High-field anomalies of equilibrium and ultrafast magnetism in rare-earth–transition-metal ferrimagnets

Abstract

Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in the equilibrium and ultrafast magnetic properties of the ferrimagnetic rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the magnetization compensation temperature, each of the magnetizations of the antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis loops. Contrary to state-of-the-art theory, which explains such loops by sample inhomogeneities, here we show that they are an intrinsic property of the rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and have a non-zero orbital momentum in the ground state and, therefore, a large magnetic anisotropy, we are able to reproduce the experimentally observed behavior in equilibrium. The same theory is also able to describe the experimentally observed critical slowdown of the spin dynamics in the vicinity of the magnetization compensation temperature, emphasizing the role played by the orbital momentum in static and ultrafast magnetism of ferrimagnets.

Unusual Field Dependence of the Anomalous Hall Effect in Ta/Tb−Fe−Co

“…Earlier studies of rare-earth ferrimagnets have found that in the case of weak rare-earth magnetic anisotropy the transition shifts towards the lower temperatures with the increase of the magnetic field 35 . On the other hand, from more general considerations we have shown 19,26,36 that if the rare-earth anisotropy is prevailing an inclination of the first-order phase transition line towards higher temperatures would occur; therefore, we assume that the surface anisotropy introduced by Ta capping influences d-states that relatively more delocalized, and thus, for Ta-coated TbFeCo the f-sublattice anisotropy doesn't dominate at least in the vicinity of the compensation temperature of the film. The physical reason behind this is that Ta, electronic configuration of which is [Xe] 4f 14 5d 3 6s 2 , has large atomic number Z = 73 and spin-orbit coupling scales approximately as Z 4 [ 37 ].…”

“…The lines are derived from the theoretical explanation that is similar to refs. [ 19,26 ]. Similarly to our earlier reports, here we also interpret a hysteresis as a result of first-order phase transition.…”

“…Noncollinear spin configurations in ferrimagnets give rise to anomalous Hall effect (AHE) 16,17 and, thus, can be sensed in transport. Earlier experimental studies of Tb(Fe x Co 1-x ) 18,19 , Gd(Fe x Co 1-x ) 15,[20][21][22][23] , HoCo 24 , DyCo4 25 revealed a rather unusual behavior of the magnetization in these compounds. Unlike a single hysteresis loop normally expected in measurements of the magnetization as a function of applied magnetic field, a triple hysteresis loops were observed.…”

“…We show that one should expect anomalous hysteresis loops below the compensation point if in the vicinity of it the magnetic anisotropy is dominated by FeCo sublattice due to interaction with Ta. 19 , here the situation is inverted that the triple hysteresis is observed below T M . More particularly, using Hall bar structures of MgO/Ta/TbFeCo we measured field dependencies…”

Laser‐Induced Transient Anisotropy and Large Amplitude Magnetization Dynamics in a Gd/FeCo Multilayer

“…[35,36] Furthermore, we assume that the anisotropy is completely dominated by the Gd sublattice, as was claimed earlier in the case of RE-TM alloys and multilayers. [45][46][47][48] More specifically, this means we replace M(T) by M Gd (T) in Equation ( 1) and let only M Gd (T) contribute to the anisotropy energy. The latter assumption, together with the Callen-Callen power law taking n = 3, will greatly reduce the anisotropy field barrier, providing the necessary "kick-start" to the precessional switching process.…”