Spin glass like state with antiferromagnetic clusters in SrRuO3/SrIrO3 Superlattice Bin Pang1, Lunyong Zhang1,2*, Y.B Chen3*, Jian Zhou1 , Shuhua Yao1, Shantao Zhang1, Yanfeng Chen1 1. National Laboratory of Solid State Microstructures & Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China 2. Max Planck POSTECH Center for Complex Phase Materials, Pohang University of Science and Technology, Pohang 790-784, Korea 3. National Laboratory of Solid State Microstructures & Department of Physics, Nanjing University, 210093 Nanjing, China * Corresponding authors: Lunyong Zhang
[email protected] and Y.B Chen
[email protected]
ABSTRACT：Heterostructure interface provides a powerful platform to observe rich emergent phenomena, such as interfacial superconductivity, nontrival topological surface state. Here SrRuO3/SrIrO3 superlattices were epitaxially synthesized. The electrical and magentic properties of these superlattices were characterized. Rapidly decreased magnetization below the spin frozen temperature and a near stable residual magnetization at the low temperature limit, as well as two steps magnetization hysteresis loops are revealed. The magnetization relaxes following the Weron interaction cluster relaxation model. These results suggest the formation of antiferromagnetic clusters in a ferromagnetic matrix in the superlattices, which give rise to the spin glass like behaviors. Moreover, correlations between the spin glass relaxation and the weak electron localization are demonstrated. Our work maybe present an experimental evidence of the theoretical prediction that Ru deficiency would induce the formation of antiferromagnetic clusters in SrRuO3. KEYWORDS: Itinerant magnet, superlattices, spin glass, antiferromagnetic clusters,
I. INTRODUCTION In the past several decades, a large number of investigations have explored the physical properties of SrRuO3 samples with/without doping. Abundant unusual magnetic and transport behaviors have been revealed, due to the complex competitions amongst the charge, spin and orbit degree of freedoms in SRO 1-2. For instance, SrRuO3 is believed as a particular metallic ferromagnetic oxide with anomalous Hall effect 3 and strong anisotropic magnetic properties4. In recent years, the researches of SRO have been extended to multilayer system owing to the potential applications in 1
spintronic devices, leading to the discoveries of exchange bias effects and antiferromagnetism (AFM) generation in the superlattices integrating SRO and ferromagnetic (FM) mangnites5-7. Spin polarized two dimensional electron gas also was proposed in the SRO/SrTiO3 superlattice interfaces8. The SRO/PbTiO3 superlattice with single unit cell layers of SRO even demonstrates ferroelectricity9. These results suggest that SRO can strongly couple to other functional perovskite oxides to generate the novel physical effects. Recently, perovskite compounds with spin orbit interaction (SOI) was extensively studied since SOI supplies another spin modulation mechanism beyond the magnetic field and Pauli exclusive interaction. SOI would delicately modify the crystal structure and electronic band structure, then lead to many emergent physical states, such as topological insulating states 10, unconventional superconductivity11-12 and improper ferroelectricity13. Naturally, an intuitive question is whether the spin polarized electrons in SRO can be affected by SOI coming from a strong SOI material in a multilayer system? Similar idea has been discussed in metallic multilayers14-15, where SOI is believed to effect the magnetic anisotropy and orbital moment. However, similar studies in oxide multilayer are not reported so far, to our best knowledge. In the present paper, we epitaxially fabricated the superlattices combining SRO (fixed as 4 nm) layers and a paramagnetic strong SOI 5d oxide, SrIrO3 (SIO, varied thickness)16-17, spin glass like state was observed in these samples. Spin glass like state maybe originate from the coexistence of AFM clusters formed by impurites and a ferromagnetic matrix. Our work maybe present an experimental evidence of the theoretical prediction that Ru deficiency would induce the formation of antiferromagnetic clusters in SrRuO3.
II. EXPERIEMTNAL SECTION All the three groups of specimens were grown with five periods of 4nm-SRO/SIO on the chemical etching prepared flat (001)-SrTiO3 substrates with Ti-rich termination18, by a pulse laser deposition system (PLD, AdNaNo) at 2Hz repetition rate with substrate temperature at 800°C and 75mTorr oxygen atmosphere. The SIO layer thicknesses are 5nm, 10nm and 13nm respectively for the specimens (hereafter labeled as SL4/5, SL4/10 and SL4/13 as respectively), much thicker than the reported thickness for canting-AFM generation in SIO film, ~ 4 unit cells 19. The thicknesses of the alter layers were measured by transmission electron microscopy (TEM, Tecnai F20). All the magnetic properties characterizations were carried out in a Quantum Design Magnetic Property Measurement System (MPMS3), and all the electron transport measurements were performed with the standard four-probe method in a 9T-Quantum Design Physical Property Measurement System (PPMS). The film surface morphology was imaged through an Asylum Cypher atomic force microscopy (AFM) system.
III. RESULTS AND DISCUSSION Figure 1a-1c present the surface morphologies of the samples, two dimensional growth model terraces were demonstrated, consistent with the distinct flat interfaces imaged by TEM, see figure 1d-1f. Orthorhombic structures of the SRO and the SIO layers were confirmed by the scanning area electron diffraction (SAED) patterns shown in figure 1g and 1f, only slight splitting can be 2
distinguished at the high index spots. Therefore, these characterizations substantiate that SIO-SRO superlattices are epitaxially constrained. Figure 2a demonstrates the temperature-dependent magnetization under zero field cooling (ZFC) and 0.1Tesla field cooling (FC) conditions. Obviously for all the samples, the ZFC- and FC-curves are split from the Curie temperature Tc to ~250K, suggesting collapse of the long range ferromagnetism ordering (FM), which was not reported in any literatures about SRO. This long range FM collapse causes seriously decrease of Tc 20, about 60K with respect to that of normal bulk SRO (～160K) 21. At low temperatures, broad cusps are observed on the ZFC curves, similar features also have been observed in both single crystal and ceramic SRO samples22, as well as films23, which is often deemed as a signature of spin glass like state 22-24 (magnetic nanoparticle associated superparamagnetic state also demonstrates spilt FC-ZFC, but does not show hysteresis opened magnetization loop which further supplies a feature to exclude superparamgnetism in our samples, as seen in following discussions). It is also noticed that the ZFC curves transit to a near-stable residual magnetization in the low temperature limit region for all the samples, especially in the SL4/5 sample, this is different from the typical spin glass type ZFC curve 24. Seemly, there exists FM contributions. The magnetization relaxation experiments were further conducted at 10K, for that the samples were first zero field cooled to 10K and then kept in an out plane magnetic field of 0.1T for 5 minutes, after that the field was removed the magnetization data were collected, see figure 2b. Distinct magnetization relaxation were demonstrated, confirming the spin-glass like state displayed by the broad cusps ZFC-MTs. Quantitatively, the relaxation data cannot be well fitted by the Stretched function M(t)~exp[-(t/)α] which is usually held in simple spin glass systems relaxed through independent relaxation processes 24-25 (see supporting materials), but can be soundly tracked by the probabilistic model proposed by Weron for dielectric relaxation based on interplay relaxation processes assumption 26 and proved validity in spin glass relaxation by Pickup et.al25, that is M(t)~[1+k(t/)α]-1/k (see the solid lines in figure 2b and supporting materials for the fitted constants values). is defined as the relaxation time, k and α are fitting parameters. The Weron model takes into account of the hierarchy of relaxation processes coming from finite size dipole clusters, reminding the similar concept in the cluster spin glass system. In fact, the observed near stable residual magnetization transition of ZFC-MTs at low temperature limit region was often seen in cluster spin glass systems 27-28. Therefore, a cluster spin glass state was observed in our SRO/SIO superlattices. This conclusion is supported by magnetization loop (MB) measurements, as shown in figure 2c. Two steps magnetization behaviors were shown in the loops for all the samples, which can be more clearly seen from the differential curves dM/dB plotted in figure 2d, where two group peaks were exhibited. The peaks at low field corresponds to the coercive field of the main step magnetization, and those at high field indicate the second step magnetization. Similar two step magnetization loops were always observed in structure stacked by multiple magnetic layers, and was interpreted as a result of the stepwise switching of the different magnetic layers 7, 29. However, the SIO layers are paramagnetic since their thicknesses are much thicker than the critical thickness for generating canting-AFM SIO, 4 unit cells about 1.6nm19. Magnetic multilayer mechanism is therefore not applicable for our observations unless that new magnetic layers were formed at the 3
SRO/SIO interfaces. Nevertheless, it is natural to extend the idea of step switching of different magnetic layers to step switching of clusters with varied magnetic interactions. Here, the ZFC-MTs suggest antiferromagnetic clusters are formed, with ferromagnetic clusters or in a ferromagnetic matrix. The former induced the rapid magnetization decrease starting from Tf (spin frozen temperature, see figure 1a), the latter contributed the near stable residual magnetization at the low temperature limit region. We further investigated the charge transport behaviors. Figure 3a gives out the temperaturedependent resistances (R-T) of the samples. Magnetic transition induced resistance suppress were observed, similar that universally observed in SRO systems21. Metal-insulator transitions were distinctly demonstrated in the spin glass like state regime. To well fit the resistance in the regime, an electron-electron scattering associated three dimensional weak localization correction (WL) term T0.5 30 and a T1.5 term are necessary, i.e. R=R0-a1T0.5+a2T1.5. R0 , a1 and a2 are constants. The T1.5 term seems a familiar contribution in SRO systems 4, 31-32 and was attributed to electron magnon (EM) scattering 33, consistent with the dramatically variations of the spin configurations in the regime, as shown in figure 2a. Magnetoresistance, defined as MR=[R-R(B=0)]/R(B=0), and Hall effect (HE) measurements shown in figure 3b and 3c further reveal the strong correlation between charge transport and the spin frozen process. Obvious hysteresis is displayed on both of the MR and the HE in the spin frozen regime. In contrast, no distinct hysteresis were shown in temperatures higher than the spin frozen regime. The hysteresis of MR and HE is induced by the magnetization hysteresis. From MR (figure 3b), the hysteresis positive peaks are formed through two steps with the field increase. The first starts from zero field corresponds to the low coercive field magnetization process shown in the M-B curves (figure 2c) and bears a lower variation rate of resistance (see figure 3d), revealing a spin re-orientation process achieved by small angle rotation which is often realized in ferromagnetic domain wall dynamics, reminding the near stable residual ferromagnetic section on the ZFC-MT curves. In contrast, the second is achieved with a higher variation rate and in a narrow field region corresponding to the high coercive field magnetization process on the MB curves (the position and the width of their field regions are perfectly same, figure 3d and figure 2c), suggesting a large angle rotation spin re-orientation process often realized in antiferromagnetic phases, reminding the rapid magnetization decrease sections started from Tf on the ZFC-MT curves. (Anomalous) Hall effect traces with hysteresis were often observed in SRO systems, the coercive field is however always small, with an order of magnitude at 0.1Tesla34-35, originated from the itinerant ferromagnetic state of SRO. Here in our samples, the coercive field of HE is at the 0.1Tesla magnitude order in the temperature regime between Tc and Tf, but larger than 1Tesla in the spin frozen regime (T
