Spin Reversal Mechanism of a Perpendicular Exchange Bias System with an Antiferromagnet Incorporating a Parasitic Ferromagnetism

Abstract

Aluminum doping produced the parasitic magnetization (M para ) with high coercivity in 20 nm thin antiferromagnetic Cr 2 O 3 films. By using the large coercive force of M para , whose direction is the same as the antiferromagnetic Cr 2 O 3 surface spin, antiferromagnetic spin reversal can be detected through the vertical shift of the hysteresis loop of an antiferromagnetic-Cr 2 O 3 / ferromagnet perpendicular exchange bias (PEB) coupling system. By analyzing both antiferromagnetic and ferromagnetic spin reversals in response to external magnetic field and temperature, each spin reversal mechanism in a PEB system is clarified. It is found that the competition among Zeeman energy due to M para , exchange coupling energy, and antiferromagnetic anisotropy energy decides the antiferromagnetic spin direction and, accordingly, PEB sign under different cooling fields, different temperatures, and different swept magnetic fields for PEB systems. This work might contribute to a deep understanding of spin reversal mechanism in a perpendicular exchange bias system.Antiferromagnetic spin is difficult to detect and control, which enables its hardness for an external magnetic field. Moreover, an antiferromagnet (AFM) also has ultrafast spin dynamics to terahertz, which makes its switching much faster than that of a ferromagnet (FM). Therefore, AFMs are widely studied in many fields. [1][2][3][4] In particular, nonvolatile magnetic random-access memory (MRAM) is expected to replace volatile DRAM and SRAM in computing systems. [5] The exchange bias at the interface between AFM and FM is an important physical phenomenon, of which the hysteresis loop shift left (negative) or right (positive) of field axis could be obtained because of exchange bias coupling. [6] The in-plane exchange bias has already been used in hard-disk drive heads. Perpendicular exchange bias (PEB) devices attract more attention because of their higher integrated density compared with that of in-plane exchange bias. PEB devices have been used for spin pinning in magnetoresistance tunneling junction devices. Also owing to the difficult detection of the AFM spin, the spin reversal during PEB has been rarely reported. X-ray magnetic circular dichroism was widely used to detect the surface spin direction of AFM at certain temperature and external magnetic field. [7] However, as discussed later, the antiferromagnetic spin direction at certain temperature and external magnetic field differs for various cooling fields and scanning fields. In addition, there are few reports on the detection of antiferromagnetic spin direction during PEB under continuous changes of temperature and magnetic field.Cr 2 O 3 , an AFM with a Neel temperature (T N ) of 307 K, has two kinds of spin directions þÀþÀ and ÀþÀþ along the c-axis direction of its corundum structure, which can store information of 0 and 1. It has a magnetoelectric effect that can switch its spin directions and work as information writing for a MRAM. Reading can be realized through PEB-coupled ferromagnetic materials (Co...

Magnetoelectric Switching Energy of Antiferromagnetic Cr2O3 Used for Spintronic Logic Devices and Memory

“…Although Cr 2 O 3 is an antiferromagnet, it has a net boundary magnetism in its surface that is independent of surface morphology and contributes significantly to its ME effect. [43][44][45][46][47][48][49] Al doping induces a high-coercivity parasitic magnetism (intrinsically, it is a volume-dependent net boundary magnetism) along the same direction to the top-surface AFM spin of Cr 2 O 3 ; [50,51] this phenomenon has been used to track AFM spin reversal during the perpendicular exchange bias process. Based on this method, [50,51] it was found that the spin direction of Cr 2 O 3 in a Cr 2 O 3 /ferromagnet exchange-coupling system is determined by the competition between the Zeeman energy of net boundary magnetism of Cr 2 O 3 and exchange-coupling energy.…”

“…[43][44][45][46][47][48][49] Al doping induces a high-coercivity parasitic magnetism (intrinsically, it is a volume-dependent net boundary magnetism) along the same direction to the top-surface AFM spin of Cr 2 O 3 ; [50,51] this phenomenon has been used to track AFM spin reversal during the perpendicular exchange bias process. Based on this method, [50,51] it was found that the spin direction of Cr 2 O 3 in a Cr 2 O 3 /ferromagnet exchange-coupling system is determined by the competition between the Zeeman energy of net boundary magnetism of Cr 2 O 3 and exchange-coupling energy. Therefore, assuming that sufficient external (magnetic or electric) fields are applied, and a single AFM domain is obtained, a downward topsurface spin of Cr 2 O 3 could result in a negative exchange bias for Cr 2 O 3 /ferromagnet exchange-coupling system (Figure 1a).…”

“…In this work, the negative (positive) direction of the magnetic or electric field is defined as the downward (upward) direction. The dotted arrows in Figure 1 show either the net boundary magnetism [43][44][45][46][47][48][49] or parasitic magnetism caused by impurity doping, [50,51] which are in the same direction as the Cr 2 O 3 top-surface AFM spin. Suppose the magnitude of magnetic or electric fields is insufficient.…”

“…[52,53] Some other experimental and theoretical works also support this viewpoint. [45][46][47][48][49][50][51] Therefore, one may consider that ME switching of Cr 2 O 3 is similar to using the magnetic field to switch a ferromagnet, and both could be strongly affected by the interface. The interface is affected by an exchange-coupled ferromagnet, a spin-polarized paramagnet, or only surface morphology to obtain the above shift (asymmetric) curves for the ME switching of Cr 2 O 3 .…”

“…The different grain structures, as schematically shown in Figure 4b,c, may form different AFM anisotropy during the ME switching process; thus, reducing the anisotropy energy of the entire multicrystal Cr 2 O 3 by controlling its microstructure is a key step for lowering the energy barrier of ME switching. In addition, impurity doping seems to decrease the AFM anisotropy of Cr 2 O 3 ; [50,51] therefore, boron doping is also expected to decrease the ME switching energy of Cr 2 O 3 , except for an increase in Néel temperature. [12,13]…”