LaCrGe3 is an itinerant ferromagnet with a Curie temperature of Tc = 85 K and exhibits an avoided ferromagnetic quantum critical point under pressure through a modulated antiferromagnetic phase as well as tri-critical wing structure in its temperature-pressure-magnetic field (T -p-H) phase diagram. In order to understand the static and dynamical magnetic properties of LaCrGe3, we carried out 139 La nuclear magnetic resonance (NMR) measurements. Based on the analysis of NMR data, using the self-consistent-renomalization (SCR) theory, the spin fluctuations in the paramagnetic state are revealed to be isotropic ferromagnetic and three dimensional (3D) in nature. Moreover, the system is found to follow the generalized Rhodes-Wohfarth relation which is expected in 3D itinerant ferromagnetic systems. As compared to other similar itinerant ferromagnets, the Cr 3d electrons and their spin fluctuations are characterized to have a relatively high degree of localization in real space.PACS numbers:
139 La nuclear magnetic resonance (NMR) measurements under pressure (p = 0-2.64 GPa) have been carried out to investigate the static and dynamic magnetic properties of the itinerant ferromagnet LaCrGe 3 . 139 La-NMR spectra for all measured pressures in the ferromagnetically ordered state show a large shift due to the internal field induction |B int | ∼ 4 T at the La site produced by Cr ordered moments. The change in B int by less than 5% with p up to 2.64 GPa indicates that the Cr 3d moments are robust under pressure. The temperature dependence of NMR shift and B int suggest that the ferromagnetic order develops below ∼50 K under higher pressures in a magnetic field of ∼7.2 T. Based on the analysis of NMR data using the self-consistent-renormalization (SCR) theory, the spin fluctuations in the paramagnetic state well above T C are revealed to be three-dimensional ferromagnetic throughout the measured p region.
We describe an oblique-incidence zero-area Sagnac interferometric microscope for studying spatial and temperature dependence of magneto-optic (MO) effects in samples under cryogenic conditions. The microscope is capable of independently measuring Kerr effects from three Cartesian components of a magnetization and thus can be used to map out the magnetization vector across the sample. For illustration, we present MO Kerr effect images of magnetic domains at 77 K of a LaCrGe3 crystal terminated with an a–c plane (the plane that contains the lattice a-axis and c-axis). We further present measurements of magnetization in these domains from 90 to 77 K during zero-field cooling and field cooling in an external magnetic field from 20 to 150 Oe. The inherently high sensitivity and the capability of detecting a magnetization without external modulation makes such a Sagnac interferometric microscope particularly useful for studying magnetic effects in novel materials at low temperatures.
Vibrations can cause noise in scanning probe microscopies. Relative vibrations between the scanning sensor and the sample are important but can be more difficult to determine than absolute vibrations or vibrations relative to the laboratory. We measure the noise spectral density in a scanning SQUID microscope as a function of position near a localized source of magnetic field, and show that we can determine the spectra of all three components of the relative sensor-sample vibrations. This method is a powerful tool for diagnosing vibrational noise in scanning microscopies.There is a large literature on detecting vibrational motion in scanning probe microscopy. Vibrations of the microscope as a whole have been determined using an acceleromater; 1 of the cantilever using piezoelectric sensing 2 or interferometry 3 ; of the sample using stroboscopic optical microscopy, 4 non-linear effects in atomic force microscopy, 5 or the cantilever deflection in scanning force microscopy. 6 In addition, a standard technique for analyzing resolution and stability in electron beam lithography is to move an anisotropically etched silicon edge relative to the beam. 7 However there has been relatively less work on using sensor-sample vibrations as a diagnostic tool of vibrations within the microscope itself.In this paper we show how one can determine all three components of the vibrations between sensor and sample by measuring the time dependence of the flux through a scanning Superconducting QUantum Interference Device (SQUID) pickup loop due to a superconducting vortex.Our measurements were made in a scanning microscope developed in a collaboration between Attocube and Stanford. Briefly, in this system the vibration isolation is provided by suspending the entire system from springs. The microscope is housed in a vacuum can that is inserted into a liquid helium dewar; cooling is provided by He 4 exchange gas. The coarse positioning and scanning of the sample are performed by an Attocube piezoelectric stack. The SQUID is mounted on a cantilever consisting of a 3 mm wide, 10 mm long, and 25 µm thick copper shim, with wire bonds making electrical contacts. The SQUID mount and Attocube stack are mounted in a massive titanium housing. The titanium housing is suspended from a copper support which is firmly clamped to the sides of the vacuum can for thermalization of the microscope wiring. All measurements reported here were made at 4.2K.The SQUID susceptometer 8 used for these measurements has an integrated pickup loop and one turn field coil in the geometry indicated by the solid lines in Fig. 1a. We scan the sample relative to the SQUID's pickup loop by applying a variable DC-Voltage to our piezobased scanners. In our coordinate system the long axis of the cantilever is in theŷ direction, as are the leads to the pickup loop in the SQUID, and the sample plane is the xy plane.The studied sample is a 0.4 µm thick superconducting niobium film (T c = 9.2K). An isolated superconducting vortex was located by repeatedly cooling the sample in ...
The high critical temperature superconductor Lanthanum Barium Copper Oxide (La2−xBaxCuO4 or LBCO) exhibits a strong anomaly in critical temperature at 1/8th doping, nematicity, and other interesting properties. We report here Scanning Superconducting Quantum Interference Device (SQUID) imaging of the magnetic fields and susceptibility in a number of thin film LBCO samples with doping in the vicinity of the 1/8th anomaly. Spatially resolved measurements of the critical temperatures of these samples do not show a pronounced depression at 1/8th doping. They do, however, exhibit strong, nearly linear modulations of the susceptibility ("striae") of multiple samples with surprisingly long periods of 1 − 4 µm. Counterintuitively, vortices trap in positions of largest diamagnetic susceptibility in these striae. Given the rich interplay of different orders in this material system and its known sensitivity to epitaxial strain, we propose phase separation as a possible origin of these features and discuss scenarios in which that might arise.
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