Clustering induced suppression of ferromagnetism in diluted magnets

Ferromagnetism in diluted magnets in the compensated regime p << x is shown to be suppressed by the formation of impurity spin clusters. The majority bulk spin couplings are shown to be considerably weakened by the preferential accumulation of holes in spin clusters, resulting in low-energy magnon softening and enhanced low-temperature decay of magnetic order. A locally self-consistent magnon renormalization analysis of spin dynamics shows that although strong intra-cluster correlations tend to prolong global order, T_c is still reduced compared to the ordered case.Comment: published version, 5 pages, 4 figure

Enhanced carrier scattering rates in dilute magnetic semiconductors with correlated impurities

In III-V dilute magnetic semiconductors (DMSs) such as Ga$_{1-x}$Mn$_x$As, the impurity positions tend to be correlated, which can drastically affect the electronic transport properties of these materials. Within the memory function formalism we have derived a general expression for the current relaxation kernel in spin and charge disordered media and have calculated spin and charge scattering rates in the weak-disorder limit. Using a simple model for magnetic impurity clustering, we find a significant enhancement of the charge scattering. The enhancement is sensitive to cluster parameters and may be controllable through post-growth annealing.Comment: 4 pages, 3 figure

Optical properties of metallic (III,Mn)V ferromagnetic semiconductors in the infrared to visible range

We report on a study of the ac conductivity and magneto-optical properties of metallic ferromagnetic (III,Mn)V semiconductors in the infrared to visible spectrum. Our analysis is based on the successful kinetic exchange model for (III,Mn)V ferromagnetic semiconductors. We perform the calculations within the Kubo formalism and treat the disorder effects pertubatively within the Born approximation, valid for the metallic regime. We consider an eight-band Kohn-Luttinger model (six valence bands plus two conduction bands) as well as a ten-band model with additional dispersionless bands simulating phenomenologically the upper-mid-gap states induced by antisite and interstitial impurities. These models qualitatively account for optical-absorption experiments and predict new features in the mid-infrared Kerr angle and magnetic-circular-dichroism properties as a function of Mn concentration and free carrier density.Comment: 10 pages, 7 figures, some typos correcte

Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N

The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is extensively studied over a vast range of Mn concentrations. We calculate the electronic structures of these materials using density functional theory in both the local spin density approximation and the LDA+U scheme, that we have now implemented in the code SIESTA. For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the ferromagnetism, with an exchange constant $N\beta$ =~ -2.8 eV. This is smaller than that obtained with LSDA, which overestimates the exchange coupling between Mn ions and the As $p$ holes. In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the double-exchange mechanism, since a hole of strong $d$ character is found at the Fermi level in both the LSDA and the LDA+U approaches. In this case the coupling between the Mn ions decays rapidly with the Mn-Mn separation. This suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a robust ferromagnetic phase at large Mn concentration coexisting with a diluted weak ferromagnetic phase.Comment: 12 pages, 11 figure

Carrier induced ferromagnetism in diluted magnetic semi-conductors

We present a theory for carrier induced ferromagnetism in diluted magnetic semi-conductor (DMS). Our approach treats on equal footing quantum fluctuations within the RPA approximation and disorder within CPA. This method allows for the calculation of $T_c$, magnetization and magnon spectrum as a function of hole, impurity concentration and temperature. It is shown that, sufficiently close to $T_c$, and within our decoupling scheme (Tyablicov type) the CPA for the itinerant electron gas reduces to the Virtual Crystal Approximation. This allows, in the low impurity concentration and low density of carriers to provide analytical expression for $T_c$. For illustration, we consider the case of $Ga_{1-c}Mn_{c}As$ and compare our results with available experimental data.Comment: 5 figures included. to appear in Phys. Rev. B (brief report

High-Temperature Hall Effect in Ga(1-x)Mn(x)As

The temperature dependence of the Hall coefficient of a series of ferromagnetic Ga(1-x)Mn(x)As samples is measured in the temperature range 80K < T < 500K. We model the Hall coefficient assuming a magnetic susceptibility given by the Curie-Weiss law, a spontaneous Hall coefficient proportional to rho_xx^2(T), and including a constant diamagnetic contribution in the susceptibility. For all low resistivity samples this model provides excellent fits to the measured data up to T=380K and allows extraction of the hole concentration (p). The calculated p are compared to alternative methods of determining hole densities in these materials: pulsed high magnetic field (up to 55 Tesla) technique at low temperatures (less than the Curie temperature), and electrochemical capacitance- voltage profiling. We find that the Anomalous Hall Effect (AHE) contribution to rho_xy is substantial even well above the Curie temperature. Measurements of the Hall effect in this temperature regime can be used as a testing ground for theoretical descriptions of transport in these materials. We find that our data are consistent with recently published theories of the AHE, but they are inconsistent with theoretical models previously used to describe the AHE in conventional magnetic materials.Comment: 6 pages, 5 figures, 1 table. Accepted to Phys.Rev.

Prospects of high temperature ferromagnetism in (Ga,Mn)As semiconductors

We report on a comprehensive combined experimental and theoretical study of Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that T_c in high-quality metallic samples increases linearly with the number of uncompensated local moments on Mn_Ga acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10% concentration of these local moments. Our magnetotransport and magnetization data are consistnent with the picture in which Mn impurities incorporated during growth at interstitial Mn_I positions act as double-donors and compensate neighboring Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I antiferromagnetic coupling. These defects can be efficiently removed by post-growth annealing. Our analysis suggests that there is no fundamental obstacle to substitutional Mn_Ga doping in high-quality materials beyond our current maximum level of 6.2%, although this achievement will require further advances in growth condition control. Modest charge compensation does not limit the maximum Curie temperature possible in ferromagnetic semiconductors based on (Ga,Mn)As.Comment: 13 pages, 12 figures, submitted to Phys. Rev.

Anisotropic Magnetoresistance and Magnetic Anisotropy in High-quality (Ga,Mn)As Films

We have performed a systematic investigation of magnetotransport of a series of as-grown and annealed Ga1-xMnxAs samples with 0.011 <= x <= 0.09. We find that the anisotropic magnetoresistance (AMR) generally decreases with increasing magnetic anisotropy, with increasing Mn concentration and on low temperature annealing. We show that the uniaxial magnetic anisotropy can be clearly observed from AMR for the samples with x >= 0.02. This becomes the dominant anisotropy at elevated temperatures, and is shown to rotate by 90o on annealing. We find that the in-plane longitudinal resistivity depends not only on the relative angle between magnetization and current direction, but also on the relative angle between magnetization and the main crystalline axes. The latter term becomes much smaller after low temperature annealing. The planar Hall effect is in good agreement with the measured AMR indicating the sample is approximately in a single domain state throughout most of the magnetisation reversal, with a two-step magnetisation jump ascribed to domain wall nucleation and propagation.Comment: 27 pages, 8 figures, accepted by Phys. Rev.

Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As

We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into account. We relate our quantum-mechanical calculation to the classical micromagnetic energy functional and determine anisotropy energies and exchange constants. We find that the exchange constant is substantially enhanced compared to the case of a parabolic heavy-hole-band model.Comment: 9 pages, 4 figure