Complex magnetic response in magnetic tunnel junctions determined via magnetic and transport measurements
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Complex magnetic response in magnetic tunnel junctions determined via magnetic and transport measurements R.Guerrero1 , V.V.Pryadun1 , F.G.Aliev1∗ , R.Villar1 , J.L.Martinez2 and J.Moodera3 (1) Dpto. de Fisica de la Materia Condensada, C-III, Universidad Autonoma de Madrid, 28049, Madrid, Spain (2) ICMM-CSIC, Cantoblanco, 28049, Madrid, Spain and (3) Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA We have studied the low frequency complex magnetization dynamics in Co/Al2 O3 /N i80 F e20 magnetic tunnel junctions (MTJs) at temperatures between 4.2K and 300K. The measurements were carried out by using two different experimental techniques. The first method probes directly magnetic properties via DC magnetization and AC susceptibility, while the second one measures AC magnetization dynamics of the ferromagnetic electrodes near the cross area, which is related to the tunnelling resistance. * - corresponding author: [email protected]
INTRODUCTION
Magnetic tunnel junctions (MTJ) with highly reliable magnetoresistance [1] are promising candidates for non-volatile memory, next generation of magnetic field sensors hard drive heads as well as important parts of more complicated magnetic nanostructures [2]. It was found recently that the magnetic domain walls (DWs) formed in the magnetic electrodes and correspondingly within cross area, may significantly affect operation of the MTJs, resulting in some cases even in long term degradation of the devices [3]. In addition, it is known that DWs strongly influence the noise of the magnetoresistive sensors [4, 5]. In their study of magnetic noise in magnetic multilayers Hardner et al. [4] proposed a simple method for evaluating dynamic magnetic properties of the systems with giant magnetoresistance (GMR) by studying the response in the electron transport of the structure subject to both DC and AC magnetic fields. Although DC transport and magnetic properties of MTJs
Q2.9.2
are reasonably well understood, little is known [6] about the AC magnetic and transport responses of MTJs. Knowledge of dynamic magnetic properties of MTJs could provide information about mechanisms of magnetization reversal, domain wall mobility, disorder and other characteristics of MTJs, important both for fundamental and applied research, and not accessible in the DC experiments. Indeed, a method proposed for investigation of the DW dynamics in magnetic nanowires by employing GMR effect as a detector [7], could not be directly applied to the standard MTJs because their geometry permits twodimensional movement of the DWs. A recent demonstration of the applicability of the tunnel magnetoresistance (TMR) signal for analysis of the DWs structure in MTJs, completed by magnetic force microscopy [8] or magnetooptic analysis [9], reveals some interesting static, but not dynamic properties of the domain walls. This paper presents an investigation of the low frequency (below 100Hz) AC magnetization response of MTJs studied both via direct magnetic measurements and by using the
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