Effect of Uniaxial Anisotropy and Demagnetizing Effects on the Magnetic Behavior of Polycrystalline Gadolinium

Gadolinium (Gd) is a soft ferromagnetic material with uniaxial magnetic anisotropy. The anisotropy is responsible for some magnetic features that are quite different from those observed in most common ferromagnetic materials. Polycrystalline Gd exhibits thermal hysteresis in which the memory of past magnetic history is stored in the buildup of magnetic charge density around grain boundaries on cooling in magnetic fields from above the spin-reorientation temperature T
_SR
~ 235 K, in the range where the uniaxial anisotropy is positive. This has the effect of narrowing domain walls (DWs) that interact more strongly with defects, such as grain boundaries. Below T
_SR
, where the negative uniaxial anisotropy is increasing in magnitude with decreasing T, the magnetic susceptibility increases by orders of magnitude. Negative uniaxial anisotropy makes it easier for DWs to move past defects, such as grain boundaries, because wider Néel walls are favored over narrower Bloch walls. Magneto-thermal measurement protocols are used to study the effect of cooling in a field μ
₀
H = -50 mT to 5 K and then measuring at lower constant fields, typically μ
₀
H = 3.5 mT, from 5 to 380 K. The protocols permit the extraction of the temperature dependencies of the susceptibility, which depends on anisotropy, and of the frozen fraction of the magnetization, which consists of two parts. One part freezes upon field cooling below T = 100 K, and a harder one freezes above 250 K. On first warming, the former unfreezes below 100 K, while the latter unfreezes above 250 K.