In this dissertation, I will delve into two themes pertinent to my research, focusing on bismuth ferrite and iron rhodium, respectively. Bismuth ferrite (BiFeO3, BFO) remains the only known multiferroic material exhibiting ferroelectricity, antiferromagnetism, and a spin-canted moment at room temperature. It serves as a versatile platform for both fundamental study and potential applications, given the interplay of diverse parameters. In this theme, I will discuss three prevalent challenges commonly found in MBE-grown BiFeO3 thin films (leakage, fatigue, imbalanced magnetic and electric energies) and how we solve the issues by material engineering (ion irradiation, stoichiometry modification, lanthanum doping). During the study, we have developed nonlinear optical second harmonic generation (SHG) and electrical PUND measurement to probe the magnetic moment and electrical polarization. These approaches allow us to gain better understanding of the magnetoelectric coupling in BiFeO3. We also conduct spin transport measurements in BiFeO3. The spin Hall magnetoresistance shows unconventional angle dependence in the in-plane magnetic field scan, which could be related to the cycloidal spin texture in the material. The second theme of this dissertation is about my research on iron-rhodium (FeRh) thin films. Fe1-xRhx alloy undergoes structural and magnetic phase transitions as the rhodium fraction x increases. We conduct stoichiometry study in MBE-grown Fe1-xRhx thin films. Using vibrating sample magnetometer (VSM), we identify a critical value at x = 0.48, where a room-temperature bistability between the ferromagnetic phase and antiferromagnetic phase emerges. Leveraging this bistability, we achieve room-temperature rewritable magnetic etch-a-sketch patterning. This is made possible by controlling phase transitions through localized heating induced by a laser.