Y. J. Cho

Carrier-mediated antiferromagnetic interlayer exchange coupling in diluted magnetic semiconductor multilayers Ga1-xMnxAs/GaAs:Be.

We report the antiferromagnetic (AFM) interlayer exchange coupling between Ga0.97Mn0.03As ferromagnetic semiconductor layers separated by Be-doped GaAs spacers. Polarized neutron reflectivity reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers is AFM. When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by doped charge carriers.

Effects of donor doping on Ga1−xMnxAs

We investigate the effect compensating Mn acceptors in Ga1−xMnxAs films by doping with Si donors. For Ga1−xMnxAs with low Mn content (e.g., x 0.10), Si doping is found to increase TC. We ascribe this to an increase in the hole mobility in high x samples due to changes in the relative occupancy of the hole impurity band associated with compensation by the Si donors.

Investigation of magnetic and electronic coupling between two (Ga,Mn)As layers in (Ga,Mn)As∕GaAs∕(Ga,Mn)As magnetic tunnel junctions

The coupling between the two magnetic layers in a series of (Ga,Mn)As∕GaAs∕(Ga,Mn)As magnetic tunnel junctions with different nonmagnetic spacer thicknesses tNM were studied by magnetization and planar Hall effect (PHE) measurements. The PHE data indicate that the magnetization reversals of the two layers are strongly correlated when the tNM is less than 3nm and are independent when tNM is larger than 15nm. From the results, it is concluded that considerable redistribution of hole wave functions plays a major role for small tNM. The PHE results for the sample with 6nm spacer also suggest an antiferromagnetic interlayer exchange coupling.

Unique properties of magnetotransport in GaMnAs films grown on vicinal and high-index planes

Ferromagnetic III–Mn–V semiconductors such as GaMnAs represent systems in which electronic and magnetic properties are closely intertwined, which results in entirely new effects in electrical transport. We illustrate this by the unique and somewhat unexpected magnetotransport phenomena observed in GaMnAs films grown on vicinal substrates (i.e., on surfaces tilted by several degrees relative to the (100) plane) as well as on substrates with high-index-plane surfaces. In particular, it will be shown that such vicinal or high-index-plane GaMnAs layers manifest a striking asymmetry in the dependence of the planar Hall resistance Rxy on magnetic field, caused by the superposition of the planar Hall effect (PHE) and the anomalous Hall effect (AHE). This asymmetry is a direct manifestation of the effect of magnetocrystalline anisotropy in GaMnAs that confines the magnetization M to a preferred crystal plane rather than to the plane of the film, resulting in turn in a finite component of M normal to the sample plane. The ability to investigate PHE and AHE occurring simultaneously in the same sample revealed a clear relationship between the two effects, suggesting that PHE and AHE are fundamentally connected. The asymmetry of the resistance Rxy occurring in the PHE geometry in these GaMnAs layers also allows one to obtain four distinct zero-field resistance states that depend on the history of the experiment, making this effect of potential interest for building a unique four-state magnetic memory device. Moreover, measurements with the magnetic field normal to the growth plane in such tilted samples have revealed a new highly complex hysteresis behaviour of Rxy. These results, together with measurements of anisotropic magnetoresistance (AMR) measured on samples grown on high-index planes, point to the intimate relationship between magnetotransport and magnetocrystalline anisotropy in GaMnAs, providing new insights into this complex interdependence, as well as opening new opportunities for exploiting magnetic anisotropy for use in spintronic devices.

Ferromagnetic behavior of CdMnCrTe quaternary system

We describe magneto-optical and magnetic properties of quaternary Cd1−x−yMnxCryTe crystals (x=0.37, 0≤y≤0.03) grown by the Bridgman method. The presence of Cr in this alloy is observed to induce ferromagnetic behavior and to enhance magneto-optical effects. Using magnetic circular dichroism and magnetization measurements, we describe the dependence of this behavior on the Cr concentration, and we discuss possible mechanisms for the observed effects.

Ferromagnetic Resonance Study of Ultra‐thin Ga1−xMnxAs Films as a Function of Layer Thickness

A series of films of the ferromagnetic semiconductor GaMnAs were grown on GaAs substrates by low temperature molecular beam epitaxy, with film thickness ranging from 10nm to 200nm. We measured the angular and temperature dependences of ferromagnetic resonance for each specimen, both as‐grown and annealed. Our experiments indicate that the in‐plane uniaxial anisotropy field strongly depends on sample thickness. Additionally, spin wave resonances (SWRs) were observed in specimens with thicknesses of 100nm and 200nm. The analysis was carried out using the SW model of Portis as well as the surface SW model of Puszkarski. Our results clearly point to the existence of surface anisotropy in ferromagnetic semiconductor films.

Magnetization reversal of Ga1−xMnxAs layers separated by a nonmagnetic spacer

We have used polarized neutron reflectometry to individually examine the magnetization reversals of ferromagnetic Ga1−xMnxAs layers separated by a nonmagnetic GaAs spacer layer of varying thickness. For each of the samples studied, the top Ga1−xMnxAs layer is adjacent to a Be-doped Al0.25Ga0.75As capping layer on one side and the GaAs spacer on the other, while the bottom Ga1−xMnxAs layer is surrounded by GaAs on either side. For samples with spacer thicknesses of 12 and 6nm, antiparallel alignment of the two Ga1−xMnxAs layer magnetizations was observed at multiple fields, implying that hole doping from the capping layer strongly affects the coercivity of the top Ga1−xMnxAs layer but has a weaker effect on the coercivity of the bottom Ga1−xMnxAs layer. However, for a spacer thickness of 3nm, both top and bottom Ga1−xMnxAs layers appear to be equally influenced by the capping layer, as virtually identical coercivities were observed. This behavior is evidence of coupling between the Ga1−xMnxAs layers across...

Magnetic anisotropy of ferromagnetic Ga1−xMnxAs formed by Mn ion implantation and pulsed-laser melting

We measured the magnetic anisotropy of nearly fully relaxed ferromagnetic Ga1−xMnxAs formed by Mn ion implantation followed by pulsed-laser melting (II-PLM) using magnetometry and ferromagnetic resonance. In qualitative terms the material formed by II-PLM exhibits all magnetic anisotropy features commonly found in Ga1−xMnxAs films fabricated by low-temperature molecular beam epitaxy (LT-MBE). Quantitatively, however, the magnetic anisotropy of II-PLM Ga1−xMnxAs is dominated by cubic anisotropy terms, which we attribute to the smaller strain in the II-PLM material due to the absence of Mn interstitials. One should note, however, that II-PLM Ga1−xMnxAs also exhibits a weak but finite uniaxial in-plane magnetic anisotropy similar to that observed in LT-MBE Ga1−xMnxAs, which can be ascribed to the small built-in compressive strain. The similarity between II-PLM and LT-MBE Ga1−xMnxAs clearly points to an intrinsic origin of this property, independent of the method of fabrication. At low temperatures the remnan...

Vanishing of ferromagnetic order in (Ga,Mn)As films at high hole concentrations: beyond the mean field Zener model

The effect of relative Mn and hole concentrations on ferromagnetic order in thin (Ga,Mn)As films is investigated in using modulation doped (Al,Ga,Be)As∕(Ga,Mn)As heterostructures grown by molecular beam epitaxy. Our Hall effect measurements indicate that the hole concentration p in the (Ga,Mn)As layers increases with the amount of Be in the (Al,Ga,Be)As barrier. At low Be amount in the barrier, the Curie temperature TC of the (Ga,Mn)As layer is enhanced compared to that observed without Be doping. However, as the Be content in the (Al,Ga,Be)As barrier is increased further, this trend is reversed: TC is observed to decrease, and eventually the ferromagnetic order in (Ga,Mn)As disappears altogether, as seen in magnetization measurements. This behavior is in disagreement with the mean field Zener model of magnetic semiconductors, since that model predicts TC to be simply proportional to p1∕3. Our results suggest that this model fails at very high values of p, especially when p exceeds the Mn concentration in...

Ultrafast Magneto‐Optical Kerr Study of Standing Spin Waves in Ferromagnetic GaMnAs Films

We report on the observation of standing spin waves in thin films of the ferromagnetic semiconductor GaMnAs in time‐resolved magneto‐optical Kerr measurements. We observe two modes, one of which is the near‐uniform spin excitation. From the magnetic‐field dependence of the frequencies and the ratio between the intensities of these modes, we obtain the spin stiffness D as well as the bulk and surface anisotropies. For as‐grown samples with ∼5–6% concentration of Mn, D = 6 ± 1 T⋅nm2 while D = 18 ± 3 (25 ± 4) T⋅nm2 for a 3% Mn as‐grown (annealed) sample. The surface anisotropy is negative.