磁性测量讲座 第二讲 研究磁结构的实验方法(I)

物质的宏观磁性来源于物质内部的磁结构,例如铁磁物质的磁化过程,就是磁畴的取向和尺寸变化的宏观反映.物质由顺磁到铁磁或反铁磁的转变是原子磁矩由无序转变为某种形式的有序结构的结果.因此,为了研究物质宏观磁性的来源,探求它的物理本质,人们总希望侥“看“到物质内部的结构     

一种研究磁性材料的核磁共振波谱仪装置

一、铁磁体核磁共振效应的实验特点 铁磁物质中许多元素的同位素(如Fe57,Ni61,Co59,Cr53等)具有核磁矩,它们在一定条件下也会产生核磁共振效应.但是,实验发现铁磁体的核磁共振效应(以下用FNR表示)与一般非铁磁物质的NMR效应不同.例如: 1.对非铁磁物质,共振核的亚能级主要受外加     

胶体铁磁流体的光子特性研究(英文)

这篇综述回顾了我们近年来在铁磁流体方面的一些研究工作。铁磁流体是一种纳米铁磁颗粒或亚铁磁颗粒悬浮在基液(如:水或煤油)中形成的悬浮液。铁磁流体之所以受到广泛关注,是因为它们在许多方面都有着潜在应用,例如在机械工程和生物医学等领域。在此综述中,我们首先介绍了铁磁流体的场感应各向异性结构,然后,介绍了几种铁磁流体基软物质材料的光学性质,即:光学负折射、磁控光子带隙和非线性光学响应。我们采用的研究方法主要是分子动力学模拟,有效媒质近似和有限元模拟。     

胶体铁磁流体的光子特性研究

这篇综述回顾了我们今年来在铁磁流体方面的一些研究工作。铁磁流体是一种纳米磁颗粒或亚铁磁颗粒悬浮在基液（如：水或煤油）中形成的悬浮液。铁磁流体之所以受到广泛关注,是因为它们在许多方面都有着潜在应用,例如在机械工程和生物医学等领域。再次综述中,我们首先介绍了铁磁流体的场感应各向异性结构,然后,介绍了几种铁磁流体基软物质材料的光学性质,即：光学负折射、磁控光子待隙和非线性光学响应。我们采用的研究方法主要是分子动力学模拟,有效煤质近似和有限元模拟.     

非磁性掺杂下的铁磁/反铁磁系统中的交换偏置和矫顽场

采用Monte Carlo方法,分别讨论了在铁磁/反铁磁双层膜和铁磁/反铁磁单层混合膜中,掺入非磁性物质后,掺杂浓度对交换偏置以及矫顽场的影响.计算结果表明:随着掺杂浓度的增大,双层膜和单层膜交换偏置都有先增大后减小的现象,而其矫顽场则先减小后增大.在相同掺杂浓度下,对随机掺杂和规则掺杂两种不同掺杂方式的结果比较发现:铁磁/反铁磁双层膜中,规则掺杂下产生的交换偏置和矫顽场都得到了增强;对于单层混合膜,随机掺杂下的交换偏置更强,规则掺杂下的矫顽场更大.研究发现对于双层膜规则掺杂可明显地导致其磁滞回线的不对称性,说明铁磁/反铁磁系统中磁滞回线的不对称性与界面自旋微结构密切相关.     

示波器观测动态磁滞回线实验中曲线描绘浅析

给出了示波器观测铁磁材料物质磁滞回线的基本原理,针对实验中常见的错误,介绍了如何选取合理的数据并快速描绘磁滞回线的技巧。     

铁基铁磁超导体

超导电性与铁磁长程序是两个"敌对"的物质状态。长期以来,探索超导与铁磁共存是十分耐人寻味的课题。文章在回顾半个多世纪以来该课题主要进展的基础上,重点介绍了作者近年来对新型铁基铁磁超导体的发现和发展,最后对这类新材料的进一步研究进行了展望。     

铁磁相变和居里温度的一种直接观测方法

铁磁相变和居里温度的一种直接观测方法王魁香，陆国会，邓京川，卢杰（吉林大学物理系长春１３００２２）一、原理在物理学中，经常遇到相变的问题，日常生活里又很少有相变现象的例子，所以人们对相变的理解很抽象．而铁磁物质（铁磁相）在居里（或临界）温度Ｔｃ转变成...     

涡流对侧向反铁磁/非磁超晶格表面推迟模的影响

使用等效介质理论研究了在Voigt位形下,由反铁磁半导体和绝缘非磁物质构成的半无限侧向反铁磁/非磁超晶格中的表面推迟模.讨论了在外磁场为零和非零两种情况下涡流对推迟模的影响,给出了同时考虑传导电流和位移电流时,侧向反铁磁/非磁超晶格的频率与波矢的关系曲线以及自旋波衰减随波矢k的变化曲线. 关键词： 推迟模 侧向超晶格 自旋波衰减     

用迈克耳孙干涉仪测量磁致伸缩系数

借助迈克耳孙干涉仪可以精密测量长度变化这一条件，对铁磁物质的磁致伸缩系数进行测量。     

Increase in temperature of a ferromagnetic substance in rf magnetic field

If one applies an rf magnetic field parallel to a strong static field, temperature of a ferromagnetic substance rises due to ferromagnetic resonance. This phenomenon finds immediate application in the field of hyperthermic oncology. In this work, we have found expression for the increase in temperature when a ferromagnetic material is placed under a static and a varying magnetic field of high frequency through spin wave approach. The numerical value of this increment of local temperature has been estimated for yttrium iron garnet (YIG). We also have examined the possibility of enhancing the temperature of a ferromagnetic material only by applying a strong static field.     

Molecular theory of nematic liquid crystals viewed as effect of collective excitation in ferromagnetic systems

We develop a microscopic theory of the nematic phase with consideration of the effect of the collective excitation on properties of nematic liquid crystals. The model is based on the Heisenberg's exchange model of the ferromagnetic materials. Since the orientation of the molecular long axis and the angular momentum of the molecule rotating around its long axis have the same direction, operators can be introduced to research the nematic liquid crystals. Using the lattice model and the Holstein--Primakoff transformation, the Hamiltonian of the system can be obtained, which has the same form as that of the ferromagnetic substance. The relation between the order parameter and reduced temperature can be gotten. It is in good agreement with the experimental results in the low temperature region, the accordance is better than that of the molecular field theory and the computer simulation. In high temperature region close to the transition point, by considering the effect of the higher-order terms in the Hamiltonian, theoretical prediction is in better agreement with the experiment. That indicates the many-body effect is important to nematic liquid crystals.     

Studies on Nematic Liquid Crystal Using Spin Wave Theory

A spin wave theory is proposed to study nematic liquid crystals. Since the orientation of the molecular long axis and the angular momentum of the molecule rotating around its long axis have the same direction, operators can be introduced to research the nematic liquid crystal. By transforming the intermolecular interaction potential,the Hamiltonian of the system has the same form as that of the ferromagnetic substance. The relation of the order parameters to the reduced temperature can be obtained. It is in good agreement with the experimental results in the low temperature region. In the high temperature region close to the transition point, by using the Hamiltonian, the transition point can be obtained, which is near to the Maier-Saupe‘s result.     

Magnetic Study of Nanostructural Composite Material Based on Cobalt Compounds and Porous Silicon

In present work, an investigation of a magnetically ordered material, which is a composite structure obtained by embedding of cobalt-containing substance into pores of silicon matrix, was performed. The samples were characterized by steady-state magnetometry and electron microscopy. The methods of longitudinal nonlinear response to a weak ac magnetic field and registration of electron magnetic resonance were used for detail study of their properties. It was established that the material forms a structure of ferromagnetic particles at the inner surface of pores. It was agued that they are mainly nonmetallic magnetic particles like Co₂B and/or Co₃B. The possibility to apply the concept of blocking temperature to specify the magnetic behavior of the compound on temperature confirmed the single-domain state of these particles.     

Magnetic contribution to the Gibbs energy of elements versus temperature and pressure

A model for the pressure and temperature dependence of the magnetic contributions to the Gibbs energy of ferromagnetic and antiferromagnetic elements is presented. These contributions are described by three parameters: (1) a critical temperature which is represented by the Curie temperature for ferromagnetic elements or Néel temperature for antiferromagnetic elements, (2) the pressure dependence of that critical temperature, (3) the average magnetic moment per atom. Using thermal expansion data, all these parameters and consequently the pressure and temperature dependence of the magnetic contribution can be calculated. Nickel, a ferromagnetic element, is used as an example.     

二维磁性系统和准一维磁性纳米管的内能

采用量子统计理论的多体格林函数法计算二维单离子各向异性海森伯铁磁体、反铁磁体以及单壁铁磁纳米管的内能,对比铁磁体和反铁磁体的结果.在相同的参量下,反铁磁能量总是低于相应的铁磁能量(相变点除外).由于反铁磁能量随温度上升的速度较铁磁能量快,当温度升高到居里点T_C和奈尔点T_N时(T_C=T_N),铁磁能量和反铁磁能量相等.横向关联效应对系统内能的影响较大,不能忽略.     

Magnetic proximity effects in antiferromagnet/ferromagnet bilayers: the impact on the Néel temperature

We present a study of the ordering temperature of an ultrathin antiferromagnetic film in the proximity of a ferromagnetic layer. The Néel temperature of a single-crystalline antiferromagnetic FexMn1-x film on Cu(001) in contact with a ferromagnetic Ni layer was monitored by the discontinuity in the coercivity as a function of temperature by magneto-optical Kerr effect measurements. It decreases by up to 60 K if the magnetization axis of the ferromagnet is switched from out of plane to in plane by deposition of a Co overlayer. These results give clear evidence for a magnetic proximity effect in which the ferromagnetic layer substantially influences the ordering temperature of the antiferromagnetic layer.     

T(3/2) dependence of the interlayer exchange coupling in ferromagnetic multilayers

The temperature dependence of the interlayer exchange coupling in ferromagnetic films coupled across nonmagnetic spacers is determined via in situ ferromagnetic resonance experiments for various systems. Clear evidence for a T(3/2) law is found over a wide temperature regime.     

Room temperature ferromagnetism in Cu-doped ZnO synthesized from CuO and ZnO nanoparticles

AC susceptibility and ferromagnetic resonance (FMR) measurements indicate that ZnO doped with Cu by a simple sintering process starting from nanoparticles of ZnO and CuO is ferromagnetic above room temperature. FMR measurements above room temperature indicate the ordering temperature to be above 520 K. The observation supports the recent theoretical calculations of Huang et al. which predict ferromagnetism in copper-doped ZnO.     

Untersuchung der Neutronendepolarisation von Dy in der Umgebung von magnetischen Umwandlungspunkten

It is possible to determine the dimensions of the ferromagnetic correlation range by depolarisation measurements of polarized thermal neutrons near magnetic transition points, where small magnetic domains (< 10^?4 cm) are present. The dimensions of the magnetic domains of Dy were determined in the temperature range from 4,2 °K to room temperature and in an external magnetic field from 0 to 2,4 kOe. The size of the domains increases with decreasing temperature and increasing external field. For low temperatures a sort of internal coercive force for the wall mobility was observed, which strongly hinders the formation of greater ferromagnetic domains. At the Curie-point the ferromagnetic correlation range shows a continuous transition and goes only slowly to zero when the temperature increases.