CN101529514B - 用于转换可磁化介质的磁光转换装置和方法 - Google Patents
用于转换可磁化介质的磁光转换装置和方法 Download PDFInfo
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Abstract
本发明涉及用于转换介质中的磁性的磁光转换装置,该装置包括可磁化介质。根据本发明,放射系统适用于向所述可磁化介质的磁自旋系统施加角动量,以对所述介质的磁性进行选择性地取向。另外,本发明还涉及转换可磁化介质的方法,包括:提供可磁化介质;提供具有选择性选择的角动量的放射光束;以及将该放射光束靶向该介质以将该角动量传递给该可磁化介质的磁自旋系统。因此,通过使用适当的角动量,能够控制磁材料中的自旋状态。有效磁场被产生以对磁畴的磁化进行取向并能够同步地用于局部加热该材料。
Description
技术领域
本发明涉及用于转换介质中的磁性的磁光转换装置,尤其是,用于信息记录的目的。另外,本发明还涉及用于转换可磁化介质的方法。
背景技术
在磁和磁光转换装置中,尤其是用于记录目的的磁和磁光转换装置中,信息比特被存储为分别代表值“0”和“1”的相反取向的磁畴。转换自旋(spin)或磁畴的传统方法是沿自旋的方向提供与其初始取向相反的外部磁场。这是一个相当慢的逆转过程。可选地,可以垂直于而非平行于自旋取向来提供外部磁场。通过围绕外部提供的磁场的自旋的旋进运动(precessional motion),这将引起快得多的逆转过程。目前有多种磁和磁光记录装置是使用这种磁性逆转原理来制造的,藉此可以写入和重新写入信息。
设计用来实现超高数据存储密度的新的类型的这种磁记录装置即是所谓的热辅助磁记录(heat assisted magnetic recording,HAMR)。此处,通过激光束对高各向异性的磁介质进行加热以使外部提供的磁场在对磁性进行逆转中仍然是有效率的。然而,该方案没有简化磁记录过程,相反还需要更多的元件,从而导致高的磁装置制造成本以及更高的功耗。另外,I.Tudosa等人在Nature428,831(2004)中以及C.H.Back等人在Nature 428,808(2004)中已经证实,由磁场引起的磁转换的极限速度被限制在皮秒的时间尺度中。由于不断增加数据存储密度的需求也需要磁性转换速度的增加,所以期望有除借助磁场以外的新的和更快的转换磁性的方法。在2006年5月25日在线公布的Nature Letters中,Kimel等人的“瞬时光磁脉冲的超快非热学磁性控制(Ultrafast non-thermal control ofmagnetization by instantaneous photomagnetic pulses)”,进一步称之为“Nature Letters论文”,其中对电介质材料的非热激发(non-thermalexcitation)进行了说明,通过引证将其结合于此。在Physical ReviewLetters第047402(2005)中,Hansteen等人的“亚铁磁石榴石膜中自旋的飞秒级光磁转换(Femtosecond photomagnetic switching of spinsin Ferrimagnetic Garnet Films)”,进一步称之为PRL论文,其中示出成功地控制了小角度磁进动(magnetization precession),其也通过引证结合于此。其中所说的光感效应材料是绝缘材料。
发明内容
期望提供一种用于转换介质中的磁性的磁光转换装置,其中,快速和可靠的转换是可能的。为此,根据本发明的一个方面,提供了一种用于转换介质中的磁性的磁光转换装置。具体地,提供了:一种用于转换介质中的磁性的磁光转换装置,包括:可磁化介质;以及放射系统,适用于向该可磁化介质的磁自旋系统施加(impart,传递)角动量,以对该介质的磁性进行选择性地取向。
因此,通过使用适当的角动量的放射,尤其是,通过圆偏振光或椭圆偏振光,能够控制磁材料中的自旋状态。
根据本发明的另一方面,提供了一种方法。具体地,提供了一种用于转换可磁化介质的方法,包括:提供可磁化介质;提供选择性选择角动量的放射束;以及将该放射束靶向该介质以将该角动量传递给该可磁化介质的磁自旋系统。
附图说明
现在将参照附图,仅通过实例的方式对本发明实施例进行描述,附图中,对应的参考标号表示对应的部分,其中:
图1示出了对作为例证的基板样本的a)磁性和b)磁化系数的温度相关性的定性示意图;
图2示出了通过法拉第效应观察并由CCD照相机捕捉到的磁畴的图像;
图3示出了实验装置的示意图;
图4示出了根据相反磁状态的两个样本的激光流结果;
图5示出了左手和右手激光束螺旋性的点图;以及
图6示出了由激光束在基板上快速扫过产生的单激光脉冲的点图。
具体实施方式
在各种磁光现象中都表明了光与磁化介质的相互作用。一个好的例子是法拉第效应,可以观察到通过磁介质传递的光的偏振面的旋转:
其中,αF是比法拉第旋转角(specific Faraday rotation),M是磁性,n是折射率,k是光的波矢量(wave vector),以及x是磁光磁化系数,该系数在各向同性介质中是标量值。诸如磁光绝缘体和 调制器的各种装置在透光磁性化合物中使用大的法拉第旋转角的值。
鲜为人知的是逆法拉第效应,其中,高密度的激光放射当作磁场作用于介质上并感应出静态的磁性M(0):
其中,E(ω)和E*(ω)分别是光波的电场及其共轭复数。根据等式2可以得出处于频率ω的圆偏振光感应出沿波矢量k的磁性。应当注意,等式2的对称描述说明圆偏振光的光激发和外部磁场的作用之间的等价。另外,右手和左手圆偏振波感应出异号(opposite sign)的磁性。等式1和等式2示出了这两种现象均由相同的磁光磁化系数x确定。具体地,在逆法拉第效应的情况下,x是感应的磁性和激光密度之间的比。所以,在每单位磁性的法拉第旋转角度值高的材料中,磁性光控制可望更有效。磁化系数x的另一个重要特性是其没有对称限制并因此适用于所有介质不用考虑其晶体结构和磁结构。另外,逆法拉第效应不需要吸收,并且被认为是基于类拉曼相干光散射过程。磁性上的光效率是非热学的与由于其看起来是发生在飞秒时间尺度而能够被认为是瞬时的之间具有重要的逻辑关系。近年来的理论工作表明了在飞秒时间尺度上的激光感应自旋逆转是可能的。然而,直到现在,这种磁性的非热学超快光控制的实验论证仍然是引起人们产生兴趣的挑战。
根据本发明的一个方面,通过放射系统将角动量传递到该可磁化磁畴的磁自旋系统以对其磁性进行取向。具体地,圆偏振光或椭圆偏振光被用于控制和/或转换磁材料中的磁性,诸如使用在磁光装置或热辅助磁随机存取存储器(MRAM)中。更具体地,左手圆偏振光将磁系统的自旋取向一个方向,而右手圆偏振光将自旋取向相 反的方向。如通过本发明所理解的,产生一个有效磁场以对磁畴的磁化进行取向并且能够(但不是必须)被同步地用于局部加热该材料。该过程显示出根本不同于根据外部磁场进行的取向,并且该过程显示出其固有的非常快的特性,并且是光学特性。另外,由于不需要外部磁场,因此本过程也减少了制造成本。
在图1中,示出了在室温下对作为例证的基板样本的a)磁性和b)磁化系数的温度相关性的定性示意图。众所周知,材料中的磁性M与作用于其上的有效磁场成比例:
M=xM·H
并且其取决于材料的磁化系数xM。在居里温度(Tc)处,xM发散(图1b)。因此,所需的用来控制磁性的磁场在居里温度附近处具有最小值,并且相关的低磁场可以仍然高于材料的矫顽磁场。因此,优选地,由激光束的螺旋性感应的磁场在居里温度附近能够更有效的使材料的磁性取向。
转向图2,其示出了通过使用传统光学显微镜配置的法拉第效应观察到的磁畴2的记录轨迹1。黑区3代表具有在垂直于样本的一个方向取向的磁性的磁畴2,该方向被称为“向下”。因此,白区4代表在相反方向取向的磁畴2,该方向称之为“向上”。样本的初始状态具有“向下”取向(黑)的磁性。通过在样本上扫过激光束并替换地在右手和左手圆偏振光之间改变光的圆环(circularity oflight)来建立磁畴2。从而圆偏振激光脉冲的一个类型建立向上磁畴,逆转样本的初始状态,而相反的圆偏振激光脉冲试图在与初始状态相同的方向对磁性取向,从而使初始状态不改变。结合与样本相关的光束的扫描速度,选择激光束的螺旋性的重复率,以使点部分重叠地写入,从而导致黑和白半圆相邻。实验是在室温下进行的,并且使用的激光量(激光能量密度,laser fluence)为大约5mJ/cm^2。 然而,本实验能够根据磁材料的特性在任何温度下重现。另外,材料属性也限定了用于有效磁性控制所需的激光量。
在扫描期间,如图3所示,通过使用四分之一波板13使激光束的偏振状态在右和左螺旋之间变换。通过使扫描速度保持相对恒定,激光的平均密度和相应的热负载均保持基本恒定。此处,记录轨迹1的宽度取决于激光束点尺寸、激光束轮廓、以及脉冲宽度和密度的组合。在实际的实施例中,可以通过扫描速度、密度、波长的组合来最优地调节用于磁转换的目标温度,并且热吸收材料可布置在基板中以提供用于目标材料的磁转换的目标温度(具体地,居里温度),以及调节吸收特性、导热、光磁响应。
激光脉冲对磁材料的影响在材料上具有加热效应,其随后导致材料的磁-晶各向异性的降低。该效应被用在传统的磁和磁光记录以及用在近年来开发出的HAMR配置中以降低样本的矫顽力以使小的外部磁场即能够改变所关心的材料的磁性状态。另一方面,如此处所述,如果激光脉冲也是圆偏振光,在加热效应之外,在不具有或具有很小的吸收的情况下,甚至在没有加热效应的情况下,也将对材料中的自旋取向。该证实不仅对自旋控制过程的简化有利,还对这些过程的速度有利。更具体地,由于磁性或自旋取向取决于光螺旋性,所以光子的角动量的取向仅在材料中存在激光脉冲时影响磁系统。所以重新取向过程的开始发生在飞秒时间尺度。
图3示出了实验装置的示意图。具体地,通过使用电磁体6来获得样本5的初始状态。在典型的情况下,由于样本的强各向异性,仅允许建立向上或向下磁畴。通过使用来自光源7的光和起偏镜8,通过传统的法拉第旋转,能够通过物镜9和检偏器10将磁畴状态分析到CCD照相机11。
为控制并转换样本5中的磁性,来自钛:兰宝石激光放大系统12的传递飞秒激光脉冲的脉冲激光束以1kHz的重复速率靶向样本5。实验是在室温下进行的,并且使用800nm波长的40fs激光脉冲激发该磁系统。四分之一波板13用于控制激光脉冲的偏振。激光束在样本5上接近法向入射。通过使用传统光学显微镜配置的法拉第效应可以观察到激光脉冲与磁系统的相互作用的结果。
图4示出了根据两种相反磁状态(M+和M-)情况下的激光量的结果,具体地,示出了在向下取向初始磁状态(前两行)上和在向上取向初始磁状态(后两行)上的两种相反圆螺旋(σ+和σ-)。在这些实验中,通过提供临时的静态磁场以建立单磁状态来获得样本的初始磁状态。
接下来,通过激光束来激发样本,并在关闭激光束后获得图片。从图4中能够看出,确实如所期望的,对于确定值(2.9mJ/cm2)的激光束能量密度,能够观察到对光的一个螺旋性(第一行)的清晰转换,而对于相反的螺旋性(第二行)则什么也不会发生。
对称地,对于相反的初始磁状态,先前成功转换样本中的磁性(第一行中)的螺旋性现在不影响磁状态(第三行),而现在相反螺旋性能够感应清晰转换区(clean switched area)(第四行)。
其示出,对于高的能量密度,激光束轮廓的一部分将样本的局部温度提高到居里温度以上,从而产生样本的消磁状态。然而,由于激光束轮廓是高斯轮廓,所以应当期望,位于激光束边缘的区正好在需要的样本中感应出正确的温度以进行清晰激光感应转换。从此处可以得出,特定的激光量确实应当感应出总转换,而在关闭激光束后应该不会产生消磁状态。实际上,理想地,为不产生消磁状态,光束的中心应该保持在低于居里温度的温度以有效地进行清晰转换。尽管较高温度下的转换是可行的,但还是期望,为了高速写 入的目的,这种较高温度是比较不利的,因为在激光束的信息离开之后,该区的温度仍然太高而不能保持消磁状态。因此,在本发明的优选实施例中,放射系统被布置以向可磁化磁畴施加保持在居里温度之下的热能。
更进一步地,具有低于居里温度的温度的退磁磁场也可以对这些区域重新取向,以使写入的信息丢失。其在图4中被示以更高的激光量(在本具体实施例中高于2.9mJ/cm^2)。因此,当点过热时,转换可根据退磁磁场发生。因此,优选地,放射系统被布置为施加足够低的热能以使磁畴的矫顽磁力高于周围磁畴的退磁磁场。
如图5所示,在本发明的另一实施例中,光束的中心可达到高于居里温度的点,并且记录信息可(仅)被一直存储在围绕该光束中心的环中,其中,激光束量,至少在环中,使得逆转的法拉第磁场高于矫顽磁场,并且环中施加的热能被保持在低于可磁化磁畴中的居里温度的温度。实际上,在图5(a)中,在每个点15中间的灰色中心区域14部分地表示由于光的加热的不具有磁反差(magneticcontrast)的顺磁状态,以及部分地表示在平均很多个脉冲后的多磁畴状态。然而,在点的外面是根据光螺旋的逆转磁性的清晰半圆16。因此,σ+(σ-)光束造成了在灰色点与黑(白)磁畴之间的白(黑)线。
图6示出了由激光束在基板上快速扫过造成的单激光脉冲的点图。因此,其说明了其中磁性产生的时间尺度至少位于几十飞秒内,尤其是,少于40飞秒。因此,通过使用本发明的该技术,以THz量级写入进行超快记录是可行的。
用于进行实验的基板材料包括金属稀土—过滤金属合金,具体地,基板包括具有大约500K的居里温度的Gd22Fe74.6Co3.4的薄膜层的典型构成。通过磁控溅射生成样本,该样本通常形成多层结 构:玻璃/AlTi(10nm)/SiN(5nm)/GdFeCo(20nm)/SiN(60nm)。AlTi被用作加热槽,同时SiN被用作缓冲器和盖层。这些样本的磁饱和度在4πm=1000G室温左右。
当如图1中所示的居里点的附近进行退磁时,系统的磁化系数发散,并因此,如所证实的,诸如逆法拉第效应的弱外部刺激可以驱使磁性进入确定状态。前面已经论述,逆法拉第效应能够导致非常高的有效场,其将扩展本发明的有效温度范围。除此之外,可期望的并且也是可能的是,为降低居里温度附近的矫顽磁场而调节基板的特性,从而能够使用更宽的温度范围,以及更具体地,使激光量能够被更容易地调节至达到条件要求以转换磁性。可以通过改变化学成分或生长参数来调节居里温度以使转换需要的激光量变少。在一个实施例中,这可以用亚铁磁合金来实现。该系统中感兴趣的另一个参数是退磁磁场。为在更宽的温度范围中保持由光感应的消磁重新取向,还应当调节退磁磁场以使其值低于在需要的温度范围内的矫顽磁场值。
如通过本发明所理解的,由于激光脉冲的两个共同协作的效应,所以发生转换。首先,对于金属介质,部分脉冲能量被金属中的电子所吸收。通过司东纳(Stoner)自旋散射机构,这个过程将导致自旋温度的超快增加。因此,磁系统的温度能够在几十飞秒内显著增加。
这也导致了该系统的有效磁化系数的增加。
其次,圆偏振激光脉冲通过自旋轨道作用于自旋,耦合为有效磁场,该效应即已知的逆法拉第效应。该磁场的幅度与磁光常数成比例,其在一级近似中不依赖于温度。
因此,在表现上,整个效应是磁系统的加热通过逆法拉第效应加上有效磁场的应用。由于磁化系数在居里温度附近发散,所以转换是非常有效率的。
40 fs的脉冲长度意味着整个转换在该时间范围内初始化。实际上,因为其基于相干散射过程,所以可以将逆法拉第效应认为是瞬时的。所以,驱动力应当与脉冲一起消失。
技术人员应当理解,在本说明书的上下文中,根据上下文,术语“基板”“介质”或“材料”的任意使用均指在为磁写入目的将激光靶向其以建立逆法拉第效应的可磁化物质。然而,另外这些基板可包括一系列的用于稳固的支撑层,并为实际的目的来调节磁材料。尽管其假设磁材料被沉积为在基板表面上的层,但其它实施例也是可行的。另外,以此处提到的磁材料作为顶层也不是必须的。
此处使用的术语“放射”和“光束”可包括所有类型的适用的电磁放射,包括红外线或紫外线辐射。
在本申请中,术语“转换”是指对介质中的磁性进行选择性的取向。具体地,磁性可被在一个状态和另一个状态之间取向,具体地,在磁性状态之间转换。同样,根据应用目的,磁性可被暂时地从基态转换到暂态。
尽管上述已经对本发明的具体实施例进行了描述,但应当理解,除上述之外,本发明还可以其它方式实施。具体地,尽管实施例着重于磁记录应用,但本发明不限于此。上述放射感应磁性也能够用于实现诸如用于激光设备的法拉第旋转器的光转换。另外,这种激光感应磁转换能够用于在例如为光通信目的的信号处理应用中控制光信号。具体地,能够使用透光电介质类型的磁材料来实现这些类型的应用,例如,诸如Nature Letters论文或PRL论文中所 描述的,但不限于此。上述描述是为了举例的目的,而不是为限制的目的。因此,对于本领域的技术人员来说,在不背离所附的权利要求的范围的情况下,能够对上述本发明做出改进是显而易见的。
Claims (20)
1.一种用于转换介质中的磁性的磁光转换装置,包括:
可磁化介质(5);以及
电磁辐射系统(12),能够产生具有右手或者左手圆偏振光或椭圆偏振光的电磁辐射束,适于通过所述电磁辐射束的电磁辐射向所述可磁化介质(5)施加角动量,以控制所述可磁化介质中的自旋状态,以便通过取决于所述电磁辐射的所述右手或者左手圆偏振光或者椭圆偏振光的有效磁场对所述可磁化介质的磁性进行选择性地取向,用于根据右手或者左手圆偏振光或椭圆偏振光而将所述可磁化介质中的磁性取向在一个方向中或者相反方向中。
2.根据权利要求1所述的装置,其中,所述电磁辐射系统(12)能够产生具有对应于待被记录的信息状态的右手或者左手圆偏振光或椭圆偏振光的电磁辐射束。
3.根据权利要求1所述的装置,其中,所述电磁辐射系统被布置为提供这样的电磁辐射束,所述电磁辐射束具有的密度使得通过逆法拉第效应而光感应的磁场强度高于需要用来转换磁性的磁场强度。
4.根据权利要求1所述的装置,其中,所述电磁辐射系统被布置为提供这样的电磁辐射束,所述电磁辐射束能够向所述可磁化介质(5)的可磁化磁区(2)施加一热能,使得所述可磁化介质(5)的可磁化磁区(2)的温度保持低于所述可磁化磁区(2)的居里温度。
5.根据权利要求1所述的装置,其中,所述电磁辐射系统被布置为提供这样的电磁辐射束,所述电磁辐射束的热能足够低以使所述可磁化介质的可磁化磁区(2)的矫顽磁力的强度高于所述可磁化磁区(2)周围的退磁磁场强度。
6.根据权利要求1所述的装置,其中,所述可磁化介质(5)的基板包括布置为用于实现所述可磁化介质的磁转换的目标温度的热吸收材料。
7.根据权利要求1所述的装置,其中,所述可磁化介质(5)包括金属稀土-过滤金属合金。
8.根据权利要求1所述的装置,其中,所述可磁化介质(5)包括亚铁磁材料。
9.根据权利要求1所述的装置,其中,所述可磁化介质的可磁化磁区(2)的磁性根据所述电磁辐射系统提供的电磁辐射束的角动量而在两个相反方向上取向。
10.根据权利要求1所述的装置,其中,所述可磁化介质(5)的基板包括底部基板层、散热层、以及包括在缓冲层和/或盖部层中的可磁化磁区层。
11.根据权利要求10所述的装置,其中,所述底部基板层包括玻璃,其中,所述散热层包括AlTi,以及其中,所述缓冲层包括SiN。
12.一种用于以相反磁性或相反自旋区来记录信息“比特”的记录装置,包括根据权利要求1所述的装置并且进一步包括用于预先确定角动量以便表示信息的装置,由此在所述介质上记录所述信息。
13.一种用于转换可磁化介质的方法,包括:
提供可磁化介质(5);
提供具有右手或者左手圆偏振光或椭圆偏振光以及预定角动量的电磁辐射束;以及
将所述电磁辐射束靶向所述可磁化介质(5)以将所述角动量传递至所述可磁化介质(5),通过取决于所述电磁辐射束的右手或者左手圆偏振光或椭圆偏振光的有效磁场来控制所述可磁化介质中的自旋状态,用于根据右手或者左手圆偏振光或椭圆偏振光而将所述可磁化介质中的磁性取向在一个方向中或者相反方向中。
14.根据权利要求13所述的方法,其中,所述电磁辐射束能够对所述可磁化介质(5)的可磁化磁区(2)施加一热能,使得所述可磁化介质(5)的可磁化磁区(2)的温度能够被保持低于所述可磁化磁区(2)的居里温度。
15.根据权利要求13所述的方法,进一步包括预先确定所述角动量以与待被记录的信息对应,从而通过将所述电磁辐射束靶向所述可磁化介质而将所述信息记录到所述可磁化介质上。
16.根据权利要求13所述的方法,进一步包括使用脉冲电磁辐射束。
17.根据权利要求16所述的方法,其中,所述脉冲电磁辐射束的脉冲持续时间在100飞秒到1飞秒范围之间。
18.根据权利要求17所述的方法,其中,所述脉冲电磁辐射束的脉冲持续时间小于40飞秒。
19.根据权利要求13所述的方法,进一步包括:提供金属稀土-过滤金属合金作为可磁化介质,并为信息记录的目的而转换所述可磁化介质的磁性。
20.根据权利要求13所述的方法,进一步包括:提供非金属可磁化介质,并为光传输和转换的目的而转换所述可磁化介质的磁性。
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US8068715B2 (en) | 2007-10-15 | 2011-11-29 | Telescent Inc. | Scalable and modular automated fiber optic cross-connect systems |
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JP5812380B2 (ja) * | 2010-07-16 | 2015-11-11 | 学校法人日本大学 | 情報記録ヘッド、情報記録装置、情報記録方法及び光デバイス |
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