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Register a new userGdRh$_2$Si$_2$: An exemplary tetragonal system for antiferromagnetic order with weak in-plane anisotropy
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The anisotropy of magnetic properties commonly is introduced in textbooks using the case of an antiferromagnetic system with Ising type anisotropy. This model presents huge anisotropic magnetization and a pronounced metamagnetic transition and is well-known and well-documented both, in experiments and theory. In contrast, the case of an antiferromagnetic $X$-$Y$ system with weak in-plane anisotropy is only poorly documented. We studied the anisotropic magnetization of the compound GdRh$_2$Si$_2$ and found that it is a perfect model system for such a weak-anisotropy setting because the Gd$^{3+}$ ions in GdRh$_2$Si$_2$ have a pure spin moment of S=7/2 which orders in a simple AFM structure with ${bf Q} = (001)$. We observed experimentally in $M(B)$ a continuous spin-flop transition and domain effects for field applied along the $[100]$- and the $[110]$-direction, respectively. We applied a mean field model for the free energy to describe our data and combine it with an Ising chain model to account for domain effects. Our calculations reproduce the experimental data very well. In addition, we performed magnetic X-ray scattering and X-ray magnetic circular dichroism measurements, which confirm the AFM propagation vector to be ${bf Q} = (001)$ and indicate the absence of polarization on the rhodium atoms.
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High-temperature indium flux growth was applied to prepare single crystals of GdRh$_2$Si$_2$ by a modified Bridgman method leading to mm-sized single crystals with a platelet habitus. Specific heat and susceptibility data of GdRh$_2$Si$_2$ exhibit a pronounced anomaly at $T_N = 107rm ,K$, where the AFM ordering sets in. Magnetic measurements on the single crystals were performed down to $T = 2$,K in external fields from B = 0 - 9,T applied along the $[100]$-, $[110]$- and $[001]$-direction of the tetragonal lattice. The effective magnetic moment determined from a Curie-Weiss fit agrees well with values from literature, and is larger than the theoretically predicted value. Electrical transport data recorded for current flow parallel and perpendicular to the $[001]$-direction show a large anisotropy below $T_N$. The residual resistivity ratio $rm RRR=rho_{300K}/rho_{0}sim 23$ demonstrates that we succeeded in preparing high-quality crystals using high-temperature indium flux-growth.
High temperature crystal structure of UPt$_2$Si$_2$ determined using single-crystal neutron diffraction at 400 K is reported. It is found that the crystal structure remains of the primitive tetragonal CaBe$_2$Ge$_2$ type with the space group P4/$n m m. Anisotropic displacement factors of the Pt atoms at the 2a (3/4 1/4 0) and Si atoms at the 2c (1/4 1/4 z) Wyckoff sites are found to be anomalously large.
Single crystals of HoIr$_2$Si$_2$ with the body-centered ThCr$_2$Si$_2$-type structure ($I4/mmm$) were grown by Bridgman method from indium flux. Single crystal structure determination yielded a Si-z position of 0.378(1) in the structure. We excluded the presence of the high temperature phase with the primitive CaBe$_2$Ge$_2$-type structure ($P 4/n m m$) by powder X-ray diffraction. Magnetic measurements on the single crystals yield a Neel temperature of $T_{rm N}=22,rm K$. In the inverse magnetic susceptibility a strong anisotropy with Weiss temperatures $Theta_{W}^{001}=26,rm K$ and $Theta_{W}^{100}=-26,rm K$ occurs above $T_{rm N}$. The effective magnetic moment $mu_{rm eff}^{001}=10.64mu_{B}$ and $mu_{rm eff}^{100}=10.53mu_{B}$ is close to the expected value for a free Ho$^{3+}$ ion, $mu_{rm eff}^{calc}=10.6mu_{B}$. The field dependent magnetization shows a step-like behaviour due to crystalline electric field effects.
The observation of Ising quasiparticles is a signatory feature of the hidden order phase of URu$_2$Si$_2$. In this paper we discuss its nature and the strong constraints it places on current theories of the hidden order. In the hastatic theory such anisotropic quasiparticles are naturally described described by resonant scattering between half-integer spin conduction electrons and integer-spin Ising moments. The hybridization that mixes states of different Kramers parity is spinorial; its role as an symmetry-breaking order parameter is consistent with optical and tunnelling probes that indicate its sudden development at the hidden order transition. We discuss the microscopic origin of hastatic order, identifying it as a fractionalization of three body bound-states into integer spin fermions and half-integer spin bosons. After reviewing key features of hastatic order and their broader implications, we discuss our predictions for experiment and recent measurements. We end with challenges both for hastatic order and more generally for any theory of the hidden order state in URu$_2$Si$_2$.
We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URuSi to probe the possible existence of a small magnetic moment parallel to tetragonal basal plane in the hidden-order phase. This small in-plane component of the magnetic moment on the uranium sites $S_parallel$ has been postulated by two recent models (rank-5 superspin/hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of-plane component $S_perp ~ approx~0.01-0.04 $mu_B$/U. In order to separate $S_parallel$ and $S_perp$ we take advantage of the condition that for magnetic neutron scattering only the components of the magnetic structure that are perpendicular to the scattering vector $Q$ contribute to the magnetic scattering. We find no evidence for an in-plane magnetic moment $S_parallel$. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is $S^{rm{max}}_parallel ~ leq~1cdot 10^{-3} ~mu_B$/U.
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