| With the development of information technology, people rely more and more on information, and more requirements are being placed on information storage devices. In the current magnetic memory devices,the information is written through changing the magnetization directions of recording bits by a magnetic field produced by current, as a result, it has the disadvantage of high power consumption. Therefore, it has become an urgent demand to find a new writing way with high-speed, high efficiency and low consumption. In this paper, we studied the electric-field (E-field) control of the magnetization states of magnetic thin films in ferroelectric/ferromagnetic heterostructures. The new way has many advantages, such as low-power, smaller-size, spot-operation et al, which has a wonderful application prospect in the future magnetic memory devices.In this work, we selected CoFe and CoNi thin films as magnetic layers, which have been widely used in magnetoresistance devices. The main contents are as follows:1) We studied E-field control of magnetic properties of CoFe thin film grown on PMN-30%PT ferroelectric substrate. In this structure, the E-field induced magnetic anisotropy field can be as large as 1.2kOe at 12kV/cm, corresponding to a magnetoelectric coefficient of 100Oe cm/kV. The large magnetoelectric effect is ascribed to the large piezoelectric coefficients of PMN-30%PT, the perfect soft magnetic properties and large magnetostriction coefficient of CoFe thin film. Under the function of E-field, the magnetization in CoFe thin film rotates 90°, namely, an interchange of hard and easy axes.2) The E-Field effect on the CoFe-SiO2 superparamagnetic nano-granular film grown on PMN-30%PT ferroelectric substrate was investigated. For the CoFe-SiO2 structure, CoFe thin film shows superparamagnetic behavior when the proportion of SiO2 is 45%. Under the function of an E-field, an in-plane anisotropy strain is induced in PMN-30%PT, which makes the CoFe magnetic moments align along the tensile strain directions. As a result, the superparamagnetic nano-granular CoFe films becomes single-domain granular thin films with in-plane anisotropy.3) The composition has a great influence on magnetism and magnetoresistance of CoNi films. We studied the E-Field effect on C043Ni57 and Co23Ni77 magnetic properties grown on PMN-30%PT and PMN-32%PT substrate. In Co23Ni77/PMN-32%PT structure, the effective magnetic anisotropy field induced by E-field can be up to 400Oe, corresponding to a magnetoelectric coupling coefficient of 40Oe cm/kV. Based on the contrary sign of magnetostriction coefficient of CoNi and CoFe, the E-field controlling of magnetization switching has opposite direction for CoNi and CoFe. In [CoNi/Cu/CoFe/Cu]n/PMN-PT multilayer heterostructure, the parallel magnetic moments between for CoFe and CoNi may turn to perpendicular under the function of an E-field, corresponding a low resistance state and a high resistance state, respectively. This effect can be used in the voltage-write magnetoelectric memory devices.4) The E-field manipulation of magnetic layer has a close relationship with the strain properties of the ferroelectric substrate. For the PMN-30%PT substrate, the E-field induced linear strain is larger than the nonlinear strain, so the magnetic properties change shows a volatile butterfly-like. For the PMN-32%PT substrate, the in-plane strain response shows large remnant strain due to the 71° and 109° ferroelectric polarization switching in the polarization process. The E-field induced magnetization switching shows a nonvolatile hysteresis loop behavior, which is very important for fabrication of voltage-write magnetic random memory devices. |
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