LaCoO3 (LCO) nanoparticles were synthesized and their magnetic and structural properties were examined using SQUID magnetometery and neutron diffraction. The nanoparticles exhibit ferromagnetic long-range order beginning at T_C approximately 87K that
persists to low temperatures. This behavior is contrasted with the ferromagnetism of bulk LCO, which also starts at T_C approximately 87K but is suppressed below a second transition at T_o approximately 37K, due to a structural phase transition. The ferromagnetism in both systems is attributed to the tensile stress from particle surfaces and impurity phase interfaces. This stress locally increases the Co-O-Co bond angle gamma, and competes with the thermal contraction of the lattice. It has recently been shown that LCO loses long-range ferromagnetic order when gamma decreases below the critical value gamma_c = 162.8 degrees. Consistent with this model, we show that gamma in nanoparticles remains larger than gamma_c at low temperatures, likely a consequence of all spins being in close proximity to surfaces or interfaces.
Elastic neutron scattering is used to study the spin correlations in the multiferroic $rm Mn_{1-x}Fe_{x}WO_4$ with $x=0.035, 0.05$ and 0.10. The noncollinear, incommensurate (ICM) magnetic structure associated with the ferroelectric (FE) phase in pur
e $rm MnWO_4$ is suppressed at $x=0.035$ and completely absent at $x=0.10$. The ICM spin order and FE phase can be restored by applying a magnetic field along the spin easy-axis. The low-$T$ commensurate magnetic structure extends in both H/T with increasing Fe concentration. The systematic evolution of the magnetic and electric properties indicates that the noncollinear ICM spin order results from competing magnetic interactions and its stabilization can be tuned by the internal ($x$) or external (magnetic field) perturbations.
Neutron scattering has been used to investigate the evolution of the long- and short-range charge-ordered (CO), ferromagnetic (FM), and antiferromagnetic (AF) correlations in single crystals of Pr1-xCaxMnO3. The existence and population of spin clust
ers as refected by short-range correlations are found to drastically depend on the doping (x) and temperature (T). Concentrated spin clusters coexist with long-range canted AF order in a wide temperature range in x = 0.3 while clusters do not appear in x = 0.4 crystal. In contrast, both CO and AF order parameters in the x = 0.35 crystal show a precipitous decrease below ~ 35 K where spin clusters form. These results provide direct evidence of magnetic phase separation and indicate that there is a critical doping x_c (close to x = 0.35) that divides the phase-separated site-centered from the homogeneous bond-centered or charge-disproportionated CO ground state.
The spin wave excitations of the geometrically frustrated triangular lattice antiferromagnet (TLA) $rm CuFeO_2$ have been measured using high resolution inelastic neutron scattering. Antiferromagnetic interactions up to third nearest neighbors in the
ab plane (J_1, J_2, J_3, with $J_2/J_1 approx 0.44$ and $J_3/J_1 approx 0.57$), as well as out-of-plane coupling (J_z, with $J_z/J_1 approx 0.29$) are required to describe the spin wave dispersion relations, indicating a three dimensional character of the magnetic interactions. Two energy dips in the spin wave dispersion occur at the incommensurate wavevectors associated with multiferroic phase, and can be interpreted as dynamic precursors to the magnetoelectric behavior in this system.
Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of special interest not only because they are a testing ground of the classical doubleexchange interaction mechanism for the colossal magnetoresistance, but also because they
exhibit an extraordinary arena of emergent phenomena. These emergent phenomena are related to the complexity associated with strong interplay between charge, spin, orbital, and lattice. In this review, we focus on the use of inelastic neutron scattering to study the spin dynamics, mainly the magnon excitations in this class of FM metallic materials. In particular, we discussed the unusual magnon softening and damping near the Brillouin zone boundary in relatively narrow band compounds with strong Jahn-Teller lattice distortion and charge/orbital correlations. The anomalous behaviors of magnons in these compounds indicate the likelihood of cooperative excitations involving spin, lattice, as well as orbital degrees of freedom.
By combining dielectric, specific heat, and magnetization measurements and high-resolution neutron powder diffraction, we have investigated the thermodynamic and magnetic/structural properties of the metastable orthorhombic perovskite ErMnO_3 prepare
d by high-pressure synthesis. The system becomes antiferromagnetically correlated below 42 K and undergoes a lock-in transition at 28 K with propagation wave vector (0,k_b,0), which remains incommensurate at low temperature. The intercorrelation between the magnetic structure and electric properties and the role of the rare earth moment are discussed.
We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$. On cooling from high temperature, the system first becomes charge- and orbital- ordered (CO/OO) near $T_{CO}=300$ K
and then develops checkerboard-like antiferromagnetic (AF) order below $T_{N}=130$ K. At temperatures above $T_{N}$ but below $T_{CO}$ ($T_N<T<T_{CO}$), the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.
