WP2 Magnetism in the vicinity of MIT (Metal-Insulating Transition)
Work package Leader PhTI (A. G. Zabrodskii)
Involved Partners PTI, LUT, UmU, NanoGUNE
Milestones
M 2.1 Achievement at least one series of IV-group samples meeting requirements for magnetic ordering
A nonmagnetic compensated nGe:As semi conductor exhibits AF ordering of impurity spins in the vicinity of the metal–insulator phase transformation, which converts into FM order when the temperature is decreased. The temperature of this transition grows with increasing degree of compensation.
The observed effect is explained on a qualitative level by the structure of spin coupling, which includes the kinetic and Coulomb exchange. As the kinetic energy decreases, the ratio of the kinetic to Coulomb exchange varies so that the latter begins to dominate, which leads to appearance of the FM order.
The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K). The transition from AF to FM spin coupling can be detected by a break in the temperature dependence of the g-factor, which corresponds to the appearance of a significant internal magnetic field. Analysis of these dependences for a series of samples has led to the conclusion that this break is observed when the degree of compensation is greater than 0.3. At a lower compensation, the spin density at low temperatures increases but insufficiently for the appearance of an internal magnetic field. At greater compensation, the transition temperature increases almost in proportion to the degree of compensation. In addition, our experiments showed that a change in only one parameter—the kinetic energy—is insuf ficient to describe the transition from AF to FM order in a real Ge:As system. An important role is also played by the degree of compensation. We have compared the behavior of uncompensated and compensated Ge:As samples with the same electron donor concentration of NAs ≈ 2 × 1017 cm–3. The g-factor increases only in a compensated sample, which is the evidence for the appearance of an internal field in this case.
M 2.2 Strategi decision on the favoured approach for realization of magnetic ordering in the vicinity of metal-insulator transition
A nonmagnetic compensated nGe:As semi conductor exhibits AF ordering of impurity spins in the vicinity of the metal–insulator phase transformation, which converts into FM order when the temperature is decreased. The temperature of this transition grows with increasing degree of compensation.
The observed effect is explained on a qualitative level by the structure of spin coupling, which includes the kinetic and Coulomb exchange. As the kinetic energy decreases, the ratio of the kinetic to Coulomb exchange varies so that the latter begins to dominate, which leads to appearance of the FM order.
The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K). The transition from AF to FM spin coupling can be detected by a break in the temperature dependence of the g-factor, which corresponds to the appearance of a significant internal magnetic field. Analysis of these dependences for a series of samples has led to the conclusion that this break is observed when the degree of compensation is greater than 0.3. At a lower compensation, the spin density at low temperatures increases but insufficiently for the appearance of an internal magnetic field. At greater compensation, the transition temperature increases almost in proportion to the degree of compensation. In addition, our experiments showed that a change in only one parameter—the kinetic energy—is insuf ficient to describe the transition from AF to FM order in a real Ge:As system. An important role is also played by the degree of compensation. We have compared the behavior of uncompensated and compensated Ge:As samples with the same electron donor concentration of NAs ≈ 2 × 1017 cm–3. The g-factor increases only in a compensated sample, which is evidence for the appearance of an internal field in this case.
Room temperature ferromagnetic-like behavior in fullerene photopolymerized films treated with monatomic hydrogen was observed. The hydrogen treatment controllably varies the paramagnetic spin concentration and laser induced polymerization transforms the paramagnetic phase to a ferromagnetic-like one. Excess laser irradiation destroys magnetic ordering, presumably due to structural changes, which was continuously monitored by Raman spectroscopy. We suggest an interpretation of the data based on first-principles density-functional spin-unrestricted calculations which show that the excess spin from mono-atomic hydrogen is delocalized within the host fullerene and the laser-induced polymerization promotes spin exchange interaction and spin alignment in the polymerized phase.
A procedure for determining the magnetic susceptibility described for the example of Ge:As. The procedure includes three parts.
(1) Use of an ESR cavity with two magnetic field antinodes, with a sample under study placed in one of these, and a reference sample into the other.
(2) Measurement of the temperature dependence of the resistivity of a sample (“poor” conductor) in order to take into account the nonuniformity of the microwave field distribution in the sample.
(3) Double integration of the measured positive part of the derivative of the Dyson resonance line, followed by application of formula (11). The procedure yields the magnetic susceptibility with an error not exceeding 15%
The microwave low temperature magnetoresistivity of the low doped (nondegenerated) p-Ge was investigated with the use of the electron spin resonance technique. This technique permits to detect the derivative of the microwave absorption with respect to magnetic field and its change with this field. In our case, this change is proportional to the magnetoconductivity of the sample under investigation. Because of the averaging time of the effective masses of the light and heavy holes is much more then microwave frequency (10 GHz), this method permits to investigate the reaction of the each kind of the holes on the magnetic field separately. The experimental results are compared with the theory of the classical magnetoresistivity effect.
Dilute magnetic semiconductors (DMS) on the base of IV group elements were studied. A small deviation of the stoihiometry results in drastic changes of magnetic properties. Critical temperature Tc reaches more than 300 K. Carriers are spin polarized. We explain experimental results within the model of exchange between local magnetic moments through the spin fluctuations. Properties of these structures strongly depend on substrate.
Si based DMSs attracted interest after the observation of a ferromagnetic (FM= state with high Curie temperature, Tc > 400 K, in these materials. This result was obtained in Si:Mn films with a relatively low (0.1 – 0.8 at.%) content of implanted Mn ions, but its origin remained mysterious.
In our experiments room temperature ferromagnetism (FM) been achieved in Si based structures with high Mn content. The important point is that the FM behavior was detected not only by magnetization measurements but also by the observation of the anomalous Hall effect. So, FM order involves charge carriers which are most probably at least partly spin polarized, and is not due to separated magnetic inclusions not interacting with carriers. The magnetic hysteresis loop as well as the temperature dependence of the saturation magnetization and coercive force measured by magnetic and transport methods are similar, and this fact proves the previous statement. The Curie temperature obtained from the temperature dependence of residual magnetization was found to be about 330 K. It is hard to explain the whole set of experimental results in the frame of the standard RKKY/Zener model of exchange between local moments of manganese. To explain the obtained experimental data, we used a more complex model of FM order, based on the conception of a two-phase magnetic material composed of defects with local magnetic moments, which are embedded in the host, assumed to be a weak itinerant ferromagnet. We argued that molecular clusters (probably, Mnx ≈ 35.0 Si-MnT or MnT-MnT dimers), containing a minority of Mn atoms, form these defects in our alloy, while the majority of Mn atoms is involved in the formation of the Mn-Si host. The observed FM ordering at high temperatures (>300 K) is due to the Stoner enhancement of the exchange coupling between local moments of defects provided by strong spin fluctuations (‘paramagnons’) in the host. Our theoretical predictions and experimental results are in good qualitative agreement.
We have performed a nonperturbative calculation of indirect exchange interaction between two paramagnetic impurities via two-dimensional (2D) free carriers gas separated by a tunnel barrier. The method takes into account the impurity attractive potential which can form a bound state. The calculations show that if the bound impurity state energy lies within the energy range occupied by the free 2D carriers the indirect exchange interaction is strongly enhanced due to resonant tunneling and exceeds by a few orders of magnitude what one would expect from the conventional Ruderman-Kittel-Kasuya-Yosida approach. We present a nonperturbative calculation of indirect exchange interaction between two paramagnetic impurities via two-dimensional (2D) free carriers gas separated by a tunnel barrier. The method takes into account the impurity attractive potential which can form a bound state. The calculations show that if the bound impurity state energy lies within the energy range occupied by the free 2D carriers the indirect exchange interaction is strongly enhanced due to resonant tunneling and exceeds by a few orders of magnitude what one would expect from the conventional Ruderman-Kittel-Kasuya-Yosida approach.
Two isomeric fulleride compounds Cs3C60which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state were studied.Using pressure (p) as a single control parameter of the C60 balls lattice spacing, one can now study the progressive evolution of the SC properties when the electronic correlations are increased towards the critical pressure pc of the Mott transition. We determine the electronic excitations in the SC state, that is 2Δ, the gap value. The latter is found to be largely enhanced with respect to the Bardeen-Cooper-Schrieffer value established in the case of dense A3C60 compounds. The 2Δ/kBTc increases regularly upon approaching the Mott transition. These results bring clear evidence that the increasing correlations near the Mott transition are not significantly detrimental to superconductivity. They rather suggest that repulsive electron interactions might even reinforce elecron-phonon superconductivity, being then partly responsible for the large Tc values, as proposed by theoretical models taking the electronic correlations as a key ingredient.