57 research outputs found
Strain-induced magnetic phase transition in SrCoO thin films
It has been well established that both in bulk at ambient pressure and for
films under modest strains, cubic SrCoO () is a
ferromagnetic metal. Recent theoretical work, however, indicates that a
magnetic phase transition to an antiferromagnetic structure could occur under
large strain accompanied by a metal-insulator transition. We have observed a
strain-induced ferromagnetic to antiferromagnetic phase transition in
SrCoO films grown on DyScO substrates, which provide a large
tensile epitaxial strain, as compared to ferromagnetic films under lower
tensile strain on SrTiO substrates. Magnetometry results demonstrate the
existence of antiferromagnetic spin correlations and neutron diffraction
experiments provide a direct evidence for a G-type antiferromagnetic structure
with Ne\'el temperatures between and depending on the oxygen content of the samples. Therefore, our
data experimentally confirm the predicted strain-induced magnetic phase
transition to an antiferromagnetic state for SrCoO thin films
under large epitaxial strain.Comment: 6 pages, 4 figure
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FeCrâSâ in magnetic fields: possible evidence for a multiferroic ground state.
We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC â 170â
K and the transition at TOO â 10â
K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9â
T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5â
T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60â
K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5â
T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10â
K
Element-Specific Depth Profile of Magnetism and Stoichiometry at the La0.67Sr0.33MnO3/BiFeO3 Interface
Depth-sensitive magnetic, structural and chemical characterization is
important in the understanding and optimization of novel physical phenomena
emerging at interfaces of transition metal oxide heterostructures. In a
simultaneous approach we have used polarized neutron and resonant X-ray
reflectometry to determine the magnetic profile across atomically sharp
interfaces of ferromagnetic La0.67Sr0.33MnO3 / multiferroic BiFeO3 bi-layers
with sub-nanometer resolution. In particular, the X-ray resonant magnetic
reflectivity measurements at the Fe and Mn resonance edges allowed us to
determine the element specific depth profile of the ferromagnetic moments in
both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a
magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in
contrast to previous observations on inversely deposited layers. Additional
resonant X-ray reflection measurements indicate a region of an altered Mn- and
O-content at the interface, with a thickness matching that of the magnetic
diluted layer, as origin of the reduction of the magnetic moment.Comment: 13 pages, 4 figures, supplemental material include
SrIrO/SrIrO superlattice for a model 2D quantum Heisenberg antiferromagnet
Spin-orbit entangled pseudospins hold promise for a wide array of exotic
magnetism ranging from a Heisenberg antiferromagnet to a Kitaev spin liquid
depending on the lattice and bonding geometry, but many of the host materials
suffer from lattice distortions and deviate from idealized models in part due
to inherent strong pseudospin-lattice coupling. Here, we report on the
synthesis of a magnetic superlattice comprising the single (=1) and the
double (=2) layer members of the Ruddlesden-Popper series iridates
SrIrO alternating along the -axis, and provide a
comprehensive study of its lattice and magnetic structures using scanning
transmission electron microscopy, resonant elastic and inelastic x-ray
scattering, third harmonic generation measurements and Raman spectroscopy. The
superlattice is free of the structural distortions reported for the parent
phases and has a higher point group symmetry, while preserving the magnetic
orders and pseudospin dynamics inherited from the parent phases, featuring two
magnetic transitions with two symmetry-distinct orders. We infer weaker
pseudospin-lattice coupling from the analysis of Raman spectra and attribute it
to frustrated magnetic-elastic couplings. Thus, the superlattice expresses a
near ideal network of effective spin-one-half moments on a square lattice
Proximate ferromagnetic state in the Kitaev model material α-RuCl3
α-RuCl is a major candidate for the realization of the Kitaev quantum spin liquid, but its zigzag antiferromagnetic order at low temperatures indicates deviations from the Kitaev model. We have quantified the spin Hamiltonian of α-RuCl by a resonant inelastic x-ray scattering study at the Ru L absorption edge. In the paramagnetic state, the quasi-elastic intensity of magnetic excitations has a broad maximum around the zone center without any local maxima at the zigzag magnetic Bragg wavevectors. This finding implies that the zigzag order is fragile and readily destabilized by competing ferromagnetic correlations. The classical ground state of the experimentally determined Hamiltonian is actually ferromagnetic. The zigzag state is stabilized by quantum fluctuations, leaving ferromagnetism â along with the Kitaev spin liquid â as energetically proximate metastable states. The three closely competing states and their collective excitations hold the key to the theoretical understanding of the unusual properties of α-RuCl in magnetic fields
Pseudospin-lattice coupling in the spin-orbit Mott insulator Sr2IrO4
Spin-orbit entangled magnetic dipoles, often referred to as pseudospins, provide a new avenue to explore novel magnetism inconceivable in the weak spin-orbit coupling limit, but the nature of their low-energy interactions remains to be understood. We present a comprehensive study of the static magnetism and low-energy pseudospin dynamics in the archetypal spin-orbit Mott insulator Sr2IrO4. We find that in order to understand even basic magnetization measurements, a formerly overlooked in-plane anisotropy is fundamental. In addition to magnetometry, we use neutron diffraction, inelastic neutron scattering, and resonant elastic and inelastic x-ray scattering to identify and quantify the interactions that determine the global symmetry of the system and govern the linear responses of pseudospins to external magnetic fields and their low-energy dynamics. We find that a pseudospin-only Hamiltonian is insufficient for an accurate description of the magnetism in Sr2IrO4 and that pseudospin-lattice coupling is essential. This finding should be generally applicable to other pseudospin systems with sizable orbital moments sensitive to anisotropic crystalline environments
Spiral magnetism, spin flop, and pressure induced ferromagnetism in the negative charge transfer gap insulator Sr2FeO4
Iron IV oxides are strongly correlated materials with negative charge transfer energy negative Delta , and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in themetallic cubic perovskite SrFeO3. Here, the spin structure of the layered negative Delta insulator Sr2FeO4 was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below TN 56K, Sr2FeO4 adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 amp; 956;B and a propagation vector k amp; 964;, amp; 964;, 0 with amp; 964; 0.137. With increasing magnetic field the spin structure undergoes a spin flop transition near 5 T. Synchrotron 57Fe Mössbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman active phonon nonintrinsic to the K2NiF4 type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe IV oxide
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