Investigation of temperature dependence of magnetic properties of Cr 2 O 3 thin film structure using a magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal Andris Berzins 1 , * Janis Smits 1,2 , Andrejs Petruhins 3 , Roberts Rimsa 4 , Martins Zubkins 5 , and G. Mozolevskis 4 1 Laser Centre, University of Latvia, Latvia 2 The University of New Mexico, Albuquerque, United States 3 Materials design group, Thin film physics division, Department of Physics, Chemistry and Biology (IFM), Linkoping University, Sweden 4 Laboratory of Prototyping of Electronic and Photonic Devices, Institute of Solid State physics, University of Latvia, Latvia and 5 Thin films laboratory, Institute of Solid State physics, University of Latvia, Latvia (Dated: May 10, 2021) This work presents a magnetic field imaging method based on color centres in diamond crystal applied to thin film structure. To demonstrate the capacity of our device we have used it for characterization of magnetic properties in microscopic scale of Cr2O3 thin film structure above and below NÃľel temperature. The obtained measurement results clearly identify the detection of the magnetic phase transition of Cr2O3 thin film with an unexpected diamagnetic like behaviour at 19 ◦ C (below the NÃľel temperature of Cr2O3). To have better insights in the magnetic fields created by the thin films we present simulations of the magnetic fields near the thin film surface. We also analysed the optically detected magnetic resonance (ODMR) profiles to be sure that the measured property is related only to the shift of the optically detected magnetic resonance. We demonstrate how Nitrogen-Vacancy centre based magnetometry can deliver a convenient platform for research of phase transition dynamics and spatial magnetic field distributions of thin films, that can rival other widely used measurement techniques. PACS numbers: I. INTRODUCTION The Nitrogen-Vacancy (NV) centres in diamond crys- tal can be used for a variety of applications, from magne- tometry [1], thermometry [2] and measurements of elec- tric field [3, 4] to strain analysis [5]. A separate sub- category of these measurements is acquisition of two- dimensional distribution of the observed effects. Mag- netic field imaging using NV centres in diamonds has a number of advantages compared to other widely used methods: possibility to measure relatively wide area of a sample simultaneously, while maintaining diffraction limited spatial resolution, diamonds can be brought into close proximity to samples, as the diamond matrix is chemically and mechanically durable, as well as non- toxic. In addition, measurements can be made over a temperature range from cryogenic to hundreds of degrees Celsius [6–8]. The high spatial resolution and wide range of operating temperatures make the NV center an excel- lent platform for analyzing properties of magnetic thin films [6, 9]. The studies of magnetic phase transitions in thin films are a widely researched effect [10–12] however in many cases vibrating-sample magnetometry is used for deter- mination of magnetic properties. This limits the mag- netic property measurements to the average value of the * Electronic address: andris.berzins@lu.lv whole sample. On the contrary magnetic field imaging based on NV centres can reveal the microstructure of magnetic interactions, while maintaining the possibility of relatively large field of view and high throughput. The other commonly used method for sensitive mag- netic field measurements is the scanning-tip measure- ments performed by superconducting quantum interfer- ence devices (SQUID) and single NV probes. These tech- niques can produce hard-to-rival magnetic measurements in the means of spatial resolution, but the main issue is a very limited field of view. NV imaging can offer sim- plicity, compactness and much higher throughput (due to simultaneous probing of relatively wide field of view). Overall the NV based magnetic field imaging can deliver a convenient platform for research of phase transition dy- namics and spatial magnetic distributions. In this work we apply the magnetic field imaging tech- nique to measure the changes in magnetic properties at phase transitions of Cr 2 O 3 thin film structures. Cr 2 O 3 is used in a number of thin film and nonstructural ap- plications: changing magnetic properties of multilayer thin film systems [13–16], enhanced magnetoresistance properties [16–18], magnetoelectric random access mem- ory [19], spin-current research [20, 21] and even exotic spin superfluid research [22]. This material is favorable for a number of properties: relatively high magnetic sus- ceptibility +1960 · 10 -6 cm 3 /mol, easy to reach NÃľel temperature at 34 ◦ C and with it transition from anti- ferromagnetic to paramagnetic properties. Cr 2 O 3 also has good adhesive properties that simplifies the manu- facturing process as many other thin film materials re- arXiv:2008.04371v1 [physics.app-ph] 10 Aug 2020