FeRh Thin Films Research Articles

Quantitative control of magnetic ordering in FeRh thin films using 30 keV Ga ion irradiation from a focused ion beam system

We investigated a low-energy ion-beam irradiation process for the magnetic modification of FeRh thin films using a focused ion beam system. Low-energy ion-beam irradiation induced ferromagnetic states in the FeRh thin films at low temperatures, that originally exhibited antiferromagnetism, as effectively as high-energy ion-beam irradiation. Because the energy deposited by the elastic collisions caused by the irradiation determined the magnetic properties of the samples, the magnetic state of the FeRh thin films could be quantitatively controlled. The low-energy ion-beam irradiation using a focused ion beam system is a potential technique to modify the magnetic properties of materials on the nano- and micro-scales, which may lead to a variety of novel spin devices and applications.

Micrometer-Sized Magnetic Patterning of FeRh Films Using an Energetic Ion Microbeam

FeRh thin films were irradiated with a focused 10 MeV I 2 ×2 µm2 ion microbeam at intervals of several µm. Following irradiation, the magnetic state at the surface was observed via magnetic force microscopy. The micrometer-sized regions irradiated with the microbeam show a ferromagnetic state while unirradiated regions remain in an anti-ferromagnetic state. Results indicate that an energetic ion microbeam can be used as a tool to produce micrometer-sized modulations of lateral magnetic states of FeRh films.

Origin of room temperature ferromagnetic moment in Rh-rich [Rh/Fe] multilayer thin films

B2 ordered FeRh thin films switch from antiferromagnetic (AFM) to ferromagnetic (FM) state on heating above 350 K and switch back on cooling, with a hysteresis. This property makes FeRh a very attractive choice as a write-assist layer material for low temperature heat assisted magnetic recording (HAMR) media. Studies have shown that as we decrease the thickness of the FeRh films, the B2 phase is no longer AFM even below 350 K and there is a thickness dependant FM stabilization of the B2 phase. It was also proved that slightly Rh-richer compositions (>50 at. % Rh) were more preferable to stabilize the AFM phase. The current study focuses on growing highly ordered FeRh films by alternate layer rf sputtering of thin layers of iron and rhodium onto a heated substrate. It has been shown that films with rhodium content beyond 55 at. % contain a disordered bcc FM phase which gives rise to residual moment at room temperature even for thicker films.

Magnetic states controlled by energetic ion irradiation in FeRh thin films

Changes in magnetic properties and lattice structure of FeRh films by 180 keV–10 MeV ion (H, He, and I) irradiation are studied. In spite of the irradiation with different ion species and wide range of energies, the changes in magnetization are dominated by solely a single parameter; the density of energy which is deposited through elastic collision between the ions and the samples. For the low deposition energy density, the magnetization increases with increasing the deposition energy density, while the lattice structure remains unchanged. When the deposition energy density becomes larger, however, the magnetization decreases after reaching the maximum value. The decrease in the magnetization accompanies the crystal structure change from B2 to A1. The present results imply that the magnetic state of FeRh films can be designedly controlled by the energetic ion irradiations.

Magnetic properties and first-order magnetic phase transition in single crystal FeRh thin film

To understand the mechanism of first-order magnetic phase transition in ordered FeRh thin films, the magnetic properties and first-order antiferromagnetic (AFM)–ferromagnetic (FM) phase transition behavior of single-crystal FeRh thin film are investigated in detail. The first-order magnetic phase transition is seen at a temperature of around 120 °C during heating and 145 °C cooling processes in perpendicular direction. The M–H loops measured isothermally amidst the AFM–FM transition regime show an opening at high magnetic field, which indicate a reversible AFM–FM transition induced by magnetic field. The clusters of the FM phase nucleate in the AFM matrix heterogeneously and vice versa during the first-order phase transition and the mechanism of nucleation and growth kinetics of the first-order magnetic phase transition in ordered FeRh thin film is quite similar to that of the crystallization of solids described by the Avrami model.

Surface influenced magnetostructural transition in FeRh films

Surface structural effects accompanying the antiferromagnetic-ferromagnetic magnetostructral transition of epitaxial FeRh thin films were investigated by grazing incidence x-ray scattering. Measurement of the film lattice parameters and variation of x-ray incident angles allow observation of the transition character on scales ranging from a few nm to the total through-thickness of the film. Out-of-plane lattice measurements confirm that the ferromagnetic phase nucleates from the surface during the heating process and is retained at the surface below the transition temperature during the cooling process. These results suggest that surface strain relief fosters nucleation of the ferromagnetic phase.

Effects of energetic heavy ion irradiation on the structure and magnetic properties of FeRh thin films

Fe–54at.%Rh thin films were irradiated with 10 MeV iodine ions at room temperature. Before and after the irradiations, the changes in magnetic properties and the lattice structure of the samples were studied by means of a SQUID magnetometer and X-ray diffraction. For the low fluence irradiation, the SQUID measurement at 20 K shows that the anti-ferromagnetic region of the thin film is changed into ferromagnetic region by the irradiation. As the film thickness is much smaller than the ion range, we can discuss the relationship between the density of energy deposited by ions and the change in magnetization quantitatively. For the high fluence irradiation, the magnetization of the film is strongly decreased by the irradiation, which can be explained as due to the change in lattice structure from B2 into A1 structure by the irradiation.

Stability of ferromagnetic state of epitaxially grown ordered FeRh thin films

We report on the magnetic properties of B2-type ordered FeRh epitaxial thin films deposited on MgO(001) substarates as a function of film thickness. All the films show a clear magnetic phase transition from the antiferromagnetic state to the ferromagnetic state with increasing temperature while the transition temperature of a 10-nm-thick film decreases down to 300K. The 10-nm-thick film also shows a large magnetization even in the antiferromagnetic state compared with other thicker films. These magnetization data indicate that the ferromagnetic state is becoming more stable than the antiferromagnetic state with decreasing film thickness. Such thickness dependent magnetic properties are qualitatively compatible with a theoretical prediction for FeRh(001) thin layers.

Magnetic and magneto-optical properties of FeRh thin films

The magnetic and magneto-optical properties of FeRh thin films epitaxially deposited onto MgO(1 0 0) substrates by RF sputter-deposition system have been investigated in conjunction with the structure. An intriguing virgin effect has been found in the M– T curves of the as-deposited FeRh thin films, which is presumably interpreted in term of a change in structural phase when heating. Also, a (negative) maximum peak of Kerr rotation at around 3.8 eV has been observed when FeRh thin films are in ferromagnetic state. The polar Kerr rotation angle is found to increase at temperatures above 100 °C, which corresponds to the antiferromagnet (AF)–ferromagnet (FM) transition of FeRh thin films.

Bulk and near-surface magnetic properties of FeRh thin films

Epitaxial Fe48Rh52 thin films of nominal thickness 500Å were grown on MgO (001) substrates via molecular beam epitaxy and capped with 20Å of either Au or MgO. The presence of the anticipated magnetostructural phase transition from antiferromagnetism to ferromagnetism at ∼350K was confirmed by superconducting quantum interference device and magneto-optic Kerr effect magnetometry. X-ray magnetic circular dichroism (XMCD) experiments were carried out at the L2,3 edges; measurements were done in both total electron yield (TEY) mode and indirect transmission mode. At room temperature, the indirect transmission XMCD data show no detectable dichroism, consistent with bulk antiferromagnetic behavior. However, room-temperature TEY data originating from the films’ surface reveal an appreciable dichroic signal indicating the presence of ferromagnetism. This near-surface/interfacial magnetism may be modified by choice of capping layer; the MgO-capped sample exhibited a considerably smaller ferromagnetic XMCD signal. The results have potential impact on the application of FeRh thin films as temperature-variable pinning layers in exchanged-biased systems.

Structural and magneto-optical properties of FeRh thin films

A systematic investigation of the magneto-optical properties in conjunction with structure and magnetic properties of FeRh thin films epitaxially deposited onto MgO(100) substrates by rf sputter-deposition system was performed. The result, for the first time, shows a (negative) maximum peak of Kerr rotation at around 3.8eV when FeRh thin films are in ferromagnetic state. The polar Kerr rotation angle is found to increase at temperatures above 100°C, which corresponds to the antiferromagnetic-ferromagnetic transition of FeRh thin films.

Magnetization behaviors for FeRh single crystal thin films

The structural and magnetic phase transition in FeRh thin films are investigated. The (001) oriented single crystal FeRh thin films fabricated onto MgO (100) substrate possess lattice parameter a smaller than c, because of the compressive stress along the a axis from MgO substrate. With increasing annealing temperature, paramagnetic FeRh thin films transform into ferromagnetic and then antiferromagnetic-ferromagnetic (AF-FM) stages. The transition temperature of AF-FM increases with annealing temperature, while the thermal hysteresis width decreases. The M-H loops of the film annealed at 700°C show an opening at high magnetic fields during the transition state. Note that the opening in M-H loops disappears when AF phase has transformed into FM phase. The origin of the opening is not known but possibly due to the combination of hysteresis loops of FM and AF phases, where FM phase is soft magnetic and the AF phase FeRh shows a hysteresis behavior with coercivity due to the reduction of magnetic anisotropy at high temperatures. The abrupt change of coercivity along the perpendicular direction during the AF-FM transition suggests that a stress-induced magnetic anisotropy exists in FeRh thin films due to the volume expansion in the phase transition process.

Identifying growth mechanisms for laser-induced magnetization in FeRh

Time-resolved pump-probe measurements of the magnetic phase change from paramagnetic to ferromagnetic and back in FeRh thin films are presented. Data are compared with simulations of laser-induced magnetization dynamics using the Landau-Lifshitz-Gilbert equation with a time dependent magnetization density, responding to the evolution of the thermal profile throughout the film. We demonstrate that the observed magneto-optical transients should be interpreted in terms of an interplay between the local magnetic moment, nonlocal domain growth or alignment, and magnetization precession as launched by the varying demagnetizing fields. Kerr rotation and reflectivity are shown to provide a complementary view on the phase transition.

Ultrafast Generation of Ferromagnetic Order via a Laser-Induced Phase Transformation in FeRh Thin Films

It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations.

Growth of FeRh thin films and magnetic properties of FePt/FeRh bilayers

A method for growing the FeRh thin films with a good (001) texture on fused silica (SiO2-glass) substrates is presented. Seed layers of Fe/Ir and the substrate temperature as low as 200°C, at which FeRh is deposited, are effective for growing the Ir-substituted FeRh films with a good (001) texture, though annealing is necessary after deposition. Furthermore, a FePt layer was deposited onto the Fe(Rh,Ir) and perpendicularly magnetized FePt film with (001)-texture was fabricated.

Ferromagnetic stabilization of ordered B2FeRhthin films

The electronic structure and the local spin polarization of the (001) FeRh thin films have been studied within density-functional formalism. Bulk B2 FeRh is an ordered alloy, with in-plane antiferromagnetism (AF-II) in the Fe layers as ground state and a ferromagnetic configuration a few millirydberg above. A transition from antiferromagnetic to ferromagnetic configuration is obtained when the temperature is increased or when an excess of Fe is introduced. Here we demonstrate that a decrease of the film thickness leads to similar transition. For Rh-terminated FeRh (001) surfaces, the calculations show that the ground state of the film is ferromagnetic for nine layers whereas for 15 layers it is antiferromagnetic as in bulk FeRh.

Features of broad magnetic transition in FeRh thin film

Abstract The first-order antiferromagnetic (AFM)-ferromagnetic (FM) transition in a Fe 47.8 Rh 52.2 thin film has been studied as a function of temperature, applied magnetic field and internal stress. Although the AFM to FM transition shows a steeper change at a certain critical temperature ( T c =320 K), the FM to AFM transition is not complete below 10 K. The field dependence of T c (d T c /d H ) for the AFM to FM transition agrees with bulk reference data. It is found that peeling from the substrate causes (1) a T c increase (about 50°), (2) an acceleration in the transformation from the FM to the AFM phase, (3) steepening of the transition, and (4) a decrease in transition width. Bulk pressure coefficient data (d T c /d P ) is used to show that the increase in T c results from the release of a measured tensile stress of 7.5 kbar. Macroscopic and locally distributed stress are both strongly related to the instability of the transformation from the FM to the AFM phase, and the steep and wide transition in FeRh thin film.

Antiferro-ferromagnetic transition and microstructural properties in a sputter deposited FeRh thin film system

Magnetic and microstructural properties are studied in sputter deposited Fe100−xRhx alloy (28.0<x<57.0) thin films. As-deposited films show a preferred (111) oriented fcc phase at x≳35.0. Annealing at 600 °C results in the CsCl type structure which exhibits a preferred (110) orientation with the same lattice constant as that of bulk. However, an incomplete antiferro-ferromagnetic transition with large thermal hysteresis is observed in a wider composition range (x≳45.4) than bulk. Furthermore, magnetization at room temperature decreases continuously from x=35.0 to x=57.0 in contrast to the sharp drop at x=50 in bulk. By using scanning and transmission electron microscopy and Mössbauer spectroscopy, it is found that the retained fcc phase and compositional fluctuation strongly affect the magnetic transition. The retained fcc phase surprisingly reduces the grain growth of the CsCl type ordered phase from 4000 to 300 Å as the Rh content increases from 45.4% to 54.0%.