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Kawasaki Y.,Tokushima University | Minami T.,Tokushima University | Minami T.,Seiryo Engineering Co. | Izumi M.,Tokushima University | And 13 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The magnetically ordered states of the A-site ordered perovskite manganites LaBaMn 2O 6 and YBaMn 2O 6 have been investigated by muon spin relaxation in zero external magnetic field. Our data reveal striking differences in the nature of the magnetically ordered state between these materials. For LaBaMn 2O 6, the μSR time spectra in the ferromagnetic state below 330 K reveal a strongly inhomogeneous phase, reminiscent of a Griffiths phase. Within this magnetically inhomogeneous phase, an antiferromagnetic state develops below 150 K, which displays well defined static internal magnetic fields, but reaches only 30% of the volume fraction at low temperatures. A broad distribution of μSR relaxation rates is inferred down to the lowest temperatures. This behavior is similar to that in the A-site disordered La 0.5Ba 0.5MnO 3. On the other hand, for YBaMn 2O 6, the μSR time spectra for both (i) the charge and orbital ordered and (ii) the paramagnetic phases reveal rather homogeneous states, namely, an exponential relaxation in the paramagnetic state and well defined muon spin oscillation in the antiferromagnetic state. © 2012 American Physical Society.

Kohama Y.,Los Alamos National Laboratory | Sologubenko A.V.,University of Cologne | Dilley N.R.,Quantum Design Inc. | Zapf V.S.,Los Alamos National Laboratory | And 10 more authors.
Physical Review Letters | Year: 2011

Several quantum paramagnets exhibit magnetic-field-induced quantum phase transitions to an antiferromagnetic state that exists for Hc1≤ H≤Hc2. For some of these compounds, there is a significant asymmetry between the low- and high-field transitions. We present specific heat and thermal conductivity measurements in NiCl2-4SC(NH 2)2, together with calculations which show that the asymmetry is caused by a strong mass renormalization due to quantum fluctuations for H≤Hc1 that are absent for H>Hc2. We argue that the enigmatic lack of asymmetry in thermal conductivity is due to a concomitant renormalization of the impurity scattering. © 2011 American Physical Society.

Journal of Nanoparticle Research | Year: 2014

The magnetic behavior of very fine metallic Ni (Ni0) particles dispersed in a matrix of 10% by mol (m/o) Y2O3-stabilized ZrO2 (10YSZ) was examined. Specimens that nominally contained 0.1, 0.5, and 1 m/o Ni were prepared by a high-purity chemical synthesis method followed by a reduction treatment in H2. A 0.5 m/o Ni specimen was also made by heating in vacuum, within a carbon environment. dc and ac susceptibility measurements revealed the presence of superparamagnetic Ni 0 as well as Ni2+. A Stoner-Wohlfarth formulation was applied to determine that the average Ni0 particle size was between 4 and 7 nm, and this was confirmed by transmission electron microscopy. The activation energy for single-domain particle spin reversal and Keff, the effective anisotropy constant, were determined. Keff was higher for the vacuum heat treatment specimen because it contained less residual organic material, and therefore a higher density of Ni0 particles. It is shown that it is possible to make very fine Ni0 particles in YSZ via a direct conversion from Ni nitrates. © Springer Science+Business Media 2014.

Journal of the American Ceramic Society | Year: 2012

The internal reduction of Ni2+ ions dissolved in 10YSZ (10 percent by mole Y2O3- stabilized ZrO2) was studied using magnetometry. Chemical methods were used to produce specimens with varying amounts of NiO (0.01-11.8% by mol), below and above the solubility limit of NiO in 10YSZ. The solubility limit was determined using x-ray diffraction measurements in which the change in lattice parameter was correlated to the amount of Ni2+ ions in solution. Vibrating sample magnetometry (VSM) was used to unequivocally establish that NiO does not exist as a second phase for specimen compositions less than or equal to 0.5 m/o NiO. Specimens with compositions 0.5, 0.1, and 0.01 m/o NiO in YSZ were then prepared and heat treated in Ar - 2% H2 at 1273 K for times ranging from 1 to 20 h. The ferromagnetic response of the reduced specimens was measured using SQUID magnetometry at 100 K to quantify the amount of Ni metal formed as a function of time. SQUID magnetometry measurements of the same specimens were performed at 5 K where the paramagnetic response may be used to quantify the decrease of Ni2+ ions during reduction. The two SQUID measurements agree well and revealed parabolic growth law kinetics. Transmission electron microscopy revealed that internal reduction in these specimens proceeds by the formation of Ni0 along the YSZ grain boundaries. The results indicate that there are two stages of internal reduction in polycrystalline NiO-containing YSZ. For short times, precipitation of Ni occurs at grain-boundary regions, and for longer times, it occurs inside the grains. © 2012 The American Ceramic Society.

Journal of Materials Science | Year: 2012

The solubility of NiO in ZrO 2 was studied by X-ray diffraction, TEM, and SQUID magnetometry. Lattice parameter measurements from a similar, established oxide system, NiO-10YSZ, were first used to show that SQUID magnetometry can effectively measure solubility. ZrO 2 specimens with 0, 0.5, 1, 2, 3, and 5 percent by mol NiO were prepared via the Pechini method. The specimens were calcined in air at 500, 600, and 1000°C. The paramagnetic response of the specimens measured with SQUID magnetometry revealed that up to 5 percent by mol NiO is soluble in ZrO 2 for specimens calcined at 500 and 600°C. The relatively large solubility compared with NiO-10YSZ occurs due to the very fine grain size (5-10 nm). The fine grain size is also responsible for stabilizing the tetragonal phase of ZrO 2. At the 1000°C calcination temperature, the ZrO 2 is entirely monoclinic, exhibits larger grains (≥45 nm), and only dissolves about 0.5 percent by mol or less NiO. The correlations between grain size, ZrO 2 polytype, and NiO addition are discussed. © Springer Science+Business Media, LLC 2011.

Munbodh K.,West Virginia University | Cheon M.,West Virginia University | Lederman D.,West Virginia University | Fitzsimmons M.R.,Los Alamos National Laboratory | Dilley N.R.,Quantum Design Inc.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Depth profiles for pinned and unpinned magnetizations were determined across the interface between a ferromagnet (F) and random and dilute antiferromagnets (RAF and DAF) exemplified by Fe 0.45Ni 0.55F 2/Co and Fe 0.34Zn 0.66F 2/Co bilayers, respectively, using polarized neutron reflectivity (PNR). PNR measurements were complemented by magnetometry using applied fields as large as 160 kOe to assure saturation of the entire sample, including magnetic moments that are normally pinned at lower fields. The locations of pinned and unpinned magnetization in the ferro- and antiferromagnets were identified. The origin of exchange bias in the RAF system is noticeably different than that of the DAF system. In the RAF system, a domain wall is formed at the RAF/F interface when the ferromagnet's magnetization is reversed. In the DAF system, some domains within the bulk of the DAF are reversed upon reversal of the ferromagnet while others remain pinned. In this case, the interface magnetization is entirely reversed. © 2011 American Physical Society.

IEEE Transactions on Magnetics | Year: 2014

We present a magneto-optic probe designed for the studies of photoinduced magnetization in the highly sensitive SQUID susceptometer MPMS 3. The apparatus is used to conduct light from an external light source into the sealed sample chamber of the susceptometer and deliver the light stimuli right onto the sample, thus allowing uninterrupted measurement as functions of temperature and field, from 1.8 to 350 K, and in magnetic field up to 7 T. The fiber optic sample holder (FOSH) consists mostly of quartz components in order to minimize magnetic background. With background subtraction, the moment resolution of the FOSH can reach $$10^{\mathrm {\mathbf {-6}}}$$ emu range even at full field. Future designs are underway to further reduce the magnetic background of the sample holder. © 1965-2012 IEEE.