Radiofrequency forward Mössbauer scattering method in study of magnetic materials

Mössbauer spectra in forward scattering scheme were measured for iron borate (FeBO ₃) exposed to radiofrequency (rf) field below the Neel temperature. The spectra have satellites spaced by doubled rf field frequency. The semiclassical model of Mössbauer transmission through a thick magnetic sample under rf reversals of a hyperfine field is proposed. This model reproduces all features of the measured spectra. Experiments and model calculations indicate additional possibilities of this measurement scheme for study the soft magnetic materials.     

A magnetic lens for cold atoms controlled by a rf field

We report on a new type of magnetic lens that focuses atomic clouds using a static inhomogeneous magnetic field in combination with a radio-frequency (rf) field. The experimental study is performed with a cloud of cold cesium atoms. The rf field adiabatically deforms the magnetic potential of a coil and therefore changes its focusing properties. The focal length can be tuned precisely by changing the rf frequency value. Depending on the rf antenna position relative to the dc magnetic profile, the focal length of the atomic lens can be either decreased or increased by the rf field. PACS 39.25.+k; 37.10.Gh     

Applications of adiabatic half passage to NQR

The application of nuclear quadrupole resonance (NQR) to the detection of materials can be hampered by the low sensitivity of the technique. The use of surface coils for remote detection only exacerbates this problem. In this paper we demonstrate the advantages of adiabatic half passage (AHP) for NQR detection ofI=1 spins in powder samples. Under optimal conditions, AHP provides a 15% sensitivity enhancement over traditional optimized, pulsed excitation. AHP excitation is independent of ω₁ over more than an order of magnitude variation in radio-frequency (rf) field strength, and can provide up to a factor of two or more sensitivity enhancement over traditional pulsed excitation in inhomogeneous rf magnetic fields. In pulsed spin-locking-type experiments, AHP as a prepulse can provide near constant signal amplitude over a factor of two variation in rf magnetic field strength.     

Radiofrequency stimulated quantum interference on Mössbauer transitions

Redistribution of the elastic and Raman channel intensities in the Mössbauer resonant scattering spectrum can be controlled by the radio frequency (rf) stimulated quantum interference of gamma-transition amplitudes. More general expressions for each channel intensity are written out exactly taking into account the influence of the rf field. Results of calculations for the simple three-level scheme can be easily generalized to the case of ⁵⁷Fe isotope in magnetic materials.     

The rf stimulation of amorphous metals

The effect of the rf modulation of the Mössbauer gamma radiation (rf sidebands) and the effect of the fast relaxation of the magnetic hyperfine field induced by the rf field (rf collapse) are described. The coexistence and separation of these effects is discussed. The rf collapse of the magnetic hyperfine structure allows a direct observation of the quadrupole interaction in ferromagnetic amorphous metals providing the method for studying the short range order. Examples of FeSiB, FeNiSiB, FeCoSiB and FeB are discussed with respect to structural models. The effect of radiofrequency enhanced crystallization of amorphous metals and its relation to rf sidebands is demonstrated.     

Optical detection of single-electron spin resonance in a quantum dot

We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few microT. In one case, the spin resonance behaves as a driven 3-level lambda system with weak damping; in another one, the dot exhibits remarkably strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with fluctuating resonant positions, evidence of unusual dynamic processes.     

Influence of external magnetic field on dust acoustic waves in a capacitive RF discharge

This paper reports experiments on self-excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low-frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E-field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B < 0.05 T only modifies the characteristics of propagating waves in dusty plasma at moderate power and pressure, P = 3.5 W and p = 27 Pa, respectively. It is found that DAWs start to be damped with increasing the magnetic field beyond B > 0.05 T and get completely damped at higher magnetic field B ∼ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma.     

A novel low-E field coil to minimize heating of biological samples in solid-state multinuclear NMR experiments

A novel coil, called Z coil, is presented. Its function is to reduce the strong thermal effects produced by rf heating at high frequencies. The results obtained at 500MHz in a 50 microl sample prove that the Z coil can cope with salt concentrations that are one order of magnitude higher than in traditional solenoidal coils. The evaluation of the rf field is performed by numerical analysis based on first principles and by carrying out rf field measurements. Reduction of rf heating is probed with a DMPC/DHPC membrane prepared in buffers of increasing salt concentrations. The intricate correlation that exists between the magnetic and electric field is presented. It is demonstrated that, in a multiply tuned traditional MAS coil, the rf electric field E(1) cannot be reduced without altering the rf magnetic field. Since the detailed distribution differs when changing the coil geometry, a comparison involving the following three distinct designs is discussed: (1) a regular coil of 5.5 turns, (2) a variable pitch coil with the same number of turns, (3) the new Z coil structure. For each of these coils loaded with samples of different salt concentrations, the nutation fields obtained at a certain power level provide a basis to discuss the impact of the dielectric and conductive losses on the rf efficiency.     

Attenuation of morphine-induced analgesia in mice by exposure to magnetic resonance imaging: separate effects of the static, radiofrequency and time-varying magnetic fields

Exposure of adult male mice to a magnetic resonance imaging (MRI) procedure has been shown to abolish the nocturnal analgesic responses observed following treatment with morphine. The field component(s) responsible for this inhibitory effect were examined by exposing mice to either the static, time-varying or rf magnetic field components associated with an MRI procedure. In the middle of the night portion of their day-night cycle, mice were exposed for 23.2 min to one of the above field components, intraperitoneally injected with morphine sulphate (10 mg/kg) and then exposed to the field conditions for another 23.2 min, after which analgesic responses were determined. Analgesia was quantitated by determining the length of time mice were content to be on a hot surface (50 degrees C) before they showed discomfort by licking their paws. It was observed that the time-varying magnetic field completely abolished, the rf field significantly reduced, while the static field component (0.15 T) had no evident effect on morphine-induced analgesia. These results indicate that the time-varying, and to a lesser extent the rf, fields associated with the MRI procedure inhibit morphine-induced analgesia in mice. These data also raise the possibility that exposure in humans to some of the magnetic field components associated with MRI may have clinically relevant effects on the actions of narcotic drugs such as morphine.     

自旋霍尔纳米振荡器的非线性动力学及其应用

自旋霍尔纳米振荡器利用电流产生的自旋轨道力矩驱动磁性薄膜中磁矩进行高频进动,能在微纳尺度下实现全电学调控的相干自旋波和微波信号,是一类新型的纳米自旋电子学器件,在信息存储、处理和通信方面具有广泛的应用前景。基于强自旋轨道矩效应,人们近期在各类铁 磁/非磁重金属构成的双层薄膜结构中,已实现了多种不同自旋波模式的电学激发和调控,并对 其复杂的非线性动力学特性进行了深入的探究。基于这些前期的研究结果与最新的进展,我们在 本综述中对“对三角”结构的纳米间隙型、“蝴蝶结”型、纳米线型、垂直纳米点接触型以及阵 列等具有各类器件结构的自旋霍尔纳米振荡器所体现出来的丰富非线性动力学特性进行了详细讨 论与归纳,并对其在新型低能耗量子磁振子自旋器件和非冯诺依曼架构的自旋型人工神经网络计 算方面的潜在应用也进行了探讨。     

联合阱中稳定囚禁离子的实验研究

实验观察到了小型联合阱稳定囚禁离子的射频信号增强现象，并以此对联合阱囚禁离子的特性进行了讨论；同时，通过实验检测到的稳定囚禁离子的工作点范围与磁场强度的关系，在一定程度上验证了联合阱囚禁离子稳定区随磁场强度的变化趋势。     

A Radio Frequency Field Guide Using Field Induced Adiabatic Potential

We study the behaviour of atoms in a field with both static magnetic field and radio frequency （rf） magnetic field. We calculate the adiabatic potential of atoms numerically beyond the usually rotating wave approximation, and it is pointed that there is a great difference between using these two methods. We find the preconditions when RWA is valid. In the extreme of static field almost parallel to rf field, we reach an analytic formula. Finally, we apply this method to 87 Rb and propose a guide based on an rf field on atom chip.     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

Linear intermode conversion of electromagnetic wave energy in a magnetically soft gyrotropic material

The excitation of axial radio-frequency (rf) magnetic induction by an axial rf current is observed in a conductor with circular magnetic anisotropy when a weak magnetizing field is applied. The conductor is an amorphous cobalt-based wire, which exhibits azimuthal magnetic anisotropy. It serves as the central conductor in a coaxial line. The axial rf magnetic induction produces an emf in an induction coil coaxial to the conductor. The induction coil is part of a matched receiving circuit. The power conversion coefficient is as high as tens of percent. The measurements demonstrate the high sensitivity of the conversion coefficient to an external field. The theory of ferromagnetic resonance faithfully describes the results of the observations. Zh. Tekh. Fiz. 69, 58–63 (March 1999)     

Giant intrinsic magnetic hardness

Crystallographically partly randomized materials with high magnetic anisotropy exhibit anisotropy and exchange fluctuations. Possibly the most dramatic consequence of these interaction fluctuations is the presence of giant intrinsic magnetic hardness observed, especially at cryogenic temperatures. This magnetic hardness (resistance to demagnetization) is anintrinsic solid state property, quite in contrast with hardness in technologically important materials based on the presence of fine particles or precipitate phases. It introduces rather a further variety in the phenomenology of interaction fluctuation materials such as spin glasses, representing the extreme case of high anisotropy and high moment concentrations. In this paper the phenomenon of strong intrinsic magnetic hardness is reviewed, and some new experimental data are presented for homogeneous pseudobinary regions of composition on the basis of such compounds as SmNi₅, SmCu₅, SmCo₅, TbFe₂ and others. Magnetic hardness in some of these cases reaches the highest values found so far for any class of materials. For instance, maximum values of coercive force of aboutH _c=230 kOe are observed for SmCo_5−xNi_x. Even higher values are extrapolated in other cases. Maximum values ofH _c are observed in materials with ordering temperatures of order 60–200 K. A strong temperature and a weaker time dependence ofH _c is observed and discussed on the basis of theories involving thermal activation of domain propagation. Comparisons are drawn with effects generally observed in magnetically hard materials, and the relationship of intrinsic magnetic hardness to technologically interesting materials is discussed. Tentative conclusions as to the details of the origins of giant intrinsic magnetic hardness are drawn and areas of future interest are indicated. This study was supported by a grant from the National Science Foundation.     

Magnetoimpedance effect in FeCoMoSiB amorphous sheets

The magnetoimpedance effect in sheets made of Fe₄Co₆₇Mo_1.5Si_16.5B₁₁ amorphous metallic alloy is studied in relation to the mutual orientations of the sheet axis, permanent magnetic field, and variable electrical current. Also, the effective permeability is studied as a function of the mutual orientations of the sheet axis, permanent magnetic field, and rf magnetic field. Under certain orientations of the rf magnetic field and rf electrical current relative to the sheet axis, experimental dependences of the magnetoimpedance effect and effective permeability on the permanent magnetic field are found to correlate qualitatively. The experimental data are explained in terms of domain reconfiguration in the alloys.     

A new rf plasma oxidation method for the insulating AlOx barrier in magnetic tunneling junctions

An rf remote plasma oxidation technique to form an insulating barrier was carried out to enhance properties of CoFe/AlO_x/CoFe magnetic tunneling junctions. The rf remote plasma method was found to reduce self-bias effect on the barrier during the rf oxidation process and to increase atomic oxygen concentration in a plasma state. Experimentally observed rms roughness of the barrier in our magnetic tunnel junction was decreased from 5 to 1.5 Å. In addition, electrical breakdown voltage and magnetoresistance of our magnetic tunnel junction devices were increased from 0.8 V up to 1.2 V and from 7% up to 30%, respectively.     

NMR Mössbauer double resonance in tantalum

Absorption spectra of Mössbauer ¹⁸¹Ta¹(W) radiation in tantalum have been measured with the source placed in a static magnetic field H_O ≈ 3400 Oe with a rf magnetic field resonant with respect to the frequency of the excited state 3 MHz with amplitude of 300 and 360 Oe, a rf field of 4 MHz with an amplitude of 300 Oe and without rf field. The dramatic change of the spectrum in the resonant field of 300 and 360 Oe, consistent with the theoretical prediction, is evidence for the observation of the NMR Mössbauer double resonance.     

Collapse in the Stoner-Wohlfarth noninteracting-particle model

A theory of Mössbauer spectra of noninteracting Stoner-Wohlfarth (SW) particles interacting with rf magnetic fields is developed. The theory makes it possible to calculate the absorption spectra for arbitrary frequency and amplitude of the rf field. The main features of the Stoner-Wohlfarth model are discussed. The Liouville superoperator formalism is used to generalize the results to the case of arbitrarily time-varying hyperfine fields at a nucleus. To understand the qualitative features of the collapse effect that are observed in the Mössbauer spectra of SW particles the particular case of a circularly polarized hyperfine field is studied, and an analytical expression is obtained describing the Mössbauer spectra for this case. An analysis is also made for weak rf magnetic fields and in this case the resonance behavior of the Mössbauer lines is traced as a function of the frequency of the rf field.