Magnetic polarons are ferromagnetic spin clusters created by the exchange interaction of a carrier spin (electron or hole) with localized spins imbedded in a semiconductor lattice. They were first studied in magnetic semiconductors [1]; more recently, there have been extensive investigations [2] of polaron behavior in diluted magnetic semiconductors (DMS), such as Cd1−xMnxTe. DMS are favorable media for magnetic polaron studies because they have simple s-p bands and excellent optical properties. Two types of magnetic polarons have been identified in DMS - the bound magnetic polaron (BMP), whose carrier is localized by an impurity [3], and the free polaron (FP) consisting of a carrier trapped by its own, self-consistently-maintained, exchange potential [4].

Mn2+ ion spins in II–VI compound semimagnetic semiconductors (SMS) such as Cd1−xMn Te interact with one another through an antiferromagnetic exchange which is responsible for many interesting properties of Mn-alloyed II–VI SMS. In particular, this interaction leads to unique magnetization (M) behavior of SMS as a function of applied magnetic field (B). The nearest-neighbor (NN) Mn2+ −MN2+ interaction has been shown to yield a series of five steps in M vs. B curves above about 10 T, providing a direct determination of the NN exchange constant JNN. A detailed analysis of the magnetization steps has yielded values for the next NN exchange constant JNNN. The magnetization steps also yield information on the distribution of Mn2+ ions. Inelastic neutron scattering and Raman scattering provide alternative methods for the direct determination of the exchange constants.

High field magnetization experiments are reported for dilute Zn1−xMnxTe alloys (x≃0.03) at T=1.3K. The first magnetization step is analyzed including effects of internal field due to the interactions of Mn-Mn pairs with the remaining spins. An accurate estimate of the nearest neighbor (NN) exchange constant is obtained which agrees quite well recent neutron scattering experiments.

Pulsed-field magnetization measurements up to 40 T by Faraday rotation were made for random alloy Cd1−xMnx Te with 0.1 < x < 0.5. At liquid helium temperature the magnetization can be separated into the sum of two components: a paramagnetic Brillouin-like part that saturates by B=15 T, and a part linear in B. For large x, the linear part dominates and becomes independent of x. Mean-field calculations suggest that an enhanced magnetization can be obtained with new ordered-alloy crystal structures which have reduced antiferromagnetic cancellations.

Universality of the spin-glass (SG) transition in bulk Cd1−xMnxTe was investigated by determining the scaling behavior of the spin-glass order parameter q as a function of the reduced temperature t = (Tg-T)/Tg, where Tg is the transition temperature. q(T) was determined from the SQUID magnetometric data both above and below the transition. It can be shown that, q(T) = 1 + T[│θ│ − C/χ(T)]−1. C and θ, the Curie and Curie-Weiss parameters, were obtained from a non-linear regression of the dc low field susceptibility χ(T) above Tg. q(T) thus obtained exhibits the cannonical order parameter criteria, viz. q(T>Tg) = 0, q(T→Tg)= 0, and q(T→Tg)→0, and q(T→0)→1. In range of Mn concentration studied (0.3 ≦ x ≦ 0.55), Tg ranged between 13 K and 23.5 K. Thus q as a function of absolute temperature behaves differently for different x. However, universality is clearly evidenced when the dependence on t is determined. q(t) exhibits a universal scaling law. We observe q ˜ tβ with β ≅ 0.95. Overall good agreement was noted with the Sherrington-Kirkpatrick (SK) infinite range SG model. Observed value of β is in excellent agreement with the model prediction β −1. But in the Cd1−xMnxTe system we find θ<0, indicating a net antiferromagnetic interaction and │θ│/Tg > 1 as opposed to the SK model. We believe this to be additional evidence that the spin interactions are not distributed as a Gaussian function.

Theoretical predictions for the magnetic specific heat of Cd1−xMnxTe:x=0.20, 0.35, 0.50, and 0.65 are reported for B=0, 10T, and 100T. The analysis applies to the low-temperature regime, T≤10K, where the fundamental excitations are harmonic magnons. The calculations use values for the exchange interactions which were inferred from fits to the dynamic structure factor describing inelastic neutron scattering at low temperatures. For x=0.35, 0.50, and 0.65 the specific heat is only weakly affected by applied fields up to 10T. At the lowest concentration the application of a field of 10T leads to a significant reduction in the specific heat. Results are also presented for the distribution of magnon modes at various concentrations and fields.

Magnetic semiconductors have enjoyed a renaissance with the discovery by Komarov et al. in 1977 of very large enhancements of magneto-optical properties in Mn doped CdTe. Although these magnetic semiconductors are dilute, that is to say they are solid solutions of II-VI or IV-VI semiconductors in which the cations have been replaced (randomly) by divalent transition metal or rare earth ions, their physical properties may be understood, in general, with concepts developed for the concentrated systems, for example the europium chalogenides. It is the purpose of this paper to review some of these ideas and point out new developments which have broadened our understanding, particularly as regards magnetic polarons, the highly correlated ferromagnetic region surrounding trapped carriers in an otherwise paramagnetic (P.M.) or antiferromagnetic (A.F.) semiconducting hosts.

Raman scattering provides significant information on the nature of the magnetic excitations of diluted magnetic semiconductors (DMS). Transitions involving an exchange of a quantum of angular momentum between the system and the radiation field result in Raman lines with shifts equal to the energy of the magnetic excitations in which the total spin of the crystal changes by ħ. Such transitions have a one-to-one correspondence with those seen in magnetic resonance but observed in the optical rather than in the microwave region of the electromagnetic spectrum. In the Mn-based II-VI DMS a variety of magnetic excitations have been observed in their Raman spectra. Microscopic models underlying these phenomena provide powerful insights into the nature of the magnetism of DMS.

Systematic masurements of energy gap and magnetic susceptibility of Zn1−xMnxTe in a composition range from 0 to 0.703 Mn mole fraction were performed as a function of temperature. The results show that the magnetic contribution to the energy gap variation is proportional to a product of temperature and susceptibility.

Magnetoreflectivity measurements in Faraday and Voigt geometry in the free exciton region (A, B and C excitons are visible) were performed on Cd0.07Mn0.13S at T=1.6 K for magnetic field up to 5.5 T. An analysis of the results in terms of a mean field model for wurtzite type crystal enabled to determine band parameters Δ1, Δ2, Δ3 and exchange integral for valence band N0ß = -1.8 ± 0.08 eV

Optically induced magnetization has been observed in a 10 micron diameter sample of Cd.8Mn.2Te. A newly developed integrated SQUID magnetic spectrometer was used to study both the magnitude and the picosecond dynamics of the magnetic response as function of the energy and polarization of the optical excitation. The dramatic energy dependence of the response provides an understanding of the way in which the overall sample magnetization changes upon illumination, and the subsequent relaxation through the spin lattice interaction.

Recent successes in molecular beam (MBE) epitaxial growth of diluted magnetic semiconductor (DMS) artificial microstructures are now generating structures in which optically excited lower dimensional electronic and magnetic phenomena can be investigated. We consider an example which illustrates the present early state of affairs: transient magnetic polarons in connection with localized quasi 2-dimensional (2D) electronic excitations. We also comment on recent measurements and antiferromagnetic ordering in ultrathin layers in magnetic semiconductors.

The characteristics of the photoluminescence of Cd(1-x)Mn(x)Se (x = 0.05, 0.10, 0.20, 0.30) and Cd(1-x)Mn(x)Te (x = 0.20, 0.30, 0.45) change considerably as the sample temperature is reduced below the exciton-magnetic polaron (EMP) threshold temperature. At low temperatures the formation of magnetic polarons has large effects on the luminescence energy, the radiative lifetime, the radiative efficiency, and the spectral half-width of the luminescence. It is also observed that the formation time of the EMP increases almost linearly with temperature. All of these effects are explained with a simple model for the EMP.

We outline a theory of the acceptor-bound magnetic polaron in cubic diluted magnetic semiconductors that exhibits a nonuniform magnetization. Calculations show that the manganese spins have an overall z-alignment, but are canted in an azimuthally symmetric way from that direction. This is new phenomenon we call BMP spin-texture. Spin-texture results from valence band degeneracy, and the important spin-orbit effects associated with it. The possibility of studying BMP spin-texture via neutron scattering is discussed.

The presence of transition metal ions (typically Mn2+) in diluted magnetic semiconductors (DMS) results in a strong spin-spin coupling between localized magnetic moments and band electrons. This leads to considerable modifications of the semiconductor band structure in the presence of strong magnetic fields, e.g., to large spin-dependent shifts of the electronic states at the band edge. This feature is of particular interest in the context of quantum wells involving DMS. Starting with the original idea of a “spin-superlattice”, we concentrate on various opportunities which arise due to the tunability of the depth of the quantum wells by the magnetic field and/or temperature associated with the aforementioned spindependent effects. Thus, we discuss boil-off and freeze-out of electrons to and from quantum wells, selective spin tunneling across the barriers, tunable infrared emitters, enhancement of electronic g-factors in shallow non-magnetic wells surrounded by DMS barriers, the possibility of transition from a type-1 to a type-il superlattice induced by the magnetic field, and quantum oscillations anomalies in DMS quantum wells.

The magnetic and electronic properties of pseudobinary alloys of the lead compounds (PbTe,PbSe)with Manganese, Europium and Gadolinium are reviewed. These materials have a direct minimum gap at the L-point of the Brillouin zone. The addition of Mn, Eu, or Gd to the diamagnetic hosts causes a rather weak antiferromagnetic exchange interaction. For sufficiently high carrier concentrations (10 2 0 -102 1 cm−3) a ferromagnetic interaction occurs due to RKKY interaction. The spin-spin exchange interaction between the Bloch states and the localized 3d or 4f states is characterized by rather small exchange integrals and the limited applicability of the mean field approximation for the description of energy band parameters like g-factors. Interband transitions in PbMnTe and PbMnS lend support to a zeromagnetic field splitting (“spin-splitting”)of band states which increases linearly with Mn-content up to several meV at temperatures below 4K.

The magnetization and magnetic susceptibility of Bridgman-grown Pb1-xGdxTe have been measured over a temperature range from 2 to 300 K and in magnetic fields from 0.01 to 50 κOe. The x-values of the crystals ranged from 0.03 to 0.07. The magnetic susceptibility followed a Curie-Weiss behavior, χ = C/(T + θ), with positive θ implying an antiferromagnetic exchange interaction between Gd ions. The magnetic field dependence of the magnetization was fitted to a modified Brillouin function with parameter values that agreed fairly well with those from Curie-Weiss plots. The magnitude of θ was comparable to the value found for Pb1-xMnxTe for similar x values; but since the ion spin is bigger for Gd this suggests that the exchange interaction in Gd-doped PbTe is roughly half the value in Mn-doped PbTe.

Magnetic resonance behavior of Sn0.98 Gd0. 02 Te has been investigated from 2.5 K down to 0.015 K using a SQUID magnetometer and a 3He-4He dilution refrigerator. Detection of magnetic resonance with a SQUID allows the detection of NMRan d EPR of some of the constituents of the sample. This method is particularly useful for broad lines. We have investigated the NMRfor 125Te, 119 Sn, and 117Sn, and the EPR for the Gd ion in a field of 265 Oe. The contribution of Gd impurities is maily important in the Knight shift of the Sn isotopes; this shift is 7%. The EPR shows 2 broad lines with structure; one line we attribute to the paramagnetic ions and the second one we attribute to Gd ions which have formed clusters.

X-band EPR measurements have been made on single crystals and powders of PbTe doped with Mn below 100 ppm. A seven-fold splitting of each Mn hyperfine structure line is shown to arise from tellurium superhyperfine structure. Microwave saturation results are presented which indicate that the spin-lattice relaxation time is different for different superhyperfine lines. This asymmetry is not readily explained by conventional models.

The EPR spectra of manganese and gadolinium doped PbTe and SnTe semiconductors were measured from 4.2K up to 400K for dopant concentrations ranging from 50 to 50000 ppm. Resolved fine, hyperfine and superhyperfine structure as well as forbidden and cluster lines were measured. The combination of Lorentzian absorption and dispersion derivatives fits the recorded spectra well. Nonlinear broadening of linewidths with temperature is characteristic in these semiconductors. A manganese-induced broad maximum in linewidth versus temperature was found in PbTe around 180K for dopant concentrations in excess of 1.5at%.