Dipolar superferromagnetism with reentrant low-temperature superspin glass behavior is observed on a randomly distributed ferromagnetic nanoparticle systems in discontinuous metal-insulator multilayers ͓Co 80 Fe 20 (t)/Al 2 O 3 ͑3 nm͔͒ 10 with nominal thickness 1.1рtр1.3 nm by use of ac susceptometry and dc magnetometry. At tϭ1.0 nm, superspin glass-like freezing is evidenced by the criticality of dynamic and nonlinear susceptibilities.

No abstract

Discontinuous magnetic multilayers [CoFe/Al 2 O 3 ] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co 80 Fe 20 nanoparticles are embedded in a diamagnetic insulating a-Al 2 O 3 matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.

No abstract

We study the magnetization damping in ion-beam deposited Co72Fe18B10 thin films as a function of film thickness and crystalline state. As-deposited amorphous layers showed low damping (αapp=0.006) that is thickness independent. 40nm Co80Fe20 with no boron content exhibited a value twice higher (αapp=0.013). Crystallization in Co72Fe18B10, triggered by annealing at 280°C, results in increased magnetization as well as a strong increase in damping, by a factor of 5 for 40nm films. For lower thicknesses the damping increase upon annealing is less pronounced. The exchange stiffness constant for amorphous films is deduced from perpendicular standing spin waves to be 28.4×10−12J∕m. The annealing dependence of damping should have consequences for the spin-transfer switching in CoFeB∕MgO∕CoFeB magnetic tunnel junctions.