Session Y8: Focus Session: Frustrated Magnetism - Experiment

Magnetic ordering in SrEr$_2$O$_4$ and SrHo$_2$O$_4$

Single crystal neutron diffraction reveals two distinct components to the magnetic ordering in geometrically frustrated compounds $\rm SrEr_2O_4$ and $\rm SrHo_2O_4$. One component, a long-range ordered ${\bf k}=0$ structure, is associated with the appearance of resolution limited Bragg peaks below the ordering temperature. Another component is a quasi 1D short-range structure which manifests itself by the presence of a strong diffuse scattering signal forming {\it planes} in reciprocal space. On cooling from higher temperatures down to 0.06~K, the partially ordered component develops gradually and does not undergo a pronounced phase transition. The magnetic moments in the long-range structure are pointing along the [001] axes in both compounds. In the short-range structure (which is incommensurate in $\rm SrEr_2O_4$ and ${\bf k} \approx 1/2$ in $\rm SrHo_2O_4$) the moments are predominantly pointing along the [001] and [010] axes in these two compounds respectively. The unusual coexistence of two magnetic structures is probed using XYZ-polarised neutron scattering techniques [1,2]. [1] T.J. Hayes {\it et al.}, to appear in Phys. Rev. B (2011). [2] O. Young {\it et al.}, in preparation (2011).   

Magnetic order in GdBiPt studied by x-ray resonant magnetic scattering

Rare earth ($R$) half-Heusler compounds, $R$BiPt, exhibit a wide spectrum of novel ground states.[1] We have employed x-ray resonant magnetic scattering to elucidate the microscopic details of the magnetic structure in GdBiPt below $T_{\textrm{N}}$ = 8.5\,K. Experiments at the Gd $L_{2}$ absorption edge show that the Gd moments order in an antiferromagnetic stacking along the cubic diagonal [1\,1\,1] direction satisfying the requirement for an antiferromagnetic topological insulator as proposed previously[2] where both time-reversal symmetry and lattice translational symmetry are broken, but their product is conserved. \\ The work at the Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, DMSE, contract DE-AC02-07CH11358. \\ \,[1] P. C. Canfield et al., J. Appl. Phys. \textbf{70}, 5800 (1991). \\ \,[2] R. S. K. Mong et al., Phys. Rev. B \textbf{81}, 245209 (2010).   

X-ray resonance Exchange scattering study of Field induced meta-magnetic phases in TbNi$_2$Ge$_2$

Rare-earth inter-metallic compound, TbNi$_2$Ge$_2$ is an interesting material for its uniaxial anisotropy and the presence of a number of meta-magnetic phases (MP), which we have investigated using x-ray resonant exchange scattering techniques. Two distinct field induced MPs have been revealed at 5 K for fields below 3 T applied along the c axis. In zero field, the magnetic structure is characterized primarily by commensurate q$_1$=(0,0,0.75), q$_2$=(0.5,0.5,0) and q$_3$=(0.5,0.5,0.5) wave vectors, implying a complex sequence of FM and AFM planes. With increasing magnetic field, a weak (0,0,1) peak starts to evolve reaching a maximum in the first MP (1.2 T $<$ H $<$ 1.6 T) and sharply disappears on entering the second MP (H $>$ 1.6 T). In the second MP, q$_1$ becomes incommensurate, (0,0,0.766), q$_2$ gets strongly suppressed, and q$_3$ peak splits into combination harmonics of q$_1$ and q$_2$. The persistence of AF peaks imply that AF planes are quite robust to spin-flip transitions and MP phases are intricate in nature due to several competing interactions in this compound.   

Inelastic neutron scattering of the itinerant magnets Cr2Te3 and tr-Cr5Te8

Itinerant magnets based on transition metal chalcogenide compounds are of current interest, in part due to their relationship to the parent compounds of Fe-based superconductors. Two particularly interesting systems in this family are the chromium tellurides Cr2Te3 and trigonal (tr) Cr5Te8. These materials crystallize in layered structures with alternating partially and fully-occupied planes of Cr atoms stacked along the c-axis. Magnetization measurements along different crystallographic directions show a net ferromagnetic response and large magnetic anisotropy. In addition, the saturation moments are smaller than predicted by an ionic model; consistent with itinerant behavior. Previous neutron diffraction results for Cr2Te3 revealed an ordered moment of $<$ 0.2 $\mu $B in the partially-occupied planes. We examined the magnetic excitations in these materials by powder neutron spectroscopy measurements using the SEQUOIA instrument at the SNS. We find similar moment sizes for the magnetic Cr atoms of both systems. However, despite their similar crystal structures, ordered moment sizes, and chemical compositions, their magnetic excitation spectra are strikingly different. We compare our data to the predictions of various models in an effort to determine the relevant exchange parameters, put constraints on their magnitudes, and understand the differences between the inelastic magnetic spectra. We find that exchange along the c-direction is critical to explain our data.   

Magnetic structure and site occupancies in Fe containing $\mu$-phases AFe (A=Ta, Nb, Mo)

Binary mu-phases are intermetallic compounds occurring in numerous systems combining heavy and light transition metals. They have compositions close to AM where A is a 4d or 5d element (Nb, Ta, Mo, W) and M is a 3d element (Fe, Co, Ni, Zn). The possibility of mixed site occupancies results in stoichiometries ranging from $A_{~7}M_{~6}$ to $A_{~6}M_{~7}$. Interestingly, only the Fe containing compounds have been found to show magnetic ordering. NbFe and TaFe are known to be antiferromagnetic, with Neel temperatures near 280 and 320 K, respectively. It has been reported that in these materials ferromagnetic kagome planes stacked antiferromagnetically along the rhombohedral (111) direction, but the exact site occupancies in these layers remain unclear. In our investigation we have found mu-phase of MoFe also to be antiferromagnetic, but with a unique magnetic structure and significantly lower transition temperature about 110 K. Here we discuss in detail the crystallographic and magnetic structures of TaFe, NbFe and MoFe mu-phases based on our recent neutron diffraction studies and physical property measurements on polycrystalline samples.   

Magnetic Excitations in the Nearly Localized, Itinerant Magnet Gd, Studied by Neutron Spectroscopy

Many of the current questions about magnetic superconductors are present when these complex materials are in the normal state. Therefore studies of simpler itinerant magnets may help provide understanding of these phenomena. We chose to study an Itinerant magnet near to the fully localized limit. The system of choice, Gd has a total moment size of $\sim 7.6 \mu_B$ of which $\sim 0.6\mu_B$ of that is itinerant. We used the SEQUOIA spectrometer, at the Spallation Neutron Source at Oak Ridge National Laboratory, to measure the magnetic excitations in a 12 gm $^{160}$Gd single crystal. The fine resolution Fermi chopper was spun at 360 Hz and phased for $E_i$ = 50 meV. The crystal was mounted with the $h0l$ plane horizontal and then rotated around the vertical axis in $1^{\circ}$ steps. This method, and the large out of plane detector coverage of SEQUOIA, provided continuous coverage of a large region of reciprocal space allowing us to map the magnetic excitations. This map provides a measured structure factor for comparison to spin wave models with and without itinerancy effects. There measurements also more clearly resolve the excitations along the $h00$ direction than in previous studies (J. W. Cable, R. M. Nicklow and N. Wakabayashi Phys. Rev. B {\bf 32}, 1710 (1985)).   

Magnetic and charge carriers properties of metamagnetic Fe$_3$Ga$_4$

Single crystals of Fe$_3$Ga$_4$ were grown via an iodine vapor transport method. Previous investigations of arc-melted polycrystalline samples identify metallic conduction with a magnetic phase transition at 400 K and interesting temperature-dependent metamagnetic behavior. The single crystal samples allow a much fuller exploration of the magnetic properties and have yielded some interesting differences with the previous data. This includes a sharp reduction of the magnetization within the magnetically ordered phase associated with a sharp onset of the metamagnetic behavior in the field dependence near room temperature. A previously identified second phase transition occurs below 50 K where the metamagnetic behavior is replaced by a ferromagnetic magnetization with little hysteresis. We find substantial anisotropy in the magnetization which is particularly apparent between 50 and 300 K. Charge transport experiments are underway to explore the magnetoresistance and Hall effect of this magnet.   

Structural and magnetic characterization of the complete delafossite solid solution (CuAlO$_2$)$_{1-x}$(CuCrO$_2$)$_x$

We have prepared the complete delafossite solid solution between diamagnetic CuAlO$_2$ and the $t_{2g}^3$ frustrated antiferromagnet CuCrO$_2$. The crystal structure and magnetism were studied with powder x-ray diffraction and magnetometry. The unit cell parameters follow the V\'{e}gard law and $\mu_{\rm{eff}}$ is equal to the Cr$^{3+}$ spin-only $S = 3/2$ value. $\Theta_{\rm{CW}}$ is negative and its magnitude increases with Cr substitution. For dilute Cr compositions, $J_{\rm{BB}}$ was estimated by mean-field theory to be 3.0\,meV. Despite the sizable $\Theta_{\rm{CW}}$, long-range antiferromagnetic order does not develop until $x$ is almost 1, and is preceeded by glassy behavior. For all samples, the 5\,K isothermal magnetization is sub-Brillouin and does not saturate in fields up to 5\,T. A scaled inverse susceptibility plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO$_2$ above its N\'eel temperature. Additionally, the Al-substituted samples exhibit uncompensated short-range behavior and $x = 0.75$ shows glassy characteristics. It is suggested that reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as do chemical disorder and dilution of magnetic exchange.   

Quasi-Spin Glass in a Geometrically Frustrated Magnet

A spin glass state is observed in the B-spinel ZnCr$_{2(1-x)}$Ga$_{2x}$O$_{4}$ for x $<$ 0.05 via low-temperature magnetization and specific heat. The spin glass phenomenology is conventional. The degrees of freedom (quasi-spins) that undergo freezing are unconventional, however, both in structure as well as their mutual interactions. In particular, below x = 0.05, the freezing temperature is independent of quasi-spin density, yielding a strong violation of mean field theory.   

Thermodynamic tuning in the dilute Ising magnet LiHo$_{0.045}$Y$_{0.955}$F$_4$

The LiHo$_x$Y$_{1-x}$F$_4$ family of dipole-coupled Ising magnets has been found to exhibit a rich variety of magnetic phases, ranging from ferromagnet to spin glass to decoupled spin clusters. We examine the behavior for x=4.5\% at dilution refrigerator temperatures as a function of thermal coupling to a heat reservoir. We show that by cooling the sample in different thermodynamic limits, we can select qualitatively different low temperature magnetic states with different quantum characters. Furthermore, we demonstrate that an external transverse magnetic field, which acts to tune the rate of quantum tunneling, can be used to switch the system between the two states.   

High Pressure $^{31}$P-NMR Study of the 2D Frustrated Square-Lattice Compound BaCdVO(PO$_{4})_{2}$ at Low Temperatures

BaCdVO(PO$_{4})_{2}$ is a $S $= 1/2 frustrated square-lattice (FSL) compound with a nearest neighbor exchange coupling $J_{1}\sim $ - 3.36 K and a next-nearest neighbor exchange coupling $J_{2}\sim $ 3.53 K bearing $J_{2}$/$J_{1}\sim $ 1.05. Based on the $J_{2}$/$J_{1}$ ratio, the system is known to be located close to the disordered ground state (known as `nematic state') regime of the phase diagram. We have carried out $^{31}$P-NMR measurements under high pressure ($\sim $ 1 GPa) and at low temperatures using dilution refrigerator to investigate the pressure effects on the magnetic properties of the system. Under ambient pressure at $H $= 2.67 T, we observed a sharp peak in $^{31}$P spin lattice relaxation rate (1/$T_{1})$ at $T_{N}\sim $ 1.05 K, which corresponds to the antiferromagnetic ordering temperature. On the other hand, under a pressure of 1 GPa, the peak in 1/$T_{1}$ is suppressed and 1/$T_{1}$ shows a sudden decrease below $\sim $ 0.75 K. This indicates that $T_{N}$ decreases with the application of pressure. We will report the temperature dependence of the NMR spectra and of the 1/$T_{1}$ under different magnetic fields and pressures.   

Triplet state and in-gap scattering from the spin-$^1$/$_2$ quantum antiferromagnet Ba$_2$YMoO$_6$

The double perovskite material Ba$_2$YMoO$_6$ is known to exhibit an absence of long-range and short-range magnetic order down to at least 2K, indicating a high degree of geometric frustration of its tetrahedrally-coordinated spin-$^1$/$_2$ Mo$^{5+}$ moments, and NMR results have implied the existence of a spin-singlet ground state. Though geometric frustration in both 3D and quasi-2D systems has been of intense interest in recent years, comparatively little attention has been given to FCC systems, which may exhibit geometric frustration as the FCC lattice can be viewed as a network of edge-sharing tetrahedra. We have conducted inelastic neutron scattering measurements using triple-axis and time-of-flight instruments, revealing a band of scattering at 28 meV which disappears above $\sim$125K; we identify this scattering band as the triplet excitation out of a singlet ground state. We also identify a weaker population of in-gap states which are reminiscent of spin-polaron states induced by weak disorder.   

Magnetothermal transport in monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$

The monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ is an interesting material due to the existence of a novel field-induced slow spin relaxation (SSR) in the paramagnetic state. The SSR is attributed to the cooperative relaxation mode formed through the spin-spin interactions between partially polarized spins. Here we report the magnetothermal transport measurement on Nd$_{2}$Ti$_{2}$O$_{7}$ single crystals. The zero field thermal conductivity is dominated by the phonon contribution. The magnetothermal conductivity reveals significant field dependence. The field and temperature dependence is discussed in connection with the field-induced slow spin relaxation. .