Strain and electric-field control of magnetism in supercrystalline iron oxide nanoparticle-BaTiO3 composites.

The manipulation of the magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO3 (BTO) single crystals is reported. We observe strain induced magnetoelectric coupling (MEC) as shown by measurements of both the magnetization and magneto-electric AC susceptibility (MEACS). The magnetization, coercivity, remanent magnetization and MEACS signal as a function of temperature show abrupt jumps at the BTO phase transition temperatures. Hereby the jump values are opposite for in-plane and out-of-plane measurements. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) confirm a hexagonal close-packed supercrystalline order of the NP monolayers. Cross-sectional scanning transmission electron microscopy (STEM) experiments provide information about the layer structure of the sample. This work opens up viable possibilities for fabricating energy-efficient electronic devices by self-assembly techniques.

Hyperfine and crystal field interactions in multiferroic HoCrO3.

We report a comprehensive specific heat and inelastic neutron scattering study to explore the possible origin of multiferroicity in HoCrO3. We have performed specific heat measurements in the temperature range 100 mK-290 K and inelastic neutron scattering measurements were performed in the temperature range 1.5-200 K. From the specific heat data we determined hyperfine splitting at 22.5(2) µeV and crystal field transitions at 1.379(5) meV, 10.37(4) meV, 15.49(9) meV and 23.44(9) meV, indicating the existence of strong hyperfine and crystal field interactions in HoCrO3. Further, an effective hyperfine field is determined to be 600(3) T. The quasielastic scattering observed in the inelastic scattering data and a large linear term [Formula: see text] mJ mol(-1) K(-2) in the specific heat is attributed to the presence of short range exchange interactions, which is understood to be contributing to the observed ferroelectricity. Further the nuclear and magnetic entropies were computed to be, ∼17.2 Jmol(-1) K(-1) and ∼34 Jmol(-1) K(-1), respectively. The entropy values are in excellent agreement with the limiting theoretical values. An anomaly is observed in the peak position of the temperature dependent crystal field spectra around 60 K, at the same temperature an anomaly in the pyroelectric current is reported. From this we could elucidate a direct correlation between the crystal electric field excitations of Ho(3+) and ferroelectricity in HoCrO3. Our present study, along with recent reports, confirm that HoCrO3, and RCrO3 (R = rare earth) in general, possess more than one driving force for the ferroelectricity and multiferroicity.

Direct observation of low energy nuclear spin excitations in HoCrO3 by high resolution neutron spectroscopy.

We have investigated low energy nuclear spin excitations in the strongly correlated electron compound HoCrO3. We observe clear inelastic peaks at E = 22.18 ± 0.04 µeV in both energy loss and gain sides. The energy of the inelastic peaks remains constant in the temperature range 1.5-40 K at which they are observed. The intensity of the inelastic peak increases at first with increasing temperature and then decreases at higher temperatures. The temperature dependence of the energy and intensity of the inelastic peaks is very unusual compared to that observed in other Nd, Co, V and also simple Ho compounds. Huge quasielastic scattering appears at higher temperatures presumably due to the fluctuating electronic moments of the Ho ions that get increasingly disordered at higher temperatures. The strong quasielastic scattering may also originate in the first Ho crystal-field excitations at about 1.5 meV.

Neutron-diffraction study of structural transition and magnetic order in orthorhombic and rhombohedral La(7/8)Sr(1/8)Mn(1-γ)O(3+δ).

We report on a systematic neutron powder diffraction (NPD) study of polycrystalline La(7/8)Sr(1/8)Mn(1-γ)O(3+δ) compounds. We investigated the structural and magnetic phases and transitions in the temperature range between 10 and 900 K for two different samples: an Ar-annealed sample with an orthorhombic Pbnm (Z = 4; O) structure at room temperature and an air-sintered sample with a rhombohedral [Formula: see text] (Z = 2;R) structure at room temperature. At higher temperatures, above 400 K, both samples exhibit a rhombohedral structure. For the Ar-annealed sample, a Jahn-Teller (JT) transition occurs in the orthorhombic phase at about 298 K with very large variations in the Mn-O2' and Mn-O2 bond lengths and Mn-O1-Mn bond angle on cooling from 298 to 180 K. For this sample the ferromagnetic moment at 10 K in the magnetic space group Pb'n'm amounts to 3.22(5) µ(B)/Mn. In contrast, the air-sintered sample undergoes on cooling a structural transition from rhombohedral (R) to orthorhombic (O) with a mixed phase of nearly equal R and O repartition at 120 K. Ferromagnetic order develops in this sample at about 240 K with a low-temperature moment of 3.35(4) µ(B)/Mn at 10 K. The coherent JT distortion in its orthorhombic phase occurs below some 170 K. In addition, we have determined the coherent JT distortion parameter Δ, the tolerance factor t and the one-electron bandwidth W of the e(g)-band.