Ferromagnetic-like Interactions Research Articles

Evidence of ferromagnetic ordering at low temperatures in epitaxial TbMnO3 thin films grown by means of DC magnetron sputtering

Thin films of the archetypal multiferroic TbMnO3 were epitaxially deposited on (0 0 1)-oriented SrTiO3 substrates by means of DC magnetron sputtering. A substrate temperature of 1053 K and an oxygen pressure of 300 Pa were determined to be appropriate growing conditions for obtaining high-quality, epitaxial TbMnO3 films using this physical growing technique. The structural and magnetic properties of ~100 nm thin TbMnO3 films were investigated. X-ray diffraction patterns showed that the films are single-phase and (00ℓ)-oriented. Rocking curves of the (0 0 2) peak confirm the good crystalline quality of the films. X-ray photoelectron spectroscopy analysis confirmed that the nominal valence of the Mn ions is 3+. The films grown on the SrTiO3 substrates displayed substantial epitaxial strain, which decidedly changed the magnetic ground state of the bulk-like counterpart. In particular, the films displayed ferromagnetic-like interactions below a temperature close to the magnetic ordering temperature of the Mn3+ spins (TN ≈ 42 K). Well-defined hysteresis loops with coercive fields as high as 0.1 T were observed at 5 K. The variation of the coercive field with the temperature clearly indicated the presence of low-temperature ferromagnetism in the TMO films. The origin of the anomalous ferromagnetism in the films is discussed in terms of the strain-induced distortion generated by the lattice mismatch between the film and substrate. The reported results can give insight into accurate strategies for controlling single-phase spin-driven multiferroic states.

Equilibrium, metastability, and hysteresis in a model spin-crossover material with nearest-neighbor antiferromagnetic-like and long-range ferromagnetic-like interactions

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical endpoints and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.

Enlarged Mn 3s splitting and room-temperature ferromagnetism in epitaxially grown oxygen doped Mn2N0.86 films

Single-phase and oxygen doped Mn2N0.86 thin films have been grown on MgO (111) by plasma-assisted molecular beam epitaxy. The films grow under tensile strain and, remarkably, they show ferromagnetic-like interactions at low temperature and ferromagnetic ordering agreed well with the Bloch-law T3/2 at room-temperature. We further demonstrate the enlarged Mn 3s splitting (6.46 eV) and its possible relation to the observed ferromagnetism. Our study not only provide a strategy for further theoretical work on oxygen doped manganese nitrides, but also shed promising light on utilizing its room-temperature FM property to fabricate spintronic devices.

Relationship between structural and magnetic properties in (Ti,Fe)O2 powders obtained by mechanical milling

Fe-doped TiO2 samples with different Fe content were prepared by mechanical alloying starting from TiO2 rutile and FeO. The samples were structurally and magnetically characterized by XRD, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), AC-susceptibility and magnetization measurements. XAS results showed that Fe ions were incorporated into the rutile phase with oxygen coordination that was lower than that expected in this phase. The oxygen coordination number decreased with the increase of Fe2+ ions such as it was previously found in the milled samples of TiO2 doped with hematite. The RT Mössbauer spectra were reproduced using two paramagnetic interactions, one corresponding to Fe2+ (δ∼0.87mm/s) and the other to Fe3+ (δ∼0.31mm/s). Magnetometry measurements showed the presence of paramagnetic and ferromagnetic-like interactions at room temperature. Although saturation and coercivity of the ferromagnetic phase increased with iron, the effective magnetic moment per iron atom decreased, probably due to the precipitation of Fe rich antiferromagnetic structures.

Hole doping into the metastable LuMnO 3 perovskite

Single-phase samples of (Lu 1− x Ca x )MnO 3 of the orthorhombic perovskite structure have been realized through solid-state synthesis under high pressures for the Ca contents 0 ≤ x ≤ 0.3 and at ambient pressure for x = 0.5 . The samples were characterized for basic structural, magnetic and transport properties with respect to x . With increasing x , Mn is gradually oxidized and the cooperative Jahn–Teller distortion appears to diminish. Increasing the concentration of Mn IV at the expense of Mn III was found to enhance the ferromagnetic-like interactions and decrease electrical resistivity. Efforts to dope the orthorhombic LuMnO 3 perovskite phase with holes by other means than the aliovalent Ca II-for- Lu III substitution turned out to be infeasible: the larger alkaline earth element, Sr, did not enter the phase and high-pressure oxygenation treatments resulted in stabilization of the Lu 2Mn 2O 7 pyrochlore phase.

Electrical resistivity and specific heat studies of NpFe 4Al 8

Electrical resistivity and specific heat of a stoichiometric NpFe 4Al 8 polycrystalline sample were measured between 2 and 300 K, and under 0–9 T applied magnetic fields. Both measurements confirm the occurrence of magnetic ordering at 135(5) K. The electrical resistivity displays a second anomaly at 113(5) K, which was ascribed to the neptunium magnetic ordering. There is a clear evidence of predominant ferromagnetic-like interactions in this compound, but with a complex magnetic structure, as previously suggested by the magnetic measurements and theoretical calculations. The high γ = 183(10) mJ mol −1 K −2 value classifies the NpFe 4Al 8 compound as a moderate heavy fermion.