Mjerenje toplinskog kapaciteta u sustavima s kompleksnim faznim dijagramom s magnetskim poljem duž osi teške magnetizacije

master's thesis

• Title (english): Measurements of heat capacity of systems with complex magnetic phase diagrams for magnetic field along hard magnetisation axes
• Author: Nives Bonačić
• Mentor: Christian Pfleiderer (mentor)
• Mentor: Mihael Grbić (mentor)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2016-06-24, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics

Abstract

Od otkrića topološki netrivijalnih uređenja, npr. faze rešetke skirmiona u MnSi-u 2009. godine [1], izrazito je povećan intenzitet istraživanja u ovom području. Ključni eksperimentalni potpis su magnetske strukture u više k smjerova, ispitivane raspršenjem neutrona pod malim kutom, dok se za k = 0 promatraju masivna magnetska svojstva poput topološkog Hallovog efekta koji proizlazi zbog pojave baždarnog polja, teorijski nađene manifestacije Berryjeve zakrivljenosti pobuđenja. Mjerenjem topoloških Hall signala identificirani su novi magnetski sustavi i skupine spojeva u kojima bi mogla nastati netrivijalna spinska stanja. Kao jedan kandidat je u našoj grupi izdvojena serija RE-Cu intermetalnih spojeva s antiferomagnetskim uređenjem u više k smjerova i vrlo velikom topološkom Hallovom otpornošću, dva reda veličine većom nego u A-fazi MnSi-a. Magnetska anizotropija je ključni sastojak energijske hijerarhije interakcija. Zbog javljanja snažnog zakretnog momenta u konačnom magnetskom polju i neprikladnosti sadašnjih mjernih postava i kalorimetrijskih tehnika, dosad nije postojala mogućnost dodatnog istraživanja mogućih potpisa spinskih tekstura masivnim mjerenjima npr. toplinskog kapaciteta. Stoga, uglavnom nema dostupnih informacija o pratećim promjenama entropije duž granica faza. U ovom radu su dizajnirani, konstruirani, karakterizirani i optimizirani sasvim novi postavi za mjerenje toplinskog kapaciteta koji prevladavaju navedeni problem osiguravaju ći mehaničku stabilnost usprkos zakretnih momenata koji djeluju na uzorak pri mjerenjima s jakim magnetskim poljem uzduž osi teške magnetizacije. Dva nova eksperimentalna postava su implementirana u komercijalno dostupan uređaj Physical Property Measurements System-a (PPMS) od Quantum Design-a, jedan koristeći vijcima stegnutu Al pločicu umjesto platforme za uzorak i drugi Kevlar postav potpuno nove strukture koja se može ugađati i podešavati za različite materijale. Detaljno su uspoređeni novi postavi s konvencionalnim kalorimetrom. Njima je određen specifični toplinski kapacitet ErCu duž tri glavne kristalografske osi u temperaturnom rasponu 2 K do 200 K i poljima do 14 T. Time su razjašnjeni bogati magnetski fazni dijagrami i utvrđen strukturni prijelaz. Ne-magnetski ekvivalent materijala LuCu je pripremljen i njegov specifični toplinski kapacitet izmjeren za utvrđivanje magnetskog doprinosa spojeva od interesa.

Abstract (english)

Since the discovery of topologically non-trivial forms of order, e.g. the skyrmion lattice phase in MnSi in 2009 [1], the intensity of research in this field increased tremendously. The key experimental signatures are multiplek structures as probed by small-angle neutron scattering, while in the k = 0 space, e.g. bulk magnetic properties, a large topological Hall effect arises due to an emergent gauge field as theoretically found to be a manifestation of the Berry curvature of the excitations involved. The latter route was followed to identify new magnetic systems or compound families, possibly hosting non-trivial spin states by measuring a topological Hall effect signal. As one class the rare-earth intermetallics series RE-Cu were recently identified in our group as having multiple-k antiferromagnetic orderings and very large topological Hall resistivity two orders of magnitude higher than in the A-phase of MnSi. One key element to the energetic hierarchy of interactions is the magnetic anisotropy. To further probe possible signatures of spin textures by other bulk measurements, e.g. heat capacity, no option so far is available since strong magnetic torques in finite magnetic fields along hard magnetisation axes forbid detailed investigations by present state-of-the-art calorimetric techniques. Therefore essentially no information is available on the underlying changes in entropy across the phase boundaries. To overcome this issue an entirely new design to measure heat capacity was invented, constructed, characterised and optimised during the course of this master thesis to ensure mechanical stability against torques acting on the sample when applying magnetic fields along the hard axes. Two setups were designed, one a screw tightened Al-plate as sample platform and second a Kevlar setup which has a completely new structure, permitting to tune and adjust the setup for different materials, both fitting to a commercially available sample puck provided by the Physical Property Measurements System (PPMS) from Quantum Design. We started with a detailed characterization of the new setups in comparison to the conventional heat pulse calorimeter technique and concentrated on the heat capacity of ErCu along the three principal crystallographic directions in the temperature range from 2 K to 200 K up to magnetic fields of 14 T. It enabled us to shed light to the rich magnetic phase diagram in identifying a structural transition. A non-magnetic equivalent of the material - LuCu - was prepared and its specific heat was used to find the magnetic contribution in the compounds of interest.

• Keywords: toplinski kapacitet; kompleksni fazni dijagrami
• Keywords (english): heat capacity; complex magnetic phase diagrams
• Language: croatian
• URN:NBN: urn:nbn:hr:217:879734
• Study programme: Title: Physics; specializations in: Research; Course: Research; Study programme type: university; Study level: integrated undergraduate and graduate; Academic / professional title: magistar/magistra fizike (magistar/magistra fizike)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access; Embargo expiration date: 2018-06-24
• Created on: 2017-08-23 12:54:16