Evolution from Ferromagnetism to Antiferromagnetism in Yb(Rh_{1-x}Co_{x})_{2}Si_{2}.

Yb(Rh_{1-x}Co_{x})_{2}Si_{2} is a model system to address two challenging problems in the field of strongly correlated electron systems. The first is the intriguing competition between ferromagnetic (FM) and antiferromagnetic (AFM) order when approaching a magnetic quantum critical point (QCP). The second is the occurrence of magnetic order along a very hard crystalline electric field (CEF) direction, i.e., along the one with the smallest available magnetic moment. Here, we present a detailed study of the evolution of the magnetic order in this system from a FM state with moments along the very hard c direction at x=0.27 towards the yet unknown magnetic state at x=0. We first observe a transition towards an AFM canted state with decreasing x and then to a pure AFM state. This confirms that the QCP in YbRh_{2}Si_{2} is AFM, but the phase diagram is very similar to those observed in some inherently FM systems like NbFe_{2} and CeRuPO, which suggests that the basic underlying instability might be FM. Despite the huge CEF anisotropy the ordered moment retains a component along the c axis also in the AFM state. The huge CEF anisotropy in Yb(Rh_{1-x}Co_{x})_{2}Si_{2} excludes that this hard-axis ordering originates from a competing exchange anisotropy as often proposed for other heavy-fermion systems. Instead, it points to an order-by-disorder based mechanism.

Doped YbRh2Si2: not only ferromagnetic correlations but ferromagnetic order.

YbRh2Si2 is a prototypical system for studying unconventional antiferromagnetic quantum criticality. However, ferromagnetic correlations are present which can be enhanced via isoelectronic cobalt substitution for rhodium in Yb(Rh(1-x)Co(x))2Si2. So far, the magnetic order with increasing x was believed to remain antiferromagnetic. Here, we present the discovery of ferromagnetism for x = 0.27 below T(C) = 1.30 K in single crystalline samples. Unexpectedly, ordering occurs along the c axis, the hard crystalline electric field direction, where the g factor is an order of magnitude smaller than in the basal plane. Although the spontaneous magnetization is only 0.1 µB/Yb it corresponds to the full expected saturation moment along c taking into account partial Kondo screening.

[Evaluation of a new device for sample collection, transport and detection of Group B Streptococcus in pregnant women]. / Evaluación de un nuevo dispositivo para la recogida de la muestra, transporte y detección del estreptococo del grupo B en mujeres embarazadas.

We have designed a new device that combines sample collection, transportation, culture and detection of Group B Streptococcus (GBS), requiring no additional processing in the clinical laboratory. The objective was to evaluate the performance of this device for GBS detection in pregnant women. The new prototype was compared to direct plating of vaginal-rectal swabs onto Granada solid media plates. Direct plating method detected 124 positive samples out of 600 (20.6%) whereas the new device detected 10 additional positive samples (134/600, 22.3%). This new device (patent-protected) could be considered for routine GBS screening.