Ho(1-x)ErxNi2B2C yapısında gözlemlenen burgaç oluşumu
Vortex formation in Ho(1-x)ErxNi2B2C
Yazar: SULTAN SÜLEYMAN GÜNDOĞDU
Danışman: PROF. DR. MEHMET KERİM RAMAZANOĞLU
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Fizik Mühendisliği Ana Bilim Dalı / Fizik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: Fizik ve Fizik Mühendisliği = Physics and Physics Engineering
Dizin: Antiferromanyetizma = Antiferromagnetism ; Araştırma reaktörleri = Research reactors ; Bor karbür = Boron carbide ; Elektron fonon etkileşmesi = Electron phonon interaction ; Faz geçişleri = Phase transitions ; Ferromanyetik = Ferromagnetic ; Nötron = Neutron ; Süper iletken bileşikler = Superconductor compounds ; Süper iletkenlik = Superconductivity ; İnelastik saçılma = Inelastic scattering

Özet:
Nadir toprak elementlerinin farklı oranlardaki katkılanmaları ile Ho(1-x)ErxNi2B2C (x = 0, 0.25, 0.50, 0.75, 1) tek kristal numunelerdeki manyetik düzen gerek manyetizasyon deneyleriyle gerekse nötron difraksiyon deneyleriyle incelenmiştir. Kristal yapıda olan numunelerimizden nötron deneyleri sırasında güçlü sinyaller elde edebilmek adına elimizdeki birden çok tek kristalin birlikte yönlendirilmesi, Laue X-ışını ölçümleriyle, Kanada'da (Hamilton, Ontario) McMaster Üniversitesi bünyesinde bulunan Brockhouse Institute for Materials Research (BIMR)'de yapılmıştır. Manyetizasyon ve manyetik duygunluk ölçümleri de yine aynı enstitünün PPMS (Physical Properties Measurement Systems), yani Fiziksel Parametreler Ölçüm Düzeneği, manyetik ve Küçük Açı Nötron Saçılması KANS (Small Angle Neutron Scattering, SANS) deneyleri ise Washington DC, ABD'de kurulu bulunan National Institute of Standards and Technology (NIST) enstitüsünün nötron kısmı olan NIST Center for Neutron Research (NCNR) laboratuvarında sırasıyla BT-9, NG-7, NG-5 ve BT-7 deney mahalleri (beam-line) kullanılarak gerçekleştirilmiştir. 
Geçiş sıcaklığı, R2CuO4 süperiletken bileşiğinde, R'nin Er ve Ho olduğu durumlar için yaklaşık 10 K'dir. Ne var ki, bu malzemeleri asıl ilginç kılan husus, bunların, içlerinde tam da bu sıcaklıklar civarında bir manyetik düzen oluşturmalarıdır. Nadir toprak kısmın yapısına bağlı olmakla birlikte süperiletkenlik ile manyetizma arasındaki bağlaşma, yeniden girilen süperiletkenliğin oranlı ve oransız antiferromanyetizma ile eşzamanlı olarak varolmasından tutun da zayıf bir ferromanyetik düzen ile tamamıyla oransız antiferromanyetik bir spin modülasyonunun birlikte varolmasına kadar çeşitli fazların oluşmasına sebebiyet verir. Tüm bu fazlar süperiletkenlik ile eşzamanlı olarak varolurlar. "Saf" bileşiklerdeki manyetik düzen RKKY manyetik etkileşmesi ile açıklanmış olup katkılı numunelerdeki manyetik yapıyla saf numunelerin manyetik yapıları üzerinde yapılan nötron saçılması deneylerinin sonuçları da karşılaştırılmıştır. Saf Ho yapısında Er katkısının artmasıyla 1. Derece düzenli fazdan 3D XY düzen değerlerine doğru bir değişim gözlemlenmiştir. Özellikle Er katkısının oranı 0.75 olduğunda manyetik pik diğer numunelerden daha farklı bir yansıma oluşturmuş ve bu pik, daha önce başka bir araştırmada R2CuO4 (R = Nd ve Pr) kuantum mıknatısında gözlemlenmiş olan manyetik tepe profiline benzetilmiştir.

Abstract:
After the excitement of finding FeAs-based metallic room temperature superconductivity in the past ~ 10 years or so, we once more turned our attention (and our hopes) toward the study of magnetic properties of some other magnetically interesting SCs. Even though these properties were studied in many known previous structures, there are still unstudied subjects or partially untouched compounds. We are currently looking for an opportunity to fill one of these gaps with (Ho and Er)Ni2B2C s and their internally doped ones. In order to study these findings in detail, however, the magnetization studies were carried out prior to neutron experiments.
Rare earth (R) nickel borocarbides RNi2B2C have managed to stay a current research topic for more than 25 years.  The reason for this longtime attraction is related to their magnetic low temperature states which occur in the vicinity of temperatures with superconductivity. Different magnetic ground states are available depending on the magnetic rare earth ion and out of these states, the most interesting one is the ferromagnetic one seen with R=Er case. Realization of a ferromagnetic ground state which orders within a superconducting phase is a rare physical event in solid state. This special condition carries an opportunity to study the spontaneous vortex lattice (VL) formation in ErNi2B2C. Spontaneous VL formation has been studied in RNi2B2C SC family with R=Er in single crystal samples. However, in almost every study, not only the directional effect but also different magnetic field strength effects were not applied. On the other hand, in HoNi2B2C VL scattering was sought by many groups and researchers with the SANS technique. However, for this SC, a VL pattern, in other words a well-formed LRO of Abrikosov lattice of vortex quanta was not observed.
Therefore, in order to study this major difference between R=Er, and Ho nickel borocarbides, we grew single crystals of Ho doped ErNi2B2C (or vice versa) single crystal samples. Thus, RKKY interactions were studied in a way of tuning them between these two rare earth elements starting with pure Er and doping it with Ho and ending with pure Ho. It is aimed to get the vicinity of a possible quantum critical doping level where we lose the VL intensity in the Ho(1-x)ErxNi2B2C  samples.
In magnetization studies it has been found that as Er amount increases in the samples, the Tc obtained under H decreases to the vicinity of T~ 5 K where reentrance is completed and the FC and ZFC values are different. This difference is more pronounced for the doped samples. The difference of FC and ZFC can be explained with the idea of having some trapped fields in the samples, this makes it even more interesting since the same difference is not so large for the pure samples. For the pure samples, the Meissner effect is almost completed in both cooling ways. Therefore, they show superconductivity. The AFM reentrance behavior for pure Ho was also observed.
The single crystal quality was studied by Laue technique. The tetragonal base of the crystals was determined. This guaranteed that our single crystal samples were free from any twinning and other imperfections. This was at least true for several 100s of micro-meter thickness of the surface. We had aligned all of our crystals and they had been made ready to be investigated by neutron studies. 
On our first neutron experiments, we focused on the pure ErNi2B2C (x=1) and doped x=0.25, 0.5 single crystals at NIST in the summer of 2019. In order to minimize the neutron absorption issue, all of our samples were grown by using 11B isotope.   We started our experiments using an Al plate to hold 6 co-aligned pure crystals. However, the Al plate was accidentally bent during the sample insertion into the magnet, so we changed it to a Si wafer holder. With GRASP analysis program, we observed that the peak positions of different temperature data are all in the vicinity of q~0.0051 Å-1 which is the calculated q position for the applied magnetic field strength of H=1180 G (0.118 T). Due to several instrumental control issues, we needed to measure the current which is fed to the superconductor magnet. This way we had found ourselves using a specific current of H=1180 G which we could not change at all. For these measurements between T=7 K down to 2.7 K, we collected a VL scattering signal with a square-like pattern. Some spots emerged on the corners of the VL which are elongated blobs with weak intensities for this range temperature, while for the low temperatures the spots become sharper. However, because of the change in the background scattering below T<2.7 K specifically for T=1.9 K, and T=2.3 K data, we could not analyze our VL with a simple T=15 K background subtraction as we did for the high temperature data which worked just fine. The I vs q profile of the observed VL pattern for T = 7K to 1.9 K has been established. Then, we observed a peak center (q0) change where q0 increases as we lower the temperature towards the weak ferromagnetic (FM) phase.  This is evidence of spontaneous VL formation due to the effect of the FM phase which sets in at T<~2.3 K. However, because of the background change observed at T< 2.5 K data, it was hard to analyze the lowest temperature data and make this figure clearer and pronounced.  
In order to collect clearer and more prominent data, we plan to have a bigger and pure ErNi2B2C single crystal. Our sample grower, Dr. Cho, prepared the sample for us and we managed to align it. With this new sample, which provided more volume, we had seen an increase in the statistics of the data. That way the increase in q0 value of the VL indicating the effect of FM phase, as if an extra H field has been applied, had been clearly seen. 
In our first experiment, we managed to work with our doped samples of x=0.5 and x=0.25 samples as well. The VL observed for the half-half sample was very weak and it emerged as a very weak intensity at the calculated q position.  The other sample; x=0.25 did not show any VL signal. This is not so surprising since by the amount of Ho ion increases in the compound, the overall behavior would end up mimicking the pure HoNi2B2C.
To conclude all that, we are already granted a 5-day experiment at ILL (Switzerland) at which we will start our investigations with the pure ErNi2B2C single crystal sample. We plan to dedicate 2 days to collect the q0 values with different temperature under B field. With this new data, we also plan to understand the reason of the square VL seen in the first run.  The observed square looking pattern might be a result of superposition of hexagonal vortices slightly misaligned to each other becoming a square lattice. 
We also plan to study the x=0.75 doped crystal –which is in fact to say Ho0.25Er0.75Ni2B2C sample- for H=1180 G and a slightly lower H value.  The difficulties we faced in the previous run taught us to use the beamtime even more wisely. Having a properly working magnetic controller would provide us a chance to collect decent H=0 background scans which we can use for the lower FM phase subtraction. We believe that with a better signal for background statistics that PSI will provide and a properly working magnet, we would be able to collect the necessary data to conclude our findings.