Anomalous metamagnetic-like transition in a FeRh/Fe$_3$Pt interface occurring at T120 K in the field-cooled-cooling curves for low magnetic fields

We report on the magnetic properties of a special configuration of a FeRh thin film. An anomalous behavior on the magnetisation vs. temperature was observed when low magnetic fields are applied in the plane of a thin layer of FeRh deposited on ordered Fe$_3$Pt. The anomalous effect resembles a metamagnetic transition and occur only in the field-cooled-cooling magnetisation curve at temperatures near 120 K in samples without any heat treatment.

treatment conditions [1][2][3] . In this cubic phase, equiatomic FeRh is antiferromagnetic and exhibits a metamagnetic magnetostructural phase transition at ambient temperature. The transition temperature is however highly sensitive to small changes out of the equiatomic stoichiometry 4 and microstructural scale 5,6 . The magnetostructural phase transition is of first order kind with a volume expansion of about 1% when entering in the ferromagnetic phase and a temperature hysteresis of the order of 10 K. 1,[7][8][9] These characteristics make thin films of the FeRh system to present a good potential for applications as microelectromechanical devices 10 . The study which motivated us, performed in the binary thin film of FePt-FeRh presented in Ref.10, is, in particular, very interesting.
In this work, we study Fe 1−x Rh x in the composition x ≈ 0.5 as obtained by electrodeposition on a foil of ordered Fe 3 Pt. This work addresses the possibility of obtaining a thin film of FeRh in the antiferromagnetic phase by electrodeposition on a foil of a ferromagnetic compound, forming a self-sustained film. We chose the ferromagnetic systems to be ordered Fe 3 Pt, which is a well known and studied ferromagnet 11,12 , for which the preparation of a thin foil is quite simple 13 . The preparation and study of such a self-sustained film stack, searching for FeRh in the antiferromagnetic phase, was our main motivation. We report here the first results obtained in the film stack Fe 3 Pt-FeRh prior to any heat treatment. The system as prepared, presents an anomaly near 120 K in the temperature dependent magnetisation curve when cooled from 300 K, in very low applied magnetic fields. The anomaly appears for fields as low as 20 Oe and it is reproducible, resembling a metamagnetic transition.
The Fe 1−x Rh x with x ≈ 0.5 thin film was obtained by electrodeposition on a 0.6 cm 2 of a thick ordered Fe 3 Pt film. The electrodeposition occurred in galvanostatic conditions from a bath composed of: 0.01 Molar Fe 2 (SO 4 ) 3 , 0.001-0.0001 Molar Rh 2 (SO 4 ) 3 , 0.1 Molar K 2 SO 4 and 0.05 Molar Sodium citrate (Na 3 C 6 H 5 O 7 ). The final film thickness of FeRh falls in the range of 30-50 nm, depending on the applied current density, which was varied from 1 to 5 mAcm −2 , and the charge for preparing all deposits was kept to 10 C. 14-16 An EG-G PAR potentiostat-galvanostat, model 273, served as a constant current source. The electrodeposition occur under previously established conditions producing an approximately equiatomic deposition of Fe and Rh. 14 The Fe 3 Pt substrate with ≈ 3 µm thickness was obtained by cold-rolling a previously prepared arc-melted sample. The ordered phase of the Fe 3 Pt foil was achieved after an appropriated heat treatment as described in Ref. 13. A foil with 0.6x0.5 cm 2 (m=0.00271 g) of the final film was used in the measurements. It should be mentioned that Energy Dispersive X-ray Spectrometry performed on many Fe 1−x Rh x thin films grown by electrodeposition under same conditions as here did not show oxygen peaks. 14 Based on that, one may speculate that Fe 3 O 4 grow on the FeRh surface as a result of surface oxidation. Despite that, it was possible to identify small peaks in the low angle diffraction curve belonging to the FeRh cubic structure (CsCl type) with lattice parameter a = 2.98Å , as calculated from the FeRh peaks indicated in Fig. 1.
Magnetisation data were obtained by using a PPMS-9T Quantum-Design magnetometer.
All data were obtained with the magnetic field applied parallel to the plane of the films.
Isofield curves, Mvs.T , and isothermal curves, Mvs.H, were all obtained after cooling the studied sample from 300 K, in zero applied magnetic field (zfc), but in the presence of the earths magnetic field, to a desired temperature. After that, for the isofield Mvs.T curves, a magnetic field was applied reaching the desired value without overshooting. The zfc (zero-field-cooled) data was then collected by heating the sample at fixed increments of temperature up to 300 K, which was followed by the fcc (field-cooled-cooling) sequence with same δT increments down to 1.8 K. Finally, a fch (field-cooled-heating) process ends the sequence at 300 K. These procedures correspond to a cycle starting with a zfc (zero-fieldcooled) curve obtained by heating from 1.8 K up to 300 K followed by a fcc (field-cooledcooling) curve followed by a fch (field-cooled-heating) respectively. These data were obtained for fields ranging from 20 Oe to 10 kOe. For   hysteresis curve) at fixed increments.    (Fig. 3c), the anomalous effect is absent (it is suggestive that this high field overcomes the antiferromagnetic-ferromagnetic exchange coupling) and, interestingly, the zfc curve appears above the fcc and fch curves while the zfc curves in Fig. 3a and 3b appears below the fcc and fch curves. These differences are probably related to the alignment of ferromagnetic domains, which depends on the strength of the magnetic field. As already mentioned, it is interesting to note that, for temperatures above 120 K, the fcc curves in Figs. 3a and 3b lie well above the zfc and fch curves, though the fcc curves match almost perfectly the fch curves below 100 K. The later suggest the existence of some antiferromagnetic ordering (within the F eRh thin layer) occurring below 100 K. One may understand this fact as a coexistence of two phases below 100 K, an antiferromagnetic phase due to the FeRh thin layer and a ferromagnetic phase due to the bulk Fe 3 Pt, which is dominant.
To check for possible effects in the temperature region around 100 K, we plot in Fig. 4a isothermal We also mention that we search for time relaxation effects when in the fcc curve near the anomalous transition above 120 K, but no magnetic relaxation was observed.
Although the metamagnetic-like transition observed here may have a different nature for bulk FeRh and thin films, it is interesting to compare the size of the jump occurring at ≈ 100 K. From Fig. 3, the jump is estimated to be ≈ 5x10 3 emu/cm 3 for a 50 nm thickness while the size of the antiferromagnetic-ferromagnetic (AF-F) transition observed in FeRh in Ref.7 is of the order of 1.2x10 3 emu/cm 3 , but at a T ≈ 350 K. It should be mentioned that an AF-F transition was observed in a Fe 48 Rh 52 film 10 at T ≈ 100 K, also with a size of the order of 1.2x10 3 emu/cm 3 . So the "size" of the metamagnetic-like transition observed here for the FeRh-Fe 3 Pt film appears to be 4 times bigger than the AF-F transition observed for pure FeRh. This discrepancy might be also in part due to an under-estimative of our FeRh sample thickness. It should be mentioned that an increasing in the FeRh magnetisation above the antiferromagnetic transition has been observed in a FeRh/FePt thin film. 10 Figure 5 depicts two measurements obtained in the same sample at same conditions but with approximately 4 months delay between each other. A comparison of both curves clearly shows the aging effect which shifted the onset temperature of the anomaly to a lower temperature (≈ 20 degrees below), broadened its width and reduced the size of the apparent discontinuity (by ≈ 30%). We lack, at moment, on an explanation for this aging effect.
In conclusion we obtained a Fe 1−x Rh x -Fe 3 Pt with x ≈ 0.5 system by electrodeposition of FeRh on a Fe 3 Pt ordered foil. The resulting film stack show, prior to further heat-treatments, an anomaly in the magnetisation when the sample is cooled in a magnetic field from 300 K down to lower temperatures, after reaching 300 K with the same applied field from a zero-field-cooled heating cycle from 1.8 K. The anomalous magnetisation is reproducible and appears at temperatures close to 120 K, from fields as low as 20 Oe up to fields as high as 1 kOe, and resembles a metamagnetic transition.