Grupo de Materiales Magnéticos (GMM). Universidad de Castilla-La Mancha

Research Line: Magnetic interactions in multilayers of transition metals and rare earths.

Gd/Co multilayeres are a system of great interest for studying magnetic interactions, as the magnetic subnetworks of both components couple antiparallel to each other, giving rise to an artificial ferrimagnet. Also, Gd subnetwork is highly dependent on temperature in the range usually accessible to the instruments (10-300K), so as a function of temperature and magnetic history, a broad range of interesting magnetic structures can take place, including those with total magnetization opposed to the external magnetic field.

Gd and Co form an amorphous alloy in a broad range of compositions. They have a strong affinity to inderdiffuse, making difficult to fabricate multilayers with clear and sharp interfaces. One of the main contributions this group has made to the study of this system was to eliminate this problem by using a GdCo alloy instead of pure Gd. The composition was chosen as to have Gd dominating the ferrimagnetic structure at all temperatures, so when interacting with a Co layer it behaves very similar to pure Gd, but reducing interdiffusion so good multilayers with sharp interfaces are achieved.

Curva de imanación frente a temperatura donde se indica la
dirección del campo externo (hacia la dcha), las orientaciones
de las subredes de Gd (gris) y Co (azul) así como el momento
total de la multicapa (en rojo), en distintos puntos característicos
de la curva. Obsérvese cómo hay amplias regiones en que la
muestra queda orientada en sentido opuesto al campo externo.

Back to Gd/Co multilayers (or rather as just seen, GdCo/Co), if thicknesses are properly chosen, it is possible to match the magnitudes of the two magnetic subnetworks, yielding a state of zero total magnetization (compensation temperature). Below that temperature, magnetization is dominated by Gd (or alloy) which implies Co layers points opposed to external field. This situation is inverted above the compensation temperature. When measuring magnetization as a function of temperature, and depending of the cooling/heating protocol, the system often gets staggered in magnetic states with negative total magnetization, jumping to more stable configurations in different ways depending on the microscopical details of the sample.

Nowadays, we work on two simultaneous topics. On the one hand we try to introduce magnetic anisotropy on the sample (which in principle is very low) as it plays a role in the way the system jumps between different magnetic states. We try to increase anisotropy by applying an external magnetic field during the growth and/or doping the structure with Pd or Pt. On the other hand, we have started the characterization of the system in a different geometric configuration: Co nanoparticles embedded in a Gd or GdCo matrix.

The experimental techniques employed for the characterization of these systems, apart from traditional magnetometry, are electric transport, conventional x-ray reflectivity, polarized neutron reflectometry and resonant x-ray reflectivity. To measure some of these, we employ big international facilities as the ESRF x-ray source in Grenoble or the ISIS neutron spallation source in the UK.