Low temperature noncollinear behavior in FePt nanogranular thin film system

Thin Solid Films 510 (2006) 280 – 285 www.elsevier.com/locate/tsf

Low temperature noncollinear behavior in FePt nanogranular thin film system A. Perumal a,b,*, Hyun-Seok Ko a, Sung-Chul Shin a a

Department of Physics and Center for Nanospinics of Spintronic Materials, Korea Advanced Institute of Science and Technology, Daejeon, 305 701, South Korea b Department of Physics, Indian Institute of Technology Guwahati, Guwahati, 781 039, India Received 9 May 2005; received in revised form 29 November 2005; accepted 9 December 2005 Available online 18 January 2006

Abstract We report the low temperature noncollinear magnetic behavior of direct current (DC) sputtered FePt thin films investigated by performing DC magnetization, thermoremanence, magnetic relaxation, and electrical transport measurements down to 1.8 K. The obtained results, interestingly, indicate a transition from ferromagnetic state to a low temperature disordered state where a collective frozen magnetic state with grain moments oriented randomly occurs. The magnetic relaxation and electrical resistivity measurements at low temperature support the spin-glass like phase, which diminishes and finally disappears with an applied field of moderate strength. We interpret the observed low temperature noncollinear magnetic behavior to be due to random freezing of grain moments. D 2005 Elsevier B.V. All rights reserved. Keywords: FePt thin films; Ferromagnetism; Magnetic relaxation; Superconducting quantum interference device magnetometer; Transmission electron microscopy

1. Introduction The understanding of magnetism in magnetic transition metals and their alloys has seen an interesting research topic in the area of condensed matter physics for more than 100 years [1– 3]. In particular the existence of disordered magnetic states at low temperature, possibly due to the frustration arising as a consequence of competing exchange interactions or random anisotropy, has been the focus of much attention of researchers in recent years. In the ultimate cases of frustration, a pure spinglass is formed [4] while the lower levels display characteristics of both extremes as long-ranged ferromagnetic order coexists with spin-glass order [5,6]. On warming such a system from low temperature, the spin-glass order melts first at a temperature, T SG, followed by the loss of ferromagnetic order at Curie temperature, T C. These properties were observed mostly in three-dimensional systems such as bulk crystalline and amorphous materials [7,8]. On the other hand, nanostructured materials (ferrofluids [9,10], interacting nanoparticles [11], and polymers containing * Corresponding author. Department of Physics, Indian Institute of Technology Guwahati, Guwahati, 781 039, India. E-mail address: [email protected] (A. Perumal). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.163

magnetic components [12]) exhibit unusual low temperature behavior suggestive of that observed in random or disordered magnetic systems due to the random distribution of anisotropy axis, interparticle interactions and surface effects [13]. However, in most cases the correlation between the magnetic behavior and the morphological origin as well as the nature of the low temperature phase for a system of interacting nanoparticles remains controversial and unclear. As the macroscopic behavior of nanostructured magnetic system is determined by the structure, size, morphology of the constituent phases and by the type and strength of the magnetic coupling between them, it is very much important to understand how such macroscopic properties arise from the interplay of microscopic parameters to design magnetic materials for specific applications. Particularly, in the area of information storage for the continuous increase of magnetic storage density it is crucial to know to what extent the independence of individual storage units can be maintained in the presence of interparticle coupling. Within this context, in this article we report the low temperature magnetic and electrical transport properties of direct current (DC) sputtered FePt alloy thin films. The choice of the materials has been dictated by the fact that these materials prepared at elevated substrate temperatures have high

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magnetic anisotropy constant (> 107 J m 3) due to the presence of L10 ordered structure, adjustable coercivity (0.5 to 1.5 T), high saturation magnetization (; 106 A m 1), and hence captured much attention of researchers in the area of ultrahigh-density information storage [14,15]. Although an extensive work has been performed from the application point of view [16 – 19], very few studies have been reported from the fundamental point of view [20 –22] due to various problems such as (i) the observation of random orientation of c-axis and/ or three-variant crystal domains in each FePt L10 particle and (ii) reduction of particle size into the nanoscale region may cause complex magnetic behavior of L10 ordered FePt, because the magnetic anisotropy of FePt L10 originates from the longrange ordering of alternately stacked Fe and Pt layers along the c-axis. Hence, there is a need to understand the thermal characteristics of the magnetization behavior of such small grains in order to achieve higher but stable recording densities. In this study, the low temperature magnetic and electrical transport properties of FePt alloy thin films were investigated by performing DC magnetization, thermoremanence magnetization, magnetic relaxation, and electrical resistivity measurements as a function of temperature down to 1.8 K. The obtained results clearly indicate a magnetic phase transition from high temperature ferromagnetic state to a low temperature disordered state similar to a spin-glass phase. Also, the aging behavior observed at low temperature fades out with increasing temperature as well as an applied magnetic field of moderate strength. This is the first experimental report that such an unusual behavior has been observed in an interacting FePt nanogranular thin film system and it is proposed that the noncollinear magnetic behavior at low temperature originates from the tendency of the freezing of random oriented magnetic moments. 2. Experimental details 50-nm-thick equiatomic FePt films were prepared directly on MgO (100) substrates using DC magnetron cosputtering of composite FePt target, made by putting Pt (4 N) chips on the Fe (4 N) target, with a base pressure of better than 1 10 4 Pa and sputtering Ar gas pressure of 0.667 Pa. The film composition was confirmed to be equiatomic by using the energy dispersive X-ray analysis. The crystal structure of the films was characterized by Rigaku X-ray diffractometer. The nanostructure and surface morphology of the films were investigated by transmission electron microscopy (TEM) and scanning electron microscopy, respectively. DC magnetization, magnetic relaxation measurements and magnetization versus field (M –H) hysteresis loops were measured using Quantum design superconducting quantum interference device (SQUID) magnetometer and vibrating sample magnetometer (VSM), respectively. The calibration on magnetization measurement was done by using standard Ni sample. The effective anisotropy energy of the samples was determined from the torque curves measured by using a torque magnetometer with an applied field of 1 T. The temperature dependent coercivity data were obtained from the M – H hysteresis loop measurements performed at different

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temperatures in the temperature range 1.8 to 300 K by using SQUID and VSM magnetometers. The electrical resistivity measurements under zero-field and in-field were performed at a constant current of 5  10 3 A in an applied field of up to 0.5 T in the temperature range 4.2 to 300 K, employing standard four-probe-DC technique. Considerable care was taken to ensure that the current– voltage contacts were collinear so that no Hall voltage component contributed to the magnetoresistive anisotropy. The cooling and heating rates for the magnetization and electrical resistivity measurements were 5 and 2 K/min, respectively. 3. Results and discussion 3.1. Magnetic properties Fig. 1(a) displays the nanostructure of 50-nm-thick FePt film revealed via TEM. The mean particle size, calculated from the corresponding histogram (Fig. 1(b)), is in the range 3 to 5 nm and separated from each other by a grain boundary with a thickness of about 1 nm. The average grain size calculated from the FePt(200) peak in the X-ray diffraction data by employing Scherrer’s formula is also in good agreement with the microstructural studies. The room temperature magnetic measurements performed by using VSM, SQUID (Fig. 1(c)) and torque magnetometers suggest that the sample has ferromagnetic nature with the coercivity of about 1.13 A m 1 in in-plane direction and with an effective anisotropy constant of 3  104 J m 3, respectively. This supports the absence of any superparamagnetic behavior at room temperature in the samples. Moreover, recent study on FePt films by means of Mossbauer measurements at room temperature suggests that the Fe moments are oriented randomly for the as-deposited films [23]. To understand the nature of the magnetic behavior at low temperature, the temperature dependent magnetization (M –T) measurement was performed. In the M –T measurement, the sample was cooled in zero-field to a lower temperature [Zerofield-cooled (ZFC)]. The magnetization was then recorded on heating the sample to high temperature under a constant applied field. Similarly, the sample was cooled in a constant applied field to a lower temperature and then the magnetization was recorded on heating the sample to high temperature with maintaining the same applied field [Field-cooled (FC)]. Fig. 2 shows the temperature dependence of M ZFC and M FC curves measured under a 2.5 A m 1 applied field. While M FC is observed to increase with lowering the temperature in the whole investigated temperature range (300 to 1.8 K), below about T = 50 K M ZFC decreases. The bifurcation between M FC and M ZFC curves indicates the existence of non-equilibrium magnetization state below 50 K in the M ZFC case. The M FC and M ZFC curves depart from one another at a temperature much higher than the maximum point, T max, in M ZFC curve (Fig. 2). In addition, the T max point shifts to lower temperature with increasing magnitude of external applied magnetic field (inset of Fig. 2) and finally disappears for the applied field more than 9 A m 1. As also seen in ferrofluids and

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interaction between the particles based on the observation that the peak position in M ZFC curve did not change with measurement time, respectively. However, the room temperature magnetic properties of the presently investigated system have ferromagnetic phase, while the other systems are superparamagnetic phase with no coercivity in magnetic hysteresis loops. In order to clarify these features and to obtain more information on the magnetic phase below the bifurcation point, the ZFC magnetic relaxation measurement was performed at different temperatures as follows: i) the sample was cooled in zero-field down to a temperature, called T mea, ii) at T mea, 10 s passed (waiting time, t wait) before the application of constant magnetic field (H e = 0.44 A m 1), and then iii) the time variation of M ZFC was recorded after the stabilization of field. The field was removed at the end of first measurement and the temperature was raised above the bifurcation point, then lowered again to the same T mea and the time variation of magnetization recorded again under the same applied field condition after passing t wait = 103 and 105 s. The relaxation measurement of M ZFC at T mea = 1.8 and 225 K with an external applied field (H e) of 0.44 A m 1 is shown in Fig. 3. When the field is first turned on, following an immediate jump (inset of Fig. 3(a)), a slow logarithmic relaxation takes place. The magnetization relaxation with time and the value of magnetization is clearly dependent on the waiting time (beyond the experimental error) at 1.8 K, i.e., the M ZFC value is lower in the measurement carried out after waiting for 103 and 105 s [Fig. 3(a)]. The waiting time effect is reduced with increasing temperature and finally disappears at T mea = 225 K (Fig. 3(b)) within the experimental error. The waiting time dependence of magnetization at 1.8 K supports the feature of collective frozen magnetic state that exists at low temperature. However, the magnetic relaxation measurements performed under an applied

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H (A m-1) Fig. 1. [a] Transmission electron micrograph of Fe50Pt50 thin film (the horizontal bar in the TEM micrograph scales 10 nm), [b] their grain size distributions and [c] room temperature M – H hysteresis loop of Fe50Pt50 thin film.

nanoparticles system, these facts distinguish the presently investigated system from a conventional spin-glass system where the M FC magnetization curve departs from the M ZFC magnetization curve just at maximum point and shows a plateau below the maximum [24]. A similar behavior of magnetization variation with temperature has been observed in pure Fe thin films grown on MgO (001) substrates at elevated temperatures [25] and frozen ferrofluid systems [26] and typically attributed to the presence of a distribution of particle size that gives rise to a corresponding distribution in relaxation time and to the dipolar

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A. Perumal et al. / Thin Solid Films 510 (2006) 280 – 285

M (105 A m-1)

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applied field of 0.5 T. At 1.8 K, the field was removed after waiting for 100 s and the remanence was measured with increasing temperature up to 300 K. The thermoremanence magnetization increases with decreasing temperature as expected for ferromagnetic system, but below about 50 K it decreases with temperature. The decrement at low temperature is consistent with the onset and development of a collective frozen magnetic state.

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The temperature dependent electrical resistivity measurement was performed under different applied magnetic fields in the range 0 to 0.5 T by employing standard four-probe technique. The temperature dependence of the normalized electrical resistivity [r(T) = q(T) / q(300 K)] for Fe50Pt50 thin film is shown in Fig. 5. This method of presenting data helps to eliminate possible uncertainty in the measurement of sample thickness. The following important observations can be made from the comparison of magnetic and transport parameters obtained from magnetization and electrical resistivity measurements, respectively:

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Time (seconds) Fig. 3. Time dependent zero-field-cooled magnetization for Fe50Pt50 thin film measured at [a] T = 1.8 and [b] T = 225 K for different waiting times (t wait = 10, 103 and 105 s). Inset: time dependent zero-field-cooled magnetization plotted in normal scale.

field larger than 1 T in M ZFC case destroy the aging feature indicating that the system transfers to an equilibrium state and no collective dynamics exists on the applied field of moderate strength. The existence of the low temperature non-equilibrium magnetic states is also supported by the thermoremanence magnetization measurement, as shown in Fig. 4. The measurement was performed using SQUID magnetometer as follows: First, the sample was cooled down to 1.8 K in an

(i) The change in resistivity (Dr(T) = r max(T) r min(T)) is about 8% under zero applied field and increases to about 10% under the application of external magnetic field of 0.5 T. (ii) Under zero-field, the resistivity decreases continuously with decreasing temperature from 300 K to a temperature, called T min, and increases with further decreasing temperature. (iii) The value of T min, interestingly, found to shift to lower temperature with increasing external applied field, i.e., the upturn behavior of the electrical resistivity disappears under the action of a magnetic field, similar to the behavior of bifurcation point observed in the magnetization. 6.0

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T (K) Fig. 4. Thermoremanence magnetization curve is plotted as a function of temperature for Fe50Pt50 thin film.

Fig. 5. Temperature dependent normalized resistivity for Fe50Pt50 thin film measured under different external applied fields. Inset: temperature dependent coercivity for Fe50Pt50 thin film.

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4. Conclusion The results obtained from both electrical resistivity and the magnetization measurements suggest that the low temperature electrical-transport behavior is dominated by the spindependent scattering [27,28], which could be systematically controlled by the application of external magnetic field. Moreover, it indicates that by decreasing the temperature below T min, the soft ferromagnetic system observed at higher temperature evolves toward a noncollinear magnetic structure. For temperature above T min, the random oriented spins [23] are magnetized along with the adjacent grains by contact with their molecular field and the whole system is expected to behave as a soft ferromagnet with lower coercivity. On decreasing the temperature below T min, the random oriented spins start to freeze toward the ground state reducing the ferromagnetic exchange coupling between grains, which is possibly a main source for this upturn in electrical resistivity due to the enhanced magnetic scattering of conduction electrons. The coercivity (H C) measurement as a function of temperature (inset of Fig. 5) shows a sudden increment in the H C values at a temperature close to the upturn in the electrical resistivity. The present experimental results can be interpreted from the generalized random anisotropy model (RAM) [29,30], as discussed previously for other nanocrystalline systems [31]. According to the RAM for two phase system, the ferromagnetic exchange interaction between two adjacent grains (n ij) and exchange correlation length through the inter-grain boundaries (n g) are defined as n ij = aA (where A is exchange coupling coefficient (A = 10 11 J m 1) and a is the factor ranging between 0 and 1) and n g = ((cA) / K) (where K is experimental anisotropy constant of about 3  104 J m 3 at room temperature), respectively. The observed ferromagnetic behavior at room temperature could be explained from the above relations, since the average crystallite size (d ; 4 nm) is smaller than the n g (> 15 nm, if c = 1) together with the assumption that the grain boundary thickness (d) is smaller than the exchange correlation length of the interface (n i). In addition, the condition n i > d implies that the spins at the grain boundary are oriented in the direction imposed by the surrounding environment and hence there exists a ferromagnetic exchange interaction between crystallites and the whole system is ferromagnetic and soft. By decreasing the temperature, (i) the n g decreases, (ii) the anisotropy at the boundary grows large, and (iii) the links holding the boundary spins aligned to the magnetization vectors of the surrounding crystallites are progressively severed. As a result, the grains are uncoupled but they interact strongly through dipole– dipole interactions [32], which can be either ferromagnetic or antiferromagnetic. The consequent mixing of these interactions which compete with local anisotropy is responsible for the random freezing of the grain moments, which leads to a noncollinear magnetic structure at lower temperature and therefore results the enhanced scattering in the transport properties at low temperature. The freezing of grain moments entails that the whole system may act as a spin-glass like system, which is fully coherent with the present experimental results.

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