E. ter Haar

Magnetization steps in Sn1-xEuxTe: Eu-Eu exchange and Eu distribution

The magnetization of Sn1-x Eux Te, with x = 0.011 and 0.042, was measured at 20 mK in magnetic fields up to 90 kOe. Magnetization steps (MSTs) from pairs and triplets were observed. The MSTs give J /kB = -0.311±0.006 K for the dominant Eu-Eu exchange constant. Comparisons of the magnetization curves with numerical simulations indicate that, instead of being distributed randomly, the Eu ions tend to bunch together. A phenomenological approach which uses the concept of a local Eu concentration xL is quite successful in describing the data for these two samples.

Magnetization steps in Pb1−xCexS at 20 mK ☆

Abstract The magnetization of Pb 1− x Ce x S single crystals was measured at 20 mK and magnetic field upto 90 kOe. Magnetization steps from two types of pairs were observed. The first has an isotropic exchange interaction with J=−0.32 K. The second type of pairs gives a structured step which can be explained by an anisotropic exchange interaction. Cluster statistics suggests that both types are nearest-neighbor pairs.

Magnetization steps from Ce3+-pairs in Pb1−xCexSe

Abstract The magnetization of Pb1−xCexSe single crystals (x=0.75% and 3.5%) was measured at 20 mK. For H || 〈1 1 1〉 two magnetization steps were observed at H1=5.7±0.2 and H 2 =9.5±0.2 kOe , which are attributed to two kinds of antiferromagnetic Ce3+-pairs. For H along 〈0 0 1〉 and 〈1 0 1〉 , the first step does not change, indicating that the associated exchange interaction is isotropic ( J=−0.26±0.01 K ). The second step, on the other hand, is split, possibly due to exchange anisotropy (with an average constant J =−0.43±0.02 K ). These two Js correspond, probably, to nearest- and next-nearest neighbor pairs. The analyses of the sizes of the steps, however, indicate that the distribution of Ce3+ in the samples studied may be non-random.

Distant-neighbor exchange constants in Mn-based dilute magnetic semiconductors

Individual exchange constants Ji between distant neighbors in Zn1−xMnxX (X=S, Se, Te) were obtained from magnetization steps at 20 mK. The results disagree with all predictions. The principles involved in this experimental work and its analysis are reviewed.

Dynamic susceptibility measurements at the spin-glass transition in the Zn1−xMnxIn2Se4 semiconductor

Measurements of the magnetic properties of the magnetic semiconductor Zn1?xMnxIn2Se4 for two Mn compositions (x = 0.87 and 1.00) are presented in this work. The localized Mn ions in the layered rhombohedral structure undergo a spin-freezing transition below 3.5?K. The frequency-dependent freezing temperature Tf varies as ?Tf/(Tf?log?)?0.022, indicating a spin-glass behaviour. From the frequency dependence of Tf, the validity of the critical slowing down associated with a true phase transition is tested. For T>Tc, the dynamic scaling of the imaginary component of ac susceptibility data yields the following set of critical parameters: z? = 9.7 ? 1.0, ? = 1.18 ? 0.20 and Tc = 2.61 ? 0.06?K for x = 0.87 and z? = 10.7 ? 0.9, ? = 1.20 ? 0.20 and Tc = 3.43 ? 0.01?K for x = 1.00. These values are in good agreement with reported values for other spin glasses.

The structure of Zn1-xMnxIn2Se4crystals grown by chemical vapour phase transport

Single crystals of Zn_(1-x)Mn_xIn_2Se_4 were grown by the chemical vapour phase transport technique (CVT). The Mn concentration was varied from x=1 to x=0.01. Through X-rays powder diffraction patterns and Laue diagrams of single crystals we studied the transformation from the hexagonal structure of MnIn_2Se_4 to the tetragonal structure of ZnIn_2Se_4.

Effect of longitudinal vibration on the final temperature of a plastic dilution refrigerator. The possibility of its use as a vibrating sample magnetometer.

Abstract We have investigated the influence of longitudinal vibrations on the final temperature of a fully plastic dilution refrigerator. We foresee the possibility of using it as a Vibrating Sample Magnetometer. At 82.7 Hz, sensitivities of 100μV/emu are possible at temperatures below 50 mK.

Phase boundary detection for dilution refrigerators

We describe a device to conveniently measure the positions of the phase boundaries in a dilution refrigerator. We show how a simple modification of a standard capacitive level gauge (segmentation of one of the electrodes) permits a direct calibration of the capacitance versus phase boundary position. We compare this direct calibration with the indirect procedure that must be adopted for a conventional capacitive level gauge. The device facilitates the correct adjustment of the 3He/4He fraction in the dilution refrigerator.

Magnetic properties of MnGa(2)Se(4)–MnIn(2)Se(4) single crystal semiconductors

Crystals of MnIn 2-2x Ga 2x Se 4 with concentrations x = 0, 0.1, 0.4, 0.6 0.8 and 1 were grown from the vapour phase by using a chemical transport technique, with the elements as starting materials. The crystals grown from the vapour phase are platelets-type and show a black colour. X-ray powder diffraction data could be indexed following the structure of MnIn 2 Se 4 for low Ga concentration samples. The magnetic susceptibility data indicate predominant antiferromagnetic interactions in all the studied samples. In samples with Ga concentration >0.8 was possible to observe a magnetic transition: a spin-glass like transition for sample with x = 0.8 and an antiferromagnetic transition for the sample with x = 1.