A parallel set of two low-temperature optical experiments are performed to explore the connection between magnetic and electronic properties in Cd1-xMn(x)Te diluted magnetic semiconductor quantum structures. Using a combination of integrated DC SQUIDs and ultrafast optical techniques, dynamical measurements reveal the magnetic manifestations of carrier scattering and localization in quantum wells. These magnetic spectroscopy studies show entirely different time scales for magnetic interactions as compared to electronic spin behavior, and are seen to persist long after the carriers have disappeared from the system. In addition, femtosecond spin-polarized luminescence experiments record unexpectedly rapid electronic spin scattering of charge carriers (< 150 fs) with a dramatic dependence of these times on the dimensionality.

Diluted magnetic semiconductor Cd1−xMnxTe-Cd1−yMnyTe heterostructures of various layer thickness, where 0 ≤ x, y ≤ 0.4, were grown by molecular beam epitaxy for optical and magnetic studies of reduced-dimensional systems. X-ray diffraction patterns, low-temperature photoluminescence spectra (PL), and AC magnetic susceptibility measurements were used to verify the integrity of these structures. The carrier quantization in the quantum wells were revealed by the magnetic response in a magneto-optic microsusceptometer as well as by the results of PL measurement. These SLs were used to study the dimensional cross-over of the spin-glass phase. In addition, a magnetic-field-induced transition from type I to type II superlattice was observed.

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A scanning tunneling microscope is used to locally deposit arrays of nanometer-scale magnets directly within the superconducting planar input coil of an integrated dc SQUID microsusceptometer. Low-temperature frequency-dependent magnetic-susceptibility measurements reveal a narrow resonance which grows and becomes independent of decreasing temperature. Studies as a function of magnet volume, spacing, and applied field are compared to recent predictions for magnetic macroscopic quantum tunneling in small particles.

Charge-carrier spin scattering in diluted-magnetic-semiconductor multiple quantum wells is directly observed through the circular polarization of luminescence in a femtosecond time-resolved measurement. This represents a new short-time regime for direct spin-relaxation studies. Separate observations of electron and hole relaxation are accomplished by varying the applied magnetic field. Surprisingly, the spin-flip scattering rates in these systems appear to be independent of the extremely strong spin-spin--exchange interaction between the charge carriers and the magnetic ions.

Direct time-resolved optical and magnetic studies of charge carriers and ionic spins in diluted magnetic semiconductors are reported. These measurements are made on a series of CdTe/Cd1 − xMnxTe superlattices, in order to probe the spin dynamics in thin magnetic layers by utilizing the overlap of the quantum-confined carrier wavefunctions and the Mn-containing barriers. We discuss representative results for the thinnest (9 Å) CdTe quantum wells.

Diluted magnetic semiconductors have been shown to be excellent hosts in which to study the fundamental exchange interaction of a carrier with localized fluctuating magnetic spins. New low-temperature optical spectroscopy techniques using integrated DC SQUIDs have allowed sensitive magnetization studies in Cd1−x MnxTe superlattices as the magnetic layer thickness was systematically reduced towards the two-dimensional limit. Static and picosecond measurements reveal the magnetic manifestations of carrier localization in these electronic quantum wells.

Advanced VLSI (very large scale integration) technology was used to fabricate ultraminiature integrated SQUID (superconducting quantum interference device) susceptometers. With the appropriate design parameters, the sensitivity of the devices approaches the quantum limit. The use of integrated circuits in conjunction with pulsed optical techniques allows magnetic systems to be probed with a picosecond time resolution. The response can be mapped out as a function of the energy of the optical excitation, providing detailed spectroscopic information. Applying these techniques to the study of II-VI dilute magnetic semiconductors has yielded new insight into the mechanics of magnetic polaron formation and the dynamics of the magnetic spins. First experiments were carried out on a small ~10×10×1-μm3 single crystal platelet of Cd0.8Mn0.2Te. The results of the time-averaged magnetic spectroscopy at two different temperatures are presented, displaying the magnetic response to optical excitation at constant intensity from a photon energy of 1.83 to 2.0 eV

It is well established that quantum size effects play an important role in modifying the electrical and optical properties of microscopic systems. Recently it was shown that diluted magnetic semiconductors1 (DMSs) can be incorporated into superlattice structures,2,3 offering the possibility of changing the confining potential by the application of an external magnetic field.

The design, construction, and performance of miniature SQUID (superconducting quantum interference device) susceptometers is discussed. Spins (in units of μβ) per √Hz has been identified as an important figure of merit. Simple expressions for Sn (spin sensitivity) in miniature SQUID susceptometers are developed and the implications of dimensional scaling explored. The details of several existing and proposed designs are reviewed, including versions that utilize commercial SQUIDs. With thin-film DC SQUIDs, Sn values of a few thousand spins/√Hz have already been obtained, and it is projected that values of a few hundred will be achieved soon

A combination of superconducting integrated circuits was used to construct a very sensitive magnetooptic microsusceptometer for observations of the optically induced magnetization in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ multiple quantum wells. Spin-polarized electrons and holes are created optically, and the quantum mechanically localized wave functions of the carriers then serve as microscopic probes of the magnetic behavior. Both the magnitude and the picosecond dynamics of the magnetic response have been studied and reveal the effects of carrier quantization.

Diluted magnetic semiconductors exhibit a variety of interesting and unique optical phenomena generated by the strong spin exchange interaction between charge carriers and the magnetic ions in the materials. Recent successes in epitaxial growth of these compounds have enabled study of the effects of quantum confinement, and of size dependence of the magnetic interactions, on the physics of these systems. This review of optical studies discusses static and time-resolved measurements, both of luminescence and of optically induced magnetization. We focus on the wide-gap, II–VI zincblende crystal Cd1−xMnxTe which is, apart from the magnetic species, much like GaAs.

The authors have developed a novel type of magnetic spectroscopy which relies on an integrated multichip SQUID (superconducting quantum interference devices) microsusceptometer to obtain nearly quantum-limited spin resolution. The susceptometer, including two DC SQUIDs, pickup loops, and field coils, was fabricated using VLSI technology to achieve an unprecedented scale of magnetic sensitivity, op...

A new planar DC SQUID with integrated pick-up loops and field coils has allowed detailed AC magnetic susceptibility measurements of Cd 0.85 Mn 0.15 Te/CdTe superlattices. Systematic dimensional studies show that reduced geometries can suppress the three-dimensional spin glass transition and produce nonequilibrium magnetic states.

A flexible combination of superconducting integrated circuits was used to construct a low-temperature magneto-optic microsusceptometer utilizing a dc superconducting quantum inteference device (SQUID) detector operating near the quantum limit (coupled energy sensitivity of 1.7ℏ). Miniature pick-up loop assemblies on transparent substrates were joined by superconducting interchip connections to a thin-film dc SQUID, which is in turn read out by a second dc SQUID connected to room-temperature electronics. Measurements on an 8.5-μm-diam titanium dot evaporated directly into the pick-up loop demonstrate a spin sensitivity of ∼103 spins/(Hz)1/2 at T=290 mK.

The macroscopic behavior of a restricted liquid has been studied through ultrasonic velocity and attenuation measurements utilizing a picosecond colliding-pulse optical technique. Transparent porous sol-gel glasses allowed a study of the collective fluid dynamics as a function of confining pore radius and temperature. The acoustic response of the supercooled state was found to be strikingly different from that of any ordinary liquid, and consistent with a simple model.

An integrated dc SQUID magnetic spectrometer has been developed to obtain direct high-resolution measurements of optically induced magnetization in a 10-μm-diam sample of Cd0.8Mn0.2Te. The static and picosecond dynamics of the magnetic response have been studied and are seen to be strikingly dependent on the energy and polarization of the optical excitation. The entire sample magnetization changes upon illumination, and the disturbed spins relax via an efficient spin coupling to the lattice.

Sol-gel methods combined with specific thermal treatments were used to fabricate porous glasses with extremely uniform pore diameters and high internal surface areas. Low-temperature adsorption isotherm measurements reveal the dynamic and thermodynamic stability limits of the liquid films as a function of evolving pore diameter. Contrary to classical predictions, the results are in remarkable agreement with a universal hydrodynamic theory of film behavior which includes substrate interactions.

New techniques have been developed for measuring time-dependent magnetization in picosecond time scales. These methods have been employed to explore the dynamics of bound magnetic polaron formation in the dilute magnetic semiconductor Cd1-xMnxTe. The data reveal real-time formation and evolution of local magnetically organized states with characteristic times of 300 ps.