On metamagnetic phase transition in 150 nm B2-like FeRh film of nanoclusters assembled.

Abstract

The FeRh alloy in CsCl-type (B2) phase is well-known for its metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) state when the temperature increases over 370 °C, which comes with a volume expansion of 1% [1,2]. Nowadays, magneto-structural transition near room temperature are of great interest for potential applications to the control of nanostructures, magnetocaloric refrigeration and antiferromagnetic spintronics [3,4]. However, there are still open questions at nanoscale, including the finite size effects on magnetic FeRh phase transition, which require to continue efforts.The aim of this work is to explore the magnetic behavior of a 150 nm-thick FeRh nano-structured film prepared from Low Energy Cluster Beam Deposition (LECBD). Using a laser vaporization source, the initial diameter of the deposited FeRh nanoparticles is ranging from 2 nm to 10 nm in a log-normal distribution according to previous transmission electron microscopy observations [5]. The sample was annealed at 700 °C for 3 hours under vacuum in order to achieve the chemically ordered B2 phase. Then X-ray Diffraction spectrum reveals all α’-bcc peaks with a lattice parameter of 2.98 Å as expected in the B2-FeRh phase for a concentration of 48%-56% of Rh. The full width at half maximum (FWHM) of the α’(001) peak gives us an estimated crystallite size of about 30 nm in agreement with a limited coalescence upon annealing, leading to a surface granular thick-sample as observed from Atomic Force Microscope observations (Fig. 1).To study the magnetic behavior, various SQUID magnetometry measurements have been performed, including thermal dependence magnetization m(T) in the heating and cooling mode, for different values of applied magnetic field. As reported in Fig. 2, one can observe a broad and asymmetric metamagnetic transition with a significant fraction of residual magnetization at low temperature. This persistent FM state is attributed to the presence of non-switchable FM Grains. By increasing the external field, the metamagnetic transition is pushed to lower temperature following a linear correlation which is observed from the temperatures of maximum and minimum magnetization evolution as a function of field (insert Fig. 2). From Hysteresis magnetization m(H) loops a decreasing coercive field as the temperature increases has also been observed sustaining the evidence of FM regions down to low temperature. This behavior supports the coexistence of these two magnetic orders in thick nanogranular B2-like FeRh film. Moreover, field cooling - zero field cooling (FC-ZFC) susceptibility measurements have revealed the presence of single FM domains with a superparamagnetic (SPM) like-transition at temperatures higher than 130K. But because there is no exchange bias measured in m(H) loops at low temperature, there is no strong FM-AFM exchange coupling as could be expected for FM core@AFM shell morphology.In this work, the magnetic behavior of such thick nanogranular B2-like FeRh film will be described as a competition between different magnetic orders as a function of nanoparticle size, temperature and applied magnetic field. Several thermal regimes in the heating and cooling modes will be proposed as 1) Low temperature regime for T<50K where small persistent FM nanoparticles co-exist with large AFM nanoparticles, 2) Intermediate regime with AFM-FM switching events and 3) at T>300K the whole film is in the FM state. In the cooling mode, the FM dragging effect will be related to the applied magnetic field. A simple model could be considered with two distinct nanoparticles populations: one with smaller nanoparticles (FM/SPM) where relaxation effects avoid reaching AFM states at low temperature, and one with larger nanoparticles with AFM-compressed core@FM-relaxed shell morphology up to room temperature.To go a step further, X-ray magnetic circular dichroism results on mass-selected FeRh nanoparticles embedded in matrix recently measured at DEIMOS-SOLEIL will be discussed to illustrate finite size and surface/interface effects on metamagnetic FeRh transition. **