Kumar Ashok, Sharma G L, Katiyar R S, Pirc R, Blinc R, Scott J F
Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, PR 00931-3343, USA.
J Phys Condens Matter. 2009 Sep 23;21(38):382204. doi: 10.1088/0953-8984/21/38/382204. Epub 2009 Aug 27.
Numerous authors have referred to room-temperature magnetic switching of large electric polarizations as 'the Holy Grail' of magnetoelectricity. We report this long-sought effect, obtained using a new physical process of coupling between magnetic and ferroelectric nanoregions. Solid state solutions of PFW [Pb(Fe(2/3)W(1/3))O(3)] and PZT [Pb(Zr(0.53)Ti(0.47))O(3)] exhibit some bi-relaxor qualities, with both ferroelectric relaxor characteristics and magnetic relaxor phenomena. Near 20% PFW the ferroelectric relaxor state is nearly unstable at room temperature against long-range ferroelectricity. Here we report magnetic switching between the normal ferroelectric state and a magnetically quenched ferroelectric state that resembles relaxors. This gives both a new room-temperature, single-phase, multiferroic magnetoelectric, (PbFe(0.67)W(0.33)O(3))(0.2)(PbZr(0.53)Ti(0.47)O(3))(0.8) ('0.2PFW/0.8PZT'), with polarization, loss (<1%), and resistivity (typically 10(8)-10(9) Ω cm) equal to or superior to those of BiFeO(3), and also a new and very large magnetoelectric effect: switching not from +P(r) to -P(r) with applied H, but from P(r) to zero with applied H of less than a tesla. This switching of the polarization occurs not because of a conventional magnetically induced phase transition, but because of dynamic effects: increasing H lengthens the relaxation time by 500 × from<200 ns to>100 µs, and it strongly couples the polarization relaxation and spin relaxations. The diverging polarization relaxation time accurately fits a modified Vogel-Fulcher equation in which the freezing temperature T(f) is replaced by a critical freezing field H(f) that is 0.92 ± 0.07 T. This field dependence and the critical field H(c) are derived analytically from the spherical random bond random field model with no adjustable parameters and an E(2)H(2) coupling. This device permits three-state logic (+P(r),0,-P(r)) and a condenser with >5000% magnetic field change in its capacitance; for H = 0 the coercive voltage is 1.4 V across 300 nm for +P(r) to -P(r) switching, and the coercive magnetic field is 0.5 T for +P(r) to zero switching.
众多作者将室温下大电极化的磁开关效应称为磁电效应的“圣杯”。我们报道了通过磁纳米区域与铁电纳米区域之间耦合的新物理过程获得的这一长期寻求的效应。PFW[Pb(Fe(2/3)W(1/3))O(3)]和PZT[Pb(Zr(0.53)Ti(0.47))O(3)]的固态溶液表现出一些双弛豫特性，兼具铁电弛豫特性和磁弛豫现象。在PFW含量接近20%时，铁电弛豫态在室温下对长程铁电性几乎不稳定。在此，我们报道了在正常铁电态和类似于弛豫体的磁淬灭铁电态之间的磁开关。这产生了一种新型的室温单相多铁性磁电体，(PbFe(0.67)W(0.33)O(3))(0.2)(PbZr(0.53)Ti(0.47)O(3))(0.8)（“0.2PFW/0.8PZT”），其极化、损耗（<1%）和电阻率（通常为10(8)-10(9)Ω·cm）等于或优于BiFeO(3)，并且还产生了一种新的且非常大的磁电效应：施加磁场H时，不是从+P(r)切换到 -P(r)，而是在小于1特斯拉的磁场H作用下从P(r)切换到零。这种极化的切换不是由于传统的磁诱导相变，而是由于动态效应：增加磁场H会使弛豫时间延长500倍，从<200纳秒增加到>100微秒，并且它强烈地耦合了极化弛豫和自旋弛豫。发散的极化弛豫时间精确地符合一个修正的Vogel-Fulcher方程，其中冻结温度T(f)被临界冻结场H(f)所取代，H(f)为0.92±0.07特斯拉。这种场依赖性和临界场H(c)是从无可调参数的球形随机键随机场模型和E(2)H(2)耦合中解析推导出来的。该器件允许三态逻辑（+P(r),0,-P(r)）以及一个电容在磁场变化时变化>5000%的电容器；对于H = 0，在300纳米厚度上从+P(r)到 -P(r)切换时的矫顽电压为1.4伏，从+P(r)到零切换时的矫顽磁场为0.5特斯拉。
