A magnetic field driven quantum critical point (QCP) is studied experimentally in the Ce3Al magnetically anisotropic intermetallic compound, which shows both antiferromagnetic (AFM) ordering and heavy–fermion behavior. Measurements of the magnetic susceptibility, the magnetoresistance and the specific heat on a Ce3Al monocrystalline sample performed down to 0.35 K in magnetic fields up to 9 T demonstrate that the QCP is anisotropic regarding the orientation of the magnetic field relative to the magnetically easy direction. External magnetic field drives the AFM transition continuously toward zero temperature when applied in the (a, b) easy plane, reaching the QCP at the critical field B c a , b = 4.6 ± 0.4 T, where a quantum phase transition from the AFM to the paramagnetic state takes place. The magnetoresistance experiments below 1 K indicate that intermediate magnetic states may have formed near the QCP. For the field applied along the c hard direction, the QCP has not been observed within our experimental range of the magnetic field. The anisotropic, magnetic field driven QCP in the Ce3Al results from competition of the exchange interaction with the Zeeman interaction in the presence of a large magnetocrystalline anisotropy. The anisotropy of the QCP is a consequence of the fact that the magnetic anisotropy locks the magnetization into the easy plane and cannot be pulled out of the plane by the available laboratory field. Consequently, only the component of the magnetic field vector that lies in the easy plane participates in the QCP formation. In AFM systems with a large magnetic anisotropy, the magnetic field driven QCP is a continuous variable of the magnetic field vector orientation relative to the easy direction.

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We report on synthesis and characterization of a novel group of compounds based on copper, gadolinium, and calcium. Cu-Ca and Cu-Gd binaries were previously studied while Ca and Gd are known to be immiscible themselves. The effects of substituting Gd with Ca in Cu5Gd1-xCax compounds (0≤x≤1) were studied by investigating the phase stability and crystal structure of the resulting new compounds in five specimens with x = 0, 0.33, 0.50, 0.66, and 1, respectively. The samples produced by melt-spinning had hexagonal P6/mmm structure, irrespective of Ca amount (x), where lattice parameters varied with x linearly. This is an indication of good solid solubility under the preparation conditions. A slower cooling upon arc-melting caused the liquid phase separation into Cu4.5Gd and Cu-Ca compounds. Using TEM, rapidly solidified ribbons (Cu5Gd0.5Ca0.5) were investigated and the formation of a homogeneous ternary phase with a nearly nominal stoichiometric composition and minor amounts of Cu-Ca secondary phase was observed. Using DSC and HT XRD, we found that these systems are stable at least up to 400°C. Upon a 16-hour hydrogenation at 1 bar and 300°C, all specimens absorbed about 0.5 wt.% of hydrogen. This caused changes in structure with the formation of pure Cu and H2Gd1-xCax solid solution.

We studied the effects of substituting gadolinium in the compound Cu5Gd with Ca by investigating the phase stability and crystal structure of the resulting new compounds. For rapidly quenched materials produced by melt spinning, the crystal structure was always hexagonal P6/mmm, irrespective of the Ca addition (x) in alloys with the formula Cu5Gd1-xCax, indicating good solid solubility under these conditions, which was additionally confirmed by Vegard’s law. Slower cooling upon arc-melting process caused the phase separation into Cu4Gd and CuCa. Using TEM, we investigated rapidly solidified ribbons of Cu5Gd0.5Ca0.5 and observed co-existence of Cu–Ca secondary phase with a matrix phase having the nominal stoichiometric composition. The oxygen level in this sample was found to be within 2–5 at. %, which was attributed to surface oxidation during or after TEM sample preparation. Upon hydrogenation, the crystal structure of all samples changed from hexagonal to cubic (F-43m), which is the thermodynamically stable polymorph of Cu5Gd compound. Strong catalytic activity of water formation from gaseous H2 and O2 was coincidentally discovered during dehydrogenation experiment, thus making this material as potential candidate for zero-platinum oxygen reduction catalyst.