D.C. Crew

Measurement of magnetic viscosity in a Stoner-Wohlfarth material

Abstract A Stoner-Wohlfarth model has been used to simulate conventional magnetic viscosity experiments. The magnetic viscosity parameter Λ was found to vary depending on the method used for measurement of the irreversible magnetisation. The difference in Λ is found to be related to a parameter η which describes the dependence of reversible magnetisation on irreversible magnetisation. The correct value of Λ at any field has been related to the population of particles which reverse at that field and it is revealed that distributions of volume and orientation can obscure the interpretation of the measured Λ. Thus the measurement of Λ as an indicator of reversal mechanism is questioned.

The interpretation of magnetic viscosity

The consideration of a simple model, based on the Stoner and Wohlfarth model of magnetic reversal including time-dependence, allows the measurement of the key viscosity parameter to be related to the micromagnetic changes occurring in the material. An aligned system with a log-normal distribution of volumes has been studied and the measured value of at each field related to the mean volume of the particles which are reversing at that field. It is also shown that, when the system is not aligned, there is no longer a simple relationship between and the physical processes occurring. This is because in this instance is a complex average over the ensemble.

Optic and acoustic modes measured in a cobalt/Permalloy exchange spring bilayer using inductive magnetometry

The resonance frequencies of a cobalt (100nm)/Permalloy (50nm) exchange spring bilayer have been measured using a pulsed inductive microwave magnetometer. Two field dependent frequencies are detected which vary in relative intensity as a function of applied magnetic field. The two frequencies are identified as the optic and acoustic spin wave modes of coupled ferromagnetic films. Only over a limited range of fields are both frequencies observable. In low fields the acoustic mode is dominant, and in higher fields the optic mode gives the largest intensity. These intensity variations are explained in terms of off-resonant driving of cobalt modes by the uniform mode in Permalloy and a field dependent interfacial pinning.

Ferromagnetic resonance in exchange spring thin films

The ferromagnetic resonance frequencies have been calculated for a soft/hard Co/CoPt exchange spring thin film system. In this geometry the magnetostatics are crucial to understanding the variation of resonant frequency with field due to the out-of-plane component of magnetization of the CoPt hard layer. When exchange coupling between films exists, significant changes in the resonant frequency occur. For in-plane reverse applied fields, a spiral is formed in the soft Co layer. This appears as a double minimum in the resonant frequency as a function of the in-plane applied field. The positions of the frequency minima correspond to the beginning and end of the spiral formation in the Co layer. The frequency of the maximum provides a sensitive measure of the perpendicular anisotropy in the cobalt film.

Interphase exchange effects in CoPt/Co bilayer thin films

The condition of the interphase interface in model exchange-spring CoPt/Co bilayers was correlated with the magnetic response. Polycrystalline bilayer samples of 16.7-nm Co and 25-nm L10-type CoPt were fabricated and subsequently annealed to vary the condition of the interphase interface. Although the thickness of the soft phase component exceeded that for ideal (i.e. completely reversible) interlayer exchange coupling, this thickness allowed correlations between the interfacial condition and the magnetic quality of the exchange spring components to be observed and quantified via magnetic recoil measurements. Annealing the bilayers at 300°C for 20 min produced a 63.0% increase in the coercivity of the soft component, while at the same time the coercivity of the hard component was decreased by 18.5%.

Temperature dependence of the magnetic viscosity parameter

Abstract The temperature dependence of the magnetic viscosity parameter Λ, also known as Sv, has been examined using two theories based on thermal energy barrier activation and thermally induced fluctuation fields. For aligned, non-interacting Stoner-Wohlfarth particle assemblies it is shown that both theories predict the same temperature dependence for Λ but that Λ ∞ T or T1/2 depending on how the field at which Λ is measured is varied with temperature. It is also shown that for other mechanisms other temperature variations are possible.

Effect of soft-phase anisotropy on reversal behavior in two-phase exchange-coupled bilayer ferromagnetic thin films

A two-dimensional micromagnetic simulation has been used to study a thin-film bilayer consisting of two ferromagnetic phases with different magnitudes of magnetocrystalline anisotropy. The aim was to investigate the effect the magnitude of magnetocrystalline anisotropy of the soft-phase has upon the resulting hysteresis properties. For low soft-phase anisotropy, reversal of the soft phase occurs in low fields while the hard phase remains unreversed until higher fields, whereupon the domain wall becomes unpinned from the interface. For higher values of the soft-phase anisotropy, the soft phase is more resistant to change in magnetization, but once reversal begins the domain wall propagates more easily into the hard phase. The result is that the coercivity of the hard phase decreases with increasing soft-phase anisotropy. The ultimate point is reached when the nucleation field of the soft phase is larger than the pinning field of the interface anisotropy mismatch and an almost single-phase reversal process ...

Coercivity, time dependence and reversible magnetization in Nd rich Nd-Fe-B alloys

The coercivity, time dependent magnetic properties and reversible magnetization behavior for a series of neodymium enriched alloys based on Nd/sub 2/Fe/sub 14/B has been investigated. These studies were carried out in an attempt to elucidate the dramatic increase in the coercivity in these materials with increasing dilution of the magnetic phase (Nd/sub 2/Fe/sub 14/B). Good correlation between the coercivity and the relative average linear sparing between magnetic grains was found for all but the lowest Nd content. As the Nd content is increased the reversible magnetization associated with domain wall pinning decreases. At high dilutions the reversible magnetization behavior is consistent with the predictions of a Stoner-Wohlfarth model of an assembly of noninteracting single domain particles. The magnetic viscosity parameter, /spl Lambda/, does not vary consistently with dilution or coercivity.