Fast sweep-rate plastic Faraday force magnetometer with simultaneous sample temperature measurement

Development of high-resolution capacitive Faraday magnetometers for sub-Kelvin region

High-sensitivity capacitive Faraday magnetometers were developed for static DC magnetization measurements in a sub-Kelvin region that can be used with ³He-⁴He dilution refrigerators (∼50 mK) and ³He refrigerators (∼0.28 K). For high-resolution magnetization measurements, the background magnetization of the force-sensing capacitor should be as small as possible, compared with the magnetization value of a measured specimen. In this study, we succeeded in reducing the background of the capacitor in both low- and high-field regions by compensating for the diamagnetic response of a thin quartz plate, making use of Pauli-paramagnetic alloys and Van Vleck paramagnets as a counter magnetization for a diamagnetic signal. Having established an ultra-high-sensitivity capacitor, we achieved a resolution of 10^-5 (∼10^-5-10^-6) emu in the low- (high-) field region below (above) 1 T. In particular, the newly developed capacitors with a Van Vleck paramagnet Pr_0.1La_0.9Be₁₃ and paramagnetic MgAl alloys are demonstrated to be very useful for high-resolution magnetization measurements at milli-Kelvin temperatures in low and high magnetic fields, respectively.

Anomalous out-of-equilibrium dynamics in the spin-ice material Dy2Ti2O7under moderate magnetic fields

We study experimentally and numerically the dynamics of the spin ice material Dy2Ti2O7in the low temperature (T) and moderate magnetic field (B) regime (T∈ [0.1, 1.7]  K,B∈ [0, 0.3]  T). Our objective is to understand the main physics shaping the out-of-equilibrium magnetisation vs temperature curves in two different regimes. Very far from equilibrium, turning on the magnetic field after having cooled the system in zero field (ZFC) can increase the concentration of magnetic monopoles (localised thermal excitations present in these systems); this accelerates the dynamics. Similarly to electrolytes, this occurs through dissociation of bound monopole pairs. However, for spin ices the polarisation of the vacuum out of which the monopole pairs are created is a key factor shaping the magnetisation curves, with no analog. We observe a threshold field near 0.2 T for this fast dynamics to take place, linked to the maximum magnetic force between the attracting pairs. Surprisingly, within a regime of low temperatures and moderate fields, an extended Ohm's law can be used to describe the ZFC magnetisation curve obtained with the dipolar spin-ice model. However, in real samples the acceleration of the dynamics appears even sharper than in simulations, possibly due to the presence of avalanches. On the other hand, the effect of the field nearer equilibrium can be just the opposite to that at very low temperatures. Single crystals, as noted before for powders, abandon equilibrium at a blocking temperatureTBwhich increases with field. Curiously, this behaviour is present in numerical simulations even within the nearest-neighbours interactions model. Simulations and experiments show that the increasing trend inTBis stronger forB‖[100]. This suggests that the field plays a part in the dynamical arrest through monopole suppression, which is quite manifest for this field orientation.

Miniature capacitive Faraday force magnetometer for magnetization measurements at low temperatures and high magnetic fields

A Faraday force magnetometer is presented for measurements of magnetization at temperatures down to 100 mK and in magnetic fields up to 14 T. The specimen is mounted on a flexible cantilever forming a force-sensing capacitor in combination with a fixed back plate. Two different cantilever designs are presented. A torsion resistant cantilever allows us to measure the magnetization of highly anisotropic single crystal samples. Measurements of the metal organic quantum magnets (C₅H₁₂N)₂CuBr₄ (BPCB) and NiCl₂ · 4 SC(NH₂)₂ (DTN) demonstrate the device's capabilities. Routinely, a specimen's magnetic moment is measured with a resolution better than 10^-7 A m² (10^-4 emu). The device is miniaturized to fit in almost any cryostat.