Haizhong Guo

Raman spectroscopy evidence of strong spin-phonon coupling in epitaxial thin films of the double perovskite La2NiMnO6

The Raman spectra of epitaxial thin films of La2NiMnO6 on (001)-oriented LaAlO3 substrates have been measured in several exact scattering configurations between 12 and 773K. The evolution of the spectra with increasing temperature is consistent with a transition from the monoclinic P121∕n1 to rhombohedral R3¯ structure. The spin-phonon coupling results in significant softening of the phonon mode involving stretching vibrations of the (Ni∕Mn)O6 octahedra. The anomalous softening extends above TC=275K, indicating the presence of short range ordering up to 400K.The Raman spectra of epitaxial thin films of La2NiMnO6 on (001)-oriented LaAlO3 substrates have been measured in several exact scattering configurations between 12 and 773K. The evolution of the spectra with increasing temperature is consistent with a transition from the monoclinic P121∕n1 to rhombohedral R3¯ structure. The spin-phonon coupling results in significant softening of the phonon mode involving stretching vibrations of the (Ni∕Mn)O6 octahedra. The anomalous softening extends above TC=275K, indicating the presence of short range ordering up to 400K.

Growth of epitaxial thin films of the ordered double perovskite La2NiMnO6 on different substrates

Epitaxial thin films of La2NiMnO6, a ferromagnetic semiconductor, have been fabricated on different substrates by pulsed laser deposition. The x-ray diffraction and Raman scattering observations reveal that the films are single crystalline and have an orthorhombic structure. The magnetic properties of the films, including the coercive field, remanent magnetization, and Curie temperature, are strongly dependent on the choice of the substrate. The optimized films exhibit a magnetic moment of 4.63μB∕f.u. at 5K, with a Curie temperature close to 280K. The film characteristics are promising for potential device applications in information storage, spintronics, and sensors.

Structural and magnetic properties of epitaxial thin films of the ordered double perovskite La2CoMnO6

Epitaxial thin films of the ordered ferromagnetic double perovskite La2CoMnO6 have been grown on (001)-oriented SrTiO3 substrates by pulsed laser deposition. X-ray photoemission spectroscopy analysis suggests that the Mn and Co ions are primarily in the 4+ and 2+ valence states. Magnetic measurements indicate the presence of two phases: a majority and a minority ferromagnetic phase with transition temperatures of 230 and 80K, respectively. A spin-glass-type magnetic behavior is also indicated for the films based on time-dependent measurements. The spin-glass behavior likely arises from a competition between ferromagnetic and antiferromagnetic interactions because of local disorder, even in films grown under optimal conditions.

Effect of oxygen concentration on the magnetic properties of La2CoMnO6 thin films

The dependence of the magnetic properties on oxygen concentration in epitaxial La2CoMnO6 thin films deposited on (100)-oriented SrTiO3 substrates has been investigated by varying the oxygen background pressure during growth using pulsed laser deposition. Two distinct ferromagnetic (FM) phases are revealed, and the relative fraction varies with the oxygen concentration. The existence of oxygen vacancies induces the local vibronic Mn3+–O–Co3+ superexchange interactions in direct competition with the static FM Mn4+–O–Co2+ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions.

Preparation and characterization of epitaxial ilmenite-hematite films

The solid solution system ilmenite-hematite [(FeTiO3)1−x–(Fe2O3)x] is a potential candidate for applications in spintronics due to its intrinsic ferrimagnetic and semiconducting properties. Epitaxial ilmenite-hematite films with x=0.33, which have the highest room temperature magnetic moment, were grown on α-Al2O3 (0001) substrates using pulsed laser deposition technique with varying oxygen partial pressure in the ambient gas. Structural, magnetic, electrical, and optical properties are found to be largely dependent on the oxygen content of the films which is controlled by substrate temperature and ambient gas composition. The highest crystalline and magnetic ordering and the lowest resistivity values could be obtained for growth at high temperatures and under low oxygen pressure. A narrowing of the band gap (to around 2.4eV) was observed for films grown under high oxygen pressure in comparison with films grown in vacuum or argon (around 3.3eV).

The effect of matrix and substrate on the coercivity and blocking temperature of self-assembled Ni nanoparticles

We have shown that the magnetic properties of nanoparticles may be tuned from superparamagnetic to ferromagnetic by changing the substrate or thin film matrix in which they are embedded. Nickel nanoparticles were embedded into alumina, titanium nitride, and cerium oxide matrices on both silicon and sapphire substrates via pulsed laser deposition. The laser ablation time on the nickel target was kept constant. Only nickel nanoparticles in cerium oxide showed characteristics of ferromagnetism (room temperature coercivity and remanence). Ni nanoparticles, in either alumina or titanium nitride, possessed blocking temperatures below 200 K. Detailed scanning transmission electron microscopy analysis has been conducted on the samples embedded into cerium oxide on both substrates and related to the magnetic data.

Magnetic properties of ilmenite-hematite films and bulk samples

Ilmenite-hematite [(FeTiO3)1−x–(Fe2O3)x] is a solid solution system with intrinsic ferrimagnetic and semiconducting properties. Bulk ceramic samples and epitaxial thin films have been prepared with magnetization values of up to 1500G (bulk) and 700G (thin films) at room temperature. Superparamagnetic behavior was discriminated from ferrimagnetic properties using field cooled and zero field cooled measurements. The blocking temperature exceeds room temperature for samples with composition x>0.25. Using magnetic force microscopy, the domain structure in the thin ilmenite-hematite films was detected.