Transition of ferromagnetism to superparamagnetism in Fe/Tb multilayers

Abstract

Fe/Tb (1.2 nm) multilayer films with various Fe layer thicknesses were prepared by vapor deposition. Magnetic and microstructure of the films were characterized by various experimental methods. The results show that the films evolve from ferromagnetic to superparamagnetic, and Fe in the multilayers transforms from bcc crystalline to amorphous state. The transition of magnetic property and structure is found to be related to the rough interface associated with the amorphous phase formation in Fe/Tb interface. In the [Tb(1.2 nm)/Fe(0.5 nm)]30 multilayer, Fe layers are separated into discontinuous superparamagnetic clusters by three-dimensional Tb crystal aggregates, which are responsible for paramagnetic-like spins at the interface that strengthen the spin-dependent scattering.

Structure and magnetic properties of Gd/Fe layers grown by MBE

“…In addition, high-resolution TEM images of samples (C) and (D) show that the Gd layer is partially amorphous. This fact is consistent with some reported results of amorphous Fe layers with thicknesses smaller than 2 nm in Fe/RE multilayer structures [14,15].…”

“…We analyzed that the small Gd magnetization was due to the superparamagnetic state of the discontinuous Gd layers, as observed in the TEM images. There are some reports about superparamagnetic behavior in Fe layers with thicknesses less than 2 nm [15,16], where the amorphous state of Fe creates ferromagnetic clusters. In a similar scheme, we can explain the small Gd magnetization of samples (C) and (D) as follows: in the thin discontinuous amorphous Gd layers, some ferromagnetic clusters of Gd form, and superparamagnetic coupling between these Gd clusters makes the Gd magnetization much smaller than expected.…”

“…Four samples in Table 1 15 . The thickness of each layer was monitored in-situ by a quartz oscillator.…”

Magnetic Transition and Structural Evolution in NiCo/Ag Multilayers

“…Note, however, that these NiCo/Ag multilayers were prepared with a fixed t NiCo of 1.8 nm, larger than the magnetic layer thickness reported before. [1][2][3][4][5][6][7][8][9][10][11] The difference may be due to the magnetic layer made of a Ni-Co solid solution with an extra energy, which turns uniform magnetic layers into an assembly of nanosized particles easily.…”

“…In recent research of some magnetic films, a transition of the films from ferromagnetic to superparamagnetic was frequently observed as magnetic layer thickness (t M ) decreased to a critical thickness, which was commonly less than 1.0 nm. [1][2][3][4][5][6][7][8][9] According to the above references, the appearance of superparamagnetism was related to interface roughness, such as the islands of face-centered-tetragonal-Fe, 1) and an amorphous structure with an entirely disordered atoms 4,5) at interfaces. Furthermore, structural imperfections can lead to the formation of an island structure with dislocated submultilayers for ultrathin t M .…”