Frustrated magnetism and caloric effects in Mn-based antiperovskite Nitrides : Ab Initio theory

We model changes of magnetic ordering in Mn-antiperovskite nitrides driven by biaxial lattice strain at zero and at finite temperature. We employ a non-collinear spin-polarised density functional theory to compare the response of the geometrically frustrated exchange interactions to a tetragonal symmetry breaking (the so called piezomagnetic effect) across a range of Mn3AN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) at zero temperature. Building on the robustness of the effect we focus on Mn3GaN and extend our study to finite temperature using the disordered local moment (DLM) first-principles electronic structure theory to model the interplay between the ordering of Mn magnetic moments and itinerant electron states. We discover a rich temperature-strain magnetic phase diagram with two previously unreported phases stabilised by strains larger than 0.75\% and with transition temperatures strongly dependent on strain. We propose an elastocaloric cooling cycle crossing two of the available phase transitions to achieve simultaneously a large isothermal entropy change (due to the first order transition) and a large adiabatic temperature change (due to the second order transition)

Ab-initio theory of magnetic ordering : electronic origin of pair- and multi- spin interactions

We present an ab initio theory to describe magnetic ordering and magnetic phase transitions at finite temperatures from pairwise and multi-spin interactions. Our formalism is designed to model thermal fluctuations of disordered local moments associated with atomic sites and adequately describes how these emerge from the glue of many interacting electrons. The key ingredient is to assume a time-scale separation between the evolution of the local moment orientations and a rapidly responsive electronic background setting them. This is the Disordered Local Moment picture grounding the framework of our theory. The method uses Density Functional Theory calculations constrained to specific local moment configurations to model the electronic structure and exploits Green's functions within a Multiple Scattering Theory to solve the Kohn-Sham equations. Two central objects are calculated as functions of magnetic ordering: internal magnetic fields sustaining the local moments and the lattice Fourier transform of the interactions in the paramagnetic state. We develop a methodology to extract the pairwise and multi-spin constants from the first and use the second to study the magnetic interactions in the reciprocal space and gain information of the type and extent of most stable magnetic order. These quantities are directly related to the first and second derivatives of the free energy of a magnetic material, respectively. Hence, our approach is able to provide thermodynamic quantities of interest, such as temperature and entropy changes for the evaluation of caloric effects, and magnetic phase diagrams for temperature, magnetic field, and lattice spacing studies can be constructed. Transition temperatures and their order, as well as tricritical points, are obtainable. We apply the theory to carry out major investigations on long-period magnetic phases in the heavy rare earth elements (HREs) and magnetic frustration in the Mn-based antiperovskite nitride Mn₃GaN. The mixing of both pairwise and four-site magnetic interactions have been found to have profound consequences on the magnetism of both systems. We have obtained a generic HRE magnetic phase diagram which is consequent on the response of the common valence electronic structure to the f-electron magnetic moment ordering. We also present a modelling based on the lanthanide contraction to describe ferromagnetic, helical antiferromagnetic, and fan phases in Gd, Tb, Dy, and Ho, in excellent agreement with experiment. Our study of Mn₃GaN shows that its first-order paramagnetic-antiferromagnetic triangular transition originates from the fourth order terms and that the effect of biaxial strain to distort the compensated antiferromagnetic interactions has a large impact on the frustrated magnetism. As a consequence, new collinear magnetic phases stable at high temperatures are predicted and a very rich temperature-strain phase diagram is obtained. We also show how to get the best refrigerating performance and design a novel elastocaloric cooling cycle from the features of the diagram

Studies of spin-orbit coupling phenomena in magnetic semiconductors

Hard disk drives (HDDs) have been the dominant secondary memory device in computing for over 50 years, while more recently magnetoresistive random access memory (MRAM) has emerged as a candidate for primary computing memory. Both HDDs and MRAM store information in the polarity of a magnetic layer, which is written and read by non relativistic mechanisms. There is now gathering interesting in using relativistic mechanisms whose origins lie with spin-orbit coupling (SOC) for MRAM writing because of potential benefits in terms of scalability, device design, and efficiency. This thesis investigates the fundamental physics of SOC phenomena that can write (spin-orbit torque (SOT), Neel order SOT) or read (anisotropic magnetoresistance (AMR), magnetic gating) the magnetic state by the application of electrical current. These phenomena are studied in ferromagnetic and antiferromagnetic semiconducting materials that offer a relevant electrical conductivity for integration into commercial electronic devices. Effective magnetic fields which parametrise the SOT phenomenon are measured in the diluted magnetic semiconductor (Ga,Mn)As using a technique based upon experimental planar Hall effect measurements and analytical fitting with a free energy equation for coherent magnetization rotation. It is found that effective magnetic fields which originate from Dresselhaus SOC increase in magnitude with increasing temperature, whereas those originating from Rashba SO have no significant temperature dependence within experimental uncertainty. The size of the measured effective fields per unit of current density, as well as the ratio of Dresselhaus to Rashba effective field magnitudes averaged over all temperatures are comparable to previous experimental measurements. Sb-based diluted magnetic semiconductors (Ga,Mn (As0.9,Sb0.1) and (Ga,Mn)Sb are characterised by magnetic and transport measurements. The Curie temperature (Tc) of (Ga,Mn)(As0.9,Sb0.1) increases from 28K to 55K upon sample annealing. The Tc of as-grown (Ga,Mn)Sb is found to be 34K, and in contrast to (Ga,Mn)(As0.9,Sb0.1) does not change upon annealing, indicating a lack of interstitial Mn in (Ga,Mn)Sb. Field rotation transport measurements for current along various crystalline directions reveal significant crystalline and non crystalline contributions to the AMR of both as-grown and annealed (Ga,Mn)(As0.9,Sb0.1). An anomalous temperature dependence of the AMR of the annealed (Ga,Mn (As0.9,Sb0.1) sample for current along the [110] crystalline direction is accounted for by considering the relative sizes of the individual AMR contributions as a function of temperature. Results are shown of an attempt to vary the current flow through a non-magnetic GaAs/AlGaAs 2D electron gas (2DEG) by changing the magnetization orientation of an electrically insulated Fe gate layer. Such magnetic gating of electrical current is based upon the principle that, as a result of SOC, the electrochemical potential of a ferromagnet is anisotropic with respect to its magnetization orientation. The magnetic gating experiment proved to be unsuccessful due to an AMR-like signal arising in field rotation measurements of 2DEG samples both with and without the gate layer. The origins of this AMR-like signal are unknown, and it cannot not be accounted for by fitting analysis

Spartan Daily, December 10, 1963

Volume 51, Issue 54https://scholarworks.sjsu.edu/spartandaily/4396/thumbnail.jp

Zapisywanie miejsca : szkice o Śląsku w literaturze przełomu wieków XX i XXI

The book comprises drafts devoted to selected literary works in which Silesia is „saved” in different ways. Among others, interpretations of the following books are presented: Urodzony w Święto Zmarłych by Feliks Netz, Finis Silesiae by Henryk Waniek, Piąta strona świata by Kazimierz Kutz, Mariacka by Jacek Durski, Przygoda na Tylnej Mariackiej by Witold Turant. The author makes an attempt to answer the question on how the issues typical of this area were reflected in works belonging to the trend of „small homelands”, regional literature, a circle of popular publications (retro detective stores), and in esseistics (Nagrobek ciotki Cili by Stefan Szymutko, Lajerman by Aleksander Nawarecki). The subject of interest constitutes both popular books and those less-known though worth attention for various reasons. Interpretations were placed in broader contexts of changes in Polish literature and the development of studies on the role of space in the culture (a „topographic turning point”). Starting from reading selected works, from details to concretes (also geographical ones), the author aims at outlining a more general and universal problem connected with a relationship between the literature and the place, man and space experienced

Pochwa\u142a malarstwa: studia z historii i teorii sztuki

S\u142owa kluczowe: malarstwo w Polsce i w Europie XIX-XX w., wizerunek artysty i artystki w polskiej literaturze i sztuce XIX i XX w., teoria sztuki XIX-XX w., j\u119zyk historii sztuki, j\u119zyk polityk