Search

The spatial and temporal correlations of magnetic moments in the paramagnetic regime of ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ have been investigated by means of polarized neutron diffraction and muon-spin relaxation techniques. Short-range magnetic correlations are present at temperatures far above the ferromagnetic transition temperature $({T}_{C})$. This leads to deviations of paramagnetic susceptibility from Curie-Weiss behavior. These short-range magnetic correlations extend in space, slow down with decreasing temperature, and finally develop into long-range magnetic order at ${T}_{C}$.

New inelastic neutron scattering data for ErFe2Si2 and Tm 169 Mossbauer spectra for TmFe2Si2 are presented. These data are analysed in terms of the crystal field acting at the rare earth site and new crystal field parameters are presented. The new crystal field parameters are compared with previous parameters to show the systematics associated with a change in transition metal.

The hexagonal TmCuAl is studied with magnetic specific heat, resistivity and Tm 169 Mossbauer spectroscopy measurements. TmCuAl orders antiferromagnetically at T N = 1.9 K. The distance between the two levels of the quasi-doublet ground state of TmCuAl, determined with the Tm 169 Mossbauer pseudo-quadrupole shift, amounts to 2.8(3) K. Based on the temperature dependence of the Tm 169 quadrupole splitting a provisional calculation has been made for the crystal field diagram with a coupling constant α , which is non zero.

We show that, in rare earth based insulators, measurement of the thermal dependence of the muon spin-lattice relaxation rate at high temperature provides information on the nature of the magnetic correlations and on the crystal-field energy splitting, if any.

Gd 2 BaCuO 5 is studied with 57 Fe Mossbauer spectroscopy on a 57 Fe (1/2%) doped sample and with muon spin relaxation on an undoped sample. Cu–Cu and Gd–Gd magnetic order are found to occur simultaneously at T N =11.8 K. The Mossbauer results suggest a first-order magnetic transition for the Cu sub-lattice.

In GdNi5, the muon localizes at the 3f interstitial site and below ∼80 K a second muon site becomes populated, either at the 6m or 6k site. These latter sites are located in a ring surrounding the (0, 0, 1 2 ) site and the muon occupies all six sites within the time range of the experiment (10−9 s). This ring site is metastable and the muon hops to the preferred 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be Ea=272 (10) K and the tunneling matrix element J=0.11 (2) meV.

We have studied DyFe4Ge2 and ErFe4Ge2 with 57Fe, 161Dy and 166Er Mossbauer spectroscopy. DyFe4Ge2 has a c axis anisotropy, whereas ErFe4Ge2 could be interpreted with two subspectra with both an angle with respect to the c axis. In ErFe4Ge2 a magnetic phase transition is observed at T=8 K.

In GdNi5, the muon localizes at the 3f interstitial site and below ∼80K a second muon site becomes populated, the so-called 6ring site. This ring site is metastable and the muon hops to the 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be E a =272(10)K and the tunneling matrix element J=0.11(2)meV.

The magnetic ordering and crystal field interactions of the Pnma phases of both Tm2BaCoO5 and Tm2BaNiO5 are investigated by 169Tm Mossbauer spectroscopy and the temperature-dependent hyperfine interactions are compared with those obtained elsewhere for Tm2BaCuO5. The Pnma phases are shown to order magnetically at temperatures of 3.5(2) K (Tm2BaCoO5) and 4.85(5) K (Tm2BaNiO5), the order being induced by the transition metal. For Tm2BaNiO5 an additional first-order transition observed at T≤1.4 K is identified with the independent magnetic order of the thulium sub-lattice.

We report zero-, longitudinal-, and transverse-field positive muon spin relaxation ({mu}SR) measurements in single crystals of the heavy-fermion superconductor UBe{sub 13} [T{sub c}=0.83(2) K]. Our zero-field measurements provide an upper limit for any quasistatic magnetic moments in this system below 100 K of 0.001{mu}{sub B}, consistent with previous measurements on polycrystals. From the transverse-field measurements we find a lower bound for the London penetration depth {lambda}{sub L}=1210 nm. This value implies a lower limit for the effective mass consistent with the low-temperature specific heat. (c) 2000 The American Physical Society.

A combined Tm Mossbauer spectroscopy and mSR ~muon spin rotation! study is presented on a polycrystalline sample of TmNi2B2C. The first experimental technique shows the coexistence of two types of Tm 4 f magnetism at 0.3 K, one related to a magnetic Tm moment of 4.3(1)mB and the other to a small Tm moment of ;0.1mB . This second type of magnetism is suggested to be caused by carbon vacancies present in TmNi2B2C. These C vacancies locally modify the crystalline electric field and therefore the Tm 4 f magnetism. Since these vacancies are ideal cavities in which muons can reside, it is suggested that mSR only probes this ;0.1mB magnetism. @S0163-1829~98!05217-5#

Specific heat and muon spin rotation and relaxation data are presented for two geometrically frustrated systems: Gd2Ti2O7 which antiferromagnetically orders and Yb2Ti2O7 which presents dynamical short range correlations at low temperature. The muon data help to characterize the spin dynamics of these two compounds.

Zero-field muon spin rotation experiments provide evidence for an antiferromagnetic component in the magnetic structure of the intermetallics ZrZn2., 5 pages, 2 figures

The magnetic properties of PrRu${}_{2}$Si${}_{2}$ have been investigated experimentally by specific heat, single-crystal magnetization, ${}^{141}$Pr M\"ossbauer and muon spectroscopies, neutron powder diffraction, and inelastic neutron scattering, leading to the determination of its zero-field phase diagram and its crystal electric-field energy levels below 40 meV. PrRu${}_{2}$Si${}_{2}$ undergoes a magnetic phase transition at ${T}_{N}$ $\ensuremath{\simeq}$ 16 K to an axial incommensurate sine-wave magnetic structure characterized by a wave vector $\ensuremath{\tau}$ = (0.133, 0.133, 0), followed by a first-order phase transition at ${T}_{C}$ $\ensuremath{\simeq}$ 14.0 K to an axial ferromagnetic structure. The lowest crystal electric-field states are the two singlets $|{\ensuremath{\Gamma}}_{t1}^{(1)}〉$ and $|{\ensuremath{\Gamma}}_{t2}〉$ separated by 2.25 meV. The low-temperature properties are described by a Hamiltonian identical to that of an Ising system with a transverse magnetic field. Since the ratio of the exchange energy to the energy splitting between the singlets is sufficiently large, it exhibits spontaneous magnetization. The nature of the two singlet states explains the giant magnetic anisotropy. The random-phase approximation predicts the value of the high-field magnetization but yields a low-field magnetization too small by $\ensuremath{\sim}$ 15$%$. Possible application of our results to uranium intermetallic compounds is pointed out.

From magnetic, specific heat, 170Yb Mossbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb2Sn2O7, we show that below the first order transition at 0.15 K all of the Yb3+ ions are long-range magnetically ordered and each has a moment of 1.1?B which lies at ?10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [ Phys. Rev. Lett. 108 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.

Muon-spin-relaxation experiments and ac low-field magnetization measurements have been carried out on the superconductor Ce${\mathrm{Ru}}_{2}$. The relaxation rate of the muon-spin polarization in zero field exhibits a small but significant increase below ${T}_{M}\ensuremath{\simeq}40$ K, which is suppressed by applying a longitudinal field. This result taken together with magnetization measurements provides definite evidence for the occurrence of static electronic magnetism involving extremely small magnetic moments. Our work shows that Ce${\mathrm{Ru}}_{2}$ is a member of a restricted family of superconducting compounds that order magnetically with extremely small magnetic moments at a temperature much higher than that at which they become superconducting.

We present zero-field muon-spin-relaxation data on the ferromagnetic fluctuations near the Curie temperature of the intermetallic GdNi5, which is a Heisenberg magnet with strong dipolar interaction. Our data show that the critical longitudinal ~along the wave vector qO spin fluctuations above and below the Curie temperature are similar. They are an experimental proof of this similarity. This result is explained using the dynamical scaling theory of Halperin and Hohenberg. Although the transition from the paramagnetic to the ordered state of a simple ferromagnet is an archetypical second-order phase transition, the dramatic effect of the dipolar interaction on the nature of the critical paramagnetic spin fluctuations has been fully understood only recently. An almost quantitative agreement has been achieved between the experimental data from magnetization, 1 neutron, 2 and local probe 3,4 techniques and mode coupling theory. 5 In contrasts below the Curie temperature TC the experimental data are scarce 6,7 and a complete theory is still lacking. 8 This report presents a detailed study on crystals of the spin dynamics for a dipolar Heisenberg ferromagnet by a local

We have studied by zero-field and longitudinal fieldμSR measurements the uniaxial antiferromagnetic intermetallicsNdRh2Si2. The measurements have been performed on single crystals. Our data show that the magnetic field at the muon site is parallel to thec axis. Below the Neel temperatureTN the spin-lattice relaxation is driven by a two magnon process. In the paramagnetic region nearTN we observe, a critical divergence of the damping rate.

The141Pr Mossbauer isomer shift and hyperfine field spectrum of PrBa2Cu3O7−y were measured at 25 and 4.2 K, respectively. The isomer shift corresponds to an intermediate valency of 3.4(1) and the hyperfine field spectrum at 4.2 K seems to indicate a hyperfine field of 29(3) T.

We report a zero fieldμSR study of single crystals of theRENi5 ferromagnets where RE=Gd, Er, Dy or Tm. Our work points out the strong influence of the crystal electric field on the rare earth total momentum dynamics: whereas the spin-lattice relaxation process belowTc is controlled by a two magnon mechanism for a weakly anisotropic magnet (GdNi5), the phonons drive the relaxation for a strongly anisotropic magnet such asErNi5. ForTmNi5 a comparison is made between the fluctuation time measured byμSR and169Tm Mossbauer spectroscopy.

Results of a detailed investigation of the ferromagnetic superconductor UGe2 using positive muon spin rotation and relaxation techniques are presented. The pressure and temperature dependences of the frequencies and related spin-spin relaxation rates show that the transition from the weakly to the strongly polarized magnetic (WP-SP) phases is still observable at TX?3?K under a pressure of 1.33(2) GPa. Thus this transition survives at higher pressures than previously believed. The temperature TX at 1.00(2) GPa corresponds to a thermodynamic phase transition rather than a crossover. No such statement can be given reliably at lower pressure. A substantial shrinking of the component along the easy axis of the diagonal hyperfine tensor, at the muon site where it is large, is observed in the SP phase relative to the WP phase. This corresponds to an appreciable decrease in the electronic density at the Fermi level in the SP phase. The investigation of the paramagnetic-ferromagnetic critical spin dynamics at ambient pressure and at 0.95(2) GPa shows that the simple one-band model is an oversimplification inconsistent with our critical spin-dynamics results. Data from specific heat, Fermi-surface studies, Hall effect, neutron form factor, and spectroscopic techniques supports this conclusion. Even at 0.95(2) GPa the conduction electrons are characterized by a small magnetic moment, relative to the bulk magnetization per uranium atom.

The 57 Fe Mossbauer effect in Tm 2 Fe 17 N χ was measured in the temperature range 4.2–600 K. The temperature dependence of the 57 Fe Mossbauer spectra shows a hyperfine field anomaly around the spin reorientation temperature ( T SR ≈ 190 K) where the easy magnetization direction changes from parallel to c ( T T SR ) to perpendicular to c ( T > T SR ). At 4.2 K the 57 Fe hyperfine field was found to have slightly increased after charging Tm 2 Fe 17 with nitrogen. The 169 Tm Mossbauer spectrum of Tm 2 Fe 17 N χ was measured at 4.2 K. From the quadrupolar splitting it was derived that interstitial solution of nitrogen atoms considerably increased the second-order crystal field parameter ( A 2 o ≈300 Ka o −2 ) although the value reached is not much larger than found before in Tm 2 Fe 17 C.

We have performed a detailed study of the magnetic properties of GdCrO 4 at low temperatures by complementary use of different macroscopic and microscopic physical techniques. A ferromagnetic order is established in this oxide below TC ≤ 22K. The ordered magnetic moments of the Cr5+ ions are located along the crystallographic c-axis, forming an angle of ≈24° with the ordered moments of the Gd3+ ions. Surprisingly, only 20% of the Gd3+ sublattice orders at the Curie temperature of 22K, while the remaining 80% stays paramagnetic down to around 10K. This 80% of the Gd3+ ions may be ascribed to a low-temperature orthorhombic phase. Moreover, the Gd3+ magnetic order seems to be triggered by a relatively large transferred hyperfine field coming from the Cr5+ sublattice. © 2006 IOP Publishing Ltd.

We report a study of the geometrically frustrated magnetic material ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ by the positive muon-spin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of ${\mathrm{Tb}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$: the ${\mathrm{Tb}}^{3+}$ magnetic moment characteristic fluctuation time is $\ensuremath{\simeq}{10}^{\ensuremath{-}10}\text{ }\text{ }\mathrm{s}$. The strong effect of the magnetic field on the muon-spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy.

Three-dimensional neutron depolarization experiments have been performed on RCrO4 (R=Y, Er, Tm, Yb) powder samples in order to gain insight into their magnetic domain structure in the submicrometer range. The temperature evolution of both the average domain size and the net magnetization of each compound has been studied for different applied magnetic fields. The largest average domain size at zero external magnetic field was found in YbCrO 4. The effect of an applied magnetic field on the magnetic domain structure is relatively small in ErCrO4 and TmCrO4, when compared to YCrO4 and YbCrO4 where the average domain size even surpasses the average particle size determined by Scanning Electron Microscopy studies. © 2004 Elsevier B.V. All rights reserved.

We report positive muon spin measurements of the spin–lattice relaxation rate, λZ ,f or the weakly helimagnetic metal MnSi performed at ambient pressure and covering the temperature range from 2 to 280 K. The self-consistent renormalization theory is unable to explain the temperature dependence of λZ , in particular far below the ordering temperature and in the critical paramagnetic regime. A temperature independent length scale is required to account for these data. (Some figures in this article are in colour only in the electronic version) The weakly helimagnetic metal MnSi is becoming the model system for the investigation of anomalous metallic properties. Thanks to recent developments of physical measurements under high pressure, anomalous properties have been detected over a wide region of its phase diagram near the pressure-induced first-order magnetic–non-magnetic transition [1]. More surprisingly, even at ambient pressure the expected metallic behaviour is not observed with the optical conductivity [2]. Here we report an investigation of the spin dynamics by the muon spin relaxation (µSR) technique; for references to this technique see e.g. [3–5]. It confirms that at ambient pressure the physical properties of MnSi are still not understood: we find the predictions of the conventional theory for a weakly magnetic metal, i.e., the self-consistent renormalization (SCR) theory [6], at variance with the experimental results in a large temperature range. These results are explained if a new length scale is introduced. MnSi is a metallic compound which undergo a second order magnetic phase transition at ∼29. 5Ki nto a helical magnetic structure characterized by a small wavevector Q0 = 0.035 A −1

We performed temperature-dependent 3D neutron-depolarisation measurements on an ErCrO4 powder sample to study the ferromagnetic order below T C ≃15 K . From the determinant of the depolarisation matrix, the domain size is calculated as a function of temperature. It is discussed in terms of the applied magnetic field and field history.

The compound TmCrO4 has been studied with bulk magnetization measurements, 169 Tm Mossbauer spectroscopy and muon spin relaxation (μSR). The Cr sublattice in TmCrO4, which orders with a second-order magnetic phase transition at T C =18.75 K , gradually induces magnetic order in the non-magnetic crystal field ground state of the Tm sublattice.

The magnetic properties of the compound DyFe 4 Ge 2 have been studied using 57 Fe Mossbauer spectroscopy and using magnetic measurements in high magnetic fields. The compound DyFe 4 Ge 2 orders magnetically below 65 K. At 4.2 K it displays a strong field-induced first order magnetic phase transition at B crit = 1.6 T and a second weaker transition at B crit = 6.0 T .

From muon spin relaxation spectroscopy experiments, we show that the sharp peak ($\ensuremath{\lambda}$-type anomaly) detected by specific heat measurements at 54 mK for the ytterbium gallium garnet compound, ${\mathrm{Y}\mathrm{b}}_{3}{\mathrm{G}\mathrm{a}}_{5}{\mathrm{O}}_{12}$, does not correspond to the onset of a magnetic phase transition, but to a pronounced building up of dynamical magnetic pair correlations. Beside the $\ensuremath{\lambda}$ anomaly, a broad hump is observed at higher temperature in the specific heat of this garnet and other geometrically frustrated compounds. Comparing with other frustrated magnetic systems we infer that a ground state with long-range order is reached only when at least $1/4\ensuremath{-}1/3$ of the magnetic entropy is released at the $\ensuremath{\lambda}$ transition.

In DyNi5, the muon localizes at the 6 i ~1/2,0,0.1! interstitial site and below ;80 K a second muon site becomes populated, the metastable 6 ring site. We determine the mean residence time of the muon in this ring site and analyze its temperature dependence with a multiphonon quantum diffusion process. The coincidence energy is measured to be Ea /k B5400 ~20! K and the tunneling matrix element J50.21 ~7! meV. A comparison with GdNi5 gives insight in the muon potential well scheme in these compounds. We attribute the large diversity of muon sites in RNi5 series, where R is a rare earth element, to the relatively low density of states at the Fermi level.

The temperature dependence of the 166 Er and 169 Tm hyperfine fields in Er 2 Fe 17 C and Tm 2 Fe 17 C was studied in extended temperature regions on both sides of the spin reorientation temperature. From fits of these temperature dependences we obtained values of the second-order crystal field parameter ( A 2 o = − 300 K a 0 −2 ) and the intersublattice coupling constants ( J ErFe / k = −6.3 K , J TmFe / k = −6.0 K ). The value of A 2 o obtained in this way agrees with an earlier determination based on the quadrupolar splitting of the 166 Er and 169 Tm Mossbauer spectra.

Positive muon spin relaxation measurements performed on the ferromagnet $\mathrm{U}\mathrm{G}{\mathrm{e}}_{\mathrm{2}}$ reveal, in addition to the well-known localized $5f$-electron density responsible for the bulk magnetic properties, the existence of itinerant quasistatic magnetic correlations. Their critical dynamics is well described by the conventional dipolar Heisenberg model. These correlations involve small magnetic moments.

Using neutron diffraction, ${}^{170}\mathrm{Yb}$ M\"ossbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, where the ${\mathrm{Yb}}^{3+}{S}^{\ensuremath{'}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ ground state has planar anisotropy. Below $\ensuremath{\sim}0.24\mathrm{K}$, the temperature of the known specific-heat $\ensuremath{\lambda}$ transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the ${\mathrm{Yb}}^{3+}$ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime.

The magnetic response of the heavy fermion superconductor UPt_3 has been investigated on a microscopic scale by muon Knight shift studies. Two distinct and isotropic Knight shifts have been found for the field in the basal plane. While the volume fractions associated with the two Knight shifts are approximately equal at low and high temperatures, they show a dramatic and opposite temperature dependence around T_N. Our results are independent on the precise muon localization site. We conclude that UPt_3 is characterized by a two component magnetic response., 5 pages, 4 figures

The magnetic properties of TmFeAl and ErFeAl have been studied using neutron depolarization and diffraction. The first compound exhibits an extended first order magnetic transition between 10 and 60 K and its ordered state consists of magnetic nanodomains. Neutron depolarization displays the formation and growth of small magnetic domains as the temperature decreases. The average domain size is clearly smaller than 30 nm at 4 K and the existence of magnetic nanodomains is confirmed with neutron diffraction. The magnetic properties of ErFeAl do not display this anomalous behaviour but show a magnetic domain size of 0.4 μm. The structural disorder of the Fe atoms in the lattice combined with the high anisotropy of the Tm moments gives rise to the observed magnetic behaviour.

We present muon spin relaxation measurements performed on crystals of the heavy fermion superconductor UPt$_3$. In zero applied field, contrary to a previous report, we do not observe an increase of the internal magnetic field in the lower superconducting phase (the B phase). Our result gives an experimental upper bound of the magnetic field that could be associated with the superconducting state., Comment: 4 pages, REVTeX 3.0, 2 PostScript figures

Muon relaxation measurements on the uniaxial commensurate antiferromagnet PrCo 2 Si 2 show that whereas the relaxation rate probing the fluctuations along the c -axis has a strong maximum at T N , the rate probing the c -plane fluctuations presents only a weak maximum which occurs at ∼ 3 K below T N .

With Mossbauer effect and X-ray diffraction on magnetically aligned ErFe3 it has been shown that the easy axis of magnetization turns from the crystallographic c-axis to the b-axis at 50 K with increasing temperature. In the Er(Fe1-xCox)3 series from x = 0 up till x = 0.4 almost the same spin reorientation temperature is found. For higher Co concentration the spin reorientation temperature increases drastically.