We report measurements of optically detected ferromagnetic resonance (ODFMR) in thin films using nitrogen-vacancy (NV) centers in diamond, whose fluorescence intensity changes in response to the ferromagnetic resonance (FMR) excitation of a proximal ferromagnet. Here, we extend the study of the off-resonant and broadband detection of FMR, first observed in the magnetic insulator YIG to a diverse set of ferromagnetic materials. We measure ODFMR signals from several technologically relevant metallic ferromagnetic materials including Py, Co, Co${}_2$ (Mn${}_{0.6}$ Fe${}_{0.4}$ )Ge, and an insulating NiZnAl ferrite. These results show the generality of the spin-relaxation based coupling of the NV spins to the ferromagnetic dynamics that enables this detection. The observable field-frequency range of the ODFMR signal is dependent on material parameters such as saturation magnetization and damping. These results imply that NV-based ODFMR can give insights into nanoscale ferromagnetic dynamics and its damping processes, especially for samples with low magnetization and in the low field-frequency regime, which is important for several technologies.

The near-future ultra-high-density magnetic recording with an assist recording technology requires a substantial evolution of the read-head technology. A read sensor with a small sensor resistance-area product (RA), a large magnetoresistance ratio (Δ R/R), and a thin total sensor thickness is required for the low bit error rate and high-spatial resolution of reading. We developed current-perpendicular-to-plane giant magnetoresistance sensors using highly spin-polarized Heusler alloy ferromagnetic layers with the spin polarization of the conducting electron β ~ 0.8. A reasonably large Δ R/R up to 14% was obtained by a practical bottom-pinned spin-valve structure with a conventional metallic spacer layer of an Ag₉₀n₁₀ (AgSn) alloy. By replacing the spacer layer with a conductive oxide-based AgSn/InZnO bilayer, a large Δ R/R up to 32% was realized at RA value of ~ 0.1 Ω · μm², which is optimal for the read sensor application. A large voltage output up to 6 mV was obtained for a sensor utilization of ~ 33% at a bias voltage of 100 mV.

Magnetic field sensors using the tunnel magnetoresistance (TMR) effect require linear resistance–magnetic field ( R – H ) response curves with small hysteresis, for which the soft magnetic property of the free layer (FL) is critical. In this work, we investigated amorphous CoFeBTa (CFBT) as a soft magnetic layer of the FL of CoFeB/MgO/CoFeB-based magnetic tunnel junctions in view of magnetic, TMR, and low-frequency noise properties. A two-step annealing process enabled an orthogonal magnetization configuration between the FL and the reference layer, by which linear R – H curves with small hysteresis were realized. The change in the shape of the R – H curve depending on annealing temperature is explained by the Stoner–Wohlfarth model. The highest TMR ratio of ∼160% and sensitivity of ∼70%/mT were obtained with a CFBT (20 nm)/Ta (0.3 nm)/CoFeB (3 nm) FL. The noise of the TMR devices are dominated by 1/ f noise below ∼10 kHz in frequency, which limits the detectivity ( D ) of the magnetic field of the sensor. The sensor devices patterned to 50 µ m diameter circular shapes showed a minimum D of ∼2 nT/ Hz at 10 Hz, which is superior to the previously reported values for the TMR sensors with NiFe soft magnetic layers.