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Structural Dependent Ferromagnetic-Nonmagnetic Phase Change in FePtRu Films.

1. 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 [1-3] 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 [L1.sub.0] [CrPt.sub.3] films [9] with an uniaxial magnetocrystalline anisotropy constant ([K.sub.u]) of ~[10.sup.6] erg [cm.sup.-3], which is not sufficient for the realization of high magnetic data storage density of over 2 Tb [in.sup.-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 ([K.sub.u]V; V: volume of an isolated magnetic grain or dot) becomes lower relative to the thermal fluctuation energy ([k.sub.B]T; [k.sub.B]: Boltzmann constant, T: 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 [K.sub.u], satisfying the thermal stability factor requirement of ([K.sub.u]V/[k.sub.B]T) > 60 (this metric was derived using the Sharrock equation [10]) which is typically regarded as a minimum requirement for magnetic data storage [1, 11].

[L1.sub.0] FePt (CuAu-I type, fct, a = b > c) films with ordered structures are possible candidates for BPM due to their high [K.sub.u] of 7.0 x [10.sup.7] erg [cm.sup.-3], high saturation magnetization ([M.sub.s]) of 1100 emu [cm.sup.-3], high corrosion resistance, and low resistivity [12-14], which leads to excellent thermal stability of magnetization in nanometer-size structures. After ion irradiation, the ordered [L1.sub.0] FePt films undergo transformation from the hard-FM phase (high [K.sub.u]) to the soft-FM phase (low [K.sub.u]) with the disordered A1 structure (fcc, a = b = c) [15-20]. Although the [L1.sub.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 ([M.sub.0]) of the soft-FM region (interdot spacing), wherein the ions are irradiated, leads to spike noise in the storage media. Therefore, developing high-[K.sub.u] and high-[M.sub.s] materials, whose [K.sub.u] and [M.sub.s] 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 [M.sub.s] to ion irradiation. The [L1.sub.0] [Fe.sub.1-x][Mn.sub.x]Pt films with x [less than or equal to] 0.44 exhibit FM properties corresponding to [K.sub.u] > 2.1 x [10.sup.7] erg [cm.sup.-3], whereas, disordered A1 films with x [greater than or equal to] 0.44 possess PM properties at room temperature. These films change from a FM to PM state as the [L1.sub.0] structure transforms into the A1 structure due to ion irradiation. However, the high Mn contents could decay corrosion resistance and [K.sub.u]. By replacing Pt with Ru in the [L1.sub.0] FePt films, which could be uninfluential to its corrosion resistance, a reduction of Curie temperature ([T.sub.c]) has been reported [23].

In this study, by replacing Pt with Ru in A1 and [L1.sub.0] FePt films, correlations between crystal structures and magnetic properties were investigated.

2. Materials and Methods

[Fe.sub.50] [([Pt.sub.1-x][Ru.sub.x]).sub.50] films with the thickness of 6.0 nm were deposited by magnetron cosputtering at a base pressure of [10.sup.-5] Pa using Ar gas at 0.5 Pa on a single-crystalline MgO (100) substrate at 298 K. The Ru composition (x) 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.sup.-1] under a vacuum of 2 x [10.sup.-4] Pa. The crystalline structure was studied using out-of-plane X-ray diffraction (XRD) measurement and in-plane XRD measurement with CuK[alpha] 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 50k0e were used to assess the magnetic properties of the films.

3. Results and Discussion

Figure 1(a) shows out-of-plane XRD patterns for the as-deposited [FePt.sub.1-x] [Ru.sub.x] films of thickness 6.0 nm. The Ru composition x of the [FePt.sub.1-x][Ru.sub.x] 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 A1 structure appear at the same angle as the MgO (100) substrate.

Figures 1(b)-1(e) show magnetization curves of the films with x = 0, 0.10, 0.20, and 0.30, measured by the VSM with a maximum field of 18 kOe applied perpendicular ([perpendicular to], 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 (T) dependence of magnetization (M) (M-T curve) for the film with x = 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. M decreases with an increase in T and approaches 0 at around [T.sub.c] of 200 K. [T.sub.c] was estimated from the [M.sup.2]-T curve (not shown). This indicates that the film with x = 0.30 is in the PM phase at room temperature.

The x-dependence of [M.sub.0], determined using the Arrott plot [24] at 298 K, is shown in Figure 1(g). [M.sub.0] decreases with increasing x, and an abrupt decrease in [M.sub.0] is found at x = 0.20, caused by the decrease in [T.sub.c]. This implies that the films with 0 [less than or equal to] x < 0.20 are in the FM phase, and the films with x [greater than or equal to] 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.sub.1-x][Ru.sub.x] films after annealing at 1173 K for 4 h with a heating rate of 300 [Ks.sup.-1] by RTA. In Figure 2(a), only superlattice (001) and fundamental (002) reflection peaks for the ordered [L1.sub.0] structure are observed for the films with 0 [less than or equal to] x [less than or equal to] 0.70, whereas only background peaks were observed for the films with 0.80 [less than or equal to] x [less than or equal to] 1.00 due to overlapping of the fundamental (111) reflection peak for the disordered A1 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 [less than or equal to] x [less than or equal to] 0.60, whereas only background peaks are observed for the films with 0.70 [less than or equal to] x [less than or equal to] 1.00. These reflection patterns indicate that the (001) crystalline texture is normal to the film plane in the films with 0 [less than or equal to] x [less than or equal to] 0.70, whereas the disordered A1 structure is obtained in the films with 0.80 [less than or equal to] x [less than or equal to] 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 [less than or equal to] x [less than or equal to] 0.60.

The lattice constants (a, c, and c/a) are plotted as functions of x in Figure 2(c). c and a values were obtained from the out-of-plane and in-plane XRD measurements, respectively. a and c values of the film with x = 0 are about 0.3867 nm and 0.3726 nm, respectively, and are close to those of the FePt bulk alloy (a = 0.3852 nm, c = 0.3713 nm; PDF number 43-1359). The values of a are ~0.39 nm for each x which could be kept by the epitaxial growth, whereas c and the axial ratio (c/a) values decrease with increasing x. By considering the layer-by-layer atomic configuration in [L1.sub.0] FePt and the differences of atomic radiuses of Pt, Ru, and Fe (Pt > Ru > Fe), the decrease in the lattice constant c indicates the substitution of Ru for Pt.

Figures 3(a)-3(e) show magnetization curves of the films with x = 0, 0.10, 0.20, 0.40, and 0.60, measured by the SQUID magnetometer with a maximum field of 50 kOe applied perpendicular ([perpendicular to], filled symbols) and parallel (//, open symbols) to the film plane at 298 K. The magnetic easy-axes of all the films were perpendicular to the film plane. [K.sub.u] values were evaluated using the magnetization curves [25]. The magnetic anisotropy field ([H.sub.k]) 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, [K.sub.u] was obtained using the relation [K.sub.u] = ([M.sub.s] x [H.sub.k]/2) + [K.sub.shape], where [K.sub.shape] is the shape anisotropy calculated using the demagnetization factors (N) of the film shape (N[perpendicular to] = 4[pi], [N.sub.//] = 0). The films with 0 [less than or equal to] x [less than or equal to] 0.50 have high magnetization of 500 [less than or equal to] [M.sub.0] [less than or equal to] 800 emu [cm.sup.-3], high coercivity of 15 [less than or equal to] [H.sub.c] [less than or equal to] 43 kOe, and high anisotropy of 2.6 x [10.sup.7] [less than or equal to] [K.sub.u] [less than or equal to] 5.0 x [10.sup.7] erg [cm.sup.-3] in the perpendicular direction, and these results are broadly consistent with the previous report [23].

The x-dependence of [M.sub.0] and [K.sub.u] is plotted in Figure 3(f). [M.sub.0] and [K.sub.u] decrease with increasing x and reach almost 0 at x = 0.80, which is the critical composition for the transition from the [L1.sub.0] structure to the A1 structure. For instance, [L1.sub.0] film with x = 0.20 has FM properties with a [K.sub.u] = 3.5 x [10.sup.7] erg [cm.sup.-3], whereas, the disordered A1 film with x = 0.20 has PM properties at room temperature. The film can change from a FM to PM state as the [L1.sub.0] structure transforms into the A1 structure due to ion irradiation. By using the thermal stability factor ([K.sub.u]V/[k.sub.B]T) > 60 and [K.sub.u] = 3.5 x [10.sup.7] erg [cm.sup.-3], the smallest diameter of the spherical shape was estimated to be ~5.1nm, which indicates that this material system has sufficient thermal stability for use in high-density magnetic data storage media of ~10 Tb [in.sup.-2].

Figures 4(a)-4(d) show the M-T curves of the films with x = 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 x [less than or equal to] 0.40, M increases with a decrease in T due to the typical FM properties. However, at x = 0.60, M reaches its maximum value at 210 K ([T.sub.0]) and decreases with a decrease in temperature below [T.sub.0]. This indicates that the film with x = 0.60 can contain an antiferromagnetically ordered phase (spin-glass or canted-AF etc.) at temperatures below [T.sub.0]. In the films with x [greater than or equal to] 0.80, M is almost 0 at each T. These results imply that the films with 0 [less than or equal to] x [less than or equal to] 0.40 should have the FM-PM transition above 300 K, the films with 0.40 < x < 0.80 have the antiferromagnetically ordered phase FM-PM transition, and the films with 0.80 [less than or equal to] x [less than or equal to] 1.00 have the FM-PM transition below 300 K due to the A1 structure.

Finally, we propose a nanopatterning method for this material system. According to our previous results, the ordered [L1.sub.0] structures of FePtX were easily transformed to the disordered A1 structure under ion irradiation [15-21]. Figure 5 shows the schematics of a nanopatterning method using an FM-PM phase change due to the [L1.sub.0]-A1 structural transformation caused by ion irradiation. After ion irradiation, FM dots having [L1.sub.0] structure should be surrounded by PM spacing having A1 structure with a smooth disc surface.

4. Conclusions

Correlations between crystal structures and magnetic properties of [FePt.sub.1-x][Ru.sub.x] films (6.0 nm thick) were investigated.

(1) Magnetic properties of A1 disordered structure (as-deposited films): [M.sub.0] decreased with increasing x, and an abrupt decrease in [M.sub.0] was found at x = 0.20 at room temperature, due to the decrease in Tc. In the range of 0 [less than or equal to] x < 0.20, the films had FM properties (100 [less than or equal to] [M.sub.0] [less than or equal to] 1050 emu [cm.sup.-3]) with their magnetic easy-axes parallel to the film plane. In the range of x [greater than or equal to] 0.20, the films had PM properties ([M.sub.0] [approximately equal to] 0 emu [cm.sup.-3]) at room temperature.

(2) Magnetic properties of [L1.sub.0] ordered structure (annealed films): [M.sub.0] and [K.sub.u] decreased with increasing x. In the range of 0 [less than or equal to] x [less than or equal to] 0.50, the films had FM properties (500 [less than or equal to] [M.sub.0] [less than or equal to] 800 emu [cm.sup.- 3], 15 [less than or equal to] [H.sub.c] [less than or equal to] 43 kOe, 2.6 x [10.sup.7] [less than or equal to] [K.sub.u] [less than or equal to] 5.0 x [10.sup.7] erg [cm.sup.-3]), with their magnetic easy-axis perpendicular to the film plane. In the range of 0.40 < x < 0.80, the films could contain an antiferromagnetically ordered phase at temperatures below room temperature.

For instance, the [L1.sub.0] film with x = 0.20 had FM properties with a [K.sub.u] = 3.5 x [10.sup.7] erg [cm.sup.-3], whereas the disordered A1 film with x = 0.20 had PM properties at room temperature. The film could change from a FM to PM state as the [L1.sub.0] structure transforms into the A1 structure due to ion irradiation. These results suggest the possibility of applying the material system for nanopatterning method for high-density magnetic storage media.

https://doi.org/10.1155/2017/8949458

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this article.

Acknowledgments

This work was supported by the Japan Society for the Promotion of Science KAKENHI through its Grant-in-Aid for Young Scientists (A) (ID: JP15H05518). A part of this work was performed under the Interuniversity Cooperative Research Program (ID: 16K0099) of the Institute for Materials Research, Tohoku University. The authors thank Editage for English language editing.

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Takashi Hasegawa and Kosuke Ito

Department of Materials Science, Akita University, 1-1 Tegata Gakuen-machi, Akita 010-8502, Japan

Correspondence should be addressed to Takashi Hasegawa; takashi@gipc.akita- u.ac.jp

Received 22 March 2017; Revised 25 May 2017; Accepted 4 June 2017; Published 4 July 2017

Academic Editor: Gianluca Gubbiotti

Caption: Figure 1: Crystal structure and magnetic properties of disordered A1 [FePt.sub.1-x][Ru.sub.x] films before annealing (as-deposited films). (a) Out-of-plane XRD patterns. Magnetization curves of the films with x = (b) 0, (c) 0.10, (d) 0.20, and (e) 0.30. Magnetic fields (H) were applied perpendicular ([perpendicular to], dashed line) and parallel (//, solid line) to the film plane at 298 K. (f) Temperature dependence of magnetization (M-T curve) for the film with x = 0.30, measured from 300 K to 30 K with a field of 1 kOe applied parallel to the film plane after saturation. (g) %-dependence of [M.sub.0] at 298 K.

Caption: Figure 2: Crystal structure of [FePt.sub.1-x][Ru.sub.x] films after annealing at 1173 K for 4 h. (a) Out-of- plane XRD patterns and (b) in-plane XRD patterns. (c) Lattice constants (a, c, and c/a) as a function of x.

Caption: Figure 3: Magnetic properties of the [FePt.sub.1-x][Ru.sub.x] films after annealing at 1173 K for 4 h. Magnetization curves of the ordered [L1.sub.0] [FePt.sub.1-x][Ru.sub.x] films with x = (a) 0, (b) 0.10, (c) 0.20, (d) 0.40, and (e) 0.60. H was applied perpendicular ([+ or -], filled symbols) and parallel (//, open symbols) to the film plane at 298 K. (f) x-dependence of [M.sub.0] and [K.sub.u] at 298 K.

Caption: Figure 4: Temperature dependence of magnetization (M-T curves) for the [FePt.sub.1-x][Ru.sub.x] films after annealing at 1173 K for 4 h with x = (a) 0.20, (b) 0.40, (c) 0.60, and (e) 0.80, measured from 300 K to 30 K with a field of 1 kOe applied perpendicular to the film plane after saturating.

Caption: Figure 5: Schematics of the nanopatterning method using FM-PM phase change.
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Title Annotation:Research Article
Author:Hasegawa, Takashi; Ito, Kosuke
Publication:Advances in Materials Science and Engineering
Date:Jan 1, 2017
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