W. D. Hutchison

Tensor operator formalism for multiple-quantum NMR. 1. Spin-1 nuclei

A mathematical treatment of multiple-quantum nuclear magnetic resonance (MQ NMR) for I = 1, based on irreducible tensor operators, is presented and discussed. In particular, stress is laid on the multipolar form of the density matrix during every stage of the MQ NMR experiment. It is argued that in the description of such experiments, tensor operators confer significant advantages over fictitious spin-12 matrices, and simple matrix transformations.

Giant reversible magnetocaloric effect in ErMn2Si2 compound with a second order magnetic phase transition

The magnetic properties and magnetocaloric effect (MCE) in the ternary intermetallic compound ErMn2Si2 have been studied by magnetization and heat capacity measurements. A giant reversible MCE has been observed, accompanied by a second order magnetic phase transition from paramagnetic to ferromagnetic at ∼4.5 K. Under a field change of 5 T, the maximum value of magnetic entropy change (−ΔSMmax) is 25.2 J kg−1 K−1 with no thermal and field hysteresis loss, and the corresponding maximum value of adiabatic temperature change (ΔTadmax) is 12.9 K. Particularly, the values of −ΔSMmax and ΔTadmax reached 20.0 J kg−1 K−1 and 5.4 K for a low field change of 2 T, respectively. The present results indicate that the ErMn2Si2 compound is an attractive candidate for low temperature magnetic refrigeration.

Tensor operator formalism for multiple-quantum NMR. 2. Spins 32, 2, and 52 and general I

Multiple-quantum nuclear magnetic (MQ NMR) experiments for I = 32, 2, and 52 nuclear assemblies are discussed within the framework of irreducible tensor operators. In general, it is shown that it is difficult to produce pure multipolar Tnn(s, a) states, where n = 2I, using simple nonselective rf pulses. However this problem can sometimes be remedied by adding NMR signals from two or more pulse sequences. In particular, signals originating from the pure T44(s) state, for I = 2 nuclei, can be easily obtained using signals from two simple MQ NMR pulse sequences. On the basis of the I = 1, 32, 2, and 52 results, some general comments are also made concerning (i) optimal MQ NMR pulse sequences, and (ii) “constants of the motion,” formed after the second rf pulse. In addition, general expressions are given for the evolution of the density matrix under (i) hard rf pulses, and (ii) the action of an axially symmetric quadrupole interaction, which hold for all I.

Electrically detected magnetic resonance in ion-implanted Si:P nanostructures

The authors present the results of electrically detected magnetic resonance (EDMR) experiments on ion-implanted Si:P nanostructures at 5K, consisting of high-dose implanted metallic leads with a square gap, in which phosphorus is implanted at a nonmetallic dose corresponding to 1017cm−3. By restricting this secondary implant to a 100×100nm2 region, the EDMR signal from less than 100 donors is detected. This technique provides a pathway to the study of single donor spins in semiconductors, which is relevant to a number of proposals for quantum information processing.

Large magnetocaloric effect in La2/3Ca1/3Mn1−xSixO3 (x = 0.05–0.20) manganites

The magnetocaloric effect (MCE) in Mn site Si substituted La2/3Ca1/3Mn1?xSixO3 (x = 0.05, 0.10, 0.15 and 0.20) was investigated. All the samples have a single paramagnetic to ferromagnetic transition and show a large magnetic entropy change (??SM) in the vicinity of the Curie temperature (TC). With the increases in Si content x, the ??SM peak position as well as TC gradually shift to a low temperature, while the maximum value of the magnetic entropy change remains at nearly the same high value, in the range 4.88?5.48?J?Kg?1?K?1 and 8.78?10.32?J?Kg?1?K?1 for a magnetic field variation of 2?T and 7?T, respectively. The origin of the large MCE and its potential application were discussed. The present system appears to be a good candidate for magnetic refrigerant materials.

MQ NMR hard and soft pulses for I = 1 spin systems. Raman multiple-quantum NMR

Abstract The general problem of applying either soft, intermediate, or hard RF pulses to an I = 1 spin ensemble is discussed, within the framework of irreducible tensors. The nuclei in question are subject to a magnetic field, directed along the axis of an axially symmetric quadrupole interaction, plus an RF field applied along the x′ axis in the rotating frame. General results are given which hold for all values of the RF field ω1 chemical shift Δω, and the quadrupole beating parameter ωQ. These results are subsequently used to discuss the recently reported “Raman Magnetic Resonance” experiment, in which double-quantum coherence is both created and detected in a single-shot NMR experiment. This is achieved by first preparing a T 22(a) multipolar state, then applying a weak cw field along the x′ axis, while detecting NMR signals along the y′ axis. It is shown that action of the weak RF field is to cause double-quantum signals to appear, along all three axes. These signals oscillate at twice the Larmor offset because they originate from the T 22(a) multipolar state. Explicit expressions are given which describe the time behavior of both the NMR signals and the T 22(a) multipolar state, as a function of the strength of the cw RF field ω1. In particular, it is shown that for a weak cw RF field, the strength of the x′ and y′ signals is proportional to ω1 whereas the depletion of the T 22(a) state is proportional to ω12. Thus detectable NMR signals, at a frequency of twice the Larmor offset, can be produced, while avoiding rapid conversion of the T 22(a) into other multipolar states. These conclusions however only hold provided the specific resonance condition Δω = ± ωQ is avoided.

The magneto-structural transition in Mn1-xFexCoGe

Large refrigeration capacities, between 212(30) J kg−1 and 261(40) J kg−1 for a magnetic field change from 0 T to 5 T, were obtained in Mn1−x Fe x CoGe (x = 0.01, 0.02, 0.03 and 0.04) compounds. A partial magnetic phase diagram has been derived on the basis of magnetic transition and martensitic transformation temperatures determined from differential scanning calorimetry (200 K to 450 K), variable temperature x-ray diffraction (20 K to 310 K) and magnetisation measurements (5 K to 340 K; 0.01 T). Mn1−x Fe x CoGe compounds with compositions in the range x = 0.01 to 0.03 exhibit magneto-structural transitions. Neutron diffraction experiments were carried out on the Mn0.98Fe0.02CoGe sample over the temperature range of 5 K to 450 K. The diffraction patterns were analysed based on irreducible representation theory which confirms a ferromagnetic structure in the sample with an atomic magnetic moment of 3.7(1)μ B at 5 K on the Mn sublattice, oriented along the orthorhombic c axis. More significantly, a magneto-structural transition around T M ~ 297(1) K with a full width at half maximum of 29 K is demonstrated directly via neutron diffraction. Larger magnetic entropy changes are obtained for the Mn1−x Fe x CoGe (x = 0.01, 0.02 and 0.03) samples than for Mn0.96Fe0.04CoGe which has separate structural and magnetic transitions. In addition, it is noted that standard Arrott plots do not provide unambiguous insight to the nature of the magneto-structural transition in the Mn1−x Fe x CoGe compounds.

Temperature-dependent electrical, elastic and magnetic properties of sol?gel synthesized Bi0.9Ln0.1FeO3 (Ln?=?Nd,?Sm)

This report details correlated electrical, mechanical and magnetic behaviour in BiFeO(3) ceramics doped with 10% Ln (Ln = Sm, Nd) ions on the Bi, or perovskite A, site and synthesized by a sol-gel method. The ceramics exhibit bulk piezoelectric and ferroelectric properties and clear ferroelectric domain patterns through piezoresponse force microscopy. Resonant ultrasound spectroscopy, dielectric spectroscopy and magnetometry studies show correlated magnetoelectromechanical behaviour and the existence of weak ferromagnetism for both compositions. An anomaly with simultaneous mechanical and magnetic signatures is discovered in both materials near room temperature, while previously reported transitions and anomalies are found to exhibit electro- and/or magnetomechanical coupling. Magnetism is significantly enhanced in the Sm doped sample, which is a promising multiferroic material.

Recent experiments with mapon spectroscopy

Electric quadrupole interactions of impurity nuclei in single crystals of54MnNi,125SbNi and125SbFe have been studied as a function of crystal field direction and applied magnetic field using MAPON. Distributions are in all cases broad compared with the mode values of the EQIs. For54MnNi the mode efg is isotropic to better than 5% between the easy <111> axis and a hard <100> axis. The efg is +0.88 (15)x 1019 Vm−2. The mode efg for125SbFe along its easy <100> axis is one half of that along a hard <112> axis, and one third of that measured along the easy <111> axis of125SbNi. The much larger efg mode and distribution seen in125SbNi, for four to eight times greater dilution than for125SbFe, suggests intrinsic contributions due to valence screening effects in the more itinerant nickel host.

Element-specific depth profile of magnetism and stoichiometry at the La0.67Sr0.33MnO3/BiFeO3 interface

Depth-sensitive magnetic, structural and chemical characterization is important in the understanding and optimization of novel physical phenomena emerging at interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant X-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3 / multiferroic BiFeO3 bi-layers with sub-nanometer resolution. In particular, the X-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers. Additional resonant X-ray reflection measurements indicate a region of an altered Mn- and O-content at the interface, with a thickness matching that of the magnetic diluted layer, as origin of the reduction of the magnetic moment.