Temperature dependence of the antiferromagnetic resonance linewidth in MnF2

The temperature dependent contribution to the antiferromagnetic resonance linewidth in MnF₂ has been measured at K-band frequencies (～ 23 GHz) between 5° and 40°K. It can be closely approximated by a T⁴ power law except at the highest temperatures.     

Critical behavior of electric field gradient in MnSi studied by muon level-crossing resonance

The electric quadrupole interaction of⁵⁵Mn nuclei was studied in the weakly ferromagnetic system MnSi using muon level-crossing resonance (LCR) technique. The temperature dependence of the electric field gradient (EFG) shows a critical behavior near the ferromagnetic transition temperature, indicating that the EFG due to the conduction electron is strongly correlated with the magnetic susceptibility in the itinerant electron magnetism. The temperature dependence of EFG is in reasonable agreement with the self-consistent renormalization theory developed by Moriya and coworkers.We gratefully acknowledge helpful discussion with Dr. N. Nishida. We also wish to thank Keith Hoyle and Curtis Ballard for technical support.     

Resolved nuclear hyperfine structure of muonium in CuCl by means of muon level-crossing resonance

Datailed muon level-crossing resonance measurements of Mu¹ and Mu¹¹ centres in single crystals of CuCl are presented. The hyperfine and nuclear hyperfine parameters of the closest two shells of nuclei are remarkably similar for the two centres, indicating that both are located at the same tetrahedral interstitial site with four Cu nearest neighbours and six Cl next-nearest neighbours. About 30% of the total unpaired-electron spin density is located on the muon, about 60% on the four nearest neighbours and the rest on the six next-nearest neighbours, with nothing observable for any other shell.     

Spontaneous surface magnetisation of single crystal MnF2 in the antiferromagnetic state

Spontaneous magnetisation of (100) and (010) surfaces of single crystal MnF₂ in the antiferromagnetic state has been discovered. The sign of the surface magnetisation is determined by the difference in dielectric constants of MnF₂ and ambient matter: magnetisation is directed to the substance with smaller . Received 28 August 1998 and Received in final form 15 December 1998     

Observation of “decoupled” diamagnetic muon states in Alkali halides by muon spin resonance

The states of positive muons in KCl, NaCl and KI were studied with the muon spin resonance method under a 3 kG decoupling longitudinal field, revealing a considerably larger fraction of diamagnetic muon state than observed by the conventional spin rotation method. The origin of this fraction, which increases with temperature, is attributed to a muonium to muon transition in solids.     

Muon level-crossing resonance in Si∶Al

A systematic investigation of five sherds from the Celtic oppidum of Manching by refiring in different atmospheres is presented. The data are part of an extended and general study of Celtic ceramics in early Europe.     

Muon level-crossing resonance in magnetically ordered materials

The positive muon is widely used as a microscopic probe of internal fields at interstitial sites in magnetically ordered materials. Recently, we have demonstrated that the hyperfine fields on the neighboring host nuclear spins can be measured using a novel muon level-crossing resonance technique, thus providing a more detailed picture of the electronic and magnetic environment around the muon. In this paper I will describe the fundamentals of muon level-crossing resonance as applied to magnetically ordered materials, and report an example in MnF₂.     

Theory of muon spin relaxation by critical fluctuations in antiferromagnetic salts

Relaxation rates for positive muons implanted in antiferromagnetic salts are predicted on the basis of calculations performed with a coupled-mode theory of critical fluctuations in isotropic and uniaxial magnets. Anisotropy suppresses fluctuations transmitted by the isotropic (hyperfine) relaxation mechanism. The dipolar mechanism leads to a relaxation rate which increases on lowering the temperature to the critical point; the predicted form, proportional to the critical correlation length, yields the same temperature dependence as for the NMR linewidth. For a magnet with isotropic spin interactions, both contributions (hyperfine and dipolar) to the relaxation rate diverge with a temperature dependence given by the square-root of the correlation length.     

Magnetostatic modes in antiferromagnetic resonance

Measurements of the antiferromagnetic resonance (AFMR) spectra of MnF₂ spheres are reported, and the principal magnetostatic modes (MSMs) in these spectra are identified. To interpret the observed spectra it is necessary to take into account, in addition to the theoretical MSM solution, the effects of impurities, finite sample size (propagation effects), and radiation damping. Expressions for the predicted relative intensities of the modes are developed for particular choices of symmetry of the exciting rf field and compared to observations. The observed shifts and intensities are larger than predicted. This discrepancy is explained by analysis of the role of the magnetic susceptibility of a sample near resonance. New observations are reported which improve the understanding of radiation damping in AFMR.     

Local hyperfine field vector of positive muon in mono-domain single crystal of antiferromagnetic La2CuO4

By using a monodomain single crystal La₂CuO₄ grown by the travelling solvent floating zone method, the strength and direction of the local hyperfine field vector at positive muon sites were determined. Combining with the ⁺ location determined separately, we found the importance of the local hyperfine field contributions with dipolar symmetry as well as the distant dipolar field from surrounding point dipoles of Cu atomic moments.     

A study of diamagnetic muon bonding in α-quartz using muon spin resonance

Diamagnetic muon spin resonance experiments were carried out in a-quartz at two temperatures: 300 K and 130 K. A strong diamagnetic resonance peak was observed at both temperatures. The spectrum exhibits an anisotropy in the chemical shift of 15 ppm (300K) and 25 ppm (130K).     

铁磁和反铁磁双层膜中铁磁共振的研究

采用微磁学理论研究了铁磁/反铁磁双层膜中的铁磁共振现象.本模型将铁磁薄层抽象为一个单晶,具有立方磁晶各向异性和单轴磁晶各向异性,而反铁磁层视为厚度趋近于半无穷,且只有单轴磁晶各向异性.推导出了该系统的铁磁共振频率和频率谱宽度的解析式.数值计算表明,铁磁共振模式分两支,取决于立方磁晶各向异性.而界面的交换耦合,是磁易轴具有单向性的起因.     

Observation of antiferromagnetic correlations in UBe13

The wavevector and energy dependence of the paramagnetic response in the normal phase of the Heavy Fermion system UBe₁₃ has been investigated between 10 K and 300 K using polarized neutrons and polarization analysis. At 10 K the response was found to be enhanced at non zero wave-vectors indicating the presence of strong antiferromagnetic correlations. The peaks in the scattering occured at positions expected for incipient type G antiferromagnetism of the simple cubic uranium sublattice. At room temperature the spatial correlations completely disappeared and the response was wave vector independent. Constant Q scans carried out at 10 K confirmed the Lorentzian dependence proposed by Goldman et al. [1].