La0/ YBa2Cu3O7/La0 .67Ca0.33MnO3异质结构中静磁耦合与交换耦合的交叉
Crossover from magnetostatic to exchange coupling in La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3 heterostructures
摘要：
研究了不同厚度（d-YBCO = 20 ~ 50 nm）的YBa_2Cu_3O_4（YBCO）超导层（S层）对两层La_（0.67）Ca_（0.33）MnO_3（LCMO）铁磁层间磁耦合的影响（F层，厚度d-LCMO = 50 nm）的F/S/F异质结构（HS）的研究，300 K和磁场（H）范围= 0 - 10 kOe。所有的HS超导临界温度（Tc）从= 78降低到36 K的d-YBCO的减少，而铁磁有序温度Tm = 250 K没有改变太多。系统地测量了所有HS在T > Tc和T < Tc时的M-H回线，结果表明：（a）d-YBCO >= 30 nm时，高于Tc时的两步磁反转转变为低于Tc时的四步磁反转;（b）对应于额外的两个切换步骤的磁场及其幅度示出了随T和d-YBCO的特征演化，以及（c）具有d-YBCO = 20 nm显示出完全不同的行为，其中高于Tc的两步磁反转继续在低于Tc时持续，并在T << Tc时转化为单步反转。前两个结果表明，在两个F/S界面上的磁畴和涡旋之间的静磁耦合，导致反转动力学不同于LCMO层内的深处。而c的结果揭示了LCMO层之间通过超导YBCO层的间接交换耦合，这是C.A.R.理论上所预期的纳米级F/S/F HS中铁磁性和超导性共存的明确实验证据。Sa de Melo（Physica C 387，17-25（2003））。
英文摘要：
The influence of YBa2Cu3O4 (YBCO) superconductor layer (S-layer) with varying thickness d-YBCO = 20 to 50 nm on the magnetic coupling between two La0.67Ca0.33MnO3 (LCMO) ferromagnet layers (F-layer, thickness d-LCMO = 50 nm) in F/S/F heterostructures (HSs) was investigated by measuring global magnetization (M) in a temperature (T) range = 2 - 300 K and magnetic field (H) range = 0 - 10 kOe. All the HSs were superconducting with critical temperature (Tc) decreasing from = 78 to 36 K with decrease in d-YBCO, whereas the ferromagnetic ordering temperature Tm = 250 K did not change much. Systematically measured M-H loops of all HSs at both T > Tc and T < Tc show three main results- (a) the two step magnetic reversal above Tc converts into a four step reversal below Tc in HSs with d-YBCO >= 30 nm, (b) the magnetic field corresponding to the additional two switching steps and their magnitude show characteristic evolution with T and d-YBCO and (c) the HS with d-YBCO = 20 nm shows radically different behaviour, where the two step magnetic reversal above Tc continues to persist below Tc and converts into a single step reversal at T << Tc. The first two results indicate magnetostatic coupling between the magnetic domains and the vortices across the two F/S interfaces resulting in reversal dynamics different from that deep within the LCMO layers. Whereas, the result c reveals indirect exchange coupling between LCMO layers through the superconducting YBCO layer, which is a clear experimental evidence of coexistence of ferromagnetism and superconductivity in nm scale F/S/F HSs expected theoretically by C.A.R. Sa de Melo (Physica C 387, 17-25 (2003)).
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