Structural Dependent Ferromagnetic-Nonmagnetic Phase Change in FePtRu Films

Herein, we studied correlations between crystal structures and magnetic properties of FePt1−xRux films. At room temperature, the chemical disordered A1 films with 0 ≤ x < 0.20 and 0.20 ≤ x ≤ 1.00 exhibited ferromagnetic properties and paramagnetic properties, respectively. Curie temperature of the disordered film with x = 0.30 was 200K. In contrast, the ordered L10 films had ferromagnetic properties in a wider range of 0 ≤ x < 0.80 with the magnetic easy-axis perpendicular to the film plane. For 0 ≤ x ≤ 0.50, with the ordered structure, the films had high magnetization and high uniaxial magnetic anisotropy of over 10 erg cm. For x = 0.60–0.70 with the ordered structure, a temperature-dependent magnetic phase transition appeared, and magnetization reached its maximum value at around 200K. Using this material system, we proposed a nanopatterning method involving a ferromagnetic-paramagnetic phase change due to the ordered-disordered structural transformation.


Introduction
Increased power consumption in data centers having millions of hard disk drives (HDDs) has emerged as a serious issue. Decreasing the number of HDDs by increasing their magnetic storage density can be a simple and efficient means to mitigate increased power consumption. Ferromagnetic (FM) thin films prepared using nanofabrication techniques can be used for high-density magnetic data storage [1][2][3] and spintronic devices [4].
Bit-patterned media (BPM) is a candidate for the creation of ultra-high-density magnetic data storage devices [1,2]. BPM consist of multiple phases, which include the FM phase (dots) characterized by a high uniaxial magnetic anisotropy along with, in principle, diamagnetic, antiferromagnetic (AF), and paramagnetic (PM) phases. Ion milling is conventionally used for nanofabrication of two-dimensional FM films [5,6], which produces physically separated FM regions. After ion milling, backfill and polishing stages are required to obtain BPM with smooth surfaces, which is a prerequisite for read/write heads to be able to fly at a few nanometers above the medium surface [1].
Another BPM fabrication method, using ion irradiation, has also been proposed [7,8]. This method can replace the three processes of ion milling, backfill, and polishing steps, thereby streamlining the production of BPM. Ion irradiation results in structural disordering. Co/Pt multilayers [7,8] and 1 0 CrPt 3 films [9] with an uniaxial magnetocrystalline anisotropy constant ( ) of ∼10 6 erg cm −3 , which is not sufficient for the realization of high magnetic data storage density of over 2 Tb in −2 , are transformed from FM to PM phase after ion irradiation.
A problem faced by high-density magnetic data storage is the thermal fluctuation effect of magnetic grains or dots. The magnetic anisotropy energy ( ; : volume of an isolated magnetic grain or dot) becomes lower relative to the thermal fluctuation energy ( ; : Boltzmann constant, : temperature) when the grain and/or dot size is reduced for increasing storage density. The thermal fluctuation can be sufficiently reduced using materials with high , satisfying the thermal stability factor requirement of ( / ) > 60 (this metric was derived using the Sharrock equation [10]) which is typically regarded as a minimum requirement for magnetic data storage [1,11].
2 Advances in Materials Science and Engineering 1 0 FePt (CuAu-I type, fct, = > ) films with ordered structures are possible candidates for BPM due to their high of 7.0 × 10 7 erg cm −3 , high saturation magnetization ( ) of 1100 emu cm −3 , high corrosion resistance, and low resistivity [12][13][14], which leads to excellent thermal stability of magnetization in nanometer-size structures. After ion irradiation, the ordered 1 0 FePt films undergo transformation from the hard-FM phase (high ) to the soft-FM phase (low ) with the disordered 1 structure (fcc, = = ) [15][16][17][18][19][20]. Although the 1 0 FePt is suitable for BPM, modifying its properties using ion irradiation is difficult due to the insensitivity of magnetization with ion irradiation. The high spontaneous magnetization ( 0 ) of the soft-FM region (interdot spacing), wherein the ions are irradiated, leads to spike noise in the storage media. Therefore, developing highand high-materials, whose and values decrease upon ion irradiation with high sensitivity, is required.
In our previous study, nonmagnetic phases were observed by replacing Fe with Mn [21] and Pt with Rh [22] in FePt films, and the former improved the sensitivity of to ion irradiation. The 1 0 Fe 1− Mn Pt films with ≤ 0.44 exhibit FM properties corresponding to > 2.1 × 10 7 erg cm −3 , whereas, disordered 1 films with ≥ 0.44 possess PM properties at room temperature. These films change from a FM to PM state as the 1 0 structure transforms into the 1 structure due to ion irradiation. However, the high Mn contents could decay corrosion resistance and . By replacing Pt with Ru in the 1 0 FePt films, which could be uninfluential to its corrosion resistance, a reduction of Curie temperature ( ) has been reported [23].
In this study, by replacing Pt with Ru in 1 and 1 0 FePt films, correlations between crystal structures and magnetic properties were investigated.

Materials and Methods
Fe 50 (Pt 1−x Ru x ) 50 films with the thickness of 6.0 nm were deposited by magnetron cosputtering at a base pressure of 10 −5 Pa using Ar gas at 0.5 Pa on a single-crystalline MgO (100) substrate at 298 K. The Ru composition ( ) was controlled by varying the sputtering rates of the Pt and Ru targets and was detected using an electron probe X-ray microanalyzer. The films were annealed by rapid thermal annealing (RTA) with a heating rate of 300 K s −1 under a vacuum of 2 × 10 −4 Pa. The crystalline structure was studied using out-of-plane X-ray diffraction (XRD) measurement and in-plane XRD measurement with CuK radiation. A vibrating sample magnetometer (VSM) with a maximum field of 18 kOe and a superconducting quantum interference device (SQUID) magnetometer with a maximum field of 50 kOe were used to assess the magnetic properties of the films.

Results and Discussion
Figure 1(a) shows out-of-plane XRD patterns for the asdeposited FePt 1− Ru films of thickness 6.0 nm. The Ru composition of the FePt 1− Ru films was changed from 0 to 1.00 and is shown from 0 to 0.30 with the background pattern (sample holder and substrate) in the figure. Only the background peaks were observed for all the films, since the fundamental (111) reflection peaks for the disordered 1 structure appear at the same angle as the MgO (100) substrate.
show magnetization curves of the films with = 0, 0.10, 0.20, and 0.30, measured by the VSM with a maximum field of 18 kOe applied perpendicular (⊥, dashed line) and parallel (//, solid line) to the film plane at 298 K. The magnetic easy-axes of all the films are parallel to the film plane. Figure 1(f) shows the temperature ( ) dependence of magnetization ( ) ( -curve) for the film with = 0.30 at temperatures at or below room temperature (300 K down to 30 K), measured using a SQUID magnetometer with a field of 1 kOe applied parallel to the film plane after saturating the magnetization by a field of 50 kOe at 298 K. decreases with an increase in and approaches 0 at around of 200 K. was estimated from the 2 -curve (not shown). This indicates that the film with = 0.30 is in the PM phase at room temperature.
The -dependence of 0 , determined using the Arrott plot [24] at 298 K, is shown in Figure 1(g). 0 decreases with increasing , and an abrupt decrease in 0 is found at = 0.20, caused by the decrease in . This implies that the films with 0 ≤ < 0.20 are in the FM phase, and the films with ≥ 0.20 are in the PM phase at room temperature.
Figures 2(a) and 2(b) show out-of-plane XRD patterns and in-plane XRD patterns, respectively, for the FePt 1− Ru films after annealing at 1173 K for 4 h with a heating rate of 300 K s −1 by RTA. In Figure 2(a), only superlattice (001) and fundamental (002) reflection peaks for the ordered 1 0 structure are observed for the films with 0 ≤ ≤ 0.70, whereas only background peaks were observed for the films with 0.80 ≤ ≤ 1.00 due to overlapping of the fundamental (111) reflection peak for the disordered 1 structure and the reflection peaks for the MgO (100) substrate. In Figure 2(b), only the fundamental (200) reflection peak is observed for the films with 0 ≤ ≤ 0.60, whereas only background peaks are observed for the films with 0.70 ≤ ≤ 1.00. These reflection patterns indicate that the (001) crystalline texture is normal to the film plane in the films with 0 ≤ ≤ 0.70, whereas the disordered 1 structure is obtained in the films with 0.80 ≤ ≤ 1.00. This result is broadly consistent with the previous report [23]. The degrees of long-range chemical order parameter were estimated to be ∼0.90 in the films with 0 ≤ ≤ 0.60.
The lattice constants ( , , and / ) are plotted as functions of in Figure 2 values were evaluated using the magnetization curves [25]. The magnetic anisotropy field ( ) was determined from the intersection point of the extrapolated magnetization curves of the magnetic fields applied parallel and perpendicular to the film plane. Only the linear part of the in-plane magnetization curve was extrapolated. Consequently, was obtained using the relation = ( × /2) + shape , where shape is the shape anisotropy calculated using the demagnetization factors ( ) of the film shape ( ⊥ = 4 , // = 0). The films with 0 ≤ ≤ 0.50 have high magnetization of 500 ≤ 0 ≤ 800 emu cm −3 , high coercivity of 15 ≤ ≤ 43 kOe, and high anisotropy of 2.6 × 10 7 ≤ ≤ 5.0 × 10 7 erg cm −3 in the perpendicular direction, and these results are broadly consistent with the previous report [23].
The -dependence of 0 and is plotted in Figure 3(f). film with = 0.20 has FM properties with a = 3.5 × 10 7 erg cm −3 , whereas, the disordered 1 film with = 0.20 has PM properties at room temperature. The film can change from a FM to PM state as the 1 0 structure transforms into the 1 structure due to ion irradiation. By using the thermal stability factor ( / ) > 60 and = 3.5 × 10 7 erg cm −3 , the smallest diameter of the spherical shape was estimated to be ∼5.1 nm, which indicates that this material system has sufficient thermal stability for use in high-density magnetic data storage media of ∼10 Tb in −2 . Figures 4(a)-4(d) show the -curves of the films with = 0.20, 0.40, 0.60, and 0.80 at temperatures at or below room temperature (300 K down to 30 K), measured using the SQUID magnetometer with a field of 1 kOe applied perpendicular to the film plane after saturating the magnetization by a field of 50 kOe at 298 K. In the films with ≤ 0.40, increases with a decrease in due to the typical FM properties. However, at = 0.60, reaches its maximum value at 210 K ( 0 ) and decreases with a decrease in temperature below 0 . This indicates that the film with = 0.60 can contain an antiferromagnetically ordered phase (spin-glass or canted-AF etc.) at temperatures below 0 . In the films with ≥ 0.80, is almost 0 at each . These results imply that the films with 0 ≤ ≤ 0.40 should have the FM-PM transition above 300 K, the films with 0.40 < < 0.80 have the antiferromagnetically ordered phase FM-PM transition, and the films with 0.80 ≤ ≤ 1.00 have the FM-PM transition below 300 K due to the 1 structure. Finally, we propose a nanopatterning method for this material system. According to our previous results, the ordered 1 0 structures of FePtX were easily transformed to the disordered 1 structure under ion irradiation [15][16][17][18][19][20][21]. Figure 5 shows the schematics of a nanopatterning method using an FM-PM phase change due to the 1 0 -1 structural transformation caused by ion irradiation. After ion irradiation, FM dots having 1 0 structure should be surrounded by PM spacing having 1 structure with a smooth disc surface.

Conclusions
Correlations between crystal structures and magnetic properties of FePt 1− Ru films (6.0 nm thick) were investigated.
(1) Magnetic properties of 1 disordered structure (asdeposited films): 0 decreased with increasing , and an abrupt decrease in 0 was found at = 0.20 at room temperature, due to the decrease in . In the range of 0 ≤ < 0.20, the films had FM properties (100 ≤ 0 ≤ 1050 emu cm −3 ) with their magnetic easy-axes parallel to the film plane. In the range of ≥ 0.20, the films had PM properties ( 0 ≈ 0 emu cm −3 ) at room temperature.
For instance, the 1 0 film with = 0.20 had FM properties with a = 3.5 × 10 7 erg cm −3 , whereas the disordered 1 film with = 0.20 had PM properties at Advances in Materials Science and Engineering 7 room temperature. The film could change from a FM to PM state as the 1 0 structure transforms into the 1 structure due to ion irradiation. These results suggest the possibility of applying the material system for nanopatterning method for high-density magnetic storage media.