nanometers for most common magnetic materials. As the size of a magnetic system goes from macroscopic (a few mm in size) to nanometer size it suffers several systematic changes in its magnetic properties [1-4]. These changes can be schematically divided in three regimes: (i) as its size becomes of the order of dFM the system goes from multidomain to closure domain, (ii) as the size becomes smaller than dFM, the system evolves from closure domain to single domain and (iii) if the size becomes sufficiently small, the systems will finally go from single domain to superparamagnetic (see figure 1). Another interesting feature of magnetic nanostructures is that, in artificially fabricated nanostructures (e.g. by electron beam lithography), the distance between nanostructures can be easily controlled. The change in interparticle distance can modify the interaction between nanostructures, which in turn can also alter the properties of the magnetic systems [1]. Thus, controlling the size and shape of the nanostructures and the distance between them, some of the magnetic properties of arrays of nanoparticles can be tailored to achieve an improved performance of the system. Aside from patterning (varying the size, shape or distance), the magnetic properties of nanostructures can also be considerably modified by different types of interactions with other magnetic materials. Examples of these are exchange biased systems [5] and exchange spring magnets [6] where there is an exchange coupling between a ferromagnet (FM) and an antiferromagnet (AF), between a “hard” FM and a “soft” FM. Both patterning, exchange coupling can considerably alter the magnetic properties, and all have been studied extensively on their own. However much less effort has been devoted to the understanding magnetic properties in patterned and exchange-coupled heterostructures, where all mechanisms may be equally important. The competition between magnetostatic constraints caused by patterning and interfacial exchange coupling can generate unexpected magnetic behavior.
[1] J.I. Martín, J. Nogués, K. Liu, J.L. Vicent, I.K. Schuller, J. Magn. Magn. Mater. 256, 449 (2003).
[2] C.Ross, "Patterned Magnetic Recording Media" Annu. Rev. Mater. Res. 31, 203 (2001).
[3] D.D. Awschalom, S. von Molnár, "Physical Properties of Nanometer Scale Magnets", in: G. Timp (Ed.) Nanotechnology (Springer-Verlag, New York, 1998) Chap.2
[4] R.P. Cowburn, J. Phys. D: Appl. Phys. 33, R1 (2000).
[5] J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J. S. Muñoz, M. D. Baró, Phys. Rep. 422, 65 (2005).
[6] E. E. Fullerton, J. S. Jiang, S. D. Bader, J. Magn. Magn. Mater. 200, 392 (1999).
Fig. 1 Schematic illustration of some of the typical effects that size can produce on the magnetic state at zero field.
Our interest in this field resides manly in three topics:
Exchange coupling: Study the coupling of magnetic nanostructures with antiferromagnetic materials to have an extra degree of freedom to control the magnetic properties of magnetic nanostructures
Morphological effects (shape, size, etc.): Study different types of nanostructures with diverse shapes, sizes, and distances to understand the role of the different parameters on the magnetic properties
Novel fabrication methods: Seacrh for novel, fast or cheap fabrication methods to produce magnetic nanostructures or arrays of magnetic nanostrutcures.
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AFM Image MFM Image |
Ferromagnetic nanostructures fabricated by nanoindentaion of a
non-magnetic alloy.
MAGNETIC NANOPARTICLES
Magnetic nanoparticles have long been studied due to their wide range of novel fundamental properties, e.g. enhanced magnetic moments, superparamagnetism, surface effects, giant anisotropies, quantum tunneling of the magnetization, or magnetocaloric effects [1-7]. Moreover, the interest in magnetic nanoparticles has dramatically increased in the last decade catalyzed by their rapidly expanding areas of potential technological applications, ranging from biomedicine and biodiagnostics (e.g. as contrast agents in nuclear magnetic resonance imaging, targeted drug delivery or local hyperthermia) to engineering and industrial applications (e.g. ferrofluid seals, nanophase hard magnets or high density magnetic recording) [1-7]. Interestingly, some of these applications require the nanoparticles to be in the superparamagnetic state, for example, to avoid interparticle aggregation. However for applications like recording media the nanoparticles must remain ferromagnetic. Moreover, the use of magnetic nanoparticles in sensors calls for the particles to be magnetically soft without being superparamagnetic. Thus, the magnetic properties of nanoparticles often need to be adjusted to match specific purposes.
[1] J.L. Dormann, D. Fiorani, E. Tronc, Adv. Chem. Phys. 98, 283 (1997).
[2] A. Hernando, J.M. González, Hyperfine Interact. 130, 221 (2000).
[3] J. González, O. Chubykalo, J.M. González, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 10, American Scientific Publishers, Stevenson Ranch, 2004, p. 1.
[4] X. Batlle, A. Labarta, J. Phys. D: Appl. Phys. 35, R15 (2002).
[5] P. Tartaj, M.P. Morales, S. Veintemillas-Verdager, T. González-Carreño, C.J. Serna, J. Phys. D: Appl. Phys. 36, R182 (2003).
[6] M.A. Willard, L.K. Kurihara, E.E. Carpenter, S. Calvin, V.G. Harris, Int. Mater. Rev. 49, 125 (2004).
[7] S.B. Darling, S.D. Bader, J. Mater. Chem. 15, 4189 (2006).
Our interest in this field resides manly in two topics:
Novel materials: Study different novel types of magnetic “nanoparticles” that present some unusual feature – size, shape, material, ...
Fe nanowire (1.5 nm in diamter) in a single
wall carbon nanotube.
Bi-magnetic Core-shell nanoparticles: Study nanoparticles with different combinations of magnetic cores and magnetic shells or magnetic nanoparticles embedded in magnetic matrices to tailor their magnetic properties.
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Fe nanoparticles (7 nm) embedded in an Cr2O3 matrix
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Co-CoO core-shell nanoparticle |
OTHER SYSTEMS
We are interested in the study of different magnetic systems even they may not be nanometric.
For example: Exchange Bias in Ferromagnetic-Antiferromagnetic Bilayers
Magnetic thin films and multilayers are of much scientific and technological interest. From the technological point of view, they form part of a large number of devices such as sensors or recording media. From the scientific point of view, surface and interface effects can dominate over bulk effects, leading to a large variety of new properties, sometimes radically different from those of the bulk. These new properties can sometimes be tuned to satisfy specific technological needs. One property, which due to its important role in many new devices, has triggered a significant interest in thin film systems in recent years, is exchange bias [1-4]. Briefly, when materials with ferromagnetic (FM) - antiferromagnetic (AFM) interfaces are cooled through the transition temperature of the AFM, Néel temperature (TN), a new anisotropy is induced in the FM. Exchange bias is one of the phenomena associated with the exchange anisotropy created at the interface between an AFM and a FM material. The primary manifestation of this anisotropy is a shift of the hysteresis loop in the field axis (see Fig. 1) [1-4].
Fig. 1 Schematic hysteresis loop for a AFM/FM system after field cooling from above TN.
[1] J. Nogués and I.K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999).
[2] A.E. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200, 552 (1999).
[3] R.L. Stamps, J. Phys. D: Appl. Phys. 33, R247 (2000).
[4] M. Kiwi, J. Magn. Magn. Mater. 234, 584 (2001).