S. M. Zhou

Magneto-optical Kerr effect in perpendicularly magnetized Co/Pt films on two-dimensional colloidal crystals

Magneto-optical Kerr effect and optical reflectance are measured in the visible region for perpendicularly magnetized Co/Pt films on self-assembly array of two-dimensional closely packed polystyrene spheres. Peaks and valleys are observed in the magneto-optical Kerr and the optical reflectance spectra and their positions scale with the sphere diameter. In explanation of the above scenario, not only the Mie optical scattering of single nanocaps but also the surface plasmon resonance should be considered. This work will facilitate development of magnetoplasmonic nanosensors.

Fabrication and optical properties of metastable Cu-Ag alloys.

A series of single-phase metastable Cu-Ag solid solutions was deposited by magnetron sputtering at liquid-nitrogen temperature. The lattice constant increases with increasing Ag content. The spectroscopic ellipsometer measurements clearly show that with increasing Ag content, the Cu-like and Ag-like absorption edges in the visible wavelength range shift continuously toward the high-energy and the low-energy sides, respectively.

Longitudinal and perpendicular exchange bias in FeMn/(FeNi/FeMn)n multilayers

Exchange bias in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is usually investigated in the longitudinal configuration with the exchange coupling established in the film plane. In this work, we report on the perpendicular exchange bias in FeMn(8 nm)/[FeNi(2 nm)/FeMn(8 nm)]n multilayers induced by perpendicular field cooling. The thin FeNi layers give rise to large values of the exchange field and coercivity, and n=15 allows a sufficiently large magnetization for the measurements. Even though the soft FeNi layers have an intrinsic in-plane anisotropy, perpendicular exchange bias has been observed after cooling in a perpendicular external field. The exchange field in the perpendicular configuration is about 0.85 that of the longitudinal case. In both the longitudinal and perpendicular configurations, the exchange field decreases quasilinearly with temperature. The squareness of perpendicular hysteresis loops decreases with increasing temperature.

New design of the variable angle infrared spectroscopic ellipsometer using double Fourier transforms

A new type of incidence-angle-variable infrared spectroscopicellipsometer working in the 2100–10 000 wave number range has been designed and constructed. For the system, the analyzer and polarizer were driven by two microstepping motors having hollow shafts and rotating synchronously with a speed ratio of 2:1, i.e., A=2P. The incidence angle can be varied from 30° to 90° with an accuracy of 0.01°. The doubled Fourier transforms as both functions of the wave number and the azimuthal angles of the polarizer and analyzer were carried out and integrated in the system. Two sets of ellipsometric parameters measured in the experiment have been used to test the data self-consistency of the system. In data reduction, the error arising from the slight anisotropy of the stray light was corrected. In application, the complex dielectric function of the Au film was measured with a data accuracy better that 1% in the entire spectral range.

Spin waves and small intrinsic damping in an in-plane magnetized FePt film

Laser-induced spin-wave dynamics in an in-plane magnetized FePt film is studied using all-optical pump-probe magneto-optical Kerr spectroscopy under different external fields and pump fluences. Uniform precession spin wave is observed. Nonlinear external field dependence of its frequency is found and well explained by the macrospin model of uniform precession. The effective damping shows a significant external-field dependence. Calculation taking account for magnetic inhomogeneity fits the external-field dependence well, revealing main magnetic inhomogeneity origin of the extrinsic damping. An intrinsic Gilbert damping parameter of <0.028 is inferred and shows potential applications of this film in magnonics.

Exchange bias, training effect, hysteretic behavior of angular dependence, and rotational hysteresis loss in NiFe/FeMn bilayer: Effect of antiferromagnet layer thickness

For NiFe/FeMn bilayers, the correlation among the exchange field, the coercivity, the training effect, the hysteretic effect of the angular dependence of the exchange bias, and the rotational hysteresis loss has been studied as a function of the antiferromagnet layer thickness tAFM. With increasing tAFM, all these quantities undergo nonmonotonic variations, except for the monotonic change in the exchange field. The maximal values of the coercivity, its relative change, and the rotational hysteresis loss are almost located at the same tAFM of 3.8 nm. The maximal values of the relative change in the exchange field and of the hysteretic effect of the angular dependence are located at 2.5 and 3.0 nm, respectively. The rotational hysteresis loss and the hysteretic behavior of the angular dependence of the exchange bias have different characteristics. The variations of all physical quantities with tAFM can be ascribed to the irreversible reversal of the antiferromagnet spins, which are governed by the Arrhenius...

Anomalous training effect of perpendicular exchange bias in Pt/Co/Pt/IrMn multilayers

For perpendicularly magnetized Pt/Co/Pt and Pt/Co/Pt/IrMn multilayers, the magnetization reversal process is accompanied by the pinned domain wall motion. For Pt/Co/Pt/IrMn multilayers, the asymmetry of hysteresis loops is always equal to zero and the exchange field HE decreases during consecutive hysteresis loops. It is interesting to find that the variation in HE with the cycle number n cannot be fitted by the empirical 1/n function. Meanwhile, the coercivity almost does not change. The unique feature of the training effect is caused by the magnetization reversal mechanism of the pinned domain wall motion in the Pt/Co/Pt multilayers.

Magnetization dependence of training effect of exchange coupling in ferromagnet/FeMn bilayers

The exchange coupling and its training effect are studied as a function of the ferromagnetic layer magnetization by using various ferromagnet/FeMn bilayers with ferromagnetic materials Ni, Ni81Fe19, Ni50Fe50, Co, and Fe. The exchange coupling energy Jex increases with increasing MFM as Jex∝MFM. The training effect of the exchange field is related to both the ferromagnet magnetization and the magnetization reversal mechanism. For ferromagnet/FeMn bilayers with similar magnetization reversal mechanisms, the relative change of the exchange field decreases with increasing magnetization in an exponential manner.

A STUDY OF INTERLAYER COUPLING IN CO/CU MULTILAYERS

Co(22 A)/Cu multilayers with Cu thicknesses varying from 6 to 40 A are studied by vibrating‐sample magnetometry and ferromagnetic‐resonance spectroscopy (FMR). The magnetization was found to increase with decreasing Cu thickness, whereas the linewidth of the uniform mode of the FMR was found to decrease. This is attributed to the spin polarization of the Cu layers due to interlayer coupling and its effect on the lowered dimensionality of the Co layers. We also observed a peak on the high‐field side of the uniform FMR mode, which we attribute to antiparallel coupling of adjacent ferromagnetic layers producing an ‘‘optical’’ mode.

Dynamics of magnetization, reversal, and ultrafast demagnetization of TbFeCo amorphous films

Static and dynamic hysteresis loops of TbFeCo amorphous films are measured by vibrating sample magnetometry (VSM) and magneto-optical Kerr rotation. The static VSM loop shows a faster magnetization reversal than the static Kerr loop does. The dynamic Kerr hysteresis loop in a sinusoidal alternating magnetic field at 1.14kHz shows a large increase in coercivity with respect to the static coercivity. The ultrafast dynamics of laser-induced demagnetization and magnetization recovery is studied by femtosecond time-resolved polar Kerr rotation spectroscopy, and shows a subpicosecond demagnetization and several hundreds of picosecond magnetization recovery process, which suggests gigahertz writing rate possible.