Gd bilayers with TC≪TN

Materials Letters 168 (2016) 200–202

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Anomalous exchange bias in FeMn/Gd bilayers with TC {TN Zhiwei Jiao a,n, Huanjian Chen a, Weidi Jiang a, Jianfeng Wang a, Dan Cao a, Yun Zhou a, Yanliang Hou b, Quanlin Ye b a b

Department of Physics, China Jiliang University, Hangzhou 310018, People's Republic of China Department of Physics, Hangzhou Normal University, 310036, People's Republic of China

art ic l e i nf o

a b s t r a c t

Article history: Received 8 September 2015 Received in revised form 19 November 2015 Accepted 16 January 2016 Available online 18 January 2016

Exchange coupling has been investigated in FeMn/Gd bilayers where the Curie temperature (TC) of Gd is much lower than the Néel temperature (TN) of FeMn. The coercivity (HC) exhibits unusual temperature dependence. As temperature increases, it decreases at first, then goes up to the maximum and keeps it in medium temperature range, finally drops rapidly and disappears at TN. The exchange bias field (HE) is negative initially and its magnitude decreases monotonically with increasing temperature, then it turns to be positive, finally disappears at high temperatures. The anomalous HC(T) behavior may be ascribed to the non-collinear coupling interaction at the FeMn/Gd interface. The positive HE can be attributed to the antiferromagnetic coupling between the interfacial Gd and FeMn moments. Moreover, the antiferromagnetic ordering adjacent to the FeMn layer may be responsible for inducing the exchange coupling at TC oT oTN. & 2016 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials Thin films Sputtering Interfaces

1. Introduction Exchange bias (EB) has attracted a lot of attention in the past decades, mainly due to the complicated mechanism for the ferromagnet (FM)/antiferromagnet (AF) coupling and its technological applications in high density magnetic recording, magnetic resonance settings and spin-valve field-sensing devices [1–8]. When a FM/AF EB system is cooled across the Néel temperature (TN) of the AF in an applied field, the FM hysteresis loop is shifted along the field axis by an amount. This shift is named as the exchange bias field (HE). In previous studies of FM/AF bilayers, dependencies of HE on FM or AF layer thickness and temperature have been widely investigated [3,6,9]. Additionally, the significant role of cooling field (HFC) plays in the EB systems has been researched as well. Ambrose and Cai observed a strong dependence of exchange bias on HFC when the magnitude of cooling field was small [10,11]. Nogués et al. found that the sign of HE changed from negative to positive with increasing HFC [4,12,13]. To date, in most experimental and theoretical exchange bias researches on FM/AF systems, the Néel temperature (TN) of AF is much lower than the Curie temperature (TC) of FM [2,4,9,10]. Recently, many studies of FM/AF thin films with TC oTN have been established. Wu et al. investigated the temperature dependence of exchange coupling in a-(Fe0.1Ni0.9)80B20/CoO bilayers where TC is n

Corresponding author. E-mail address: [email protected] (Z. Jiao).

http://dx.doi.org/10.1016/j.matlet.2016.01.084 0167-577X/& 2016 Elsevier B.V. All rights reserved.

slightly lower than TN. They found that HE still existed at TC o To TN [14]. Cai et al. observed that HE persisted to TN, whereas HC disappeared at TC in a-Fe4Ni76B20/CoO bilayers with TC { TN [11]. In our previous studies, we reported the anomalous exchange coupling in Cr/Gd bilayers where TC of Gd is close to TN of Cr and an unexpected coercivity enhancement in Tb/Cr multilayers where the TC of Tb is much lower than the TN of Cr [15,16]. In this work, we have investigated the magnetic properties of FeMn/Gd bilayers with TC { TN, in order to provide a new insight into the features of exchange coupling. Gd is a ferromagnetic heavy rare-earth metal with complex magnetic structure in which the relative angle between the c axis and the easy axis of magnetization varies with temperature [17– 19]. The Curie temperature of bulk Gd is about 293 K [17,18]. The γ -FeMn with (111) texture has a long-range antiferromagnetic ordering with TN ¼425 K [20,21]. Neutron scattering result shows that the Fe-Fe spins align parallelly, whereas the neighboring MnMn and Mn-Fe spins align anti-parallelly [6,22].

2. Experimental details The Cu(15 nm)/FeMn(15 nm)/Gd(tGd)/Cu(30 nm) films were grown on Si(001) substrates by dc magnetron sputtering in a 0.5 Pa Ar atmosphere with a base pressure of 5  10  5 Pa. The thickness of Gd layer tGd are 20 and 35 nm. The thicknesses of each single layer were measured by SEM (supra 55). The bottom and top Cu layer is used to promote the growth of fcc-FeMn

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antiferromagnetic phase and protect the sample from oxidation, respectively. The crystalline structure of the samples were investigated by Rigaku/SmartLab diffractometer with Cu Kα radiation (λ ¼ 0.15418 nm). The magnetic hysteresis loops of the films were measured with a vibrating sample magnetometer (VSM, Quantum Design) from 60 K to 440 K. Prior to the magnetic measurement, the samples were heated to 450 K in zero field and cooled to the target temperatures in an external field of 10 kOe parallel to the film plane.

3. Results and discussions Fig. 1 shows X-ray diffraction pattern of the Cu(15 nm)/FeMn (15 nm)/Gd(35 nm)/Cu(30 nm) film. The (100), (101) and (110) peaks of Gd indicate that the Gd layer exhibits poly-crystalline structure. It shows a FeMn (111) peak at 43.4°, which facilitates the exchange coupling [6]. The inset in Fig. 1 shows the cross-sectional SEM image of the film by which the thickness of each single layer can be obtained. Fig. 2 shows the representative hysteresis loops for Cu(15 nm)/FeMn(15 nm)/Gd(20 nm)/Cu(30 nm) film. Obviously, a shifted hysteresis loop with a large value of HE appears at low temperature, as shown in Fig. 2(a). Both HC and HE decrease at higher temperatures. It is striking that the ferromagnetic behavior of the thin film still exists at T4 TC, as exhibited in Fig. 2(c) and (d). Similar phenomenon were observed in Refs. [11,16,21,23]. As discussed in Refs. [11,23], when temperature ranges from TC to TN, the effective field produced by AF ordering can induce the paramagnetic moments at the interface to be strongly coupled to the AF spins, giving rise to the ferromagnetic behavior and exchange coupling. This mechanism can be applicable for the FeMn/Gd bilayers. The temperature dependence of HC and HE for the FeMn (15 nm)/Gd(20 nm) and FeMn(15 nm)/Gd(35 nm) bilayers are presented in Fig. 3. Obviously, for different thickness of Gd layer, both HC(T) and HE(T) of the bilayers show similar tendency, respectively. HC exhibit anomalous behavior vs. temperature, which is completely different from what has been revealed in traditional FM/AF films [6]. As temperature increases, HC initially decreases

Fig. 1. X-ray diffraction pattern of Si/Cu(15 nm)/FeMn(15 nm)/Gd(35 nm)/Cu (30 nm) film. Inset shows the cross-sectional SEM image of the film.

Fig. 2. Representative hysteresis loops measured after field cooling in an applied field of 10 kOe at 60 K (a), 160 K (b), 300 K (c), 400 K (d) for the Cu(15 nm)/FeMn (15 nm)/Gd(20 nm)/Cu(30 nm) film. Inset shows the amplified part of the loop at 300 and 400 K, respectively.

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large value. At T 4320 K, HC decreases rapidly, which may be ascribed to the thermal fluctuations [25]. The temperature dependence of HE for the bilayers exhibit some striking features as well. When temperature increases from 60 to 240 K, HE is negative and shows usual behavior [6]. Its magnitude decreases to zero monotonically. At 240oT o360 K, HE turns to be positive. And the strength of the positive HE in the FeMn(15 nm)/Gd(20 nm) bilayer is larger than that of the FeMn (15 nm)/Gd(35 nm) bilayer, which suggests that the antiferromagnetic coupling is an interface effect as well. Finally, it vanishes at T Z360 K. As discussed above, the Gd moments may align antiparallelly to the FeMn net spin in the temperature range of 240–360 K, which induce the positive HE.

4. Conclusions In summary, we have investigated the exchange coupling of FeMn/Gd bilayers with TC { TN. The anomalous temperature dependence of HC and the crossover from negative to positive HE have been found. Both HC and HE exists not only in ToTC, but also in TC oT oTN. The unusual HC(T) behavior is discussed in terms of the non-collinear coupling at the FeMn/Gd interface and the antiferromagnetic spins close to the FeMn layer. The positive HE might result from the antiferromagnetic coupling interaction between the Gd and FeMn spins at the AF/FM interface.

Acknowledgments Fig. 3. Temperature dependence of the coercivity HC and exchange bias HE of the FeMn (15 nm)/Gd (35 nm) and FeMn (15 nm)/Gd ( 20 nm) bilayer, respectively.

quasi-linearly at 60 oT o140 K, then increases to the maximum at 240 K, and varies very little in the temperature range of 240– 320 K, then decreases rapidly and disappears at T ¼430 K (close to the TN of FeMn). The anomalous HC(T) behavior may be attributed to the non-collinear interfacial exchange coupling. The easy axis direction of magnetization in Gd layer varies with temperature [15,17,18,24]. Therefore, the interfacial Gd spins and FeMn spins may be not always collinear in the whole temperature range, bringing about a non-collinear exchange coupling at Gd/FeMn interface. At T o165 K, as the angle between the c axis and the easy axis of Gd magnetization ranges from 34° to 90° with the increasing temperature [15], the exchange coupling between the interfacial Gd and FeMn spins may progressively reduce, leading to the decrease of HC. At 165 oTo 225 K, the easy axis gradually turns to be perpendicular to the c axis [17,24]. In this temperature range, the projection of Gd magnetization into the film plane might increase with temperature due to the thermal activation, which enhances the interfacial exchange coupling, then resulting in the increase of HC. When temperature increases from 225 K to 245 K, the easy axis departs from the basal plane to the c axis continuously [17,24]. The spins in Gd layer may turn to antiparallel to the interfacial FeMn spins, which increases the strength of exchange coupling and gives rise to an enhancement of HC. At T 4245 K, the c axis is the easy axis [24] and the Gd spins may align antiparallelly to the FeMn spins. Hence the interaction between the interfacial Gd and FeMn moments turns to be antiferromagnetic coupling. This strong coupling interaction brings about a large HC and a positive HE. When temperature is near but larger than TC, the FeMn net spins adjacent to the interface may cause strong interfacial exchange coupling, then making HC remain

This work is supported by the National Natural Science Foundation of China (Grant nos. 51271172 and 11204283) and Science Foundation of Zhejiang province (Grant nos. LQ14A040007 and LY13E020005).

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