Development of ultrasensitive calorimeters: Studies of new superconducting pnictides and molecular magnets

Abstract

The measurement of low temperature heat capacities gives a great deal of information about the properties of a material, such as the density of states of electrons or phonons and about structural, electronic, or magnetic phase transitions. In recent years new phenomena have been evidenced in systems of reduced dimensionality. For instance, superconductivity in high critical temperature superconductors seems to be related to a kind of low dimensionality effects. However, the measurements with commercially available calorimeters have limited sensitivity because of the big size that gives a large background contribution. Systems such as superconducting materials or assemblies of magnetic molecules often have too small mass to be measured with conventional devices. Typically, in materials where low dimensionality effects are due to a certain crystalline arrangement, it is often very difficult to obtain a homogeneous structure over bulk sample. So in these cases there is great deal of interest in measuring samples as small as possible for probing their microscopic properties. My thesis work deals with two main activities: the development of ultrasensitive calorimeters and the physical measurements on the new family of superconducting materials Iron based pnictides and on the two new molecular magnet clusters. I first report the development of a series of calorimeters based on thin membrane of Si and Si3N4 by using MEMS technique, which have small addenda heat capacity and high sensitivity for studying small samples typically small single crystals with mass less than 1mg. Totally five different types of device were successfully fabricated. They all have extremely small addenda heat capacity with respect to that of commercial calorimeters, about 104 times smaller at room temperature. The development of the calorimeters implied the design of the device, choosing materials for each components, fabrication and characterization of the devices. The devices were installed into a commercial Quantum Design PPMS cryomagnetic system. Measurements based on thermal relaxation method were done automatically with our electronic set-up and controlled by Labview program developed by me. By using two different materials for the membrane, we obtained two classes of devices which exhibit different thermal conductance.

The first with rather high thermal conductance made based on Si membrane offer the possibility to measure proper samples in thermal relaxation method. The second class of devices based on thin Si3N4 membrane has a very small thermal conductance. They allow heat capacity measurements on small samples (typically less than 0.5mg) by thermal relaxation method. In this class, I have developed two designs of devices. One is made on 250 nm Si3N4 membrane. By using the same materials (Platinum) for all the electrical components of the device leads to a simple and effective fabrication process. A tested measurement on a known sample ((NaMn3)Mn4O12 cluster) shows a good agreement with the result obtained from PPMS calorimeter. This type of device offers possibility to carry out the measurement in a wide range of temperature (between 20K and to room temperature). The second type of device in this class also the final device that I developed is made based on a 2μm Si3N4 membrane. This type of device has a special design which offers possibilities of making both heat capacity and thermal conductivity measurements on small (sub millimeter) sample. The thermometer of this device is made from Nb1−xNx. Therefore, by choosing the right concentration of Nitrogen, we can have devices with different working temperature range (from liquid Helium up to room temperature). In 2008, a new family of high temperature superconducting materials was first reported by group of of Prof. Hideo Hosono in Japan, the Oxy - pnictides REFeAsO(F) (RE is rare earth). I made specific heat measurements on two elements of this family including SmFeAsO(F) and CeFeAsO(F) by using commercial PPMS calorimeter. The two materials show some common properties: A spin density wave transition at high temperature and an antiferromagnetic ordering transition at about 3 - 5K on the undoped samples, the spin density wave anomaly is reduced gradually by introducing Fluorine and it completely disappeared when the doping is optimal for a highest superconducting critical temperature. Among all the components of this superconducting family, SmFeAsO is an odd case, since the peak at low temperature (around 5K) is not sensitive to the applied magnetic field. In order to understand this strange behavior of SmFeAsO, we have measured specific heat under high magnetic field (up to 35T). The investigation of the evolution of specific heat on the applied magnetic field revealed the antiferromagnetic ordering nature of this peak combined with spin-reorientation due to the high anisotropy of the Sm3+ ions. Recently, Iron-Arsenide based superconducting materials are available in form of the single crystal. We have measured the specificheat of small BaFeCoAs single crystal and we were able to measure the superconducting anomaly by using our home - made devices. The measurement revealed a small bump related to superconducting transition of the sample at about 22K. Molecular magnetic materials are the subject of an active research in recent years because of their possible applications in future information processing and storing device. In 2007, the group of Prof. Euan K. Brechin introduced new molecular magnets, Ni12 and Mn4 molecular magnets. In my thesis work I contributed to characterize new molecular derivatives and to investigate their thermodynamic properties at low temperatures. The Ni12 clusters show single molecular magnet behavior of Ni12 with some evidences of weak AF intermolecular interaction. The thermodynamic studies onMn4 clusters shows an antiferrogmanetic long range ordering and 2D spin wave below TN ~ 0.72K.