My name is Gonzalo Pérez Blasco and I am a research student at the Aragón Nanoscience and Materials Institute (INMA) which is a joint research center between the Spanish National Research Council (CSIC) and the University of Zaragoza under the supervision of Prof Javier Campo. I carried out a research stay at CROSS from July 1st to August 28th under the supervision of Dr. Kazuki Ohishi.
The main objective of the stay was to translate a program written by Dr. Miguel Pardo Sainz for the analysis of μSR experimental data from MATLAB to GNU Octave and to use said code to analyze the remaining set of measurements. This code was designed only for MnSi so another task was to generalize the program so that it could also analyze measurements for Fe0.75Co0.25Si. The interest in studying these two materials is that a new phase (called the B-phase) was discovered at low temperatures and high external magnetic fields using AC susceptibility measurements. However, SANS measurements performed at the instruments D33 (Institut Laue-Langevin, France) and TAIKAN (Materials and Life Science Experimental Facility, Japan) observed magnetic Bragg reflections due to the conical phase; no additional reflections attributable to a possible new magnetic state were observed in the B-phase. To solve this issue, μSR experiments were performed at M15 and M20 (TRIUMF in Canada) to analyze the local magnetic fields of the B-phase.
The first objective was to translate the current version of the code into Octave and to write the remaining compatibility functions to use it for automated fitting. This task included automatizing the calculation of the local magnetic field at the muon implantation sites with the updated parameters produced during the non-linear fitting procedure. Once the program was running, the next step was to understand the underlying physics assumptions performed in order to verify that calculations were physically meaningful. Finally, the final task was to use the program to try to replicate the already analyzed sets of data to verify that both programs performed the same calculations.
The project proposed was quite ambitious as it was my first time working with μSR data and magnetic materials. Dr. Kazuki Ohishi and Dr. Kenji Kojima monitored the project and patiently explained the theory behind positive muon science for condensed matter research while Dr. Javier Campo helped me understand the physics behind magnetic materials. Thanks to their help I was able to give my first steps on both fields and to understand what physical constraints are meaningful when analyzing real data.
In addition, I took part on the 1st New Horizons in Negative Muon Science Area Meeting & JMMS Meeting from August 3rd to August 7th. This congress was an incredible experience to meet muon researchers and other students studying in the same field. Furthermore, the talks given at that seminar helped me further to understand muon science and the large variety of topics where these techniques can be used. On August 6th I also presented my work with a digital poster and got incredible feedback on the topic that I used during the rest of my stay at CROSS.
Finally, I presented on July 29th a seminar titled "Empirical machine learning for 3He spin-filter polarization decay" in which I explained the project performed at the Institut Laue-Langevin during the summer of 2025 for predictions on MEOP 3He spin-filters for polarized neutrons. Most polarizer cells used at J-PARC use the SEOP technique but the seminar showcased how Machine Learning can help with automated pipelines for data treatment.
My stay at CROSS was an incredible experience and I am extremely grateful to Dr. Kazuki Ohishi for his constant support and guidance. I am looking forward to continue researching magnetic materials with him whenever possible.
