Jauyn Grace Lin

Structure-dependent magnetization in helimagnetic YMn/sub 2/O/sub 5/

YMn/sub 2/O/sub 5/ compounds have been important for industrial applications due to their unique nature of ferroism. In this study, two YMn/sub 2/O/sub 5/ samples with different phases are prepared, one is orthorhombic (OT) and the other is hexagonal (HX) in structure, for investigating the structure dependency of magnetic properties. By changing the sintering temperature from 1100 to 1400/spl deg/C, the crystal structure of YMn/sub 2/O/sub 5/ is transformed from OT phase to HX phase. Based on the data of temperature-dependent magnetization M(T), the magnitude of magnetic moment in the HX phase is one order higher than that of the OT phase. Furthermore, the field-cool and zero-field-cool M(T) show a splitting around 50 K for both samples, indicating a common feature of spin-canted state. In conjunction with the results of low-temperature M--H curves, it is concluded that the YMn/sub 2/O/sub 5/ sample with the HX structure has one order higher magnetization at low temperature than that of the OT structure, which should have wide impact to future applications.

FMR study on some magnetic granular films

We have measured the angular dependence of the ferromagnetic resonance (FMR) spectra for two systems of magnetic granular films: Ag-Co and Ni-SiO/sub 2/. In these two systems, the concentration of the magnetic component was varied to tune the magnetic properties, which were reflected from the FMR spectra. Our results have shown that the angular dependences of resonance field and the line width of these samples are changed significantly with the concentration of magnetic component.

Magnetotransport Behavior and the Phase Diagram of Nd

In this work, the temperature dependent magnetization M(T) and the electrical resistivity rho (T) of Nd1-xCaxMnO3 (x=0.1-0.5) are measured. From the M(T) curves, the ferromagnetic (FM) phase is observed for samples of x=0.1 and 0.2. However, the canted-antiferromagnetic (CAF) phase which coexists with the charge-ordering (CO) phase occurs at low temperature for samples x=0.3-0.5. By systematically changing the Ca concentration from x=0.1 to 0.5, a detailed phase diagram is established. Based on the result of magnetic field dependent rho (T), the magnetoresistance ratio is calculated, which shows a maximum in the x=0.4 sample. These results are helpful for improving the MR effect in these manganites.

_{1 - {\rm x}}

The inverse spin-Hall effect (ISHE) which predicts the transferring of spin current to electrical voltage has received extensive attention due to its experimental realization in the ferromagnetic(FM)/paramagnetic(PM) bilayer systems. The ISHE induced spin voltage is measured via the technique of spin pumping in association with a mechanism of driving the spin angular momentum from the FM layer into the PM layer via ferromagnetic resonance. Among many novel metals, platinum (Pt) is widely used as PM layer because of its strong spin-orbit coupling. In this work, we study the FM-thickness dependence of ISHE in two bilayer films, La0.7Sr0.3MnO3(10 nm)/Pt(10 nm) and La0.7Sr0.3MnO3(23 nm)/Pt(10 nm). Our experimental results show that the ISHE voltage is larger in the film with 10 nm-FM layer, and the fitting results indicate that there may be a direct relation between the magnitude of ISHE voltage and the damping constant of FM layer.

Ca

Ferromagnetic resonance (FMR) was used to study the magnetic properties of Fe3O4 films to characterize the Verwey transition. Epitaxial, smooth films were deposited by molecular beam epitaxy. FMR spectra were recorded using a 9.49 GHz Brucker EMX system from room temperature to 80 K with a magnetic field applied perpendicular to the film surface. A single resonance feature was observed above the Verwey transition temperature (Tv~ 105 K). However below Tv additional features were observed indicating the presence of complex magnetic interactions below Tv.

_{\rm x}

The heterogeneous spin states of non-stoichiometric LaMnO3+δ with low (annealed in argon) and high oxygen (prepared in air) are studied with X-ray diffraction (XRD) and electron spin resonance (ESR). The competition between Mn+3–O–Mn+4 and Mn+3–O–Mn+3 spin states results in the coexistence of ferromagnetic (FM) and canted antiferromagnetic (CAF) states, which are precisely identified by XRD and ESR. FM order from the double-exchange interaction of Mn+3–O–Mn+4 dominates in high-oxygen LaMnO3+δ, and CAF state from Mn+3–O–Mn+3 dominates in the samples with low oxygen. Our results further demonstrate the high sensitivity of ESR for the heterogenous sample with complex spin states.

MnO

Manganites with small rare earths (RE = Ho, Er, Tm, Yb, Lu, or Y) and hexagonal structures belong to a class of materials known as hexagonal ferroelectrics, in contrast to those with large rare earths and orthorhombic, perovskite-related structures showing different physical properties. In this work, we study the effects of growth conditions on the stabilization of HoMnO3 films with both hexagonal and orthorhombic structures. Based on our experimental results, a HoMnO3 film with a hexagonal structure can be stabilized on Si(111) at 850 °C and above. Pure orthorhombic phases grow on SrTiO3(100), SrTiO3(110), LaAlO3(100), and LaAlO3(110) in a temperature range from 750 to 850 °C. Multiphase samples can be grown on MgO(100) at all temperatures.

_{3}

The hexagonal (HX) YbMnO₃ thin films were synthesized on the substrates of (111)Y₂O₃:ZrO₂ and (111)Pt/(0001)Al₂O₃ by pulsed laser deposition (PLD), and their structural, magnetic, and electrical properties are measured. The temperature-dependent magnetization clearly displays an antiferromagnetic ordering near 80 K with field perpendicular to the film surface. In the curve of polarization-voltage, the HX-YbMnO₃ thin films show distinct ferroelectricity with the room-temperature remanent polarization and coercive field being 1.1 muC/cm² and 58 kV/cm, respectively. These results indicate that the HX-YbMnO₃ thin films are prominent candidates for the applications of multiferroic devices.

Manganites

Ferromagnetic resonance (FMR) is used to investigate the magnetic anisotropy of a 25.5 nm thin Fe₃O₄ film above and below its Verwey transition temperature (TV). Epitaxial smooth film of Fe₃O₄ is deposited by oxygen-assisted molecular beam epitaxy. FMR spectra are recorded using a 9.49 GHz Brucker EMX system from 80 K to room temperature with a magnetic field applied along [100] direction. The resonance field (Hr) and linewidth (AH) are extracted by fitting the FMR line with a Lorentzian function. Temperature dependence of H_r and ΔH shows a transition at T_V. Furthermore, the data of in-plane angular dependence of H_r at 300 and 83 K indicated a change in symmetry from fourfold to twofold below TV, consistent with a transformation of Fe₃O₄ from cubic (fourfold) to monoclinic (twofold) structure with its easy axis switching from [110] to [100]. Complex symmetry pattern is found in angular dependence of ΔH below TV, which is attributed to the in-plane twinning of Fe₃O₄ lattice.

Thickness Dependent Spin Pumping Effects in La

In this work, different metal electrodes (ME) are sputtered on the surface of Nd0.7Ca0.3MnO3 thin films. The behavior of current-voltage (I-V) characteristics of these films is investigated. Among different metal electrodes, Au yields the smallest electrical hysteresis loop, while Cu generates the largest one. According to our analysis on the work function of each ME material, the result of ME-dependent I-V hysteresis loop can not be simply explained by the Schottky barrier. We propose that the electronnegativity of each ME material also plays a crucial role for the oxygen diffusion at interface. In conclusion, this study provides useful information for understanding the origin of I-V hysteresis loop and the principle of resistance switching in CMR manganites