Time-dependent magnetic phenomena and particle-size effects in recording media

Abstract. The coercivity and energy losses in superparamagnetic magnetite and FePt nanoparticle composites subjected to an external, alternating magnetic field have been calculated as a function of the mean particle-size and packing density. The effect of interactions has been investigated by fitting the Sharrock law to the coercivity results as a function of the field cycle frequency of the magnetic field. This fitting leads to effective parameters for the anisotropy field H ef f K and β ef f = KV /k B T , which are themselves dependent on the interaction strength. The increase or decrease of the coercivity with interactions depends upon the relative change of H ef f K and β ef f , thus demonstrating the complex effect that interactions have in these nanoparticle composites. The interparticle interactions have a non-trivial effect on the energy loss per cycle. The energy loss is reduced for systems with larger particles since the reduction in coercivity together with a corresponding reduction in the remanence dominates. For small particle sizes, the energy loss is increased. The primary mechanism here seems to be an enhancement of the energy barrier due to interactions, which changes the nature of the particles from superparamagnetic to being thermally stable.

show abstract

“…In Ref. [21] it is shown that as long as the SPM fraction is negligible, the coercivity can be given by the so-called Sharrock law [22], where,…”

Section: Sweep Rate Dependence Of Magnetic Propertiesmentioning

confidence: 99%

Energy losses in interacting fine-particle magnetic composites

Burrows¹,

Parker²,

Evans³

et al. 2010

J. Phys. D: Appl. Phys.

View full text Add to dashboard Cite

show abstract

“…This value is below the applied field amplitudes, and it is an upper bound to the real dipolar field since the probability of all the neighbors of a given island being aligned is low. The temperature dependence of the switching field is approximately given by [49,50] …”

Section: Quantifying the Contribution Of Perimeter Vs Surface Atoms mentioning

confidence: 99%

Magnetic anisotropy from single atoms to large monodomain islands of Co/Pt(111)

Gambardella

Rusponi

Cren

et al. 2005

Comptes Rendus. Physique

View full text Add to dashboard Cite

By studying the magnetic behavior of self-assembled Co islands on a single-crystal metal surface, Pt(111), we show how the magnetic anisotropy evolves from isolated atoms to monolayer islands and films. Single Co adatoms are found to have a giant magnetocrystalline anisotropy energy of E a = 9.3 ± 1.6 meV/atom arising from the combination of partly preserved orbital moments (m L = 1.1 µ B ) and strong spin-orbit coupling induced by the Pt substrate. Combined scanning tunneling microscopy and X-ray magnetic circular dichroism experiments performed for differently sized small two-dimensional Co islands establish a clear connection between E a and m L , both quantities decrease sharply with the lateral coordination of the magnetic atoms. In accordance with this, Kerr magnetometry experiments, again performed in conjunction with scanning tunneling microscopy, reveal that the anisotropy energy of large two-dimensional Co islands is almost entirely determined by the relatively low number of perimeter atoms, having E a = 0.9 ± 0.1 meV/atom. These results confirm theoretical predictions and are of fundamental value the understanding of how the magnetic anisotropy develops in finite-size magnetic particles. Identification of the role of perimeter and surface atoms opens up new opportunities for engineering the anisotropy and the moment of a magnetic nanostructure. RésuméAnisotropie magnétique de l'atome isolé aux îlots monocouches de Co/Pt(111). En étudiant les propriétés magnétiques de particules de Co auto assemblées sur une surface d'orientation (111) d'un monocristal de Pt, nous montrons comment l'anisotropie magnétique évolue de l'atome isolé aux îlots monocouches et aux films ultraminces. Nous trouvons que les atomes de Co isolés adsorbés en surface ont une énergie d'anisotropie magnétocrystalline de E a = 9.3 ± 1.6 meV/atome provenant de la combinaison d'un moment orbital conservé (m L = 1.1 µ B ) et d'un fort couplage spin-orbite induit par le substrat de Pt. Des expériences combinées de microscopie à effet tunnel et de dichroïsme circulaire magnétique à rayons X, effectués sur des îlots de Co de petit taille établissent une connexion claire entre E a et m L , chacune de ces quantités décroissant rapidement avec la coordination latérale des atomes magnétiques. En conformité avec ceci, nous trouvons en employant la magnétométrie par effet Kerr et le microscope à effet tunnel que l'énergie d'anisotropie de grands îlots bidimensionnels de Co est presque entiè-rement déterminée par le nombre relativement faible d'atomes au périmètre, ayant E a = 0.9 ± 0.1 meV/atome. Ces résultats confirment les prédictions théoriques et sont d'un intérêt fondamental pour comprendre comment l'anisotropie magnétique se développe dans les particules magnétiques de taille finie. L'identification du rôle des atomes au périmètre et des atomes de surface ouvre de nouvelles opportunités pour l'ingénierie de l'anisotropie et du moment de nanostructures magnétiques. Pour citer cet article : P. Gambardella et al., C. R. Physique 6 (200...

show abstract

“…This is simply done by noting that from equation (4) the M = o state is obtained when precisely half the distribution has switched which, from the definition of the median energy barrier occurs for Ye = l. This leads to the expression where V m is the volume of the particles corresponding to the median diameter. Equation (9) was first derived for a single spin by Sharrock [13].…”

Section: Analytical Models For Time Dependencementioning

confidence: 99%

“…The effect of the damping term is to produce a damped precessional motion which eventually leads to an equilibrium position in which the magnetisation is collinear with the local magnetic field. Following It is straightforward to show under the assumption that the random process results from the superimposed effects of a large number of independent random events, then by virtue of the central limit theorem the statistical distribution of the random variable is Gaussian with a variance of J.I.. Standard numerical techniques exist for picking a normal deviate with a specified variance, and using these techniques it is possible to randomly choose the fluctuating field using equation ( (13) where V is the particle volume. It has been shown [14] that the introduction of thennal activation gives rise to a random walk toward equilibrium, followed by a random precession about the local field direction.…”

Section: Description Of the Modelmentioning

confidence: 99%

Thermally Activated Magnetisation Reversal at High Frequencies

Chantrell

Hannay

1997

J. Magn. Soc. Jpn.

View full text Add to dashboard Cite

-A computational model of thermally activated magnetisation reversal in particulate materials is described. The model allows the study of the detailed relaxation behaviour of isolated (non-interacting) particles. Computed results show non-exponential relaxation over small energy barriers attributable to the absence of a unique relaxation time. The coercivity shows a rapid increase at very short measurement time which is important as regards ultimate switching speeds in recording. The effects of interparticle dipolar magnetic interactions are also introduced into the model. These have the effect of increasing the spread of relaxation times via the inevitable dispersion in the magnitude and direction of the local field.

show abstract

Time-dependent magnetic phenomena and particle-size effects in recording media

Cited by 254 publications

References 26 publications

Energy losses in interacting fine-particle magnetic composites

Energy losses in interacting fine-particle magnetic composites

Magnetic anisotropy from single atoms to large monodomain islands of Co/Pt(111)

Thermally Activated Magnetisation Reversal at High Frequencies

Contact Info

Product

Resources

About