Glassy ferromagnetism and magnetic phase separation in La1-xSrxCoO3
Glassy ferromagnetism and magnetic phase separation in La1-xSrxCoO3
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DOI：
10.1103/physrevb.67.174408
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发表时间：
2003-05
影响因子：
3.7
通讯作者：
J. Wu;C. Leighton
中科院分区：
文献类型：
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作者：
J. Wu;C. Leighton
我们提出了一个全面的调查结果的直流磁化强度，交流磁化率，和磁输运性质的玻璃铁磁体La 1 - x Sr x CoO 3。研究的组成范围从端元LaCoO 3（x=0.0）到x=0.7。这些材料最近引起了人们的关注，主要是由于LaCoO 3中的自旋态转变现象和有限x下磁基态的不寻常性质。在本文中，我们提出了一个一致的图片的磁性行为的La 1 -xSrxCoO 3的短程铁磁有序和本征相分离。在高Sr掺杂（x>0.2）下，该体系表现出非常规的铁磁性（居里温度高达250 K），这可以用短程有序铁磁团簇的聚结来解释。Brillouin函数拟合了磁化强度的温度依赖关系，高温Curie Weiss行为表明Co 3 +和Co 4 +离子均处于中间自旋态.在较低的Sr掺杂（x<0.18）下，系统进入显示自旋玻璃和铁磁体两者的特性的混合相。在零场冷却的直流磁化尖点，在交流磁化率和时间依赖性的影响，在直流和交流磁性能的频率依赖性的峰值都指向玻璃态行为。另一方面，场冷却导致相对较大的类铁磁矩，零场冷却和场冷却磁化在不可逆点分叉。即使在该区域以上的x=0.2的交流磁化率的异相分量显示频率依赖的峰值低于居里温度（玻璃态行为的指示），这在以前被解释为冻结的集群。所有的结果是一致的存在一个强烈的趋势，在这种材料中的磁相分离，这是进一步加强考虑的电子性能的结论。金属-绝缘体转变被观察到与铁磁有序（x=0.18）的发病是一致的，并具有在低温电导率的掺杂依赖性的行为，这是强烈暗示渗滤。这可以解释为简单的铁磁集团模型内的渗流转变。在过渡的金属侧，该系统表现出巨大的磁阻型行为，在居里温度附近的负磁阻（在90 kOe的10%）的峰值。作为过渡接近从金属侧，我们观察到的发病率的负磁阻的幅度随着温度的降低而增加，在90千奥斯特场达到高达90%的值。这种磁电阻在金属-绝缘体转变时增强，在那里它甚至持续到室温。
We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet La 1 - x Sr x CoO 3 . The compositions studied span the range fromthe end-member LaCoO 3 (x=0.0) through to x=0.7. These materials have attracted attention recently, primarily due to the spin-state transition phenomena in LaCoO 3 and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of La 1 - x Sr x CoO 3 in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping (x>0.2) the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the Co 3 + and Co 4 + ions are both in the intermediate spin state. At lower Sr doping (x<0.18) the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above x=0.2 the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering (x=0.18) and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (∼10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.