X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ show that the ferromagnetism below $T_{textrm{C}}approx$ 100 K arises in the As $4p$ conduction band. No XMCD signal is
observed at the Mn x-ray absorption edges. Below $T_{textrm{C}}$, however, a clear XMCD signal is found at the As $K$ edge which increases with decreasing temperature. The XMCD signal is absent in data taken with the beam directed parallel to the crystallographic $textrm{c}$ axis indicating that the orbital magnetic moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant ferromagnetism is associated with the As $4p$ conduction band and that distinct local-moment antiferromagnetism and itinerant ferromagnetism with perpendicular easy axes coexist in this compound at low temperature.
We present experimental results for the heavy-electron compound CeCu$_{4}$Ga which show that it possesses short-range magnetic correlations down to a temperature of $T = 0.1$ K. Our neutron scattering data show no evidence of long-range magnetic orde
r occurring despite a peak in the specific heat at $T^{*} =1.2$ K. Rather, magnetic diffuse scattering occurs which corresponds to short-range magnetic correlations occurring across two unit cells. The specific heat remains large as $Tsim0$ K resulting in a Sommerfeld coefficient of $gamma_{0} = 1.44(2)$ J/mol-K$^{2}$, and, below $T^{*}$, the resistivity follows $T^{2}$ behavior and the ac magnetic susceptibility becomes temperature independent. A magnetic peak centered at an energy transfer of $E_{rm{c}}=0.24(1)$ meV is seen in inelastic neutron scattering data which shifts to higher energies and broadens under a magnetic field. We discuss the coexistence of large specific heat, magnetic fluctuations, and short-range magnetic correlations at low temperatures and compare our results to those for materials possessing spin-liquid behavior.
We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, $tau_{rm{AFM}}$ = ($frac{1}{2} frac{1}{2} frac{1}{2}$), and ordered moments that align
along the [1 1 1] direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below $T_{rm{N}}~approx 0.4$ K and corresponds to a magnetic correlation length of $xi_{rm{n}} approx$ 80 $rm{AA}$, and a broad component that persists up to $T^*approx$ 0.7 K and corresponds to antiferromagnetic correlations extending over $xi_{rm{b}} approx$ 20 $rm{AA}$. Our results illustrate the fragile magnetic order present in YbBiPt and provide a path forward for microscopic investigations of the ground states and fluctuations associated with the purported quantum critical point in this heavy-fermion compound.
A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase which can lead to a Hall voltage independent of the spin-orbit coupling within the material--a geometrical Hall
effect. We show that the highly-correlated metal UCu5 possesses an unusually large controllable geometrical Hall effect at T<1.2K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20% of the u=1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in UCu5 opens a new field of inquiry into the importance of the role of frustration in highly-correlated electron materials.
Neutron scattering and magnetization measurements have been performed on the stuffed pyrochlore system Tb2+xTi2-2xNbxO7. We find that despite the introduction of chemical disorder and increasingly antiferromagnetic interactions, a spin glass transiti
on does not occur for T >= 1.5 K and cooperative paramagnetic behavior exists for all x. For x = 1, Tb3NbO7, an antiferromagnetically ordered state coexisting with cooperative paramagnetic behavior is seen without applying any external fields or pressure, a situation advantageous for studying this cooperative behavior.
