Persistence of Magnetism in Atomically Thin MnPS3 Crystals

The magnetic state of atomically thin semiconducting layered antiferromagnets such as CrI3 and CrCI3 can be probed by forming tunnel barriers and measuring their resistance as a function of magnetic field (H) and temperature (T). This is possible because the spins within each individual layer are ferromagnet cally aligned and the tunneling magnetoresistance depends on the relative orientation of the magnetization in adjacent layers. The situation is different for systems that are antiferromagnetic within the layers in which case it is unclear whether magnetoresistance measurements can provide information about the magnetic state. Here, we address this issue by investigating tunnel transport through atomically thin crystals of MnPS3, a van der Waals semiconductor that in the bulk exhibits easy-axis antiferromagnetic order within the layers. For thick multilayers below T similar to 78 K, a T-dependent magnetoresistance sets in at mu H-0 similar to 5 T and is found to track the boundary between the antiferromagnetic and the spin-flop phases known from bulk measurements. We show that the magnetoresistance persists as thickness is reduced with nearly unchanged characteristic temperature and magnetic field scales, albeit with a different dependence on H, indicating the persistence of magnetism in the ultimate limit of individual monolayers.

Temperature and field dependence of magnetic domains in La1.2Sr1.8Mn2O7

Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic magnetic force microscopy, domain structures for the layered manganite La1.2Sr1.8Mn2O7 as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature T-C, and appear either via the application of a c-axis magnetic field, or by decreasing the temperature further. At temperatures well below T-C, new domain walls, stable at zero field, can be formed by the application of a c-axis field. Magnetic structures are seen also at temperatures above T-C : these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.

Amer Physical Soc2015

Study of novel electronic conductors

Neven Barisic

This thesis presents the results of a concerted effort to understand the complex array of physical properties exhibited by the BaVS3 family of materials. As a 3d1 system, BaVS3 displays a unique collection of correlation-driven phenomena, including a metal-insulator transition driven by spin-density waves and or charge-density waves as well as a pressure-dependent crossover between the non-Fermi-liquid and Fermi-liquid behaviors. In an attempt to better understand these and many other properties, we have undertaken a systematic experimental study of BaVS3 and related compounds. The primary measurements carried out were those of the transport properties, resistivity and thermoelectric power (TEP). Through the construction of specialized measuring apparatuses, we were able to measure simultaneously these transport properties under conditions of variable temperature (from 2 to 600 K), pressure (up to 3 GPa) and magnetic field (up to 12 T). At ambient pressure and in the range of 250 to 600K, BaVS3 shows nearly isotropic but poor metallic behavior with linear temperature dependences of resistivity and TEP and a Curie-like magnetic susceptibility. The nearly isotropic conductivity contrasts with the 1D 2kF fluctuations observed by lowering the temperature below 250 K (at which the first Jahn-Teller structural phase transition occurs) deep in the metallic phase. The fluctuations reveal the 1D aspects of the electronic character, originating from the chain like structure of the material. The salient feature of BaVS3 at ambient pressure is the second order metal-to-insulator (MI) transition at TMI = 69 K, accompanied by the tetramerization (doubling of the 2V unit cell in the chain direction). In addition to the transport measurements, the strong changes in the electrical and magnetic properties of the system around TMI were followed by magnetic susceptibility, angle-resolved photoelectron spectroscopy and frequency-dependent conductivity. By increasing the pressure, the three-dimensionality of BaVS3 is enhanced and the MI phase transition is suppressed to lower temperatures. TEP and magneto TEP measurements in this pressure range revealed the existence of polarons and of spin fluctuations in the metallic phase. The latter can be identified as precursors to the MI transition. Around 1.8 GPa, where TMI ~15 K, the system enters a strongly fluctuating regime, highly sensitive to the magnetic field, the amplitude and the frequency of the measuring current and to a further increase of the pressure. Closely related to these features, the phase boundary collapses and a hysteretic behavior appears in the transport properties and their magnetic-field-dependent counterparts. At a critical pressure of ~2 GPa, a non-Fermi liquid (NFL) state arises (with n~1.5 in the Tn resistivity law between 1 and 15 K) in relation to the proximate Quantum Critical Point. The p-H-T phase diagram of BaVS3 in this region has been explored in some detail with particular emphasis placed upon the relevance of the spin degrees of freedom (on the insulator side) and the role of quantum fluctuations above the critical pressure. Finally, as the pressure is increased further, the conventional Fermi-liquid exponent of n = 2 is obtained. In order to delve further into the subtleties of the MI transition and the NFL behavior, a comparative study was carried out on the sister compounds Bax-1SrxVS3 and BaVS3 and on sulfur-deficient BaVS3. From this study it became apparent that the imposed chemical substitution manifests itself as an additional effective pressure. The inherent disorder of these samples was observed to affect the NFL behavior by reducing the exponent n towards a value of 1, while the apparent ferromagnetic order below 15 K was seemingly independent of pressure. All the observed features are consistent with a relatively simple two-band tight-binding, vanadium-based, model consisting of a wide quasi one-dimensional band hybridized with a rather isotropic narrow band. Within this context, it is concluded that the MI transition is controlled by the Coulomb-interacting electrons of the wide quasi one-dimensional band. Based on this model a new description of the NFL regime has been proposed.

EPFL2005

Spin dependent thermoelectric effects in magnetic nanostructures

Santiago Serrano Guisan

The aim of this thesis work was to obtain a deeper understanding of spin-dependent transport by the study of mixed effects of heat and charge currents in magnetic nanostructures. Indeed, even thermodynamics establish a relationship between both currents, no previous works were performed in this sense up to now. In order to do this, an original experimental setup was developed which allows to carry out a novel transport measurement called the thermogalvanic voltage (TGV). It measures the AC voltage response of the sample driven by an oscillatory variation of the temperature induced by a chopped laser beam focused on the nanostructure under a direct current flow. The main advantage of this new experimental setup is that it can be used to perform three different transport measurements: resistance, thermoelectrical power and TGV measurements. Furthermore, the well-defined junctions of our magnetic nanowires, in addition to the lock-in detection of signals, allows to obtain a better quality of thermoelectrical measurements than those observed by other groups. Different kind of magnetic nanowires like ferromagnetic/non-magnetic multilayers and ferromagnetic homogeneous nanowires were studied by these three different transport measurements. All these nanowires were 6μm in length and 50nm in diameter. All TGV measurements reveal a linear dependence with currents. This behaviour can be interpreted in terms of adiabatic conductivity for homogeneous nanowires and in terms of Peltier effect for multilayer nanowires produced in ferromagnetic/non-magnetic interfaces. Furthermore, the magnetic field dependence of TGV (MTGV) cannot be accounted for by GMR and MTEP effects. Therefore, a novel spin-dependent mechanism is invoked in such measurements. A simple phenomenological model, ascribed in the framework of thermodynamics of irreversible processes, was developed to explain this effect. It allows to identify this phenomena to a novel spin-dependent variable: The asymmetry of spin-mixing mechanisms (ΔL). This variable will be related to the difference of transition rates between both spin channels (τ-1+- and τ-1-+) developed in ferromagnetic material due to electron-magnon interactions. A large magnetic field dependence of TGV signals were observed in magnetic granular systems. Moreover, MTGV shows a different dependence from GMR on magnetic field, grain size and temperature. This effect is identified as a spin-mixing mechanism produced by the electron spin precession about the magnetic grain moments.